TECHNICAL MEMORANDUM

Date: November 3, 2015 Document No.: 1524433 002 TM01 Rev1

To: File Company: North Poudre Irrigation Company

From: Jean Kugel and Stephen Rogers Email: SWRogers@Golder.com

RE: BOXELDER B-2 AND B-3 EMBANKMENT STABILITY ANALYSIS

1.0 INTRODUCTION North Poudre Irrigation Company (NPIC) is performing a planning study relative to possible rehabilitation

of the Boxelder Creek Watershed Dams B-2 and B-3. Both embankments are located about 15 miles

north of Fort Collins, Colorado, within approximately 5 miles of one another. The embankments were

constructed as flood control facilities to alleviate the potential for flooding in downstream areas. With

recent urban development in downstream areas, both embankments are now classified as high hazard

structures. As part of the watershed planning phase services that Golder Associates Inc. (Golder) is

providing, the stability of the existing structures requires evaluation. This technical memorandum presents

the stability assessment for the B-2 and B-3 embankments.

2.0 STABILITY CRITERIA The stability of B-2 and B-3 was assessed in accordance with the Rules and Regulations for Dam Safety

and Dam Construction set forth by the State of Colorado Department of Natural Resources, Division of

Water Resources, Office of the State Engineer, Dam Safety Branch (DWR 2007).

2.1 Minimum Factors of Safety Per the Rules and Regulations for Dam Safety and Dam Construction (DWR 2007), the minimum factors

of safety (FS) for slope stability of embankments for loading conditions applicable to B-2 and B-3 are:

Case 1: FS > 1.5 for steady seepage with phreatic surface fully developed for reservoir at normal pool elevation

Case 2: FS > 1.2 for rapid draw-down (upstream slope)

Case 3: FS > 1 for pseudostatic analysis

Each of the three stability cases was evaluated for both B-2 and B-3 as summarized in this technical

memorandum. The following sub-sections highlight key assumptions and frame the methodology for each

of the three cases.

I:\15\1524433\0100\0122\002 TM01 Rev1\1524433 002 TM01 Rev1 BoxelderDamStability 03NOV15.docx Golder Associates Inc.

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Tel: (303) 980-0540 Fax: (303) 985-2080 www.golder.com

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File November 3, 2015 North Poudre Irrigation Company 2 1524433 002 TM01 Rev1 2.1.1 Case 1: Full Reservoir Steady-state seepage analyses were performed to establish phreatic surfaces for use in the stability

modeling. The “normal pool elevation” was assumed to be equal to the elevation of the spillway crest.

This is a conservative assumption, because it is unlikely that the flood-control embankment structures will

remain full for sufficient periods of time to develop the phreatic surface at the normal pool elevation.

2.1.2 Case 2: Rapid Drawdown Two methods were used to evaluate rapid drawdown. The first method used was the staged undrained

method (Duncan et al. 1990). The initial (before drawdown) phreatic surface was assumed equal to the

steady-state seepage phreatic surface used in Case 1. The final (after drawdown) phreatic surface

mimicked empty reservoir conditions. The second method used was the simple effective strength method.

This method assumes rapid drawdown occurs instantaneously. The phreatic surface follows the upstream

slope surface of the embankment and then remains unchanged within the body of the embankment from

the steady-state seepage phreatic condition. The simple effective strength method provides a more

conservative mechanism of failure and, therefore, provides a lower factor of safety than the staged

undrained method.

2.1.3 Case 3: Pseudostatic (Seismic Stability) Per section 5.9.2 of the Rules and Regulations for Dam Safety and Dam Construction (DWR 2007), high

hazard dams are required to be analyzed for seismic stability. The embankment model used for seismic

stability was based on the following recommendations in the regulations:

Because both the B-2 and B-3 embankments are flood control embankments with ungated outlets, the seismic analyses presented herein were for empty reservoir conditions and did not consider steady-state seepage conditions

Because the sole purpose of the embankments is flood control, the stability was analyzed for a 2% chance of exceedance in 50 years (approximately 2500-year return frequency)

Based on the “Two-percent probability of exceedance in 50 years map of peak ground acceleration” (USGS 2014), both embankments are located in the region with a PGA of approximately 0.1g (see Attachment 1)

The pseudostatic analysis for these embankments was conducted using a pseudostatic coefficient equal to 0.05g (i.e., equal to 50% of the design peak bedrock acceleration, but not less than 0.05g)

Reducing the design peak bedrock acceleration is prescribed by the regulations and is consistent with the methodology presented by Hynes-Griffin and Franklin (1984)

The factor of safety for pseudostatic loading conditions is required to be 1.0 or greater

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File November 3, 2015 North Poudre Irrigation Company 3 1524433 002 TM01 Rev1 3.0 INPUTS AND ASSUMPTIONS

3.1 Embankment Geometry Golder analyzed the maximum cross-section for each of the two embankments. The as-built drawings

provided by NRCS were used to define the geometry for B-2 and B-3.

For B-2, the as-built C-1568 Sheet 17 of 52 shows the maximum cross-section located at Station 25+00

For B-3, the as-built C-1488X Sheet 9 of 18 shows the maximum cross-section located at Station 14+83

The geometries for the maximum cross-sections for B-2 and B-3 are presented on Figures 1 and 8,

respectively. The following sub-sections summarize the basic material zonation for each of the cross-

sections. The information and level of detail presented was limited by the information available.

3.1.1 B-2 Based on Golder’s review of the drawings and specifications provided by NRCS, B-2 consists of the

following zones of material:

Zone I: Low-permeability upstream portion of embankment

Silty clays and clays with silts

Zone II: Downstream portion of embankment

Silty sand and coarser

The specification designated this zone as less compacted than Zone I

Zone III: 2-ft cover layer

Silty clays and clays with silts

The specifications for this zone are the same as for Zone I

Drain

A four foot thick vertical drain comprised of drain fill was constructed approximately 40 feet downstream of the centerline of the dam

The top elevation of the drain fill in the critical cross-section is called at out at an elevation of 5549 ft

A horizontal eight inch diameter perforated asbestos cement pipe was installed near the bottom of the drain running parallel to the dam crest to conduct flow laterally

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File November 3, 2015 North Poudre Irrigation Company 4 1524433 002 TM01 Rev1 3.1.2 B-3 The NRCS provided less information for B-3 than for B-2. Based on Golder’s review of the drawings

provided by NRCS, the B-3 embankment consists of the following zones of material:

Zone I: Low-permeability core

Clay and silt

Zone II: Outer Shell

Silty sand

Zone III: Intermediate shell zone, transition between outer shell and low-permeability core, likely has properties somewhere between Zone I and Zone III

Drain

A five foot thick sloped vertical drain comprised of drain fill was constructed along the downstream face of Zone I

The top elevation of the drain fill in the critical cross-section is called at out at an elevation of 5470 ft

A horizontal 6-inch diameter perforated asbestos cement pipe was installed near the bottom of the drain running parallel to the dam crest to conduct flow laterally.

3.2 Material Properties Field programs were conducted at each of the embankment sites in the late 1970s (based on information

provided in the NRCS transmittal). The programs included test pits, boreholes, and in-situ hydraulic

conductivity testing. The findings were summarized by other consultants to NRCS and compiled for the

embankment design reports. Laboratory testing was conducted on samples collected from the field

programs. Laboratory testing included index tests (USCS classification, particle size distributions,

Atterberg limits, and specific gravity), compaction tests, and strength tests. Relevant laboratory results to

the stability model developed for B-2 and B-3 are presented in Tables 1 and 2, respectively.

Golder developed material properties for the stability models using information provided in the NRCS

transmittal including the laboratory test results, other documents provided by NRCS, and previous stability

analyses for the embankments. Golder used engineering judgment where measured or previously

specified properties were unavailable. The material properties used as input to the stability analysis for

B-2 and B-3 are presented in Tables 3 and 4, respectively. The material properties used to develop the

phreatic surface used in the stability analysis for B-2 are presented in Charts 1 and 2. Similarly, the

material properties used to develop the phreatic surface for B-3 are presented in Charts 3 and 4.

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File November 3, 2015 North Poudre Irrigation Company 5 1524433 002 TM01 Rev1 3.3 Software Stability analyses presented herein were conducted using the Geostudio (GEO-SLOPE 2014) suite of

computer software programs. The stability analysis was conducted using SLOPE/W, a two-dimensional

limit equilibrium slope stability program. The analyses were conducted using the Spencer method

(Spencer 1967), as this procedure satisfies both force and moment equilibrium, thereby yielding a

rigorous solution. The phreatic surface based on steady-state seepage was developed using SEEP/W.

4.0 RESULTS The factor of safety values from the stability analyses are summarized in Table 5. The following figures

have been compiled to illustrate the location of the slide surface for the factor of safety presented in each

case:

B-2

Figure 1: Geometry and material zonation

Figure 2: Steady-state seepage analysis and phreatic surface

Figure 3: Case 1 stability analysis

Figure 4: Case 2 stability analysis – simple effective strength

Figure 5: Case 2 stability analysis – staged undrained strength

Figure 6: Case 3 stability analysis – downstream failure

Figure 7: Case 2 stability analysis– upstream failure

B-3

Figure 8: Geometry and material zonation

Figure 9: Steady-state seepage analysis and phreatic surface

Figure 10: Case 1 stability analysis

Figure 11: Case 2 stability analysis – simple effective strength

Figure 12: Case 2 stability analysis – staged undrained strength

Figure 13: Case 3 stability analysis – downstream failure

Figure 14: Case 3 stability analysis– upstream failure

5.0 CONCLUSIONS AND RECOMMENDATIONS The stability analyses conducted for the as-built configuration of the existing B-2 and B-3 embankments

indicate that the factors of safety are acceptable in accordance with the Rules and Regulations for Dam

Safety and Dam Construction (DWR 2007).

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File November 3, 2015 North Poudre Irrigation Company 6 1524433 002 TM01 Rev1 6.0 REFERENCES GEO-SLOPE 2014. Geostudio 2012, December 2014 Release, version 8.14.1.10087. GEO-SLOPE

International. Calgary, Alberta, Canada.

Division of Water Resources (DWR), 2007. State of Colorado Department of Natural Resources, Division of Water Resources (DWR), Office of the State Engineer, Dam Safety Branch. Rules and Regulations for Dam Safety and Dam Construction. 2-CCR 402-1. Effective Date: January 1, 2007.

Duncan, J.M., S.G. Wright, and K.S. Wong. 1990. Slope Stability during Rapid Drawdown. Proceedings of H. Bolton Seed Memorial Symposium. Vol. 2.

Hynes-Griffin, M.E., and A.G. Franklin. 1984. Rationalizing the Seismic Coefficient Method. Waterways Experiment Station, U.S. Army Corps of Engineers. Misc. Paper GL-84-13.

Spencer, E. 1967. A Method of Analysis of Embankments Assuming Parallel Interslice Forces. Geotechnique, Col 17 (1), pp. 11-26.

United Stated Geological Survey (USGS), 2014. National Seismic Hazard Maps. Two-percent probability of exceedance in 50 years map of peak ground acceleration. website: http://earthquake.usgs.gov/hazards/products/conterminous/

NRCS transmittal. Please note that all references to the information provided by NRCS refer to the

electronic transmittal of the following documents:

1. Boxelder Creek B-2 Design and Design Review Report for Phase I, II

2. Boxelder Creek B-2 Design Reports for Irrigation Diversion Structure downstream of dam

3. Boxelder Creek B-2 Design Phase II – Emergency Spillway

4. Boxelder Creek B-2 As-Built Plans, Specifications, and Contract Information

5. Boxelder Creek B-2 Dam Breach Information – Original and 2009 Analysis

6. Boxelder Creek B-2 Geology Reports and Test Hole Information

7. Boxelder Creek B-2 Miscellaneous Design and Construction Data

8. Boxelder Creek B-2 Design Computations for Dam and Irrigation Diversion Structure

9. Boxelder Creek B-2 Construction Survey Information

10. Boxelder Creek B-3 As-Built Plans, Specifications, and Contract Information

11. Boxelder Creek B-3 Design Report

12. Boxelder Creek B-3 Geology Report and Test Hole Information

13. Boxelder Creek B-3 Dam Breach Information – 2009 Analysis

14. Boxelder B-2, B-3 and B-4: Probable Maximum Flood Analyses prepared by the NRCS and dated August 2010

15. Boxelder B-3: Dam Breach Analysis prepared by the NRCS and dated January 2011

16. The approved Boxelder Creek Watershed Work Plan

17. Lidar Data from Boxelder Creek B-3 dam to Fort Collins

18. Draft Plan of Work Spreadsheet for Boxelder Creek B-2 dam

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File November 3, 2015 North Poudre Irrigation Company 7 1524433 002 TM01 Rev1 7.0 TABLES AND CHARTS Table 1: B-2, Laboratory Test Results (from NRCS Transmittal)

Sample Depth

Index Compaction

Strength

Total Stress Effective Stress

Class LL PI Gs ρd, max (pcf)

wopt (%)

φ (°)

c (psf)

φ’ (°)

c’ (psf)

123A-1 3.0 – 8.0 ft CL 40 21 2.70 105 18 - - - - 123A-2 18.0 – 23.0 ft SC-SM 21 5 2.70 123.5 10.5 - - - - 128A-1 0.0 – 4.0 ft CL 38 18 2.69 101 20 15 775 31.5 125 128A-2 9.0 – 13.0 ft SC 22 8 2.69 127 9.5 - - - - 188A-1 3.0 – 8.0 ft CL 39 19 2.70 107 18 - - - - 191A-1 2.5 – 6.0 ft CL 39 23 2.73 109.5 16.5 13.5 550 29.5 125 264.1 5.0 – 12.0 ft SC or SC-SM 25 7 2.67 118 12.5 14 1050 32 225 264.2 20.0 – 28.0 ft CH 51 33 2.71 108 18 11.5 900 24.5 300

Table 2: B-3, Laboratory Test Results (from NRCS Transmittal)

Sample Depth

Index Compaction Strength (Total Stress)

Class LL PI Gs ρd, max (pcf)

wopt (%)

φ (°)

c (psf)

Comp 1 1.0 – 6.0 ft CL 37 14 2.71 105.9 17.1 8 288 Comp 2 3.0 – 13.0 ft SM NP NP 2.63 130.6 8.2 - - Comp 3 1.0 – 6.0 ft ML 34 8 2.70 104.5 19.7 15 158 Comp 4 1.0 – 7.0 ft ML 31 7 2.68 110.1 15.2 - - Comp 5 6.0 – 14.0 ft SM 24 3 2.61 128.9 8.5 - - Comp 6 1.0 – 5.0 ft SM 23 3 2.64 117.9 13.2 - - Comp 7 1.0 – 10.0 ft SW-SM NP NP 2.66 125.1 10.3 - - 20.1 3.0 – 4.5 ft ML NP NP 2.74 18.5 0 206.1 1.0 – 5.0 ft SM NP NP 2.68 110 16.5 - - 207.1 1.5 – 5.0 ft SM NP NP 2.69 106 18 16 1008

Table 3: B-2, Material Properties for Stability Model

Material Bulk Unit Weight (pcf)

Total Stress (Rapid Drawdown, Staged Undrained) Effective Stress

Effective Stress Scaled by 80% (Pseudo-static)1

Ksat (ft/s)

φ (°)

c (psf)

φ’ (°)

c’ (psf)

φ’ (°)

c’ (psf)

Zone I 120 13.5 550 29.5 125 24 100 3.3×10-9

Zone II 125 14 1,050 32 225 27 180 3.3×10-6 Zone III 120 13.5 550 29.5 125 24 100 3.3×10-9 Drain Fill 125 Drained 30 0 Not scaled, free draining 3.3×10-4 Foundation 100 13.5 820 33 0 27 0 4.5×10-4 Shale Impenetrable and impermeable (assumed)

Notes: 1. The strength function used for the pseudo-static analysis were scaled by 80%, as recommended by Hynes-Griffin and Franklin

(1984). 2. See Section 3.2 for discussion of material properties.

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File November 3, 2015 North Poudre Irrigation Company 8 1524433 002 TM01 Rev1 Table 4: B-3, Material Properties for Stability Model

Material Bulk Unit Weight (pcf)

Total Stress (Rapid Drawdown, Staged Undrained) Effective Stress

Effective Stress Scaled by 80% (Pseudo-static)1

Ksat (ft/s)

φ (°)

c (psf)

φ’ (°)

c’ (psf)

φ’ (°)

c’ (psf)

Zone I 134 11.5 223 29 180 24 144 3.3×10-9

Zone II 134 16 1,008 32 225 27 180 3.3×10-6 Zone III 127 11.5 223 29 180 24 144 1×10-7 Drain Fill 125 Drained 30 0 Not scaled, free draining 3.3×10-4 Foundation (ML, CL)

82 11.5 223 29 180 24 144 1.5×10-4

Foundation (SM)

110 16 1,008 32 0 27 0 1.5×10-4

Shale Impenetrable and impermeable (assumed) Notes: 1. The strength function used for the pseudo-static analysis were scaled by 80%, as recommended by Hynes-Griffin and Franklin

(1984). 2. See Section 3.2 for discussion of material properties.

Table 5: Stability Results, Factor of Safety Values

Stability Case B-2 B-3

Case 1: Full Reservoir 2.14 1.82 Case 2: Rapid Drawdown Simple Effective Strength Method 1.52 1.36

Staged Undrained Strength 1.80 1.62 Case 3: Pseudo-static Downstream 1.61 1.44

Upstream 1.77 1.75

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File November 3, 2015 North Poudre Irrigation Company 9 1524433 002 TM01 Rev1

Chart 1: B-2, Storage Functions

Chart 2: B-2, Horizontal Hydraulic Conductivity Functions

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.01 0.1 1 10 100 1000

Volu

met

ric W

ater

Con

tent

(ft3 /f

t3 )

Matric Suction (psf)

Zone I and Zone III Zone II Drain Fill

1.00E-14

1.00E-13

1.00E-12

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

0.01 0.1 1 10 100 1000

Hor

izon

tal H

ydra

ulic

Con

duct

ivity

(ft/s

)

Matric Suction (psf)

Zone I and Zone III Zone II Drain Fill

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File November 3, 2015 North Poudre Irrigation Company 10 1524433 002 TM01 Rev1

Chart 3: B-3, Storage Functions

Chart 4: B-3, Horizontal Hydraulic Conductivity Functions

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.01 0.1 1 10 100 1000

Volu

met

ric W

ater

Con

tent

(ft3 /f

t3 )

Matric Suction (psf)

Zone I Zone II Drain Fill Zone III

1.00E-14

1.00E-13

1.00E-12

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

0.01 0.1 1 10 100 1000

Hor

izon

tal H

ydra

ulic

Con

duct

ivity

(ft/s

)

Matric Suction (psf)

Zone I Zone II Zone III Drain Fill

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FIGURES

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-2 Sta. 25+00.gszAnalysis Name: Key

Zone IZone II

Zone III Drain Fill

Foundation

Shale

-270 -250 -230 -210 -190 -170 -150 -130 -110 -90 -70 -50 -30 -10 10 30 50 70 90 110 130 150 170 190 210 230 250 2705,480

5,500

5,520

5,540

5,560

5,580

5,600

Figure 1

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-2 Sta. 25+00.gszAnalysis Name: Steady-State Seepage

-270 -250 -230 -210 -190 -170 -150 -130 -110 -90 -70 -50 -30 -10 10 30 50 70 90 110 130 150 170 190 210 230 250 2705,480

5,500

5,520

5,540

5,560

5,580

5,600

Total Head

5,515 - 5,520 ft5,520 - 5,525 ft5,525 - 5,530 ft5,530 - 5,535 ft5,535 - 5,540 ft5,540 - 5,545 ft5,545 - 5,550 ft5,550 - 5,555 ft5,555 - 5,560 ft5,560 - 5,565 ft

Figure 2

2.14

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-2 Sta. 25+00.gszAnalysis Name: Downstream

-270 -250 -230 -210 -190 -170 -150 -130 -110 -90 -70 -50 -30 -10 10 30 50 70 90 110 130 150 170 190 210 230 250 2705,480

5,500

5,520

5,540

5,560

5,580

5,600

Figure 3

1.52

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-2 Sta. 25+00.gszAnalysis Name: Upstream - Rapid Drawdown - Simple Effective Strength Method

-270 -250 -230 -210 -190 -170 -150 -130 -110 -90 -70 -50 -30 -10 10 30 50 70 90 110 130 150 170 190 210 230 250 2705,480

5,500

5,520

5,540

5,560

5,580

5,600

Figure 4

1.80

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-2 Sta. 25+00.gszAnalysis Name: Upstream - Rapid Drawdown - Staged Undrained Strength (Duncan et al., 1990)

-270 -250 -230 -210 -190 -170 -150 -130 -110 -90 -70 -50 -30 -10 10 30 50 70 90 110 130 150 170 190 210 230 250 2705,480

5,500

5,520

5,540

5,560

5,580

5,600

Figure 5

1.61

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-2 Sta. 25+00.gszAnalysis Name: Downstream - Seismic: 0.05

-270 -250 -230 -210 -190 -170 -150 -130 -110 -90 -70 -50 -30 -10 10 30 50 70 90 110 130 150 170 190 210 230 250 2705,480

5,500

5,520

5,540

5,560

5,580

5,600

Figure 6

1.77

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-2 Sta. 25+00.gszAnalysis Name: Upstream - Seismic: 0.05

-270 -250 -230 -210 -190 -170 -150 -130 -110 -90 -70 -50 -30 -10 10 30 50 70 90 110 130 150 170 190 210 230 250 2705,480

5,500

5,520

5,540

5,560

5,580

5,600

Figure 7

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-3 Sta. 14+83.gszAnalysis Name: Key

Shale

Foundation (SM)

Zone IZone II

Zone III

Drain Fill

Foundation (ML, CL)

-240 -220 -200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 1805,380

5,400

5,420

5,440

5,460

5,480

5,500

Figure 8

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-3 Sta. 14+83.gszAnalysis Name: Steady-State Seepage

-240 -220 -200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 1805,380

5,400

5,420

5,440

5,460

5,480

5,500

Total Head

5,440 - 5,445 ft5,445 - 5,450 ft5,450 - 5,455 ft5,455 - 5,460 ft5,460 - 5,465 ft5,465 - 5,470 ft5,470 - 5,475 ft5,475 - 5,480 ft5,480 - 5,485 ft

Figure 9

1.82

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-3 Sta. 14+83.gszAnalysis Name: Downstream

-240 -220 -200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 1805,380

5,400

5,420

5,440

5,460

5,480

5,500

Figure 10

1.36

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-3 Sta. 14+83.gszAnalysis Name: Upstream - Rapid Drawdown - Simple Effective Strength Method

-240 -220 -200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 1805,380

5,400

5,420

5,440

5,460

5,480

5,500

Figure 11

1.62

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-3 Sta. 14+83.gszAnalysis Name: Upstream - Rapid Drawdown - Staged Undrained Strength (Duncan et al., 1990)

-240 -220 -200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 1805,380

5,400

5,420

5,440

5,460

5,480

5,500

Figure 12

1.44

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-3 Sta. 14+83.gszAnalysis Name: Downstream - Seismic: 0.05

-240 -220 -200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 1805,380

5,400

5,420

5,440

5,460

5,480

5,500

Figure 13

1.75

Project Name: Boxelder Embankment StabilityProject Number: 152-4433File Name: B-3 Sta. 14+83.gszAnalysis Name: Upstream - Seismic: 0.05

-240 -220 -200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 1805,380

5,400

5,420

5,440

5,460

5,480

5,500

Figure 14

ATTACHMENT 1 PGA MAP

Two-percent probability of exceedance in 50 years map of peak ground acceleration

0 500 1,000 KILOMETERS

0 500 1,000 MILES

70°80°90°100°110°120°

45°

40°

35°

30°

25°

Areas where suspected nontectonic earthquakes have been deleted

0.80.40.30.20.140.10.060.040.020

EXPLANATIONPeak acceleration, expressed as a fraction of standard gravity (g)

B-2 and B-3 Dams