Initial Safety Factor Assessment, Ponds M5 and M7, Reid Gardner Generating Station

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1 TECHNICAL MEMORANDUM, Reid Gardner Generating Station PREPARED FOR: PREPARED BY: REVIEWED BY: NV Energy John Barker/CH2M Dean Harris/CH2M DATE: August 2, 26 CH2M PROJECT NUMBER: APPROVED BY: Nathan Betts, PE /CH2M Introduction This technical memorandum summarizes the methods and findings of the initial (slope stability) safety factor assessment for Ponds M5 and M, which are both existing coal combustion residuals (CCR) surface impoundments at the Reid Gardner Generating Station (Station). This assessment was performed to satisfy the requirements of Section 25.3(e) of the U.S. Environmental Protection Agency s CCR Rule. The CCR Rule requires that this initial assessment be certified by a qualified professional engineer and placed in the Station s operating record by October, 26 ( 25.3(e)(2) and 25.3 (f)()). Within 3 days of placement the State Director must be notified as required by 25.6(f)() and 25.6(d). Also within 3 days of placement, the assessment must be placed on a publicly accessible Internet site per 25.(f)() and 25.(d). Per 25.3 (f)(3) this assessment must be performed every 5 years starting from the date that this initial assessment is placed in the operating record. Future assessments must meet the same requirements for certification, record keeping, notification, and posting on a publically accessible Internet site. Background Ponds M5 and M are classified as existing CCR surface impoundments according the definitions in of the CCR Rule because they received CCR both before and after October 9, 25. Published on April, 25, the CCR Rule regulates the disposal of CCRs as solid waste under Subtitle D of the Resource Conservation and Recovery Act. 25.3(e) contains the requirements for safety factor assessments for existing CCR surface impoundments. The ponds are also permitted dams by the Nevada Department of Resources under the State s dam safety program (permit J-652). Project Setting The Station is a coal-fired, steam-turbine, electric generating facility located approximately 5 miles northeast of Las Vegas near Moapa, Nevada. The Station is located in a valley near the banks of the Muddy River. Ponds M5 and M are located on a mesa south of the Station, approximately feet Per a July 2, 25, revision, the CCR Rule took effect on October 9, 25. EN44648BOI

2 INITIAL SAFETY FACTOR ASSESSMENT, PONDS M5 AND M, REID GARDNER GENERATING STATION above the bottom the valley. The area immediately around the ponds is sparsely vegetated, undeveloped, open desert land. Constructed in 2, Ponds M5 and M are earthen structures in cut and fill, and are lined with two layers of high-density polyethylene geomembrane liner. An interstitial leak detection and collection system is located between the liners. Each pond has a water surface area of approximately 3 acres when filled to the permitted maximum operational water level, and each impounds approximately 2 acre-feet of water when full. The southwestern corner of Pond M5, and the southern and western sides of Pond M, are situated in cuts below previous existing grade; the rest of the ponds are contained by fill embankments. A common fill embankment separates Pond M5 and Pond M. The exterior embankment side slopes are 3:, and the maximum height of embankment is 24 feet above existing ground surface at the northeast corner of Pond M5. The interior side slopes are 3: and extend 23 to 26 feet above the pond bottom. Regulatory Requirements In accordance with 25.3(e), the assessment must demonstrate that the calculated factors of safety for Ponds M5 and M achieve the minimum factors of safety as follows: The calculated static factor of safety under the long-term, maximum storage pool loading condition must equal or exceed.5. The calculated static factor of safety under the maximum surcharge pool loading condition must equal or exceed.4. The calculated seismic factor of safety must equal or exceed.. Section 25.3(e) does not specify the seismic loading event that should be used in the assessment to calculate the seismic factor of safety. However, page 2384 of the Rule s preamble states that CCR surface impoundments must be assessed for the seismic loading event with a 2 percent probability of exceedance in 5 years, which equates to a return period of approximately 2,5 years, based on the U.S. Geological Survey (USGS) seismic hazard maps for the region where Ponds M5 and M are located. This is in keeping with the location restrictions which require a demonstration that CCR impoundments are designed to resist the maximum horizontal acceleration in lithified earth material ( 25.63) which is defined as the 2 percent in 5 year event ( 25.53) The calculated liquefaction factor of safety must equal or exceed.2 for dikes constructed of soils that have susceptibility to liquefaction. Technical Data This section summarizes the data used to perform the safety factor assessment. Subsurface Conditions As summarized in the Geotechnical Investigation Report completed for pond design (Converse, 29), subsurface conditions on the mesa consist of predominantly silty to poorly graded sands and gravels that are occasionally partially cemented to cemented, intermingled with lean and fat clays and silts. Based on a site reconnaissance, conversations with Converse, and construction observations, the cemented material is anticipated to behave as a low-strength (relative to intact rock) conglomerate rock that can be crushed and processed to remove the coarse-grained aggregate material from the cemented binder. Some exposed cemented sands and gravels are visible in existing washes on the mesa, and layers of cemented sands and gravels were encountered during construction. It was observed that the coarsegrained material in the cemented zones may contain particle sizes up to 5 inches in diameter, with the predominant materials being coarse sand and fine gravel. 2 EN44648BOI

3 INITIAL SAFETY FACTOR ASSESSMENT, PONDS M5 AND M, REID GARDNER GENERATING STATION The lean and fat clays encountered in the subsurface investigations are very stiff to hard and desiccated. Field and laboratory tests indicate that the native soils exhibit low compressibility and have moderate to high internal angles of friction and low to moderate cohesive strengths. Groundwater Groundwater was not encountered within the upper feet in the subsurface explorations on the mesa. There are also six groundwater monitoring wells installed around the footprint of Ponds M5 and M. Readings taken in July of 26 found depths to groundwater ranged from approximately 8 to 63 feet below ground surface, with an average depth of 4 feet. Material Properties Material properties used in the safety factor assessment are based on investigation and laboratory testing performed by Converse (29). Testing on the subgrade soils at the pond locations included moisture content and dry density, Atterberg limits, consolidation, grain size distribution, laboratory maximum density, direct shear strength, and permeability. Table lists the material properties used in the safety factor evaluation. Table. Material Properties Material Internal Friction Angle Reference 3 psf 2 pcf Converse, 29 Native Soil 3 psf 2 pcf Converse, 29 Notes: psf pounds per square foot pcf pounds per cubic foot Evaluation In general, the dense soil conditions, relatively flat slopes, and configuration of the impoundments indicate favorable slope conditions. Slope stability calculations were performed to quantitatively evaluate the conditions and document compliance with the technical requirements of the CCR Rule. The sections below summarize the technical considerations and approaches used to complete the safety factor assessment. Seven cross-sections (A through G) were cut through the existing pond berms and surrounding topography. From these seven sections, a single critical cross-section was selected for Ponds M5 and M (for two total cross-sections in the assessment). The critical cross-sections were analyzed for the safety factor assessment because they represent the most severe case for each impoundment. The critical cross-sections of Ponds M5 and M were selected based on a review of the conditions and the finding that these appear to be most susceptible to structural failure among all cross-sections of the embankment. The cross-section locations are shown on Figure ; Sections A-A and C-C are the critical cross sections for Pond M and Pond M5, respectively. Evaluation Criteria The stability evaluation is an analysis of potential sliding of the pond embankment material on a hypothetical or trial failure surface passing through the embankment, at the interface of the embankment and native subgrade soils, through native subgrade soils, or a combination of the three. Minimum required factors of safety for the conditions analyzed are listed in the CCR Rule and presented in Table 2. EN44648BOI 3

4 INITIAL SAFETY FACTOR ASSESSMENT, PONDS M5 AND M, REID GARDNER GENERATING STATION Table 2. Safety Factor Criteria Loading Condition Required Safety Factor Static, long-term maximum storage pool.5 Static, maximum surcharge pool.4 Seismic. Liquefaction.2 Note: The liquefaction loading condition was not analyzed as explained herein. Storage Pool and Surcharge Pool Elevations The ponds are completely enclosed by the pond berms. As such, they were designed to meet the State s dam safety requirements to prevent run-on of stormwater runoff into the ponds. The ponds are not provided with spillways. There is no hydraulic interconnection between ponds (no overflow pipe from one pond to the next). The permitted minimum freeboard for the ponds is 4.3 feet, which takes into account a storm surcharge and wave run-up (CH2M HILL, 29). For a dam without a spillway, the storm surcharge elevation must be the peak maximum precipitation, which was calculated to be.25 feet (5 inches) for this site. State regulations require a minimum of 3 feet of freeboard to account for wave run-up. Calculations of wave run-up based on pond fetch, depth, wind, and appropriate safety factors were less than 3 feet; therefore, the minimum 3 feet must be used (CH2M HILL, 29). On the basis of the design criteria for the purposes of this safety factor assessment, CH2M has set the long-term maximum storage pool elevation at 4.3 feet below the top of the pond berm (embankment) elevation. For the maximum surcharge pool, the elevation is set at 3 feet below the top of the pond berm (4.3 feet of permitted freeboard minus 5 inches for direct precipitation caused by the probable maximum precipitation). This is shown graphically in the output of the global stability analysis (Attachment ). Steady-State Seepage Page 2348 of the Rule s preamble states that it is generally accepted practice to analyze the stability of the downstream slope of the dam embankment for steady-state seepage (or steady seepage) conditions with the reservoir at its normal operating pool elevation (usually the spillway crest elevation) since this is the loading condition that the embankment will experience most. Ponds M5 and M include a double liner system (both primary and secondary liners are 8-mil highdensity polyethylene geomembrane) with a leak detection and recovery system that greatly reduces or eliminates the possibility for a condition of significant steady state seepage from developing as long as the pond is operated and maintained.. Because of the double liner system and the leak detection system, the maximum surcharge pool case did not include steady-state seepage through the embankment. Seismicity The USGS online U.S. Seismic Design Maps (USGS, 26) application was used to estimate peak ground acceleration (PGA) at the project site using location coordinates (latitude , longitude 4.634). The U.S. Seismic Design Maps application provides parameter values from the 29 National Earthquake Hazards Reduction Program s Recommended Seismic Provisions for New Buildings and Other Structures. This design code reference document provides seismic design parameter values that are used in major U.S. building codes (International Building Code and the American Society of Civil Engineers ASCE Standard). 4 EN44648BOI

5 INITIAL SAFETY FACTOR ASSESSMENT, PONDS M5 AND M, REID GARDNER GENERATING STATION The USGS application provided a PGA for the maximum considered earthquake event. The maximum considered earthquake event corresponds to an event with a 2 percent probability of exceedance in 5 years, which equates to a return period of approximately 2,5 years. The USGS application defines the PGA as.26g for Site Class B (the application references the USGS 28 ground motion database). On the basis of standard penetration test blowcount data obtained during the geotechnical exploration, the site should be classified as Site Class C (Converse, 29). The site coefficient at zero-period, F pga, for Site Class C is.24, such that the PGA at the ground surface (A s) is.3g (A s =.24 x.26g =.3g). For the evaluation of seismic stability of slopes and embankments, a pseudo-static stability analysis is commonly conducted. The pseudo-static stability analysis consists of conventional limit equilibrium static slope stability analysis completed with horizontal and vertical pseudo-static acceleration coefficients (k h and k v) that act upon the critical failure mass. A horizontal pseudo-static coefficient, k h, of.5 x A s and a vertical pseudo-static coefficient, k v, equal to zero should be used when seismic stability of slopes is evaluated, not considering liquefaction. A seismic acceleration of.6g (roughly half of.3g) was used in a pseudo-static analysis to evaluate the seismic loading scenario. Note that the use of a reduced acceleration coefficient considers the ability of the slope to displace laterally, thereby reducing the acceleration that can be experienced by the slope failure mass. Liquefaction Scenario Liquefaction is considered highly unlikely, given the soil and groundwater conditions at Ponds M5 and M. Liquefaction is defined as the loss of shear strength in a saturated soil due to excess hydrostatic pressure. In saturated, cohesionless soils, such a strength loss can result from loads that are applied instantaneously or cyclically, particularly in loose fine to medium sands that are uniformly graded. All of the following general conditions are necessary for liquefaction to occur: A sustained ground acceleration that is large enough and acting over a long enough period of time to develop excess pore-water pressure, thereby reducing effective stress and soil strength. Predominantly cohesionless soil that has the right gradation and composition. Liquefaction has occurred in soils ranging from low plasticity silts to gravels. Clean or silty sands and non-plastic silts are most susceptible to liquefaction. The state of the soil is characterized by a density that is low enough for the soil to exhibit contractive behavior when sheared undrained under the initial effective overburden stress. The presence of groundwater, resulting in a saturated or nearly saturated soil. Ponds M5 and M are founded on dense, cemented sands and gravels of the mesa, and the depth to groundwater below the mesa is more than feet (Converse, 29). Thus, the subsurface profile is not susceptible to liquefaction, and no safety factor assessment has been performed for the liquefaction scenario. Method of Analyses CH2M used the limit equilibrium methods of slope stability analysis to perform the safety factor assessment using the program Slide Version. (Rocscience, 26). CH2M used the Spencer Method (a limit equilibrium method) to perform the safety factor assessment. Limit equilibrium methods compare forces, moments, and stresses that cause instability of the mass of the embankment to those that resist the instability. The principle of the limit equilibrium method is to estimate the ratio of the available shear strength of the soil to the average shear stresses along assumed failure surface, without directly estimating the magnitude of displacement of the slope. Limit equilibrium methods include, but are not limited to, methods of slices. These methods are considered conventional stability analyses by the U.S. Army Corps of Engineers Slope Stability Engineer Manual EN44648BOI 5

6 INITIAL SAFETY FACTOR ASSESSMENT, PONDS M5 AND M, REID GARDNER GENERATING STATION (23), which is generally considered by the geotechnical community as a summary of the state of the practice for these analyses. These methods are also considered to be conventional by the preamble to the CCR Rule (page 2383). Slide allows the user to control the generation of potential failure surfaces and their locations within the embankment or foundation materials. The analysis included an evaluation of circular and block-shaped surfaces to consider each configuration (through embankment, interface, through foundation, and combination embankment and foundation) stipulated by the CCR Rule. Critical Sections Figure presents a plan view of the Mesa Ponds and the cross-sections that were considered for the safety factor assessment. The critical cross-section for Pond M is Section A-A, and the critical cross section for Pond M5 is Section C-C. The height of the downstream toe of slope these two crosssections, which are the maximum heights for for each pond, differentiate them from the other sections considered for analysis. Thus, Section A-A and Section C-C were selected as the critical sections. Conclusions The computed safety factors from the global stability analyses are listed in Table 3. In Table 3, for each section analyzed, the loading condition, failure surface type, analysis method, required factor of safety, and computed factor of safety are presented. The loading conditions are described in the Evaluation Criteria section of this memorandum. The failure surface type describes whether a circular or a noncircular (wedge) failure surface is used and what limitations might be applied to the entry and exit point of the failure surface. The analysis method refers to the technical approach selected in Slide to compute factor of safety. Generally, Slide revealed that the lowest factors of safety were associated with circular surfaces that extended through the embankment material and foundation material. Copies of the Slide output plots are included as Attachment to this technical memorandum. Table 3. Safety Factor Assessment Results Section Loading Condition Failure Surface Type Analysis Method Required Safety Factor Computed Safety Factor Pond M Section A-A Scenarioa Static, long-term maximum storage pool Circular No Limitation on Failure Surface Spencer Pond M Section A-A Scenariob Static, long-term maximum storage pool Circular Failure Surface Within Embankment Spencer Pond M Section A-A Scenarioc Static, long-term maximum storage pool Non-circular Failure Surface at Embankment/Native Interface Spencer.5 3. Pond M Section A-A Scenario2a Static, maximum surcharge pool Circular No Limitation on Failure Surface Spencer Pond M Section A-A Scenario2b Static, maximum surcharge pool Circular Failure Surface Within Embankment Spencer Pond M Section A-A Scenario2c Static, maximum surcharge pool Non-circular Failure Surface at Spencer EN44648BOI

7 INITIAL SAFETY FACTOR ASSESSMENT, PONDS M5 AND M, REID GARDNER GENERATING STATION Table 3. Safety Factor Assessment Results Section Loading Condition Failure Surface Type Analysis Method Required Safety Factor Computed Safety Factor Embankment/Native Interface Pond M Section A-A Scenario3a Seismic Circular No Limitation on Failure Surface Spencer..6 Pond M Section A-A Scenario3b Seismic Circular Failure Surface Within Embankment Spencer..59 Pond M Section A-A Scenario3c Seismic Non-circular Failure Surface at Embankment/Native Interface Spencer..95 Pond M Section A-A Liquefaction Not Applicable Not Applicable.2 No Liquefaction Pond M5 Section C-C Scenarioa Static, Long-term maximum storage pool Circular No Limitation on Failure Surface Spencer Pond M5 Section C-C Scenariob Static, Long-term maximum storage pool Circular Failure Surface Within Embankment Spencer Pond M5 Section C-C ScenarioC Static, Long-term maximum storage pool Non-circular Failure Surface at Embankment/Native Interface Spencer Pond M5 Section C-C Scenario2a Static, maximum surcharge pool Circular No Limitation on Failure Surface Spencer Pond M5 Section C-C Scenario 2b Static, maximum surcharge pool Circular Failure Surface Within Embankment Spencer Pond M5 Section C-C Scenario2c Static, maximum surcharge pool Non-circular Failure Surface at Embankment/Native Interface Spencer Pond M5 Section C-C Scenario3a Seismic Circular No Limitation on Failure Surface Spencer..39 Pond M5 Section C-C Scenario3b Seismic Circular Failure Surface Within Embankment Spencer..4 Pond M5 Section C-C Scenario3c Seismic Non-circular Failure Surface at Embankment/Native Interface Spencer..89 Pond M5 Section C-C Liquefaction Not Applicable Not Applicable.2 No Liquefaction EN44648BOI

8 INITIAL SAFETY FACTOR ASSESSMENT, PONDS M5 AND M, REID GARDNER GENERATING STATION Professional Engineer Certification This section contains the written certification by a qualified professional engineer as required by Section 25.3(e)(2) of the CCR Rule. This initial safety factor assessment meets the requirements of Section 25.3(e) of the CCR Rule. 8 EN44648BOI

9 References INITIAL SAFETY FACTOR ASSESSMENT, PONDS M5 AND M, REID GARDNER GENERATING STATION CH2M HILL, Inc. 29. Basis of the Preliminary Design for Mesa Evaporation Ponds, M3, M5, and M, NV Energy, Reid Gardner Station Facility Improvement Program. May Converse Consultants, Inc. 29. Geotechnical Investigation, Mesa Evaporation Ponds, Reid Gardner Station. Converse Project No May. Rocscience, Inc. 29. Slide Computer Software. Version., Build date: February 2, 26. U.S. Army Corps of Engineers. 23. Slope Stability Engineer Manual. EM October. U.S. Geological Survey (USGS). U.S. Seismic Design Maps. Accessed March 4, 26. EN44648BOI 9

10 EN44648BOI Figure

11 N Scale In Feet LEGEND APPROXIMATE BLM ROW APPROXIMATE BLM ROW ec A i ca ls Cr it 5 5 Cri 2 2 C l Se tica C tio n B n D B ctio 2 D A 5 POND M POND M G E G F E M esa_ponds_fi gur e. dgn F FIGURE MESA PONDS CROSS SECTION LOCATIONS SAFETY FACTOR ASSESSMENT REID GARDNER STATION DATE: 3/3/6

12 EN44648BOI Attachment Slide Output Plots

13 85 Safety Factor Nave Earth 2 Mohr-Coulomb Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:2:59 PM Stability Factor Assessment - Static, Long-term maximum storage pool Scale: :4 Pond M_Section A-A_Scenarioa.slim

14 85 Safety Factor Nave Earth 2 Mohr-Coulomb 3 2 Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:29:35 PM Stability Factor Assessment - Static, Long-term maximum storage pool Scale: :4 Pond M_Section A-A_Scenariob.slim

15 Safety Factor Nave Earth 2 Mohr-Coulomb 3 2 Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 5:5:43 PM Stability Factor Assessment - Static, Long-term maximum storage pool Scale: :4 Pond M_Section A-A_Scenarioc.slim

16 Safety Factor Nave Earth 2 Mohr-Coulomb 3 2 Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:36:8 PM Stability Factor Assessment - Static, maximum surcharge pool Scale: :4 Pond M_Section A-A_Scenario2a.slim

17 Safety Factor Nave Earth 2 Mohr-Coulomb 3 2 Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:38:4 PM Stability Factor Assessment - Static, maximum surcharge pool Scale: :4 Pond M_Section A-A_Scenario2b.slim

18 85 Safety Factor Nave Earth 2 Mohr-Coulomb 3 2 Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 5:24: PM Stability Factor Assessment - Static, maximum surcharge pool Scale: :4 Pond M_Section A-A_Scenario2c.slim

19 Safety Factor Nave Earth 2 Mohr-Coulomb 3 2 Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:42:2 PM Scale: :4 Stability Factor Assessment - Seismic Pond M_Section A-A_Scenario3a.slim

20 85 Safety Factor Nave Earth 2 Mohr-Coulomb Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:56:24 PM Scale: :4 Stability Factor Assessment - Seismic Pond M_Section A-A_Scenario3b.slim

21 85 Safety Factor Nave Earth 2 Mohr-Coulomb Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 5:26:54 PM Scale: :4 Stability Factor Assessment - Seismic Pond M_Section A-A_Scenario3c.slim

22 Safety Factor Nave Earth 2 Mohr-Coulomb 3 2 Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:2:5 PM Stability Factor Assessment - Static, Long-term maximum storage pool Scale: :45 Pond M5_Section C-C_Scenario.slim

23 Safety Factor Nave Earth 2 Mohr-Coulomb Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:6:34 PM Stability Factor Assessment - Static, Long-term maximum storage pool Scale: :45 Pond M5_Section C-C_Scenariob.slim

24 85 Safety Factor Nave Earth 2 Mohr-Coulomb 3 2 Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4::38 PM Stability Factor Assessment - Static, Long-term maximum storage pool Scale: :45 Pond M5_Section C-C_Scenarioc.slim

25 Safety Factor Nave Earth 2 Mohr-Coulomb Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:4:39 PM Stability Factor Assessment - Static, maximum surcharge pool Scale: :45 Pond M5_Section C-C_Scenario2a.slim

26 Safety Factor Nave Earth 2 Mohr-Coulomb 3 2 Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:6:2 PM Stability Factor Assessment - Static, maximum surcharge pool Scale: :45 Pond M5_Section C-C_Scenario2b.slim

27 85 Safety Factor Nave Earth 2 Mohr-Coulomb Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:2:23 PM Stability Factor Assessment - Static, maximum surcharge pool Scale: :45 Pond M5_Section C-C_Scenario2c.slim

28 9 Safety Factor Nave Earth 2 Mohr-Coulomb 3 2 Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:22:5 PM Scale: :5 Stability Factor Assessment - Seismic Pond M5_Section C-C_Scenario3a.slim

29 9 Safety Factor Nave Earth 2 Mohr-Coulomb 3 2 Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:24:3 PM Scale: :5 Stability Factor Assessment - Seismic Pond M5_Section C-C_Scenario3b.slim

30 Safety Factor Nave Earth 2 Mohr-Coulomb 3 2 Mohr-Coulomb ft SLIDEINTERPRET.3 Date: 3/3/26 4:26:8 PM Scale: :5 Stability Factor Assessment - Seismic Pond M5_Section C-C_Scenario3c.slim