8.4.0 WOLFORD MOUNTAIN SITE C 8.4.1 LOCATION AND TOPOGRAPHY Wolford Mountain Site C is located on Muddy Creek, 5 miles north of Kremmling, Colorado and 3.8 miles upstream from Site A'. The dam site is a 250-foot wide canyon with sides that rise steeply to a height of approximately 80 feet. At that point, the left abutment is relatively flat for about 500 feet, continuing east to the base of Wolford Mountain. The right abutment slopes gently upwards towards Highway U.S. 40, located 0.7 miles west of the site. Figure 8.4.1 presents a plan view of the Wolford Mountain Site C. A 120-foot high dam at Site C would impound a reservoir of 60,000 af capacity with a water surface elevation of 7485 feet. This is 10 feet higher than the water surface elevation proposed for an alternative reservoir at Site A'. Approximately 1900 acres would be occupied by the reservoir and lake shore area. An area-capacity curve is presented on Figure 8.4.2. 8.4.2 PRIOR STUDIES Wolford Mountain Site C has been under investigation by the CRWCD. A report titled "Rock Creek Dam Project", prepared by Morrison-Knudsen Engineers, Inc. (1986) for the River District, considered three reservoir capacities for this Muddy Creek Site: 60,000, 46,800 and 30,400 af. The feasibility report illustrates a preliminary design for the selected 46,800 af size. Information from this report has been liberally incorporated into this section on Wolford Mountain Site C. The report titled "Seismotectonic Hazard Evaluation, Rock Creek Project Near Kremmling, Grand And Routt Counties, Colorado", was prepared by Michael West and Associates (1986) for Morrison-Knudsen Engineers as part of their study for the CRWCD. A comparative review of capacities and costs of a dam at Site C and the proposed Rock Creek Dam, located 13 miles to the west, was conducted for the Municipal Subdistrict of the Northern Colorado Water Conservancy District (Swaisgood, 1983). 8-4-1 The "Wolford Mountain Reservoir Project Feasibility Report", prepared by Western (1983), provided reconnaissance-level geological, hydrological and construction cost information about the original Site C and three other sites downstream. Because of an old landslide on the left abutment of the original site, the "C" site was moved 1500 feet upstream to the present location. B.4.3 FIELD AND LABORATORY INVESTIGATION As part of the study of Wolford Mountain Site C by Morrison-Knudsen Engineers (1986), field and laboratory investigations were performed. These included a geophysical survey in the form of seismic refraction lines along the proposed centerline of the dam, across the valley downstream of the dam and on the left abutment along a possible spillway alignment. Three core borings were drilled along the proposed dam axis, with one on each abutment and one in the floor of the valley. Core samples were recovered and in-place permeability measurements taken of the foundation bedrock. Two test pits were excavated downstream, and two upstream, in potential embankment material borrow areas. A 5-foot interval contour map was developed from aerial photographs. Laboratory tests were conducted on samples of the material obtained from the exploratory program. Standard properties and strength characteristics were evaluated. Sources of construction materials were identified. Roller compacted concrete trial mixes using aggregates from the site were evaluated. Exploratory boring locations, logs and test results are included in the report, Rock Creek Dam Project (Morrison-Knudsen Engineers, 1986). During this Joint-Use Reservoir and Green Mountain Exchange Study, Chen & Associates reviewed and reported upon geological information from prior studies and other published sources. This was compared with surficial observations during two site visits. Conclusions are included in the geological and geotechnical portions of this study. B-4-2 B.4.4 REGIONAL GEOLOGY Wolford Mountain Site C is located near the western edge of the Middle Park Basin, a structural sag that formed as the Park Range was uplifted to the west and the Front Range was uplifted to the east. These major geologic structures, along with several north-trending thrust faults, were formed during a period of mountain building, known as the Laramide Orogeny, that occurred in the late Cretaceous to Eocene geologic epochs about 40 to 70 million years ago. A time scale and list of geologic formations in the Kremmling area are presented on Table B.4.1. After the Laramide Orogeny, the Middle Park Formation, which is comprised largely of sandstone, conglomerate and shale, was deposited as basin fill derived from materials eroded from the adjacent uplifted formations. Igneous rock in the form of volcanic flows and breccias were deposited over the Middle Park Formation. Following this, the Troublesome Formation, which consists primarily of tufaceous siltstone and claystone, was deposited. This constitutes the youngest rock in the basin. The youngest geologic deposits in the basin, however, are alluvium, colluvium and glacial deposits. Under the Middle Park Formation are older, pre-Laramide sedimentary rocks. Underlying these sedimentary rocks are Precambrian crystalline rocks, over 600 million years old. Within the basin, the Precambrian rocks are exposed in places where they have been brought up by overthrusting along the Laramide thrust faults. This is displayed on Wolford Mountain, east of the site, where the Williams Range Thrust Fault has brought older, dark Precambrian granite over the younger, light tan shale. B.4.5 SITE GEOLOGY The Site C Dam would be constructed across a 250 foot wide valley, with nearly vertical shale cliffs. Valley alluvium covers bedrock at the present location of Muddy Creek. Terrace alluvium overlies bedrock above the valley floor. Lower units of the Pierre Shale bedrock are exposed in the cliffs on the valley sides, but in most places the valley sides are overlain by colluvial deposits derived from upslope rock as a result of slope wash processes. 8-4-3 TABLE B.4.1 Stratigraphic Column of the Kremmling Area (showing years before present) ERA PERIOD EPOCH FORMATION C Alluvium E Holocene N 0 Quaternary Landslide Deposits Z (10,000 years) 0 Pleistocene Terrace Deposits I {1 miJlion years} Pediment C Pliocene Miocene Troublesome Tertiary Oligocene Rabbit Ears Eocene Paleocene Middle Park {65 million years} M Upper Pierre Shale E Cretaceous Niobrara S Benton Shale 0 Lower Dakota Sandstone Z 0 Jurassic Morrison I C Triassic (245 miJlion years} P Permian A Pennsylvanian L Mississippian E Devonian 0 Silurian Z Ordovician 0 Cambrian I C (570 miJlion years} .p R E C A M B R I A N B-4-4 8.4.5.1 Rock Units and Structures The bedrock is a hard, gray mudstone or shale. Surface outcrops are weathered to a friable, flaky, slivery material with separations along bedding planes. Since the rock has been gently folded, the strike and dip of the bedding planes vary from 2 0 to 150 within 1000 feet of the dam axis. The predominant strike near the axis of the dam is N.450 W. and the corresponding dip is 50 to the northeast. This is in the upstream direction. Three sets of principal joints were identified in the bedrock. One set is parallel to bedding and is probably open and more pronounced at the surface due to stress relief and weathering. Occasionally these joints are oxidized for a width of about one half inch and can be followed for some distance. Spacing of these weathered joints is irregular but generally greater than 10 feet. A second principal joint set is oriented approximately vertically and strikes to the northwest. At the dam axis, the strike is nearly perpendicular to the cliff. Frequency of these joints is about 10 feet at the closest spacing, but they are generally more widely spaced. The third principal joint set is associated with the stress relief that has formed the vertical cliffs. These joints have vertical orientation and are parallel to the canyon. The frequency of these joints cannot be determined by surface mapping since they occur parallel to the outcrops. Other randomly oriented, discontinuous joints can also be observed (Morrison-Knudsen Engineers, 1986). 8.4.5.2 Surficial Deposits The cliffs bordering the valley bottom are capped by alluvial terrace deposits that form pediments, approximately 30 feet thick, which slope gently towards the abutments. The terrace deposits consist of well rounded to subrounded cobbles of sandstone, basalt and crystalline rock covered with two to three feet of weathered, light brown clay and silt. The terraces pinch out abruptly toward the abutments. The left abutment bedrock is partially overlain by landslide debris originating from high above on Wolford Mountain. The slides appear to be associated with slabbing and toppling of the outcrops of crystalline rock above the Williams Range Thrust Fault (Morrison-Knudsen Engineers, 1986). 8-4-5 B.4.6 DESIGN CONSIDERATIONS This section addresses site factors which affect dam design. Included are foundation conditions, earthquake design, spillway requirements and construction materials. Preliminary evaluation indicated that impermeable earth materials suitable for the core of an embankment dam are available. The topography is suitable for an abutment spillway location. These are factors favorable to selection of an earth embankment design. A roller compacted concrete (RCC) design was also considered, however, laboratory tests of on-site aggregates indicated marginal durability (Morrison-Knudsen Engineers, 1986). B.4.6.1 Foundation Conditions Selection of dam type and design is affected by depth and properties of the surface material, strength of bedrock and permeability of the foundation as well as the availability of construction materials. The foundation conditions described below are based on the interpretation by Morrison-Knudsen Engineers (1986) of the subsurface investigations, outlined .in Section B.4.3 above, performed as part of that study.