Engineering aspects of the St. Peter sandstone in the Minneapolis-St. Paul area of Minnesota

Item Type text; Thesis-Reproduction (electronic)

Authors Payne, Charles Marshall, 1937-

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/551926 ENGINEERING ASPECTS OF THE ST. PETER SANDSTONE IN THE MINNEAPOLIS - ST. PAUL AREA OF MINNESOTA

by Charles Marshall Payne

A Thesis. Submitted to the Faculty of. the DEPARTMENT OF GEOLOGY In Partial Fulfillment of the Requirements. For the Degree of - —• j ' MASTER OF SCIENCE

In the Graduate College THE UNIVERSITY OF ARIZONA

1 9 6 ? The St. Peter sandstone at the Minnesota Silica Company quarry, Minneapolis. STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfill­ ment of requirements for an advanced degree at The Univer­ sity of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Li brary.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknow­ ledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department of the Dean of the Graduate College when in his judgment the proposed use of the material is in the inter­ est of scholarship. In all other instances, however, per­ mission must be obtained from the author.

SIGNED:

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below

Date Professor of Mining & Geological Engineering ACKNOWLEDGMENTS

The writer wishes to acknowledge with gratitude the able assistance given by Dr. George M0 Schwartz in the col­ lection of studies and literature needed to compile this p&- per. The writer wishes also to express his indebtedness to

Dr. Paul K. Sims, director of the Minnesota Geological Sur­ vey, for allowing the time and facilities to conduct this

study and fo r the preparation and use of the Bedrock Geologic

Map of Minneapolis, St. Paul and Vicinity. Dr. John E. Stone, of- the Minnesota Geological Survey, was' very helpful in supplying general information regarding the engineering

properties of the St. Peter sandstone. Recognition is given to' Dr. Willard C. Lacy, professor at the University of Ari­

zona, for his patience and assistance in organization and preparation of this thesis.

i l l TABLE OF CONTENTS

P a g e

LIST OF ILLUSTRATIONS ...... vl

LIST OF TABLES ...... v i i i

J—_L 1^ ...... ^ . . . e o o . ". o o o o . . 2-5C

INTRODUCTION 1. Description of the Thesis Area ...... 2 General Geology ...... 4 PHYSICAL CHARACTERISTICS OF THE ST. PETER SANDSTONE . . 9

General Description ...... 9 F r ia b ility and Cementation ...... 13 Mineralogy and Lithology ...... 16 Textural and Grain Size Analyses ...... 19 Roundness5 F rosting, and P ittin g ...... 25 Silt Horizon ...... 26 Green Sand Horizon . ^ . 27 Shakopee Dolomite - St. Peter Sandstone Contact . . 28 Glenwood Shale - St. Peter Sandstone Contact . . . 29 Jointing and. Fracturing ...... 29 Sandstone Caves ...... 36 Origin of the St. Peter Sandstone 37 HYDROLOGIC ASPECTS OF THE ST. PETER. SANDSTONE ..... 4l

GENERAL GROUNDWATER CONDITIONS IN THE ST. PETER SANDSTONE ...... 44 ENGINEERING ASPECTS OF THE ST. PETER SANDSTONE .... 53

Previous Engineering Studies ...... 53 Sampling Techniques ...... 54- Unit Weight ...... 56 Unconfined Compression Tests ...... 56 Triaxial Compression Tests ...... 62 Plate Bearing Tests ...... 70 Consolidation and Settlement ...... 73 Penetration Resistance Tests ...... 75 Bearing Capacities ...... 78 FOUNDATION USE OF THE ST. PETER SANDSTONE ...... 80

i v V

TABLE OF CONTENTS--Continued Page

Potential Problems 80 Bridges 83 Locks and Dams „ „ 85 Buildings „ „ „ „ , 87 E O a d S o o o o e o o o \ 91 TUNNELING IN THE ST. PETER SANDSTONE 92 Problems in Tunneling . . . . „ ...... < 92 Tunnel Description ...... 95 Arching and Stress Distribution ...... 96 Tunnel Supports ...... 99 Mining Methods ...... 104 Case Histories of Tunneling in the St. Peter 1C ^3 S o 1C ...... 108 Eastman Tunnel ...... 108 Wabasha S tre e t In tercep to r Sewer . . . . , 109 Roblyn Avenue Interceptor Sewer, St. Paul , 110 Dayton's Bluff Interceptor Sewer, St. Paul 111 Rondo Street Tunnel Cave-in ...... ill Stevens Avenue Sewer Tunnel Cave-in, Minneapolis ...... 113 CONCLUSIONS AND RECOMMENDATIONS ...... 119

LIST OF REFERENCES ...... 124 LIST OF ILLUSTRATIONS

Figure Page

1. Index map of Minnesota showing location of thesis area <,„<,.<, «».<>„ 6 „ = <,«>.<, „ 3 2 „ Rock formations of the Minneapolis-St.Paul

3. Photograph of a fresh exposure of the St-. ' Peter sandstone ...... 10

4. Photograph of a fresh exposure of the St. Peter sandstone 10 5. Photograph of the river bluff at Holiday Harbor in St, Paul ...... 11 6 . Photograph of a mine d r i f t a t Holiday Harbor in St. Paul ...... 12

7. Photomicrograph showing grain relationships in the St. Peter Sandstone ...... 15 8 . Grain size analysis, vertical section ...... 21

9. Grain size analysis, horizontal section ..... 23 10. Histogram showing the bimodal distribution of grain sizes ...... 24

1 1 . Rose diagram showing joint directions in the St. Peter sandstone ...... 31 12. Photograph of the river bluff below the Soldier’s Home in Minneapolis (1932) ...... 7 . 34

13. Photograph of the river bluff below the Soldier's Home in Minneapolis (1964) ...... 35 14. Sketch of the natural sandstone cave, Minneapolis ...... 38 15. Moisture - density relationships ...... 57

1 6 . Stress-strain curves for unconfined compression

te s ts . 0 . a . . . e .0 o o o o 0 o 00000 59

v i v i i

LIST OF ILLUSTRATIONS--Continued

Figure Page 17. Undisturbed triaxial compressions tests 64

18. Triaxial compression tests--disturbed O OOOOO 69 19o .Triaxial compression tests5 disturbed sand - S tone OOO 0 90 0 00 OO OOC OO o'o 71 20. Field bearing tests 72

21 o Consolidation tests 0.=., ,,o . 76 22. Photograph of an excavation in the St. Peter sandstone at the Dayton’s Department Store in St. Paul o,. ..L 90 Photograph of placing lagging in a sandstone

t unne1 ...... a.... 101 24. .Photograph of an unsupported tunnel driven in competent sandstone ...... 102

25. Photograph of full ring beam supports in a sandstone tunnel ...... 102

2 6 . Photograph of hydraulicly mining the St. Peter sandstone ...... 105

27. Sketch and photograph of the Rondo Street Tunnel Cave-in ...... 114

28. Geologic section, Stevens Avenue Sewer Tunnel, Minneapolis ...... 117 1 . 29. Photograph of the Stevens Avenue cave-in .... 118 30. Bedrock geologic map of Minneapolis- St. Paul and v ic in ity ...... Map Pocket

31. North-South cross section of the Minneapolis-St. Paul area ...... Map Pocket 32. East-West cross section of the Minneapolis-St. Paul area ...... Map Pocket LIST OF TABLES

T a b le P a g e

1 . Textural characteristics of the St. Peter sandstone ...... 25

2. Coefficient of Transmissibility ...... 4-2 3. Test results - unconfined compression tests . . . 60 4. Triaxial compression tests conditions, undisturbed St. Peter sandstone ...... 66

v i i i ABSTRACT

In the Twin City area a variety of heavy structures and tunnels have been built on or in the St„ Peter sand­ stone. The rock in situ, though friable, is competent and capable of withstanding high bearing loads. Because the soft sandstone is easily excavated and supported,.tunnels can be constructed- at below normal tunneling costs.

Buried river channels, groundwater problems, and the unique physical properties of the sandstone have caused numerous engineering perplexities. The sandstone normally lacks cohesion, has a low shear stren g th , but has a high angle of internal friction. If the rock is disturbed or exposed to running water, the material loses its strength, and structural failure results.

The engineering properties of the sandstone have not been thoroughly defined. The formation is unusual be­ cause it is neither a soil nor rock. The elastic properties of the sandstone categorize it as a rock; cohesionless characteristics are more typical of soils.

The objectives of this paper have been to compile all available engineering data in an attempt to better un­ derstand the engineering properties of the St. Peter sand­ stone.

i x X

Several case histories are reviewed to demonstrate the necessity of adequate exploration and testing of the sandstone. In addition 5 a subsurface geologic map of the

Twin City area was made to show the distribution of the various rock types present. INTRODUCTION

For over one hundred years the St. Peter sandstone5

In the Twin City area, has been utilized for foundation purposes and for the excavation of utility and sewer tun­ nels . As the cities expand and grow skyward, an ever in­ creasing use of the sandstone will become evident. As succeeding projects become larger and more extensive, the number and complexity of engineering problems w ill increase. City governments, among other organizations, are concerned with construction costs, and for practical pur­ poses they must understand the engineering and geologic parameters of these projects.

A considerable amount of research has* been conduct­ ed on the engineering properties of the St. Peter sandstone by foundation engineers, usually with the aid of a consult­ ing geologist. To date, testing techniques and experience have not thoroughly defined the geologic and engineering characteristics of the sandstone, but it is vital that re­ search be continued.

This discussion is presented with the hope of clar­ ifying engineering aspects of the St. Peter sandstone as related to tunnel construction and foundation conditions.

I t is also hoped th is study w ill demonstrate how modern en­ gineering geology and rock or soil mechanics techniques may

1 $

2 be used to obtain more precise design data and to control construction costs. The area studied and mapped covers approximately 265 square miles, which includes the cities of Minneapolis and St. Paul with add itio n al portions of Ramsey, Dakota, Hennepin, and Anoka Counties. The boundaries of the mapped area are those of the U. S . Geological Survey Map, Minneapo lis - St. Paul and Vicinity, 1952. The subsurface geology of the study area was re­ mapped and brought up to date. Data pertaining to the en­ gineering aspects of the sandstone were collected only w ithin the mapped boundaries. However, due to the co n sis­ tent physical distribution, similar engineering properties of the formation can be expected in other localities.

Description of the Thesis Area

Glacial features form much of the Twin City land­ scape. Except for the valleys of the Mississippi and Min­ nesota Rivers, ground moraines mask the bedrock. The land surface is gently undulating with many aligned elongate de­ pressions in the drift. Lakes, swamps, and stream courses now occupy these drift filled depressions which suggest the location of preglacial and interglacial river channels.

The Mississippi River separates the Cities of Min­ neapolis and St. Paul. As the River flows south from St. 3

ST. PAUL

MINNEAPOLIS

INDEX MAP OF MINNESOTA

SHOWING LOCATION OF THESIS AREA

FIGURE I Anthony Fallsa a youthful and narrow gorge has been formed, characterized by high vertical bluffs of exposed bedrock. At Fort Snelling, where the Minnesota River converges with the Mississippi River, the river valley broadens as does its flood plains. The river bluffs still persist intermittently on each side of the valley for many miles south of the con­ fluence.

Except for the Minnesota and Mississippi Rivers, stream degradation, since the last glacial retreat, has not been rapid. Reasonably good percolation, heavy vegetation, and numerous local pondages have minimized flu v ia l erosion. Climatic conditions in the Metropolitan area are typical of the Northern Middle Western States. Long win­ te rs , short summers, and extremes in temperature are common.

Normal average temperatures range from 1 5 °F in January to

7^°F in July. The normal annual rainfall is 25 inches and relative humidity 68 p e rc en t.

General Geology

Minneapolis - St. Paul and the surrounding area are situated in the center of a flat structural basin encircled by gently dipping sediments. The basin is elongate south­ west-northeast. with the center located south of the Univer­ sity of Minnesota in Minneapolis. Dips of the sediments vary but average less than one degree or twenty feet per m ile . 5 Minor structures in the Twin City area have been observed at several locations. The Glenwood shale - St.

Peter sandstone contact may.vary as much as three feet with­ in the area of a block. This phenomenon is probably due to differential settlement or minor faulting. A more pro­ nounced structure was noted in the Roseville area. The con­ tact of the Platteville limestone - St. Peter sandstone, paralleling New Brighton Boulevard, varied over 100 feet in elevation within a few hundred yards on either side of the boulevard. More well data are needed to determine whether this structure is a fault or a monoclinal flexure. Recent aeromagnetic surveys of the basin, conducted by the U„ S. Geological Survey, suggest the thickness of sediments to be in excess of 4,000 feet.. The aeromagnetic data also indicate a possible five mile thick section of

Keweenawan basalt flows beneath the sediments (Sims, 1962).

The stratigraphy in the Twin City area has been reasonably well determined as a result of recent studies of well cuttings and over 5,000 drillers' logs. The Bedrock Geologic Map of Minneapolis, St. Paul and Vicinity, Figure

3 0 , was compiled from an analysis of these subsurface data.

The map shows the topography of the bedrock surface and'the distribution of the major bedrock units. North and east trending s tru c tu ra l sections are shown on Figures 31 and 32 respectively. 6

The ages of sediments range from Precambrian to mid­ dle Ordovician. The formations are easily distinguishable and readily correlatable between wells. Also, thickness of the essentially continuous formations beneath Minneapolis and St. Paul changes little. A complete description of the lithology of each stratigraphic unit is unnecessary for this investigation. However, a revised stratigraphic column has been made based on recent well cutting studies, and a brief description of the formations are tabulated (Figure 2).

Four formations are recognized on the surface in the

Twin City area. All rock outcrops are in the form of River bluffs along the Mississippi and Minnesota Rivers. The

Decorah shale, Platteville limestone, Glenwood shale, and

St, Peter sandstone crop out together discontinuously along the river bluffs throughout the metropolitan area. The bedrock surface is mantled by 10 to 350 feet of unconsolidated glacial materials. Glacial and interglacial rivers shaped this surface to resemble a. canyon and plateau type of environment. A complex system of deep, narrow can­ yons, resembling the present day narrow gorge of the Missis­ sippi River below St. Anthony Fails, was cut into the under­ lying sediments. Figure 30 shows with reasonable accuracy the configuration, size, and extent of these channels.

During the summer of 1964 a Warden Geodetic Gravity Meter was usedtto help locate buried channels in the north­ ern half of the New Brighton Quadrangle. The low density. PERIOD FORMATION LITHOLOGY THICKNESS

Pleistocene Undifferentiated drift and glacial 50-350 • outwash. More recent deposits: (fe e t) loess, talus, river sands and gra­ vels , and lake deposits.

Ordovician Galena limestone and Argillaceous green shale with in- 0-60 Decorah terbedded fossiliferous limestone.

Platteville limestone 50 Caramona member Medium bedded, fine grained lime­ stone with interbedded shale. Oju imby * s M il l me mb e r Fine grained, conchoidal fractured lim estone. McGregor member Very fine grained limestone with a minor clay fraction.

Glenwood shale Shale grading into sandy shale 6 downward.

St. Peter sandstone Medium to fine grained clean 150 quartzose sandstone.

P ra irie du Ohien Shakopee dolomite Fine to medium grained dolomite. 45 New Richmond ss Fine to medium grained sandstone. 11 Oneota dolomite Fine to medium grained dolomite, 80 compact and vuggy. « Cambrian Jordan sandstone 90 Van Oser member Coarse grained quartzose sand­ stone. Norwalk member Fine grained quartzose sand­ stone.

St. Lawrence formation 50 Lodi member Fine grained dolomitic sandstone and siltstone.

- Black Earth member Sandy and glauconitic dolomite with interbedded dolomitic silt- stone beds.

Franconia formation 200 Mazomanie member Facies - eastward only. Fine to medium grained, thin bedded sand­ stone. Some cross bedding. Reno member Glauconitic fine sandstone. Tomah member Very fine sandstone and inter­ bedded shale. Birkmose member Glauconitic fine sandstone. Woodhill member Cross bedded medium to coarse grained snadstone.

Dresbach formation 400 Galesville member Massive to cross bedded fine to medium grained sandstone. Eau C lair member Thin bedded, very fine sandstone, siltstone, and shale. Mt. Simon member Medium to coarse white quartzose sandstone.

Precambrian Hinckley sandstone Medium grained, salmon colored 40 (Keweenawan) quartzose sandstone. Fond du Lac (red beds) Arkosic and lithic silty sandstone 3000 / B asalt Basalt flows. 5

j ROCK FORMATIONS OF THE MINNEAPOLIS-ST. PAUL AREA

FIGURE 2 -4 8 unconsolidated glacial materials resting on the higher den­ sity sediments gave a density contrast great enough to pro­ vide the gravity method with a fairly high degree of resolution. Two six mile traverses were made normal to the assumed channels and with sta tio n s 100 yards a p a r t„ Results were recorded on two profiles which correlated remarkably well with the mapped subsurface topography of the southern half of the New Brighton Quadrangle. PHYSICAL CHARACTERISTICS OF THE ST. PETER SAMDSTONE

General Description The St. Peter sandstone (Ordovician) is one of the most ‘conspicuous rook formations in the Mississippi River

Valley. The formation is very widespread and displays rea­ sonably consistent physical and mineralogic characteristics.

Perhaps the most unusual properties of the formation is that i t is composed of 99 percent pure quartz sand grains and lacks cementation. In the Metropolitan area the St. Peter everywhere lies unconformably over the Shakopee dolomite member of the

Prairie du Chien formation. The sandstone formation aver­ ages 150 feet in thickness and is uniform except for a dis­ continuous shale layer located approximately 50 feet above the Shakopee contact. The top of the sandstone is charac­ terize d by a tra n s itio n zone th at grades upward in to the

Glenwood shale. The shale varies from three to five feet in thickness in the Twin City area. Conformably overlying the Glenwood is ten to thirty feet of Platteville limestone.

The best exposed and most complete section of the St. Peter sandstone is located at Holiday Harbor. The har­ bor is situated across the Mississippi River from the type section of the St. Peter sandstone at Fort Snelling. The section is a seventy-five foot high river bluff and is

9 1 0

Figures 3 and 4. Fresh exposures of the St. Peter sandstone at the Minnesota Silica Company quarry, Minneapolis. Platteville limestone, Glenwood shale, green sand horizon also are shown. 11

FIGURE 5. River bluff at Holiday Harbor, St. Paul exposing the St. Peter sandstone. Opening at right was caused by the caving of an unsupported mine drift. FIGURE 6. Mine drift in the St. Peter sand­ stone. Opening was purposely arched. (Holiday Harbor, St. Paul) 1 3 easily accessible. Several large horizontal drifts have been driven as much as fifty yards into the bluff, permit­ ting observation of freshly exposed rock.

Friability and Cementation The St. Peter sandstone can normally be described as a fria b le white sandstone which lacks cementation. However, locally within the formation irregular shaped masses of sandstone are cemented with iron oxide, forming a very hard and dense sandstone. Occasionally, the iron oxide appears to have formed concretionary masses which are spheroidal in shape. In examination of polished sections made of the sandstone, under a petrographic microscope, no obvious ce­ menting agents were observed except in those cases where iron oxide had indurated the specimens. Secondary growth of silica, which was in optical continuity with the grains, was observed on less than 5 percent of the grains. The growths were characterized by discontinuous bands around the coarser grains and were usually term inated by one or more of the faces of the individual grains. This secondary growth may have been formed by a previous rock cycle, prior to the sed­ imentation of the St. Peter sandstone. In addition, the low percentage of grains displaying overgrowths is unusual for a rock material that has undergone the introduction of silica in solution. The bonding force that keeps the sandstone together as a unit can best be explained mechanically by an inter­ locking grain effect„ The finer grains that make up the sandstone are angular in shape and have become interlocked with each other to form an aggregate. The shape of these aggregates conform to the shape of the interstitial openings between the larger, more rounded grains. In addition, the coarser grains, being frosted and pitted, allow the finer angular grains to use these indentations as foot holes in which to help keep the grains interlocked. Therefore, it can be assumed that these effects substantially keep the ma­ terial together. The photomicrograph (Figure 7) will help emphasize this theory. Perhaps the higher percent of the rounded grains in the sandstone would cause the material to be more friable and conversely, the higher percent of smaller angular grains would cause the sandstone to be less friable.

The force that caused the interlocking of grains was probably due to the compaction of the formation after sedimentation.

The sandstone generally becomes harder arid more dense with depth in the formation. However, exposed sand­ stone, even though more friable in the upper part of the formation, withstands the effect of weathering well. Where the material stands unsupported in high river bluffs or ex­ cavations, the strength needed to support the sandstone is mostly the result of compaction and the presence of a pro­ tective limestone cap. With the cap rock removed, the IUE . htmcorp soig ri rltosis n h St. the in relationships grain showing Photomicrograph 7. FIGURE «y * ore gan so ay ere f rounding. is of degree matrix any show grains fine coarser indistinguishable The sandstone. Peter opsd f nelcig ie ad ris Ol the Only grains. sand fine interlocking of composed 5 microns 350 SCALE

16 sandstone is very soft and friable„ Occasionally, the sand­ stone may be found to be quite friable in an exposure even with the cap rock in place. This phenomenon is possibly due to an excessive amount of water which once flowed through the material causing it to lose its strength by grain rear­ rangement (Schwartz, 1965), Case hardening on some sand­ stone outcrops has given added protection against weather­ ing.

Mineralogy and Lithology

The most outstanding lithologic feature of the St. Peter sandstone is its purity and white color on freshly ex­ posed surfaces. For all practical purposes, the formation can be considered monomineralic. A chemical analysis of the sandstone at the type locality. Fort Smelling, showed the following results: SiOg - 9 7 . 6^, Al^Og - 1.3$, Fe^Og -

0 .-55$ , CaO - 0.41$, Wa20 - 0,15$, and Kg© - 0.02$ (Winchell, 1884). In the Twin City area these figures can be consider­ ed average. Percentages will not vary over a few tenths of a percent from location to location.

Numerous heavy mineral analyses have been made on the St. Peter sandstone. These, show an average composition of the heavy mineral fraction of the sandstone as: zircon -

48$, tourmaline - 35$ j r u tile - 8$, leucoxene - 6$, and a trace of garnet (Stauffer and Thiel, 1941), and constitute approximately 1 percent of the total mineral composition. 1 7 There is an increase in content jand an obvious tex­ tural change in the heavy minerals near the top of the for­ mation. Normally, the garnet content increases, and the grains become larger and more angular as the transition zone is approached and crossed into the Glenwood shale. This is due to an environmental change in the sedimentation history of the area. The heavy mineral content of the St. Peter sandstone is one of the main criteria used to differentiate between the St. Peter and Jordan sandstones. Thd Jordan sandstone, which underlies the Prairie du Ohien formation, is very sim­ ilar to the St. Peter, and often the two are confused with each other in the study of well cuttings in interpreting sand intrusion problems of water wells. The heavy minerals are easily separated from the sandstones with high density liquids. Since there is usually an obvious difference in heavy mineral composition between the two formations, this comparison is used to distinguish between the two sandstones in question. Heavy mineral content in the,Jordan sandstone is as follows 5 garnet - 6l$, tourmaline - 16$, zircon -

180, rutile - 20, and leucoxene - 10 (Stauffer and Thiel, 1941).

Iron in the St. Peter sandstone usually occurs as a coating on the individual grains and results in a yellow or reddish brown stain on the grains. Occurrence of iron in the formation is very erratic and has locally formed stained 1 8 bands which tend to follow bedding planes in the formation. The bands may be an inch to a few fe e t in thickness and may represent old, static water tables, or preferred bedding planes for the lateral movement of water. Irregular shaped iron stained bodies of all sizes and shapes are also common. The iron is probably of second­ ary origin deposited by downward percolating iron charged w aters5 which followed irregular courses through the sand­ stone. The irregular courses may have been caused by dif­ ferences in permeability in the formation.

Pyrite and marcasite grains have also been recog­ nized near the top of the formation. Crystal faces on these particles are well developed and show no signs of being transported, which indicates that the minerals are of sec­ ondary origin. Nodules of pyrite with disseminated quartz grains are not uncommon, and are found throughout the forma­ tion. The nodules also are obviously of secondary origin because of well formed crystal faces on the pyrite grains.

Authigenic and detrital grains of feldspar were rec­ ognized in the lower part of the formation near Newport,

Minnesota (Thiel, 1935* p. 590)- The angular feldspar grains are largely due to authigenic growth or secondary en­ largement of detrital grains. Thiel recorded 3 to 5 percent feld sp ar content in samples from the Newport areaj however, no feldspar grains could be isolated in the thesis study 1 9 area. Therefore, the existence of feldspar grains may only be a local phenomenon. Due to the homogeneous character of the St. Peter sandstone, bedding is not conspicuous. Upon close examina­ tion, the bedding planes can be distinguished and range from a few inches to a few feet apart. Iron stained exposures of the rock seem to be the best location.to study the bedding planes. Textural studies among beds show no obvious differ- \ ences.

Some cross bedding has been reported in the St.

Peter. It is thought to be mostly of fluvial origin with only minor evidence of eolian deposition.

Textural and Grain Size Analyses Grain size analyses of the St. Peter sandstone indi­ cate the material to be extremely well sorted and restricted to a limited size range. Ninety percent of the sand grains f a l l between the range of 125 and 250 microns or the 140 and 60 mesh sieve sizes respectively. The sandstone should fall into most soil classifications as a fine sand.

Numerous sedimentation analyses were made of the sandstone with a visual accumulation tube. The visual tube method is used for size analysis of suspended sediment sam­ ples of sand fractions less than one millimeter in diameter.

The theory of the accumulation tube is based on Stokes Law and was developed for sediments similar to the St. Peter 2© sand. Percentages of each grain size retained are graphi­ cally determined on a chart. This method of grain size an-, alysis was chosen because of the nature of the sandstone, and the te s t is sim ple, rapid, and accurate. Figure 8 shows ten histograms (frequency distribu­ tion curves) made with the visual accumulation tube. The specimens tested represent a vertical section of an outcrop of St. Peter sandstone taken on the east -abutment of the new Fort Snelling Bridge on the Mississippi River. Channel sam­ ples were taken at five foot intervals for the top fifty feet of the formation. The site was chosen to take samples because the rock was freshly exposed, and the location was near the type section of the sandstone. In examining the histograms, there are no great var­ iations in grain sizes vertically in the section except in the upper ten feet (transition zone) where the material be­ comes slightly coarser. Excellent sorting,is also very con­ spicuous in the section.

Samples from several horizontal sections also were taken to determine their consistency in gradation of grain size. A series of eight channel samples, one hundred feet apart and approximately twenty feet below the transition zone in the St. Peter, were collected in the Minneapolis

Storm Sewer Tunnel between Washington Avenue. North and First Street North. Samples were collected during the construc­ tion of the tunnel. Feet "below Grain size Percent Histogram G-lenwood in microns Retained contact 500 350 8 m 3 2.5 4 62.5 3.5 500 3 350 s TPHTT^TT liimmiirnTTiiiiiiimimiiiirtiu 125 • 5 88 4 62.5 3 F 500 350 I i liill III |1010 250 it tr^ 175 l 11 125 1 ! 88 mini 62.5 ii3 0 350 hm : i .m| illlillliliflll 250 III1 175 8 U 1 III III1111 1 till 1 350 Til | i L tee llilIlllil hb

5 350 III 1III u ^75 nr mlIlll111. 125 8 ] 88 6 n 2 HI || | 1 175 38 II 1M II 1 1111111,1 125 30 11 j i li 88 T] 350 3 111 | 1 ??? i I 1 1 | 0 ,, 1 II l 1 ?02 350 i Hi 1! || Tl , ., E !|l Mil lil! 1 ITHTITI 125 l E iii iul 88 6 mlj 118 .ILLL.. 1 1 i I___ 175 .1111.. i.lm Illl Ml nimmimm 125 E IT

GRAIN SIZE ANALYSIS— VERTICAL SECTION

FIGURE 8 22 The visual accumulation tube was used to determine

\ the size gradation, and the data are tabulated in Figure 9»

An even more remarkable consistency can be observed in size gradation as compared to the vertical section.

Similar series of grain size analyses were made at the following locations: Minnesota Silica Company in North Minneapolis, Stevens Avenue Storm Sewer in Minneapolis, Fort

Snelling, and Holiday Harbor in St. Paul. Gradation results from vertical or horizontal sections were very similar to those aforementioned. Therefore, the St. Peter sandstone can be considered nearly homogeneous with regard to its. grain size distribution.

Lacabanne (1964) noticed a definite bimodal and pos­ sible trimodal effect in the sandstone as shown in Figure

10. His specimens were collected from a river bluff below the University of Minnesota in Minneapolis. They were ob­ tained fifteen feet below the Glenwood - St. Peter contact.

Holder (1963, p. 5) also recognized a similar bimodal dis­ tribution of sand grains a foot below the Glenwood contact at several locations. The study with the visual accumula­ tion tube showed only one instance of this bimodal effect.

Evidently, the entire formation does not show this bimodal distribution.

A summary of textural characteristics of the St.

Peter sandstone are given in the table below for several 23

S tation Grain size Percent His togram lo cation in microns retain ed

9+00 350 3 m 250 35 11 ill I n 175 31 ' 11 n h I nn 125 20 HIL in 10+00 350 6 niln 250 36 jj_ i m 175 35 nil ilk ! ittrr 125 31 j! Ilk n" 88 4 rij 11+00 350 6 r ]______250 37 11n n hi III! nn ii i i n i 175 34 il mi IN ill nn i n n 125 17 11 Ji nil 88 6 !lll 12+00 350 8 • III IL ______39 ! i nn rimiiiii _ L _ . 31 i n n m125 16 ! j 88 6 til I 13+00 350 7 n n H ______250 37 ini n n 1 1ll| w n llli111 175 34 !li || | I till 125 19 ill llli ml 88 5 14+00 5 118 33 n n n in i ni 175 30 i in 125 25 i l 88 7 II 15+00 350 4 etl 28 llli M l 31 j Mill m125 27 in i 11TT 88 10 16+00 350 5 250 27 iiil m l ill! III i Im il 175 ilk k k Ill n i n 125 11 kii n nil In ill 1n r 88 7 mi il

GRAIN SIZE ANALYSIS — HORIZONTAL SECTION

FIGURE 9 PERCENT RETAINED

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 28 32 35 42 48 l u 60 % 65 uj 80 y io o cn 115 150 170 200 PAN

HISTOGRAM SHOWING THE BIMODAL DISTRIBUTION OF GRAIN SIZES

IN THE ST PETER SANDSTONE rv FIGURE 10 -t=r 2 5 locations in the greater metropolitan area (Theila 1935* P»

584). Table 1. Textural Characteristics

Medium Coeff. Coeff. E ffective Uniformity Location Diameter of of Size Coeff. (mm) Sorting Skewness (mm) Mendota 0.235 " 1.41 1.09 0.142 ... 1.99 North Mpls. 0.235 1.48 0.96 0.107 2.65 St.- Paul Park 0.178 1.32 1.20 0.119 2.28 Zumbrota 0.201 1.32 1.06 0.114 2.00 South Mpls. 0.219 1.55 0.94 0.086 3.02 Mounds Park 1.970 1.37 1.07 0.121 ■ 1.92 Washington Co. 0.281 1.27 1.11 0.175 . 3.66

Roundness , F ro s tin g , and P ittin g

Grain shapes in the St. Peter sandstone vary from

angular to subround with very few grains displaying a spheri­ cal form. Generally5 the coarser fraction displays the

higher degree of rounding. Theil (1935* p. 586) found that the grains retained on a y millimeter screen show an average

circularity index of 0.956, l/8 millimeter screen had a 0.925 index, and the l/l6 millimeter screen fraction was 0.917. The index is based on a circle which has a value of 1.00 and a square with an index of 0.784.

Actual grain shapes may be elongate, equidimension- al, or even conical. Variation in shape is mainly due to

the difference in origin of the original grains (Thiel,

1935* p. 587)* or the shapes may be due to corrasion during transportation.

The coarser grains show a higher degree of frosting, which is in agreement with the rounding of the coarser 26

grains„ The white color displayed by the sandstone in fresh exposures is caused by the frosting of the grains.

In conjunction with the frosting, pitting is also characteristically displayed on the grains. The pits are generally shallow and broad and may reach depths of 0.1 mil­ limeter on the larger grains. Occasionally, the pits act as

"foot holes", for grains to interlock with one another. The Origin of the frosting and pitting is probably due to grain

impact by wind action. -

Silt Horizon

A siltstone layer, located below the middle of the St. Peter formation, occurs almost continuously throughout

the Metropolitan area. The layer varies in thickness from a few inches to as much as six feet. Textural properties of

the siltstone are not consistent laterally; however, the ma­

terial is fine enough to be significant as an aquiclude.

The layer represents a period of quiet sedimentation during

the depositional history of the formation. The Army Corps of Engineers secured a cored sample of

the layer during the site examination of Lock and Dam Number 1 on the Mississippi River,;in Minneapolis. The horizon was

reported as three feet of soft laminated shale and siltstone,

dark gray in color. An X-ray defractometer analysis was

made of the material to determine the mineralogy of the clay

fraction in the sample. A high content of illite was found 2 7 with minor amounts of kaolin!te and only a trace of quartz. A petrographic examination of the coarser fraction showed the m aterial to be composed of quartz, orthoclase, micro- c lin e , and s e r ic ite grains. i Green Sand Horizon

Five to fifteen feet below the Glenwood - St.'Peter contact, a conspicuous eight' inch thick layer of green sand can normally be observed in fresh exposures throughout the Twin City area. Close examination of the material reveals a mottled texture, consisting of small, irregular patches of clean quartzose sand and quartzose sand with its interstices filled with a pale green clay. The material is very hard when dry and has to be broken with a hammer, when wet the material softens and shows plastic tendencies. An X-ray analysis of the green clay gave well defined peaks for the mineral illite.

The origin of the material-may be from the overlying Glenwood shale. The clay particles may have been transport­ ed down and accumulated at a preferred level because of a textural change in the sandstone. Since.stratification within the layer cannot be distinguished, this also might indicate that the clay is a weathered product from detrital feldspar grains deposited near the end of the deposition of the sandstone. 2 8 For engineering purposes5 this material should at le a st be considered, avoided, or removed fo r construction or tunneling projects when encountered near a site. • Even though the layer is thin, its high clay content can cause settlement or shear failure problems.

Shakonee Dolomite - St. Peter Sandstone Contact

Only one locality in the State of Minnesota is known where the Shakopee - St. Peter contact is exposed (Austin,

1964). The outcrop is located one-tenth of a mile north of the south border of Section 25 in Stanton Township (112

North, 18 West) and 200 feet northwest- of the township road in Goodhue County. The contact is exposed in a river bluff, three feet above the present Cannon River.

The contact is sharp and conspicuous and can be con­ sidered a disconformity. The break is characterized by a generally horizontal, but undulating surface, presumably . rip p le marked. The rip p le marks are composed of a w eather­ ed, soft silty dolomite, and are approximately two inches . high and one-half to two fe e t between c re sts. Immediately above the contact are lenses of coarse angular quartz sands interbedded with black and red clay lenses. Pyrite crystals are fin e ly dissem inated throughout these lenses. Twelve inches above the contact a - twenty ?-four inch horizon of gray silty sand predominates. Above this zone, and exposed for 2 9 forty feet upward in the outcrop, is typical clean St. Peter sandstone.

Glenwood Shale - St. Peter Sandstone Contact

The Glenwood - St. Peter contact is transitional and conformable. .The transition zone is usually three to four fe e t thick and grades upward from a pure q u art2rose sandstone to a sandy shale. The Glenwood shale, which is normally considered as being closely related to the Platteville limestone, does contain a low percent of oalcite. An X-ray analysis showed the shale to contain mostly quartz with minor amounts of il-

\ l i t e , feld sp ar, and c a l c i t e „

The shale has been responsible for several settle­ ment problems in the City of St. Paul, suggesting that the shale is compressible upon loading. Experience has also shown the shale is susceptible to caving when encountered in tunnels. Therefore, this material should be avoided for foundation use or tunneling unless adequate support is planned.

Jointing and F racturing

Jointing of rock in the Twin City area, is conspicu­ ously displayed in the Platteville limestone. The exposed joints are pronounced, systematic, and are traceable over tens of yards. They are normally vertical, open fractures which penetrate the entire thickness of the formation but 3 0 are terminated at the underlying shale. Joint widths vary from l/l6 of an inch to three inches and are usually filled with glacial debris„ Jointing in the St. Peter, though unrelated to

jointing in the Platteville, is severe. In contrast, the fractures in the sandstone show no preferred orientation or

spacing. The only consistent property of the joints is that their planes dip vertically or near vertical (70 to 90 de­ grees). When observed on the surface, the joint planes are

tight and follow irregular paths. Figure 11 represents three locations where joint directions were measured. The rose diagrams graphically show the number of joints striking

in a particular direction at a site. Ho correlation between

strike directions can be made at any of the locations. The first two diagrams show joint directions mea­

sured at the Minnesota Silica Company Quarry. Diagram one represents the Platteville limestone, and the second is of

the St. Peter. Very little similarity of joint directions can be observed. Therefore, it can be concluded that joint­

ing in the St. Peter is unrelated to jointing in the Platte­

ville except for the possible causes for jointing. It can

further be said that existing or buried drainages in the re­ gion are not controlled by jointing in the rocks.

The majority of joints observed in tunnels under­

ground are tight, have regular planes, dip at high angles, and are randomly orientated. Spacing of the larger and more N

20 15 10 5 0 5 10 15 20 Number of Joints Number of Joints MINNESOTA SILICA CO. QUARRY MINNESOTA SILICA CO. QUARRY (Plotteville limestone)

Number of Joints RONDO ST. STORM SEWER, ST PAUL LEXINGTON AVE., NO. RIVER BLUFF, ST. PAUL u> ROSE DIAGRAMS SHOWING JOINT DIRECTIONS H IN THE ST. PETER SANDSTONE FIGURE II 3 2 prominent joints varies from a few feet to several hundred feet. Occasionally, two or more have been known to inter­ sect each other at high angles causing a keystone effect in the rook. Joints encountered underground are not always recog­ nizable after they have been exposed unless sloughing along the planes appears. The obvious joints are those which are open and yield water or tig h t jo in ts whose planes have been iron stained or filled with a clay material. Gore drilling the sandstone to locate joints is un­ satisfactory unless the material is cemented enough to allow , i - " " for good core recovery. Geophysical methods of finding planes of weakness is equally unsatisfactory because the joints are normally tight. Undoubtedly, the most effective way to prevent structural failure, due to jointing, is to take precautionary measures during excavation. -

Occasionally, some joints near the top of the forma­ tion are found to be filled with a brown clay material„ An

X-ray analysis indicated the clays to be illite and mont- morillonite. It is possible that the clay was derived from the overlying Glenwood shale, fo r the main clay mineral in the shale is illite. If clay filled joints are encountered during excavation of the sandstone, failure often occurs along these planes of weakness due to the low internal fric­ tio n . 3 3 Joints in the sandstone were created as a result of probably both tensil and shearing stresses. The type of jointing suggests many small scale repeated stresses, such as earth tides', earthquake shocks, atmospheric movements, isostatic adjustment in the basin, and loading and unloading of glacial ice. Very few of the jo in ts have been bonded by a cement­ ing agent. Therefore, the friction between grains along the planes and the lateral confinement of the joints are the forces that restrict any shearing action along the planes. Once a joint is exposed, it is quite possible that it may have lost its confining pressure and is therefore suscepti­ ble to failure.

Lamar (1928) describes sheeting in the St. Peter sandstone as being developed on both weathered and freshly exposed surfaces. The sheeting planes are inclined at steep angles and strike parallel to the face of an exposure.

Lamar stated th at the phenomenon is probably due to irre g u ­ larities in the bedding or just typical weathering effects of a homogeneous rock. However, perhaps an even more important cause of sheeting in the sandstone is pressure release in the rock adjacent to an exposure. Assuming the rock a t an outcrop has lost its triaxial confinement, then elastic re­ lease to areas of lower pressure, which would be the exposed face, may cause tensional forces back of the face. These stresses would result in development of fractures, striking 34

FIGURE 12. Unprotected river bluff below Soldier's Home, Minnea­ polis (1932). Exposure of Platteville limestone, Glen - wood shale, and St. Peter sandstone. 3 5

FIGURE 13. Concrete crib retaining wall built against river bluff below Soldier's Home to prevent spalling of the sandstone. 3 6 p a ra lle l to the omtorop* Frost wedging* gravity* and flow - age of water would cause enlargment of the fractures* and eventually large blocks of sandstone would be freed from the outcrop.

Sandstone Caves .

During construction of utility tunnels in the St.

Peter sandstone* several natural caves have been encounter­ ed. The caves are irregular in shape and vary in size. In every instance they have been found near the top of the for­ mation* and the Platteville limestone was acting as a roof rock for the openings. The cave walls are normally verti­ cal* and the floors are laden with rubble. No natural en­ trances have been found to the caves with the exception of one in St.. Paul; however* all the caves located thus'far are located near one of the present river channels.

Poor cementation or mechanical bonding of the sand­ stone in the upper portion of the formation has been the cause for the occurrence of the caves. Water evidently had the opportunity to flow along joints with sufficient hy­ draulic gradient to remove large volumes of sand. Outlets to a river must have existed and later backfilled (Schwartz* 1936* p. 46).

The large cave* located in downtown Minneapolis* r e ­ quired reinforcing due to heavy structures build directly over the cave. Concrete and brick p iers and bearing wall 3 7 were systematically placed in the opening to help distribute the overlying weight evenly. Figure 1% shows the configura­ tion and location of the cave. The cave has been considered for use as a parking, ramp or fall-out shelter.

Origin of the St, Peter Sandstone

Probably the most disputed problem of the St. Peter is its origin. The present, accepted theory is that the sandstone was deposited in a marine environment. Mew evi­ dence of the bimodal grain distribution in the St. Peter may give a better understanding to the formation’s depositional h isto ry . Holder (1963, p. 3) stated that the sand grains ex­ hibit far too high a degree of rounding, sorting, and pit­ ting to be created by normal action of water. Therefore, one might conclude that the sand was windblown, at least in its initial phase of transportation. If a land mass of con­ siderable size were to undergo wind erosion, the grains from which, after being shifted about for a considerable time, would have become well rounded or would have d isin teg rated to a very fine m aterial. Assuming a gradually transgressing sea, the grains would have been deposited in the water adja­ cent to the coast. Thus, the marine depositional theory and the eolian grain rounding theory are both satisfied. Today a similar type of depositional environment exists along the northern coast of the Libyan Desert. NATURAL SANDSTONE CAVE, MINNEAPOLIS

/--> x

CAVE LOCATION

Brick or Concrete Piers Broken off old r artesian well

Bearing Retaining8 viSi Walls £

'Entrance

Scale l"=80'

CAVE CONFIGURATION FIGURE 14 3 9 After deposition of the Shakopee dolomite in Ordo­ vician time, there was a period of uplift followed by exten­ sive erosion on the dolomite surface. A transgressing sea advanced over the land mass and deposited the S t. Peter. This sea must have drowned the source area of the sandstone before regressing. Erosion, of presumably the same land mass that supplied the sand, now began supplying the materi­ al for the Glenwood shale, Platteville limestone, and

Decorah shale before the. sea completely regressed. The source material for the St. Peter has not been definitely recognized. Many feel that the sand grains, due to their character, were derived through a. multicycle pro­ cess. This would mean that the grains originated from a quartz rich rook. The grains were eroded, transported, and deposited as a sediment, then eroded .again and redeposited, perhaps several times. It is very likely that the sand could have been derived from older formations in the sec - tion, such as the Jordan, Mt. Simon, or Hinckley sandstones, a l l of which are somewhat sim ila r to the St. P eter sand - stone.

Location of the source area for the sand was prob­ ably to the north, in the Canadian Shield area. This was obviously a land mass of extensive size in past geologic i time. Environmental and climatic conditions during deposi­ tion of the St. Peter are also not thoroughly understood be­ cause of the near absence of fossils in the formation. Only #0 a few fossil forms of pelecypods, gastropods, and cephalo­ poda have been found near Dayton's Bluff in St„ Paul„ The

St„ Peter was not a satisfactory media for the preservation of fossilsc HYDROLOGIC ASPECTS OF THE S T , PETER SANDSTONE

The St. Peter sandstone has a high porosity as a re­ sult of its .high degree of sorting and the absence of a ce­ menting agent to fill the interstitial spaces between grains. Kamb (1932) ran ten porosity tests from specimens collected from several locations in the metropolitan area.

Porosities ranged from 27 to 31;percent with an average por­ osity of 28.3 percent. The tests were made by the immersion method using acetylene tetrachloride. The consistency of test results indicates the porosity of the sandstone to be fairly, uniform.

The sandstone has low permeability, which is expect­ ed for a fine sandstone having an average effective size of

0.122 millimeters. According to Buchanan (l937)» a very fine sand to a silt size material should have a permeability ^r7 ranging from 10 to 10 ' centimeters per second. Permeabil­ ity test results run on the St, "Peter, by various organiza­ tions, have not been consistent„ Laboratory tests indicate -3 an average permeability of 3.5 x 10 centimeters per second.

For the disturbed sand a value of 1.0 x 10 can be consider­ ed average (Corps of Engineers, 1938).

Much discretion should be used when applying the co­ efficient of permeability, determined from laboratory tests, to actual field problems. Many variables exist in the field 4l %2 that cannot be duplicated in the laboratory. For example. Jointing in the sandstone will greatly influence the flow of subsurface water; this factor is difficult to reproduce in laboratory tests. Leisch (1964) stated th a t the most e ffe c tiv e method for determining the rate of flow of water through an aquifer is controlled pumping te s ts in the fie ld . In te s ts p e r­ formed by Leisch (1962, pp. 9-10), the coefficient of trans- 1 ’ missibility for the sandstone was determined by means of pumping te s ts . In calcu latin g the c o e ffic ie n t of perm eabil­ ity, Leisch found that the permeability of the St. Peter sandstone is higher than previous laboratory tests have shown. Results of three test locations are given below.

Table 2. Coefficient of Transmissibility

Coefficient of Coefficient of Test Location Transmissibility Permeability■ 5th Ave. between 18th 20.800 gpd/ft 9 . gxlO"^ cm/sec. & 19th Aves., Mpls. 9th St. between 13th 20.800 gpd/ft. 5.5x10 ^ om/sec. & l4 th Aves., Mpls. Loring Park between 22,400 gpd/ft. 6 .6 x 1 0 cm/sec. 14th & 15th Sts.. M-ols.

The re su lts of the pumping te s ts given above .are

^The coefficient of transmissibility is defined as: The quantity of water, in gallons per day, that is transmit­ ted under a unit hydraulic gradient, through a vertical strip of aquifer that is one foot wide and extends the full ' thickness of the aquifer. The units of the coefficient are given in gallons per day per foot.

The coefficient of permeability is easily determined by dividing the transmissibility coefficient by the saturat­ ed thickness of the aquifer. 4 3 very consistent, Leisch concludes that the overall trans- missibility of the St. Peter sandstone is remarkably uni­ form. GENERAL GROUNDWATER CONDITIONS IN THE. ST, PETER SANDSTONE.

The normally saturated St„ Peter sandstone includes both confined and unconfined water. As previously men­ tioned, the Twin City area is located in a structural basin 1 which has created a natural gradient in the formation. Ar­ tesian water is locally found where the St. Peter is capped with the impervious Glenwood shale. However, unconfined water normally exists due to the lack of an adequate head in the formation. This lack is due to heavy pumpage from wells in the sandstone, discharge into unlined sewer tunnels, and discharge from springs along the rivers„ Although most of the drawdown in the formation has been caused as a result of leakage into the. buried channels that have cut the sandstone and to excessive pumping of the P ra irie du Chien and Jordan

Formations, both of which receive their water from the St.

Peter. Over the past few years discharge from the sand­ stone has exceeded its recharge rate. This has resulted in a slowly declining water table and a disappearance of con­ fined water at the top of the formation.

Slight variation in lithology of the sandstone such as grain size, bedding, cementation, or grain orientation has created local artesian conditions at various levels

4 4 4 5 within the formation. The aforementioned siltstone horizon, fo r example, can be considered an aquiclude and has created localized artesian water.

In the metropolitan area water is fed to all the aquifers above the St. Lawrence Formation directly from the glacial drift. The drift is charged from precipitation in the local upland areas. Water not only moves laterally into the St. Peter Formation from the drift, but also downward through joints in the Platteville limestone. Minor amounts of water infiltrate the sandstone directly from outcrops.

Movement of water through the sandstone is laminar and mostly downward. Smaller q u a n titie s of water move laterally toward the major drainages. The drainages, in some areas, which cut the sandstone are low enough to cause a hydraulic gradient toward the rivers. Where the St.

Peter sandstone is exposed along the river bluffs, the sand­ stone is usually dewatered. This condition normally exists for several hundred yards back into the bluffs.

Abundant water is available from the sandstone pro­ viding wells are properly developed. For high yield wells as much hole is left open in the sandstone as possible. If the sandstone is found to disintegrate easily under normal pumping, well screens and gravel packing are employed. Sand intrusion into wells in the St. Peter is a common occur­ rence . Normally, wells that cannot be cleaned out and the sandstone stabilized, have to be driven into the Prairie dm

Chien or Jordan Formation and the St. Peter cased off.

The Minnesota State Board of Health restricts the use of water extracted from the St. Peter to industrial use, due to contamination. The contamination is caused by leak­ age from sanitary sewer tunnels in the formation and in fil­ tration of sewage from outdoor sanitary facilities in areas outside the metropolitan area. Water from the St. Peter is used mostly for air conditioning purposes for large build­ ings because of the water's low temperature.' During the hot summer months heavy pumpage from the aquifer has caused as much as thirteen feet of fluctuation in the water table. Unfortunately, the water used for air condition is not re­ claimed and is therefore wasted. Summer drawdown of the water table has caused wells to go dry to private owners in the outlying areas of the Twin City. However, the drawdown has been beneficial in alleviating dewatering operations for construction of utility and sewer tunnels in the sandstone.

Leisch (1962, p. 17) has stated that since the St.

Peter is a homogeneous aq u ifer, wells used fo r dewatering areas of construction can be properly arranged to obtain de­ sired drawdown with maximum efficiency. Theoretical draw­ down curves fo r any number of wells pumping a t a given rate can be estim ated by means of pumping te s ts . C alculation of the coefficient of transmissibility and storage, at a H-7 p a rtic u la r s ite , can in d icate optimum spacing and yield of dewatering wells„ Other methods of determining boundary conditions in the St. Peter sandstone can be employed. Mathematical anal­ yses are feasible but are only beneficial for simple boun­ dary cases. Graphical methods, such as electric analogies and image well in te rp re ta tio n s , have given reasonable two dimensional re s u lts . However, to accurately locate dewater­ ing wells, well points, drainage galleries, and underdrains with two dimensional solutions, is difficult. Leisch's method of using pumping te s ts in the fie ld is a true three dimensional procedure and will give the most reliable re­ sults for hydraulic flow conditions in the St. Peter sand­ stone.

If turbulent water is allowed to come in contact with a fresh surface of the sandstone, disintegration of the sandstone is usually rapid. Therefore, water should be cut off before it reaches the boundary with the use of well - points, deep wells, underdrains, or by grouting if possible.

Experience has indicated that if water in the sand­ stone can be removed without disturbing the granular s t r u c ­ ture of the sand, a higher stability of the. material will be obtained than if it is left saturated. If water is removed too rapidly from the zone of dewatering, slight agitation and reorientation of the sand grains may result. This 4 8 ' condition may cause a loss in physical strength to the sur­ rounding sandstone (Schwartz, 1964)„ The greatest difficulty encountered during construc­ tion activities below the water table is heavy discharge of water from fissures„ The fissures can be visualized as an interconnecting system, which draws water from a large area in. the sandstone and turbulently discharges the water into a tunnel or excavation. If water continues to flow turbulent - ly for any length of time, enlargement of the fissures will result, followed by the danger of cave-ins. Normally, the discharge from fissures is heavy at first as a cone of de­ pression is created. In time the flow decreases consider­ ably and may even stop. The fissures that show no decrease in flow with time presumably are obtaining their water from a considerable distance from the exposure. It is possible that the fissures may intersect a surface source of water or a perched water table. If discharging fissures are encount­ ered during excavation and erosion can be kept to a minimum at the opening, this may prove to be an excellent means to rapidly lower the water table in the vicinity. A close ob­ servation of the surrounding water table should be made dur­ ing this operation to make sure the area is actually being . dewatered.

Dewatering operations for tunnel construction vary from location to location and depend on the water table lev­ el, permeability of the sandstone, and the number of joints 4 9 bearing water. Saturated sandstone usually warrants the use of high yield wells drilled from the surface prior to tunnel construction. When a tunnel is driven near the water table surface, perforated pipe drains buried beneath the invert are employed to remove unwanted water.

Leisch (1962, p. 12) made several pumping te s ts to determine the proper spacing and yield of dewatering wells. An invert of a proposed tunnel in the St. Peter was to be located twenty to thirty-five feet below the piezometric or water pressure surface. The tests involved a group of four pumping w ells, discharging sim ultaneously, a minimum of 600 gallons per minute. The wells were spaced along the tunnel alignment at 1,000 and 1,200 foot intervals and approximate­ ly 75 feet below the tunnel invert. A trough was created on the piezometric surface that produced the required drawdown.

When fewer than four wells were operating, the desired draw­ down was not obtained. More than four wells operating cre­ ated a longer trough but did not increase the drawdown within the spread. The drawdown estimates were based on the assumption th a t the sandstone is homogeneous and receives no recharge. To compensate for recharge, each well was de­ signed to yield 900 gallons per minute for that particular spread.

During construction of the Twin City Lock and Dam, cement grout was used to seal discharging fissures

(Schwartz, 1938, p. 58). The grouting was done in stages. 5 0 allowing the grout to set before more was forced into the openings5 otherwise too large a quantity of slurry would be required„ The method proved effective, but a complete seal was not obtained„ The City of Minneapolis was also successful in seal­ ing fissures behind sewer tunnel linings with the use of ce­ ment and bentonite g ro u ts„ The Joosten process of chemical grouting was used to construct a grout curtain for the lower St. Anthony Falls

Lock and Dam. Pipes, were je tte d down to a desired depth, and the chemicals were injected through the pipes as they were jacked up out of the holes. The purpose of solidifying the sandstone was to prevent seepage of water under the cof­ ferdam and to seal joints and bedding planes. Pressure grouting tests in the St. Peter sandstone and using AM-955 (nitrilotrispionamid) chemical grout, have been conducted. The tests were made to determine the poten-. tial of chemical grout to be used in the construction of the Upper Locks at St. Anthony Falls (Corps of Engineers, 1957)° A one shot process was used with injection pressures ranging from thirty-five to forty-five pounds per square inch. Pres­ sures above forty-five pounds caused a temperature rise and a quick polymerization of the chemicals.

Density determinations, before and after the injec­ tion of AM-955^ indicated very little change. Only a small quantity of grout penetrated the pore spaces of the 51 sandstone. It was originally thought that the chemical grout would penetrate the sandstone in a uniform, spherical pattern away from the injection point. Instead, the grout flowed laterally along uncemented fissures and soft layers of sandstone. The sealing of the bedding planes and joints was the main objective of the grouting work needed for the construction project, and the results were considered favor­ able. However, the grout was not used because i t was be­ lieved that it would become ineffective if it were to dry out under a fluctuating water surface. On a cost estimate the grouting method would have saved $110,000 over the use of sheet piling for this project if the grout had been ef­ fective. The cost of chemical solidification is about 0.6 as much as the cost of a sheet piling cut off.

For deep vertical shafts where a high inflow of water is anticipated, freezing methods may prove satisfac­ tory to temporarily seal off the water. The method involves circulating a cold brine solution through closely spaced pipes around the perimeter of the proposed excavation and to a depth of the proposed shaft depth. During excavation and construction the water in.the sandstone will remain frozen.

After the shaft has been completed and lined, the process can be discontinued and the ice allowed to thaw (Legget,

1962). Due to the expansive properties of water upon freez­ ing, structural rearrangement of the sand grains may result and cause loss of strength to the sandstone in the immediate .. . 52 vicinity of the excavation. In addition, as soon as the ice thaws, a head of water will exist on the lining and must be considered in the design of the lining. There are no re­ ports of th is method having been used t-o con tro l water in the formation. This is probably because the process is costly and cannot be used where subsurface water is continu­ ally flowing. Water in the St. Peter sandstone is one of the main factors that has and will cause many engineering problems. A thorough study of the existing hydrologic conditions at a site must be conducted in advance of any excavation or con­ struction project in this formation. ENGINEERING ASPECTS OF THE ST..PETER SANDSTONE

Previous Engineering; Studies

Numerous private and civil organizations have con­ tributed greatly to narrowing down the engineering proper­ ties of the sandstone. Perhaps the organization that should receive the most recognition is the U. 3., Corps of Engi­ neers. Previous to construction of the Upper Locks a t S t.

Anthony Falls5 an extensive exploration and testing program was conducted to determine the feasibility of using the St. Peter as a suitable foundation material for the locks. Many of the test results from the report are included on the fol­ lowing pages.

Recent exploration and testing have been conducted, and conclusions from these later works are remarkably simi­ lar. In part, this may be due to similar testing techniques or similar study approaches.

Actual experience during construction projects prove results from tests and studies can be used with confidence and safety. In the past, experience has been the main cri­ teria for safe construction techniques in the St. Peter sandstone. It is difficult to correlate between experience and the number of fa ilu re s th at have occurred because of poor judgment. The cause of a failure is not easily isolat­ ed or assignable to a single geologic phenomenon. Conclusions

53 5% drawn from experience are mostly a m atter of judgment and therefore debatable as to the degree of applicability„ Previous engineering studies of the St. Peter have been conducted using techniques of soil mechanics. Due to possible elastic behavior in the sandstone, methods used in. the field of rook mechanics should definitely be considered for future studies. To the geologist the St. Peter sandstone is classi­ fied as a massive sandstone, which is homogeneous, isotrop­ ic, and competent. The engineer would consider the St. Peter as an uncemented, friable, fine quartz sand. To pro­ perly study the St. Peter sandstone, both the engineer and geologist have to realize that the material lies in a trans­ itio n zone between a s o il and rock m aterial.

Sampling Techniques

Suitable undisturbed samples of the sandstone for laboratory testing are extremely difficult to obtain. Split tube and Shelby tube sampling devices have not been success­ fully used because the tubes have to be hydraulically pushed or driven into the sandstone. Penetration resistance values are too high to allow for this type of sampling. The Denni­ son core barrel sampler, which is a rotary sampler, has also proven unsatisfactory due to disintegration o f.the sample by the coring fluid, water. 5 5 The only undisturbed sandstone samples that have been obtained Intact from coring methods were from the site examination of the new Dayton's Department Store In St. Paul. A M diamond core barrel was used with a moderately dense drilling mud. Specimens were obtained at depths up to forty feet in the sandstone. Shaved test specimens from chunks of sandstone, ob­ tained from test pits, have been attempted. The Minnesota State Highway Department has endeavored to carefully shape blocks of undisturbed sandstone1 from several locations, by shaving the blocks down to test specimen size. They have also tried to push, by hand, cylinders into an undisturbed chunk of sandstone. Both means failed to produce usable test specimens. The Corps of Engineers (1958, p. 6) was successful in hand trimming trlaxial test specimens from chunk samples obtained from test pits in the middle of the formation below the Ford Motor Company in St. Paul. Because the sandstone becomes harder with depth in the formation, hand trimmed or cored specimens should be easier to obtain in the, middle or below the middle of the formation. Most structures and tunnels are constructed near the top of the formation, which does offer problems in obtaining suitable test specimens. If usable specimens cannot be obtained for laboratory use, then it is quite obvious that field testing methods are inevitable. 5 6

Unit Weight

To substantiate the fact that the St„ Peter has been highly densified, several inplace density tests and labora­ tory compaction tests' were made of the sandstone from the Minnesota Silica Company's quarry in north Minneapolis„ The field density tests were made with a sand funnel apparatus„

The Modified Procter test, with a compactive force of

515j>625 foot pounds, was used for the compaction tests„ The field dry density, from an average of four tests, was 126.4- pounds per cubic foot. The recompacted dry density was

11.37 pounds per cubic foot with an optimum mositure content of 4-. 6 percent. Figure 15 shows the typical moisture-densi­ ty relationship for the recompacted disturbed sandstone. The above results indicate that even with a great deal of compactive force, the disturbed sandstone cannot be raised to its original field density. The density of the

St. Peter, however, is low when compared to many sandstones.

This can be explained when one considers the high porosity of the material (25 percent). The apparent specific gravity of the sandstone, when using a bulk density of 126.4- pounds per Cubic foot, is 2.02. The true specific gravity of the sandstone itself is that of quartz, 2.66.

Unconfined Compression Tests

The first strength determination tests of the St.

Peter sandstone were made by Schwartz (1939* p. 4-0). Three 57 122 "1? 121 i

120

119

X 118 _ > / O/ f & 117 oy —<6>-

- - 116

115 1u ——— 114 4 V 2a, rV 113 s V 1 yvt\ 1 12 * :=Mr V-iL I 11 4 I 10 j —V 109 4 r t : 108 6> 107 I 2 3 4 5 6789 10 PERCENT MOISTURE

MOISTURE-DENSITY RELATIONSHIP FIGURE 15 5 8 two inch cubes were shaved from an outcrop of sandstone and tested, unconfined, by loading the blocks until failure oc­ curred „ The first two cubes were tested wet and failed at

0.547 and 1.06 pounds per square inch. The third cube was tested dry and failed at 144 pounds per square inch, which was considerably higher than the wet specimens but low as far as a crushing strength of any rock material. These com­ pression tests indicate the potential weakness of the sand­ stone and not its bearing strength. When the sandstone is in place and confined, it is not free to yield in any direc­ tion j therefore, unconfined compression tests on the St. Peter sandstone mean very little.

The Twin City Sanitary D istrict made one compression test on the sandstone during the site examination of a grit chamber on Pigs Eye Lake site. The specimen was obtained from a lower portion of the formation and withstood sixteen tons per square foot crushing strength. Evidently, the ma­ terial was fairly well cemented as indicated by the high compressive strength.

Eleven unconfined compression tests were conducted by the Corps of Engineers for the site examination at St.

Anthony F alls Lower Lock and Dam (Corps of Engineers, 1958). The stress strain curves for the tests are shown in Figure

16, and the test data is tabulated in Table 3. The tests were made in an universal hydraulic testing machine and loaded at a rate of 200 pounds per square inch per minute. 1000 —L!-L 1400 41 1000 i t S # / i - Jxr: ' Xft ■Ui: / M 800 # i 1200 , / 1 | z 1 1 # zprq: 1 S; s i ': ! W: « 600 —A-1 / 1000 600 / .i;u± s / / # | / # rd+tf o/ s :cp:i: # t 400 I y !/ tn ^ 800 r / ' f 1 .b / / •2 :/ 200 t ” 600 T01 ;eee: xirx ;±d: cn / m :b:q: :Lt±r :cctc f / r __ -hZ / 1 0 400 \ i 0.0 01 02 03 04 05 j 0.2 0 3 Percent Strain y Percent Strain

(I) 200 1 / (3) 1 f 0 t . 0 0.5 1.0 1.5 Percent Strain

(2 ) 2500 1000 | ,/ 1 1 2000 800 / C :1 1! / :ru"r 600 / i l xGi /

i3 r x Z 400 / / -Hdi- 200 i , z g y Q0 l/ B B Z:',- F! 0.2 0.3 0.2 0.4 0 o.l 02 03 0.4 Percent Strain Percent Strain Percent Strain (4) (5) ( 6)

3 000

2500 .ml:; 2500 •fix •J+ht # ■b; # p i ±f p s 2500 # » / 2000 : 2000 y > / f z tfH y 2000 § | y 1 / :x d 4XX # i _ SR # ^ 1500 :s 3 15 0 0 / 4 g y z vf -fen V i 1 * Z V) 1000 p A 55 1000 c- * o y :q Z j-U-'-E S' • 4 f c > V/ z i 1000 # / .:.x . .--if d : x T# - f / s 500 500 r f 1 | ± jg i / :adt x Z i # •Bti •if: o f e . 0 0 o.l 0.2 0.3 0.4 0.5 D 0.2 04 0.6 0.8 Percent Strain Percent Strain 0 2 0.4 Percent Strain (7) 18) 19)

2500 -fH-r 1000 rxxi -UJJ- # Ex # xffi dti: IW m ■Z4 m y r x g - . : :J $ # E - • s # 1 : 2000 800 m / A~rr~ :-pFf \ ::;z / S } J :i±bj: difb u I / gi XL i1 E 1500 600 1— J 'Zjx .% STRESS-STRAIN CURVES l P I f ( .te: E 1 x d f ±i±t Ett b e .xix / FOR i ; buE i 400 k :cE: / E UNCONFINED COMPRESSION TESTS to *000 'XX -dx± > / tr | | " X: tttj: ±i±i: # i S FIGURE 16 500 i f 7 r 200 y E ZE 4 eGx E j xpx EEi: > Ittf x:pb. A , — 0 w m E E 'r - . # 0 I D 02 04 0 6 0.8 10 D 02 04 06 08 10 Percent Strain Percent Strain d o ) (II) TABLE 3

TEST RESULTS - UNCONFINED COMPRESSION TESTS ( p s f ) Sample Sample Number Percent Strength (psi) Modulus Modulus of Axial (psi) Poisson*s Moisture Dry Density Compressive Deformation Deformation Lateral (psi)* Ratio Modulus of Remarks 1 14.6 115.2 850 336,000 714,000 0.46 Medium to low dry strength, fairly well cemented. 2 10.4 125.1 1370 151,000 2,800,000 0.05 High dry strength, hard at field m oisture. 3 13.2 114.6 990 247,000 1,280,000 0.19 Medium to low dry strength, fa irly well cemented. 4 9.4 122.9 2100 518,000 1,370,000 0.38 High dry strength, hard at field m oisture. 5 3.8 113.4 677 173,000 2,220,000 0.08 Medium to low dry strength, hard at field moisture. 6 5.0 135.0 990 424,000 4,550,000 0.09 Medium to high dry strength, hard at field moisture. 7 5.3 134.0 2370 542,000 4,550,000 0.12 High dry strength, hard at field m oisture. 8 10.1 123.1 2010 695,000 2,480,000 0.28 High dry strength, hard at field m oisture. 9 10.1 118.7 2820 741,000 2,480,000 0.30 High dry strength, hard at field moisture. 10 6.4 127.3 2070 426,000 5,440,000 0.08 High dry strength, hard at field moisture. 11 3.5 141.1 940 185,000 9,190,000 0.02 High dry strength, hard at field moisture o\ -x-The lateral modulus of deformatiom is the axial stress divided by o the lateral strain. 61

The specimens were cylindrical in shape, approximately twice as long as their diameters„ The axial strain was measured between the ends of the specimen while the lateral strain was measured with four dial gages, graduated to 0,0001 inches, and placed 90 degrees apart at the mid point of the specimens„ The modulus of deformation (modulus of compression) was calculated by dividing the axial stress by the lateral strain. The specimens were only loaded once and taken to failure. This loading makes it only possible to obtain the modulus of deformation and includes both elastic and plastic deformation. In practice, where a heavy structure is to have a high dead load and a small live load, such as a powerhouse, the modulus of deformation is used in designing a proper foundation. When high live loads are anticipated, for exam­ ple a bridge, the modulus of e la s tic ity should be used in design work.

As indicated in Table 3, there was a considerable variation in the compressive strengths, moduli of deforma­ tion, moistures, and dry densities in the tests. Only one conclusion can be made when analyzing the results - there was an increase in the axial modulus of deformation with an

> increase in compressive strength,

Poisson’s ratios, also tabulated in Table 3s were computed by dividing the axial modulus of deformation by the 6 2

lateral modulus„ This relationship is the ratio of lateral strain to the., longitudinal strain of the specimen. Normal­ ly, most rock materials have a Poisson's ratio that fall in

a range between 0.11, for a tu f f, and 0.38 fo r a dense mar­ ble. A ratio of 0.5 is a limiting value and indicates hy­ drostatic conditions. A typical sandstone should have a

value near 0.17; however, as indicated in Table 3, a wide range of ratios from 0.02 to 0.4-6 were calculated for the St., Peter sandstone. This v a ria tio n is due to the amount of bonding between grains and the water content of the test specimens. The Poisson's ratios given, if considered valid, indicate an-anisotropic condition in the sandstone.

Compressive strengths -of the sandstone are extremely

low for a competent rock material. The strengths obtained

from the tests ranged from 677 to 2,820 pounds per square inch. An average sandstone has a range between 6,100 to

12,000 pounds per square inch (Krynine, 1957)» The strengths obtained are not unusual when one considers the

lack of cohesion in the rock.

Triaxial Compression Tests

Triaxial compression tests have been aimed at deter­ mining the bearing capacity of the sandstone. The results

of the triaxial tests are tabulated on Mohr envelopes in P i-'

gures 17, 18, and 19. In general, the strength envelopes revealed very high angles of internal friction and a lack of 63 cohesion. Therefore5 the shear strengths displayed by the tests are mostly due to'friction, and only minor amounts, if any, due to cohesion. The shearing resistance is the property of a materi­ al to withstand loading without objectionable lateral de­ formation, In the case of the St. Peter sandstone, a force applied to the granular mass is transmitted throughout the mass' by contact pressure between.individual grains. The loading will cause the particles to rearrange and tend to slide with respect to each other. The sliding is opposed by frictional resistance between the particle surfaces and the pressure between the surfaces. In addition, the sliding is also restricted by mechanical interlocking of the particles and by a small, cohesive factor which the sandstone locally possesses.

Figure 17 shows eight triaxial compression.tests run on hand trimmed, undisturbed sandstone specimens (Corps of

Engineers, 1958). The specimens were trimmed to 2 3/4- inches in diameter and inches long, encased into a thin rubber membrane, and sealed into the tr ia x ia l machine, A vacuum was applied to the specimens for five minutes j then without releasing the vacuum, water.was allowed to flow in to the specimen from the bottom of the sample under a three foot head until the specimen was saturated. A pressure chamber was put around the samples, filled with glycerin and pres­ sure applied to the fluid to a desired level. After the 36 48 „ NORMAL STRESS KG/cm2 , UNDISTURBED TRIAXIAL COMPRESSION TESTS 6 5 chamber pressure was applied, the vacuum was released, and the vertical load was increased progressively. The load, deformation, and volume change fo r each increment was r e ­ corded until the specimen failed. A te stin g machine with a 3,000 pound capacity was used on the first two specimens (Figure 17)° The. machine was unable to cause failure until the lateral pressure was reduced to twenty pounds per square inch. Tests number 3 through 8 were tested on a 20,000 pound capacity machine in order that higher lateral pressures could be obtained.

The stress-strain curves obtained for all tests had a characteristic shape which is shown in Table 4. In all instances the specimens were allowed to rebound one or more times before rupture. To obtain the modulus of deformation, the curves were measured at three different positions: 0A, AB, and CD (Table 4), and were designated: First, Second, and Third Stages respectively. Table 4 gives the modulus of deformation for the three stages, as well as the maximum principal stress, angle of internal friction, minor princi­ pal stress, maximum percent strain, and the initial void ra­ tio s fo r each t e s t . As would be expected, the modulus of deformation increases as the chamber pressure increases. A shear failure was characteristic for all the specimens test­ ed.

The angle of shearing resistance (internal friction) is represented on the Mohr's diagrams. If a straight line 66

TABLE 4

-TTIAXIAL COMPRESSION TEST CONDITIONS- UNDISTURBED ST. PETER SANDSTONE Modulus of Deformation in Kg.cm.-2 (Kg.cm.-2 ) Intern al Test Number Angle of Stress I n itia l Void F ric tio n Major Principal Stress(Kg. cmr2) Ratio Minor Principal Stage F irs t Second Stage Stage Third S train Maximum PercentMaximum 1 690251 43.29 1.40 6600 15,000 24,000 0.46 0.445 2 67o50l 35.07 1.27 5000 8000 13,000 0.40 0.414 3 60o401 40.90 2.81 2000 5000 12,000 1.24 0.461 4 60°551 98.35 6.67 4700 9000 13,000 0.85 0.408 5 63°25' 12.50 0.70 2500 5000 8000 0.35 0.447 6 64°45' 14.06 0.70 3000 6000 10,000 0.29 0.453 7 60°151 29.96 2.11 4000 7500 12,000 0.36 0.431 8 58050* 134.10 10.50 5500 11,000 22,500 i . i i 0.445

CHARACTERISTIC SHAPE OF TRIAXIAL STRESS-STRAIN CURVES

MAJOR PRINCIPAL STRESS

First S tage

Second Stage

Third Stage 67 Is drawn tangent to 'the c irc le s a t the point where stre ss conditions caused failure, the angle this line makes with the abscissa is the angle of internal friction. In practice when several tests are conducted, a curved envelope results, which is an average of all the straight envelopes, The straight line envelope does not represent the true angle of internal friction except in the case of an individual test. The curved line is thought to be the result of ductile and brittle properties of the material, which is typical for a rock substance. This may be additional evidence to conclude that the St. Peter sandstone possesses elastic properties. The undisturbed sandstone specimens, when tested, revealed a high angle of internal friction. An average val­ ue for the curved envelope and resulting from the eight tests, was sixty degrees. Individual tests ranged from fif­ ty-nine to sixty-nine degrees. Mo cohesion was recorded for this set of specimens. The high angle of shearing resis­ tance answers the question of why the St. Peter can form high vertical faces such as is exhibited in the river b lu f f s .

V ictor Gruen Associates (1 961) also ran. several tri- axial tests on undisturbed sandstone^ however, no informa­ tion was.available on the test methods used. Presumably, their methods were similar to those of the Corps of Engi­ neers, since they obtained similar test results. 68

Three sets of test specimens were obtained by coring methods for the site examination of the new Dayton's Depart­ ment Store in St. Paul. In all instances the tests were run on undisturbed and undrained specimens. Pore pressures were measured on only one specimen; however, these test data were not available. The strength envelopes for the three sets of tests also revealed high angles of internal friction, ranging from forty-eight to sixty degrees. These values were lower than those obtained by the Corps of Engineers, but they are still higher than for an average sandstone. A small cohesion fac­ tor was recorded for one specimen (800 pounds per square inch). Results of these triaxial tests are given in Figure

1 8 , Triaxial compression tests run on disturbed St.

Peter sand were prepared by disaggregating chunks of sand­ stone by washing the m aterial through a 35 mesh sieve. The sand was then dried and packed into rubber -cylinders. . After packing the sand as densely as possible, the cylinders were sealed and exposed to a vacuum of one atmosphere. A void ratio of 0.4% was obtained, and the first set of tests was conducted at this value. A second set of tests was run at a looser state, with a void ratio of O.69, Thiel (1935, P, 589) reported an average void ratio of 0.394 for ten samples of undisturbed St. Peter sandstone from Minnesota. SHEAR STRESS KG/cm2 ircle C 8 6 .. . 24 . . . . 16 8 0 Mo* Note: 7 6 2 5 4 3 1 O ML TES KG/cm2 STRESS NORMAL Strength Envelope Content atural N pe o Stai - in tra S of Speed Water 70132.81 113 17.0 12.6 17.9 17.9 16.0 17.0 12.6 8 6 4 2 8 56 48 0 4 32 24 16 8 0 ET 65ft. DEPTH % Density Natural Envelope Strength 113 115 114 1 2.11 117 114 1 3.51 117 Dry PCF TRIAXIAL COMPRESSION TESTS - DISTURBED - TESTS COMPRESSION TRIAXIAL Pressure 0.5/o/minute. Chamber KG/cm2 2.82 1.41 0 14.80 0 OM L TES Gc2; KG/cm2 STRESS NORMAL Deviator Deviator Maximum S tress tress S 28.00 57.30 66 2.1 16.60 19.60 12.75 KG/cm2 3.07 IUE 18 FIGURE DEPTH 75 75 ft. DEPTH % Maximum -[Strength [[Envelope train S 2.3 0.9 2.6 2.3 1.2 1.0 1 24 16 8 OML TES KG/cmZ STRESS NORMAL aturation S Degree 100 100 100 79 79 94 94 % of DEPTH 70 ft. 70 DEPTH rw snsoe ' sandstone Brown ■ 1 ■■ .■ ry Sandstone Gray Unconfined y r, lit p s ert, V by ape f l d ile fa Samples Compression Remarks it it , ■ 7 0

Therefores attempts to recompact the sand to the void ratio of undisturbed sandstone were not attained.

The modulus of deformation for the densely compacted sand was as high as the modulus fo r the undisturbed sand­ stone. This was found to be true when the major principal stress did not exceed twice the lateral stress for the re - compacted sandstone. A rapid increase in strain made it im­ possible to measure the modulus of deformation except in the initial stage of the test. The actual failure of the speci­ mens was characterized by bulging around the periphery in ­ stead of a true shear failure. The angle of internal friction for the disturbed sand was thirty-two degrees in the loose state and forty-one degrees for the dense state. Results of these tests are shown on Mohr's diagrams in Figure 19, along with the condi­ tions under which the specimens were run.

Plate Bearing Tests

Field plate bearing tests have been, conducted on the sandstone in an attempt to determine the ultimate bearing strength of the material. Nine tests were conducted by the

Corps of Engineers (19583 p. 6)5 but none were carried to failure because of the excessive leads needed to induce failure. The maximum load obtained during testing was 200 pounds per square inch (l4.4 tons per square foot). As in­ dicated from the load deflection curves in Figure 20s the

Z Shearing Stress KG/cm2 Shearing Stress EOPCE LOE AD (Void Ratio-0.69) SAND LOOSE RECOMPACTED EOPCE DNE AD Vi Ratio-0.47) (Void SAND DENSERECOMPACTED 1 16 12 8 oml tes KG/cm2 Stress Normal oml tes KG/cm2 Stress Normal 12 16 20 20 16 12 TRI TRI COMPRESSIONAXIAL TESTS DSUBD SANDSTONE)' (DISTURBED IUE 19 FIGURE H

D ensity IRD IRD RUNS S p e c ific G rav ity D D 6 TH SECON F ines SECON Mechanical Analysis G lass (SP) White (SP) Light fin e sand. sand. L aboratory Yellow fine Elev, REBO 30 inch tonsdiameter per bearingsquare plate foot. was used with a maximum load of lU.U T est FIELD FIELD PLATE BEARING TESTS PRESSURE IN POUNDS PER SQUARE INCH FIRST RUN ; lu

— =r o =r m o 000 010 030 0£0' OfrO' 090" 090' 0/0 080' 060 7 3 deflection at the maximum load varied between .02 and .10 inches5 and with almost complete rebound after removal of the load. The tests were conducted in place on undisturbed sandstone a t the S t. Anthony F a lls Lower Lock. There was no.information available regarding the ■ loading rates of the plate bearing tests. The deflection was instantaneous as the sandstone was being loaded. Bend­ ing of the plates was noticed during testing, and results may contain errors. Results of these tests indicate elastic settlement will result upon loading the sandstone. The settlement should take place entirely during construction and initial loading of a structure unless a high live load is anticipat­ ed. Safe allowable bearing values should be calculated for any structure to eliminate excessive displacement or detri­ mental settlement.

Consolidation and Settlem ent Approximate settlement estimates, for any structure, are normally based on consolidation tests. Settlement is caused by the addition of loads to a foundation material, removal of lateral support, fluctuation of the water table, crushing of grains, or shear failure. Assuming the St.

Peter sandstone has been exposed to extreme compactive forces, the material should be considered incompressible for all practical purposes except for its elastic properties. 7 4 Unless rearrangement of the sand grains was to occur, for example by vibratory action,- densification of the material would not result. This is because weight added to the sand­ stone is transmitted from grain to grain, and the grains are interlocked to such a degree that they will not become dis­ aggregated under normal loading. Under unconfined water table conditions, settlement of a stru c tu re located on the sandstone and caused by a fluctuation in the water table should be negligible. This is providing the sand grains remain undisturbed, and the ap­ plied stresses are transmitted from grain to grain. If water comes in contact with the foundation of a structure, hydrostatic pressure beneath the structure will cause a buoyant effect. This effect exists when .the pore water pressure (neutral pressure) is high enough to partially car­ ry the applied load. If the hydrostatic pressure were to drop, either naturally or by artificial means, the buoyancy in the dewatered area would disappear. The load, after de­ watering, will be supported by the skeletal structure of the sandstone. ' In such cases a foundation in contact with a fluctuating water table may experience differential settle­ ment .

Only two known consolidation tests have been con­ ducted on the St. Peter sandstone (Gruen, I 96I, p. 9)» The tests performed were made on undisturbed samples and were true one dimensional consolidation tests, in that the 7 5 samples were adequately confined laterally. One of the two tests was partially confined at the surface and therefore represents a combination consolidation and plate bearing test (Figure 21).

The pressure-void ratio curves are fairly linear, which suggests a very small but uniform densification of thee sandstone during loading. Apparently, changes in the void ratios of the specimens were instantaneous upon loading.

In computing the settlement from the tests, loads varying from fifteen to twenty-five tons per square foot produced the same total displacement. The settlement analy­ sis of the two tests revealed a total computed settlement of l/lO of a foot with a load of twenty-five tons per square foot. The rebound portion of the curves indicates that most of the settlement is permanent, and only minor amounts are elastic. The consolidation tests are not in agreement with the field plate bearing tests in regard to permanent settle­ ment or the amount of settlem ent. This may be caused by the consolidation test specimens not being adequately confined and thereby causing some grain rearrangement upon loading. This would effect the difference in void ratios and increase the total settlement of the consolidation test specimens.

Penetration Resistance Tests

The standard penetration test is basically a shear test and can give a quick, inexpensive estimate of the Legend Natural Natural I n i t i a l , Remarks , Water Dry Void Content fo Density Ratio CONSOLIDATION TESTS O- 16 114 pcf .420 Totally Confined. FIGURE 21 5 124 pcf .377 Confined at sides and Oi -A- bottom, 80/o Confined a t top. 7 7 stability of a foundation material. Blow counts5 measured from penetration tests5 according to Terzaghi (1948), can be used to determine the permissible soil pressures for foot­ ings on sand. A high blow count will generally indicate good foundation conditions. The Minnesota Highway Department has conducted num­ erous penetration tests on the St. Peter (Ford, 1964). The tests were made by driving a split tube sampler into the sandstone with a 140 pound weight, dropped 30 inches. With

100 blows penetration was nowhere greater than 0.2 feet, which was found to be typical for the sandstone throughout the Metropolitan area. Rate of penetration was also found to be the same for all hardnesses of sandstone. For this reason the test cannot be used as a measure for hardness or degree of friability of the sandstone.

The values obtained from the penetration tests are relative, and, normally, resistances increase with depth.

Assuming penetration rates for the sandstone to be 0.2 with 100 blows, this would indicate that the sandstone is an ex­ cellent foundation material with a high bearing capacity.

Ford (1964) has devised a modified penetration test for which he feels will be useful in measuring the hardness of the sandstone. Friability determinations are very impor­ tant when estimating costs of excavations. Instead of a split, tube sampler, which is normally used for the test.

Ford has used a tool steel probe attached to the end of the 7 8

drilling tools„ The probe is one foot long, one-half inch in diameter, and has a blunt working end. The standard 100 blows are made with a 1%0 pound weight and dropped 30 inches. The test has only been used on hard sandstone, and the re­

sults are similar to the standard penetration tests.

Bearing Capacities

In reviewing the engineering properties of the St. Peter sandstone, a high bearing capacity of the material is

obvious. Since so few shear strength tests have been con­ ducted on the sandstone, no conclusions can be made as to

the consistency of the rock in its load carrying power.

Adequate support can be safely estimated for any proposed structure by a site examination, accompanied by an accepta­ ble testing program. Bearing capacity estimates are only- valid if the material remains undisturbed, confined later­ ally, and the groundwater conditions can be controlled at the s ite . The C ity of St. P aul's Building Code recommends a

safe bearing value of fifteen tons per square foot for

structures being built on the St. Peter sandstone. The Min­ neapolis Code has no specifications; however, the same value of fifteen tons is used when a problem arises in the City.

The value of fifteen tons per square foot was established be­ cause at the time no methods had been developed to measure the ultimate or safe bearing power of the sandstone. The 79 existing figure in the code is antiquated and should be con­ sidered a conservative value for most conditions„ - Plate bearing tests used by the Corps of Engineers

(1958S p. 6)5 revealed bearing capacities for the sandstone to be l4„4 tons per square foot, with no signs of failure„

In design of their locks an allowable safe bearing pressure of ten tons per square foot was used„ This very conserva­ tive estimate was based on the available data at the time.

Victor Gruen Associates (1962, p. 11.) computed a l­ lowable bearing pressures of twenty-five tons per square foot, determined from triaxial compression test results. This figure is also conservative but is probably the highest value that can be obtained without the use of an ultra-high load testing machine and special made triaxial loading cells. With the proper testing equipment5 ultimate bearing capacities in excess of 100 tons per square foot can be ex­ pected. The safe bearing capacity of a material is the ul­ timate bearing capacity divided by a safety factor of two or three. In the field the bearing capacities will increase with depth below the undisturbed surface, which is due to increasing confining pressures. In placing extremely high static loads on the sandstone, it may be advantageous to use deep footings to insure for a sound foundation. FOUNDATION USE OF THE ST. PETER SANDSTONE

Potential Problems

The St. Peter sandstone has been used as a founda­ tio n m aterial for bridges5 dams, locks, and buildings. The high bearing capacity of the rock has allowed for excellent foundation conditions. Depending on the nature of the ma­ terial at a building site, local variations in the bearing power can be expected. Therefore, as much geologic informa tion as possible should be collected from local sources prior to construction. The Minnesota Geological Survey has compiled borings and well logs for the entire Twin City area, and the records are available to the public. These records will help an investigator in the initial stages of exploration but will not provide sufficient detail to elim­ inate the necessity of a site examination.

Preliminary selection of a foundation type to be used on the St. Peter can be intelligently determined when the properties, of the sandstone are known at a particular site. The high angle of internal friction and the limited settlement of the sandstone suggest spread footings for the foundation. From experience, adverse and v ariab le condi­ tions in the sandstone may warrant the usecf other types.

The adverse conditions most commonly found are summarized below:

8 0 8 1

1) The sandstone can be expected to be so ft and unconsoli­ dated where the Platteville limestone cap has been nat­

urally removed„ 2) Freeze-thaw conditions in saturated sandstone near the surface may cause detrimental effects to a foundation.

3) When encountered, open fissures in the rock may allow

large quantities of groundwater to flow through the

openings, thus weakening a foundation. 4) Vibratory action, such as driving piles, may cause rear­ rangement of the sand grains and result in a loss of

. strength to the material. 5) Submerged and irre g u la r surfaces of the sandstone in the

riv e r channels are common. This is due to the numerous plunge pools and pot holes. Difficulties may arise in

locating bridge.piers on sound rock. 6) Buried river channels in the sandstone beneath a pro­

posed building site will cause difficulties in locating sound rock for a foundation. Water problems can also be expected in the channels.

7) The sandstone may be found to be saturated during exca­ vation at a building site. Thus, adequate dewatering

processes have to be made to prevent damage by erosion when flowing water is encountered.

8) Near river channels the sandstone may have lost its con­

fining pressure. Thus, heavy applied loads in such

areas may result in shear failure to the sandstone. 8 2

9 ) Natural caves In the sandstone are uncommonj however, sites where heavy structures are to be built should be

■ thoroughly explored at depth to assure that such hazards

do not exist. 10) Joints are potential planes of weakness, especially

those that are filled with clay materials„ Such struc­

tures should be avoided or properly secured in advance

of. construction. 11) Once the sandstone has been disturbed, the sand will not . rebind by normal compaction methods to form a hard work­

ing surface for the placement of a foundation. There­

fore, it is necessary to protect a freshly exposed

excavation until the foundation is poured.

12) Grouting the sandstone to give added strength has not

been successful because of the low permeability in the

rock. 13) High penetration resistance restricts the use of fric­

tion piles that would have to be driven into the sand­ stone.

Various types of foundations have been used on the

St. Peter sandstone in the metropolitan area. The type of foundation to be used varies with the type of structure, the structure’s use, and the physical conditions of the sand­ stone at the site. The types of structures and their foun­ dations are considered separately below. 8 3 Bridges

The majority of bridges crossing the Mississippi

River in the study area are in part supported by the St. Peter sandstone. Normally, where possible, a concrete pier is poured directly on a hard, fresh surface of the sand­ stone. The piers are constructed by first placing caissons. This is accomplished by first driving sheet piling into the riv e r gravels and sands beneath the riv e r and around the pier area. The enclosed area is dewatered, and then the . river alluvium is excavated to a depth where undisturbed sandstone is encountered. A reinforced concrete pier is then poured, after which the base of the pier is protected from scour with rip rap. A second type of foundation used for bridges is a spread foot foundation. The spread foot foundation is placed on the unconsolidated fluvial materials and is usual­ ly supported by pilings driven to the solid rock below. The main problem encountered in the placement of bridge piers is the lack of a suitable sandstone or the ab­ sence of sandstone at a reasonable depth. The river chan­ nel, especially between St. Anthony Falls and Fort Smelling, is known for its many plunge pools and pot holes, created by the retreat of St."Anthony Falls. For example, during the site examination of the new Tenth Avenue Bridge, the State

Highway Department (Ford, 1964) found a fo rty foot d i f f e r ­ ence in elevation of the sandstone surface across the 8 4

Mississippi River. Borings proved that two large plunge popls existed on the center line of the bridge, and, there­ fore, the piers had to be located accordingly. In another instance during construction of the Cedar Avenue Bridge, the northeast p ie r had to be placed fo rty -fiv e fe e t below the river bed, because soft (unconsolidated) sandstone was encountered. The southeast pier of the same bridge.was placed on wooden piles driven twenty feet below the water level, because no undisturbed sandstone was encountered while driving sheet piling to a depth of 105 feet. A simi­ lar situation occurred during construction of the Great Northern Stone Arch Bridge. The piers for the bridge have had to be reinforced several times due to settlem ent of the structure. The situation was not realized when the bridge' was built, and suitable pilings were not provided for the unconsolidated materials on which the piers were built

(Schwartz, 1939, pp. 23-24). The total vertical load on the foundation of a bridge is the sum of the vertical live load plus the dead load. Live loads such as flowing water, drifting ice, wind, and traffic on the bridge will not cause settlement. This- - is providing the bridge piers are seated on sound sandstone or are well seated on the river alluvium. Moderate elastic and plastic settlement can be expected during the construc­ tion of the bridge. . : 8 5 Locks and Dams Mat or raft type foundations were used in the con­ struction of the three locks and two dams in the Twin City area. For all the structures a reinforced concrete mono­ lithic slab was poured on a fresh, excavated surface of un­ disturbed sandstone. During construction of the lower lock and dam at St,

Anthony Falls, the placing of forms and steel disturbed sev­ eral inches of material on the excavated surface. Consider­ able difficulty was encountered in removing the loose sand beneath the heavy steel mats„ Because of this, recommenda­ tions were made by the Corps of Engineers (1958, p, 8) for the construction of the upper lock at St, Anthony Falls, It was suggested that excavation be carried to grade during the initial operation, and a six inch thick construction slab be placed over the entire area immediately. This would provide a firm working surface, which would not be subject to ero­ sion by traffic during construction operations,

Dewatering the sandstone In the vicinity of the• locks offered additional problems. It was felt that because of the presence of uncemented bedding planes in the sand­ stone, they would blow and become open fissu re s i f dewater­ ing were attempted by open sump methods. Therefore, for construction of the upper locks it was recommended that well points be used and spaced at three foot centers. Deep wells 8 6 would not be satisfactory because most of the water in the sandstone was derived from the river. Foundation difficulties were encountered during the

construction of the Second Twin City Lock (Schwartz, 1 9 3 9 a p. 29)o During excavation, a leak developed in the sand­ stone near the lower end of the middle wall and a short dis­ tance above the lower gate. Provisions to prevent erosion, under the middle wall, were made by sealing both sides of the wall with a steel diaphram. After completion of the lock,, two months later, water again was noticed leaking un­ der the same wall. The leak was finally secured after rein­ forcement of the steel diaphram.

Coffer dams had to be constructed to permit excava- . tio n fo r the locks. Because- a llu v ia l f i l l mixed with lim e­ stone slabs and glacial boulders underlie the location of the dams, sheet p ilin g could not be driven to a s u ffic ie n t depth to secure cut off of subsurface flow. At the Twin City Locks no difficulty was encountered with leakage be­ cause of a ten to fifteen foot thick blanket of river silt which accumulated on the upstream side of the coffer dam.

The silt effectively sealed the area. For construction of the St. Anthony Locks, well points along the center line of the dam were recommended to elim inate seepage in to the con­ struction area.

Bearing values of ten tons per square foot were used in design of the locks, enough to insure safety of loading 8 7 failure. A stress zone (pressure bulb) of up to 300 feet below the foundation of the locks was c a lc u la te d . This zone would project through the St. Peter, Prairie du Chien forma­ tions, and well into the Jordan sandstone.

Consolidation and plate bearing tests may give an estim ate of the amount of settlem ent to a n tic ip a te to the structures. Heavy structures, such as locks and dams, will not normally experience high live loads, and most of the settlement will be experienced during construction.

Buildings

Whenever possible, the St. Peter sandstone is used as a foundation base, because of its high bearing strength and small settlement. Reinforced concrete spread founda­ tions are normally used where the sandstone is undisturbed and is free from running water. Where the sandstone is not saturated, the rock acts as an excellent underdrainage for a foundation. In addition, the sandstone can be cheaply exca­ vated at costs no higher than to remove unconsolidated soil m aterials.

Concrete pier or caisson foundations also have been used to give adequate support to multilevel buildings. Oc­ casionally, where structures are to be built on the sand­ stone, considerable amounts of disturbed sand or alluvium overlie the undisturbed material. Under these conditions it has been necessary to drill and place caissons at a O

88 sufficient depth in the sandstone to insure adequate con­ finement for the foundation. Several foundations have been built on steeply dip­ ping surfaces of the formation. Uneven surfaces of the sandstone can usually be attributed to as sides of buried channels. This offers additional problems of not only re­ moving so ft sand and alluvium, but water in tru sio n may be considerable. The buried channels are well known water courses. Under these conditions, caisson casing has to be extended through the saturated zones and into hard sand­ stone. The casing will relieve the inflow of water from the unconsolidated materialsj however, if the sandstone is also saturated, dewatering operations will probably be needed be­ fore concrete can be placed. Pile foundations in the sandstone have been consid­ ered but have never been used. The sandstone has too high a penetration resistance for piles to be driven. If struc­ tures are not to be built directly on the sandstone, piles can sometimes be driven through unconsolidated materials to refusal in the St. Peter. This may offer additional prob­ lems, because the unconsolidated materials - in.the Twin City area are well known for the large block of limestone and glacial boulders. Many heavy structures, especially in the downtown portion of St. Paul, are supported on footings over compressible soils. Vibration from trying to drive piles to 8 9 the sandstone and over an extended length of time may cause differential settlement to nearby structures„

Jetting piles into place has not been attempted, be­ cause the jetting action may cause excessive erosion by over enlargement or loss of the hole. The success of jetting piles into the sandstone depends on the hardness and fria­ bility of the rock at the site. In the City of St.' Paul, where buildings are to be constructed on a thin layer of Platteville limestone or Glenw.ood shale, additional support to the foundation is re­ quired by the- City Building Department. ' This is because the Glenwood shale is not capable of supporting heavy applied loads without excessive settlement. Ford (1964) stated that additional support to such structures is obtained by driving piles through the shale to refusal in the sandstone below.

The area of shear influence below a structure, built on the sandstone, can be safely estim ated at one and one- half times the width of the structure. This rule of thumb can be used if the material is considered homogeneous, con­ fined, and the applied load is a dead load. High live loads will vary the depth of shear influence. If the depth is calculated to be deeper than the thickness of the St. Peter, then the Prairie du Chien and Jordan formations will absorb a part of the shearing force of the overlying structure.

Shear strengths-of the underlying formations should be as great, if not greater, than the St. Peter. 9 0

FIGURE 22 Excavation in the St. Peter sand­ stone for the Dayton's Deport­ ment Store in St. Paul. Numer­ ous tunnels are exposed near the Glenwood- St. Paul contact. 9 1 The First National Bank Building in St. Paul is a typical example of a building built using the St. Peter as a foundation (Schwartz, 1936, p. 97)° The depth to sandstone was not over thirty feet, and the designers planned to place the structure on eighty-four piers placed on the sandstone. To locate a suitable location to place the piers, some of them had to be sunk through fifty feet of soft sandstone to a depth of eighty feet, where hard sandstone, was found and was suitable to place the piers. This particular building site was located on a buried river channel, which is the reason for the soft sandstone.

Roads

The St. Peter sandstone, when properly drained, is , an excellent sub-base for roads and highways. According to

Ford (1964), after the sandstone has been removed to road grade, an a d d itio n al foot is removed and replaced with a properly graded subgrade material. No road failures in the past have been recorded 'due to settlement, shear failures, or freeze-thaw conditions in the 'sandstone.

The sandstone itself is not considered a good con­ struction material due to its well sorted character and lack of fines. Without a fine fraction In the sand, the material will not bind or compact to give a suitable building m aterial. TUNNELING IN THE ST„ PETER SANDSTONE

Problems in Tunneling;

The'reader, at this,point, should be well aware of the geologic and engineering characteristics of the St.

Peter sandstone„ Obviously, because of these characteris­ tics and the distribution of the formation, the sandstone is an excellent medium in which to construct sewer and utility tu n n els. Cost of tunnel excavation is the main reason for placing tunnels in the St. Peter sandstone rather than the overlying glacial drift. Driving tunnels in the drift costs over $600.00 per footj tunneling in the St. Peter Formation averages around $250.00 per foot.

The St. Peter sandstone is far from being trouble free in regard to tunnel construction. However, the only exploration or testing performed prior to construction is the drilling of alignment holes along the proposed routes of the tunnels.

The best tunneling, conditions are found where the formation is free from water and where the sandstone is com­ petent and capped with the Platteville limestone. To some degree, this is normally the case; however, problems do ex­ i s t and are summarized below:

1) High jointed, friable, or saturated sandstone may create 92 9 3 heavy ground conditions. The type of tunnel lining is directly related to the competency of the rook. Steel arch supports5 liner plates, or skin tight lining may be required in caving ground. Where the sandstone is com­

petent, no lining is necessary. 2) Jointing problems can be placed into two categories:

water bearing joints which may be enlarged when encount­

ered unless water flow is immediately arrested, and joints that dip at high angles and are obscured from sight. Clay filled joints can also offer hazards be­

cause of a lack of friction between surfaces.

3) A high water table may necessitate the use of dewatering w ells, underdrains or w ell points. Heavy pumping may

cause grain rearrangement in the rock structure and re­

duce the competency of the sandstone.

4) Soft, and caving ground is usually encountered near bur­

ied channels. The sandstone can also be expected to be

saturated in these locations, which will require dewater ing. When these-conditions exist, costly mining methods

are required and will depend on the type of channel

filling and the yield of water.

5) The ease of excavation in the sandstone is a day to day

problem, and rate of tunnel advancement depends on the

friability and degree of cementing of the sandstone.

Under ideal conditions a tunnel face can be advanced

from four to six feet in an hour by hydraulic mining 9 4

methodsj however, when cemented or hard sandstone is en­ countered, jack hammers and blasting are employed, which

is time consuming and costly. 6) Excessive arching or over break, caused by the natural

balancing of shear stresses around a tunnel opening, may require costly and elaborate tunnel supports.

7) Near the banks of buried or exposed river channels, nat­ ural sandstone caves may exist. These may be a hin­

drance, or they may be beneficial in which to dispose of tunnel muck.

8) A sudden rise of the water table over the invert level

will cause flooding of the pore spaces in the sandstone.

The added weight of the water may overcome the shear strength of the rock and result in failure. This situ­

ation is most common when the sandstone is being a rti­

ficially dewatered and then for natural or unnatural reasons the area is rewatered.

9) Buried channels th at have cut and removed large portions of sandstone are found in many localities. Unless the

channels are located prior to their encounter by a tun­ nel, additional costs will be required for the special mining methods needed.

10) Due to the depth of the St. Peter Formation below ground,

it is most desirable to locate the invert as close to

the Glenwood-St.' Peter contact as possible. The 95 sandstone is known to be soft near the top of the forma- tion* and therefore, caving problems are not uncommon.

Tunnel Description Tunnels driven in the St. Peter sandstone are usual­ ly located w ithin a few fe e t below the Glenwood shalej how­ ever, inverts have varied from 20 to 200 feet below the ground surface, depending on the surface topography. Most of the older tunnels are rectangular in shape, with their backs being one-third to one-half higher than their widths.

The more recent storm sewers have a circular cross section with diameters up to fourteen feet. The sewer tunnels are usually lined" with reinforced cast-in-place concrete for maintaining proper tunnel grade (one foot per 1,000 feet), and to insure tunnel stability. In contrast, many of the older tunnels beneath St. Paul have stood for over fifty years unlined and with no support.

In the metropolitan area over 145 miles of sewer tunnels alone have been excavated. The City of Minneapolis has 65 miles of tunnels, of which 33 miles are sanitary sew­ ers, averaging four by six feetj 17.6 miles of interceptor sewers up to th irte e n feet in diam eter; and 15-4 miles of storm sewers, averaging four by eight feet. The University of Minnesota in Minneapolis has five miles of tunnels under the campus, which are used for heating and sewage disposal. v The tunnels under the campus average four by six feet and 9 6

100 fe e t below the ground surface. The City of St, Paul has a total of seventy-five miles of tunnels, five miles of which belonged to the street car company but have been aban­

doned. The deepest tunnel in the Twin City area, located at

Griggs and Rondo Streets in St. Paul, is 12 feet in diameter and 205 feet below the ground surface. Built in 1935-1936, it is the main trunk sewer for the Minneapolis-St. Paul San­

itary District. Tunnels are either driven in from an existing river bluff where the sandstone outcrops, or vertical shafts are

driven down from the surface to intersect the tunnel„ The

Vertical shafts are used to remove tunnel muck, give access and ventilation to the tunnels, and bring in the - needed

utilities. Small diameter alignment holes are spaced 500 to 2,000 feet apart along the center line of the tunnel and are sometimes used to bring in utilities and., pipes to pump in concrete for the tunnel lining.

Arching, and S tress D istrib u tio n

Rib supports and lagging in the tunnels are mainly

used to limit excessive overbreak in the tunnel back. When an opening is made in the sandstone, there is a tendency for

the adjacent rock to move into the tunnel. The displacement of rock is caused by the incompetency of the sandstone to withstand the overlying weight above the opening. Shear 9 7 stresses are built up in the rock in an attempt to redistri­ bute the overburden weight to the sides of the tunnel. Re­ sistance to this arching effect is provided by the friction and the shear strength of the sandstone. The tunnel sup­ ports need only be designed to withstand the weight of the calculated overbreak and not the entire rock load (thickness of overburden).

The magnitude of arching stresses that will exist around an opening in the sandstone is one of the main prob­ lems in designing a tunnel. I t is inadvisable to determine the actual stresses by assuming data without the verifica­ tio n of fie ld data. However, large sums of money can be saved in lining the tunnel if the stresses can be calculated quantitatively. "Rules of thumb" for lining tunnels, for example, one inch of concrete lining per foot of tunnel di­ ameter, can be a costly overdesign aspect. To illustrate how stre sse s and rock loads can be computed fo r an under­ ground opening, the following discussion is offered.

Using Terzaghi's formulas (Proctor and White, 1958) 3 the following exerted pressures can be calculated for a dense sand:

Hp(max) = K(B * Hfc)

TT Where: p(max) ~ Load thickness carried by roof supports K = Constant, depends on the degree of compactness. The St. Peter has a value of .60 9 8 (B + Kj.) - Width plus height ©f tunnel ©r two ■ times the diameter for a.circular , cross section. For this ease a. diameter of 13 feet is used.

Therefore: ^pfmax) = 0.06(2 x 13 ft,) Hp(max) = 15.6 feet of sandstone load

And the weight of the overbreak = Vertical load = (sandstone density) x (load thickness)

V Vertical load =150 psf x 15.6 ft.

Vertical load & 2,340 psf

And the sand pressure on the sides of the tunnel (horizontal thrust) = ph = 0.30 w (0.5Ht + Hp)

Where: p^ = H orizontal th ru st 0.30 = Constant w = psf of sandstone (150 psf) = Height of tunnel (l2 feet) Hp s Load Thickness carried, by supports

Therefore: '

Ph = 0.30 x (150 p sf) X (12/2 + 15.6 ft.) P'n = 1,080 psf For added protection a safety factor of two should be c a lc u la te d : Thus s Vertical Load = 4,600 psf or 32 psi

Horizontal Thrust ="2,100 psf or 15 psi

For this particular case, the tunnel lining should be designed to withstand the above given vertical load and horizontal thrust unless the -tunnel is to have internal pressures. The safety factor will allow for any increase of

^p(max) ^ water level rises and saturates the load sup-* ported by the tunnel lining. These values are rough 99

\ estimates and cannot be considered valid if the sandstone is. not considered massive, iso tro p ic , and homogeneous with r e s ­ pect to its elastic properties„ Other criteria that have to be assumed are: l) stress distribution is uniform; 2) there are no residual stresses in the sandstone; 3) vertical pres­ sure on the tunnel back is independent of the tunnel depth below ground; 4) the tunnel lining has to only support the mass of the overbreak; and 5) the sandstone has not been disturbed or is highly jointed.

Tunnel Supports

The final support system should be designed to sup­ port the total load of the calculated overbreak. This is the most adverse load the tunnel supports will be subjected to, unless large, joint derived blocks of sandstone are en­ countered. Therefore, dimensions and spacing of the sup­ ports depend on the anticipated rock loads.

If the sandstone is very competent and will stand without sloughing, the tunnels can be left unsupported.

Normally, steel arch supports are used if there is any sign of overbreak. The supports that have been used are five inch wide steel channels that are formed into a semi-circle with a radium equal to that of the tunnel. Foot plates, welded to the end of the channels, are placed in notches cut in the tunnel walls. Weight from the back solidly seats the footings of the arch against the sandstone walls and 100 distributes the weight into and away from the tunnel open­ ing, As a general rule, the supports are spaced on six foot centers, and lagging is placed between the ribs. Steel liner plates have been used in the tunnels when excessive arching is anticipated or where the sandstone is highly jointed. The plates used have been made of four­

teen gauge corrugated steel. The plates are bolted together to form a solid semi-circle, the size of half the diameter of the tunnel. A sill cut in the wall of the tunnel sup­ ports the plates„ "Skin tight" lining also is used under heavy ground conditions. This type of lining is where the upper half of

the tunnel is lined with timber. The timbers are usually planks, three inches thick and eight inches wide. Their lengths depend on the tunnel's dimensions. The planks are

joined together to form a half decagon, and the ends rest on a sill cut into the tunnel wall.

The most expensive type of lining, but the strong­ est, that has been used to support the sandstone is steel

ring beams. The ring beams are six inch wide "l" beams that

completely circumscribe the tunnel opening. Spacing of the

beams has varied from three to ten feet depending on the competency of the rock. Hard wood lagging or spiling is

placed between the. beams. If the tunnel is horse shoe

shaped, s te e l ribs and in v ert s tru ts are normally employed in heavy ground. 1 0 1

FIGURE 23. Placing lagging on the back of a sandstone tunnel be­ tween steel arch supports. Nozzle in foreground is used to cut the sandstone. 1 0 2

FIGURE 2 4. Unsupported tunnel driven in competent sand stone. (Rondo St. storm sewer, St. Paul)

FIGURE 25. Full ring beam supports used to support heavy ground. (Rondo St. storm sewer, St. Paul) 1 0 3 When the excavation of a tunnel has been completed, a concrete lining is poured in place over the temporary steel or wood lining. In the larger and more recent tun­ nels, 100 foot lengths are poured at one time with the use of a track mounted form. The concrete is mixed on the ground surface and pumped through pipes placed on alignment holes or vertical shafts that intersect the tunnel at approx-

' , imate 1,600 foot intervals. When a tunnel is being driven through heavy ground,„concrete is usually poured as soon as possible after excavation.- Under these conditions, only ten to twenty foot spans are poured at one time. Tunnel support requirements can be determined by.a rock mechanics instrumentation program. The investigation should include the time-strain loading properties of the rock and an evaluation -of the mode of loading. The mode is the process through which rock strain is translated into load on the support system.

Deformation measurements are helpful in detecting convergence, compression, floor heave, and other unwanted indications of tunnel failure. Recording this data over a period of time would forewarn against failure long before visual evidence is obvious. Advance warning would permit the placing of adequate support in time. Instruments, used in the tunnels, by no means should replace the miner's in­ sight and understanding of rock conditions but siiould sup­ plement his skills. io4 Various types of load cells, extensionometers, and strain gages have been developed to give very good strain

information in tunnels and eliminate the educated guesses, now used in mining. In situ testing also eliminates obtain­ ing undisturbed samples and laboratory testing which takes time, and results from the test may be too late to be of any good in the field. Such in situ load measurements will de­ termines 1) how the load varies with time and face advance­ ments 2 ) quality of constructions 3 ) changes in load on installed supports as the face approaches zones of weakness. Perhaps the greatest waste of today’s construction dollar is over design of underground structures and sup­ ports. Underdesign can be equally as bad because of person­ a l in ju ry and co stly delays.

Mining Methods

Until 1934, conventional mining methods were used to excavate tunnels in the St. Peter sandstone. In 1934, hy- draulic mining was adopted in order to expedite and cut cost of mining. This method advances tunnels by undercutting the

Sandstone with a stream of water under a pressure of 300 pounds per square inch, forced through a 3/ l 6 inch nozzel.

The sandstone is broken into small particles, partly by di­

rect impact of the je t and p a rtly by undercutting, which forces the tunnel face to cave. 1 0 5

FIGURE 26. Hydraulic mining the St. Peter sandstone. Operator is undercutting the face which will cause it to cave into the tunnel. 106

When the jet is moved slowly across the face of the tunnel, a two to six inch wide and three to four foot deep cut is made in the rook, unless the material is quite hard„

The face of the tunnel is first cut into several rectangles with the water jet„ Undercutting the lower section of the face causes' the hanging block to cave. Once caved, the blocks are broken up into a fine uniform sand by the same high pressure jets. Water supplied through a two inch wash hose flushes the loose sand from the face to a small slurry pump, maintained in the tunnel floor a few feet back from the face„ From the sump the slurry is pumped out of the tu n n el, , A crew of four men is capable of handling the mining operation. Two men direct high pressure jets, a. third man directs the two inch wash hose, and the fourth man tends the pumps„ Under good conditions a. crew can advance a face from four to six feet in an hour. An adaptation of the hydraulic mining method is the hydraulicing machine called the "jet" or "jet jumbo," The machine is track mounted and advances a tunnel by the r o ta ­ tion of a cutting bar, rotating parallel to the face. The bar is about the same length as the tunnel diameter and ro­ tates at two revolutions per minute by a two horse power compressed air motor. Water is discharged at pressures be­ tween 300 and 350 pounds per square inch through holes in the cutting bar at three inch intervals. The rotating bar 1 0 7 continually cuts and disintegrates the sandstone as the face is advanced„ The machine can usually advance a face faster than the conventional hydraulic method. The■"mole” or tunnel boring machine also has been used to excavate tunnels in the Twin City area. The boring machines that have been used were hydraulically operated fan type cutters with rotating blades. The machines are usually mounted on four jacks secured to the floor and back of the tunnel. Four blades with fifty cutting teeth, on each blade, rotate at approximately twenty revolutions per minute and have cut tunnels up to fourteen feet in diameter. The cost of operating the "mole” is comparable to that of the hydraulic mining method. The ”mole" is not adaptable to all mining conditionsj soft sandstone caves easily, and it is very difficult to place lagging above.the machine in the arching sandstone. Normally, the "mole" can advance a face faster than any other method if the rock conditions are fa­ vorable. As much as 40 feet per shift has been excavated with a total of 120 feet per day. Excavated material from the machine is removed from the tunnel by a narrow gauge ra ilro a d .

Compressed air, under high pressure and released through hose lines, has been used to cut the sandstone.

This method was employed to mine glass sand fo r the Ford

Motor company beneath their plant in St. Paul. 1 0 8 When heavy ground or burled river channels are en­ countered 5 hydraulic shields have been used to temporarily support the sandstone while mining. The shields used con­ sisted of a circular bulkhead with the working end having several large cutting teeth. The back of the rig is secured, to the tunnel lining with hydraulic jacks. The jacks push the shield into the tunnel face several inches or feet so th a t the m aterial can be mined with safety . A fter the shield has been pushed forward, the jacks are retracted so more supports can be installed. This process is repeated • until the tunnel has-been driven through the problem area.

A pressure caisson has been used only once for tun­ nel construction in the St. Peter. The caisson was used while driving a tunnel across the buried channel beneath Lexington Avenue in St. Paul. A high inflow of water from glacial debris necessitated this operation.

Case Histories of Tunneling in the St. Peter Sandstone

Considerable experience in tunneling methods in the sandstone has been gained over the last 100 years. The col­ lection of case histories that follow briefly describes some of the engineering and geologic problems that were encount­ ered while driving tunnels in the Metropolitan area.

Eastman Tunnel (Schwartz, 1933, pp. 25-26)

The first attempt to construct a tunnel in the St.

Peter sandstone was to drive a shaft from the foot of St. l o g Anthony Falls at Hennepin Island up to and under Nicollet

Island„ The tunnel was to be used to Increase the availa­ bility of water power. When the tunnel was one-third com­ pleted <, water broke into the tunnel from the river above and eroded away large amounts of friable sandstone and under­ mined the above lying Platteville limestone. This caused a cave-in and a break in the crest of St, Anthony Falls which resulted in the retreat of the falls 300 feet up stream on the east bank of the Mississippi liver. The cave-in was attributed to a large quantity of water percolating through joints in the Platteville lime­ stone and into the underlying sandstone. Additional amounts of water were encountered discharging into the tunnel along - ~ 7 -1 horizontal fissures, presumably bedding planes. The added weight of the water and the rapid erosion of the sandstone caused a loss of strength to the m aterial .'""The collapse of the tunnel caused the abandonment of the project on October

4, 1869.

Wabasha S tre e t In tercep to r Sewer (Ackermann, 1938)

The Wabasha S tre et tunnel was driven under downtown

St. Paul at a depth below the water table. A water inflow of 4,00© gallons per minute was encountered from numerous seams and fis s u re s , causing extreme erosion problems. The concrete lining of the tunnel was not poured until practic- ally the entire 3,000 feet of tunnel was completed. By the 110 time the lining was constructed, the side walls of the shaft had become dry. The entire inflow by that time was being intercepted by an anderdrain. The earlier inflow into the tunnel was water drain­ ing from the surrounding sandstone about the opening. As the saturated zone above the invert was being dewatered to the underdrain level, a cone of depression was being cre­ ated „ After the cone was established, the inflow decreased to the natural flow velocity of water through the sandstone in that locality.

Roblyn Avenue Interceptor Sewer, St. Paul Sandstone encountered west of the Pairview Avenue shaft was found to be hard and self sustaining. The hy­ draulic method of mining was inadequate, and b lastin g te c h ­ niques had to be used to loosen the sandstone.

An average of 0 .69 pounds of 30 percent gelatin dy­ namite was required to loosen one yard of sandstone. The depth of heading pulled by a single round was 8.4 feet and required 33<.12 pounds of explosives. A round consisted of three cut holes, nine reliever holes, and seven trimming h o le s.

After the rook was loosened from the face, it was washed down to a dredge pump with a six inch hose. A second breaking of the rock was required before the pumps could I l l handle the material„ The blasting operation caused a delay In time and additional cost to the project„

Dayton's Bluff Interceptor Sewer,, S t„ Paul (Ackermannj, 1938) Water bearing fissures were encountered during con­ struction of the sewer under Dayton’s Bluff in St. Paul.

Initially, the fissures in the tunnel back yielded large quantities of water, but the inflow rapidly decreased, and the wall of the tunnel dried up. In this case the fissures were beneficial for dewatering the sandstone. Erosion was negligible, thus indicating that the sandstone was compe­ te n t.

Rondo Street Tunnel Gave-in (Schwartz, 1962)

As the Rondo Street relief sewer tunnel was being advanced up from the Mississippi River in St. Paul, a cave- in occurred at approximately 3*065 fee t from the portal and on a PI in the tunnel. The cause of the rock fall was due to the convergence of two high angle intersecting joints above the crown of the tu n n el. Only one of the jo in ts was visible during the mining operation. The second joint ap­ peared as a horizontal line along the tunnel wall and was mistakenly interpreted as a bedding plane in the sandstone. In addition, the sandstone in the cave-in area of the tunnel was reported to be friable, and water courses were noted in the rock. 112 Schwartz (1962, pp„ 1 -7 ) made the following observa­ tions after the cave-In occurred; from the portal to 1,725 feet the rock was competent and stood without support„ Be­

yond th is point the sandstone became soft, and arching in the

crown was evident„ At 1,780 feet from the portal, six inch wide steel ring beams were placed from three to ten feet

apart, depending on the softness of the sandstone. Lagging was placed between the beams. At 1,885 feet, flattening of the ring beams was noticed at the tunnel crown. This loca­ tion was the first evidence of vertical pressure on the beam supports. An arching effect at the 2,100 foot mark relieved a mass of sandstone to a height of five feet above the back,

twenty feet long and six feet wide. This caused two sets of

ring beams to collapse. The hole above the crown was

cribbed over, and from 2,275 feet on, spiling replaced the lagging method previously used. As the tunnel advanced and the ground became heavier, double steel ring beams were em­

ployed, and the spiling was widened to catch the caving rock on the shoulders of the tunnel.

Numerous joints along the first 3,000 feet of tunnel

were noticedj however, none caused any cave-in problems or

inflow of water. The joints that caused the large cave-in were tight and did not arouse suspicion any more than those

in from the portal. No way to predict the trouble was found. 1 1 3 Figure 27 shows the main cave-In area. The walls on the right and left, A and B respectively, are the plane sur­

faces of the two major joints responsible for the cave-in.

The attitudes of the planes are shown by the dip and strike

symbols in the photograph. The black triangular patch at location C in the photograph, and the line extending down from the patch, show a joint filled with two inches of clay.

This surface intersects surfaces A and B roughly at right angles. The letter D marks the upper surface of the cave-in

and is inclined downward toward the tunnel face.

Stevens Avenue Sewer Tunnel Cave-in, Minneapolis

During construction of the Stevens Avenue, fourteen foot diameter sewer tunnel between 39th and 35th Streets in Minneapolis, the sandstone in which the tunnel was being driven unexpectedly changed to glacial drift. Prior to con­

struction of the tunnel, alignment holes were placed 900 feet apart and inadvertently on either side of a buried

channel. A normal sequence of material was logged in both holes, thus inferring the rock strata was continuous between them. When the buried channel was encountered underground,

approximately 300 feet south of 38th Street, mining opera­ tions were stopped, for. it was necessary to redesign the

tunnel to cross the buried channel.

Over fo rty auger and core borings were d rille d from

the surface to delineate the size and shape of the channel rH i—I

rQ-Q Joint Plone

//A\v/V//xv///N\Ny//-^v//^/ / '//^ "W /*$ //A $ // K$/ r ^///^//^V/A^ ///^//^^//^ LONGITUDINAL SECTION Scole-I =12

Photograph of Caved Zone

CROSS SECTION Scale"!"3 6'

ST PETER-RONDO STREET TUNNEL CAVE-IN FIGURE 27 1 1 5 and to log the types of materials present. Figure 28 Is a cross section of the channel. While exploration of the burled channel and redesign of the tunnel was In progress, another segment of the tunnel was being advanced southward to intersect the north boundary of the channel. The sandstone in this portion of the tunnel was saturated and was being dewatered by high yield pumps. Eight hundred feet north of the buried channel ex­ cessive arching in the crown of the tunnel occurred. The sandstone was caving upward to within a few feet of the base of the Platteville limestone, located twenty-two feet above the invert. At the first sign of caving, men. and equipment were removed from the tunnel. At about the same time a shift in the casing of one of the dewatering wells made it necessary to shut the pump down. This caused the water tab­ le to rise and cause unstable conditions in the sandstone.

The unloading effect, when the water table rose, evidently caused a loss of confining pressure to the sand­ stone, thereby causing the back to cave. In addition, rapid inflow of water probably caused some disintegration of the sandstone in the opening, destroying some of the natural support' for the tunnel. It was later found that sixty feet of the tunnel back had caved, burying invert forms that were being readied for pouring the concrete lining. Immediately north of the caved-In area, for a distance of sixty feet. 1 1 6 displacement had bowed lagging and twisted some of the steel, su pports. Recommendations were made to put the dewatering well back in operation immediately and drain the tunnel with ditches„ The area of the tunnel that still threatened to cave was to be reinforced and concreted before the caved-in area was cleaned out„ To recover the caved area of the tunnel, the loose sandstone was first removed, forms were placed, and then a concrete lining was poured„ The excavated sandstone was then pumped back into the cavity around the lining„ After the danger area had been secured, a hydraulic shield was used to continue the tunnel southward to the south bank of the buried channel„ Conventional methods were used to exca­ vate the material while the shield was in service. r9 0 0 Access Shaft Alignment Alignment 37th Street Hole Hole 36th Street 35th Street Ground Surface

Glacial Drift

Static Water Level -Platteville limestone -Glenwood shale Sewer Tunnel x Cave-in

Buried Channel Approximate Bedrock Surface ELEVATION SEA ABOVE ELEVATION LEVEL IN FEET

St. Peter sandstone

5 221 219 217 215 TUNNEL S T A T IO N S -S cale lin.= 2 0 0 ft

GEOLOGIC SECTION - STEVENS AVE. SEWER TUNNEL, MINNEAPOLIS FIGURE 28 1 1 8

FIGURE 29. Stevens Ave. tunnel cave-in. The sandstone caved up to the base of the Platteville limestone. Iron stained bedding planes in the sandstone are exposed. ■ CONCLUSIONS AND RECOMMENDATIONS

1) Lithologically and hydrologically the sandstone is homo­ geneous and locally isotropic„ Isotropic conditions ex­ ist only when the friability of cementation of the

sandstone is consistent within a prescribed area„ 2) The formation is weakly cemented or more commonly, lacks cementation altogether. The only obvious cementing agent is iron oxide, and this occurs locally in the sandstone, Interlocking grains are the main bonding

agent keeping the sand grains together as a unit„ 3) Friability or hardness of the sandstone varies laterally

and v e rtic a lly w ithin the form ation. This e ffe c t is due to the degree of interlocking or compaction of the sand

grains., 4) High natu ral compactive forces have densified the sand­

stone to a degree that is impossible to reproduce by

normal compaction methods in the.laboratory. An excep­

tion to this occurs when a load great enough to cause crushing of the sand grains occurs,

5) The upper portion of the sandstone, in the Twin City area, is discontinuously saturated. Locally, the satur­

ated sandstone may possess artesian water.

6) The average porosity of the sandstone has a high average

value of 28.3 percent. In contrast, the permeability is

1 1 9 1 2 0

low with a value of 1 x lO-^ centimeter per second„ 7) Grain size analyses indicate a high degree of sorting

and uniformity, with grain sizes falling in the fine

sand range (0.15 to 0.4-0 millimeters). Effective size

in millimeters is 0.122. 8) Undisturbed St. Peter sandstone was found to have a high angle of internal friction (fifty-five degrees) but has

a low shearing strength. This high value is a function of the size and shape of grains, as well as the degree

of compaction the formation has undergone. 9) The ultimate bearing capacity of the sandstone is in ex­

cess of twenty-five tons per square foot. The maximum

load carrying capacity will be found to be much higher

when adequate methods are developed for testing shearing strengths of granular materials with high angles of in­

ternal friction. .Bearing values up to 100 tons per square foot may not be uncommon. 10) Field plate bearing tests indicate settlement to heavy

structures to be less than 0.1 of a foot with applied loads of twenty-five tons per square foot. The same

tests have shown almost complete rebound of the consoli­

dated material after removal of the load. This indi­ cates the sandstone to possess elastic properties.

11) Unconfined compression tests indicate a wide but low

variation in compressive strengths and moduli of defor­

mation. Axial moduli of deformation ranged from 151,000 1 2 1

pounds per square inch with a compressive strength of

1,370 pounds, to 7^1 j>000 pounds per square inch with a

compressive strength of 2,810 pounds per square inch„ The moduli for the tests made to date indicate both

elastic and plastic deformation, because they were only loaded once and taken to failure. When future compres­ sion tests are made, they should be loaded and unloaded

several times until a straight, line is obtained on the stress strain curve. A straight line, if possible to

obtain, in d icates the modulus of e la s tic ity and is a very important characteristic when designing heavy live

loads on the sandstone. 12) Poisson's ratios, calculated from unconfined compression

tests, showed a wide variation from .02 to .46. This

indicates that inconsistent ratios between lateral and

vertical stresses will result between locations. 13) Test data obtained from plate bearing tests show the modulous of e la s tic ity to be very low. A value of 383 pounds per square inch was computed. A normal sandstone will average 1,000 kips per square inch.

14) Geophysical methods may prove beneficial to locate bur­

ied channels or irregularities in the sandstone surface.

The gravity method w ill prove the most s a tis fa c to ry . I t

is a relatively rapid and inexpensive survey tool.

Electrical and seismic methods will only be of value if there is a thin cover. 1 2 2

15) Instrumentation of tunnels to obtain stress-time rela­

tionships is highly recommended. This inform ation can be used to design for the proper supports needed in a tunnel. ■Down hole, in situ testing with seismic, elec­ trical, or audible devices may reveal the degree of fri­ ability or jointing in the sandstone. Down hole

pressure meters w ill give important design data (modulus

of deformation). Pump tests will disclose the type of

ground water problems that'can be anticipated along a tunnel route or in an excavation. 16) Low permeability of the sandstone has restricted pres­ sure grouting techniques for solidifying the rock or

creating grout curtains.

17) Friction piles cannot be driven into the sandstone, be­ cause of the high penetration resistance of the rock..

Point bearing piles driven to refusal in the sandstone are commonly used in the Twin City area.

18) Extreme caution should be exercised when clay filled joints, closely spaced tight systematic j_oints, or

joints bearing water are encountered during excavation. These are zones of potential weakness due to lack of

friction between surfaces.

19) Undisturbed test specimens are difficult to obtain of

the St. Peter sandstone. In situ testing techniques

should be employed where p o ssib le . 1 2 3 In general, the St. Peter sandstone,, being unique in its geologic and engineering propertiess is an excellent me­ dia in which to construct tunnels or for the use as a foun­ dation base. Problems will continue to arise, but with the proper use of data obtained from site examinations and test­ ing programs, these perplexities will be reduced. LIST OF REFERENCES

Ackermann5 J. O., Apr. 18., 1938, Engineering problems, con­ cerning' the,St. Peter sandstone: Report to W. Z, Lidicker, Corps of Engineers, St. Paul, Minnesota, p. 8 . Austin, Go, June 1964, Personal communication: University of Minnesota, Mpls„, Minn. Buchanan, 8. T., Aug. 1937, The techniques of soil testing: Civil Engineering, pp. 568-572.

Corps of Engineers, U.S. Army, 1938, St. Peter sandstone specimens: Lab Report No. 35, Corps of Engrs., Iowa City, Iowa.

______1957, Chemical solidification of St. Peter sand­ stone: Design Memo. No. 3, Upper Locks, Pari UL-9, River Control, Corps of Engineers, S t. Paul, Minn.

1958, S t. Anthony F alls Upper Lock and Dam: Report UL-3, Corps of E ngrs., S t. Paul, Minn. Ford, G. R., July 1964, Personal communication: Minn. State Dept, of Highways, S t. Paul, Minn.

Gruen, Victor, 1961, Soils report for Dayton’s Dept. Store, St. Paul, Minnesota: Victor Gruen Associates, Edina, Minn.

Kamb, H. R., Aug. 1932, Porosity tests of the St. Peter sandstone: Report to the City Council of Mpls., Minn. Krynine, D. P. & Judd, M. R., 1957, Principles of engineer­ ing geology and geotechniques: McGraw-Hill Book Co., In c.,. New York, p. 52.

Lacabanne, ¥. D., Aug. 1964, Personal communication: Univ. of Minn., Mpls,, Minn. ,•

Lamar, J. E ., 1928, Geology and economic resources of the St. Peter sandstone of Illinois, 111. Geol. Surv., . Bull.. 53, p. 33

Legget, R, F., 1962, Geology and engineering: McGraw-Hill Book Co., Inc., p. 3 3 6 ,

1 2 4 1 2 5 Lelsch, Bo A.j, 19625 Ground water hydrology and hydraulics of the St. Peter sandstone In Minneapolis3 Hennepin County, Minnesota: Minn. S tate Div. of Waters, Report on State ProJ. 2781-51 and 2781-52. July, 1964, Personal communication: Minn. State D lv. of Waters, St. Paul j, Minn. Holder, N. D., March 1963, The upper boundary of the St. Peter sandstone deposit: Report to P. E. Cloud, Prof, of Geol. at the Univ. of Minn., for Geol. Course 1-H, Mpls., Minn.

Proctor, R. V. & White, T. L., 1956, Rock tunneling with steel supports: The Commercial Shearing and Stamping Co., Youngstown, Ohio, p. 6 3 . Schwartz, G. M., 1936, The geology of the Minneapolis-St. Paul area.: University Press, Mpls., Minn., Minn. Geol. Surv. Bull. No. 27. ______1939, Final report of foundation conditions at the site of the proposed St. Anthony Falls locks, Mpls., Minn.: Report to the Corps of Engrs., U. S. Army, St. Paul, Minn. ' 1962, Summary of observations in the St. Peter- Rondo relief sewer: Report to Foley, Hurley, and Winston, St. Paul, Minn.

■ Aug. 1964, Personal communication: Univ. of Minn., Mpls., Minn.

Feb. 1965, Personal communication: Univ. of Minn., Mpls., Minn.

Sims, P. K.Apr. 1962, Personal communication: Univ. of Minn., Mpls., Minn. S tau ffer, C. R ., . and T heil, G ..A,, 1941, The Paleozoic and related rocks of southeastern Minnesota: University Press, Mpls., Minn., Minn. Geol. Surv. Bull. No. 29,. P- 75. Terzaghi, K„, and Peck, R. B., 1948, Soil mechanics in engi­ neering practice: John Wiley and Sons, Inc., New York. •

Theil, C. A., 1935, Sedimentary petrographic analysis of the St. Peter sandstone: Geol. Soc. of Amer. Bull., Vol. 46, No. 4. 126

Wlnchell, N„ H„, 1884, Geology of Minnesota - final report, geology and natural history survey of Minnesota; Vol, 1, p. 202=

) -4 3 5 0

- 4 0 0 0

-3 6 5 0

Datum sea level -L -3 3 0 0 RKReed A' CROSS SECTION HORIZONTAL AND VERTICAL SCALE = 1:8,450

PLA TE 17 f ? / f / 10 0 0 - Rice Creek

9 0 0 -

300 NORTH SOUTH NORTH-SOUTH CROSS SECTION OF THE MINNEAPOLIS-ST PAUL AREA I Mile VIEW LOOKING EAST SCALE FIGURE 31 Z f f 7 f / By-C. M. PAYNE AUG. 1964 r $:• Xv "r-zm xwmzsmii

FIGURE 32 By-C M. PAYNE AUG. 1964 ■eweie GEOLOGICAL SURVEY STATE OF MINNESOTA MISCELLANEOUS MAP Paul K. Sims, Director UNIVERSITY OF MINNESOTA M-l ANOKA 14 Ml. ANOKA 10 Ml. FOREST LAKE 20 Ml FOREST LAKE 14 Ml. EXPLANATION

5 5 DECORAH FORMATION Green, fissile to blocky argillaceous shale. Contains many ir­ *21 regularly distributed coquina lenses and thin limestone beds that increase in abundance upwards. Thickness ranges from 0 to 90 feet.

Op

PLATTEVILLE FORMATION Gray to brown dolomite and limestone; average thickness 25 feet. Consists of following members, from youngest to oldest (not distinguished separately on map): Carimona Member; brown, medium-bedded, fine-grained limestone with some interbedded shale; average thick­ ness 3.5 feet. Magnolia Member; massive micragranular, dolomitic lime­ stone. Weathers to yellowish-gray; average thickness 8 feet.

Hidden Falls Member; greenish-gray, microgranular, argill­ aceous dolomic limestone; average thickness 6 feet. Mifflin Member; very thin, crinkly-bedded limestone and do­ lomite-mottled limestone; weathers very light-gray with grayish-orange dolomitic patches; average thickness 6 feet. Pecatonica Member; buff-weathering dolomite or dolomitic limestone; average thickness 1 foot.

Osp

ST. PETER SANDSTONE (Includes Glen wood Formation at top) White, fine- to medium-grained, well-sorted, friable sand­ stone; locally is iron-stained and well-cemented; rounding and frosting of the grains are common. Except for a thin gray argillaceous siltstone bed below the middle of the forma­ tion, the sandstone is 99 percent quartz; average thickness 150 feet. The formation discomformably overlies the Shakopee Dolomite. Glen wood Formation; Greenish-gray, arenaceous to argill­ aceous shale; average thickness 5 feet. Formation represents a transition from sandy shale to overlying carbonate rocks.

S Opc

PRAIRIE DU CHIEN FORMATION Dominantly dolomitic limestone and dolomite. Consists of following members, from youngest to oldest (not distinguish­ ed separately on map): Shakopee Dolomite; buff, massive-to thin-bedded, dolomitic limestone; commonly cherty, sandy, and oolitic. Thin beds of fine- to medium-grained sandstone and greenish- gray shale are irregularly distributed through the mem­ ber; average thickness LO feet. New Richmond Sandstone; fine- to medium-grained quart- zose sandstone, locally absent; thickness ranges from 0 to 10 feet. Oneota Dolomite; brownish-gray or buff, fine- to medium­ grained, locally vuggy, thin- to thick-bedded dolomite; average thickness 80 feet.

€1

JORDAN SANDSTONE Generally white and friable but locally cemented and iron- stained, massive- to well-bedded, fine- to coarse-grained sand­ stone. Conformably overlies the St. Lawrence Formation. Gen­ erally consists of very clean, well-rounded and frosted quartz grains that grade upward from fine sand at the base to coarse sand at the top; average thickness 90 feet.

Note: Formations below the Jordon Sandstone have been penetrated by drilling, and are important sources of ground water.

Geologic contact

------45° ------Topographic contours (Drawn on top of bedrock surface; contour interval 20 feet)

Water well or drill hole

U. $ till

Z

00

>CN

Ul - j I II oz *^ 25

57' 30"

O) CXI h-

k 1

52'

\ FARMINGTON 23 Ml 5 ML TO MINN 218 4.1 Ml. TO U.9. 65 RMINGTON 18 Ml Williams & Heintz Map Corporation, Washington 27, D.C. Geology compiled 1962-63 Base from U.S. Geological Survey Topographic map, 1952 BEDROCK GEOLOGIC MAP, MINNEAPOLIS, ST. PAUL AND VICINITY by C. Marshall Payne

SCALE 1:24000 o 1 MILE

1000 1000 2000 3000 4000 5000 6000 7000 FEET APPROXIMATE MEAN MAP LOCATION DECLINATION, 1952 CONTOUR INTERVAL 10 FEET FIGURE NO. 3 0 Ip 7 DATUM IS MEAN SEA LEVEL A 1 Qfig;