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1 Uinta Basin Replacement Project Big Sand Wash Reservoir Enlargement Geology Report Prepared for Central Utah Water Conservancy District Prepared by December 2003

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3 Central Utah Water Conservancy District Uinta Basin Replacement Project Big Sand Wash Reservoir Enlargement Geology Report December 2003 Reviewed and Recommended by: Michael J. Mickelson, P.E. CH2M HILL Project Manager Approved by: Kirk D. Beecher, P.E. Central Utah Water Conservancy District UBRP Project Manager Prepared by 4001 South 700 East, Suite 700 Salt Lake City, Utah 84107

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5 Contents Section Page 1 Introduction Geological Reconnaissance Site Geology General Geologic Setting Geologic Map Units Geologic Structure Geologic Hazards Recommendations General Conditions for the Proposed Project Dam Type and Construction Materials Foundation Conditions for the Main Dam Foundation Conditions for the East Saddle Dam Foundation Conditions for the West Ridge Dike and West Saddle Dam Foundation Conditions for the Spillway Seepage / Leakage from the Reservoir Potential Problems During Reservoir Operation References Appendices A B C D Site Geologic Map Geologic Cross Sections and Profiles Photographic Profiles of the Left Abutment (East Butte) Regional Fault Map Figure 1 Existing and New Structures CVO\ III

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7 SECTION 1 Introduction

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9 SECTION 1 Introduction The Big Sand Wash Reservoir Enlargement project forms a major part of the Lake Fork Section 203(a) Uinta Basin Replacement Project. In addition to the reservoir enlargement, the project also includes (1) a new diversion structure on the Lake Fork River to divert water from the Lake Fork River, (2) a new feeder pipeline to deliver water from the Lake Fork River to the Big Sand Wash Reservoir, and (3) a new pipeline to deliver water to Roosevelt and irrigation water to the State Road area near Roosevelt. The contract documents for the Big Sand Wash Dam Enlargement project are presented in five volumes: Volume 1 contains the general conditions, the Division 1 specifications (special conditions), 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 and construction of the existing dam and information collected during CH2M HILL s investigations for the new dam. Volume 5, the Geotechnical Baseline Report for Outlet Works, contains a summary of the geologic and geotechnical information, a description of the anticipated ground conditions, and a prediction of the ground behavior during construction of the outlet conduit and vertical shaft. In addition to these five volumes, three additional design reports were prepared. These design reports are being made available to the Contractor, but are not part of the contract documents and are for the Contractor s information only. These reports are: The Geology Report (this report), which describes the geologic reconnaissance and mapping conducted at the site, provides engineering geologic evaluation of the site conditions, and presents a site geologic map, geologic cross sections and profiles, and seismotectonic 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 outlet works, and (3) recommendations for design. The Hydrology and Hydraulic Structures Report, which describes the hydrologic and hydraulic engineering analyses and results, and the hydraulic and structural design of the spillway and the outlet works. The existing Big Sand Wash Reservoir (Figure 1) is owned by the Moon Lake Water Users Association and was designed by Todd and Horrocks, Inc. with geotechnical investigation CVO\

10 BIG SAND WASH GEOLOGY REPORT and design by Fuhriman, Rollins and Company. The reservoir is contained by one main dam, a natural ridge on the west (right abutment) and a natural butte on the northeast (left abutment), and two constructed saddle dams (east and west). The dams were constructed in and the final inspection was on December 21, It is proposed that the Big Sand Wash Reservoir will be enlarged from 12,050 acre-feet to 24,100 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 using microtunneling techniques and a vertical construction shaft A new concrete-lined spillway will be constructed consisting of a concrete gravity dam overflow 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 the majority 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 dam as 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 detailed description of the enlargement project. 1-2 CVO\

11 SECTION 2 Geological Reconnaissance

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13 SECTION 2 Geological Reconnaissance An initial geological reconnaissance was performed in February 2002 to establish boring locations for the geotechnical exploration and to identify potential borrow areas. Later, in March and April 2002, a CH2M HILL engineering geologist performed geologic mapping at the site. The geologic mapping included observations and descriptions of bedrock and unconsolidated geologic deposits, and their expected effects on dam reconstruction and use as 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 Design Report and the contract documents for a description of the main dam options), a desire was expressed to obtain more specific geologic data concerning the left abutment (northeast butte) of the dam. Because the left abutment experienced seepage problems during first filling, and the stability of the proposed cutoff wall heavily depends on the geologic stability of 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 CH2M HILL engineering geologists. In addition, geotechnical exploration programs were conducted in 2002 and 2003 that included 36 borings, 44 test pits and 4 test trenches located in the vicinity of the main reservoir, and 67 test pits located off site. The borings were advanced to determine the rock stratigraphy, conduct in situ water pressure (Packer) tests, and obtain samples for laboratory testing. The test trenches were dug to examine the bedrock surface for use in designing the surface preparation, and the test pits were dug to obtain samples for laboratory testing and to examine potential borrow sources for embankment materials. The Geotechnical Data Report contains the full geotechnical exploration results. From the information obtained from the geological reconnaissance and geotechnical explorations, a site geologic map was produced (Appendix A). Geologic profiles were produced 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 2003 geological reconnaissance (Appendix C). One photographic profile is of the entire left abutment, looking northeast from the right dam abutment. The locations of Boring D-8 and D-9 are shown on the profile. The second photographic profile is a modified black and white photograph taken during construction of the key trench for the existing main dam. This photograph is looking northeast and shows the excavation of the key trench, the installed grout 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, and were based on the response of the rock core to resistance, and scratching and/or breakage tests. 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 CVO\

14 BIG SAND WASH GEOLOGY REPORT 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 2-2 CVO\

15 SECTION 3 Site Geology

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17 SECTION 3 Site Geology 3.1 General Geologic Setting The project site is located within a regional geologic feature known as the Uinta Basin. The geologic setting consists primarily of near-horizontally bedded sedimentary rocks incised by rivers draining from the Uinta Mountains to the north. The bedrock in the area consists primarily 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 in the 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 stream activity and much of the outwash is found as isolated remnants on flat-topped buttes and mesas in the area. River-transported (alluvial) deposits are present in the flat-bottomed valleys. Wind-blown (eolian) deposits have covered the bedrock and alluvial deposits in some areas. The geologic units are described below in more detail, and shown on the geologic 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 of gravel, sand, and silt. Qe + Qal: Eolian and Alluvial deposits. Eolian and alluvial deposits were found in the valley bottoms along Big Sand Wash and its tributaries. These deposits consist primarily of 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 units were mapped together in Big Sand Wash because of the absence of a distinction between the two units. The eolian deposits consist primarily of very fine silty sand deposited by wind, whereas the alluvial deposits in the valley bottom consist largely of reworked eolian deposits. In addition, eolian deposits cover much of the alluvial deposits in the valley 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 the reservoir. These deposits were thinner at the valley edges where they pinched out against the sandstone. Test pits and boreholes indicated that this material exceeded 14 feet in thickness in some parts of the valley downstream of the dam. The geologic cross sections in Appendix B show this relationship and the geometry of the eolian/alluvial deposits. CVO\

18 BIG SAND WASH GEOLOGY REPORT Qe: Eolian deposits. A thick eolian (wind-blown) deposit not mixed with alluvial deposits was observed on the valley wall southeast of Big Sand Wash Dam. This material consists primarily of tan, poorly graded, non-plastic, fine silty sand to sandy silt. These deposits were distinctive from the mixed alluvial/eolian deposits described previously because they were deposited higher on the valley walls and consist of purely wind-blown deposits. Test pits indicated that this deposit is at least 13 feet thick, with a maximum thickness estimated to be approximately 30 feet, based on interpretation from topographic and geologic cross sections. Qgo: Glacial Outwash deposits: Glacial outwash deposits were mapped in several locations at the site. The glacial outwash deposits consist of rounded quartzite gravels within a fine to coarse sandy matrix. The gravels are typically up to 12 inches in diameter, but occasionally boulders in excess of 2 feet were observed. The upper and lower 2 to 3 feet of the glacial outwash deposits appear to be cemented in some areas and are more typical of conglomerate bedrock. The glacial outwash was deposited by post-glacial streams and was found primarily on the butte between the dam and east dike, on the linear northwest-southeast-trending ridge 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 the Pleistocene on surfaces with differing elevations (Bryant, 1992). Although the glacial outwash deposits in the vicinity are of different ages, there is little or no difference in material properties. The estimated thickness of the outwash deposits varies. A portion of the outwash deposits 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. Much of the outwash on the linear ridge southwest of the dam and reservoir was also removed for use as riprap on the dam and dikes. However, on the ridge south and southeast of the dam, glacial deposits between 8 and 10 feet thick were observed. Glacial outwash materials 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 unit surrounds much of the reservoir and underlies all of the previously described unconsolidated geologic units. Bryant (1992) describes the Brennan Basin Member as Tertiary-age, moderate red, grayish-red, reddish-brown, yellowish-brown, and yellowish-orange sandstone and less-abundant siltstone and claystone. Jones (1957) described the Duchesne River Formation as fluvial channel sandstone, sandy conglomerates, and sandy siltstone. This formation was formed by erosion of the Uinta Mountain uplift and subsequent deposition of sediments in the Uinta Basin as it was subsiding. The sediments were deposited by braided streams and thus consist of sandy channel-fills and discontinuous lensatic beds, and siltstone and claystone where the sediments were deposited in shallow lakes. Because of the depositional nature of the bedding, the stratigraphy and lithology of the beds varies horizontally. Numerous lens-shaped channel cuts filled with cross-bedded sandstone was observed in the vicinity. Also, claystone layers have been scoured away and truncated by fluvial processes. During the geologic reconnaissance, 3-2 CVO\

19 3. SITE GEOLOGY numerous profiles were constructed between borings. It was evident from these profiles that 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 Sand Wash typically consists of reddish to pinkish-purple, fine-grained, slightly to moderately weathered quartzitic sandstone with occasional layers of medium-grained sandstone. The hardness of the sandstone is typically in the R1 to R2 range, which indicates very weak to weak rock. Often the core samples of sandstone were easily eroded and could be broken down to sandy material by hand. In some locations the sandstone eroded to sandy material in the borehole during drilling and required grouting to keep the holes open. The sandstone layers were observed to be up to 25 to 35 feet thick in some of the borings. Several interbedded layers of fine sandy siltstone, siltstone, silty claystone and claystone were also observed in outcrops and core samples. The siltstone could typically be described 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, portions of the claystone were highly weathered to clayey material and clayey rubble. Based on the boring logs, claystone and siltstone layers are more prevalent in the valley near the toe of the dam than sandstone layers. The contacts between the sandstone, siltstone, and claystone were often gradational and would transition gradually between the different rock types. However, some contacts of the sandstone overlaying the claystone were abrupt. Sequences of interbedded siltstone and claystone more than 40 feet thick were observed in some of the borings. Geologic units not shown on the geologic map: Thin and discontinuous geologic units including colluvial deposits, eolian deposits, and glacial deposits were also observed but are not shown on the map. These units were not mapped because either they were too small to show, or not available in sufficient quantities to be useful. Colluvial deposits consist of unsorted gravel, sand, and silt that mantle steep slopes typically underlain by sandstone. The gravel is typically comprised of well-graded angular sandstone clasts, but also contains rounded quartzite gravels eroded down from the glacial outwash deposits. Some colluvial materials in claystone deposits appear to have a higher percentage of clay. The colluvial deposits formed as a result of mass wasting processes and weathering of the underlying bedrock. The colluvial deposits form a thin (usually less than 5 feet thick or so), discontinuous cover on the slopes of the butte northeast of the dam, the valley walls downstream from the dam, and the sides of the ridge southwest of the reservoir. It is not anticipated that the colluvial materials would be useful as borrow materials. Eolian deposits, in addition to those described previously, were also found throughout the area. These deposits are typically thin (less than approximately 5 feet thick) and discontinuous and overlay the sandstone bedrock. The bulk of these deposits are scattered in the flat areas southeast of the east dike, and northeast and northwest of the CVO\

20 BIG SAND WASH GEOLOGY REPORT reservoir covering the sandstone. It did not appear that there are adequate quantities of these scattered eolian deposits to be used as borrow materials, and mining these deposits for a material source would likely be impractical and not cost-effective. Remnants of glacial outwash deposits are present on the ridge southwest of the reservoir. Much of the original material was stripped away for use during the original dam construction. The material left is typically less than 2 to 3 feet thick and only discontinuous patches remain. It appears there is an inadequate quantity of outwash material in this area to consider as a material source and it was therefore not shown on the map. 3.3 Geologic Structure Dividing planes in the rock mass, such as joints and faults, are referred to as the rock structure. Rock structure influences rock mass strength and seepage properties. No faults were 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 in the sandstone. The bedding planes at the site are essentially horizontal, although it appears there is a slight regional dip to the north (less than 10 degrees or so). The bedding planes are continuous over large distances; however, the bedding planes observed in the vicinity of the dam are typically wavy because of cross-beds in the sandstone, particularly where the sandstone was deposited directly on top of the claystone. The amplitude of these asperities is up to 2 or 3 feet in some areas, and some of the claystone beds are actually truncated by erosional channels filled with sandstone. In addition to the major bedding planes, some areas of very thin bedded cross beds were observed within the sandstone cliffs and outcrops. The cross-bedding resulted in thin planes of sandstone. The bedding planes, in particular the contact point between sandstone and claystone, have an effect on the mass hydraulic conductivity and especially the horizontal and vertical movement of groundwater. Horizontal bedding results in higher horizontal hydraulic conductivity values. During the geologic mapping, joints were observed in the sandstone outcrops. Most of the joints observed are roughly vertical, generally widely spaced, and open in the vicinity of the outcrops. Two sets of joints were identified: a northwest-trending vertical joint set, and a northeast-trending vertical joint set. The northwest-trending joints are likely responsible for the geomorphic expressions of the distinctive northwest-trending linear ridge on the southwest side of the reservoir, and the oval-shaped northwest-southeast trending butte northeast of the left abutment. Erosional patterns often follow discontinuities in rocks, and joint sets often form anisotropic lines of weakness and thus contribute to structural control of drainage patterns. The large apertures (openings) of the joints observed near outcrops are likely caused in part by expansion of the rock from stress relief and lack of overburden pressure, and possibly spreading of the sandstone on weaker claystone layers. The joints likely have smaller apertures deeper in the subsurface as a result of higher confining stresses. The apertures of the joints are important because joints act as seepage conduits for water transmission. It appears that the high water take in some sandstone layers observed during the water pressure testing may have been related to jointing in the sandstone beds. 3-4 CVO\

21 3. SITE GEOLOGY 3.4 Geologic Hazards Landslides Small 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, the general location of these slumps is shown on the map. These slumps originated in eolian and colluvial materials that covered the valley walls and appear to have been initiated by seepage emanating from the valley walls. These slides appear to be relatively shallow and limited to surficial materials. Although small, these slides represent unstable areas and would not provide suitable foundation conditions for dam abutments. The surficial materials in which these slides occurred would have to be removed down to competent bedrock where the main dam contacts the abutments. Faults/Earthquakes No faults were observed in the vicinity of the dam site during the geologic mapping, and no evidence of active faulting was evident in the immediate vicinity. However, according to published geologic mapping, faults that are considered active and that may contribute to seismic hazards have been mapped within a 50-mile radius of the dam site. Four different faults and groups of faults may cause concern. These include the Duchesne-Pleasant Valley fault 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) included in Appendix D. The following is a brief description of each fault or potentially seismogenic feature. The Duchesne-Pleasant Valley fault system is a system of east-west trending faults approximately 20 miles long that approaches within 11 miles south of the Big Sand Wash Dam 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 that scarps along the faults are fault line features but not late-quaternary scarps. In addition, the trend of these faults is roughly perpendicular to the stress regime in the region (that is, most of 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 miles northwest of the Big Sand Wash Dam site. These faults offset late Quaternary deposits. The age of most recent activity on this fault system is estimated to be 130,000 to 500,000 years ago. However, based on field evidence, scarps along this fault may not be seismogenic in origin 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 miles west of the dam site. This fault is estimated to be late-quaternary in age based on the rather ambiguous criteria of range-front morphology and drainage disruption. This fault lacks direct evidence of Holocene movement; however, the inferred rupture length suggests that displacement may occur in events with an estimated magnitude of around 6.5. The recurrence interval is not well-constrained on this fault (Nelson and Martin, 1982; Van Arsdale, 1979). CVO\

22 BIG SAND WASH GEOLOGY REPORT The Strawberry Fault is a large, north-trending normal fault located approximately 46 miles west of the dam site. The age of recent displacement on this fault is estimated to be early to middle Holocene, which would make it the most active fault within a 50-mile radius of the site. In addition, the data and age constraints for movement on this fault are more accurate than for the other faults. A minimum of two to three events have occurred on this fault since 15,000 to 30,000 years before present, based on carbon dating. Based on the offset of the movements and the recurrence interval, the slip rate of this fault is estimated to be 0.04 to 0.17 mm/year over the last 15,000 to 30,000 years. The estimated maximum credible earthquake 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 and Martin, 1982; Nelson and Van Arsdale, 1986). According to the U. S. Geological Survey s Earthquake Hazards Program, the probabilistic Peak Ground Acceleration at the site is 0.08g for a 500-year return period, 0.12g for a 1,000-year return period, and 0.2g for a 2,500-year return period. Liquefaction Liquefaction hazards appear to exist in the vicinity of the main dam. Based on geologic mapping and drilling information, it appears that liquefiable soils are present in the eolian/alluvial deposits found in the valley bottom downstream from the dam. These deposits 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. In addition, 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 the potentially liquefiable soils appears to be 10 to 15 feet. Potentially liquefiable soils were also observed in Boring D-21, which was drilled from a barge in the forebay of the dam. SPT N-values indicate very loose fine silty sand to silt, with blow counts between 3 and 8 blows per foot. The thickness of these potentially liquefiable soils is approximately 7 to 8 feet. It is likely that this layer is recently deposited sediment within the reservoir, and not the noted alluvial layer. Springs/Seepage Springs and areas of seepage were observed in the valley walls downstream from the Big Sand Wash Dam. The primary mechanism for the seepage appeared to be water moving horizontally along sandstone layers underlain by claystone layers. Downstream of the right abutment, springs were emanating from the valley wall at an elevation between approximately 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, it appears that the springs in this area pre-date the dam construction and may be related to irrigation in the field southwest of the reservoir, storage of water within the ridge, or they may emanate from a regional aquifer. The groundwater appeared to be seeping in an east to northeast direction through the narrow ridge and discharging in the valley wall. Although the likely transmission mechanism for the seepage appeared to be open joints, it was observed that the sandstone itself was saturated in some areas, indicating that seepage is occurring through the sandstone mass as well as through the fractures. Downstream from the left (northeast) abutment, diffuse seepage was evident in several locations. The highest observed level of seepage was occurring at an elevation between 5, CVO\

23 3. SITE GEOLOGY and 5,840 feet, and appeared to be seeping from the same sandstone/claystone contact where the springs were discharging on the opposite valley wall. However, the flow appeared to be at a lower elevation in the northeast valley wall. Other areas of seepage were observed 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 distance of the horizontal seepage appeared to be the result of the water flowing horizontally through the sandstone above the claystone layers and eventually discharging from the valley walls. Horizontal drains were installed in the vicinity of the left abutment to help drain 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 left abutment. This area was very soft and unstable, and pumping was observed when heavy equipment was driven across it. This area coincides with the area potentially susceptible to liquefaction. 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 the overall performance of the dam. Water losses appeared to be relatively small and seepage and 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 little danger of significant deterioration of the dam or dike foundations, or the potential for foundation failure. However, piping at the interface of the dike core and bedrock surface may be a concern. Seepage may be occurring through the west ridge through highly permeable sandstone layers. It appears that saturated ground is present, and riparian vegetation is growing on the southwest toe the west ridge. However, it could not be determined if the saturated ground conditions were caused by leakage and overflow from a nearby ditch. Minor seepage may also be occurring under the southwest portion of the east dike. The ground surface is covered with grasses and riparian vegetation. This portion of the east dike is underlain by fine silty sand, which may be more permeable than the surrounding silty sand; however, no actual seepage or springs were observed. Erosion Erosion of weak claystone layers was observed in the spillway area at the southwest side of the west dike. During past water flow over the natural rock spillway channel, the weaker claystone layers eroded more quickly than the more resistant sandstone layers and undermined the sandstone layers. This phenomenon is known as headward erosion. Although this activity does not immediately endanger the reservoir, continued headward erosion may undermine rock layers closer to the reservoir and result in subsurface leakage in the vicinity of the spillway. Typically, if constant leakage is allowed to begin, the result is rapid erosion of the material, which could potentially be disastrous. Areas around the shoreline of the reservoir, in particular the base of the butte and along the west dike, show evidence of erosion of the claystone layers. This erosion has resulted in rockfalls of large sandstone blocks. Although this process is not necessarily a concern at present, if dikes or structures are reconstructed near shoreline areas, armoring weak layers is recommended to prevent future undermining and erosion. CVO\

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25 SECTION 4 Recommendations

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27 SECTION 4 Recommendations 4.1 General Conditions for the Proposed Project Overall, the topographic, geologic, and geotechnical conditions appear to be suitable for the proposed embankment raises of the Big Sand Wash Reservoir Enlargement project. The existing dam has performed fairly well during its operating lifetime. However, important issues for consideration include: (1) obtaining the large volume of materials required for embankment construction, (2) liquefaction potential of foundation soils downstream of the main dam, (3) liquefaction of foundation soils beneath the existing main dam and east saddle dam, (4) stability of the west ridge, and (5) zones of high water take observed during drilling and water pressure testing. No other geologic hazards, such as active onsite faults, major landslides, or other slope stability problems, were identified for the natural formations. Evaluations of the embankments are in the Geotechnical Design Report. 4.2 Dam Type and Construction Materials Three 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 sufficient percentage of fines in some areas to meet the requirements of core material for the embankments. The volume of this material will have to be evaluated carefully, because the 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 variable nature of these deposits. The material suitable for core construction will have to be segregated and stockpiled during excavation. In addition, removing the eolian/alluvial materials from the valley downstream from the dam will probably create ponds and wetlands because of the shallow water table, and may therefore require reconstruction of the outlet canal. These factors may limit the amount of available material in this region. Eolian materials: These poorly graded fine sands and silty sands appear to meet the requirements for shell materials for the embankments, but lack the percentage of fines required for core materials. One potential limiting factor of this material is a possibly low angle of internal friction resulting from the poorly graded, fine-grained nature of the material. This could be compensated for by flattening the slopes of the dam. Another limiting factor for this material is a possibly insufficient quantity, given the large volume anticipated for construction of the embankments. Glacial outwash gravels: These hard, quartzite gravels may be suitable for armoring the exterior of the dam. These gravelly deposits also appear suitable for aggregate and rock fill, if crushed and processed. The volume of these deposits will also have to be evaluated in detail to determine whether there are sufficient quantities. If the quantity in CVO\

28 BIG SAND WASH GEOLOGY REPORT 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 west ridge. 4.3 Foundation Conditions for the Main Dam The sandstone and claystone bedrock of the Duchesne River Formation will provide a suitable foundation for the main dam. This bedrock underlies the valley at the dam site, but is covered by 15 to 20 feet of eolian and alluvial soils. The core/cutoff trench should be excavated through the soils down to competent bedrock. In addition, potentially liquefiable soils should be removed from the dam footprint. It is anticipated that liquefiable soils are approximately 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 are exposed. Claystone exposed in the excavations may require special protection prior to dam construction to prevent desiccation and degradation of this material upon being exposed to weathering elements. Profiles A and D (Appendix B) show estimated bedrock and subsurface conditions beneath the dam and abutments. 4.4 Foundation Conditions for the East Saddle Dam Foundation conditions for the east saddle dam appear favorable. The existing dam is underlain by eolian soils and bedrock. However, to prevent leakage beneath the dam, the eolian soils underneath portions of the dam should be excavated down to competent bedrock. Based on drilling information, these eolian soils are up to 10 feet thick. It appears some subsurface flow is occurring beneath the existing dam. If the reservoir level is raised 30 feet, the flow underneath the dam will likely increase as a result of the elevated reservoir and higher hydraulic head. The non-plastic eolian soils may also be subject to piping. The surface of the dam should be protected with cobble- to boulder-sized gravel, especially on the reservoir side, to limit wave erosion. Profile E in Appendix B parallels the east saddle dam and shows the estimated subsurface conditions beneath the dam. 4.5 Foundation Conditions for the West Ridge Dike and West Saddle Dam Foundation conditions for the west ridge dike appear to be favorable. The dike will be underlain by bedrock including claystone, siltstone, and sandstone. One potential concern is that the weak claystone layers could reduce the stability of the west dike by acting as sliding failure planes, although during the subsurface investigation no extremely weak, weathered claystone layers were identified as potential sliding planes. In addition, preliminary laboratory 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 4-2 CVO\

29 4. RECOMMENDATIONS in detail. Geologic cross sections have been constructed to support the stability analyses. In addition, zones of high potential seepage will have to be addressed to prevent excessive leakage through the west ridge and dike. Northwest of the west ridge, the foundation conditions for the west saddle dam appear favorable. The existing saddle dam is underlain by sandstone and claystone near the surface and 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 (limited seepage expected). Profile C in Appendix B parallels the west ridge and shows subsurface conditions beneath the ridge. Profile B is a cross section perpendicular to the west ridge. 4.6 Foundation Conditions for the Spillway Foundation conditions for the spillway appear favorable. The existing spillway flows over a natural channel underlain by interbedded sandstone and claystone. However, erosion of weak claystone layers has occurred during past flows, leading to undermining of the sandstone layers and resulting in headward erosion toward the reservoir. It appears this erosion of the claystone occurs extensively during each flow. A new spillway in the same location would have to be constructed to accommodate the raised dike. The new spillway should be constructed to avoid continued erosion of the existing channel. Erosion during future flood events could undermine more claystone layers and potentially endanger the integrity of the east dike and the reservoir in general. The existing channel could be armored and/or retrofitted with flow energy dissipators to limit future channel erosion. 4.7 Seepage / Leakage from the Reservoir As previously discussed, leakage around the main dam abutments is ongoing. The proposed reservoir enlargement should address leakage in the area of the main dam to prevent potential stability and piping problems on the downstream side of the dam. The abutment foundations should be excavated to competent bedrock to avoid founding the dam on fractured bedrock or soils that may contain zones with high hydraulic conductivity values. A core/cutoff trench excavated through soils down to the underlying competent bedrock would reduce the potential for seepage under the dam. Water pressure testing in borings at the dam abutments showed zones of high water take in the sandstone layers of the abutments. The mass hydraulic conductivity of these zones may be reduced by installing a grout curtain. A test-grouting and water pressure testing program should 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 the reservoir 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 the southwest side of the west ridge. It appears that seepage is also occurring under portions of the east saddle dam, where riparian vegetation is growing in an otherwise sand- and CVO\

30 BIG SAND WASH GEOLOGY REPORT sagebrush-covered area. Water pressure testing in borings drilled into the west ridge and below the east saddle dam showed zones of potentially high seepage in the sandstone layers that underlie these areas. Grouting may reduce the mass hydraulic conductivity of these zones. As with the main dam, a test-grouting and water pressure testing program should be used to determine the effectiveness of a grouting program. 4.8 Potential Problems During Reservoir Operation No major problems are anticipated during reservoir operation. Minor problems that may occur include slope stability and slumping in saturated sediments around the rim of the reservoir. However, these stability issues should not affect the overall performance of the dam. 4-4 CVO\

31 SECTION 5 References

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33 SECTION 5 References Bryant, B., Geologic and Structure Maps of the Salt Lake City 1 X 2 Quadrangle, Utah and Wyoming. U. S. Geological Survey Map I Scale 1:125,000. Hecker, S., Quaternary Tectonics of Utah with Emphasis on Earthquake-Hazard Characterization. Utah Geological Survey Bulletin 127. Jones, D. J., Geosynclinal Nature of the Uinta Basin in "Guidebook to the Geology of the Uinta Basin." Intermountain Association of Petroleum Geologists Eighth Annual Field Conference, Otto G. Seal, ed. Martin, R. A., Nelson, A. R., Weisser, R. R., and Sullivan, J. T., Seismotectonic Study for Taskeech Dam and Reservoir site, Upalco Unit and Upper Stillwater Dam and Reservoir Site, Bonneville Unit, Central Utah Project, Utah: Denver, U. S. Bureau of Reclamation Seismotectonic Report 85-2, 95 p. Nelson, A. R. and Martin, R. A. Jr., 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., Quaternary Faulting on Towanta Flat, Northwestern Uinta Basin, Utah, in Picard, M. D., editor, Geology and Energy Resources, Uinta Basin of Utah: Utah Geological Association Publication 12, p Nelson, A. R. and Van Arsdale, R. B., Recurrent late Quaternary Movement on the Strawberry Normal Fault, Basin and Range Colorado Plateau Transition Zone, Utah: Neotectonics, v. 1, p Osborn, G. D., Quaternary Geology and Geomorphology of the Uinta Basin and the South Flank of the Uinta Mountains: Berkeley, University of California, Ph. D. dissertation. Sullivan, J. T Seismotectonic Study for Starvation Dam, Bonneville Unit, Central Utah Project, Utah: Denver, U. S. Bureau of Reclamation Seismotectonic Report 88-7, 14 p. Van Arsdale, R. B., Geology of Strawberry Valley and Regional Implications: Salt Lake City, University of Utah, Ph. D. dissertation, 65 p. CVO\

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36 APPENDIX A Site Geologic Map

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38 Qf Qe+Qal Qe Qgo Tdb N Qe + Qal Qe + Qal Qgo

39 APPENDIX B Geologic Cross Sections and Profiles

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47 APPENDIX C Photographic Profiles the Left Abutment (East Butte)

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49 ACCESS ROAD FOR D-8 VARIATIONS IN ROCK COLOR ACCESS ROAD FOR D-9 TALUS LOOSE BLOCKS AND TALUS SANDSTONE COVERED BY TALUS CONTINUOUS SANDSTONE LAYERS D-9 D-8 SPOILS FROM ACCESS ROAD SPOILS HIGH WATER MARK SANDSTONE CHANNEL SANDSTONE CLAYSTONE SANDSTONE CHANNEL SANDSTONE WAVY CONTACT BETWEEN SANDSTONE AND CLAYSTONE TALUS CLAYSTONE SANDSTONE LOOSE BLOCKS DAM CREST NOTE LENS-SHAPED CHANNEL CUT; DISCONTINUOUS SANDSTONE LAYERS LOOSE SURFICIAL BLOCKS AND TALUS CONTINUOUS THICK SANDSTONE LAYER

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51 D-8 EL D-9 EL SANDSTONE CLAYSTONE APPROX. DAM CREST EL SANDSTONE SANDSTONE CLAYSTONE SANDSTONE GROUT TRENCH THICK SANDSTONE NO OFFSET APPARENT CLAYSTONE FLOOR OF CUTOFF TRENCH EL FT.

52 APPENDIX D Regional Fault Map

53 Scale in Miles 50 miles TOWANTA FLAT GRABEN ROOSEVELT, UTAH BIG SAND WASH RESERVOIR ENLARGEMENT PROJECT SITE STRAWBERRY FAULT DUCHESNE-PLEASANT VALLEY FAULT SYSTEM STINKING SPRINGS FAULT REGIONAL FAULT MAP MODIFIED FROM HECKER (1993) GEOLOGY REPORT UINTA BASIN REPLACEMENT PROJECT BIG SAND WASH RESERVOIR ENLARGEMENT Geology Report - Appendix D - Faults.vsd 10/08/ :15