CONCEPTUAL DAM DESIGN

Transcription

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2 II. CONCEPTUAL DAM DESIGN The geologic assessment resulted in moving the dam site several thousand feet upstream from the original proposed location. The most favorable of the sites considered, from a geological standpoint, is designated as 3A on Figure I-5 in Section I. The alignment was rotated upstream on the left abutment to avoid impacting the U.S. Forest Service land. We computed a reservoir capacity of 8660 acre feet with a dam height of 198 feet at the original site. The selected site has a reservoir capacity of 8650 acre feet plus an additional 1,050 acre feet from excavation of embankment fill from the reservoir basin, resulting in a total reservoir capacity of about 9,700 acre feet with a dam height of 158 feet at dam centerline. Initially, consideration was given to three potential dam types. These included a zoned Earthfill embankment, a Roller Compacted Concrete (RCC) embankment, and a Rockfill embankment with an impervious core. Upon completion of the field investigations, it became readily apparent that a zoned Earthfill embankment will be the most efficient dam for this site. The depth to bedrock and poor quality of the bedrock result in very poor foundation conditions for an RCC type structure. Mitigation of the foundation conditions to support an RCC dam, if feasible, would also increase the cost significantly. A Rockfill embankment with an impervious core allows the slopes to be steepened, reducing total embankment volumes. This is a viable option where a rock quarry can be established in close proximity to the site. The borrow investigations show there to be excellent sources of material for a zoned Earthfill dam within the reservoir basin. There does not appear to be a sufficient quantity of rock within the basin for a Rockfill embankment. This site is best suited technically for construction of a zoned Earthfill dam, with an impervious core and gravelly outer shells. The Earthfill dam also represents the most cost effective structure of the three structures considered. Shown in Figure II-1 is the proposed embankment at the maximum section. II.1 FOUNDATION TREATMENT II.1.1 REMOVAL OF OVERBURDEN SOILS Drill hole 15-2 was located in the valley floor near the downstream toe of the proposed dam embankment. This drill hole encountered relatively loose deposits of sandy gravel to silty sandy gravel with cobbles in the upper 15 feet. Below 15 feet the sand and gravelly soils ranged from dense to very dense. Based upon liquefaction analyses, the soils above 15 feet will liquefy during Page II-1

3 the design seismic event. Liquefaction of these deposits will cause a substantial loss of shear strength and result in slope failures. To mitigate the liquefaction potential, we have assumed that the upper 20 feet will need to be removed from beneath the embankment footprint throughout the valley floor. Based upon the results of standard penetration tests in Drill Hole 15-2, it appears that the granular deposits below 15 feet will not liquefy during a seismic event. A rigorous subsurface investigation program will be required during final design to determine if the feasibility drill hole is representative of conditions beneath the embankment in the valley floor. II.1.2 SEEPAGE CUTOFF To force the seepage away from the foundation-embankment contact and reduce seepage, it is proposed that a cutoff trench extend through the gravelly deposits and about 5 feet into bedrock. Based on the feasibility drill holes, this will require excavating approximately 20 feet on the right abutment and 75 to 120 feet across the valley floor and up the left abutment. It is recommended that the cutoff trench extend approximately 200 feet beyond the high water level on the left abutment to minimize end around seepage. Groundwater was encountered within 10 feet of the surface in the valley floor. Excavation will require a significant dewatering program consisting of a number of deep wells. The proposed cutoff trench is excavated with slopes of 1.5 horizontal to 1 vertical, extending about 5 feet into bedrock, with a bottom width of 40 feet as shown in Figure II-1. This excavation slope assumes that the foundation will be dewatered. Where feasible, dental concrete will be used to seal significant fractures and joints in the bedrock. The bedrock encountered in Drill Hole 15-1 and 15-2 was highly fractured and broken. We have assumed that a 6 inch cap of dental concrete may be required to prevent piping at the contact between the core and the bedrock. Consideration should be given in final design to using a flexible mix of cement-bentonite for the dental concrete, based on conditions encountered. For feasibility design, we have assumed that a grout curtain will extend about 125 feet into the bedrock, flaring into the right abutment to reduce end around seepage. The design intent is to extend the grout curtain to a depth where the rock permeability is below 300 ft/yr. II.2 EMBANKMENT SECTION A drawing showing the proposed zoned Earthfill dam section near the maximum section is presented in Figure II-1. The section includes a 40 foot crest width with side slopes of 3 Page II-2

4 horizontal to 1 vertical on the upstream side and 2 horizontal to 1 vertical on the downstream site. II.2.1 ZONE I IMPERVIOUS CORE Sources of lean clay have been identified in the reservoir basin and cutoff excavation which will serve well as an impervious core. The plasticity index of samples tested ranges from 13 to 31 with an average of 18. The material tested has very low amounts of soluble solids and is nondispersive. Two consolidated drained direct shear tests performed on this material resulted in a friction angle of about 32 degrees with 1 psi cohesion. It will be observed from Figure II-1 that the core begins 2 feet below the top of dam, is 20 feet wide, and slopes at 0.5 horizontal to 1 vertical both upstream and downstream. The core width to water head ratio is about 1 at the maximum section. II.2.2 FILTER AND DRAIN To protect against internal erosion and piping of the impervious core material, a 5 foot wide sand filter is shown on the downstream side of the core (Figure II-1) within the embankment. To prevent the downstream slope from becoming saturated, should cracking of the core occur, a 5 foot wide aggregate drain is shown downstream of the sand filter. A 2 foot wide sand filter is shown on both sides of the clay core in the cutoff trench to prevent piping. A 1 foot layer of sand filter and 2 foot layer of aggregate drain, constituting a downstream drainage blanket, have been placed between the downstream foundation and downstream embankment to collect any foundation seepage and protect the downstream embankment from becoming saturated. The drain will extend to a collection toe drain for monitoring. It is anticipated that the filter and drain material can be processed from on site deposits of granular material encountered in the drill holes and test pits. II.2.3 ZONE II EMBANKMENT UPSTREAM AND DOWNSTREAM OF ZONE I Sources of gravel with sand, silt, cobbles and occasional boulders have been identified in the reservoir basin. Between 1 and 3% of the material encountered in the feasibility investigation was greater than 12 inches. It is anticipated that the minus 12 inch outer Zone II material can be Page II-3

5 placed in lifts 15 to 18 inches thick before compaction. During final design, consideration should be given to providing a thin layer of downstream slope protection using the plus 12 inch size boulders. II.2.4 RIPRAP As shown in Figure II-1, we have included a 3 foot thick layer of riprap extending along the upstream slope to provide protection of the slope due to wave action. It is anticipated that this material can be obtained by quarrying bedrock from the areas shown in Figure I-7. II.3 PRELIMINARY SEISMIC DESIGN PARAMETERS The Provo section of the Wasatch fault zone has a straight-line end-to-end surface rupture length of approximately 59 km (37 mi). The length measured along the surface trace of the fault is approximately 77 km (48 mi). For a surface rupture length in the range of 59 to 77 km, the estimated maximum earthquake moment magnitude associated with the Provo section is in the order of 7.1 to 7.3. The USGS 2008 and 2014 updates to the US National Seismic Hazard Maps use a fault length of 77 km and a maximum magnitude of 7.27 (Peterson et al., 2008, USGS, 2015). Other faults, including sections of the Wasatch fault zone adjacent to the Provo section, contribute to the seismic hazard at the site; however, the Provo section is the controlling fault from a deterministic standpoint. Seismic design criteria, fault models, attenuation relationships, and geological studies are subject to relatively frequent updates and revisions. It is anticipated that the parameters discussed below will be updated and revised at the time of final design to account for such updates, along with the results of more detailed subsurface investigations. II.3.1 MAXIMUM CREDIBLE EARTHQUAKE The Maximum Credible Earthquake (MCE) parameters for design of high hazard dams in the State of Utah are required to be at least equal to the mean plus one standard deviation (84 th percentile) deterministic predictions. For this feasibility study, the 2008 NGA-West2 ground motion prediction equations published by the Pacific Earthquake Engineering Research Center (Abrahamson et al.; Boore et al.; Campbell and Bozorgnia; Chiou and Youngs; and Idriss; all Page II-4

6 2013) were used to estimate the median and 84 th -percentile Peak Ground Acceleration (PGA) and Peak Ground Velocity (PGV) for the site. Earthquake and fault parameters were selected to be consistent with the source parameters selected in the 2014 USGS maps. Sensitivity analyses were conducted to evaluate the sensitivity of the NGA calculations to the fault dip, which the 2014 USGS models assume may vary from 35 to 65 degrees, and the average site shear wave velocity in the upper 100 feet. The average shear wave velocity in the upper 100 feet may vary substantially by location within the dam footprint, and is also dependent upon the foundation elevation. A copy of the NGA-West2 calculation summary sheet for a dip of 50 degrees and a shear wave velocity of 550 m/s is included at the end of this section, along with a summary of the sensitivity analysis for the PGA and PGV values. Considering a most likely fault dip of 50 degrees and a most likely average shear wave velocity of 550 m/s (1800 ft/s), median and 84 th -percentile PGA values of 0.60g and 1.07g, respectively, were selected for feasibility-level seismic design analyses. The corresponding values of PGV are approximately 42 cm/s (17 in/s) and 73 cm/s (29 in/s), respectively. The NGA-West2 equations were also used to develop response spectra for simplified seismic deformation analyses. II.3.2 OPERATING BASIS EARTHQUAKE At the feasibility study stage, the Operating Basis Earthquake may be considered the probabilistic ground motion having a return period of 200 years. The 224-year mapped PGA determined using the 2008 USGS Interactive Deaggregation tool is 0.079g. It should be noted, however, that the 475-year and 2475-year probabilistic PGA values mapped in the 2014 USGS maps are 30 to 60 percent larger than those indicated in the 2008 deaggregations. This increase is in part attributable to revisions to the annual rate and slip rate parameters used in the 2014 USGS models (Peterson et al., 2015). It is anticipated that interactive deaggregation capabilities using the 2014 (and possibly future iterations) of the USGS models will be available at the time of final design. For the feasibility study, the 2008 mapped PGA for a 224-year return period was increased by 58 percent to estimate an approximate 2014 mapped 200-year PGA value of 0.12 g. To adjust for dense soil to soft rock conditions in the upper 100 feet, this value was multiplied by a Site Class C site factor of 1.2, resulting in a site-adjusted estimated OBE PGA of 0.15g. Page II-5

7 A summary page is attached which shows a comparison of the mapped PGA values for the project coordinates and a range of return periods, based on versions of the USGS deaggregations from the years 1996, 2002, and The approximated 2014 values, estimated by scaling the 2008 values to two points available for 2014, are also shown on the summary. It will be noted that the deterministic median PGA has a return period in the order of 2,500 to 3,000 years based on the 2008 deaggregation and a return period between 1,500 and 2,000 years based on the estimated 2014 hazard curve. The 84 th -percentile PGA corresponds to return periods of about 12,000 years on the 2008 curve and approximately 5,500 years on the estimated 2014 curve. II.4 EMBANKMENT STABILITY ANALYSIS This section describes the feasibility level slope stability analyses that were conducted for the conceptual earth fill embankment section described in this report. Plots showing the output from the slope stability analyses are included at the end of this section. The material properties used in each zone were estimated based on feasibility-level field and laboratory testing of the materials, experience with similar materials, and engineering judgment. Specific properties assumed for the various embankment and foundation zones are shown on the plots at the end of this section. Stability analyses were conducted using the computer program SLOPE/W. Spencer s method which satisfies both force and moment equilibrium was used to compute factors of safety. Circular trial failure surfaces were evaluated using a grid and radius search approach. After the critical surface was identified, the program performed an optimization routine in which points on the selected failure surfaces were iteratively adjusted to examine the potential for more critical failure surfaces having geometry that varied from the critical circular surface. Because the anticipated embankment and foundation zones do not include thin layers of distinctively weaker or stronger soil, the differences between the circular factors of safety and the optimized factors of safety were not dramatic. It is anticipated that the looser shallow soils will be removed from the entire dam footprint. The remaining foundation materials are relatively dense, and the embankment will consist of wellcompacted select clay core and gravel shell materials. Based on these conditions, it was assumed that soil strength loss in the foundation and embankment soils due to seismic ground shaking will not exceed 10 percent in the dynamic seismic and post-earthquake conditions. Page II-6

8 II.4.1 RESULTS OF SLOPE STABILITY ANALYSES The critical factor of safety calculated for each of the static stability cases is listed on the table below. The slope stability plots included at the end of this section show the critical failure surfaces determined for each case. It will be noted that the minimum required factors of safety are satisfied for a 3H:1V upstream slope and the 2H:1V downstream slope. The critical failure surfaces are in most cases relatively shallow. However, it was found in review of other nearcritical failure surfaces that the potential exists for significantly deeper failure surfaces having factors of safety only slightly greater than those determined for the critical shallow surfaces. Condition Static Steady-State Seepage Required Minimum Factor of Safety 1.5 Slope Assumed Reservoir Level Calculated Factor of Safety Upstream 3H:1V Full 2.44 Downstream 2H:1V Full /3 drawdown 1.66 Instantaneous Drawdown 1.2 Upstream 3H:1V 2/3 drawdown 1.52 Full Drawdown 1.46 Post-Earthquake 1.2 Upstream 3H:1V Downstream 2H:1V Full Full II.4.2 DYNAMIC SEISMIC STABILITY AND DEFORMATION A simple indication of the dynamic seismic stability of an embankment can be obtained by applying a pseudostatic coefficient equal to one half the design PGA value in the static slope stability analyses. Studies by Hynes-Griffin and Franklin (1984) found that earth dams having a factor of safety greater than 1.0 under a pseudostatic force of 0.5 times the peak acceleration would not undergo dangerously large deformations under the peak seismic acceleration. The pseudostatic load corresponding to a factor of safety of 1.0 is considered the yield acceleration of the slope. The results of the pseudostatic embankment stability analyses indicate that the upstream and downstream slopes have similar yield accelerations, ranging from as low as 0.18g if the soil strengths are reduced by 10 percent, to as large as 0.24g if the strengths are not reduced. Because the preliminary OBE PGA of 0.15g is less than the yield acceleration, the OBE is not expected to cause dangerously large deformations of the proposed embankment section. However, the MCE PGA of 1.07g is significantly greater than the calculated yield accelerations. Page II-7

9 Furthermore, a deformation analysis is required for the MCE by Utah Dam Safety regulations because the MCE PGA exceeds 0.35g. Seismic deformation estimates have been calculated using simplified approaches developed by Makdisi and Seed (1978), Jansen (1988), Swaisgood (2003), and Bray and Travasarou (2007). Dynamic parameters for the zones in the dam, including shear moduli and damping ratios, were estimated using correlations with laboratory test results and general properties of the proposed borrow materials (Seed and Idriss, 1970). The maximum acceleration and natural period of the embankments were estimated using the simplified methods of Makdisi and Seed (1977). A yield acceleration of 0.21g (average yield acceleration for upstream and downstream slopes considering 0 to 10 percent strength reductions) was used for the methods requiring an estimate of the yield acceleration. Embankment accelerations were estimated based on the NGA-West2 84 th -percentile deterministic response spectra computed for a site shear wave velocity of 550 m/s. The deformation estimates calculated using these methods are tabulated below. For the Makdisi and Seed method and the Bray and Travasarou method, the deformations shown are the sum of deformations calculated for the upstream slope and the downstream slope, while the deformations determined using the Jansen and Swaisgood relationships are more generic total crest settlements. Estimate Level Method Estimated Permanent Deformation (ft) for Median PGA (0.60g) for 84 th Percentile PGA (1.07g) Maximum Makdisi & Seed 4 14 Jansen 2 3 Average Makdisi & Seed 1 6 Jansen 1 2 Bray & Travasarou 1 4 Swaisgood 2 n/a* Minimum Makdisi & Seed <1 1 Jansen <1 1 *Swaisgood method applies up to about 0.7g, but predicts unreasonably large settlements for PGA in the order of 1g. Of the simplified methods used, the Makdisi and Seed approach is the most rigorous and likely to be the most applicable to the large MCE PGA applicable to the site. In final design, dynamic computer modeling of the embankment and foundation response to simulated earthquake ground shaking should be conducted to estimate deformations using the Newmark method. Page II-8

10 At the feasibility stage, it may be assumed that permanent embankment deformations associated with the 84 th -percentile deterministic ground shaking will average in the range of 4 to 6 feet. For the median deterministic ground shaking, the estimated permanent deformation averages 1 to 2 feet. Seismic performance will be a critical consideration if design of the project moves forward. In accordance with Utah State Dam Safety Rules, a factor of safety of 3 should be provided against overtopping due to seismic deformation. Application of a factor of safety of 3 to the largest calculated deformation of 14 feet, which is the cumulative deformation of upstream and downstream slopes computed for the 84 th -percentile deterministic ground motion using the upper-bound Makdisi and Seed parameters, would be very conservative. In our opinion, it would be appropriate for the factor of safety requirement to be applied to the average or best-estimate deformation calculated for the MCE (84 th -percentile ground shaking), which at this feasibility stage is 4 to 6 feet and would require 12 to 18 feet of freeboard (roughly equal to the maximum calculated deformation). The preliminary deformation estimates therefore indicate that, depending on the results of final analyses, it may be necessary to provide additional freeboard above the 10-foot freeboard height assumed in this feasibility study. II.5 EMBANKMENT SETTLEMENT The majority of the subsurface materials underlying the proposed dam site consist of relatively dense granular soils, underlain by rock. These materials are not susceptible to large settlements. The limited compression that does occur in the granular foundation soils and underlying rock materials will be relatively immediate, and construction of the embankment will automatically adjust for these relatively small settlements as they occur. Some loose soils were encountered in the upper 15 feet below the existing ground surface in Boring The conceptual dam design presented in this report assumes that these materials will be excavated from the dam footprint. A zone of clay encountered at depths of about 82 to 112 feet in Boring 15-1 could contribute to longer-term consolidation settlements; however, this clay will be excavated and removed from beneath the central clay core section of the dam. Excavation and replacement of the loose shallow soils and the deeper clay soils will prevent these settlement-prone materials from contributing significantly to settlements of the dam embankment. Page II-9

11 The clay core of the embankment section will experience some consolidation as the embankment is constructed and for several months to years thereafter. This consolidation of the embankment itself is typically accommodated by building camber into the dam crest. The amount of camber should be evaluated at final design, but will likely be in the order of 1 percent of the maximum vertical thickness of the clay core (approximately 30 inches). Settlement monuments installed along the dam crest will allow the progress and magnitude of this consolidation to be monitored, and grading of the crest can be adjusted after the majority of the settlement is complete. II.6 SEEPAGE ANALYSIS A seepage analysis was performed using the finite-element program SEEP2D to develop a preliminary estimate of the seepage which can be expected through the maximum section of the dam and the underlying foundation. The hydraulic conductivity values used in the model for the various embankment and foundation materials are shown along with the results of this analysis on a figure at the end of this section. With the reservoir full and under steady-state seepage conditions, the calculated total flow rate is approximately 72,000 cubic feet per year per foot of dam length, at the maximum section. This flow rate equates to about 1 cfs (500 gallons per minute) of seepage flowing through each 500- foot length of full-height dam. It would be conservative to apply this seepage rate to the entire length of the dam (approximately 1500 feet), since the seepage rate will be smaller through lower portions of the dam away from the maximum section. As a preliminary estimate, the total seepage through the dam and foundation is expected to be in the range of ½ to 2 cfs under steady-state seepage conditions with the reservoir full. The seepage analyses should be updated to reflect the results of additional field and laboratory testing at the time of final design. II.7 APPURTENANT STRUCTURES II.7.1 OUTLET WORKS It is anticipated that the outlet works will consist of a 36 inch diameter encased steel pipe placed on bedrock at the approximate location shown in Figure I-5. The concrete encasement for the steel pipe will require heavy reinforcement to resist the loads imposed by the approximately 100 foot of embankment fill. A hydraulically operated upstream gate will likely be the most efficient guard and regulating gate for the structure, with the hydraulic lines extending to a gate house on Page II-10

12 the dam crest. The outlet conduit will extend to an energy dissipater at the downstream end before entering the stream channel. II.7.2 SPILLWAY The most efficient location for a principal emergency spillway is around the right abutment, as shown in Figure I-5. The spillway must be designed to pass the inflow design flood (IDF) as determined using criteria established by the Utah State Dam Safety Engineer. A report prepared for Payson City in 2013 states that the watershed basin encompasses about 26.3 square miles and that the IDF has a maximum flow rate of about 1219 cfs. Based on our experience, it appears that this report significantly underestimates the IDF, and we recommend that the IDF be re-evaluated during final design. II.8 INSTRUMENTATION Instrumentation will be required to monitor dam performance. Settlement monuments are proposed at about 200 foot intervals across the dam crest. Piezometers are proposed to monitor the water level in the foundation and embankment. Two banks of piezometers with one set (foundation and embankment) located at the crest, one set mid slope and one piezometer at the toe are planned across the valley floor. In addition, piezometers on each abutment will be required. Toe drain collection pipes and monitoring structures will be required for each abutment and across the valley floor. Page II-11

13 II.9 OPINION OF PROBABLE COST Payson City Dam Crest Elev. 5,278 Ft. HW Elev. 5,268 Ft. Dam Ht. 178 Ft. HW 9,700 Ac. Ft. Approx. Length Along Cutoff 1,800 Ft. ENGINEERS OPINION OF PROBABLE COST Item No. ITEM Qty. Unit Unit Price Cost 1 Mobilization 1 Lump $1,000,000 $1,000,000 2 Clearing, Grubbing and Stripping 22 Acres $5,000 $110,000 3 Removal of Water 1 Lump $700,000 $700,000 4 Excavation, Unclassified 1,400,000 Cu. Yds. $6 $8,400,000 5 Excavation, Rock 12,200 Cu. Yds. $15 $183,000 6 Foundation Preparation 9,500 Sq. Yd. $40 $380,000 7 Dental Concrete 1,600 Cu. Yds. $250 $400,000 8 Grouting 56,000 Ln. Ft. $100 $5,600,000 9 Zone I 960,000 Cu. Yds. $5.50 $5,280, Zone II 2,100,000 Cu. Yds. $4.50 $9,450, Filter 69,100 Cu. Yds. $40 $2,764, Drain 54,200 Cu. Yds. $40 $2,168, Riprap 51,300 Cu. Yds. $30 $1,539, Toe Drain 1,700 Lin. Ft. $30 $51, Instrumentation 1 Lump $75,000 $75, Outlet Works 1 Lump $750,000 $750, Spillway 1 Lump $1,500,000 $1,500,000 Subtotal $40,350,000 15% $6,052,500 Construction Contract Total $46,402,500 Engineering 2% $807,000 Engineering 2% $807,000 Construction 4% $1,856,100 PROJECT COST $49,872,600 Page II-12

14 II.10 CONCLUSIONS AND RECOMMENDATIONS Based upon the results of this geologic and geotechnical feasibility study, the following conclusions and recommendations are provided. It must be recognized that the scope of work requested for this feasibility study resulted in limited geologic and geotechnical data upon which to draw conclusions. Additional investigations may identify conditions that could not be identified with the scope of work requested for this study. We reserve the right to modify conclusions and recommendations based upon the data obtained from future investigations. II.10.1 CONCLUSIONS We conclude, based upon studies performed, that it is feasible to construct a dam in Payson Canyon which will meet or exceed dam safety requirements regulated by the Utah State Engineer-Dam Safety Division. It is our opinion that locating the dam such that it does not impact the U.S. Forest Service lands is critical to the future success of the project. If the project impacts the Forest Service land, it will require a NEPA compliance study that far exceeds that which is required if federal lands are not affected. The NEPA compliance associated with impacted federal lands would delay the project for several years and add millions of dollars to the project cost. The dam site selected avoids impacting Forest Service lands. A storage capacity of 9700 acre feet with a maximum dam height of 158 feet at dam centerline is feasible at the selected dam site. Due to the geologic conditions described in Section I-Geologic Assessment, extensive foundation treatment will be necessary. The dam foundation will require a deep cutoff excavation extending into bedrock (ranging from 20 to 120 feet deep) and an extensive grout curtain in the bedrock extending approximately 100 to 150 feet in depth to reduce seepage to an acceptable level. The difficult foundation conditions and materials readily available for dam construction make a zoned Earthfill dam the preferred dam type for the site. Sufficient granular soils appear to exist within the dam foundation excavation and reservoir basin to construct the granular zone (Zone II) of the embankment, including the filter and drain material. Sources of lean clay have been identified in the left abutment foundation excavation and reservoir basin which can be used to construct the impervious section (Zone I) of the embankment. During feasibility design, it is preferable to identify at least 1.5 times the quantity required for construction. We have identified about 1.2 times the required Page II-13

15 quantity. Additional borrow investigations will determine if the remaining materials are available within the basin, or if it will be necessary to import off site materials. Sufficient quantities of riprap appear to be available on the right abutment for required spillway excavation and upstream of the right abutment in or adjacent to the reservoir basin. The preferred location for the outlet works and spillway is along and around the right abutment of the dam where the structures can be founded on bedrock. The engineer s opinion of probable cost for the embankment dam and appurtenant hydraulic structures including preliminary and final investigations, engineering design and contingencies is approximately $50 million dollars. Costs for property acquisition, gas line relocation and road relocation are not included in this cost. This results in a cost per acre foot of about $5155. This cost is approximately 1.5 times the cost of other recently constructed dams in the State of Utah with a capacity less than 10,000 acre feet. II.10.2 RECOMMENDATIONS Additional investigations are recommended prior to a decision to proceed with Final Design and Construction of the project. It is recommended that Preliminary Design Data Collection include the following: o Perform detailed trenching of the lineament feature located along the left side of the reservoir basin and left abutment of the dam is required to determine if it is depositional or a fault trace. We suggest that the City request the Utah Geological Survey update the published geologic map of the area and investigate the lineament feature. o Complete sufficient drill holes and trenching to verify location and age of faults in close proximity to the dam site. o Drill an additional 8 holes along the dam axis and embankment toes to more completely detail foundation conditions at the dam site. o Perform additional borrow investigations to validate material quantities and properties. This will require drill holes in the proposed riprap source and both Zone I and Zone II borrow areas to determine thickness of available materials, as well as additional test pits throughout each borrow area. Seek potential funding sources prior to initiating Preliminary Design Data Collection. Page II-14

16 II.11 REFERENCES Abrahamson, NA, Silva, WJ, and Kamai, R, 2013, Update of the AS08 Ground-Motion Prediction Equations Based on the NGA-West2 Data Set, Pacific Earthquake Engineering Research Center Report PEER 2013/04. Bray, JD, and Travasarou, T, 2007, Simplified Procedure for Estimating Earthquake-Induced Deviatoric Slope Displacements, Journal of Geotechnical and Geoenvironmental Engineering, American Society of Civil Engineers, 133, Boore, DM, Stewart, JP, Seyhan, E, and Atkinson, GM, 2013, NGA-West2 Equations for Predicting Response Spectral Accelerations for Shallow Crustal Earthquakes, Pacific Earthquake Engineering Research Center Report PEER 2013/05. Campbell, KW, and Bozorgnia, Y, 2013, NGA-West2 Campbell-Bozorgnia Ground Motion Model for the Horizontal Components of PGA, PGV, and 5%-Damped Elastic Pesudo- Acceleration Response Spectra for Periods Ranging from 0.01 to 10 sec, Pacific Earthquake Engineering Research Center Report PEER 2013/06. Chiou, BSJ, and Youngs, RY, 2013, Update of the Chiou and Youngs NGA Ground Motion Model for Average Horizontal Component of Peak Ground Motion and Response Spectra, Pacific Earthquake Engineering Research Center Report PEER 2013/07. Hynes-Griffin, M.E., and Franklin, A.G., 1984, Rationalizing the Seismic Coefficient Method, Miscellaneous Paper GL-84-13, US Army Corps of Engineers Waterways Experiment Station, Vicksburg, Mississippi, 21 p. Referenced by Kramer (1996). Idriss, IM, 2013, NGA-West2 Model for Estimating Average Horizontal Values of Pseudo- Absolute Spectral Accelerations Generated by Crustal Earthquakes, Pacific Earthquake Engineering Research Center Report PEER 2013/08. Jansen, R.B., 1988, Special considerations subsection in Chapter 9: Earthfill Dam Design and Analysis, Advanced Dam Engineering for Design, Construction, and Rehabilitation, Robert B. Jansen ed., Van Nostrand Reinhold, pp Kramer, S.L., 1996, Geotechnical Earthquake Engineering, Prentice Hall, Upper Saddle River, NJ, 653 p. Makdisi, F.I., and Seed, H.B., 1977, Simplified Procedure for Computing Maximum Acceleration and Natural Period for Embankments, Report No. UCB/EERC-77/19, Earthquake Engineering Research Center, University of California, Berkeley. Page II-15

17 Makdisi, F.I., and Seed, H.B., 1978, Simplified Procedure for Estimating Dam and Embankment Earthquake Induced Deformations, Journal of the Geotechnical Engineering Division, ASCE, Vol. 104, No. GT7, pp Petersen, MD, Frankel, AD, Harmsen SC, Mueller, CS, Haller, KM, Wheeler, RL, Wesson, RL, Zeng, Y, Boyd, OS, Perkins, DM, Luco, N, Field, EH, Wills, CJ, and Rukstales, KS, 2008, Documentation for the 2008 Update of the United States National Seismic Hazard Maps, US Geological Survey Open-File Report , 61 p. Petersen, MD, Moschetti, MP, Powers, PM, Mueller, CS, Haller, KM, Frankel, AD, Zeng, Y, Rezaeian, S, Harmsen, SC, Boyd, OS, Field, N, Chen, R, Rukstales, S, Luco, N, Wheeler, RL, Williams, RA, and Olsen, AH, 2008, Documentation for the 2014 Update of the United States National Seismic Hazard Maps, US Geological Survey Open-File Report , 243 p. Seed, H.B., and Idriss, I.M., 1970, Soil Moduli and Damping Factors for Dynamic Response Analyses, Report No. EERC 70-10, Earthquake Engineering Research Center, University of California, Berkeley. Swaisgood, J.R., 2003, Embankment Dam Deformations Caused by Earthquakes, Proceedings, 2003 Pacific Earthquake Conference on Earthquake Engineering, Paper No USGS, 2008, 2008 Interactive Deaggregations, US Geological Survey, < accessed July 30, USGS, 2014, 2014 National Seismic Hazard Maps Source Parameters, US Geological Survey, < hp?a=1>, accessed July 31, Page II-16

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