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CVO\032860004 III

ContentsSection Page

1 Introduction..........................................................................................................................1-1

2 Geological Reconnaissance ...............................................................................................2-1

3 Site Geology .........................................................................................................................3-13.1 General Geologic Setting...........................................................................................3-13.2 Geologic Map Units ...................................................................................................3-13.3 Geologic Structure .....................................................................................................3-43.4 Geologic Hazards.......................................................................................................3-5

4 Recommendations...............................................................................................................4-14.1 General Conditions for the Proposed Project ........................................................4-14.2 Dam Type and Construction Materials ..................................................................4-14.3 Foundation Conditions for the Main Dam.............................................................4-24.4 Foundation Conditions for the East Saddle Dam..................................................4-24.5 Foundation Conditions for the West Ridge Dike and West Saddle Dam..........4-24.6 Foundation Conditions for the Spillway ................................................................4-34.7 Seepage / Leakage from the Reservoir...................................................................4-34.8 Potential Problems During Reservoir Operation ..................................................4-4

5 References.............................................................................................................................5-1

Appendices

A Site Geologic MapB Geologic Cross Sections and ProfilesC Photographic Profiles of the Left Abutment (East Butte)D Regional Fault Map

Figure

1 Existing and New Structures ............................................................................................... 1-3


SECTION 1

Introduction


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SECTION 1

Introduction

The Big Sand Wash Reservoir Enlargement project forms a major part of the Lake ForkSection 203(a) Uinta Basin Replacement Project. In addition to the reservoir enlargement, theproject also includes (1) a new diversion structure on the Lake Fork River to divert waterfrom the Lake Fork River, (2) a new feeder pipeline to deliver water from the Lake ForkRiver to the Big Sand Wash Reservoir, and (3) a new pipeline to deliver water to Rooseveltand irrigation water to the State Road area near Roosevelt.

The contract documents for the Big Sand Wash Dam Enlargement project are presented infive volumes:

• Volume 1 contains the general conditions, the Division 1 specifications (specialconditions), and instructions to the bidders, including the notice inviting bids,instructions for preparing the bid, etc.

• Volume 2 contains Divisions 2 through 16 of the specifications.

• Volume 3 contains the design drawings.

• Volume 4, the Geotechnical Data Report, contains information on the design andconstruction of the existing dam and information collected during CH2M HILL’sinvestigations for the new dam.

• Volume 5, the Geotechnical Baseline Report for Outlet Works, contains a summary of thegeologic and geotechnical information, a description of the anticipated groundconditions, and a prediction of the ground behavior during construction of the outletconduit and vertical shaft.

In addition to these five volumes, three additional design reports were prepared. Thesedesign reports are being made available to the Contractor, but are not part of the contractdocuments and are for the Contractor’s information only. These reports are:

• The Geology Report (this report), which describes the geologic reconnaissance andmapping conducted at the site, provides engineering geologic evaluation of the siteconditions, and presents a site geologic map, geologic cross sections and profiles, andseismotectonic information, including a regional fault map.

• The Geotechnical Design Report, which includes (1) interpretations of geotechnical data,(2) the results of geotechnical evaluations of the embankment dams, spillway, and outletworks, and (3) recommendations for design.

• The Hydrology and Hydraulic Structures Report, which describes the hydrologic andhydraulic engineering analyses and results, and the hydraulic and structural design ofthe spillway and the outlet works.

The existing Big Sand Wash Reservoir (Figure 1) is owned by the Moon Lake Water UsersAssociation and was designed by Todd and Horrocks, Inc. with geotechnical investigation

BIG SAND WASH GEOLOGY REPORT

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and design by Fuhriman, Rollins and Company. The reservoir is contained by one maindam, a natural ridge on the west (right abutment) and a natural butte on the northeast (leftabutment), and two constructed saddle dams (east and west). The dams were constructed in1963–1964 and the final inspection was on December 21, 1964.

It is proposed that the Big Sand Wash Reservoir will be enlarged from 12,050 acre-feet to24,100 acre-feet. The enlargement will include the following elements:

• The main dam and the saddle dams will be raised by 26 feet

• A dike will be added to the top of the natural west ridge

• The existing outlet works will be abandoned

• A new outlet works will be constructed through the right abutment usingmicrotunneling techniques and a vertical construction shaft

• A new concrete-lined spillway will be constructed consisting of a concrete gravity damoverflow structure with a stepped downstream face and an ogee crest

• State Highway 87 will be realigned

The main dam will be constructed by one of two options. One option is to replace themajority of the existing dam and construct the new dam with a clay core as seepage control.The second option is to construct the new dam on the downstream side of the existing dam,and to install a deep concrete cutoff wall through the new and existing portions of the damas seepage control. The locations of the existing and new structures are shown in Figure 1.See the Geotechnical Design Report and the contract documents for a more detaileddescription of the enlargement project.

SECTION 2

Geological Reconnaissance


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SECTION 2

Geological Reconnaissance

An initial geological reconnaissance was performed in February 2002 to establish boringlocations for the geotechnical exploration and to identify potential borrow areas. Later, inMarch and April 2002, a CH2M HILL engineering geologist performed geologic mapping atthe site. The geologic mapping included observations and descriptions of bedrock andunconsolidated geologic deposits, and their expected effects on dam reconstruction and useas construction materials. Other geologic features observed and mapped included faults,landslides, springs and seeps, erosion features, and rock structure.

During the design process for the main dam cutoff wall option (see the Geotechnical DesignReport and the contract documents for a description of the main dam options), a desire wasexpressed to obtain more specific geologic data concerning the left abutment (northeastbutte) of the dam. Because the left abutment experienced seepage problems during firstfilling, and the stability of the proposed cutoff wall heavily depends on the geologic stabilityof the abutments, a more detailed geologic evaluation of the left abutment was undertaken.The evaluation occurred in August 2003 and included a field reconnaissance by two CH2MHILL engineering geologists.

In addition, geotechnical exploration programs were conducted in 2002 and 2003 thatincluded 36 borings, 44 test pits and 4 test trenches located in the vicinity of the mainreservoir, and 67 test pits located off site. The borings were advanced to determine the rockstratigraphy, conduct in situ water pressure (Packer) tests, and obtain samples forlaboratory testing. The test trenches were dug to examine the bedrock surface for use indesigning the surface preparation, and the test pits were dug to obtain samples forlaboratory testing and to examine potential borrow sources for embankment materials. TheGeotechnical Data Report contains the full geotechnical exploration results.

From the information obtained from the geological reconnaissance and geotechnicalexplorations, a site geologic map was produced (Appendix A). Geologic profiles wereproduced from the surface and subsurface information and are presented in Appendix B.

Two photographic geologic profiles were developed based on the results of the August 2003geological reconnaissance (Appendix C). One photographic profile is of the entire leftabutment, looking northeast from the right dam abutment. The locations of Boring D-8 andD-9 are shown on the profile. The second photographic profile is a modified black and whitephotograph taken during construction of the key trench for the existing main dam. Thisphotograph is looking northeast and shows the excavation of the key trench, the installedgrout curtain/trench, and the prepared contact area between the core and the left abutment.

Estimates of rock core hardness were determined during the geotechnical explorations, andwere based on the response of the rock core to resistance, and scratching and/or breakagetests. The following rock hardness scale was used:

• R0 - Indented by a fingernail; very soft• R1 - Peeled by a pocketknife and crumbles under a hammer; very soft


2-2 CVO\032860004

• R2 - Peeled with difficulty by a pocketknife; soft• R3 - Fractured by a hammer; medium hard• R4 - Fractured by repeated hammer blows; hard• R5 - Fractured by repeated geologic pick blows; very hard• R6 - Chipped by a geologic pick; very hard

SECTION 3

Site Geology


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SECTION 3

Site Geology

3.1 General Geologic SettingThe project site is located within a regional geologic feature known as the Uinta Basin. Thegeologic setting consists primarily of near-horizontally bedded sedimentary rocks incised byrivers draining from the Uinta Mountains to the north. The bedrock in the area consistsprimarily of the Tertiary-age Brennan Basin Member of the Duchesne River Formation(Bryant, 1992). This formation was shed from the Uinta Mountain uplift and deposited inthe Basin.

Gravelly glacial outwash deposits are present on geomorphic surfaces throughout the area.After the deposition of the outwash, the bedrock was dissected by more recent streamactivity and much of the outwash is found as isolated remnants on flat-topped buttes andmesas in the area. River-transported (alluvial) deposits are present in the flat-bottomedvalleys. Wind-blown (eolian) deposits have covered the bedrock and alluvial deposits insome areas. The geologic units are described below in more detail, and shown on thegeologic map in Appendix A.

3.2 Geologic Map Units• Qf: Fill deposits. Fill deposits include man-made fills such as dams, dikes, and other

disturbed areas. These were constructed from local deposits consisting primarily ofgravel, sand, and silt.

• Qe + Qal: Eolian and Alluvial deposits. Eolian and alluvial deposits were found in thevalley bottoms along Big Sand Wash and its tributaries. These deposits consist primarilyof tan to reddish-tan, poorly graded, non- to low plastic, fine silty sand to sandy silt.Occasional gravelly sand deposits and clay layers also appear to be present. These unitswere mapped together in Big Sand Wash because of the absence of a distinction betweenthe two units. The eolian deposits consist primarily of very fine silty sand deposited bywind, whereas the alluvial deposits in the valley bottom consist largely of reworkedeolian deposits. In addition, eolian deposits cover much of the alluvial deposits in thevalley bottom. The alluvial deposits also contain clasts of eroded sandstone, siltstone,claystone and occasional quartzite gravels in a silty sand to sandy silt matrix.

Test pits indicated that this material exceeds 15 feet in thickness at the upper part of thereservoir. These deposits were thinner at the valley edges where they pinched outagainst the sandstone. Test pits and boreholes indicated that this material exceeded14 feet in thickness in some parts of the valley downstream of the dam. The geologiccross sections in Appendix B show this relationship and the geometry of theeolian/alluvial deposits.


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• Qe: Eolian deposits. A thick eolian (wind-blown) deposit not mixed with alluvialdeposits was observed on the valley wall southeast of Big Sand Wash Dam. Thismaterial consists primarily of tan, poorly graded, non-plastic, fine silty sand to sandysilt. These deposits were distinctive from the mixed alluvial/eolian deposits describedpreviously because they were deposited higher on the valley walls and consist of purelywind-blown deposits. Test pits indicated that this deposit is at least 13 feet thick, with amaximum thickness estimated to be approximately 30 feet, based on interpretation fromtopographic and geologic cross sections.

• Qgo: Glacial Outwash deposits: Glacial outwash deposits were mapped in severallocations at the site. The glacial outwash deposits consist of rounded quartzite gravelswithin a fine to coarse sandy matrix. The gravels are typically up to 12 inches indiameter, but occasionally boulders in excess of 2 feet were observed. The upper andlower 2 to 3 feet of the glacial outwash deposits appear to be cemented in some areasand are more typical of conglomerate bedrock.

The glacial outwash was deposited by post-glacial streams and was found primarily onthe butte between the dam and east dike, on the linear northwest-southeast-trendingridge southwest of the dam, and in the broad alluvial valley southwest of the reservoir.These deposits are Pleistocene in age but were deposited at different times during thePleistocene on surfaces with differing elevations (Bryant, 1992). Although the glacialoutwash deposits in the vicinity are of different ages, there is little or no difference inmaterial properties.

The estimated thickness of the outwash deposits varies. A portion of the outwashdeposits from the butte northeast of the dam was used to armor the east dike. However,it appears that the outwash on this butte is still up to 10 feet thick in some areas. Muchof the outwash on the linear ridge southwest of the dam and reservoir was also removedfor use as riprap on the dam and dikes. However, on the ridge south and southeast ofthe dam, glacial deposits between 8 and 10 feet thick were observed. Glacial outwashmaterials have also been mapped in the floodplain southwest of the reservoir (Bryant,1992) in an area currently used for agriculture.

• Tdb: Brennan Basin Member of the Duchesne River Formation: This bedrock unitsurrounds much of the reservoir and underlies all of the previously describedunconsolidated geologic units. Bryant (1992) describes the Brennan Basin Member asTertiary-age, moderate red, grayish-red, reddish-brown, yellowish-brown, andyellowish-orange sandstone and less-abundant siltstone and claystone. Jones (1957)described the Duchesne River Formation as “fluvial channel sandstone, sandyconglomerates, and sandy siltstone.”

This formation was formed by erosion of the Uinta Mountain uplift and subsequentdeposition of sediments in the Uinta Basin as it was subsiding. The sediments weredeposited by braided streams and thus consist of sandy channel-fills and discontinuouslensatic beds, and siltstone and claystone where the sediments were deposited inshallow lakes. Because of the depositional nature of the bedding, the stratigraphy andlithology of the beds varies horizontally. Numerous lens-shaped channel cuts filled withcross-bedded sandstone was observed in the vicinity. Also, claystone layers have beenscoured away and truncated by fluvial processes. During the geologic reconnaissance,

3. SITE GEOLOGY

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numerous profiles were constructed between borings. It was evident from these profilesthat the thickness and lithology of bedding could change over a short distance;therefore, correlating stratigraphy between borings was difficult.

Based on field observations and core samples, the bedrock in the vicinity of Big SandWash typically consists of reddish to pinkish-purple, fine-grained, slightly tomoderately weathered quartzitic sandstone with occasional layers of medium-grainedsandstone. The hardness of the sandstone is typically in the R1 to R2 range, whichindicates very weak to weak rock. Often the core samples of sandstone were easilyeroded and could be broken down to sandy material by hand. In some locations thesandstone eroded to sandy material in the borehole during drilling and requiredgrouting to keep the holes open. The sandstone layers were observed to be up to 25 to 35feet thick in some of the borings.

Several interbedded layers of fine sandy siltstone, siltstone, silty claystone and claystonewere also observed in outcrops and core samples. The siltstone could typically bedescribed as purple with gray and yellow mottling, massive, fresh to slightly weathered,with a hardness in the R1 to R2 range.

The claystone could typically be described as purple red with mottled gray and yellow,fresh to slightly weathered, and a hardness of R1 to R2. In some core samples, portionsof the claystone were highly weathered to clayey material and clayey rubble. Based onthe boring logs, claystone and siltstone layers are more prevalent in the valley near thetoe of the dam than sandstone layers. The contacts between the sandstone, siltstone, andclaystone were often gradational and would transition gradually between the differentrock types. However, some contacts of the sandstone overlaying the claystone wereabrupt. Sequences of interbedded siltstone and claystone more than 40 feet thick wereobserved in some of the borings.

• Geologic units not shown on the geologic map: Thin and discontinuous geologic unitsincluding colluvial deposits, eolian deposits, and glacial deposits were also observed butare not shown on the map. These units were not mapped because either they were toosmall to show, or not available in sufficient quantities to be useful.

Colluvial deposits consist of unsorted gravel, sand, and silt that mantle steep slopestypically underlain by sandstone. The gravel is typically comprised of well-gradedangular sandstone clasts, but also contains rounded quartzite gravels eroded down fromthe glacial outwash deposits. Some colluvial materials in claystone deposits appear tohave a higher percentage of clay. The colluvial deposits formed as a result of masswasting processes and weathering of the underlying bedrock. The colluvial depositsform a thin (usually less than 5 feet thick or so), discontinuous cover on the slopes of thebutte northeast of the dam, the valley walls downstream from the dam, and the sides ofthe ridge southwest of the reservoir. It is not anticipated that the colluvial materialswould be useful as borrow materials.

Eolian deposits, in addition to those described previously, were also found throughoutthe area. These deposits are typically thin (less than approximately 5 feet thick) anddiscontinuous and overlay the sandstone bedrock. The bulk of these deposits arescattered in the flat areas southeast of the east dike, and northeast and northwest of the


3-4 CVO\032860004

reservoir covering the sandstone. It did not appear that there are adequate quantities ofthese scattered eolian deposits to be used as borrow materials, and mining thesedeposits for a material source would likely be impractical and not cost-effective.

Remnants of glacial outwash deposits are present on the ridge southwest of thereservoir. Much of the original material was stripped away for use during the originaldam construction. The material left is typically less than 2 to 3 feet thick and onlydiscontinuous patches remain. It appears there is an inadequate quantity of outwashmaterial in this area to consider as a material source and it was therefore not shown onthe map.

3.3 Geologic StructureDividing planes in the rock mass, such as joints and faults, are referred to as the rockstructure. Rock structure influences rock mass strength and seepage properties. No faultswere identified at the site during the geologic reconnaissance or on published maps;therefore, the geologic structure of the site consists primarily of joints and bedding planes inthe sandstone. The bedding planes at the site are essentially horizontal, although it appearsthere is a slight regional dip to the north (less than 10 degrees or so). The bedding planes arecontinuous over large distances; however, the bedding planes observed in the vicinity of thedam are typically wavy because of cross-beds in the sandstone, particularly where thesandstone was deposited directly on top of the claystone. The amplitude of these asperitiesis up to 2 or 3 feet in some areas, and some of the claystone beds are actually truncated byerosional channels filled with sandstone. In addition to the major bedding planes, someareas of very thin bedded cross beds were observed within the sandstone cliffs andoutcrops. The cross-bedding resulted in thin planes of sandstone. The bedding planes, inparticular the contact point between sandstone and claystone, have an effect on the masshydraulic conductivity and especially the horizontal and vertical movement ofgroundwater. Horizontal bedding results in higher horizontal hydraulic conductivityvalues.

During the geologic mapping, joints were observed in the sandstone outcrops. Most of thejoints observed are roughly vertical, generally widely spaced, and open in the vicinity of theoutcrops. Two sets of joints were identified: a northwest-trending vertical joint set, and anortheast-trending vertical joint set. The northwest-trending joints are likely responsible forthe geomorphic expressions of the distinctive northwest-trending linear ridge on thesouthwest side of the reservoir, and the oval-shaped northwest-southeast trending buttenortheast of the left abutment. Erosional patterns often follow discontinuities in rocks, andjoint sets often form anisotropic lines of weakness and thus contribute to structural controlof drainage patterns.

The large apertures (openings) of the joints observed near outcrops are likely caused in partby expansion of the rock from stress relief and lack of overburden pressure, and possiblyspreading of the sandstone on weaker claystone layers. The joints likely have smallerapertures deeper in the subsurface as a result of higher confining stresses. The apertures ofthe joints are important because joints act as seepage conduits for water transmission. Itappears that the high water take in some sandstone layers observed during the waterpressure testing may have been related to jointing in the sandstone beds.

3. SITE GEOLOGY

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3.4 Geologic HazardsLandslidesSmall landslides were observed on the steep valley walls south of the right dam abutment.Although individual slumps were too small to effectively show on the geologic map, thegeneral location of these slumps is shown on the map. These slumps originated in eolianand colluvial materials that covered the valley walls and appear to have been initiated byseepage emanating from the valley walls. These slides appear to be relatively shallow andlimited to surficial materials. Although small, these slides represent unstable areas andwould not provide suitable foundation conditions for dam abutments. The surficialmaterials in which these slides occurred would have to be removed down to competentbedrock where the main dam contacts the abutments.

Faults/EarthquakesNo faults were observed in the vicinity of the dam site during the geologic mapping, and noevidence of active faulting was evident in the immediate vicinity. However, according topublished geologic mapping, faults that are considered active and that may contribute toseismic hazards have been mapped within a 50-mile radius of the dam site. Four differentfaults and groups of faults may cause concern. These include the Duchesne-Pleasant Valleyfault system, the Towanta Flat graben, the Stinking Springs Fault, and the Strawberry Fault.These faults are indicated on the Regional Fault Map (adapted from Hecker, 1993) includedin Appendix D. The following is a brief description of each fault or potentially seismogenicfeature.

The Duchesne-Pleasant Valley fault system is a system of east-west trending faultsapproximately 20 miles long that approaches within 11 miles south of the Big Sand WashDam site. The age of the most recent movement on these faults is not well constrained.However, it appears these faults do not offset ±250,000 year old deposits. Sullivan (1988)suggests that these faults are not a potential source for large earthquakes, and believes thatscarps along the faults are fault line features but not late-Quaternary scarps. In addition, thetrend of these faults is roughly perpendicular to the stress regime in the region (that is, mostof the active faults trend north-south). However, contrary to Sullivan, Martin and others(1985) and Osborn (1973) believe these faults may exhibit late Quaternary movement.

The Towanta Flat graben is a northeast-trending fault system approximately 13 milesnorthwest of the Big Sand Wash Dam site. These faults offset late Quaternary deposits. Theage of most recent activity on this fault system is estimated to be 130,000 to 500,000 yearsago. However, based on field evidence, scarps along this fault may not be seismogenic inorigin and the faults may not be capable of significant future surface-rupturing events(Nelson and Weisser, 1985).

The Stinking Springs fault is a north-trending normal fault located approximately 43 mileswest of the dam site. This fault is estimated to be late-Quaternary in age based on the ratherambiguous criteria of range-front morphology and drainage disruption. This fault lacksdirect evidence of Holocene movement; however, the inferred rupture length suggests thatdisplacement may occur in events with an estimated magnitude of around 6.5. Therecurrence interval is not well-constrained on this fault (Nelson and Martin, 1982; VanArsdale, 1979).


3-6 CVO\032860004

The Strawberry Fault is a large, north-trending normal fault located approximately 46 mileswest of the dam site. The age of recent displacement on this fault is estimated to be early tomiddle Holocene, which would make it the most active fault within a 50-mile radius of thesite. In addition, the data and age constraints for movement on this fault are more accuratethan for the other faults. A minimum of two to three events have occurred on this fault since15,000 to 30,000 years before present, based on carbon dating. Based on the offset of themovements and the recurrence interval, the slip rate of this fault is estimated to be 0.04 to0.17 mm/year over the last 15,000 to 30,000 years. The estimated maximum credibleearthquake generated by this fault is 7.0, with a recurrence interval of 5,000 to 15,000 years.It appears this fault has the highest potential for seismic activity in the region (Nelson andMartin, 1982; Nelson and Van Arsdale, 1986).

According to the U. S. Geological Survey’s Earthquake Hazards Program, the probabilisticPeak Ground Acceleration at the site is 0.08g for a 500-year return period, 0.12g for a1,000-year return period, and 0.2g for a 2,500-year return period.

LiquefactionLiquefaction hazards appear to exist in the vicinity of the main dam. Based on geologicmapping and drilling information, it appears that liquefiable soils are present in theeolian/alluvial deposits found in the valley bottom downstream from the dam. Thesedeposits consist of interbedded, loose fine sands, silty sands, clay, and gravelly fine sands.SPT N-values indicate very loose soils with blow counts of 4 to 12 blows per foot. Inaddition, the groundwater appears to be relatively close to the ground surface in the area,and the ground was saturated downstream from the left abutment. The thickness of thepotentially liquefiable soils appears to be 10 to 15 feet.

Potentially liquefiable soils were also observed in Boring D-21, which was drilled from abarge in the forebay of the dam. SPT N-values indicate very loose fine silty sand to silt, withblow counts between 3 and 8 blows per foot. The thickness of these potentially liquefiablesoils is approximately 7 to 8 feet. It is likely that this layer is recently deposited sedimentwithin the reservoir, and not the noted alluvial layer.

Springs/SeepageSprings and areas of seepage were observed in the valley walls downstream from the BigSand Wash Dam. The primary mechanism for the seepage appeared to be water movinghorizontally along sandstone layers underlain by claystone layers. Downstream of the rightabutment, springs were emanating from the valley wall at an elevation betweenapproximately 5,830 and 5,840 feet. The surficial material in this area was saturated,resulting in slumping of surficial materials. However, based on historical evidence, itappears that the springs in this area pre-date the dam construction and may be related toirrigation in the field southwest of the reservoir, storage of water within the ridge, or theymay emanate from a regional aquifer. The groundwater appeared to be seeping in an east tonortheast direction through the narrow ridge and discharging in the valley wall. Althoughthe likely transmission mechanism for the seepage appeared to be open joints, it wasobserved that the sandstone itself was saturated in some areas, indicating that seepage isoccurring through the sandstone mass as well as through the fractures.

Downstream from the left (northeast) abutment, diffuse seepage was evident in severallocations. The highest observed level of seepage was occurring at an elevation between 5,830

3. SITE GEOLOGY

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and 5,840 feet, and appeared to be seeping from the same sandstone/claystone contactwhere the springs were discharging on the opposite valley wall. However, the flowappeared to be at a lower elevation in the northeast valley wall. Other areas of seepage wereobserved at lower elevations along this valley wall at lower sandstone/claystone contacts.Seepage was observed as much as 1,000 feet downstream from the dam. The long distanceof the horizontal seepage appeared to be the result of the water flowing horizontallythrough the sandstone above the claystone layers and eventually discharging from thevalley walls. Horizontal drains were installed in the vicinity of the left abutment to helpdrain seepage in the vicinity of the abutment during first filling of the existing reservoir.

An area of saturated ground was present in the lower valley near the toe of the leftabutment. This area was very soft and unstable, and pumping was observed when heavyequipment was driven across it. This area coincides with the area potentially susceptible toliquefaction. This saturated area is noted on the geologic map in Appendix A.

Overall, the seepage from the reservoir did not appear to be significantly detrimental to theoverall performance of the dam. Water losses appeared to be relatively small and seepageand spring flows are collected in the outlet canal, horizontal drains, and creek downstream.Because the leakage occurred from the bedrock formations, there appeared to be littledanger of significant deterioration of the dam or dike foundations, or the potential forfoundation failure. However, piping at the interface of the dike core and bedrock surfacemay be a concern.

Seepage may be occurring through the west ridge through highly permeable sandstonelayers. It appears that saturated ground is present, and riparian vegetation is growing on thesouthwest toe the west ridge. However, it could not be determined if the saturated groundconditions were caused by leakage and overflow from a nearby ditch.

Minor seepage may also be occurring under the southwest portion of the east dike. Theground surface is covered with grasses and riparian vegetation. This portion of the east dikeis underlain by fine silty sand, which may be more permeable than the surrounding siltysand; however, no actual seepage or springs were observed.

ErosionErosion of weak claystone layers was observed in the spillway area at the southwest side ofthe west dike. During past water flow over the natural rock spillway channel, the weakerclaystone layers eroded more quickly than the more resistant sandstone layers andundermined the sandstone layers. This phenomenon is known as headward erosion. Althoughthis activity does not immediately endanger the reservoir, continued headward erosion mayundermine rock layers closer to the reservoir and result in subsurface leakage in the vicinityof the spillway. Typically, if constant leakage is allowed to begin, the result is rapid erosionof the material, which could potentially be disastrous.

Areas around the shoreline of the reservoir, in particular the base of the butte and along thewest dike, show evidence of erosion of the claystone layers. This erosion has resulted inrockfalls of large sandstone blocks. Although this process is not necessarily a concern atpresent, if dikes or structures are reconstructed near shoreline areas, armoring weak layersis recommended to prevent future undermining and erosion.


SECTION 4

Recommendations


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SECTION 4

Recommendations

4.1 General Conditions for the Proposed ProjectOverall, the topographic, geologic, and geotechnical conditions appear to be suitable for theproposed embankment raises of the Big Sand Wash Reservoir Enlargement project. Theexisting dam has performed fairly well during its operating lifetime. However, importantissues for consideration include: (1) obtaining the large volume of materials required forembankment construction, (2) liquefaction potential of foundation soils downstream of themain dam, (3) liquefaction of foundation soils beneath the existing main dam and eastsaddle dam, (4) stability of the west ridge, and (5) zones of high water take observed duringdrilling and water pressure testing. No other geologic hazards, such as active onsite faults,major landslides, or other slope stability problems, were identified for the naturalformations. Evaluations of the embankments are in the Geotechnical Design Report.

4.2 Dam Type and Construction MaterialsThree types of construction materials suitable for dam construction were located at the site:

• Eolian/alluvial soils: These fine silty sands and sandy silts appear to have a sufficientpercentage of fines in some areas to meet the requirements of core material for theembankments. The volume of this material will have to be evaluated carefully, becausethe eolian and alluvial deposits with a high percentage of fines were of limited quantity.It will be difficult to assess the available quantity of this material given the variablenature of these deposits. The material suitable for core construction will have to besegregated and stockpiled during excavation. In addition, removing the eolian/alluvialmaterials from the valley downstream from the dam will probably create ponds andwetlands because of the shallow water table, and may therefore require reconstructionof the outlet canal. These factors may limit the amount of available material in thisregion.

• Eolian materials: These poorly graded fine sands and silty sands appear to meet therequirements for shell materials for the embankments, but lack the percentage of finesrequired for core materials. One potential limiting factor of this material is a possiblylow angle of internal friction resulting from the poorly graded, fine-grained nature ofthe material. This could be compensated for by flattening the slopes of the dam. Anotherlimiting factor for this material is a possibly insufficient quantity, given the large volumeanticipated for construction of the embankments.

• Glacial outwash gravels: These hard, quartzite gravels may be suitable for armoring theexterior of the dam. These gravelly deposits also appear suitable for aggregate and rockfill, if crushed and processed. The volume of these deposits will also have to beevaluated in detail to determine whether there are sufficient quantities. If the quantity in


4-2 CVO\032860004

the immediate vicinity of the dam site (on the butte and west ridge) is inadequate,gravelly material may be available from the agricultural field southwest of the westridge.

4.3 Foundation Conditions for the Main DamThe sandstone and claystone bedrock of the Duchesne River Formation will provide asuitable foundation for the main dam. This bedrock underlies the valley at the dam site, butis covered by 15 to 20 feet of eolian and alluvial soils. The core/cutoff trench should beexcavated through the soils down to competent bedrock. In addition, potentially liquefiablesoils should be removed from the dam footprint. It is anticipated that liquefiable soils areapproximately 10 to 20 feet deep in the vicinity of the main dam.

Colluvial and eolian soils on the valley walls should be removed from the abutment areas.Loose rock should also be scaled off until fresh, unweathered sandstone and claystone areexposed. Claystone exposed in the excavations may require special protection prior to damconstruction to prevent desiccation and degradation of this material upon being exposed toweathering elements.

Profiles A and D (Appendix B) show estimated bedrock and subsurface conditions beneaththe dam and abutments.

4.4 Foundation Conditions for the East Saddle DamFoundation conditions for the east saddle dam appear favorable. The existing dam isunderlain by eolian soils and bedrock. However, to prevent leakage beneath the dam, theeolian soils underneath portions of the dam should be excavated down to competentbedrock. Based on drilling information, these eolian soils are up to 10 feet thick. It appearssome subsurface flow is occurring beneath the existing dam. If the reservoir level is raised30 feet, the flow underneath the dam will likely increase as a result of the elevated reservoirand higher hydraulic head. The non-plastic eolian soils may also be subject to piping. Thesurface of the dam should be protected with cobble- to boulder-sized gravel, especially onthe reservoir side, to limit wave erosion.

Profile E in Appendix B parallels the east saddle dam and shows the estimated subsurfaceconditions beneath the dam.

4.5 Foundation Conditions for the West Ridge Dike and WestSaddle Dam

Foundation conditions for the west ridge dike appear to be favorable. The dike will beunderlain by bedrock including claystone, siltstone, and sandstone. One potential concern isthat the weak claystone layers could reduce the stability of the west dike by acting as slidingfailure planes, although during the subsurface investigation no extremely weak, weatheredclaystone layers were identified as potential sliding planes. In addition, preliminarylaboratory testing indicates the claystone has relatively high residual frictional strength.However, during the design of this portion of the dike the stability will have to be evaluated

4. RECOMMENDATIONS

CVO\032860004 4-3

in detail. Geologic cross sections have been constructed to support the stability analyses. Inaddition, zones of high potential seepage will have to be addressed to prevent excessiveleakage through the west ridge and dike.

Northwest of the west ridge, the foundation conditions for the west saddle dam appearfavorable. The existing saddle dam is underlain by sandstone and claystone near the surfaceand is buttressed by a sandstone ridge on the northwest and the west ridge on the south.Most of the bedrock in this area is siltstone and claystone that appear to be tight (limitedseepage expected).

Profile C in Appendix B parallels the west ridge and shows subsurface conditions beneaththe ridge. Profile B is a cross section perpendicular to the west ridge.

4.6 Foundation Conditions for the SpillwayFoundation conditions for the spillway appear favorable. The existing spillway flows over anatural channel underlain by interbedded sandstone and claystone. However, erosion ofweak claystone layers has occurred during past flows, leading to undermining of thesandstone layers and resulting in headward erosion toward the reservoir. It appears thiserosion of the claystone occurs extensively during each flow.

A new spillway in the same location would have to be constructed to accommodate theraised dike. The new spillway should be constructed to avoid continued erosion of theexisting channel. Erosion during future flood events could undermine more claystone layersand potentially endanger the integrity of the east dike and the reservoir in general. Theexisting channel could be armored and/or retrofitted with flow energy dissipators to limitfuture channel erosion.

4.7 Seepage / Leakage from the ReservoirAs previously discussed, leakage around the main dam abutments is ongoing. Theproposed reservoir enlargement should address leakage in the area of the main dam toprevent potential stability and piping problems on the downstream side of the dam. Theabutment foundations should be excavated to competent bedrock to avoid founding thedam on fractured bedrock or soils that may contain zones with high hydraulic conductivityvalues. A core/cutoff trench excavated through soils down to the underlying competentbedrock would reduce the potential for seepage under the dam.

Water pressure testing in borings at the dam abutments showed zones of high water take inthe sandstone layers of the abutments. The mass hydraulic conductivity of these zones maybe reduced by installing a grout curtain. A test-grouting and water pressure testing programshould be instituted to determine the effectiveness of a grouting program.

Seepage through the east saddle dam and west ridge may also be a concern when thereservoir is raised to higher levels. Seepage is possibly occurring through the west ridge,resulting in saturation of surface soils and riparian plant growth immediately below thesouthwest side of the west ridge. It appears that seepage is also occurring under portions ofthe east saddle dam, where riparian vegetation is growing in an otherwise sand- and


4-4 CVO\032860004

sagebrush-covered area. Water pressure testing in borings drilled into the west ridge andbelow the east saddle dam showed zones of potentially high seepage in the sandstone layersthat underlie these areas. Grouting may reduce the mass hydraulic conductivity of thesezones. As with the main dam, a test-grouting and water pressure testing program should beused to determine the effectiveness of a grouting program.

4.8 Potential Problems During Reservoir OperationNo major problems are anticipated during reservoir operation. Minor problems that mayoccur include slope stability and slumping in saturated sediments around the rim of thereservoir. However, these stability issues should not affect the overall performance of thedam.

SECTION 5

References


CVO\032860004 5-1

SECTION 5

References

Bryant, B., 1992. Geologic and Structure Maps of the Salt Lake City 1 X 2 Quadrangle, Utahand Wyoming. U. S. Geological Survey Map I-1997. Scale 1:125,000.

Hecker, S., 1993. Quaternary Tectonics of Utah with Emphasis on Earthquake-HazardCharacterization. Utah Geological Survey Bulletin 127.

Jones, D. J., 1957. Geosynclinal Nature of the Uinta Basin in "Guidebook to the Geology ofthe Uinta Basin." Intermountain Association of Petroleum Geologists Eighth AnnualField Conference, Otto G. Seal, ed.

Martin, R. A., Nelson, A. R., Weisser, R. R., and Sullivan, J. T., 1985. Seismotectonic Studyfor Taskeech Dam and Reservoir site, Upalco Unit and Upper Stillwater Dam andReservoir Site, Bonneville Unit, Central Utah Project, Utah: Denver, U. S. Bureau ofReclamation Seismotectonic Report 85-2, 95 p.

Nelson, A. R. and Martin, R. A. Jr., 1982. Seismotectonic Study for Soldier Creek Dam,Central Utah Project: Denver, U. S. Bureau of Reclamation Seismotectonic Report 82-1, 115 p.

Nelson, A. R. and Weisser, R. R., 1985. Quaternary Faulting on Towanta Flat, NorthwesternUinta Basin, Utah, in Picard, M. D., editor, Geology and Energy Resources, UintaBasin of Utah: Utah Geological Association Publication 12, p. 147 – 158.

Nelson, A. R. and Van Arsdale, R. B., 1986. Recurrent late Quaternary Movement on theStrawberry Normal Fault, Basin and Range – Colorado Plateau Transition Zone,Utah: Neotectonics, v. 1, p. 7 – 37.

Osborn, G. D., 1973. Quaternary Geology and Geomorphology of the Uinta Basin and theSouth Flank of the Uinta Mountains: Berkeley, University of California, Ph. D.dissertation.

Sullivan, J. T. 1988. Seismotectonic Study for Starvation Dam, Bonneville Unit, Central UtahProject, Utah: Denver, U. S. Bureau of Reclamation Seismotectonic Report 88-7, 14 p.

Van Arsdale, R. B., 1979. Geology of Strawberry Valley and Regional Implications: Salt LakeCity, University of Utah, Ph. D. dissertation, 65 p.


APPENDIX A

Site Geologic Map


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APPENDIX B

Geologic Cross Sections and Profiles


APPENDIX C

Photographic Profiles the Left Abutment(East Butte)


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APPENDIX D

Regional Fault Map

REGIONAL FAULT MAP

GEOLOGY REPORTUINTA BASIN REPLACEMENT PROJECTBIG SAND WASH RESERVOIR ENLARGEMENT

0 5 10 15

Scale in Miles

STRAWBERRY FAULT

STINKING SPRINGS FAULT

TOWANTA FLAT GRABEN

DUCHESNE-PLEASANT VALLEYFAULT SYSTEM

ROOSEVELT, UTAH

BIG SAND WASH RESERVOIRENLARGEMENT PROJECT SITE

MODIFIED FROM HECKER (1993)

50m

iles

Geology Report - Appendix D - Faults.vsd 10/08/2003 04:15

Report Covers Dec 03 - Ira A. Fulton College of ...kmiller/projects/Big Sandwash Dam/Report/BSW... · BIG SAND WASH GEOLOGY REPORT ... sand, and silt. ... claystone and occasional

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