Geology and Tectonic Framework of the Keowee- Toxaway Region

The 24th Annual David S. Snipes/Clemson Hydrogeology Symposium Field Trip Guidebook

Geology and Tectonic Framework of the Keowee- Toxaway Region, Northwestern South Carolina and Engineering Geology of the Bad Creek Pumped Storage Project, Northwestern South Carolina

Malcolm F. Schaeffer HDR Engineering, Inc. 440 South Church Street Suite 900 Charlotte, NC 28202-2075

Field Trip Leaders: Malcolm Schaeffer and Scott Brame March 30, April 1, and April 28, 2016

GEOLOGY AND TECTONIC FRAMEWORK OF THE KEOWEE-TOXAWAY REGION, NORTHWESTERN SOUTH CAROLINA

Malcolm F. Schaeffer, HDR Engineering, Inc., 440 South Church Street, Suite 900, Charlotte, North Carolina 28202-2075, [email protected]

INTRODUCTION

A significant amount of geologic work has been completed in the last fifteen years in the Keowee-Toxaway Region of northwestern South Carolina and vicinity (Garihan 2001, 2005, 2008; Garihan et al. 2005; Hatcher 2000, 2001a, 2001b; Hatcher and Lin 2001; Hatcher et al. 2001; Garihan and Ranson 2003; Merschat et al. 2003; Merschat and Hatcher 2007; Clendenin and Garihan 2004, 2007a, 2007b, 2008; Garihan and Clendenin 2007; Merschat et al. 2010) that builds on and extends the ground-breaking work by Villard Griffin (1967, 1969, 1971, 1973, 1974a, 1974b, 1975, 1993) and Bob Hatcher (1969, 1970, 1971, 1972, 1973, 1977, 1978a, 1978b, 1984; Hatcher et al. 1973; Hatcher and Acker 1984) and work by others in the area and region (Brown and Cazeau 1964; Cazeau 1967; Acker and Hatcher 1970; Lemmon 1973, 1981; Lemmon and Dunn 1973a, 1973b; Odom and Fullagar 1973; Roper and Justus 1973; Bond 1974; Horton 1974; Odom and Russell 1975; Clark et al. 1978; Fullagar et al. 1979; Harper and Fullagar 1981; Bobyarchick 1983, 1984; Edelman et al. 1987; Bobyarchick et al. 1988; Horton and McConnell 1991; Nelson et al. 1998)1. Hatcher (1993, 2002), Garihan et al. (2001), and Garihan and Ranson (2012), in field guides for the Carolina Geological Society, summarized the state of knowledge and advances in the understanding of the geology/tectonics of the region, in particular the Inner Piedmont at the time of those trips.2 Recent David S. Snipes/Clemson Hydrogeology Symposium field trips have examined the geology of the region in the light of the recent geologic work and interpretations (Goforth et al. 2012; Clendenin and Garihan 2013; Goretoy and Brame 2014; Sellers and Brame 2015). In the last twenty-five years major changes have occurred in geologic concepts and methods of analysis resulting in new theories of the tectonic development of the Appalachians. These geologic concepts and methods of analysis are continuously being updated as additional data are collected and different and sometimes contradictory interpretations are developed. A number of tectonic models of the southern and central Appalachians have been developed based on new data, including but not limited to state-of-the-art sensitive high-resolution zircon age-dating (SHRIMP), laser ablation inductively coupled plasma mass spectrometer (LAICP-MSI), new isotope data (Pb isotope and Sm-Nd), chemical analysis of metavolcanic and plutonic rocks, new detailed geologic mapping, an improved understanding of tectonic links between internal and external parts of the Appalachians, and modern geophysical data, primarily aeromagnetic and gravity (Hatcher et al. 2007) Recent models include those by Rankin et al. (1989), Horton et al (1989), Hatcher et al. (1990), Hibbard et al. (2002, 2006), Hatcher et al. (2007) and Hatcher (2010)3. This review seeks to summarize and synthesize the work in the Keowee-Toxaway Region in the context of a

1 This is by no stretch of the imagination all the work that has been done in the area; they are mainly the ones I am familiar with and have referred to in the past when working in and writing about the Bad Creek project and region. 2 A number of the Carolina Geological Society Annual Field Trip Guides are available on the Society’s website: http://carolinageologicalsociety.org/CGS/Guidebooks.html 3 Again, not an exhaustive list of models developed over the last twenty-five years. 1

Southern and Central Appalachians tectonic framework model developed by Hatcher et al. (2007) that incorporates the advances made by geologic investigations and increased knowledge of the region over the last twenty-five years.

GEOLOGY OF THE KEOWEE-TOXAWAY REGION

Introduction

The crystalline rocks of the southern Appalachians occur in northeast-trending parallel geologic terranes. The Keowee-Toxaway Region is within the Tugaloo terrane that includes rocks of the eastern Blue Ridge northwest of the Brevard zone as well as the rocks of the western Inner Piedmont southeast of the Brevard zone (Figure 1). The Tugaloo terrane maintains the same stratigraphic sequence (Tallulah Falls-Ashe Formation) across the Brevard zone (Figure 2; Hatcher et al. 2007) and contains detrital zircons of Laurentian provenance (Bream et al. 2004; Bream 2003, in Hatcher et al. 2007). The late Cambrian to early Ordovician Chauga River Formation and middle Ordovician Poor Mountain Formation sedimentary and volcanic rocks overlie the Tallulah Falls-Ashe sequence and are intruded by various granitoid rocks (Henderson Gneiss, Table Rock suite, and others) in the Inner Piedmont south of the Brevard zone (Hatcher 2002; Bream 2003; in Hatcher et al. 2007). A compiled geologic map of the Keowee-Toxaway Region is presented in Figures 3a, 3b, and 3c.

Blue Ridge - Introduction

The Blue Ridge is a mountainous zone that extends from southern Pennsylvania to central Alabama and varies in width from less than 24 km to about 100 km. Its greatest width is in the Tennessee/Carolinas/North Georgia segment. It is a complex crystalline terrane consisting of Precambrian gneissic basement core structurally overlain by a vast thickness of metasedimentary and metavolcanic rocks of Precambrian to lower Paleozoic age (Hatcher 1978a, 1978b). Numerous igneous bodies of mafic to felsic composition intrude into the basement core and the overlying metasedimentary and metavolcanic sequence. The structure of the Blue Ridge is controlled by major thrust faults, associated complex polyphase folding, and later brittle faulting (Hatcher 1978a). The southern Blue Ridge is divided into three belts: 1) a western belt of imbricate thrust sheets involving upper Precambrian and lower Paleozoic rock and some basement rocks, 2) a central belt containing most of the basement rocks exposed in the Blue Ridge terrane along with higher grade upper Precambrian and possible lower Paleozoic metasedimentary rocks, and 3) an eastern belt of high grade early Paleozoic metasedimentary and metavolcanic rocks (Hatcher 1978a, 1978b; Hatcher et al. 2007). The eastern belt of the southern Blue Ridge comprises those portions of the Tugaloo terrane that occur northwest of the Brevard zone (Figure 1).

Blue Ridge – Keowee-Toxaway Region

The principal rock units of the western Tugaloo terrane (eastern Blue Ridge belt) within the Keowee-Toxaway Region are the Tallulah Falls Formation (Cambrian) and the underlying Toxaway Gneiss (Hatcher 1977; Figure 2) which underlie the upper portion of Lake Jocassee northwest of the Rosman fault and Brevard zone (Figure 3c). Rocks of the Tallulah Falls

Keowee-Toxaway Region

Figure 1: Tectonostratigraphic terrane map of the southern and central Appalachians (from Hatcher et al. 2007) and approximate location of the Keowee-Toxaway Region (Figures 3a, 3b, and 3c).

Formation consists of metagraywacke, pelitic schist, mafic volcanic rocks, quartzite and are migmatitic. They are intruded by Paleozoic granitoid rocks and overlie 1,000 to 1,200 million years ago (Ma) Grenville basement in the Toxaway Dome (Hatcher 1977) and Paleozoic oceanic crust (Figure 2; Hatcher 2002). The formation consists of four members: 1) the quartzite-schist member, 2) the lower graywacke-schist-amphibolite member, 3) the garnet-aluminous schist member, and 4) the upper graywacke-schist member (Hatcher 1977). The lower member contains quartzite with interlayered schist. The lower graywacke-schist-amphibolite member contains metagraywacke (quartz-biotite-plagioclase-muscovite gneiss), amphibolite, muscovite schist, and biotite schist. Layers of granitic gneiss and pegmatites also occur in the lower member. Overlying this member is the garnet-aluminous schist member. It consists of muscovite-garnet-kyanite schist with interlayered amphibolite, muscovite schist, metagraywacke, granitic gneiss, and pegmatites. It is generally easily recognizable by abundant garnet and kyanite. The upper graywacke-schist member contains metagraywacke, muscovite schist, muscovite-biotite schist, and minor amounts of amphibolite, granitic gneiss, and pegmatites. In some areas surrounding the Toxaway Dome, the lower members are absent, suggesting either non-deposition or faulting out of the lower members (Hatcher 1977; Merschat et al. 2003; Schaeffer 2007; Clendenin and Garihan 2007a). Mylonitic fabrics, up to several thousands meters thick, in the Talluluh Falls rocks northwest of the Brevard zone are attributed to deformation along the zone (Clendenin and Garihan 2008, Cattanach et al. 2012). Although recent mapping (Merschat et al. 2003; Clendenin and Garihan 2007a; Cattanach et al. 2012) of the Tallulah Falls Formation in the area does not distinguish the members described by Hatcher (1977), they describe similar lithologic sequences.

Figure 2: Stratigraphic relationships in the Tugaloo terrane (from Hatcher 2002).

The Toxaway Gneiss, part of the Precambrian basement of the eastern Blue Ridge, is exposed in the core of the Toxaway Dome. It is typically a medium- to coarse-grained banded biotite- plagioclase-microcline-quartz gneiss with some massive and augen varieties present, that do not appear to be significantly different in composition (Schaeffer 1987, 2007; Merschat et al. 2007).

Walhalla Nappe

Six Mile Thrust Sheet

Figure 3a: Geologic Map of the Keowee- Toxaway Region (Griffin 1967, 1973; Kenwill 1982; Schaeffer 1987). Sheet 1 of 3

Jocassee Thrust Sheet

Walhalla Nappe

Six Mile Thrust Sheet

Figure 3b: Geologic Map of the Keowee- Toxaway Region (Griffin 1967, 1973; Kenwill 1982; Schaeffer 1987; Clendenin and Garihan 2007a). Sheet 2 of 3

Brevard Zone

Eastern Blue Ridge

Jocassee Thrust Sheet

Figure 3c: Geologic Map of the Keowee- Toxaway Region (Hatcher 1977; Kenwill 1982; Schaeffer 1987; Clendenin and Garihan 2007a). Sheet 3 of 3

The Toxaway Gneiss has a Rb/Sr whole-rock isochron age of 1203+54 Ma (Fullagar et al. 1979). A zircon age for the Toxaway Gneiss is 1.15 Ga (Carrigan et al. 2003, in Hatcher et al. 2007). The migmatitic Tallulah Falls Formation rocks are metamorphosed to the upper amphibolite facies (Hatcher 1977). Dominant metamorphic fabric and peak metamorphism in the eastern Blue Ridge, dominantly ca 450 Ma based on metamorphic ages of detrital monazite and zircon grains from the Talluluh Falls rocks, is interpreted to be the result of middle Ordovician Taconian orogenesis (Moecher et al. 2011; Cattanach et al. 2012). The Grenvillian basement rocks of the Blue Ridge terrane, including the Toxaway Gneiss, were subjected to granulite facies metamorphism about 1000 Ma (Fullagar and Odom 1973; Hatcher and Butler 1979). Crystalline thrust sheets dominate the Blue Ridge terrane of the southern Appalachians with ages ranging from pre-Taconic (>480 Ma) to Alleghanian (~265 Ma) in age (Hatcher 1978a; Hatcher and Odom 1980; Hatcher et al. 2007). The earliest thrusts were complexly deformed by later deformation and overprinted by the main metamorphic event. The western boundary thrust (Chattahoochee-Holland Mountain Fault) of the Tugaloo terrane truncates the Rabun Granodiorite (335 Ma; Miller et al. 2000 and Mapes et al. 2002 in Hatcher et al. 2007) indicating final emplacement of the terrane occurred during the Alleghanian orogeny (Hatcher et al. 2007).

Inner Piedmont - Introduction

The Inner Piedmont is located southeast of the Blue Ridge and is separated from it by the Brevard zone. The Brevard zone extends from the coastal plain overlap in Alabama to near the North Carolina – Virginia state line where it may merge with the faulting along the northwest side of the Smith River allochthon. In the region the Brevard zone is characterized by a linear topographic expression with relief of about 300 meters. Within South Carolina and North Carolina, the Inner Piedmont has a maximum width of approximately 100 km, but narrows both to the northeast and southwest. The Inner Piedmont terrane is a fault-bounded composite stack of crystalline thrust sheets of medium- to high-grade metamorphic rock assemblages, overlapping northwest to southeast, and containing a variety of gneisses, schists, amphibolites, sparse ultramafic bodies, and intrusive granitoids. The general structure of the thrust sheets is characterized by irregular, generally shallow-dipping foliation, and folds transverse to the northeast regional geologic trend. The stratigraphy of the thrust sheets southeast of the Brevard zone in the eastern Tugaloo terrane consists of the Tallulah Falls Formation (Cambrian) overlain by the Chauga River (Cambrian – early Ordovician) and Poor Mountain Formations (middle Ordovician; Figures 1 and 2; Hatcher 2001; Hatcher et al. 2007). The stratified rocks of the belt consist of thinly layered mica schist and biotite gneiss interlayered with lesser amounts of amphibolite, calc-silicate rocks, hornblende gneiss, and quartzite. Protoliths of these rocks were largely sedimentary and in part volcanic. Large and small masses of granite and granodiorite in the belt form concordant to semi-concordant bodies in the country rock. Some of these granitoid bodies are gneissic and are probably older than the poorly foliated to nonfoliated granitoid facies. Small pods of metamorphosed ultramafic rocks are present in the western Inner Piedmont of the region and are dispersed throughout the Walhalla nappe with a few bodies present in the Six Mile thrust sheet (Griffin 1974b; Bridgeman and Ranson 2012; Hatcher et al. 2007). Bridgeman and Ranson (2012) interpreted the pods and bodies as metamorphosed oceanic crustal basalts faulted into place, consistent with the interpretation that the Inner Piedmont overlies oceanic crust with scattered Grenvillian age basement rocks (Figure 2; Hatcher 2002). The rocks of the central core of the Inner Piedmont reach sillimanite grade zone of metamorphism. Rocks along

the flanks are primarily staurolite-kyanite grade. Lower grade metamorphic rocks occur along the northwestern edge of the Inner Piedmont in the Brevard zone. The Inner Piedmont is stratigraphically linked to the eastern Blue Ridge (Figure 2; Hatcher 2002 and references therein; Hatcher et al. 2007). However, its structural style, migmatitic nature, and kinematics of the Inner Piedmont set it apart from the eastern Blue Ridge (Hatcher 2002; Hatcher et al. 2007).

Inner Piedmont - Early Work

Griffin (1969, 1971, 1974b)4 recognized that the structure of the Inner Piedmont is a series of crystalline thrust sheets directed to the northwest (antiformal reclined nappes separated by tectonic slides) and identified two nappes, the Walhalla nappe and the Six Mile nappe5. The Walhalla nappe is comprised of hornblende gneiss/amphibolite, feldspathic quartzite, biotite gneiss, and granitic gneiss (Griffin 1969). The Six Mile nappe overlies the Walhalla nappe across a tectonic slide (Seneca Fault) and consists of muscovite-biotite schist, biotite gneiss, biotite schist, and hornblende gneiss (Griffin 1969). Hatcher (1969, 1970, 1972) defined the Chauga belt, the westernmost subdivision of the Inner Piedmont as a belt of low to medium grade metasedimentary and metavolcanic rocks overlain by the Henderson Gneiss. The Chauga belt extends from the vicinity of Atlanta, Georgia to near Marion, North Carolina, obtaining a maximum width of 10 km. The Brevard zone occupies the northwest limb of the Chauga belt and is bounded to the northwest by the Blue Ridge. The Chauga belt was originally referred to the Low Rank or Non-Migmatitic belt (Griffin 1969; Hatcher 1969). Hatcher (1969, 1970) recognized three principal units in the Brevard/Chauga belt, the Chauga River Formation, the Poor Mountain Formation, and overlying Henderson Gneiss (via faulting over the Chauga River and Poor Mountain Formations (Hatcher 1993; Figure 4) with the Chauga River and Poor Mountain Formations as stratigraphic equivalents (Figure 4). The Chauga River Formation contains four members, the basal graphitic phyllite, the lower Brevard phyllite, the carbonate member, and the upper phyllite member (Hatcher 1969). The Poor Mountain Formation contains three members, the basal Poor Mountain (with lithologies similar to the lower Brevard phyllite member of the Chauga River Formation), the amphibolite member, and the marble-quartzite unit (Hatcher 1969; Figure 2). Mapping to the northeast in North Carolina found that the Poor Mountain amphibolite is in unconformable contact with the underlying Chauga River and Talluluh Falls Formations (Hatcher 2002; Figure 2). Hatcher and Acker (1984) interpreted the Henderson Gneiss – Chauga River contact as a premetamorphic fault with Henderson Gneiss over Chauga River with later northwest-verging, overturned folds deforming the earlier faulting. The boundary between the Chauga belt and the Inner Piedmont was initially interpreted as a tectonic boundary (Griffin 1969; Hatcher 1969). Griffin (1969; 1974a) interpreted the boundary as a large recumbent anticline that overrode the Chauga belt to the northwest (Walhalla nappe). Later, Hatcher (1972; 1978a) interpreted the boundary to be a metamorphic gradient with local tectonic movement.

4 It should be noted that Villard Griffin’s (1969, 1971, 1974b) work revealed the basic structural style (series of crystalline thrust sheets) of the Inner Piedmont that is still valid today although refinements have been made over the years by various workers.. 5 Now referred to as the Six Mile thrust sheet. 9

The Brevard zone is one of the most controversial structures of the southern Appalachians and summaries concerning its origin can be found in Roper and Justus (1973) and Hatcher (1978). The zone has a lengthy and complex history of deformation including multiple ductile events involving both prograde and retrograde metamorphic processes followed by brittle events (Hatcher 2001c; Clendenin and Garihan 2008). The Brevard zone will be discussed in more detail in the following sections.

Figure 4: Original interpretation of the stratigraphic relationships between the Chauga River and Poor Mountain Formations (from Hatcher 1993). Note that in this interpretation the Henderson Gneiss is thrust over the Chauga River and Poor Mountain Formations.

The work of Griffin and Hatcher summarized above is the primary foundation for subsequent work and geologic interpretations in the Keowee-Toxaway Region although they emphasized different approaches; Griffin stressed the need to establish the structural relationships in the area before setting up stratigraphic relationships, whereas Hatcher stressed the importance of understanding the rock composition and stratigraphic order. As pointed out by Garihan and Clendenin (2007), after 40 plus years of work in the area, the relationships between stratigraphy and structure remain controversial.

Inner Piedmont - Keowee-Toxaway Region

The most recent interpretation of Inner Piedmont stratigraphy and structure in the Keowee- Toxaway region is by Clendenin and Garihan (2007a). They describe three thrust sheets within the eastern Tugaloo terrane: the Jocassee thrust sheet, the Walhalla nappe, and the Six Mile thrust sheet (Figure 5). Each thrust sheet includes deformed early-middle Paleozoic-age metasedimentary, metavolcanic, and metaigneous rocks of greenschist or amphibolite metamorphic grade (Garihan 2012). Jocassee thrust sheet - The Jocassee thrust sheet is southeast of the Blue Ridge and separated from it by the Brevard zone (Figure 3c). The sheet is comprised of the Henderson Gneiss, part of a large, regional igneous body that is granodioritic to granitic in composition (Figure 6). It is coarse-grained biotite augen gneiss characterized by large microcline crystals (augen) up to 3.0

cm in a fine-grained biotite quartzo-feldspathic matrix (Lemmon 1981; Clendenin and Garihan 2007a). Interlayered within the Henderson Gneiss are muscovite-quartz-feldspar gneiss, quartz- feldspar gneiss, aplite, pegmatite, quartz veins, and minor biotite amphibole gneiss. The Henderson Gneiss is variably mylonitic with shearing expressed as thinly layered, schistose, fine-grain biotite-muscovite-quartz-feldspar gneiss with locally interlayered muscovite schist (Hatcher and Butler 1979; Clendenin and Garihan 2007a). The high-temperature mylonitic overprint is intensely developed southwest of the Keowee River with the microcline megacrysts

Figure 5: Regional tectonostratigraphy of the eastern Tugaloo terrane in the Keowee- Toxaway Region as interpreted by Clendenin and Garihan (2007a). TF – Tallulah Falls Formation, Hgn – Henderson gneiss, CRfm – Chauga River Formation, PMa(w) – Poor Mountain Formation amphibolite, TRg – Table Rock gneiss, PM(s) – Poor Mountain Formation. flattened into the foliation producing a northeast-southwest trending lineation (Hatcher 2002). This fabric is overprinted by later deformation and retrograde metamorphism (Hatcher 2002). In the Brevard zone, the Henderson Gneiss is a fine-grained leucocratic gneiss (Clendenin and

Garihan 2007a). The Henderson Gneiss has a zircon age of 470 Ma (early middle Ordovician; Stahr et al. 2005, in Hatcher et al. 2007). Garihan and Clendenin (2007) recognized three types of contacts between the Henderson Gneiss and Chauga River Formation rocks in the Jocassee thrust sheet, intrusive contacts (intrusion into Chauga River rocks along axial planar schistosity), Chauga River thrust over Henderson Gneiss (ductile thrust sheet emplacement), and Henderson Gneiss thrust over Chauga River (brittle faulting and duplication of the earlier formed contacts).

Figure 6: Outcrop distribution of the Henderson Gneiss (from Hatcher 2002).

Walhalla nappe - The Walhalla nappe overlies Henderson Gneiss of the Jocassee thrust sheet. The contact between the nappe and the thrust sheet is the Eastatoee fault (Figure 3b; Figure 5: Garihan and Clendenin 2007). The fault is recognized by the abrupt change in lithology across the sharp contact and the marked grain size reduction in the underlying Henderson Gneiss (Garihan and Clendenin 2007). The Walhalla nappe is comprised of phyllonite, the Table Rock gneiss, the Poor Mountain Formation, the Chauga River Formation, and a biotite-porphyroclastic feldspar gneiss (Clendenin and Garihan 2007a). The phyllonite is a resistant, fine- to medium-grained schistose muscovite-quartz phyllonite which occurs in discontinuous zones up to several feet wide and up to 65 ft long. The phyllonite occurs within the formations of the Walhalla nappe and along the contacts of the formations (Clendenin and Garihan 2007a). The Table Rock gneiss is a metamorphosed suite of intrusive granitoid rocks (Clendenin and Garihan 2007a). It consists primarily of a well-foliated biotite quartzo-feldspathic gneiss or a biotite granitoid, but also includes migmatitic biotite-quartz-feldspar gneiss, fine-grained muscovite-biotite-quartz-feldspar gneiss, poorly to well-foliated biotite and muscovite granitoid gneisses, migmatitic biotite schist, aplite, pegmatites, and quartz veins (Clendenin and Garihan 2007a). A member of the Table Rock suite has a 450 Ma (late Ordovician) zircon age (Ranson et al. 1999). The Chauga River Formation was described by Hatcher (1969) and recently re-evaluated by Clendenin and Garihan (2007a) and consists of schist, metagraywacke (biotite gneiss), and mica

gneisses. A fine-grained, button mica schist with mm-size garnets and locally migmatitic garnet- muscovite biotite metagraywacke are common lithologies (Clendenin and Garihan 2007b). The formation also includes schistose, fine-grained muscovite-quartz-feldspar gneiss with localized areas of muscovite schist and fine-grained biotite-quartz-feldspar gneiss and fine-grained biotite gneiss (Clendenin and Garihan 2007a). The Poor Mountain Formation was described by Hatcher (1969, 1970) and recently re- evaluated by Clendenin and Garihan (2007a). Several mafic rock types make up the formation; fine-grained, thinly layered amphibolite, epidote amphibolite, biotite amphibolite, porphyroclastic plagioclase amphibolite, fine-grained porphyroclastic amphibolite with prismatic hornblende idioblasts, fine- to medium-grained hornblende gneiss, fine-grained porphyroclastic plagioclase hornblende gneiss, and ultramafic schist (Clendenin and Garihan 2007a). Felsic rocks interlayered with the mafic rocks include migmatitic biotite gneiss, garnet-muscovite- biotite schist, garnet-mica gneiss, quartz-feldspar gneiss, fine-grained biotite quartz-feldspar gneiss, fine-grained schistose muscovite-quartz-feldspar gneiss, biotite granitoid gneiss, pegmatite, and minor muscovite phyllite (Clendenin and Garihan 2007a). Biotite-porphyroclastic feldspar gneiss is present in several locations in the Walhalla nappe (Tallulah Falls Formation (?); Clendenin and Garihan 2007b). The gneiss contains irregularly shaped feldspar porphyroblasts in a fine-grained biotite schist matrix. The gneiss is either above or below the Poor Mountain Formation amphibolites or the Chauga River Formation schists (Garihan and Ranson 2003; Garihan 2005, 2008; Clendenin and Garihan 2007a, 2007b). Complex structural and intrusive relationships in the Walhalla nappe record a complex history of polyphase folding, intrusion, and ductile and brittle faulting (Garihan 2012). Early development of macroscopic folds was affected by intrusion of the Middle Ordovician Table Rock suite (Taconian (?); Ranson et al. 1999). The Table Rock gneiss, Chauga River and Poor Mountain Formations were folded into a series of northwest-verging isoclines and subjected to high grade metamorphism (Garihan 2012). Regional structural relationships suggest this deformation took place in the Neoacadian (late Devonian to Mississippian; Merschat and Kalbas 2002; Bier et al. 2002; Hatcher and Merschat 2006; Hatcher et al. 2007). Six Mile Thrust Sheet - The Six Mile thrust sheet is southeast of the Walhalla nappe and has been thrust over the Walhalla nappe along the Seneca fault (Figure 3a; Figure 5; Griffin 1971; 1974; Garihan 2005; Clendenin and Garihan 2007a). The Seneca fault is a knife-edge sharp, subhorizontal fault with marked grain size reduction in the footwall. Garihan (2001) showed that the Six Mile thrust sheet is the southwestern continuation of the Sugarloaf Mountain thrust sheet described by Davis (1993) in the Columbus Promontory area of western North Carolina. The Six Mile thrust sheet is comprised of rocks of the Tallulah Falls and Poor Mountain Formations (Garihan 2005). Garihan (2005) divided the Tallulah Falls Formation into two map units, a porphyroclastic feldspar gneiss and a non-porphyroclastic mica amphibole gneiss and schist. The porphyroclastic feldspar gneiss is a fine-grained, schistose garnet-sillimanite- muscovite-biotite porphyroclastic plagioclase quartz gneiss with lesser amounts of fine-grain muscovite-biotite gneiss, schistose sillimanite-muscovite gneiss, medium-grained muscovite- quartz schist, medium-grain migmatitic sillimanite-biotite schist, coarse-grain biotite gneiss, and minor granoblastic calc-silicate rocks and amphibolite (Garihan 2005). The mica amphibole gneiss unit contains coarse-grained biotite schist and medium-grained, poorly layered, biotite- hornblende gneiss with minor medium-grained migmatitic sillimanite-garnet-muscovite-biotite schist, medium-grained garnet—muscovite-biotite schist, medium- to coarse-grained sillimanite- muscovite schist, medium-grained graphite-garnet-mica quartzite, medium-grained amphibolite,

and fine-grained hornblende gneiss (Garihan 2005). The other major lithology in the Six Mile thrust sheet is a biotite-garnet quartzite of the Poor Mountain Formation (Garihan 2005). Timing of major deformation in the Six Mile thrust sheet is similar to that described above for the Walhalla nappe. Brevard zone - Along the northwestern edge of the Jocassee thrust sheet, the primary structure is the Brevard zone (Figure 3c). The Brevard zone, 1 to 2 km wide, is a linear belt of mylonitic, folded, and faulted rocks (Hatcher 1978a, 2001c; Clendenin and Garihan 2008; Nelson et al. 1998). The Brevard zone is not a suture as similar stratigraphic sequences (Figure 2; Tugaloo terrane) occur on either side of the zone (Hatcher 2002; Hatcher et al. 2007). The northwestern portion of the Brevard zone, southeast of the Blue Ridge terrane, is a narrow belt of Chauga River Formation low grade metasedimentary and metavolcanic rocks referred to as the Brevard fold belt and characterized by asymmetric northwest-verging folds (Clendenin and Garihan 2007a; 2008). The Rosman fault occurs along the northwest limb of the Brevard fold belt and separates rocks of the Jocassee thrust sheet from Tallulah Falls Formation rocks. The Rosman fault is a distinctive tectonic mélange that consists of clasts of mylonite and slices of exotic carbonate rocks that chemically resemble platform-type carbonates of the Knox Group (Hatcher et al. 1973; Horton 1974; Hatcher 1978a; Clendenin and Garihan 2008). Southeast of the Brevard fold belt is the Brevard imbricate thrust stack consisting of thrust sheets comprised of Chauga River Formation rocks previously thrust over the Henderson Gneiss (Clendenin and Garihan 2007a, 2008). The Brevard zone has a lengthy and complex history of deformation including multiple ductile events involving both prograde and retrograde metamorphic processes followed by brittle events (Hatcher 1978a; 1989; 2001c; Bobyarchick 1983; 1984; Bobyarchick et al. 1988; Hatcher et al. 2007; Clendenin and Garihan 2008). Early left-lateral movement that affected Chauga River Formation rocks southeast of the zone is overprinted by two sets of right-lateral shear indicators (Clendenin and Garihan 2007b). Bobyarchick et al. (1988) related the right-lateral shear to the Alleghanian orogeny, However, some workers believe that all or part of the right-lateral shear may be the result of the older Neoacadian orogeny (Hatcher 2001c; Clendenin and Garihan 2008). The final, brittle faulting including, the development or reactivation of the Rosman fault as an out of sequence thrust is associated with the final head-on collision of Laurentia and Gondwana during the final stage of the Alleghanian orogeny as shown by seismic reflection studies that indicate parts of the Brevard zone, including the Rosman fault, are splays off the major decollement that underlies the Blue Ridge - Piedmont megathrust sheet (Clark et al. 1978; Cook et al. 1979; Hatcher et al. 2007). This event thrust the combined Blue Ridge – Piedmont megathrust sheet over platform sedimentary rocks (Hatcher 2001c; Hatcher et al. 2007). Tugaloo terrane - The difference in the structure between the western and eastern Tugaloo terrane is related to the Brevard zone (Hatcher et al. 2007). Davis (1993) demonstrated that the dominant foliation (S2) in the Inner Piedmont is a pervasive C-foliation that formed near the peak of metamorphism, that the kinematics indicates a west- directed transport direction in the central portion to a southwest-directed transport direction in the western Inner Piedmont, and interpreted the entire Inner Piedmont as a crustal-scale shear zone. The shear zone was buttressed against the present site of the Brevard zone and was decoupled from the eastern Blue Ridge (Davis 1993; Hatcher 1993; 2002). The primordial Brevard zone buttressed the obliquely converging Inner Piedmont thrust sheets causing them to extrude (lateral extrusion or escape in a oblique transpressional zone) to the southwest (Hatcher et al. 2007; Clendenin and Garihan 2008). This buttressing effect is shown by the strong northeast-southwest mineral lineation southeast of the

zone as noted in the Henderson Gneiss and elsewhere in the Inner Piedmont southeast of the zone (Hatcher 2002) and by backfolding in the Brevard zone (Clendinen and Garihan 2008) The latest faulting (Mesozoic or younger) to affect the rocks of the region are northeast- and northwest-trending brittle faults with a complex history of deformation (Garihan et al. 1990; Garihan et al. 1993; Clendenin and Garihan 2007a).

Inner Piedmont – Northeast of the Keowee-Toxaway Region

Mapping by Cattanach et al. (2012) of the Henderson Gneiss – Chauga River Formation rocks in the Horse Shoe, Pisgah Forest, and Brevard quadrangles in Henderson and Transylvania Counties, North Carolina and Greenville County, South Carolina is consistent with

Figure 7: Conceptual stratigraphic and structural cross-section of rock units in the Columbus Promontory and adjacent Blue Ridge. MST – Mill Springs Thrust, SMT – Sugarloaf Mountain thrust (Seneca fault to the southwest), TCT – Tumblebug Creek thrust, BFZ – Brevard fault zone; Hg – Henderson Gneiss (from Davis 1993). The Lower and Upper Mill Spring complexes are equivalent to the lower and upper Tallulah Falls Formations (from Davis 1993).

Clendenin and Garihan’ s (2007a) interpretation. In the Columbus Promontory to near Marion, North Carolina, the Henderson Gneiss is overlain by the equivalent of the Six Mile thrust sheet along the Sugarloaf Mountain thrust, the northeast continuation of the Seneca fault (Figures 7 and 8; Davis 1993; Howard 2001; Bier et al. 2002). The Henderson Gneiss, and a younger granitoid, are thrust over Poor Mountain Formation rocks along the Tumblebug Creek thrust (Davis 1993). Similar thrust sheet relationships are present in the Marion – Brushy Mountains area of North Carolina where Henderson Gneiss is thrust over rocks of the Chauga River Formation and Tallulah Falls Formation rocks along the Tumblebug Creek thrust (Figure 8; Bier et al. 2003). The origin of the Henderson Gneiss was termed an enigma by Hatcher (2002) for a number of reasons, including the size of the body, along-strike differences of rock lithologies in contact at the southwest and northeast ends of its exposure, and an absence of contact aureoles suggesting it is not an in-situ pluton. As previously discussed, the contact of the Henderson Gneiss with surrounding rocks is generally mylonitic suggesting to a number of workers that the contact is faulted (Davis 1993; Garihan 2001; Garihan and Clendenin 2007). Liu (1991, in Bier et al. 2002) and Davis (1993) interpreted the Henderson Gneiss as a single large thrust sheet. Hatcher (2002), based on several lines of field evidence, stated that the Henderson Gneiss is not an “in situ” pluton and was therefore faulted into place and then penetratively deformed into its present shape by southwest transport.

Figure 8: Western Inner Piedmont generalized stratigraphy with major faults. T-Toward; A-Away (from Bier et al. 2002).

Inner Piedmont – Deformational Events

Table 1 presents an interpretation of the deformational events that have affected Inner Piedmont rocks of the Keowee-Toxaway Region based on papers reviewed as part of this summary/synthesis, and on tables provided in Bier et al. (2002) and Merschat and Kalbas (2002). Details can be found in references previously provided and those cited in the table.

DEFORMATION STRUCTURES METAMORPHIC REGIONAL EVENTS OROGENIC EVENTS REFERENCES EVENTS FABRICS FOLDS FAULTS CONDITIONS Davis (1993); Hopson and Hatcher (1988); S 1 Moderate to high Howard (2001); Rootless intrafolial Pre- to early D -- pressure and - Merschat and Kalbus 1 folds, Foliation Neoacadian temperature (2002); Clendenin and preserved in boudins Garihan (2004); Hatcher et al. (2007) S F Davis (1993); Hatcher 2 2 Emplacement of Penetrative foliation Inclined to recumbent, Initial NW and then SW (1993; 2002); Peak metamorphism, crystalline thrust L tight to isoclinal passive deflected movement of Neoacadian Merschat and Kalbas D 2 upper amphibolite sheets of the IP and 2 Mineral lineation and flexural flow folds, the Eastatoee fault and ~350 MA (2002); Clendenin and facies SW deflection along map-scale sheath folds, Seneca fault Garihan (2004; Initial S-C fabrics in CRF primordial BFZ rocks and HG trend NE, SW, and SE 2007b); Davis (1993); Hopson F Continued SW and Decreasing and Hatcher (1988); 3 Final emplacement S-C fabrics in CRF rocks inclined to upright, NW(?) movement on the conditions, high to Late Neoacadian or Hatcher (1993; 2002); D and exhumation of IP 3 and HG closed to open folds, Eastatoee fault and moderate pressure early Alleghanian Clendenin and thrust sheets trend NW-SE Seneca fault and temperatures Garihan (2004; 2007b; 2008) Bobyarchick et al. F Low pressure and Initial placement of (1988); Davis (1993); S-C and related fabrics 4 Ductile reactivation of D Upright open folds, temperature - the composite Blue Alleghanian Hatcher (2001c), 4 in the BFZ the Brevard zone trend NW-SE greenschist facies Ridge-IP thrust sheet Clendenin and Garihan (2008) Brittle movements in the Continued Brevard zone including Regional broad, open emplacement of the Davis (19930; Hatcher D Joints development of Brittle conditions Late Alleghanian 5 folds composite Blue Ridge- et al. (2007) reactivation of the IP thrust sheet Rosman fault Garihan et al. 1990; Garihan et al. 1993; Rifting Mesozoic extension Clendenin and Garihan 2007a Regional broad, open Meso- and macroscopic Citron and Brown D Joints Brittle conditions 6 folds faults (1979); Gable and Hatton (1983), Clark Uplift Cenozoic uplift (1993); Garihan (2002); Clendenin and Garihan (2004) Table 1: Deformational characteristics in the Keowee-Toxaway Region (modified from tables in Bier et al. (2002) and Merschat and Kalbas (2002) and cited references). 16

SUMMARY OF THE TECTONIC HISTORY OF THE KEOWEE-TOXAWAY REGION

The tectonic history of the Southern and Central Appalachian orogen is bracketed by the formation of two supercontinents (Proterozoic Rodinia and late Paleozoic Pangea) and is the result of the amalgamation of complex tectonic terranes during the Paleozoic (Hatcher et al. 2007; Hatcher 2010). A tectonic map of a portion of the Southern and Central Appalachians (from Hatcher et al. 2007) is shown in Figure 1. Tectonic maps are by definition interpretative compared to geologic maps with respect to both the types of boundaries shown and emphasis on tectonostratigraphic units (e.g. terranes of Laurentian affinity, suspect terranes, and exotic terranes; Hatcher et al. 2007). The terranes shown in Figure 1 represent a complete Wilson cycle (complete opening and closing of an ocean basin; Hatcher et al. 2007). The cycle began with the ~565 Ma rifting of the supercontinent Rodinia (amalgamation completed ~1.1 Ga) and ending ~265 Ma with the super-continent Pangea (Hatcher et al. 2007). The following sequence of tectonic events is taken or simplified from Gillon and Wooten (2010)6 from their summary of Hatcher et al. (2007). Tectonic events summarized by Gillon and Wooten (2010) are shown in italics. Geological manifestations of these events represented in the Keowee-Toxaway Region are briefly summarized in bold font with associated field trip stops: 1) Final amalgamation of supercontinent Rodinia (the basement of the Appalachian orogeny), occurring in the late Mesoproterozic 1.2-1.1 Ga Grenvillian orogeny. This event included the intrusion and metamorphism of the Toxaway Gneiss. The Toxaway Gneiss is exposed in the Toxaway Dome and underlies most of the Bad Creek Pumped Storage Project site (STOP 6). 2) An initial, failed Neoproterozoic rifting event ~735 Ma, depositing sediments, volcanics, and (intrusion) of alkalic A-type plutons. These rocks are exposed in the Grandfather Mountain Window 3) Neoproterozoic (~565 Ma) rifting of Rodinia evidenced by deposition of Laurentian rift margin sediments, and further east, deposition of deeper water facies of sediments and volcanics on old continental crust and newly created Paleozoic ocean crust (e.g. Ashe- Alligator Black, Tallulah Falls Formation, Sandy Springs Group). This rifting and some associated arc development ended with the onset of the Taconic orogeny in the Middle Ordovician (~480 Ma). Deposition of the Tallulah Falls metasedimentary and metavolcanic rocks (STOP 6) from a Laurentian source (Bream et al. 2004) overlain by Cambrian-Ordovician Chauga River Formation metasedimentary rocks (STOPS 2. 4, and 5) and middle Ordovician Poor Mountain Formation arc-volcanic rocks on oceanic crust with fragments of Rodinian (Grenvillian) basement (Eastern Blue Ridge and Western Inner Piedmont - Tugaloo terrane; Figures 1 and 2; Hatcher et al. 2007).

6 While reviewing the literature for preparation of this synthesis/summary of the tectonics of the region, in particular the paper by Hatcher et al. (2007), I realized I could not summarize the tectonic history outlined in that paper better than already done by Ken Gillon and Rick Wooten in 2010 for an Association of Environmental and Engineering Geologist Field Trip Guidebook that included a stop at the Bad Creek Project. This is their summary of the paper, in italics. The bold inserts are my interpretations of the tectonics of the region and where they fit into Hatcher et al.’s (2007) model. I am responsible for any errors that may be present as I did a thorough comparison of their summary with the sequences described by Hatcher et al. (2007) and for any errors in my interpretations of events in the K-T Region.

4) Distal Laurentian margin deposition of the Cowrock and Cartoogechaye terranes comprised of sediments and an assemblage of mafic and ultramafic rocks having an oceanic origin. These associations suggest an environment where ocean floor sediments were deposited on ocean crust and mantle material. The Cartoogechaye terrane also contains some Grenvillian basement rocks, suggesting it was deposited in part on older continental crust. 5) Distal Laurentian margin (and locally, a peri-Gondwanan, or other, pre-Laurentian crustal source) deposition of Dahlonega gold belt terrane sediments, lesser abundant mafic units, mafic-ultramafic complexes, and minor Middle Ordovician (480-460 Ma) arc metavolcanics 6) Neoproterozoic to Cambrian (?) deposition of Kings Mountain terrane Laurentian affinity sediments, and associated arc-volcanic rocks having a peri-Gondwanan affinity, specifically, that of the Avalonian terrane (a peri-Gondwanan terrane accreted to the Northern Appalachians during the Paleozoic). 7) Neoproterozoic (625-550 Ma) deposition, metamorphism, and amalgamation of individual peri-Gondwanan terranes, producing the Carolina superterrane. These individual terranes, consisting of volcanic arcs, volcanogenic sediments, and related subvolcanic plutonic complexes, formed in the oceans that separated Laurentia from Gondwana. 8) Early Cambrian to earliest Middle Ordovician deposition of sediments and carbonates on the Laurentian platform, representing a rift-to-drift transition, and development of a stable ocean platform. 9) The middle Ordovician (~480-420 Ma) Taconian Orogeny, involving subduction of the Laurentian margin, penetrative deformation and amphibolite to granulite facies metamorphism, and thrust emplacement of the Dahlonega gold belt, Cowrock, Cartoogechaye, and Tugaloo terranes. Grenvillian basement massifs within the Tugaloo Terrane were also redeformed and remetamorphosed. Initial emplacement of the Tugaloo terrane; the western Tugaloo terrane was mobilized at this time although not yet accreted to Laurentia (Hatcher et al. 2007). The Toxaway Gneiss is one of the redeformed and remetamorphosed Grenville basement massifs (STOP 6). Intrusion of several large granitic plutons during the middle or late Paleozoic after the intrusion of middle Ordovician plutonic rocks (Hatcher et al. 2007) including the Henderson Gneiss (~470 Ma; STOPS 3 and 5) and the Table Rock plutonic suite (~450 Ma; STOP 1). 10) Westward transport of Taconian thrusts resulting in eustatic uplift of the Laurentian platform sediments and carbonates and the resulting Middle Ordovician unconformity, the formation of a rapidly subsiding foredeep and the deposition into this foredeep of Ordovician to Lower Silurian clastic wedges. 11) Lower to Middle Silurian (~430 Ma) deposition of Cat Square terrane sediments in an open remnant ocean between the Laurentian margin and approaching Carolina superterrane to the east. 12) Neoacadian (380-355 Ma) orogeny subduction of the Cat Square and Tugaloo terranes beneath the advance of Carolina superterrane thrust sheets to the east. Occurring in a north-to-south, zipper fashion, this docking of the Carolina superterrane was accompanied by amphibolite to granulite facies metamorphism. Distal effects of this orogeny also occurred in the Laurentian margin, where Upper Devonian to Lower Mississippian sediments were deposited unconformably on deformed Silurian and older

rocks. Metamorphism of the Tugaloo and Cat Square terranes to upper-amphibolite grade corresponds to the docking of the Carolina superterrane (Hatcher et al. 2007). Emplacement of Jocassee thrust sheet, Walhalla nappe, and Six Mile thrust sheet (Davis 1993; Hatcher 2002; Clendenin and Garihan 2007a). The crustal flow pattern in the Cat Square and eastern Tugaloo terranes shows that the thrust sheets were northwest-directed in the southeastern portion of the terranes and became southwest-directed by buttressing against the eastern Blue Ridge rocks along the Brevard zone (STOP 5; Hatcher 2001c; Hatcher et al. 2007). The docking was therefore zippered (oblique-transpressional) beginning in the north and closing southwestward (Hatcher et al. 2007). 13) Late Mississippian to Middle Permian (325-265 Ma) collision of Laurentian and peri- Gondwana terranes with Gondwana, forming supercontinent Pangea. This Alleghanian orogenic event was an oblique, north-to-south collision that closed in a zipper-like fashion. During the early to middle stages of this orogeny, dextral fault systems developed in response to SW movement of crustal blocks escaping the collision further north. As the mountain chain continued to rise, Late Mississippian to Permian deltaic clastic wedges, sourced from the east, were deposited on the on the Laurentian margin. Reactivation of the Brevard zone with right-lateral ductile to brittle movement under greenschist metamorphic conditions (STOP 5; Bobyarchick et al. 1988; Hatcher 2001c; Hatcher et al. 2007; Clendenin and Garihan 2008). 14) Final, head on collision with Gondwana, transporting Blue Ridge-Piedmont megathrust sheets at least 350 km onto the North American platform, and deforming Laurentian platform sediments into a foreland thrust belt. By Middle Permian (~265 Ma), assembly of Pangea and the Wilson cycle were complete. The Brevard zone was reactivated as a brittle out-of-sequence thrust in the megathrust sheet including the Rosman fault on the northwest boundary of the zone (Hatcher 1971; 2001c). 15) Late Triassic (~215 Ma) breakup of Pangea, evidenced by rifting, the development of Triassic-Jurassic basins, mafic plutonism, volcanism, and zeolite facies metamorphism. Northeast- and northwest-trending brittle faults with a complex history of deformation (Garihan et al. 1990; Garihan et al. 1993; Clendenin and Garihan 2007a). 16) Upper Cretaceous to Holocene (present day) fluvial and marine sediments deposited on the peneplained Piedmont and Coastal Plain physiographic provinces. 17) Pleistocene (2.6 Ma) to Holocene uplift, weathering and erosion, shaping the landscape into its current geomorphic character.

ACKNOWLEDGEMENTS

This manuscript has benefited by reviews by Dave Campbell, John Charlton, and Steve Godfrey. Given the short span of time I had available to prepare this summary/review, I trust I have not committed too many errors and misinterpretations of researcher’s work referenced herein. I have not worked in the Keowee-Toxaway Region in over twenty-five years and pulling together so many quality contributions by (literally) several generations of notable geologists for this paper has been both a learning experience and a pleasure. It has also been my pleasure over the past twenty-nine years to lead several field trips on the engineering geology aspects of the Bad Creek Pumped Storage Project including three trips (2000, 2004, 2007) as part of the annual David S.

Snipes/Clemson Hydrogeology Symposium. I felt that as part of this trip, being the final field trip I will lead to Bad Creek for the symposium, it was appropriate to focus part of the trip on the tectonic context in which the geology of the Bad Creek Project resides. I recommend to all who have attended the Bad Creek trip this year and/or in years past, to read the original references cited herein to learn more about the geology of the region, and how concepts and interpretations of the geology and tectonics have evolved over the past 40 years.

REFERENCES

Acker, L. L. and R. D. Hatcher, Jr. 1970. Relationships between structure and topography in northwest South Carolina: South Carolina Division of Geology, Geologic Notes, v. 17, p. 19-25. Bier, S. E., B. R. Bream, and S. D. Giorgis. 2002. Inner Piedmont stratigraphy, metamorphism, and deformation in the Marion-South Mountains area, North Carolina, in, Hatcher, R.D., Jr., and B. R. Bream, eds., Inner Piedmont geology in the South Mountains-Blue Ridge Foothills and the southwestern Brushy Mountains, central-western North Carolina: North Carolina Geological Survey, Carolina Geological Society Annual Field Trip Guidebook, p. 65-99. Bobyarchick, A. R. 1983. Structure of the Brevard zone and Blue Ridge near Lenoir, North Carolina, with observations on oblique crenulation cleavage and a preliminary theory for irrotational structures in shear zones: Ph.D. dissertation, State University of New York at Albany, 306p. Bobyarchick, A. R. 1984. A late Paleozoic component of strike-slip in the Brevard zone, southern Appalachians (abs.): Geological Society of America, Abstracts with Program, v. 16, p. 126. Bobyarchick A. R., S. H. Edelman, and J. W. Horton, Jr. 1988. The role of dextral strike-slip in the displacement history of the Brevard zone, in, Secor, D. T., Jr., ed., Southeastern Geological Excursions, Geological Society of America Southeastern Section, Columbia, South Carolina, 4-10 April 1988, Columbia, South Carolina, Geological Survey, p. 53- 154. Bond, P. A. 1974. A sequence of development for the Henderson augen gneiss and its adjacent cataclastic rocks: M. S. Thesis, University of North Carolina at Chapel Hill, 53p. Bream, B. R. 2003. Tectonic implications of geochronology and geochemistry of para- and ortho-gneisses from the Southern Appalachian crystalline core (Ph.D dissertation): Knoxville, University of Tennessee, 296p. Bream, B. R., R. D. Hatcher, Jr., C. F. Miller, and P. D. Fullagar. 2004. Detrital zircon ages and Nd isotopic data from the southern Appalachians crystalline core, GA-SC-NC-TN: New provenance constraints for part of the Laurentian margin, in, Tollo, R. P., L. Corriveau, J. McLelland, and M. J. Bartholomew, eds., Proterozoic Evolution of the Grenville Orogen in North America: Geological Society of America Memoir 197, p. 459-475. Bridgeman, J. L. III. and W. A. Ranson. 2012. Petrology of the Salem ultramafic body, Oconee County, South Carolina, in, Garihan, J. M. and W. A. Ranson, eds., Geologic studies in the Inner Piedmont, Brevard zone, and Blue Ridge, South and North Carolina: Guidebook for the Seventy-fifth Anniversary of the Carolina Geological Society, October 12-14, 2012, p. 45-51.

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ENGINEERING GEOLOGY OF THE BAD CREEK PUMPED STORAGE PROJECT, NORTHWESTERN SOUTH CAROLINA

Malcolm F. Schaeffer HDR Engineering, Inc. 440 South Church Street Suite 900 Charlotte, NC 28202-2075

ENGINEERING GEOLOGY OF THE BAD CREEK PUMPED STORAGE PROJECT, NORTHWESTERN SOUTH CAROLINA

TABLE OF CONTENTS

INTRODUCTION………………………………………………………………………...1 BAD CREEK PUMPED STORAGE PROJECT…………………………………………1 Facility Description………………………………………………………………..1 Site Geology……………………………………………………………………....4 Aboveground Structures and Works………………………………………………9 Main Dam – West Abutment………………………………….…………..9 Main Dam – Fault Zone Grouting……………………………………….19 West Dam – East Abutment Leakage……………………………………21 East Dike – Shear Zone…………………………………………………..27 Intake Channel…………………………………………………………...32 Engineering Geology of the Access Road Along Lake Jocassee………..35 Discharge Structure Area………………………………………………...42 Underground Structures and Works……………………………………………...47 Introduction………………………………………………………………47 Underground Layout and Design – Powerhouse………………………...50 Underground Layout and Design – Tunnels……………………………..51 Underground Construction – Geology Program…………………………53 Penstock Bypass Portal/Tunnel Relocation……………………………...54 Manifold – Penstock Area……………………………………………….54 Vertical Access Shaft…………………………………………………….63 Powerhouse Excavation – Rock Wedges and Stress Relief……………...66 Draft Tube Gate Shaft #1………………………………………………...70 ACKNOWLEDGEMENTS……………………………………………………………...71 REFERENCES……………………………………………………..……………………71

INTRODUCTION

The 24th Annual David S. Snipes/Clemson Hydrogeology Symposium Field Trip will examine aspects of the Bad Creek Pumped Storage Projects in northwestern South Carolina. At the Bad Creek Project, both aboveground and underground aspects of the project will be examined and discussed with emphasis on the geologic influences on the location and construction of the major plant structures and the continued operation of the facility.

BAD CREEK PUMPED STORAGE STATION

Facility Description

The Bad Creek Pumped Storage Station is located in Oconee County, northwestern South Carolina approximately 6 miles northwest of the Jocassee Powerhouse (Figure 1). The upper reservoir is located on the Bad Creek and West Bad Creek tributaries of Howard Creek, about 1 mile west of Lake Jocassee and within several thousand feet of the North Carolina state line. Lake Jocassee, a 7565 acre reservoir impounded in 1974, serves as the lower reservoir.

Figure 1: Generalized Geologic Map of the Carolinas and Location of the Jocassee and Bad Creek Pumped Storage Projects.

The upper reservoir is created by two large dams and a saddle dike (Figure 2). It has a surface area of 367 acres and a storage capacity of 33,900 acre feet of which 31,400 acre feet is usable storage between minimum drawdown elevation of 2150 ft-MSL and

1 full pond elevation of 2310 ft-MSL. Maximum drawdown is 160 ft with 2500 acre feet of dead storage below elevation 2150 ft-MSL. The Main Dam across Bad Creek consists of an impervious central core supported by a rockfill shell. Between these is a layered filter system which also functions as a blanket drain downstream of the core. The internal filter system collects and controls the seepage through the dam and abutments. The dam utilized approximately 12, 650, 385 yd3 of material. It has a crest width of 30 ft, maximum base width of 2851 ft, maximum height of 360 ft at the crest, and is approximately 2600 ft long. The nominal crest elevation of 2315 ft-MSL allows for 5 ft of freeboard. The Main Dam, as well as the West Dam and East Dike, are instrumented for performance measurement and design verification. The West Dam is similar to the Main Dam in construction and was built across West Bad Creek. Crest width is 30 ft with a maximum base width of 1080 ft, maximum height of 170 ft, and is approximately 900 ft long with a crest elevation of 2315 ft-MSL. About 1,000,129 yd3 of material were utilized in construction of the dam. The East Dike is approximately 900 ft long and 90 ft high across a natural depression on the eastern rim of the reservoir. The dike has a maximum base width of 580 ft with a crest width of 30 ft. A section of the dike has a crest elevation of 2313 ft- MSL allowing that portion of the dike to act as an emergency spillway. It has a constructed volume of about 444,601 yd3 of material. The Intake Structure for the Power Tunnel consists of a submerged reinforced concrete Bellmouth Inlet with one 51 ft diameter opening tapering down to 30 ft. The tapered inlet serves as a means of transition from the slower velocities of the intake channel to the higher velocities of the Power Tunnel. Maximum flow through the structure is 16,000 cfs generating and 12,080 cfs pumping. A reinforced concrete Discharge Structure is located on the discharge channel on the west shore of the Whitewater River arm of Lake Jocassee (Figure 2). The structure is equipped with four, 18 ft wide by 30 ft high bulkhead gates and four trashracks approximately the same size. A gantry crane is provided to lift the gates. The invert elevation of the openings is at 1031.5 ft-MSL. Lake Jocassee at full pond is 1100 ft-MSL Rock excavated from the underground works was used to construct a submerged weir in Lake Jocassee with a crest elevation between 1060 and 1070 ft-MSL. It is located about 1800 ft downstream of the Discharge Structure. The weir prevents the mixing of warmer water from the pumped storage discharge with the cooler water in the lower layer of the lake in order to reduce adverse effects to the cold water fish habitat of Lake Jocassee. In the generating mode, water is taken from the Bad Creek Reservoir via the Intake Structure inlet though a 30 ft diameter shaft (Figure 3). The shaft drops vertically 850 ft and then elbows into the Power Tunnel which is sloped toward the Powerhouse at a seven percent grade. Near the Powerhouse, the Power Tunnel curves into the Manifold Tunnel from which four 13.75 ft diameter Penstock Tunnels emerge. The flow passes from these tunnels through a reducer cone into 8.4 ft steel-lined penstocks, then through the turbines and out of the Powerhouse cavity by way of four 16.4 ft diameter Draft Tube Tunnels. The Draft Tube Tunnels bifurcate into two 26.2 ft diameter Tailrace Tunnels which discharge into Lake Jocassee through the Discharge Structure. In addition to the concrete-line water tunnels, three bypass tunnels, Powerhouse, Penstock, and Tailrace,

2 were excavated to provide access for construction and for use in monitoring the underground during plant operation.

Figure 2: Site Plan of the Bad Creek Pumped Storage Project.

Main access to the Powerhouse is supplied by a 30 ft wide by 26.25 ft high access tunnel. The tunnel is about 1200 ft long and enters the Powerhouse at elevation 1015 ft- MSL. The tunnel invert accommodates a two-lane paved road used during construction and for present day access. Access is also provided by way of a stairway and elevator in the Vertical Access Shaft. The shaft is recessed in the downstream face (east) of the Powerhouse Cavern. The shaft services all four elevations in the area of the service bay and terminates at the equipment building on the ground surface directly above. It houses the Isophase Bus and major HVAC ducts. Relatively small diameter construction shafts were installed to facilitate the construction effort and for ventilation. The Powerhouse Cavern is located about 540 ft underground and 1200 ft upstream of the Main Access Tunnel entrance. The cavern is 75 ft wide by 165 ft high by 430 ft long. It contains the service bays and four unit blocks. The powerhouse is constructed of reinforced concrete up to and including the operating floor at elevation 1015 ft-MSL. There are intermediate floors at elevations 992, 966, and 943 ft-MSL that house miscellaneous mechanical and electrical equipment. The four vertical shaft, Francis-type, reversible pump/turbines are nominally rate 309 Mw each and are capable of generating 360 Mw each at maximum head. They are supported on mass concrete foundations that transfer the operating loads to the surrounding rock. The water tunnels, access tunnels, shafts, and powerhouse cavern were excavated using drill-and-blast techniques. As tunnels were excavated, pattern rock bolts (5 ft x 5 ft) 10 ft long were set into the crown to help stabilize the rock mass. When geologic conditions warranted, additional random rock bolts were used in the ribs and crown to

3 insure the integrity of the excavations. Shotcrete was applied to the crown after bolting to provide additional safety for the workers from spalling that occurred between rock bolts due to the high in-situ stresses. Stresses were relieved generally with several weeks of excavation. However, minor spalling in the underground continues to the present day. Minimal amounts of water were encountered in the underground works during construction. The Powerhouse was excavated by tunneling a center cut down the long- axis of the cavern and slashing out the rock to the east and west springlines. The rest of the cavern was excavated by bench blasting down to grade.

Figure 3: Plan of the Underground Structures, Tunnels, Powerhouse, and Vertical Shaft at Bad Creek.

Planning and licensing activities for the Bad Creek Project began in the early 1970's, including subsurface investigation in the Upper Reservoir and the pilot tunnel in 1976-77. The access road into the project and a test quarry/test fill program was begun in 1984. Project construction began in 1985 with construction completed in early 1991. Initial reservoir filling, by pumping, began on March 15, 1991. Units 1 and 2 went into commercial operation on May 15, 1991 followed by Unit 3 on September 3, 1991, and Unit 4 on September 13, 1991.

Site Geology

The Bad Creek Project is located immediately northwest of the Brevard zone in the Blue Ridge Physiographic and Geologic Province within the Toxaway Dome (Figure 4). The Blue Ridge is characterized by mountainous terrain consisting of closely spaced ridges trending in a northeasterly direction. Streams are deeply incised and average relief is about 1800 ft. The Toxaway Dome consists of a core of Toxaway Gneiss and a sliver

4 of Tallulah Falls Formation. It is an elongate feature that has a steeply dipping to overturned northwest limb and a more moderately inclined southeast limb. At the ends, the structure plunges gently northeast and southwest, resulting in a structural dome defined by the upward arching of the dominant foliation. At least two episodes of flowage folding have been recognized in the dome (Hatcher, 1977). The first set is isoclinal and recumbent, trending east to northeast and verging north to northwest. The second set is more upright, isoclinal to open, trending northeast and verging northwest. Later mesoscopic crenulation cleavage and macroscopic northeast and northwest trending folds are also present. The dominant northeast outcrop pattern of the Toxaway Gneiss and Tallulah Falls Formation is due to the northeast-trending structures, principally the second set of flowage folds. Detailed mapping performed during the construction of the Bad Creek Project indicates that the basement (Toxaway Gneiss)/cover (Tallulah Falls Formation) contact is repeated several times due to isoclinal folding and transposition. Textural evidence (grain size reduction and truncated foliation and fold axis in the Toxaway Gneiss at the contact) suggests that the original basement/cover contact was a premetamorphic (Post Grenville metamorphism fault. The majority of the site is underlain by Toxaway Gneiss (Figure 4). All of the tunnels, shafts, and the powerhouse cavern were excavated in the Toxaway Gneiss. The Main Dam and East Dike are founded on the gneiss. The West Dam and a portion of the reservoir are underlain by a sequence of schistose rocks belonging to the Tallulah Falls Formation (Figure 4). The Tallulah Falls rocks are predominantly the garnet-aluminous schist member, however, in places portions of the upper graywacke-schist member is present. This belt of rocks is isolated from similar rocks on either side of the Toxaway Dome by the refolding of earlier folds. The Toxaway Gneiss, part of the Precambrian basement of the eastern Blue Ridge, is a medium- to coarse-grained gneiss of granitic to quartz monzonitic composition. It is composed of microcline, plagioclase, quartz, and biotite with minor amounts of epidote, garnet, allanite, muscovite, zircon, sphene, apatite, and opaques. The Toxaway Gneiss can be divided into two major types: 1) a banded, medium- to coarse- grained granitic gneiss composed of alternating light-colored quartz-feldspar rich bands and dark biotite-quartz-feldspar bands and 2) a coarse-grained augen granitic gneiss consisting of a poorly foliated feldspar-quartz-biotite gneiss with feldspar and locally hornblende augen up to 3 cm across and a medium- to coarse-grained quartz-feldspar- biotite gneiss with a more distinct foliation and feldspar augen up to 1 cm. The Toxaway Gneiss has an Rb/Sr whole-rock isochron age of 1203+54 Ma (Fullager and others, 1979). Layers of biotite-hornblende schist (sills or dikes, possibly feeders for the mafic volcanic rocks of the Tallulah Falls Formation) are present with thicknesses up to 20 ft. Their orientation is parallel to the foliation in the Toxaway Gneiss. At least two generations of quartz-feldspar-mica pegmatites occur within the gneiss. They are distinguished by the fact that the later generation is undeformed except by fracturing, whereas the earlier generation is folded. Most of the early pegmatites parallel the dominant foliation; the later generation cuts across foliation. Small quartz veins are also present. The Tallulah Falls Formation consists of three members in the site vicinity (Hatcher, 1977). The lower graywacke-schist-amphibolite unit consists of meta-

5 graywacke (biotite gneiss), amphibolite, muscovite schist, biotite schist, pegmatites, and minor granitic gneiss. The garnet-aluminous schist member includes muscovite-garnet- kyanite schist with minor interlayered amphibolite, muscovite schist, and metagraywacke. The upper graywacke-schist member consists of metagraywacke (biotite gneiss), muscovite schist, muscovite-biotite schist with minor amounts of interlayered amphibolite, granitic gneiss, and pegmatite. The units have undergone regional metamorphism to the kyanite zone of the amphibolite facies.

Figure 4: Geologic Map of the Bad Creek Pumped Storage Project Site.

The geologic program conducted during the construction of the project was developed to provide additional geologic information for construction and design personnel and to document the conditions encountered. The geologic studies included observation, measurement, sampling, photographs, mapping, and evaluation of the exposed rock and foundation surfaces. The geologic conditions encountered in the underground works were documented by geologic mapping of at least one rib of the tunnel, the walls of the two vertical shafts, and the walls, crown, and floor of the Powerhouse cavern at a scale of 1 in = 6.56 ft (1.27 cm = 1 m). The aboveground structures including dam foundations, intake excavation, and discharge excavation were

6 mapped at a scale of 1 in = 20 ft (1 cm = 2.4 m). The upper reservoir area was mapped at a scale of 1 in = 200 ft after all excavation and borrow work was completed. The mapping was the primary input into construction and design considerations as work progressed and was supplemented by additional studies as needed. Detailed mapping in the underground structures resulted in a detailed subdivision of rock types within the Toxaway Gneiss. The following units were recognized and mapped: 1) Granitic Gneiss, medium light gray to light gray, medium- to coarse-grained gneiss consisting of alternating layers of light colored quartz-feldspar bands and darker biotite-quartz-feldspar bands, well-foliated, 2) Banded Augen Granitic Gneiss, medium light gray to light gray, medium- to coarse-grained gneiss consisting of a foliated (banded) quartz-feldspar-biotite gneiss containing feldspar augen up to 1 cm long, 3) Augen Granitic Gneiss, medium light gray, coarse-grained gneiss consisting of a coherent, massive, poorly foliated feldspar-quartz-biotite gneiss with feldspar and locally hornblende augen up to 3 cm long, 4) Biotite Schist, medium dark gray to dark gray, coarse-grained biotite-hornblende schist, 5) Biotite Gneiss, medium dark gray to dark gray, medium- to coarse-grained biotite-hornblende gneiss, 6) Biotite Augen Gneiss, medium gray to medium dark gray, medium- to coarse- grained, foliated biotite-feldspar-quartz gneiss with feldspar augen up to 1 cm long, biotite content generally greater than 30%, 7) Quartz-Feldspar Gneiss, very light gray to white, very coarse-grained, distinctly foliated quartz-feldspar gneiss with minor biotite (less than 10%), 8) Very Coarse-Grained Granitic Gneiss, light gray, very coarse-grained, distinctly foliated quartz-feldspar-biotite gneiss, biotite content greater than 10%, 9) Weathered Sheared Rock, moderate to moderately severe weathering, light gray to yellowish gray to greenish gray, original rock type granitic or augen granitic gneiss, and 10) Hard Sheared Rock, medium light gray to light gray, medium- to coarse-grained rock, original rock type granitic or augen granitic gneiss. Detailed mapping of the Tallulah Falls Formation rocks in the West Dam Foundation and reservoir area identified the following major rock types: 1) Mica Schist, varies from medium dark gray to dark gray, very light gray to light gray, brownish gray to olive gray to grayish black, coarse-grained schist consisting of muscovite-quartz-feldspar-garnet, garnets range in size from pinhead to 3 cm in diameter, thin interlayers of schistose biotite-quartz-feldspar gneiss and muscovite quartzite occur in the mica schist unit, 2) Granitic Gneiss, light gray to medium light gray, medium- to coarse-grained biotite-quartz-feldspar and quartz-feldspar layers, 3) Biotite Gneiss, light gray to medium light gray to yellowish gray, fine- to medium-grained quartz-biotite-feldspar gneiss, fine banding, 4) Muscovite-Biotite Schist, pale yellow orange to pale brown, coarse-grained schist with muscovite, biotite, quartz, trace feldspar,

7 5) Muscovite-Biotite Gneiss, very light gray to light yellowish gray, medium- grained gneiss with muscovite-biotite, contains very light gray to white, coarse- to very coarse-grained quartz-feldspar interlayers, and 6) Augen Gneiss, medium dark gray to medium gray, medium-grained augen gneiss with biotite-muscovite, equal amounts of quartz and feldspar, quartz and feldspar augen to 1 cm long aligned parallel to foliation. Foliation in the Toxaway Gneiss and Tallulah Falls Formation rocks is defined by the parallel orientation of platy minerals and by compositional layering. The average orientation of foliation in the reservoir area is N37E; 38SE and varies from N35-50E; 28- 41SE in the underground works. Minor folds are present; some lie within foliation whereas others fold the dominant foliation. The earliest set of folds are isolated “z-”, “s- ”, and crescent-shaped fragments and they are axial planar to the dominant foliation. The presence of these isolated fold fragments indicates that transposition of an older foliation has occurred. The second set of folds are isoclinal to open with variable development of a secondary foliation. In areas where this folding is isoclinal, an axial planar foliation (defined by secondary biotite) is present. Later open folding has been recognized in several tunnels. Shear zones with thicknesses up to 200 ft occur throughout the Toxaway Gneiss and generally parallel the dominant foliation. Three major shear zones are present in the reservoir and dam areas. The zones consist of hard sheared rock with layers of weathered sheared rock present. The zones are mineralized with chlorite, epidote, calcite, and quartz in various combinations. Originally white feldspars have been discolored to a pink or light orange-pink color within and adjacent to the shear zones. Along some of the shear planes, breccia is present with thicknesses of less than 1 inch to about 12 inches. The breccia consists of rock, coarse quartz/feldspar (pegmatites), and vein quartz fragments in a matrix of fine-grained chlorite and epidote. Several of the shear zones have associated weathered zones up to 12 inches thick. Within the weathered zone there is up to 2 inches of gouge-breccia composed of rock, coarse quartz/feldspar, and vein quartz in a clay matrix. The hard sheared rock exhibits tight, complex isoclinal folding with sheared out limbs with a secondary axial planar foliation defined by biotite. This relationship indicates that the major shearing is related to the second fold event although some of the shear zones my have been reactivated from the first fold event. The brecciation and mineralization of the zones is a later event. There are three dominant joint sets in the reservoir area: 1) N77E; 82 NW, 2) N42E; 74NW (strike joints), and 3) N47W; 88SW (dip joints). In the underground works, the predominate joint set varies between N70W and N70E with steep north and south dips. Another set strikes N60E with moderate to steep northwest dips and a weakly developed set oriented N45W with steep southwest dips is present. All joint sets have some degree of mineralization, but the northeast and particularly the east-west set (N77E in the reservoir area) contain a greater percentage of mineralized joints. The dominant mineral fillings are quartz, chlorite, epidote, biotite, and calcite in various combinations. Iron oxide and manganese staining is present along weathered joint surfaces. Spacing within the joint sets varies from less than 1 inch to greater than 50 ft. Fault and fault zones are present and are generally associated with the NE-striking joint sets. Single fault planes with few associated fractures have offsets up to 6 inches (vertical separation). The fault zones have complex fracturing with several planes and

8 offsets ranging from less than 1 inch to greater than 12 ft. Breccias up to 6 inches thick is developed along some of the fault planes and consist of rock, quartz/feldspar, and vein quartz fragments in a fine-grained matrix of chlorite-epidote. Discoloration of feldspars to pink occurs along some of the fault planes. All fractures within the zones are mineralized by combinations of epidote, chlorite, quartz, and calcite. Along some of the fault planes, chlorite up to 2 inches thick is present. Subhorizontal slickensides on the chlorite indicate the primary movement was strike-slip. The thicker chlorite mineralization has a secondary shear foliation indicating minor movement after the primary mineralization. In some fault zones the rock is shattered between fault planes with chlorite-quartz mineralization throughout the fracture zone. The brecciation and mineralization of the fault zones occurred at the same time as the brecciation along the shear zones. The faults and shear zones are similar to others within the southern Appalachians that have been healed under greenschist metamorphic conditions suggesting the last movement occurred at least 300 Ma (Gilbert and others, 1982).

Aboveground Structures and Works

Main Dam – West Abutment

Standard foundation treatment/preparation for embankment dam construction was utilized with the requirements depending on the location within the cross-section of the dam (i.e. core, upstream or downstream shell). Foundation preparation for the Main Dam (and the West Dam and East Dike) consisted of excavation and removal of sod, topsoil, boulders, soft pockets, alluvium, colluvium, and organic or other objectionable material. The final foundation is a combination of rock, weathered rock, and saprolite. Rock surfaces in the core area are cleaned by air blasting and hand picking of all loose rock fragments, sand, gravel, and other materials. The depth required for cleaning joints and fractures was three times the width of the feature. Dental concrete was used when proper compaction of the embankment material against the foundation was impractical. Rock surfaces, shallow depressions, and areas filled with dental concrete were slush grouted before fill placement. The foundation rocks in the Main Dam are primarily foliated granitic gneisses varying from very hard rock to saprolite (completely weathered bedrock but retaining the rock structure). Thin layers of biotite schist, quartz-feldspar gneiss, and pegmatites are present as well as structural features including joints, faults, and shear zones. In the Main Dam foundation the strike of foliation is more or less parallel to the creek bed. The dip is toward the southeast. In the narrow valley bottom, the west or right abutment is a dip slope. Strike joints (N42E; 74NW set) also parallel the valley bottom perpendicular to the axis of the dam. Stress relief and local buckling/sliding of the rock slabs defined by the foliation and joint sets produced voids several inches wide that lead into the abutment and which trend upstream-downstream. Grout pipes were set into the voids and dental concrete place around the pipes and over the voids. They were grouted with low pressure when the fill was high enough above them to resist heaving. On some of the larger voids, the extent, location, and orientation were noted and additional vertical holes drilled into them and grouted.

9 Two major geologic features required additional treatment beyond the normal foundation preparation on the West Abutment, an old colluvial slide in the downstream rock shell area (West Abutment Buttress Area) and a zone of blocky rock in the core area downstream of the centerline of the dam (Blocky Rock Zone). The initial work activities in the area of the Main Dam downstream shell (West Abutment) began in the spring of 1986. Following construction of a temporary construction road and initial stripping of the slope, tension cracks indicative of slide movement were noted (April, 1986) on the West Abutment within the theoretical dam footprint. The movement progressed over time. In July 1986, because of continued deterioration of the abutment slope, an exploration program was undertaken which included soil borings, installation of crack monitors, shear tubes, and inclinometers. The investigation determined that the entire area was an old landslide (colluvial) area bounded topographically by two drainages. The borings indicated a shallow zone, 10 to 15 ft, of relatively weak material underlain by more competent material. The visible appearance of a “tombstone” multiple wedge sliding mechanism indicated a shallow failure plane that was confirmed by data from the shear tubes and inclinometers and was consistent with the boring data. An area of wet and organic material at the toe of the slope was removed and replaced with a small random rock buttress that became a permanent part of the dam. A stability analysis of the dam modeling the weak foundation material in the slide zone yielded an acceptable (>1.5) safety factor. It was decided to treat the colluvial material in place by extending the random rock buttress. The buttress was extended outside the nominal dam limits and effort was made to expedite its construction ahead of the dam fill. Extending the buttress up the slope was chosen over a retaining wall system above the trace of the downstream shell primarily due to economics. Within the dam limits, the normal blanket drain system was eliminated within the colluvial zone. The trench drain and relief well system was extended under the buttress to intercept abutment seepage. The completed buttress configuration along with the location of the permanent instrumentation (inclinometers, surface monuments, and observation wells) is shown on Figure 5. Since completion of the buttress to the present day, no discernable movements of the slope or fill have been observed aside from buttress and foundation consolidation (settlement). On December 9, 1987, a rock slide occurred on the west abutment of the Main Dam (Figure 6). The possible extent of the slide was determined based on the projection of the failure plane along a weathered zone parallel to foliation separating hard rock from above from saprolite below (Area C on Figure 6). The decision was made to remove the material (+3000 yd3) that had already moved. Nine simple shear tube type slope indicators were located on the abutment above the slide area (SPD designation on Figure 6). These consisted of one inch diameter PVC tubes which were routinely probed with steel rods of varying lengths. Shearing due to slide movement creates tube deformation and the depth can be determined by tight or blocked areas.

Figure 5: Plan of the Main Dam West Abutment Stability Buttress.

On December 17, 1987, the contractor started excavation at elevation 2120 ft- MSL on the abutment in the core area to uncover and prepare final foundation grade. The initial Standard Penetration Tests (SPT) of the prepared surface yielded values less than the required 20 blows for foundation acceptance. Two inspection pits were excavated in order to determine the depth of the excavation required over the area to reach acceptable foundation. Both pits were excavated without reaching consistently firm foundation material. In Pit #1 (see Figure 6 for location), 4 to 8 ft of colluvial material was encountered consisting of rock fragments and boulders in a red sandy silt matrix overlying a 2 to 3 foot thick saprolite layer (Figures 7 and 8). The saprolite graded downward into a relatively hard blocky rock. Joints oriented N45-60W and NS were open and infilled with red sandy silt (Figure 8). Some silt infilling and minor clay was present along foliation surfaces in the blocky rock. The infiltration of the sandy silt into the openings in the rock suggested that the cracking/opening was relatively old and that the water movement bringing the colluvial matrix material into the bedrock had been occurring over a period of time. In Pit #2 (see Figure 6 for location), no overlying colluvium layer was present, although it may have been removed by normal erosion or during the contractor's excavation work. Open joints were present in the hard rock below a 3 to 4 foot thick saprolite layer. The majority of the openings were unfilled, with a small number of openings filled with red sandy silt, suggesting that the area was at one time covered with colluvium. Examination of several access roads on the West

Figure 6: Plan and cross-section of the Blocky Rock zone on the West Abutment of the Main Dam.

12 Abutment indicated that the colluvium was not present everywhere on the slope, but occurs in odd-shaped, uphill-downhill lenses. The breaking/cracking phenomena in the first relatively hard rock layer could be present in areas not overlain by colluvium.

Figure 7: Test Pit #1 looking to the southwest. Note colluvium below the silt fence overlying saprolite which grades down into hard rock.

It was postulated that the blocky rock observed in the two test pits described above was related to the rock slide downhill in that the downhill movement associated with the open joints occurred at the contact between the hard rock and the underlying saprolite seam as noted in the active slide area. The development of the colluvium and the blocky rock is related to past downslope movement and may have been partially influenced by stress relief due to the excavation of the abutment. At this time the extent of the zone, both horizontally and vertically, was not known. The major concern was that a zone of open fractures in the core foundation would provide for a short circuiting of the grout curtain along the centerline of the dam leading to the following potential problems: 1) erosion of the foundation material from concentrated seepage, 2) erosion of the core from seepage through a foundation crack into the core (foundation pressure exceeds core pressure), and 3) seepage through the core eroding core material into a crack in the foundation (core pressure exceeds foundation pressure). Additional test pits and borings (Pits #3 to #8; holes designated WAG and WA; see Figure 6 for location) were made and data from the exploration borings (B designation on Figure 6) and from the grout curtain holes were examined to further determine the characteristics and extent of the zone. The abutment was found to be overlain by colluvium and weathered rock extending from the surface to as much as 40 ft before rock was reached. The blocky rock zone, based on the drill data, was from 15 to 25 ft thick overlying a layer of saprolite/weathered rock before relatively sound rock is

13 reached. The greatest thickness and more open fractures were in the downstream part of the core and the zone thinned upstream so that at the line of the grout curtain it is weathered and not jointed. It appears to daylight out of the slope just upstream of the centerline.

Figure 8: Test Pit #1 showing the blocky rock and infiltration of the colluvial materials (red sandy silt) into the open joints. Rule is 3 ft long.

An outline of the potential blocky rock zone was made based on the geologic projection of the known failure plane (Location A on Figure 6), possible failure planes in test pits, and interpretation of the drill data (see Section A-A on Figure 6). The best interpretation for the orientation of the failure surface was N28E; 28SE and based on this orientation a projection of the potentially affected area was made (Location D on Figure 6). It was decided to enhance the grout curtain from Stations 20+50 to 25+75. This consisted of two additional grout lines location 10 ft upstream and downstream of the existing curtain. Initial spacing was 20 ft on center with split-spacing as dictated by review of grout takes. The holes extended down to the top of the original grout curtain and penetrated the potential blocky rock zone into the sound rock below. In addition, a supplementary grout line was added at the centerline using microfine cement grout to grout the area between the top of the existing grout curtain and prepared foundation. Four alternatives were considered for remedial and control measures. They were: 1) complete removal of the blocky rock zone, 2) a partial core trench, 3) blanket/consolidation grouting of the blocky rock zone, and 4) enhanced grout curtain (work then underway) with foundation under the core prepared according to procedures defined in the construction specifications. Alternate 1 would involve the removal of 20 to 60 ft of colluvium, saprolite, weathered rock, and the blocky rock. It was rejected due to the possibility of generating new crack in the hard, brittle rock below the blocky rock

14 zone due to stress relief and due to the schedule implications and cost. Alternate 2 proposed a core trench at the centerline which would penetrate to a depth equivalent to the bottom of the blocky rock. It would have a bottom width of 20 ft with 1.5:1 side slopes and extend from approximately Station 21+00 to 26+00. The volume of excavation was estimated at 50,000 yd3. Because of the existing topography, the excavation would have a top width of 200 ft and would extend up the abutment to near Station 19+00. In the vicinity of Station 20+00 it would extend outside of the present core area into the upstream random rock zone. Most of the excavation would be upstream of the centerline resulting in excavation of much of the more impermeable material already in place. It would provide cutoff of potential water flow into the downstream open jointed blocky rock, although its configuration would not result in a significant trench. There is no source on the project site for plastic, highly impervious material for backfill. The same silty sand material of the dam core would have to be used. In the event of an open path for reservoir water through the upstream foundation, this relatively narrow core of silty sand would be subject to high hydraulic gradients and subject to potential piping into the downstream open rock. It would have no advantage over the present grout curtain and the impact on the construction schedule would have been significant. Alternate 3 would attempt to grout the open jointed, blocky rock, possibly improving structural strength and reducing permeability. Grouting to depths of about 50 ft would try to consolidate an area in which experimental grouting, at no pressure, was done (in WAG and WA borings). Some grout takes were extremely high, and grout was noted to flow freely out of the abutment on top of the underlying saprolite layer/failure plane at about elevation 2000 ft-MSL. Successful grouting would be difficult and not insure an increase in the strength of the rock mass or a reduction in rock mass permeability. It would also have the disadvantage of blocking open drainage paths for water that does get through the upstream foundation and grout curtain resulting in pressure buildup under the downstream core. This alternative was rejected. Alternative 4 was chosen for implementation. Excavation of the area started at elevation 2120 ft-MSL and proceeded down to the existing elevation of the placed core material (about elevation 1980 ft-MSL). About 50 ft downstream of the centerline, the blocky rock forms an elongated bulge about 140 ft wide, 360 ft long, and 5 to 20 ft high (Figure 9). The cracks on the upstream half of the area are up to several inches wide. The cracks become wider downstream (to several feet) and also wider along the downhill side (Figure 10). Rock bolts were installed along the downhill edge of the zone for temporary support to stabilize the area for treatment (Figure 11). The cracks were cleaned according to specifications and then backfilled with dental concrete (Figure 12). In irregular cracks, grout pipes were installed for contact grouting (Figure 12). Overhangs and the downhill edge of the zone were treated with dental concrete in order to provide an area to place and compact core material (Figure 13). The entire treated area was covered by 3 ft of fine filter material which was tied into the downstream filter system (Figure 14). This filter layer provides a controlled relief path for seepage through the blocky rock and core material on the downstream side of the core. The Main Dam instrumentation program was augmented by the addition of piezometers in the zone of blocky rock to monitor pressure at the core/foundation interface, within the blocky rock zone, and in the core material.

Figure 9: Geologic Map of the Blocky Rock Zone on the West Abutment of the Main Dam.

Figure 10: Blocky rock zone looking southeast along N45W joints. The blocks have moved downhill opening northeast striking joints with minimal rotation of the blocks.

Figure 11: Rock bolting operation to stabilize the blocky rock zone. The bolts were installed along the downslope side of the zone and were sized to penetrate the blocky rock, the sliding plane and underlying saprolite, and back into sound rock. Location of rock bolts are shown in Figure 9.

Figure 12: Backfilling of blocky rock with dental concrete. Note grout pipes (1” PVC) installed for later, low pressure grouting.

Figure 13: Dental concrete being placed on the downslope side of the blocky rock in order to allow proper compaction of the filter and core materials. Note the treatment of the zone above the dental concrete.

Figure 14: The last step in the treatment of the blocky rock zone was the placement of the filter material over the blocky rock and tying the filter into the downstream drainage system.

Main Dam – Fault Zone Grouting

A fault zone was exposed during final foundation cleanup of the creek bottom downstream of the Main Cofferdam (Figure 15). The zone strikes about N30E, approximately parallel to the creek bottom, with various splays striking N30E to N60E with steep dips generally to the southeast although vertical and steep northwest dips also occur. The fault planes are generally tight, however, along some of the planes weathering has occurred and at the time of the excavation water was moving along some of the fault planes (Spring locations shown on Figure 15). Treatment of the fault zone consisted of grouting from both the foundation surface and through a limited amount of core fill upstream of the main centerline grout curtain. The later situation existed because the contractor was allowed to place some core material prior to grouting to minimize construction delays. Grout holes were located and angled to intersect the fault planes (see Section A-A’ on Figure 15). Thirty-two holes were laid out for initial grouting. Additional grout holes were added if a hole took 10 or more bags of grout and were located by split-spacing along and between the original grout holes. Split-spacing continued until grout takes were less than 10 bags. A total of 100 grout holes were installed. Grout takes ranged from 0 to 57.5 bags with an average of 0.25 bags/linear foot and were generally small, indicating the relative tightness of the fault planes. Several of the grout holes took more than 20 bags and a number of grout “connections” between holes were observed between adjacent holes. A few interconnections greater than 40 ft occurred. Shows of grout in adjoining holes were

Figure 15: Geologic map of the Main Dam Fault Zone, location of springs and remedial grout holes.

Figure 16: Fault splay exposed during foundation cleanup after completion of the grouting operation. Grout was injected into the fault plane.

Figure 17: Fault with 6 to 10 inch breccia layer exposed during foundation cleanup after completion of the grouting operation. Note injection of grout into the fractures within the breccia. not considered unusual in the system of complex, interconnected, three-dimensional steeply dipping fault planes intercepted at varying depths by the inclined grout holes. The longer connections indicated the zone of influence of the remedial grouting program. As the work proceeded, additional foundation preparation on the west abutment exposed portions of the fault zone that had been grouted. Grout was found emplaced into single fault planes (Figure 16) and into fractures within breccias associated with some of the fault planes (Figure 17), indicating the success of the remedial grouting program.

West Dam – East Abutment Leakage

On the upper portions of the east and west abutments of the West Dam, Toxaway Gneiss is present. The rest of the foundation, including the creek bottom, is underlain by rocks of the Tallulah Falls Formation. The foundation rock varies from very severely weathered (saprolitic) to fresh. In general, the gneisses and quartzites are harder than the schist units resulting in a foundation in which hard and soft layers alternate. The layering and primary foliation is approximately parallel to the stream bed and perpendicular to the axis of the dam. The Toxaway Gneiss on the East Abutment is primarily saprolitic, but does contain hard layers of granitic gneiss and thin, less than 2 ft, discontinuous layers of quartz-feldspar pegmatites. The Toxaway Gneiss on the upper portion of the West Abutment is only slightly weathered. Jointing in the foundation is discontinuous with the primary set being approximately parallel to the dam axis. The gneiss units have a higher joint intensity than the schist units with the joints confined within the contacts of the gneiss units. No major through going faults or shear zones are present in the foundation. The contact between

21 the Toxaway Gneiss and the Tallulah Falls Formation rocks and some of the contacts within the Tallulah Falls rocks are sheared as indicated by a decrease in grain size and other mesoscopic structures. Later, planer surfaces in the shear zones as found in the major shear zones in the Toxaway Gneiss east of the West Dam are not present in the foundation. No major or significant geologic feature requiring special treatment was noted in the foundation. Based on the foundation geologic mapping, certain areas downstream of the dam were designated as areas to observe during the initial filling operation (by pumping) for seepage. During the initial filling of the reservoir, spring activity was noted in the downstream East Abutment shortly after the reservoir level reached the upstream face of the dam (elevation 2200 ft-MSL). Collection systems were installed at these locations to allow the quantitative measurement of flow (Figure 18). On February 5, 1991 when the pool reached elevation 2265 ft-MSL, small seeps and wet areas were noted further north along the East Abutment. Early on the morning of February 6th, sloughing of the overburden soil occurred (Figure 19). The filling operation was immediately stopped and the reservoir level dropped to below 2200 ft-MSL. Visual inspections indicated that flows within and under the slough were considerably higher than previously observed. This leads to the conclusion that this seepage, in the form of concentrated flows in the weathered rock, was not being relieved through the relatively impervious overburden soil thereby saturating the overburden and precipitating the slide. The loose and displaced

Figure 18: Collection system for capturing water leakage in the East Abutment of the West Dam. Six inch PVC was installed with connections for a flow meter in order to measure flow.

22 Figure 19: Slough on the East Abutment of the West Dam that developed during the initial filling of the Bad Creek Reservoir.

Figure 20: Location and original slough area repair on the East Abutment of the West Dam. material was removed and filter consisting of a needle punched non-woven geotextile and

23 gravel was installed and ballasted by clean rockfill (Figure 20). Efforts were made to collect the seepage at the base of the repair buttress to allow flow measurement. The slough/slide was just above the 2195 ft-MSL berm on the East Abutment. The entire area is in granitic gneiss saprolite/weathered rock above the toe of the dam. The entire area was saturated and six springs were noted. In one spring, the water was flowing along a 2 to 3 inch thick fractured pegmatite oriented parallel to the dominant NE striking SE dipping foliation in the gneiss (Figure 21). In the other five springs, the water was flowing along joints oriented N10-20E, 70-80NW, approximately perpendicular to the dam axis (Figure 22). The joints were Fe and Mn stained indicating past water circulation and movement. This set of joints is not well-developed in the foundation rocks and the joints are generally discontinuous with trace lengths of 2 to 20 ft. The slough/slide area is about 20 ft east of the contact between the Toxaway Gneiss (granitic gneiss) and the Tallulah Falls Formation (mica schist) and is a zone where the granitic gneiss has been sheared in the past and is consequently finer-grained than the normal Toxaway Gneiss. The granitic gneiss in the area, based on the foundation geologic mapping, has higher fracture intensity and consequently a higher rock mass permeability than the mica schist. The mica schist acts as a barrier to the water movement resulting in a concentration of water and its movement in the granitic gneiss. The relatively rapid movement of water in the area suggested that the water path is below foundation grade and above the grout curtain (starts at the top of hard, sound rock; 5 to 15 ft of 50+ blow saprolitic material at the centerline that was not grouted). The path of the water is

Figure 21: Slough area after cleanup. The location of the pegmatite with water seepage is shown by the arrow.

24 Figure 22: Water flowing out of a N10-20E, 70-80NW joint in the slough area. The flow is clear (note ripples in water), no indication of material being piped through the abutment of the dam. probably not a straight conduit, but a zigzag route through the discontinuous joints, fractured pegmatites, and weathered rock seams. The slough/slide was due to saturation of the relatively steep saprolite/weathered rock slope on the abutment. The area was identified prior to filling as a potential leakage path based on geologic conditions and was incorporated into the visual inspection procedures for the initial filling operation. Wells were installed to monitor abutment phreatic levels (Figure 20). Seven holes were drilled at the dam centerline for dye testing based on review of the geologic mapping. Rhodamine fluorescent dye was injected at various depths utilizing a packer system. A fluormeter was used to test for dye concentrations downstream. When reservoir filling resumed, seeps were noted above the repair buttress (designated TJ-1, MB-1, and RM-1 on Figure 20). These flows were isolated, contained, and collected to allow quantitative measurement. Flows were initially minimal, but increased rapidly to a maximum cumulative flow of 150 cfs. A portion of the flow from under the buttress was bypassing the filters and running into the dam shell and was collected at the 2195 ft-MSL berm. Due to the significant quantity of flow which was not passing through the filter system, it was determined that the repair buttress would be rebuilt and expanded. The upcoming, scheduled initial drawdown of the reservoir presented the opportunity to reduce the flows and provide a more favorable working environment than available during the unscheduled drawdown for the original repair work. The goals of the buttress reconstruction were to replace the original geotextile with a new “designed” product, place a diffusion system at points of concentrated flow, contain flows that were bypassing the original buttress and filter system, extend the buttress and filters up the abutment to cover the source of the upper flows, and to collect

25 all filtered seepage for quantitative measurement. A system of geosynthetics was selected based on tested soil gradations and installation parameters such as puncture and tensile strength. At points where concentrated flows were noted or suspected in the future, a system to disperse and then filter flow was installed. This system, as well as other details of the permanent buttress, is shown on Figure 23. The multi-layer system consist of a woven monofilament with a high opening percentage covered by a three dimensional open grid to distribute flows. The combination was placed in localized areas. Filtration is assured by completely covering these patches and the remaining foundation with a heavy weight calendared woven geotextile with a somewhat tighter weave. A gravel filter was then placed over this to collect and drain the water. An impervious dike with an 8 inch PVC pipe was constructed at the lower end of the buttress. The gravel filter was covered with a heavy weight non-woven geotextile as a transition and rockfill placed to ballast the filter system. The extent of the repair area, as well as sectional views, is shown on Figure 23. Performance of the system has met all needs and service requirements to date. In addition to the new buttress/filter system, additional grouting was performed in the foundation. The dye testing discussed above revealed a zone of potential water migration between Stations 10+00 and 11+00. This is near the Toxaway Gneiss/Tallulah Falls Formation contact and is an area of extensive joints and fractures. Eleven grout holes were located with only moderate to slight takes. The grouting reduced flow (seepage) in the area by approximately one-third. An additional observation well was installed between the grouted zone and the East Abutment contact.

Figure 23: Plan and final repair details for the slough area on the East Abutment of the West Dam

26 East Dike – Shear Zone

The foundation rocks for the East Dike are primarily foliated granitic gneisses varying from very hard rock to completely weathered bedrock (saprolite). Thin layers of biotite schist, quartz-feldspar gneiss, and quartz-feldspar pegmatites are present. Structural features present include joints, faults, and shear zones. The major geologic feature in the East Dike foundation requiring special consideration was the continuation of the shear zone first mapped in the Intake Structure excavation and which continues across the topographic low across which the dike was constructed (Figure 24). The shear zone is approximately perpendicular to the axis of the dike. In the Intake Excavation (where the shear zone was first mapped) and in the East Dike foundation, the shear zone consist of a weathered zone 2 to 3 ft thick with alternating layers of hard material (quartz-feldspar pegmatites and breccia with an epidote-chlorite matrix) and soft material (weathered granitic gneiss, weathered sheared rock, discontinuous layers of biotite schist, and discontinuous layers of phyllonite ½ to 12 inches thick; Figures 25 and 26). Within portions of the shear, there is up to 8 inches of gouge-breccia composed of rock, quartz/feldspar fragments, and vein quartz fragments in a clay matrix. A relatively pure clay layer, 1 to 2 inches thick, is present along the hard layer of breccia. The harder layers within the zone are highly fractured with Mn and Fe staining along the fractures indicating water percolation. During exploration (air track holes) to define the excavation limits for the East Dike foundation and during the drilling of the grout holes along the centerline of the dike, mud seams were encountered. The presence of the mud seams suggested that the shear zone mapped in the Intake Excavation continued into the East Dike foundation. The shear zone in the Intake area was projected into the East Dike foundation area using an average strike and dip of N45E; 35SE. Based on this projected layout, additional exploratory borings were laid out to better define the position of the feature in the foundation and two test pits were located to study the zone in the foundation. In the borings, several were continuously sampled (split-spoons) and permeability tests were conducted where low blow counts or mud seams (wet zone) were encountered. A sheared zone was identified in five of the borings in saprolite. The zone consists of rock fragments in a highly weathered, very wet, red clay matrix similar in appearance, except for weathering, to breccia with the epidote-chlorite matrix. In some of the borings, the zone has different characteristics, all of which could be related to previously studied shear zones at the site. Using various combinations of locations of sheared areas from the borings, the strike and dip of the possible shear planes were calculated. Solutions ranged from N45E to N59E with 14SE to 28SE dips indicating more than one shear plane is present in the zone (as noted in the Intake Excavation and verified after foundation excavation).

27 Figure 24: Location of the East Dike Shear Zone in the East Dike, Intake Structure, and Main Dam and the location of other mapped shear zones in the Reservoir.

28 Figure 25: East Dike Shear Zone exposed in the Intake excavation southwest of the East Dike. Erosion has accentuated the major shear plane in the zone. Location on the 3:1 slope of the Intake excavation looking northeast.

Figure 26: Close up of shear zone in the east wall of the cutoff trench showing a 6” thick phyllonite layer with a well-defined plane containing red clay (1/4” to ½” thick) at the end of the hammer handle.

29 The test pits verified the existence of the shear zone in the dike foundation and combined with the information from the exploratory borings showed the shear zone is continuous in the foundation. In one pit, two shear planes were present. They consist of a pegmatite above sheared rock which rests on a thin clay layer above a breccia. The breccia was highly fractured with heavy Mn staining throughout. The breccia was discontinuous along strike while the clay layer was continuous across the pit. Four shear planes were present in the second pit. They all had thin, discontinuous biotite schist and breccia layers. Associated pegmatites as well as the breccia were fractured and Mn stained. The area of concern in the foundation was between the top of the grout curtain and the bottom of prepared foundation (Figure 27). The results of four permeability tests across the shear planes in saprolite were highly variable (1.1E-1, 2.1E-4, 4.4E-5, and 2.2E-7 cm/sec). The shear planes are not uniformly permeable and the zones of high permeability are probably related to the relatively thin, hard layers of pegmatite and breccia in the otherwise weathered shear zone. The possibility existed for high seepage rates with associated piping of the shear zone and surrounding foundation materials between the top of the grout curtain and the bottom of the prepared foundation. A positive cutoff of the shear zone through the core area was recommended. Remedial action to control seepage and piping, if performed after construction of the dike and filling of the reservoir, would in all likelihood be more expensive then positive treatment during construction. In the sound rock, the grout curtain would be sufficient cutoff. The excavation of a core trench would remove the un-grouted saprolitic material though which piping could occur. High grout takes on the South Abutment are related to the shear zone (Figure 27). In the Intake excavation isolated areas of highly fractured rock occurs updip of the easternmost shear plane. These highly fractured areas are related to movement on the shear zone as suggested by the termination of the fractured zone at the uppermost shear plane. Excavation of the foundation verified this zone of highly fractured rock in the East Dike foundation (Figure 28). An additional grout line in this area showed the normal decrease in grout takes indicating a satisfactory cutoff had been achieved. As stated above, a partial cutoff trench under the core was selected as the best option for treating the shear zone. This required excavation of material proceeding south along the contact between saprolite and relatively sound rock following the dip of the rock foliation. The base width of the trench was selected to achieve a maximum hydraulic gradient of 2.0 through the core. The excavation created a relatively confined area at the intersection of the trench base with the south wall of the excavation. Additionally, significant amounts of water were encountered in the excavation. To control the water and allow placement of fill along the steep south wall, a concrete backfill ~10 ft thick was placed in the area. The balance of the cutoff trench was backfilled with core material with conventional foundation preparation (Figure 29). The south wall required the use of extensive dental concrete to fill the irregular rock surface and provide a positive slope for the placement of core material. Piezometers were installed within the shear zone under the core and downstream shell. Trench drains were installed in the saprolite and along the shear zone under the downstream shell. These drains yielded negligible flow during initial filling, early operation, and to the present.

30 Foundation preparation for the remaining areas of the dike core and shell areas followed typical procedures.

Figure 27: Cross-section A-A’ along the centerline of the East Dike, looking west into the reservoir. The area of concern for potential seepage was between the bottom of the core and the top of the rock curtain. A core trench was used to cut off this potential seepage path. Note the high grout takes on the south abutment of the dike. The takes are related to fractured rock above the main shear zone (see Figure 28).

Figure 28: South and southeast wall of the core trench exposing the highly fractured and sheared rock (area of the high grout takes) updip of the major sheared area.

Figure 29: The core trench looking north along the centerline of the East Dike. Note the rock-shell fill downstream of the core trench on the right side of the photograph. The trench was excavated to remove the shear zone in the saprolite above the top of the grout curtain.

The shear zone also is present in the East Abutment of the Main Dam (Figure 24). Treatment of the shear zone there was similar to that used at the East Dike.

Intake Channel

The geometry of the Intake Channel was determined based on the results of hydraulic model testing. The selected configuration is a rectangular basin excavated into rock with a width of 75 ft at the vertical shaft tapering out to 140 ft at the channel entrance (Figures 30 and 31). The vertical rock walls beginning at the channel base were formed by pre-split blasting techniques. Tensioned rock bolts and chain link fence were installed as required to stabilize the rock cuts. The 3H:1V cut slopes in residual soil and saprolite provide a safety factor of 1.5 for the sudden drawdown condition. To prevent erosion, cut slopes in soil/saprolite are armored with rip-rap underlain by sand-gravel bedding. The rip-rap is sized to resist wave action forces and velocities created during pumping and generation. Total excavation volume for the channel was 2,200,000 yd3 of which 900,000 yd3 was rock.

32 Figure 30: Plan and sections - Intake Structure.

Figure 31: Photograph of the Intake Structure after completion and before reservoir filling. Note the water retaining structure at the start of the channel and the anti-vortex beams over the vertical intake shaft.

Prevention of large rocks entering the shaft is a design feature because no trash rack on the intake or rock trap in the power tunnel is provided. To prevent rock entrainment the walls adjacent to the intake are covered with steel mesh. A concrete parapet wall surrounds the intake shaft above the rock wall to prevent dislodged rip-rap from entering the shaft. The entire reservoir was meticulously cleaned to minimize the potential of floating debris reaching the intake shaft.

34 To minimize vortex activity during generation, antivortex beams were constructed approximately 70 ft above the bellmouth inlet. The beams consist of two structural steel members spanning the channel and a transverse brace. Each member is a box-type section approximately 4 ft deep by 2 ft wide fabricated from 1.5 and 2 inch plate. Each beam is completely filled with concrete and non-shrink grout. The end connections are supported on reinforced concrete pads poured into notch outs and bolted to the rock walls. The beams are designed in accordance to standard provisions for applicable dead and live load combinations, including lift and drag forces, with special emphasis on the dynamic response such as frequency, vortex shedding, and torsional flutter. All materials are either noncorrosive or epoxy coated. The water retaining structure is a gravity type concrete dam, with a height of 30 ft, located midway in the Intake Channel. The purpose of the structure is to permit un- watering of the power tunnel for inspections without completely un-watering the reservoir. The reservoir would be lowered to a predetermined elevation below the top of the structure via two 42 inch diameter sluice gates. This level is selected to avoid overtopping in the event of a 100 year rainfall event, yet allow a sufficient volume of water to fill the power tunnel and restart the units. The rock abutments and foundation of the water retaining structure were pressure grouted to insure an adequate cutoff. The structure is designed in accordance with standard provisions for applicable dead and live load conditions including hydrostatic pressure and uplift. The sluice gate controls are accessible from a steel walkway affixed to the top of the concrete structure.

Engineering Geology of the Access Road Along Lake Jocassee

In 1980, a geologic survey of the alignment of the Main Access road was made to identify geologic features that could influence the stability of cuts and fills. Potential stability problems related to rock cuts and the presence of old landslides consisting of colluvial materials (boulders in a finer-grained matrix) were identified during the survey. The possibility of unstable rock wedges in the rock cuts along the last mile of the access road was investigated using a standard technique (Hoek and Bray, 1981). The average orientation of the major joint sets and bedding were used in the analysis (Figures 32 and 33). The average values of the discontinuities are plotted as great circles on a lower hemisphere-equal angle projection along with the great circle representing the orientation of the cut to check for wedges with intersections that daylight out of the proposed rock cuts. Friction angles of 30o and 40o were assumed for evaluating potential wedge failures on the stereo plots. Five rock cut orientations along the access road were analyzed (Figures 32 and 33). Wedges related to the joint sets, with a daylighting, steeply dipping line of intersection, are not formed in the various rock cuts (see projections on Figures 32 and 33). The only potential wedges are related to the intersection of foliation planes (N55E; 39SE) with Joint Set #1 (N75E; 86NW) and Joint Set #3 (N58W; 80SW). The lines of intersection for foliation with Joint Set #1 and #3 plunge at 15o and 38o, respectively. Depending on the friction angle (if less than ~38o), movement of the wedge developed by the intersection of foliation and Joint Set #3 is possible. However, it was thought that the geometry of the wedge (small angle between the strike of Joint Set #3 and the trend of the line of intersection; see Figures 32 and 33) and the relatively large spacing of joints in Set

35 #3 (> 5 meters), precluded any major stability problems related to these potential wedges. This conclusion was confirmed during construction. Failure of the rock cuts along foliation planes was a distinct possibility at Orientations 3 and 5. The design in these areas called for rock bolts as needed for stabilization of the slopes. Part of the rock slope failed at Orientation 3 during the excavation/blasting of the slopes. Because of this failure during excavation, major, additional rock support in this area was not required. This decision on rock support has been confirmed by the stability of the area to the present. During construction, cracking was noted above the road alignment within a day after the initial excavation in the area of Orientation 5. The cracking was noted on a Thursday afternoon, and directions given to Safety Personnel at the site to stop work below the area until additional evaluation could be performed. The slope failed along foliation planes sometime on the following Sunday. After cleanup, the slope was stabilized by rock bolts along the base of the slope. Four old landslides were identified on the last mile of the access road and their locations are shown on Figures 32 and 33. The depth to sound rock under Slides 1 and 2 (Figure 32) was shallow enough to allow excavation of all the colluvium under the access road (within 1 meter of grade). Cuts above the road in these slide areas were laid back at 3:1. The depth of the colluvial material in Slide 3 was up to 8 meters below road grade. Because of stability concerns, it was decided to remove all of the colluvial material down to sound rock. After this excavation there would not be enough area from the access road to Lake Jocassee to build a structural fill with the nominal 2:1 slopes. Therefore, a retaining wall was constructed across the Slide 3 area. Details of the retaining wall are shown in Figure 34. The depth of colluvial material in Slide 4 (see Figure 33) is up to 25 meters. Because of the volume of material that would have to be excavated and the height and length of the required retaining wall that would be needed across the area, an alternate approach for stabilization was considered. Seven slope indicators were installed in the slide area in 1980 (Figure 35). Very small movements at the colluvium-sound rock contact were noted in three of the slope indicators located above the access road. The movements were less than 1 mm/month and discontinuous along the contact. Boring data indicated that the water table was generally within 1 meter of the contact between the colluvium and sound rock (Figure 36). With access road construction in the area not to start until 1983, subhorizontal drains were installed in an attempt to drain the slide above the contact to stop the discontinuous slope movement (Figures 35 and 36). The drains combined made between 15 and 30 gallons/minute with the lower row of drains near the access road alignment making less than 2 to 5 gallons/minute depending on the season. The drains did stop the movement of the slope (in the 3 slope indicators that showed movement before the installation of the drains). Therefore, the access road was constructed across the colluvial material of Slide 4. No movement of the area has been noted since the completion of the access road.

36 Figure 32: Access road along Lake Jocassee showing the location of cuts, fills, and colluvium and the lower hemisphere-equal angle plots of potential wedge and planar failures in the five analyzed rock cut orientations. Shows the location of the retaining wall installed to stabilize colluvial materials.

Figure 33: Access road along Lake Jocassee showing the location of cuts, fills, and colluvium and a lower hemisphere-equal angle plot of potential wedge failures along this portion of the access road. Shows the location of horizontal drains installed in colluvium to increase stability of the mass.

Figure 34: Typical Section through the concrete retaining wall across Slide 3 (see Figure 32 for location).

Figure 35: Plan showing the location of inclinometers, piezometers, borings, and subhorizontal drains in Slide 4 (see Figure 33 for location).

Figure 36: Cross-sections showing horizontal drain construction details and the location of the water table and colluvium-sound rock contact in relation to the drains.

Discharge Structure Area

The area above the Discharge Structure is an old landslide that was reactivated during the initial portal preparation at Bad Creek before Lake Jocassee was filled in 1974. In May 1984, the slide progressed up the slope toward the switchyard (Figure 37). The area was mapped in 1986 to provide geologic input for stabilization efforts to prevent localized slides during construction and permanent plant operation (Figure 38).

Figure 37: Old landslide at the Discharge Structure Area (lower left). The switchyard is in the upper middle of the photograph. Movement in May 1985 was in the woods above the upper right portion of the cleared area below the switchyard. The overlying colluvial material of the slide has been removed below the concrete ditch running in the middle left of the photograph.

The southern boundary of the slide area (A; letters refer to locations on Figure 38) is a fault zone, associated with jointing, striking approximately E-W and dipping 60-80N (Joint Set #1). The fault zone is characterized by intense fracturing and secondary mineralization. This boundary controlled the southern limit of the colluvial material which moved during the initial portal preparation in 1974. South of this boundary, the rock is moderately severe to severely weathered (saprolitic). The failure plane in Area B is a 15 cm to 1.5 meter thick biotite schist layer. The schist is severely weathered in places and joints within it are spaced from 15 cm to 3 meters. The failure plane steps down section to Area C where the main failure plane is another biotite schist layer (see Section B-B in Figure 38). This biotite schist layer is the failure zone for the majority of the northern portion of the slide mass. The schist is severely weathered in places and varies from 15 cm to about 6 meters thick (thinner areas left may be the result of removal during sliding). This biotite schist layer is the likely failure plane for the slide that

42 occurred in May 1984 above the concrete ditch. Based on observations above and below the drainage ditch (D) and two borings installed just downhill of the ditch (see Figure 38), the depth to the failure plane in this area is about 8 meters. About one month after slope indicators were installed in the two borings, movement occurred inside the main slide mass developing a secondary scarp about 15 meters above the ditch (D) that sheared the two slope indicators. After this movement, additional borings and a seismic refraction survey was made in the area of the ditch (D) to determine the depth to sound rock in order to evaluate the possible installation of a tieback wall in that area. The final design for the stabilization of the area called for the removal of all older colluvium and more recent slide material from the slope, laying back the saprolite/soil area south of the east- west fault zone (A) on 2:1 slopes, and construction of a retaining wall along the general alignment of the ditch (D; Figures 39 and 40). The area was remapped after the implementation of the design work (Figure 42) and a photograph of the area is shown in Figure 41.

Figure 38: Geologic map of the Discharge Structure Area in 1986 before excavation of the Discharge Structure.

In the fall of 1994, a portion of the soil slope below the retaining wall and south of the east-west fault zone, failed. Examination of the area indicated that the slope, although soil/saprolite, failed along a manganese oxide coated contact between a biotite schist layer and the granitic gneiss saprolite. The concern was that continued failure up the slope along this layer would result in the undermining of the southern end of the

43 retaining wall. This portion of the original design wall was not founded on sound rock but transitioned up into alternating seams of weathered rock and saprolite. An “Insert Wall” was selected as the method to stabilize the soil-saprolite below the existing wall. The wall consists of three components, 1) grouted soil anchor bars, 8 to 15 meters long with a 25 cm x 25 cm anchor plate, 2) 20 cm thick reinforced concrete face restrained by the anchor plates, and 3) a toe buttress wall tied to the underlying rock with grouted rock anchors (Figure 43). The anchor bars are high strength, deformed threadbars set in grout in the saprolite, partially weathered rock, and rock. They transfer tension loads through the grout into the materials surrounding the drill holes in which they are inserted. The shotcrete face restrains the soil-saprolite-weathered rock between the anchors. It acts in conjunction with the anchors and the anchor plates. The anchors are not pre-tensioned, but are passive anchors. As installed they insert no restraint on the soil or rock behind the wall. As the soil and rock behind the wall expands or slides downhill, reacting to the unloading of weight by the new excavation and the previous sliding of the mass, the anchors will provide increasing resistance with increasing movement. Some movement of the materials is normal and essential in developing the restraint of the wall. The design includes vertical drains consisting of a geotextile filter against the soil and a drain grid of plastic between the filter and the shotcrete. This drainage system is to relieve water pressure against the shotcrete. Additional rock bolts were installed in the rock exposed by the removal of the overlying soil/saprolite during the repair work.

Figure 39: Plan view of the construction work above the Discharge Structure showing the location of rock excavation (and removal of older colluvium and recent slide material), location of soil excavation, and the location of the retaining wall.

Figure 40: Typical cross-section of the upper retaining wall at the Discharge Structure.

Figure 41: Photograph of Discharge Area after the design work and construction. Note retaining wall and area of soil excavation and layback.

Figure 42: Geologic Map of the Discharge Structure Area after completion of design work. Geology by M. F. Schaeffer (1987-88).

Figure 43: Typical cross-section of the “Insert Wall” at the Discharge Area.

Underground Structures and Works

Introduction

Design began with site selection when it was determined that the lower reservoir would be existing Lake Jocassee and the upper reservoir would be constructed about one mile west of Lake Jocassee’s Whitewater River Arm (Figure 2). The hydraulic head between the reservoirs is approximately 1200 feet which dictated a submergence for the pump-turbines that was too great to allow economical construction of an aboveground powerhouse adjacent to Lake Jocassee. An underground powerhouse was therefore required somewhere between the two reservoirs. In order to eliminate the need for downstream surge chambers, the Tailrace Tunnels needed to be as short as possible making a Powerhouse location near the lower reservoir most desirable. The minimum distance for the Powerhouse from the lower reservoir was then determined by the combination of the depth of submergence of the turbines below the surface of the lower reservoir and the maximum allowable grade (10%) for the Access Tunnel from the lower discharge area to the Powerhouse operating floor elevation. With the selection of a four-unit station, the number of Penstock and Draft Tube Tunnels were set. Analysis involving hydraulic transients, head losses, together with geologic, economic, and construction considerations determined the preliminary sizes and layout of the water tunnels. The final location and orientation of the Powerhouse and layout of the tunnel system was based on the results of the subsurface exploration program.

47 The subsurface exploration program was developed with the following objectives: 1) examination of the rock characteristics and geologic structure of the proposed Powerhouse location, 2) determination of the best Powerhouse orientation and location with respect to the geologic structure and in-situ stresses, 3) provide the data and experience necessary to facilitate an efficient design of the underground portions of the project, and 4) serve as a model for the instrumentation and monitoring to be incorporated into the permanent underground structures. Early in the project it was decided that a Pilot Tunnel into the proposed Powerhouse location would be the primary activity of the underground exploration program. Preliminary core drilling, laboratory testing of core samples, and a deep hole hydrofracturing test had been conducted before the design of the Pilot Tunnel Program. Data from these tests showed generally good rock conditions, but with high horizontal in- situ stresses present. However, due to the magnitude of the project, the Pilot Tunnel program was considered a prudent investment. The Pilot Tunnel excavation and testing lasted from October 1976 through September 1977. The work was divided into three main components: 1) excavation monitoring, 2) rock testing, and 3) geologic exploration. Two methods of in-situ stress measurement were employed, hydrofracture and overcoring. The hydrofracturing tests were performed in a deep borehole (B-52) from the ground surface and the overcoring technique was employed in the proposed powerhouse location in the Pilot Tunnel. Table 1 gives the in-situ stress values obtained from the hydrofracturing tests and Figure 44 gives the values from the overcoring tests. Preliminary calculations and the hydrofracturing measurements assumed a vertical stress component equal to that due to overburden. At the overcoring test depth this would be about 690 psi. The vertical stress determined from overcoring was 1476 psi and was oriented 10o south of east at an angle of 14o from vertical (Figure 63). If this higher stress magnitude had been assumed in the hydrofracturing stress calculations then there would have been good agreement with the overcoring results. The directions of the horizontal stresses were in good agreement between the overcoring and hydrofracturing tests.

Stress Pore Pressure Stress Magnitude Orientation of Principal Stress

Vertical Stress,  3 800 – 1000 psi Vertical

Maximum Horizontal,  1 0 psi 2500 – 4100 psi N60E 300 psi 2200 – 3800 psi N60E Minimum Horizontal, 1950 – 2650 psi N30W  2 Table 1: Hydrofracturing results in Borehole B-52. Several tests were performed at different depths in the vicinity of the proposed powerhouse.

The overcoring stress values provided some of the input parameters for the finite element modeling (FEM) of the powerhouse and tunnels. The results of the FEM analysis were used to determine the shape of the powerhouse and tunnels, other factors such as geologic structure, support methods, and other functional requirements played a major role. The most useful information from the FEM results is they provided an estimate of the how much rock movement should be expected during and after

48 powerhouse excavation. These estimates became the basis for evaluating the data from installed instruments during and after construction.

Figure 44: Result of overcoring in-situ stress measurements in the Pilot Tunnel.

49 The major factor affecting the design of the underground structures is the structural geology of the site. Since there was a possibility that reorientation of the Powerhouse might be required due to geologic conditions, continuous monitoring of the geology was performed as the Pilot Tunnel excavation progressed. Early analysis of the geologic data indicated that the original north-south Powerhouse orientation would be compatible with the geologic structure. The underground structures are located in the Toxaway Gneiss. The foliation is fairly consistent with an average orientation of N35E; 30SE. The dominant joint set is oriented N70E to N70W (east-west) with dips >50o north and south. Other sets are oriented N60E; 60NW, N65E; 30SE (foliation joints), and N45W; 70-90 SW or NE. The joint are tight at depth. Near the surface some joints are open and weathering resulted in blocky conditions at the tunnel portals. Occasional shear zones are present with orientations parallel to foliation and faults with minor offsets are present and related to the NE joint set. Some of the shear zones and faults made small amounts of water and were the only sources of water encountered during the Pilot Tunnel excavation and during the main construction phase. The results of the Pilot Tunnel and other subsurface exploration studies were: 1) The rock quality is good to excellent, 2) No problems were expected due to excessive water flow, 3) There are high in-situ stresses which would cause rock to spall in some locations, 4) Deep seated or large instabilities were not expected beyond 300 feet from the surface portals, 5) Blocky rock would be encountered near the tunnel portals and would require support and/or ground reinforcement measures, and 6) Minor zones of closely spaced joints, faults, and the shear zones may cause local zones of instability in the underground works.

Underground Layout and Design – Powerhouse

The Powerhouse Cavern was oriented long-axis north-south based on the geologic conditions documented during the Pilot Tunnel studies in 1976-1977 (Schaeffer and Steffens, 1979) and the results of hydrofracturing stress measurements in borehole B-52 and overcoring stress measurements in the Pilot Tunnel (Schaeffer and others, 1979). The magnitude and direction of the in-situ stresses determined by the overcoring technique are:  1 , maximum principal stress, 29.3 MPa (4253 psi) @ N57E,  2 , intermediate principal stress, 18.4 MPa (2675 psi) @ N32W, and  3 , least principal stress, 10.2 MPa (1476 psi) subvertical. All stresses are compressive. The subvertical stress is about 2 times that expected from overburden indicating the Toxaway Gneiss at this location is overstressed. The in-situ stresses are high enough that they caused shallow spalling of excavated surfaces. Most of the spalling during the Pilot Tunnel studies occurred in the enlarged Powerhouse test chamber where the shape of the crown arch was such that large tangential stresses were produced. The optimum orientation of the long-axis of the Powerhouse Cavern with respect to the in-situ stresses would be N57E-S57W, that is, the short wall would be perpendicular to the direction of the maximum stress and the long wall of the cavern would be perpendicular to the direction of the intermediate stress. The main set of discontinuities in the Powerhouse area are joints of Set #1 (N75E; 86NW – closer to east-west strike in the Powerhouse). These combined with the foliation could produce large wedges in the crown. A north-south

50 orientation of the Powerhouse minimizes the potential size of the wedges. The north- south orientation was selected as the most stable with respect to both the discontinuities and the in-situ stresses. The overall size of the Powerhouse was determined by the generation and support equipment requirements while keeping the width within manageable limits (75 feet wide). Straight sidewalls were employed due to the massive, strong, and minor fractured nature of the rock mass. With high horizontal stresses, a flat crown would be advantageous, but a crown 20 feet high above the springline was deemed necessary to provide support for potential rock blocks delineated by the east-west joint set and the foliation. In addition to the FEM studies, an analysis of potential rock blocks in the cavern walls and crown was performed using stereographic methods including rock bolt forces. Pattern rock bolts were specified in the walls and crown of the Powerhouse based on the analysis and successful experience in similar powerhouses. In the crown and wall of the Powerhouse above the structural concrete 20 foot long, #9 Grade 60 rock bolts on a 5 x 5 foot pattern were specified. In the areas of the structural concrete a 6 x 6 foot pattern was specified. The pattern in the end walls varied somewhat and was modified based on rock conditions. All the rock bolts are of the fully resin encapsulated type. The bolts were designed as dowels (no pre-stressing), but to ensure they were “snugged up” in order to mobilize their strength in case of rock movement, a two part resin system was used with nominal 350 ft-lbs of torque applied to provide nominal pretension. In addition to the pattern rock bolts, the Powerhouse crown received two 2 inch layers of shotcrete with welded wire fabric installed between the layers. The pattern rock bolts have extensions through the shotcrete allowing the false ceiling to be coupled to them for support.

Underground Layout and Design - Tunnels

Hydraulic transient analyses showed that a downstream surge chamber was not necessary if the Powerhouse was located close to the lower reservoir. The elevation of the Powerhouse was set by the required submergence of the turbine runner below the surface of the lower reservoir. The final Powerhouse location was then determined by the slope of the Main Access Tunnel from the portal down to the Powerhouse operating floor elevation. Hydraulic and economic analyses determined that a single Power Tunnel could be utilized. This then set the general configuration of the water tunnels. Tunnel diameters were chosen based on head losses, maximum velocities, hydraulic transients, and the sizes of the turbine spiral case and draft tube. Other tunnel alignments and diameters were determined by access requirements during construction and for later plant operations. Final constructed underground layout is shown in plan view on Figures 2 and 3 and in cross-sectional view in Figures 3 and 45. One of the access tunnels was relocated during construction and this relocation will be discussed in a following section. The length of the Penstock Tunnels and hence the location of the Manifold Tunnel depended on the length of steel liners required in the penstocks. The purpose of the steel liners in the four Penstock Tunnels is to prevent the leakage of high pressure water into the rock mass near the Powerhouse Cavern. With a deep underground Powerhouse, the length of the steel liners depends on the quality of the rock mass and the potential leakage/water flow paths within the rock mass (shear zones and faults as

51 discussed above). Considering the potential of these geologic structures to be in the Manifold/Penstock area, the overall excellent quality of the rock, depth of the penstocks below the ground surface, and what had been used in similar powerhouses, the steel liner length was set at 200 feet.

Figure 45: Profile of the Bad Creek Project underground works, looking north.

Starting from the Powerhouse, the steel liner is 8.4 feet in diameter to match the spherical valve. The 8.4 foot diameter section is 65 feet long and was designed as a free standing pipe to resist full internal pressure plus transients of 912 psi. Over the next 15 feet, the steel liner transitions to a 13.75 foot diameter. The remaining 120 feet of the steel liner was designed so that the loading from internal pressure is shared between the steel, concrete, and rock. The steel liner was fabricated from A516-70 steel with the thickness varying from 1.625 to 1.75 inches. The liner was checked for several possible external pressures and at 344 psi it was determined that external stiffeners were not required. The steel liner was grouted in place to within 30 feet of the Powerhouse where an unbonded length was left to allow for axial thrust related to the operation of the spherical valve. Even with the steel lining of the Penstocks, there is the possibility of high pressure water finding its way to the Powerhouse Cavern. The concrete lined portion of the Penstock and Manifold Tunnels may have some leakage due to the typical cracking of the concrete linings. These tunnels contain steel reinforcement to limit cracking, but it must be assumed that some leakage will occur. The rock mass in the Penstock area was grouted using pressures up to 600 psi to help fill and block potential seepage paths. A two stage grout curtain was placed at the end of each Penstock steel liner. The tunnel linings were contact grouted at 20 psi except for the Penstocks that were grouted at 100 psi. As a final measure, a system of drain holes was installed from the Powerhouse Bypass Tunnel located just upstream of the Powerhouse and above the Penstock Tunnels to provide a means to intercept water in the rock mass from the upstream tunnels. The installation of these drain holes will be discussed in more detail in a following section. A system of piezometers was installed in the rock mass and behind the tunnel linings to monitor water pressure. During un-watering of the tunnels, the water pressure is monitored and the rate of un-watering adjusted to prevent the buildup of excessive external pressures behind the linings. The majority of the piezometers are vibrating wire type with some pneumatic types installed for redundancy.

52 With the excellent rock at Bad Creek, the tunnels generally stood unsupported after excavation. The high in situ stresses caused some spalling to occur, primarily in the tunnel and powerhouse crowns, but also in the northwest and southeast corners of the Powerhouse excavation. The spalling was continuously observed during the Pilot Tunnel work and was noted in the Pilot Tunnel Geologic Report and in the bid documents. The spalling rock occurred as thin slabs of rock and was most prominent in the more massive gneiss bodies in the underground works. Near the ground surface where stresses had been relieved over time, spalling did not occur. Only after a depth had been reached where the stresses had not been relieved did spalling start. Other than the blocky rock near the Access Portal, no support of the Main Access Tunnel was needed (outside of spot rock bolts in the ribs) until the tunnel had advanced to about 770 feet, where the overburden was about 500 feet and spalling occurred. At this point, the spalling was controlled by using 10 foot, resin anchored rock bolts (#9 bars) on a 5 x 5 foot spacing in the tunnel crown and a 1 inch thick layer of fiber reinforced shotcrete. The need for rock bolts and shotcrete varied somewhat, depending on the rock type, but pattern bolting and shotcrete was used routinely in the tunnel crowns for safety purposes. All the water tunnels are lined with cast-in-place concrete. The linings are used for hydraulic smoothness and not as ground support since the rock is largely self- supporting. Most of the linings are un-reinforced with a required minimum thickness of 12 inches. Reinforcing was used where additional stresses develop such as the Intake elbow from the Vertical Access Shaft to the Power Tunnel, the Manifold and Tailrace bifurcations, and near the Discharge and Access Tunnel Portals. Reinforcing was used for crack control and to minimize leakage from the Manifold and Penstock Tunnels upstream of the steel liners as previously discussed and in the Draft Tube Tunnels near the Powerhouse. Concrete mixes used were designed to have a low heat of hydration to help control cracking. Consistent with the ground interaction design, contact grouting was performed in all tunnel linings to provide close contact between the rock and concrete. In addition to the contact grouting and penstock consolidation grouting, grouting was performed in the Manifold Tunnel and interface grouting (to fill voids between steel liners and concrete) in the Penstock and Draft Tube linings and along the joint between the Draft Tube Gate concrete and waterway tunnel liner to minimize flow into the Draft Tube Gate Shafts.

Underground Construction – Geology Program

The geologic program during the construction of the Bad Creek Project was designed to provide geologic information to construction and engineering personnel and to provide documentation of the conditions encountered. The program was setup to confirm design conditions or identify conditions that would require design changes during construction. The geology of the underground works was documented by the geologic mapping of at least one rib of all the tunnels, the walls, crown, and floor of the Powerhouse excavation, and the walls of the vertical shafts (Vertical Intake Shaft and Access Shaft into the Powerhouse). The detailed mapping identified factors that were not considered during the initial design phase. Most of these factors were related to geologic features known to be present from the preliminary studies, but their actual locations in the underground excavations which made them significant, were not know until identified

53 during the detailed studies. Based on the detailed and additional studies of the particular features, remedial action was planned and implemented to minimize potential effects. These geologic factors and studies included: 1) the relocation of the portal for the Penstock Bypass Tunnel from a potentially unstable area related to a weathered shear zone to a stable area, 2) projection and prediction of the locations of a water-bearing fault zone and a shear zone in the Manifold, Penstock, Powerhouse, and Draft Tube areas, design of a drainage system installed from the Powerhouse Bypass Tunnel, design of a shear zone drainage system along the Powerhouse walls and floor to control water movement through the shear zone into the Powerhouse excavation, and selection of the location of piezometers for monitoring water pressure in the rock mass surrounding the Penstock area and just upstream of the Powerhouse, 3) additional rock bolting of wedges defined by jointing, faulting, foliation, and biotite schist seams in the Powerhouse crown, and 4) stress relief and cracking of shotcrete in the Powerhouse Cavern crown during excavation and the design of additional support measures. These studies and others will be discussed in the following sections.

Penstock Bypass Portal/Tunnel Relocation

A shear zone encountered in the Main Access Tunnel near the Penstock Bypass Portal would follow and be in the crown of the Penstock Bypass Tunnel for up to 160 feet (Figure 46 shows the original location of the Portal). In the Main Access Tunnel, metal straps and wire mesh were used in addition to rock bolts and shotcrete to control spalling and fallout of a weathered zone associated with the shearing. The shear zone was projected into the planned alignments of the Draft Tube-Tailrace Tunnels, Tailrace Bypass-Penstock Bypass Tunnels, and to above the Powerhouse Cavern (Figure 46). The projection suggested the zone may be in the crown of the Tailrace 1 and 2 Tunnels and Tailrace Bypass intersection. The shear would cut across the Penstock Bypass Tunnel in the vicinity of the Tailrace Bypass Tunnel Portal, but would occur in a limited area if the original designed Penstock Bypass Portal and portions of the tunnel were moved. The data indicated the shear zone would pass above the Powerhouse cavern. Because of the stability problems encountered along the shear zone in the Main Access Tunnel and based on the projection, the Penstock Bypass Tunnel Portal was moved back up the Main Access Tunnel to keep the shear zone from being in the crown of the Penstock Bypass Tunnel for a significant distance (Figure 3 shows the as-built location). When the tunnels advanced into the Tailrace area, the excavation was closely watched for problems associated with the shear zone being at a tunnel intersection. It turned out that the orientation of the shear zone in the Tailrace area was close to the projection of the maximum strike and dip (Figure 46) and did not cross any of the tunnel intersections.

Manifold – Penstock Area

On March 3, 1986, a fault zone was exposed in the Penstock Bypass Tunnel between Stations 2+30 and 2+38 (Figure 47). The zone has complex fracturing with up to eight fault planes oriented EW to N60E and dipping north. The orientation of the zone in the tunnel is approximately N70E. Breccia up to 3 inches thick developed along portions of the fault planes and consists of rock fragments and vein quartz in a fine-

Figure 46: Projection of the shear zone in the Main Access Tunnel into the Penstock Bypass, Draft Tube, and Tailrace Tunnels and to above the Powerhouse Cavern. Original location of the Penstock Bypass Portal and Tunnel is shown on the above plan. As-built locations are shown on Figure 3. grained matrix comprised predominantly of chlorite with minor epidote. All fractures within the zone are mineralized with combinations of epidote, chlorite, and quartz. Along some of the fault planes only chlorite up to ½ inch is present. Vertical offsets are up to 2 inches, but subhorizontal slickensides on chlorite indicate the primary movement was strike-slip. The thicker chlorite mineralization has a secondary shear foliation indicating movement after the primary mineralization. Near the tunnel invert, weathering within the fault zone is advanced and clay is present along some of the fault planes. Portions of the fault zone were wet, but flowing water was not present. Due to the proximity and orientation of the fault zone to the Penstock Area and the presence of wet zones indicating a possible seepage path from the Penstock Tunnels into the rock mass upstream of the Powerhouse Cavern, the fault zone was traced and/or projected through the tunnels into the Powerhouse. On March 28, 1986, the fault zone was mapped in the Powerhouse Bypass Tunnel between Stations 1+08 and 1+16 (Figures 47 and 48). Along this portion of the zone, breccia up to 6 inches thick and comprised of broken rock fragments and quartz/feldspar grains in a fine-grained matrix of quartz, chlorite, epidote, mica (muscovite?), and feldspar (albite?). Chlorite up to 1.5 inches thick and slickensided throughout is present along some of the fault planes indicating strike-slip movement. Dripping water was present in the fault zone.

Figure 47: Geologic map of a portion of the Penstock Bypass and Powerhouse Bypass tunnels. Locations of the maps are shown on Figure 51, Sections B and C. Red rectangle shows the location of Figure 48. Geology by M. F. Schaeffer, 1986.

Figure 48: Photograph of the Penstock Area Fault Zone in the Powerhouse Bypass Tunnel. Location of photograph shown of Figure 47.

The fault zone was traced into the Powerhouse Cavern. The locations and elevations of the fault zone where known at that time are shown on Figure 51. Using these locations the fault zone was projected to the bottom of the Penstock Tunnels assuming an average dip of 75NW. The projected trace is shown on Figure 51. On March 10, 1986, a shear zone in the Manifold Tunnel at Station 0+21 was exposed (Figures 49 and 50; location shown on Figure 51). Water flow from the shear zone was between 10 and 15 gpm. It has a weathered zone up to 6 inches side with gouge (broken, angular sand-size fragments in clay) up to 2 inches thick. Where water was flowing along the zone, portions of the weathered material had been removed. The measured orientation of the zone ranged from N43E to N65E with dips between 25-52SE. As seen in Figures 49 and 50, the shear zone is undulating, possibly folded. The zone projects through Penstock Tunnels 3 to 1. The presence of flowing water and rough projection of the zone into the Powerhouse area suggested this feature was the same one that produced water in boreholes drilled from the Pilot Tunnel in 1977 (DH-110, -111, - 120, -121, and -124; see Figure 51 for locations). During the pilot tunnel studies, dye was pumped into one of these holes and came out all the others showing the interconnection of the hydrologic system. Shear zones or possible shear zones were logged in these holes. Using the elevations and locations of these logged features and the location of the zone in the Manifold Tunnel, the zone was projected through the Penstock Tunnels and into the Powerhouse Cavern. The plan view in Figure 51 shows the location and elevation of the shears logged in the boreholes along with the location and elevation of the zone in the Manifold Tunnel. A strike range of N45E to N65E is shown. The N45E projection connects the zone in the Manifold Tunnel to the zone in borehole DH-

57 Geology by M. F. Schaeffer, 1986 Figure 49: Geologic map of a portion of the Manifold Tunnel. Location shown in Figure 51, Section A. Red rectangle shows the location of Figure 50.

120. The N51E projection connects it to the zone in borehole DH-124. The N65E projection is based on the maximum strike measured in the Manifold Tunnel. The shear zones were projected to the elevations of Draft Tube Tunnels and Powerhouse Bypass

58 Tunnel. It is possible that the features in boreholes DH-120, -121, and -124 are the same shear zone within this range of possible orientations of strike. Strike and dip of the zone calculated using the data from these boreholes is N63E; 25SE, which is within the projected range. Using other combinations of data from all five boreholes yields strikes ranging from N15E to N63E with dips between 10 and 25SE.

Figure 50: Photograph of the shear zone in the Manifold Tunnel. Note the flowing water along the zone. Location of Photograph shown on Figure 49.

Several interpretations were possible, but two were considered most plausible: 1) the zone in all five boreholes and the Manifold Tunnel is the same requiring folding and/or flattening of the shear zone, and 2) the features in boreholes DH-110, -111, and possibly -120, although part of the same hydrologic system, are not the same geologic feature. The shear zone does flatten in the Manifold Tunnel, but the large variation necessary to accomplish interpretation 1 was not found elsewhere in the tunnels. For this reason, interpretation 2 was preferred and the best estimate of the shear zone is N51E. This was found later to be the correct interpretation. Several methods for treating the fault zone and shear zone in the Penstock Area were considered. They were as follows: 1) Normal tunnel grouting program per the specifications, 2) Supplemental grout programs for areas near the shear/fault zones, 3) Drilling of additional drain holes from the Powerhouse Bypass Tunnel to intercept the shear/fault zones and drilling of local drain holes in the west wall of the Powerhouse Cavern as necessary, 4) Increase reinforcing locally in the concrete lining for better crack control, 5) Increase the length of the concrete plug in the Penstock Bypass Tunnel plus extra grouting around the plug, 6) Increase the length of the Penstock Steel Liner, and 7) Installation of additional piezometers behind the concrete lining for monitoring. It was

59 decided to modify the existing grout program in the areas of the shear/fault zones to insure they would be intersected by grout holes, to install appropriate drains from the Powerhouse Bypass Tunnel, increase the length of the concrete plug with additional grouting, and to locate piezometers based on the geologic conditions. This work was based on geologic information obtained during the excavation of the remaining tunnels in the Penstock area. Structural changes, such as providing additional reinforcing steel in the concrete liners were not considered necessary. In November 1986, enough additional mapping had been performed to evaluate piezometers and drain holes in the Penstock Area. The primary reason for the installation of piezometers is to monitor pressures external to the liners. A secondary reason is to monitor the effectiveness of the grouting and drainage programs in the rock mass upstream of the Powerhouse. The purpose of the drain holes was to intercept any potential and known water-bearing features and to relieve water pressure in the rock mass. The piezometers were located to meet the following criteria: 1) Monitor pressures in the fault and shear zones in the rock around the Penstock Tunnels, 2) Monitor pressures in various rock mass conditions, 3) Monitor and compare pressures on the upstream and downstream sides of the Penstock grout curtain, 4) Monitor pressure in the rock mass just upstream of the Powerhouse Cavern (walls and concrete structures in the cavern are not designed for external water loads), 5) Provide at least two piezometers behind the Penstock steel liner of each unit, 6) Provide at least two piezometers behind the concrete lining of the Manifold Tunnel, and 7) Provide at least two piezometers behind the concrete lining of the Power Tunnel at the downstream end. In considering the piezometer locations, the rock mass condition was divided into three categories: 1) best; normal granitic gneiss, no joints, faults, shear zones, etc., 2) intermediate; normal granitic gneiss, discontinuous and/or continuous joints, and areas near the shear zone, and 3) worst; in the shear zone that it making water. To meet the purpose of intercepting any potential and know water-transmitting geologic features, all geologic data available from the underground mapping program were used to project geologic features throughout the Penstock, Powerhouse, and Draft Tube areas. Using this information and a preliminary drain hole layout as a guide, a new drain hole layout was developed. Geologic data were plotted on working drawings at several different elevations in the Powerhouse Complex and then examined to see if various geologic features mapped in the various tunnels and Powerhouse Cavern could be correlated. The shear and fault zones were traced throughout the entire area as were various biotite schist layers. The correlation of joints and/or joint sets was more difficult, but interpretations of the more continuous features were made. The shear zone and biotite schist layers were projected using the mapped strikes and an average dip calculated from measured dips in the Penstock area. The joints or joint sets were projected using either the mapped strike or an average strike when a large number of joints were measured with an average dip. The interpreted geologic features are shown along a cross-section in the plane of the Powerhouse Bypass Tunnel in Figures 52 and 53. An alternate projection of the shear zone is shown on Figure 52 based on the location of the shear zone in the Draft Tube Tunnel 1 and the borehole data from the Pilot Tunnel studies.

Figure 51: Projection of the shear and fault zones through the Underground Complex.

Using the geologic projections, drain holes were laid out so that each geologic feature would be intercepted by at least two drain holes. For the down holes in the north- south plane, the holes were angled to the south to provide positive interception of the close to vertical joints/joint sets. The depth of the holes was based primarily on the projected position of the shear zone. Figure 54 is photograph of a model of the Power-

61 Figure 52: Projection of geologic features into the plane of the Powerhouse Bypass tunnel and the location of drain holes. See Figure 53 for Explanation.

62 Figure 53: Explanation for Figure 52, Projection of geologic features into the plane of the Powerhouse Bypass tunnel and location of drain holes.

house complex showing the location of the fault and shear zones and the drain holes specifically located to intercept those features. As a final remedial measure to control the flow and pressure of water into the Powerhouse Cavern, a drain system was designed and installed along the shear zone on the walls and floor of the cavern before the placing of structural concrete. The drain system was tied into the originally designed sump systems.

Vertical Access Shaft

Geologic mapping of the Vertical Access Shaft (Figures 55 and 56) was used to locate Miradrain for water control behind the concrete lining in the shaft. The intervals chosen for Miradrain installation were based on the location and physical characteristics of geologic features such as weathered zones, shear zones, faults, and biotite schist layers. The specified intervals contained one or more of the following geologic features: 1) Features that are presently water bearing, 2) Features containing geologic conditions which indicate past water movement, and 3) Features that have potential for future water movement. The entire section shown on Figure 55 had Miradrain installed between the rock and the concrete lining based on the following features: 1) Elevations 472.5 to 458.15 m – wall drainage as required by design drawings, 2) Elevations 458.5 to 452 m – wet weathered zone, 3) Elevations 454 to 451 m – weathered zone in the northeast and northwest quadrants, 4) Elevations 453 to 444 m – shear zone with clay layers and associated weathered zones with flowing water, 5) Elevations 447 to 439 – shear zone with clay layer, and 6) Elevations 439 to 429 m – fault planes with thin clay layers and associated weathered zones.

63 Figure 54: Model of the Powerhouse Complex with the fault zone and shear zone represented by Plexiglas planes with the main intersecting drain holes shown.

Figure 55: Geologic map of a portion of the Vertical Access Shaft. See Figure 56 for Explanation.

Figure 56: Explanation for geologic map of a portion of the Vertical Access Shaft (Figure 55).

Powerhouse Excavation – Rock Wedges and Stress Relief

The Powerhouse excavation began with the development of the crown. A ramp was opened from the Main Access Tunnel elevation in the south end of the Powerhouse northward to the center of the Powerhouse following the previously excavated Pilot

66 Tunnel. The crown was next developed northward using four staggered headings. During the initial center cut along the crown, a fault zone (same zone as discussed in the Penstock area) and associated jointing oriented approximately N60E at an angle to the long axis of the Powerhouse was present (and anticipated) between Stations 8S and 32S (Figure 57). These discontinuities along with the foliation planes and discontinuous joints striking N45W resulted in blocky conditions in the crown. At this time, the decision was made to supplement the design rock bolts (5 x 5 foot pattern bolts, 20 feet long) with additional 20 foot bolts angled at 20o from vertical to intersect the fault/joint planes. These bolts were split-spaced between the design bolts. The installation of these bolts continued in that section of the crown as excavation proceeded to the east and west springlines. While the crown was being developed, the Powerhouse Bypass Tunnel was driven around the south end and parallel to the west side of the Powerhouse making an entrance in the center of the west wall. A ramp was then driven upward along the west wall of the Powerhouse to the north end. This allowed access to the Powerhouse while the south half of the Powerhouse was excavated and the Access Tunnel entrance to the Powerhouse was blocked. After the Powerhouse was benched to the Access Tunnel elevation, the remainder of the Cavern was excavated by ramping down northward from the Access Tunnel along the East Wall. Benches were about 12 feet deep with volumes from 790 to 160 yd3. Muck was carried up the ramp on the east wall and out the Access Tunnel until the Penstock Tunnels were reached. Then the east wall ramp was excavated using the Penstock Tunnels for access and mucking. As the Powerhouse crown was excavated, the design rock bolts were installed and shotcrete was applied. Shotcrete application was delayed in the northern portion of the Powerhouse to allow for distribution and relief of the in-situ stresses and temporary chain link wire was installed between the rock bolts for safety reasons. In this area, there was spalling of the rock into the temporary chain-link wire. Between Stations 26N and 32N, a rock wedge defined by east-west and N60E joints and the foliation moved slightly (~2 inches) after the crown excavation was completed (Figure 58). Due to time restraints, the weight of the block was estimated and the number of additional rock anchors needed to hold the load considering no support to the block from the surrounding rock mass was calculated and three times this number of bolts were installed in the block, with the bolts being 40 foot in length. The location of the additional rock bolts are shown on Figure 58. During excavation of the south half of the Powerhouse crown, the northern half was shotcreted. About midway in the Powerhouse excavation sequence, cracking and some spalling of shotcrete occurred in the center of the crown, primarily in the north half of the Powerhouse. This was a safety concern because the working area was about 75 feet below the crown. The primary cause of the cracking was the high horizontal in-situ stresses which squeezed the center of the crown resulting in shear cracks and some spalling in the shotcrete and rock to depths of about 3 feet. Excavation was suspended in the Powerhouse to allow for scaling and installation of a nylon safety net. A movable access platform close to the crown was suspended from cables strung length-wise in the crown. Additional extensometers were installed in the northern half of the Powerhouse to monitor crown movements. All the work was performed utilizing mobile cranes 75 to 100 feet below the crown.

67 Figure 57: Geologic map of a part of the southern portion of the Powerhouse Crown. Area of blocky rock requiring extra rock bolts between Stations 8S and 32S, Stations in meters. Reflected crown plan.

There was concern about the size of the bench blasts and how they related to the cracks in the crown since nearly all the cracking and spalling occurred in the northern half of the Powerhouse where full-width bench blasts had been made. Subsequently, the Bench blasts were reduced to a maximum size of 100 yd3, blasted to a free face and delayed to limit vibrations. Crown movement slowed and later ceased after the Powerhouse excavation was completed. The maximum downward movement measured was 1.6 inches at one location. Other movements measured from 0 to 1.2 inches at the apex of the crown. The FEM model showed the crown moving up in response to excavation in the in-situ stress field. Some small upward movements were recorded initially by the extensometers, but the rock cannot move up very far before it cracks and shears. Once this happens, the

Figure 58: Geologic map of a part of the northern portion of the Powerhouse Crown. Large rock wedge requiring extra rock bolts (red squares) between Stations 26N and 32N, Stations in meters. See Figure 57 for Explanation. Reflected crown plan.

FEM model breaks down unless cracking and shearing can be accurately incorporated into the model.

69 After the Powerhouse excavation was completed and the bridge crane installed, permanent repairs were made to the crown. A modified drill jumbo was mounted on a specially built platform which traveled on the Powerhouse crane girders in place of the normal trolley. The rig was used to install additional rock bolts and a wire rope net system set tight against the crown. Since this repair no significant cracking or movement of the crown has occurred. The direction of the in-situ stress field was confirmed by the spalling of rock in the vertical corners of the Powerhouse excavation. The Powerhouse is oriented north- south with the principal horizontal stresses measured by hydrofracturing and overcoring oriented northeast-southwest. Spalling occurred in the northwest and southeast corners of the Powerhouse Cavern in response to the squeezing from the stress. This spalling was controlled by shotcrete and rock bolts and ceased after the completion of excavation.

Draft Tube Gate Shaft #1

Spalling of rock in the underground has continued sporadically to the present. In 2002, continued spalling of rock in the Draft Tube Gate Shaft #1 had reached a critical condition in that additional spalling could damage equipment in the shaft. A repair program that included scaling, rock bolting, and the installation of mesh, chain link, and wire rope was developed to stabilize the spalling. Details of the design/repair work are shown in Figure 59.

Figure 59: Detailed drawing for the repair of Draft Tube Gate Shaft No. 1 Walls. .

70 ACKNOWLEGDEMENTS

I thank Duke Energy for continuing to let me take interested professionals to the Bad Creek Project twenty-five years after I was the Project Geologist during construction and plant startup. I, again, thank Allen Nicholson of Duke Energy who has helped make the trips possible over the years (and this year) and makes the necessary site arrangements and provides all the help, and more, that I ask for. A number of professionals, Frank Hagye, Robert Bryant, Linda McAuliffe, Buddy Davis, Christine Thompson, Everett Orr, Ron Birch, Robert Stubblefield, and Anita Holder, worked on the Bad Creek Project Geology Team during the construction and contributed greatly to the geologic work presented in this guide. Portions of this guide were re-written from the September 1991 Bad Creek Pumped Storage Project – Final Report prepared by the Bad Creek Design Engineering Project Team of which I was a member. Ed Luttrell and Dick Steffens wrote the sections on aboveground and underground structures, respectively in that report, either by themselves, with my help, or inserted sections that I wrote specifically for those sections. I would like to thank Scott Brame, the David S. Snipes/Clemson Hydrogeology Symposium organizer for the great job he does putting the field trip together, handling the trip details, and putting up with all my requests, this year for the fourth time in seventeen years. Last and most important, I acknowledge all the Bad Creek field trip participants on trips I have done for various groups over the years including the David S. Snipes/Clemson Hydrogeology Symposium in 2000, 2004, 2007, and this year, the Annual Meeting of the Association of Environment and Engineering Geologists in 1987 and 2010, and several field trips for local geology groups and universities for their interest, excellent questions, and discussions over the years. This is the last time that I will lead a trip to the Bad Creek Project. Again, thanks to all who have attended one of the past trips and I hope you have learned how geology is an important consideration and has to be understood to successfully construct a project of this magnitude.

REFERENCES

Duke Power Company, 1991, Bad Creek Pumped Storage Project - Final Report: Duke Power Design Engineering Department, September 1991.

Fullagar, P. D., Hatcher, R. D., Jr., and Merschat, C. E., 1979, 1200 M.Y.-old gneisses in the Blue Ridge Province of North and South Carolina: Southeastern Geology, v. 20, p.69-77.

Gilbert, N. J., Brown, H. S., and Schaeffer, M. F., 1982, Structure and geologic history of a part of the Charlotte belt, South Carolina Piedmont: Southeastern Geology, v. 23, p. 129-145.

71 Hatcher, R. D., Jr., 1977, Macroscopic polyphase folding illustrated by the Toxaway Dome, eastern Blue Ridge, South Carolina-North Carolina: Geological Society of America Bulletin, v. 88, p. 1678-1688.

Hoek, E. and Bray, J. W., 1981, Rock Slope Engineering: The Institution of Mining and Metallurgy, London, 3rd Edition, 358p.

Schaeffer, M. F., 1987, Geology of the Keowee-Toxaway Complex, northwestern South Carolina: Association of Engineering Geologists, Field Trip Guide No. 1, 30th Annual Meeting, Atlanta, Georgia, p. 15-93.

Schaeffer, M. F. and Steffens, R. E., 1979, Geology of the Bad Creek Pilot Tunnel, northwestern South Carolina: Geologic Notes, South Carolina Division of Geology, v. 23, p. 117-128.

Schaeffer, M. F., Steffens, R. E., and Hatcher, R. D., Jr., 1979, In-situ stress and its relationship to joint formation in the Toxaway Gneiss: Southeastern Geology, v. 20, p. 129-143.

72 FIELD TRIP STOPS FOR THE KEOWEE-TOXAWAY REGION

Scott E. Brame, Environmental Engineering and Earth Sciences, Clemson University, Clemson, SC 29634 and Malcolm F. Schaeffer, HDR Engineering, Inc., Charlotte, NC 28202-2075

The field trip stops will provide the opportunity to observe the lithology and structure of part of the Inner Piedmont, Brevard zone, and eastern Blue Ridge in the Keowee-Toxaway Region of northwestern South Carolina (Figure 1). Stops 1 and 2 are within the Walhalla nappe, Stop 3 in the Jocassee thrust sheet, Stops 4 and 5 in the Brevard zone, and Stop 6 in the Blue Ridge at the Bad Creek Pumped Storage Project. Discussions at the stops will place the geology of the area into a model of the tectonic history and framework of the southern and central Appalachians developed by Hatcher et al. (2007; see discussion in Schaeffer 2016a; this volume) At the Bad Creek Project, both aboveground and underground aspects of the project will be examined and discussed with emphasis on the geologic influences on the location and construction of the major plant structures and the continued operation of the facility (see Schaeffer 2016b, this volume).

Note: Coordinates for the stops are in Universal Trans Mercator 1983.

STOP 1: Walhalla nappe - Table Rock gneiss (330583.9E, 3868704.2N).

Large boulders and outcrops of Table Rock gneiss are found along the north side of Hwy 11 in the Sunset quadrangle (Figure 2). The exposures reveal the leucocratic, mylonitic nature of the gneiss. The light gray to tan colored fine-crystalline biotite quarto-feldspathic gneiss is compositionally layered with coarse pegmatite veins of quartz-feldspar that are oriented parallel to the foliation (Figure 3). The Table Rock gneiss is a metamorphosed suite of intrusive granitoid rocks (Clendenin and Garihan 2007a) and is the dominant unit found within the Walhalla nappe. The gneiss is an orthogneiss that was intruded/emplaced into the country rock and underwent both syn- and post-metamorphic deformation. A member of the Table Rock suite has a 450 Ma (late Ordovician) zircon age (Ranson et al. 1999). It underlies a series of prominent rocky balds including the centerpiece feature of Table Rock State Park.

Figure 1: Simplified geologic map of the upper Keowee-Toxaway Region showing the locations of the field trip stops. Geology from Schaeffer (1987), Garihan (2005), Clendenin and Garihan (2007a).

Figure 2: Boulders of Table Rock gneiss along north side of SC Hwy 11.

Figure 3: Leucocratic Table Rock gneiss with pegmatite veins running parallel to the foliation. Photo courtesy of Jack Garihan.

STOP 2: Walhalla nappe - Chauga River Formation (325045.5E, 3865319.5N).

Outcrops of the Chauga River Formation (Cambrian-Ordovician) are generally found near the west side of the Walhalla nappe and close to the Eastatoee Fault in the Salem quadrangle (Figure 4). The Chauga River Formation at this locality consists of gray, fine- to medium-grained mica button schist interlayered with fine-grained muscovite-biotite gneiss. The gneissic component is sometimes referred to as metasiltstone. The presence of the more gneissic layers indicates that at this location the unit contains a fair amount of silt and fine sand compared to other locations where the unit is more mica-rich and schistose. A main point of this stop is to demonstrate the variability of the formation with regard to composition and degree of metamorphism/deformation for comparison with the rocks of the same formation at Stops 4 and 5.

Figure 4: Stop 2 location and geologic setting along Hwy 11. Note the proximity of the Eastatoee Fault to the north. Hgn = Henderson Gneiss, CRfm = Chauga River Formation, PMa = Poor Mountain Formation, TRg = Table Rock gneiss. The Eastatoee Fault separates the Walhalla nappe and Jocassee thrust sheet. Geologic map from Clendenin and Garihan (2007a).

The Chauga River rocks in this prominent outcrop along SC Highway 11 have a northeast-strike with shallow southeast- to south-dip (toward the highway) foliation. Northwest verging overturned folds are present. The rocks were exposed during construction of the highway and drilling holes where explosives were used for rock excavation are visible (Figure 5). Well-developed C-S structures (see Figure 12 for a diagram showing idealized C-S fabrics) are present within the more schistose layers of the exposure (Figure 6). C-S structures provide shear sense indicators showing the sense of deformation (left- or right-lateral shear) that has affected the rocks and produced the button, or fish scale, texture. At this outcrop, the shear sense is in the westward direction (right-lateral slip) which is consistent with regional southwest directed shearing in the rocks of the region (see Schaeffer 2016a; this volume).

Figure 5: Outcrop of Chauga River Formation along north side of Hwy 11 showing drill holes from road construction.

Figure 6: C-S structure developed in button schist at Stop 2. This is the backside of a broken piece that was removed from the outcrop face and thus the view is to the south. The 8 mm long fish structure shown (yellow oval) was sheared to the right which is westward. Scale is in mm.

STOP 3: Jocassee thrust sheet - Henderson Gneiss (318976.2, 3866020.5N). [Note: This is private property. Please get permission from owner before entering.]

The Henderson Gneiss is exposed as discrete outcrops and boulders in Ricky Campbell’s yard along Cross Creek Road in the Salem quadrangle (Figure 7). The foliation is dipping 50o SE and the strike is N50oE. It is coarse-grained biotite augen gneiss characterized by large microcline crystals (augen) up to 5.0 cm in a fine-grained biotite quartzo-feldspathic matrix (Lemmon 1981; Clendenin and Garihan 2007a). A close inspection of the rock reveals conspicuous pink, porphyroclastic microcline with white myrmekitic rims. The Henderson Gneiss is variably mylonitic with shearing expressed as thinly layered, schistose, fine-grain biotite-muscovite-quartz-feldspar gneiss with locally interlayered muscovite schist (Hatcher and Butler 1979; Clendenin and Garihan 2007a). The high-temperature mylonitic overprint is intensely developed at this location, southwest of the Keowee River, with the microcline augen/megacrysts flattened into quarter sized features in the plane of foliation producing a northeast-southwest trending lineation (Hatcher 2002) with corresponding grain size reduction in the matrix associated with the ductile shearing. This fabric is overprinted by later deformation and retrograde metamorphism (Hatcher 2002). The Henderson Gneiss is the dominant unit within the Jocassee thrust sheet. Like the Table Rock gneiss, it is an orthogneiss with granodiorite to granite composition. After intrusion, it was metamorphosed and deformed to a gneiss and likely thrust into its present position. The

Henderson Gneiss has a zircon age of 470 Ma (early middle Ordovician; Stahr et al. 2005, in Hatcher et al. 2007).

Figure 7: Outcrop of Henderson Gneiss along Cross Creek Road exhibiting mylonitic structure with flattened augens and a distinctive northeast-southeast lineation. The end of the hammer handle (blue) is pointing north. The quartz filled vein parallels the foliation.

STOP 4: Brevard zone - Chauga River Formation (315612.9E, 3869303.9N).

This stop is Location 1 shown on the geologic map shown below (Figure 8; modified from Schaeffer 1987; Figure 9 is a field map of approximately the same area provided by the South Carolina Department of Natural Resources, S.C. Geological Survey with a recent interpretation of the geology along SC Highway 130). Approximately 1200 meters back south down SC 130 from this location is the South Boundary fault of the Brevard zone where mylonitic Henderson Gneiss (HG) is thrust over Chauga River (CR) rocks (Figure 9). We are within the Brevard imbricate stack of Clendenin and Garihan (2008). The CR rock at this location consists primarily of muscovite-chlorite phyllonite (Figures 10 and 11) with interlayered metasiltstone (fine-grained biotite gneiss). Generally only one foliation is discernible in hand specimens of the metasiltstone while two foliations are present in the phyllonite resulting in a “button” or “fish- scale” texture. Right-lateral S-C and S-C’ shear bands (Figure 12) are present in the phyllonite and are related to right-lateral strike-slip movement in the Brevard zone (Bobyarchick 1984; Bobyarchick et al. 1988; Clendenin and Garihan 2008). South on SC Highway 130 at Tommy’s

Knob, Clendenin and Garihan (2007) described an earlier left-lateral shear band fabric including S-C’ bands, buttons, and intrafolial folds that are overprinted by later right-lateral S-C’ shear bands that become more pervasive in the CR phyllonite as the Brevard zone is approached.

Rosman Fault

Southern Boundary Fault

Northern Boundary Fault Brevard Imbricate Sheet

STOP 5 Brevard Fold Belt STOP 4

Figure 8: Geologic map of the area around Stops 4 and 5. Contacts – Black dashed lines – contacts; Red dashed lines with open barbs – ductile thrust (Eastatoee fault); barbs on upper sheet; Purple dashed lines with solid barbs – brittle thrust, barbs on upper sheet; hg – Henderson Gneiss, hm – mylonitic Henderson Gneiss, cr – Chauga River Formation, cr- hm – imbricate sheets of CR-mylonitic HG; tu – Tallulah Falls Formation – greywacke schist member; tp; Tallulah Falls Formation - garnet-aluminous schist member; tl – Tallulah Falls Formation – greywacke-schist amphibolite member; tg – Toxaway Gneiss. Geology by Kenwill (1982), Hatcher (unpublished data), and Schaeffer (unpublished data) and modified by Schaeffer (2016; unpublished data).

STOP 5 STOP 4

Figure 9: Geologic sketch map of the area around Stops 4 and 5. HM – mylonitic Henderson Gneiss (light blue) ;CR – Chauga River Formation (purple); TF – Tallulah Falls Formation (orange); Dashed barbed line – thrust fault with barbs on upper sheet, dashed lines – high angle faults with arrows showing sense of strike-slip movement. Map courtesy of the South Carolina Department of Natural Resources, S. C. Geological Survey (2016).

Figure 10: Chauga River Formation muscovite-chlorite phyllonite. Location 1 on Figure 8.

Figure 11: Chauga River Formation muscovite-chlorite phyllonite showing “button” or “fish-scale” texture. Location 1 on Figure 8.

Figure 12: Idealized C-S fabrics: Top - right-lateral C- and C’-type shear bands, Bottom – left-lateral C- and C’-type shear bands (from Clendenin and Garihan 2007).

STOP 5: Brevard zone - Brevard imbricate stack in the Brevard zone (315643.2E, 3869716N).

About 300 meters north of Location 1 at Location 2 on Figure 8, a thrust fault with CR rocks thrust over a thin mylonitic, fine-grained HG is present. For the next 60 meters, the exposed rocks are an imbricate thrust stack of CR phyllonite and mylonitic HG with several thrust planes present (Figure 13 and 14; cr—hm unit on Figure 8). Brittle deformation (damage zones) has occurred in layers less than 15 cm up to 1 m thick where HG is thrust over CR rocks. The Eastatoee fault (R rocks thrust over mylonitic HG) is repeated several times by the later brittle thrusting. The CR phyllite/phyllonite has been highly sheared as indicated by flattened “buttons”. At Location 3 on Figure 8, the imbricate stack of CR rocks - mylonitic HG is thrust over approximately 200 m of predominantly mylonitic Henderson Gneiss (Figure 8; Figure 9 shows a different/alternate interpretation). Further north, several more thrusts involving the phyllonite and mylonitic Henderson Gneiss are present. See Schaeffer (2016; this volume) for more discussion of the Brevard zone.

Figure 14: Thrust faults in the Brevard imbricate stack (green dashed line - ductile faulting; yellow dashed line – brittle faulting; arrows indicatw direction of movement). CR – Chauga River phyllite/phyllonite; HG – mylonitic Henderson Gneiss. Location 2 on Figure 8 and photograph location shown on Figure 13.

STOP 6: Blue Ridge - Bad Creek Pumped Storage Project (315494.9E, 3877437.2N).

See Schaeffer (2016b; this volume) for discussion of the geology and engineering geology of the Bad Creek Project. Both aboveground and underground aspects of the project will be examined and discussed with emphasis on the geologic influences on the location and construction of the major plant structures and the continued operation of the facility

REFERENCES

Bobyarchick, A. R. 1984. A late Paleozoic component of strike-slip in the Brevard zone, southern Appalachians (abs.): Geological Society of America, Abstracts with Program, v. 16, p. 126. Bobyarchick A. R., S. H. Edelman, and J. W. Horton, Jr. 1988. The role of dextral strike-slip in the displacement history of the Brevard zone, in, Secor, D. T., Jr., ed., Southeastern Geological Excursions, Geological Society of America Southeastern Section, Columbia, South Carolina, 4-10 April 1988, Columbia, South Carolina, Geological Survey, p. 53- 154. Clendenin, C. W., Jr. and J. M. Garihan. 2007a. Geologic map of the Salem and Reid Quadrangles, Oconee and Pickens Counties, SC: South Carolina Department of Natural Resources, S. C. Geological Survey, Map Series MS-28, Scale 1:24,000. Clendenin, C. W., Jr. and J. M. Garihan. 2007b. Polyphase deformation in the basal Chauga River Formation, northwestern South Carolina: South Carolina Geology, v. 45, p. 9-16. Clendenin, C. W., Jr. and J. M. Garihan. 2008. The role of oblique transpressive strain partitioning in the development of the Brevard zone, northwestern South Carolina: South Carolina Geology, v. 46, pp. 30-42.

Garihan, J. M. 2005. Geologic map of the Sunset quadrangle, Pickens County, South Carolina: South Carolina Department of Natural Resources, S. C. Geological Survey, Geologic Quadrangle Map, GQM-28, Scale 1:24,000. Hatcher, R. D., Jr. 2002, An Inner Piedmont primer, in Hatcher, R.D., Jr., and B. R. Bream, eds., Inner Piedmont geology in the South Mountains-Blue Ridge Foothills and the southwestern Brushy Mountains, central-western North Carolina: North Carolina Geological Survey, Carolina Geological Society Annual Field Trip Guidebook, p. 1-18. Hatcher, R. D., Jr., Bream, B. R., and Merschat, A. J., 2007, Tectonic map of the southern and central Appalachians: A tale of three orogens and a complete Wilson cycle, in, Hatcher, R. D., Jr., Carlson, M. P., McBride, J. H., and J. R. Martinez Catalan, eds., 4-D Framework of Continental Crust: Geological Society of America Memoir 200, p. 595- 632. Hatcher, R. D., Jr. and J. R. Butler. 1979. Guidebook for southern Appalachian field trip in the Carolinas, Tennessee, and northeastern Georgia: International Geological Correlation Program, The Caledonides in the U.S.A., 117p. Kenwill, Inc. 1982. Jocassee-Keowee area, North and South Carolina: Report for Crescent Land and Timber Corporation, 244p. Lemmon, R. E. 1981. An igneous origin of the Henderson Augen Gneiss, western North Carolina: evidence from zircon morphology: Southeastern Geology, v. 22, p. 79-90. Ranson, W. A., I. S. Williams, and J. M. Garihan. 1999. Shrimp zircon U-Pb ages of granitoids from the Inner Piedmont of South Carolina: Geological Society of America, Abstracts with Program, v. 31, no. 7, p. A-167. Schaeffer, M. F. 1987. Geology of the Keowee-Toxaway Complex, northwestern South Carolina: Association of Engineering Geologists, Field Trip Guide No. 1, 30th Annual Meeting, Atlanta, Georgia, p. 15-93. Schaeffer, M. F. 2016a. Geology and tectonic framework of the Keowee-Toxaway Region, northwestern South Carolina: Field Trip Guidebook for the 24th David S. Snipes/Clemson Hydrogeology Symposium, March 30, April 1, and April 28, 2016, 27 p. (this volume). Schaeffer, M. F. 2016b. Engineering geology of the Bad Creek Pumped Storage Project, northwestern South Carolina: Field Trip Guidebook for the 24th David S. Snipes/Clemson Hydrogeology Symposium, March 30, April 1, and April 28, 2016, 72p. (this volume). Stahr, D. W., C. F. Miller, R. D. Hatcher, Jr., J. Wooden, and C. M. Fisher. 2005. Evidence for high-temperature ductile Acadian deformation in the eastern Blue Ridge: Implications of new structural, petrologic, and geochronological data from southwestern North Carolina: Geological Society of America Abstracts with Program, v. 37, no. 7, p.72.