COAL COMBUSTION RESIDUAL LANDFILL AT BOARDMAN GENERATION FACILITY: UNSTABLE AREA ANALYSIS

2

• 1‐Foot Contours from LiDAR (Plan View and AutoCad File), dated November 30, 2016, provided by PGE on August 8, 2018.

• Coal Combustion Residual Landfill, Annual Inspection Report, by PGE, dated January 2018. • 2017 Annual Groundwater Monitoring Report for the CCR Landfill at the Boardman Power Plant,

near Boardman, Oregon, by CH2M, dated January 2018. • Statewide Landslide Information Database for Oregon, release 3.2 (SLIDO‐3.2), interactive map,

by Oregon Department of Geology and Mineral Industries (DOGAMI), accessed June 28, 2018. • Karst in the United States: A digital Map Compilation and Database, Open‐File Report 2014‐

1156, By United State Geological Survey (USGS), dated 2014.

Site Description The Boardman Power Plant is located approximately 11 miles south‐southwest of Boardman, Oregon, and approximately 12 miles south of the Columbia River as shown in Figure 1. Figure 2 is a site map that shows the plant, located immediately north of Carty Reservoir, and the CCR Landfill, located approximately 0.25 to 0.5 mile southeast of Carty Reservoir. The normal pool elevation of the Carty reservoir is 667.5 feet.

The CCR Landfill was built on a gentle sloping ground. Based on information provided to CH2M, the original ground elevation varied between 705 feet to the south to 690 feet to the north of the landfill. In the north part of the site, the original topography was undulated, typical of sand dune topography.

The climate of the Boardman area is characterized as semi‐arid, with a mean annual precipitation of 8.4 inches per year from 1971 to 2005 (Western Regional Climate Center, accessed September 2018).

Review of Existing Data: Unstable Area Analysis Summaries of findings from historical data and publicly available and readily accessible literature are provided in the following sections.

Review of Geotechnical Investigation report by Shannon & Wilson (1979) • The general geology and subsurface conditions at the CRR Landfill site consist of a layer of

windblown silt and sand (loess) overlying sediments of the Dalles Formation and Pomona basalt flow. The Dalles Formation is an early to middle Pliocene fluvial deposit, and Pomona flow is member of the late Miocene Yakima Basalt.

• Geotechnical explorations along the north portion of the perimeter retaining dike indicated that the loess layer is about 35 to 40 feet thick at the northwest and thins to 10 to 15 feet thick under the northeast part of the dike (see Figures 3, and 4). Geotechnical explorations encountered the Dalles Formation beneath the loess layer.

• The loess layer was classified as medium dense to very dense sandy silt and silt, with low in‐situ dry density. The Dalles Formation was classified as very dense silt and sand with Basalt fragments.

• The presence of loess was considered to be a potential geologic hazard at the time of the original landfill design since saturation of some types of loess material can cause significant settlement and loss of strength due to partial collapse of soil structure. The 1979 geotechnical investigation included a series of laboratory tests, including triaxial compression tests to evaluate effects of saturation on shear strength of the loess material, and consolidation tests to evaluate collapse potential and the effects of saturation on consolidation characteristics.

• The 1979 analyses indicated that up to 6 inches of total settlement could occur in the landfill due to saturation of loess foundation material.

• The report indicated that the height of the perimeter retaining dike was designed to vary between zero feet at the south to 18 feet at the north.

COAL COMBUSTION RESIDUAL LANDFILL AT BOARDMAN GENERATION FACILITY: UNSTABLE AREA ANALYSIS

• The exterior slopes were designed to be constructed at 3 horizontal to 1 vertical (3H:1V), and interior slopes were designed to be constructed at 2H:1V.

• The 1979 slope stability analyses were conducted for the retaining dike having a height of 15 feet with exterior slope of 3H:1V and interior slope of 2H:1V. The analyses were conducted for four cases. The first two cases were completed to evaluate the stability of the upstream and downstream slopes of the retaining dike without ash behind the dike. The last two cases were completed to evaluate the stability of the landfill dike for the condition where ash was mounded to representative elevations of 712.5 feet and 725 feet. The final condition was meant to represent the case when the landfill had been filled to its capacity.

• The calculated static factors of safety for all four conditions evaluated were well above the generally accepted minimum factor of safety value of 1.5.

• The minimum static factor of safety for the exterior slope was reported 2.69, and the minimum static factor of safety for the interior slope was reported 2.28. For the loading case where ash was mounded to elevations 712.5 and 725 feet, the calculated static factors of safety were reported to be 2.84 and 2.75 (Shannon & Wilson, 1979).

Review of Annual Inspection Report Completed by PGE (2018) • Data from a site topographic survey, dated November 2017, was provided to CH2M. The survey

data is limited to the area inside of the gravel road around the landfill area. The survey data does not show any significant anomalies in the slopes of the dikes and no anomalies or areas of movement have been reported by PGE.

• The interior slopes of the dike are shown to be sloped at about 3H:1V– flatter than what was recommended by the design, which is beneficial for overall dike stability. The interior slopes are shown based on the original contours as depicted from PGE drawing D‐7400, dated January 25, 1980.

• The top of the dike elevation varies from about 697.5 to 701 feet along the north side of the retaining dike, where the height of the dike is a maximum. The overall height of the dike is less than 8 feet along the southern portion of the retaining dike, where the top of the dike elevation varies from about 705 to 714 feet.

• The maximum elevation of mounded ash inside the landfill is about 719 feet in the southeast corner of the landfill.

Review of the recent borings completed by CH2M in April 2016 (CH2M, 2016) • The findings from these borings are consistent with the 1979 geotechnical investigation.

Review of the 2017 Annual Groundwater Monitoring Report (CH2M, 2018) • Groundwater elevation varies from 660 to 650 feet in the landfill area. Therefore, the

groundwater table is at least 30 feet below the retaining dike (see Figure 7), and saturation of loess foundation material due to the Carty Reservoir is unlikely.

Review of the Statewide Landslide Information Database for Oregon • There is no landslide hazard mapped in the project area.

Review of the Karst in the United States: A digital Map Compilation and Database (USGS, 2014) • The CRR Landfill site is not located in the mapped karst and potential karst areas.

Stability Evaluation CH2M reviewed the 2016 LiDAR data provided by PGE on August 8, 2018 to evaluate the status of the exterior slope. CH2M developed five cross sections through the north part of the retaining dike where the height of the dike and ash mound is the greatest. The locations of the cross sections and the section profiles are shown in Figures 5 and 6, respectively.

COAL COMBUSTION RESIDUAL LANDFILL AT BOARDMAN GENERATION FACILITY: UNSTABLE AREA ANALYSIS

4

The steepest exterior slope of the dike is about 2.62H:1V which is slightly steeper than the design slope. The height of the retaining dike is about 10 feet in this section.

To evaluate the current stability of the retaining dike, CH2M conducted slope stability analyses for the retaining dike with exterior slope of 2.6H:1V, interior slope of 2H:1V, height of 15 feet, and using strength parameters provided in the original design (Shannon & Wilson, 1979). The analyses were conducted considering the conditions where ash is mounded to representative elevations of 712.5 feet and 725 feet. The calculated static factors of safety for all the cases were above the generally accepted value of 1.5. The minimum static factor of safety for the exterior slope was 1.9.

Summary CH2M reviewed the available historical data and publicly available literature to evaluate if the CRR Landfill is located in an unstable area.

Review and evaluation of the historical information gathered at the site during the original design indicated that loess material is presented in the foundation of the retaining dike. The presence of loess material in the foundation of retaining dike was accounted for in the original design. The 1979 settlement analyses indicated the possibility of up to 6 inches of total settlement of the retaining dike which includes settlement that could occur due to additional loading from the embankment and ash and due to the potential saturation of loess foundation material. Therefore, it is apparent that the original design considered the potential impacts that would result from saturation of loess foundation material.

The retaining dike was originally designed for the ash mounded to maximum elevation 725 feet with the exterior slope of 3H:1V. The current maximum elevation of the mounded ash is about 719 feet, which is less than the design level. The steepest exterior slope based on 2016 LiDAR data is about 2.6H:1V, however, the height of the retaining dike is about 10 feet in this section. The static slope stability analyses showed acceptable factors of safety similar to those calculated as part of the original design. Therefore, static slope stability does not appear to be a concern in this area.

Review of publicly available and readily accessible literature indicate no landslide hazards were mapped in the vicinity of the project site. In addition, karst hazards are not mapped in the vicinity of the project.

Conclusion Based on the CH2M review of existing geologic hazard mapping and data provided by PGE it is CH2M’s opinion that the CCR landfill is not located in an unstable area. Further, a review of the original design of the landfill suggests that recognized and generally accepted good engineering practices were incorporated into the design of the CCR unit.

COAL COMBUSTION RESIDUAL LANDFILL AT BOARDMAN GENERATION FACILITY: UNSTABLE AREA ANALYSIS

Figures

Carty Reservoir

FIGURE 1Vicinity Map

Boardman Power Plant FacilityMorrow County, Oregon

Source: USGS The National Map: National Boundaries Dataset, 3D Elevation Program, Geographic Names Information System, National Hydrography Dataset, National Land Cover Database, National Structures Dataset, and National Transportation Dataset; U.S. Census Bureau - TIGER/Line

0 5,000 20,000 Feet

¯10,000

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Boardman, Oregon

Boardman Plant

CCR Landfill

Carty Reservoir

SEWAGE LAGOONS

COAL YARD PONDS

LINED EVAPORATION PONDS

VEHICLE WASH POND

SETTLING PONDS

BOARDMAN POWER PLANT

CARTY GENERATING STATION LINED HOLDING PONDS

(UNDER CONSTRUCTION)

FIGURE 2Site Map

Boardman Power Plant FacilityMorrow County, Oregon

¯0 1,000 2,000 4,000 Feet

Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community

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PORTLAND GENERAL ELECTR IC C'DMPANY

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S HAN~DN & WILSON , INC , GEOTECHNI CAL CONSULTAN TS

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PORTLAND GENERAL. ELECTRIC COMPANY

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SHANNON & WILSON, INC. GEOTECHNICAL CONSULTANTS

PORTLAND, OREGON

FIG, 21

FIGURE 5Plan View Based on 2016 LiDAR Data

Boardman Power Plant FacilityMorrow County, Oregon

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SECTION A-A'

2.83:1

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SECTION B-B'

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700

705

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700

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SECTION C-C'

2.62:1

690

695

700

705

710

715715

690

695

700

705

710

715715

0+00 0+50 1+00 1+50 2+00 2+50 3+00 3+50 4+00 4+50 5+00

SECTION D-D'

2.69:1

695

700

705

710710

695

700

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710710

0+00 0+50 1+00 1+50 2+00 2+50 3+00 3+50 4+00 4+50 5+00

SECTION E-E'

3.20:1

FIGURE 6Cross-Sections Based on 2016 LiDAR Data

Boardman Power Plant FacilityMorrow County, Oregon

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MW-120660.34

AL-001649.10

AL-002658.20

AL-006648.52

AL-004647.49

AL-008663.89

AL-007655.10

655

650

660

FIGURE 7Groundwater Elevation Contours and Flow Map

Boardman Power Plant Facility Morrow County, Oregon

Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN,IGP, swisstopo, and the GIS User Community

LEGENDMonitoring Well Location (Groundwater Elevation Measured August 22, 2017, ft NAVD88)!U Upgradient

!U Cross Gradient

!U DowngradientGroundwater Elevation Contour (5 ft Contour Interval, ft NAVD88) Inferred Groundwater Flow Direction

0 1,000 2,000500 Feet¯

\\SPKFPP01\PROJ\PGE\666272\H_GIS\MAPFILES\CCR\SUMMARY REPORTS\8_FIGURE 1_ASHLFCONTOURS_082217.MXD GGEE 1/5/2018 2:31:58 PM