GM 16-24 Prelim Geotech Memo WA FLAP SKAMAN 92135-1.pdf

WA FLAP SKAMAN 92135(1)

Wind River Road Slide Reconstruction Geotechnical Memo 16-24 Preliminary Geotechnical Recommendations

DISCLAIMER:

The following memorandum includes geotechnical exploration data and initial slope stability analyses of the existing site conditions performed during a preliminary landslide assessment. This memorandum is provided for informational purposes only as it may contain information potentially pertinent to the design of the soldier pile retaining wall.

158377846

M E M O R A N D U M Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA 98661

DATE: April 15, 2024

TO: Michael Traffalis Project Manager

FROM: Mark Swank Lead Engineering Geologist

Eric Lim Senior Geotechnical Engineer

SUBJECT: Geotechnical Memo 16-24 Preliminary Geotechnical Recommendations Wind River Road Slide Reconstruction

WA FLAP SKAMAN 92135(1)

Wind River Road, Skamania County, Washington

INTRODUCTION

This memorandum summarizes the preliminary geotechnical findings and recommendations for the Wind River Road Slide Reconstruction Project (WA FLAP SKAMAN 92135(1)), located along Wind River Road in Skamania County, Washington (Figure 1, Site Vicinity Map). The recommendations include retaining wall type and location, slope grading (cuts and fills), and landslide mitigation options to support the development of the 30 percent plan set.

Additional background, subsurface information, detailed geotechnical recommendations, and construction considerations will be provided in the Final Geotechnical Report prior to completion of the PS&E package.

PROJECT DESCRIPTION

Owned and maintained by Skamania County within the project limits, the Wind River Road (Highway No. 92135) provides access to the Gifford Pinchot National Forest from SR-14, the City of Carson, Mount St Helens Monument, Swift Reservoir, Mount Adams, Pacific Crest Trail, Columbia River Gorge National Scenic Area Trails, recreational campground facilities, winter recreational parks, and local private property. Ongoing, intermittent slope movements between Mile Post (MP) 0.2 and MP 0.40, are damaging the road, causing longitudinal and lateral cracks and depressions from subsidence.

We understand this project will improve the stability and reliability of Wind River Road. The anticipated work includes stabilizing the road embankment with a retaining wall or other appropriate method, replacing the existing guardrail and constructing additional guardrail, and drainage improvements. The drainage improvements will consist of ditch regrading, replacing the existing 36-inch diameter cross culvert, and replacing two minor culverts. Additionally, a 4-foot-wide bike lane along the westbound (northside) lane, may be constructed, requiring a roadway widening of about 3 feet expected on the eastbound (southside) lane.

Due to concerns of further destabilizing the landslide beneath the road and, potentially, the surrounding slopes during construction excavation, the existing 36-inch diameter cross culvert will either be rehabilitated in-place using trenchless technologies or replaced post-support of the excavation side slopes.

SITE DESCRIPTION

Available published data, including geologic maps, aerial and light detection and ranging (LiDAR) imagery, topographic maps, and reports and plans provided by Skamania County were reviewed. The following sections describe the surface conditions, topography, geologic setting and local seismic conditions, and the subsurface conditions encountered during the geotechnical subsurface investigation.

Surface Conditions

The project is along a 200-foot-long segment of Wind River Road, a hot mix asphalt (HMA) paved roadway in southern Skamania County, between the City of Carson, 0.5 miles to the east, and State Route (SR) 14, 0.25 miles to the west. The road traverses southwest-northeast across hilly terrain that slopes toward the Columbia River approximately 0.3 miles to the south and is within the Columbia River Gorge Scenic Area. The damaged road segment is between two parallel drainage channels and constructed atop embankment fill, which primarily spans the eastern drainage and its 36-inch diameter corrugated metal pipe (CMP) culvert. The road surface includes an approximately 140-foot-long asphalt patch with sealed and unsealed longitudinal cracks.

This road segment has three lanes, one westbound and two eastbound, and gently curves through the project area, with a gradual slope down to the west that steepens towards the SR 14 roundabout. An unlined drainage ditch parallels the northside of the road and a guardrail spans the embankment where the 36-inch diameter CMP culvert crosses beneath it. A short driveway over the ditch leads to a flat, graded area and to the forestry access roads that traverse upslope for 200 feet to the north at 1V:4H (vertical:horizontal) slope to an upper bench. South of the road, the embankment slopes at 1V: 1.3H for 25 feet, then flattens to 1V:

5H down to the drainage channel eastern drainage channel.

The slopes south of the road are heavily vegetated with large deciduous and coniferous trees with abundant underbrush. The property north of the road, based on aerial photographs, was timber harvested between July 2017 and February 2018, and smaller and younger trees are re-establishing, along with shrubs and grasses.

Photographs 1 and 2. Wind River Road Looking West (Left) and East (Right)

Photographs 3 and 4. Cut-Off Trench Northside of Road (Left) and Culvert Inlet Northside of Road (Right)

Photographs 5 and 6. Upper Slope Looking South (Left) and Lower Slope Looking North (Right) secondary scarp

Geologic Setting

The project is within the South Cascades physiographic province in Washington State. The South Cascades have a complex geologic history, spanning over 200 million years of tectonic accretion, folding, faulting, uplift and erosion, magmatic intrusion and sedimentary deposition, resulting in a chaotic assembly of various terranes overlain by Tertiary volcanic and sedimentary rocks (DNR, 2018). The Columbia River has cut through the Cascade Mountain Range, exposing mainly Tertiary volcanic and sedimentary sequences in the site vicinity and, locally, portions of the uplifted core of the mountain range. The most recent volcanic episode in the Cascades Volcanic Arc began approximately 500,000 years ago, including the formation of Mount Rainier, Mount St. Helens and Mount Adams within the subject physiographic province. Local tectonism rotated these units to dip westward, which manifests itself in the en échelon linear ridges north and west of the site, with characteristic parallel north/northwest-trending ridges and valleys. The more resistant units (andesite flows) form the crests of the ridges, while the less resistant units (flow breccia, tuff, and clay) are the low areas and drainages.

Landslides have developed on the weathered and adversely dipping volcanic and sedimentary geologic units within the Columbia River Gorge, including a large complex that underlies the project site. LiDAR-based mapping indicates this landslide is approximately 10,000 feet long by 2,000 feet wide, originating at Stephenson Ridge and terminating near the Columbia River (Jacobacci and Mickleson, 2017). Pierson, et al., 2016, assigned a relative age of “Intermediate” (1,000 to 15,000 years) to this landslide due to the presence of rounded and eroded surficial features and a lack of apparent erosion of the toe area by Missoula floods.

Locally, the site geology is mapped as Quaternary unconsolidated sediments (Qs) that are locally derived, ranging from clay to boulders, and consisting of talus, alluvium, alluvial fans, colluvium, rockfall, and landslide deposits (Czajkowski, et al., 2014).

Based on nearby mapped units and explorations, these Qs deposits are derived from Miocene-Oligocene volcanic rocks of Stevenson Ridge (Mvb), consisting of interlayered sequence of basaltic andesite to andesite flows, flow breccia, tuff, and clay divided into:

− (MФvas): Andesite and basaltic andesite lava flows—Dark grayish black to dark blackish gray, seriate to porphyritic basaltic andesite to andesite with minor flow breccia and interlayered pyroclastic rocks.

− (MФvcs): Pyroclastic rocks—Light greenish gray to light green tuff breccia with minor interlayered flow breccia and basaltic andesite to andesite lava flows. Highly altered throughout and weathered to clay in upper portion of layers adjacent to overlying lava flows.

Project Landslide The active project landslide impacting Wind River Road is part of the older, larger, prehistoric landslide complex that extends further to the north and south as shown in Figure 2. The active project landslide was divided into three general areas to help with describing the location of various features (Figure 2, Active and Prehistoric Landslide Map; Figure 3, Exploration and Landslide Features Map):

− Headscarp, Crown, and Upslope—the general area at the existing road where recent or ongoing landslide movement is occurring and further upslope of the road where evidence of past landslide movement may exist. Road and embankment construction have modified and obscured these features.

− Body—the general area of the project landslide to the southeast of the road and downslope. Road and embankment construction have modified and obscured these features.

− Toe—the general area near creek where undercutting is occurring.

The active project landslide is continually causing road damage with episodic rates of movement up to several inches per year and grading and asphalt patching has been required to maintain passable, safe conditions.

Faults and Seismicity The Site is located within an area of active seismicity that is subject to earthquakes on shallow crustal faults and deeper subduction zone earthquakes. Though no fault traces are mapped as underlying the Site, several fault zones are located within 15 miles, and the Cascadia Subduction Zone (CSZ) is located approximately 120 miles west, off the Oregon coast. The following subsections describe the seismic sources and nearby faults that are pertinent to the Site and Project.

Shallow Crustal Faults

The northwest-striking faults near The Dalles in northern Oregon and southern Washington are right-lateral strike-slip and minor normal faults is mapped approximately 7 miles west of the project. These faults offset Miocene and Pliocene volcanic and sedimentary rocks near the southern margin of the Yakima fold belt. No fault scarps on Quaternary deposits have been described, but one of these faults may offset Quaternary basalt and these faults form prominent regional lineaments that also suggest they may have undergone Quaternary displacement (Personius, 2003).

The typically northwest-striking Hood River fault zone, mapped approximately 12 miles south of the project, defines the eastern margin of a half graben that forms the Upper Hood River Valley in the High Cascades of northern Oregon. This structure may be part of an extensive group of graben structures formed in response to subsidence related to extrusion of extensive volcanic rocks in the early Pliocene. The area is underlain by Miocene volcanic rocks of the Columbia Plateau and Pliocene through Quaternary volcanic rocks of the Cascade Range. No fault scarps on Quaternary deposits have been described, but prominent escarpments on Neogene volcanic rocks and a minimum offset of 2,000 feet in Pliocene volcanic rocks suggest that some displacement occurred in the Quaternary (Personius, 2002).

Cascadia Subduction Zone

The CSZ megathrust represents the boundary between the subducting Juan de Fuca tectonic plate and the overriding North American tectonic plate. Based on studies of the geologic record, recurrence interval estimates have been generated ranging from about 300 to 600 years. Geologic evidence suggests the most recent earthquake occurred in January 1700 (Atwater et al., 2015) and probably ruptured much of the approximate 620 miles length of the CSZ with estimated moment magnitudes MW 9.0. The horizontal distance from the edge of the CSZ megathrust, located offshore from Lafayette, is approximately 70 miles, with an uncertainty of ±30 miles.

Two types of subduction zone earthquakes are possible in this region: intraplate and interplate.

• A deep intraplate earthquake on the seismogenic part of the subducting Juan de Fuca Plate typically occurs at depths of between 20 and 40 miles and is capable of generating an event with a MW of up to 7.5.

• CSZ interplate earthquakes occur on the boundary between the subducting Juan de Fuca and the overriding North American tectonic plate. The magnitudes for such earthquakes are estimated to range from approximately MW 7.0 to 9.0, with the current U.S. Geological Survey (USGS) risk-based maximum credible earthquake for CSZ megathrust of MW 9.2 (Petersen et al., 2008).

SUBSURFACE CONDITIONS

Subsurface conditions at the Site were inferred from subsurface field explorations, laboratory testing on the collected soil samples, and review of applicable geologic literature. The locations of our field explorations are presented on Figure 3, Exploration and Landslide Features Map.

Detailed descriptions of the field explorations, geologic units, laboratory testing, groundwater observations, and generalized engineering properties are presented in the following sections and on subsurface exploration logs in Appendix A.

Subsurface Explorations

Between February 21 and March 8, 2023, Haz-Tech Drilling, advanced four borings (designated BH23-01 through BH23-04) using a track-mounted CME-850 with hollow stem auger (HSA) drilling and rock coring techniques. Disturbed soil samples were obtained in our borings at 2.5- to 5-foot intervals using the Standard Penetration Test (SPT) in general accordance with the ASTM International (ASTM) D1586, Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils (ASTM, 2021). Rock samples were collected using HQ wireline coring system.

Boring logs summarizing the subsurface conditions are presented in Appendix A.

Observations and tests were performed in general accordance with ASTM International

(ASTM) Methods D2488 Standard Practice for Description and Identification of Soils (Visual- Manual Procedure) (ASTM, 2022). The terminology used in the soil classifications and other modifiers are defined and presented on the attached Descriptive Terminology for Boring Logs, Boring Log Legend. Laboratory test results are presented in Appendix B.

Boring details are presented in Table 1 below.

Table 1. Boring Locations and Installations

Boring Total Depth

(ft bgs) Location Description Purpose and Installations

BH23-01 80.5 Near landslide toe

Landslide monitoring:

Inclinometer casing Vibrating-wire Piezometer

BH23-02 90 Southside, road embankment

BH23-03 125.5 Upper bench 200 feet north of road

BH23-04 61.5 Southside, road embankment

Groundwater monitoring:

Vibrating-wire Piezometer

Soils and Rock Subsurface conditions were inferred from the subsurface field explorations, laboratory testing on the collected soil samples, review of applicable geologic literature, and nearby borings completed by the Washington State Department of Transportation (WSDOT). The soil and rock units observed in the borings include the active project landslide with deposits, comprised road embankment fill and layers of pyroclastic rocks (MФvcs), underlain by pyroclastic rocks (MФvcs).

A summary of the subsurface units encountered in the borings, presented in stratigraphic order from top to bottom, is presented below.

Active Project Landslide (BH23-01 and BH23-02)

A ct iv e

Pr oj ec t L an ds lid e

Ground Surface Borings BH23-01 and BH23-02 drilled within the active project landslide below and along Wind River Road encountered topsoil in the near surface or the road embankment gravel.

Road Embankment and Culvert Fill

Approximately 20 feet of road embankment fill was encountered in borings BH23-02 consisting of brown clayey gravel (GC) and poorly-graded gravel (GP). The relative densities of the coarse-grained fill in BH23-02 are very loose to loose, with N-values between 2 and 6 blows per foot (bpf).

Volcanic rocks of Stevenson Ridge

Borings BH23-01 (from the ground surface) and BH23-02 (beneath the road embankment fill) encountered 20 feet of light brown fat clay with sand (CH) and brown fat clay (CH), respectively. The

– Pyroclastic Rocks (MФvcs) fine-grained soils are moist with consistencies of very soft to stiff (N-values between 1 and 9 bpf) and the plasticity index (PI) is between 53 and 58.

A distinct landslide shear zone consisting of bentonitic light brown fat clay (CH) was encountered in borings BH23-01 and BH23-02 and are supported by the inclinometer casing deflection measurements. The landslide shear zone is wet with a soft consistency (N-value of 3 bpf).

Table 2. Landslide Shear Zone in BH23-01 and BH23-02

Boring Depth to Top / Bottom of Shear Zone Elevation(1) to Top /

Bottom of Shear Zone BH23-01 20 / 22 feet 312 / 310 feet

BH23-02 33 / 35 feet 326 / 324 feet

Notes: 1 Based on NAVD88 taken with survey equipment

Pr eh is to ric L an ds lid e

Volcanic rocks of Stevenson Ridge

– Pyroclastic Rocks (MФvcs)

Between 45 and 55 feet of layered fat clay (CH), sandy fat clay (CH), gravelly fat clay (CH), sandy elastic silt (MH), sandy fat clay with gravel (CH), clayey sand with gravel (SC), silty sand (SM), and clayey gravel with sand (GC) derived from pyroclastic flows or air-fall was encountered underlying the landslide shear zone.

The consistencies of the fine-grained deposits generally increased with depth and are firm to hard, with N-values between 5 and 45 bpf and the plasticity index (PI) is between 25 and 30. The relative densities of the coarse-grained deposits are medium dense to dense, with N-values between 23 and 39 bpf.

Volcanic rocks of Stevenson Ridge – Pyroclastic Rocks (MФvcs)

Residual soil grading to slightly weathered andesite breccia was encountered beneath the pyroclastic deposits. The intact rock is very weak to strong (R1 to R4), with rock quality designation (RQD) between 50 and 75 percent. Discontinuities consist of joints that were in good to fair condition, closely to moderately spaced, horizontal to dipping (0 to 50 degrees), moderate open apertures (<0.02 to 0.10 inches), iron oxide surface staining; slightly rough to rough surface (JRC = 8 to 10).

Table 3. Depth to Competent Rock in BH23-01 and BH23-02

Boring Depth to Top of Rock Elevation(1) to Top of

Rock BH23-01 71 feet 261 feet

BH23-02 80 feet 279 feet

Periphery Site Borings (BH23-03 and BH23-04)

Ground Surface Borings BH23-03 and BH23-04 were drilled outside the active project landslide above and along Wind River Road and encountered topsoil or the road embankment gravel.

Road Embankment and Culvert Fill

The approximately 18 feet of road embankment fill in boring BH23-04 consisted of brown clayey gravel (GC), poorly-graded gravel (GP), and cobbles and boulders. The relative densities are loose to dense, with N-values between 6 and 38 bpf.

Underlying the road embankment fill in BH23-04, approximately15 feet of brown fat clay with gravel (CH) and 2 feet of poorly-graded gravel with clay and sand (GP-GC) surrounds the 36-inch cross culvert.

The consistencies of the fine-grained fill are firm, with N-values between 5 and 7 bpf, and the relative density of the coarse-grained fill is medium dense, with an N-value of 13 bpf.

Pr eh is to ric L an ds lid e

Volcanic rocks of Stevenson Ridge – Pyroclastic Rocks (MФvcs)

Boring BH23-03 (from the ground surface) encountered 108 feet and BH23-04 (beneath the road embankment fill) encountered up to 28 feet (total boring depth) layers of fat clay (CH), fat clay (CH) with sand, sandy fat clay (CH), gravelly fat clay (CH), sandy fat clay with and without gravel (CH), clayey sand with and without gravel (SC), silty sand (SM), and clayey gravel with sand (GC) derived from pyroclastic flows or air-fall

The consistencies of the fine-grained deposits generally increased with depth and are firm to hard, with N-values between 4 and 67 bpf and the plasticity index (PI) is between 16 and 67. The relative densities of the coarse-grained deposits are medium dense to very dense, with N-values between 22 and greater than 100 bpf.

Volcanic rocks of Stevenson Ridge – Pyroclastic Rocks (MФvcs)

Slightly weathered andesite breccia was encountered beneath the pyroclastic deposits in BH23-03. The intact rock is medium strong to strong (R3 to R4), with rock quality designation (RQD) of 100 percent. Discontinuities consist of joints that were in good to fair condition, closely to moderately spaced, horizontal to dipping (0 to 50 degrees), moderate open apertures (<0.02 to

0.10 inches), iron oxide surface staining; rough to very rough surface (JRC = 12 to 16).

Table 4. Depth to Competent Rock in BH23-03

Boring Depth to Top of

Rock Elevation(1) to Top of

Rock BH23-03 108 feet 310 feet

Groundwater During drilling, groundwater levels were measured in borings BH23-01, BH23-02, and BH23- 03 with a water level reader. Groundwater was not encountered in BH23-04. The depths to groundwater are presented in Table 5 and on the boring logs in Appendix A.

Table 5. During Drilling Groundwater Depths and Elevations

Boring Surface EL

(ft.)(1) Groundwater

Depth (ft bgs) Elevation (ft)

BH23-01 332 49 283

BH23-02 359 66 293

BH23-03 418 111 307

Notes: 1 Based on NAVD88 taken with survey equipment

Vibrating-Wire Piezometers

Unvented vibrating-wire piezometers (VWPs) equipped with single-channel dataloggers were installed in the borings to monitor long-term fluctuations in groundwater. Readings were collected at 6-hour intervals starting in mid-March 2023 and downloaded in June, August, and November. The instruments were installed after completing the drilling and installation of flush-mounted monuments to protect the dataloggers. Table 6 summarizes the installations and the collected VWP data ranges.

Table 6. VWP Groundwater Measurement Depths and Elevations

Boring Surface EL

(ft.)(1)

Groundwater Levels(2)

Date High EL Shallow Depth

Deep Depth Low EL Date

BH23-01 332 4/11/23 351.54 (+)19.54(3) 1.52 330.48 10/10/23

BH23-02 359 3/11/23 357.86 1.14 19.78 339.22 10/10/23

BH23-03 418 4/25/23 384.92 33.08 54.32 363.68 10/10/23

Notes: 1 Based on NAVD88 taken with survey equipment 2 readings between 3/10/2023 and 11/8/2023

3 (+) indicate artesian conditions

These data indicate a seasonal cycle, with groundwater levels deeper in the summer and fall and shallower in the winter and spring (Appendix C). Local groundwater levels will fluctuate seasonally with precipitation and near-Site usage.

Landslide Monitoring Slope inclinometer casings installed in borings BH23-01, BH23-02, and BH23-03 for the purpose of measuring horizontal landslide displacements consist of 2.75-inch-diameter polyvinyl chloride (PVC) pipes encased with bentonite cement grout with interior tracks to insert a monitoring probe down the casing. The inclinometer probe and data logger are used to measure and record inclinometer casing deflection, presumably related to ground movement.

Baseline readings were recorded shortly after installation with subsequent readings in June, August, and November 2023. Slope inclinometer casing readings were collected using a Slope Inclinometer Digitilt AT Inclinometer System®. Table 7 below summarizes the Slope inclinometer casing installation and displacements, and graphs are included in Appendix C.

Table 7. Slope Inclinometer Casing Installations and Deflections

Boring Installation

Depth / Elevation Shear Zone

Depth / Elevation Average Vector(1)

Displacement Total Vector(1) Displacement

BH23-01 78 feet / 254 feet 22 feet / 310 feet 0.05 inches/month 0.45 inches

BH23-02 88 feet / 271 feet 34 feet / 325 feet 0.22 inches/month 1.92 inches

BH23-03 124 feet / 294 feet N/A N/A N/A Notes: 1 Since A- and B-axes are not exactly parallel or perpendicular to landslide movement, the results are reported by the estimated vector magnitudes of the casing deflections.

GEOTECHNICAL CONCLUSIONS AND RECOMMENDATIONS

Based on our Site reconnaissance, recent subsurface borings, instrument monitoring, and review of the previous geotechnical evaluations, we present the following key findings and conclusions that should be considered in the project design and construction:

• Based on our review of GeoDesign’s memorandum (2020), and our work, the active project landslide has been affecting Wind River Road since at least 1980 that coincided with road widening.

• The active project landslide is approximately 150 feet wide between its lateral extents along the road and 300 feet long from north of the road to the slope toe at the creek channel, moving with translational movement driven by toe erosion from the creek and likely triggered by intense precipitation and elevated groundwater pressures.

• The active project landslide deposits are comprised of road embankment fill and recently disturbed pyroclastic deposits with a relatively distinct shear zone.

• The stratigraphy surrounding and underlying the active project landslide consists of road embankment and culvert fill atop older, prehistoric landslide deposits derived from pyroclastic deposits.

• Based on the borings and slope inclinometer casing deflections, the landslide is active with the basal shear zone in BH23-01 at 22 feet bgs, approximately 75 feet north of the creek channel, and in BH23-02 at 35 feet bgs along Wind River Road, projecting a landslide shearing surface angle of approximately 6 degrees with movement in the southeast direction.

• Groundwater was encountered during drilling in BH23-01, BH-23-02, and BH23-03 between EL 283 and EL 307 feet with none recorded in BH23-04. The installed vibrating-wire piezometers measured porewater pressures between March and November 2023 with the maximum elevation in BH23-01 of EL 351.54 feet and in BH23-03 of EL 384.92 feet, indicating confined aquifer conditions beneath the site.

Seismic Design

The American Association of State Highway and Transportation Officials (AASHTO) Load Resistance Factor Design (LRFD) Bridge Design Specifications (BDS) (AASHTO, 2020) uses an earthquake design ground motion with a 7 percent probability of exceedance in 75 years, approximately equivalent to a return period of 1,000 years. The procedures outlined in Section 3.10 of the AASHTO LRFD Bridge Design Specifications were followed to characterize the seismic hazard. Based on Table C3.10.3.1-1—Steps for Site Classification of the AASHTO LRFD Bridge Design Specifications, the seismic Site Class for the Wind River Road Slide Reconstruction Project area is Site Class D.

The U.S. Seismic Design Maps website (USGS, 2022) was used to determine the peak ground acceleration and spectral accelerations at the project site based on Site Class B rock. The spectral accelerations were then scaled to Site Class D using the site factors recommended by AASHTO to estimate the seismic accelerations based on project site ground conditions. The results of this analysis are presented below in Table 8.

Table 8. AASHTO Seismic Design Parameters AASHTO BDS Parameter(1) Recommended Value

Site Class D – “Stiff Soil” Short Period Spectral Acceleration, Ss (g)2 0.406

1-Second Period Spectral Acceleration, S1 (g) 0.151

Peak Ground Acceleration, PGA (g) 0.172

Site Coefficient (Fa) 1.475

Site Coefficient (Fv) 2.195

Site Coefficient (FPGA) 1.455

Design Short Period Spectral Acceleration, SDS (g) 0.599

Design 1-Second Period Spectral Acceleration, SD1 (g) 0.332

Acceleration Coefficient, AS (g) 0.251 Notes: 1 Based on the latitude and longitude of the Site: 47. 717°N, 124. 826°W

2 g = acceleration due to gravity

The scaled peak ground acceleration, As, for an earthquake with a return period of approximately 1,000 years was determined to be 0.251g using the USGS Seismic Design Maps website. Seismic design for retaining walls is typically not required by the AASHTO BDS where the As is under 0.4g. Embankment and cut slopes for roadways are generally not designed for earthquake loading, except when in close proximity to bridge abutments.

No bridges are proposed for this project.

Slope Stability Analyses

The stability of the slope descending to the south toward the creek channel, including the areas above, within, and below the active project landslide, was evaluated with information developed from the available data, including the 2024 borings and instrumentation observations, surveys, and surficial movement data. Based on critical cross section A to A’ (refer to Figure 3), a limit-equilibrium slope stability was modelled using the Slide2 software (RocScience, 2022).

The model is specific to the upper active project landslide and does not evaluate the deeper-seated, large-scale, prehistoric landslide deposits underlying the region and we have assumed these large blocks are generally stable. Limited effort was given to analyzing and assessing the deeper landslide risk, as any mitigations would be infeasible due to their costs.

Several soil units are described within each boring, which were grouped based on type and origin into:

• Road embankment fill

• Upper clay layer that includes the soils in the active project landslide and the surrounding surficial soils

• Landslide shear zone correlated with movement measured in the inclinometer casings

• A lower clay layer and a sandy clay/clayey sand layer below the landslide shear zone comprised of the Prehistoric Landslide deposits

• Underlying andesite rock layer that is a unit within the Volcanic rocks of Stevenson Ridge – Pyroclastic Rocks (MФvcs)

The material properties in the model are based on typical values for the observed soil types and relative densities, the laboratory index test results, and shear strength correlations (NAVFAC, 1983) (Appendix B). The soil units and layers units within the stability model were selected to best represent the conditions encountered during the investigation and instrumentation observations.

The groundwater encountered during drilling and measured with the deeper borehole instruments suggest a confined aquifer at depth, with a likely seasonal, perched zone above the landslide shear zone. Due to the chaotic nature of landslide deposits, perched groundwater is not easily delineated between bore holes; so a simplified groundwater table is used in the cross section under undrained conditions. Saturated surficial soils were observed at the toe of the landslide and appear to be shallower groundwater tied to precipitation and poor soil infiltration.

The soil engineering properties used in the analyses are summarized in Table 9 and were modeled using Mohr-Coulomb failure criterion.

Table 9. Soil Units and Engineering Properties

Soil Unit Unit Weight

(pcf)

Saturated Unit Weight

(pcf)

Strength Parameters Cohesion

(psf) Friction Angle

(degrees) Road Embankment Fill 130 0 40

Fat Clay (CH) – Upper (Landslide Deposits) 100 105 0 25

Fat Clay (CH) – Ash (Landslide Shear Zone) 80 90 0 --

Fat Clay (CH) – Lower 100 500 25

Sandy Clay / Clayey Sand (SC/CH) 115 100 26

Andesite / Andesite Breccia 145 5000 50 Notes: pcf = pounds per cubic foot psf = pounds per square foot

Slide® performs slope stability computations based on the modeled conditions and calculates a factor of safety against slope failure, which is defined as the ratio of resisting forces to driving forces. A factor of safety of 1.0 indicates a “just-stable” condition, and a factor of safety less than 1.0 would indicate unstable conditions. The typical factor of safety for a landslide slope mitigation is 1.25. Risk reduction efforts are considered at a reduced factor of safety when appropriate.

Existing conditions were modeled using the engineering properties in Table 9 above, the recently surveyed surface topography, the interpreted subsurface stratigraphy along cross section A–A’ (Figure 3), and the slope inclinometer and vibrating wire piezometer monitoring in borings BH23-01, BH23-02, and BH23-03. The parameters were adjusted under static conditions until the FOS was 1.0 or less at depths between 20 and 22 feet bgs in the lower landslide body (BH23-01) and between 33 and 35 feet bgs beneath the roadway, respectively.

Table 10. Summary of Profile A-A’ Slope Stability Analysis Results

Scenario

Calculated Factor of Safety

Undrained Drained Existing 0.96 1.29

The existing conditions slope stability models are shown in Appendix D, Figures D-1 through D- 3.

REFERENCES

American Association of State Highway and Transportation Officials (AASHTO), 2020, AASHTO LRFD Bridge Design Specifications, 9th Edition.

ASTM International (ASTM), 2021, 2021 Annual Book of ASTM Standards, West Conshohocken, Pennsylvania.

Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuji, Y., Ueda, K., and Yamaguchi, D.K., 2015, The orphan tsunami of 1700—Japanese clues to a parent earthquake in North America, 2nd ed.: Seattle, University of Washington Press, U.S. Geological Survey Professional Paper 1707, 135 p.

Czajkowski, J.L., Bowman, J.D., Fusso, L.A., and Boschmann, D.E., 2014, Geologic mapping and geothermal assessment of the Wind River valley, Skamania County, Washington:

Washington Division of Geology and Earth Resources Open File Report 2014-01, March, geologic map scale: 1:24,000.

Federal Highway Administration (FHWA), 2014, Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14, Updated: Thursday, March 24, 2022.

GeoDesign, Inc., 2020, Field Reconnaissance and Preliminary Evaluation, Wind River Road Slide, Skamania County, Washington, GeoDesign Project: Skamania-10-01, dated November 2, 2020.

Jacobacci, K.E.; Mickleson, K.A., 2017, Landslide Inventory for the Washington Side of the Columbia River Gorge: Clark, Skamania and Klickitat Counties: Washington Department of Natural Resources GSA Poster Session T244: Booth #7.

Naval Facilities Engineering Command Design Manual (NAVFAC DM-7.3), 1983, Soil Dynamics, Deep Stabilization, and Special Geotechnical Construction, United States Naval Facilities Engineering Command.

Personius, S.F., compiler, 2002, Fault number 866, Hood River fault zone, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 12/05/2023 12:14 PM.

Personius, S.F., and Lidke, D.J., compilers, 2003, Fault number 580, Faults near The Dalles, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 12/05/2023 12:11

PM.

Pierson, T.C., Evarts, R.C., and Bard, J.A., 2016, Landslides in the Western Columbia Gorge, Skamania County, Washington: U.S. Geological Survey Scientific Investigations Map 3358, pamphlet 22 p., scale 1:12,000, http://dx.doi.org/10.3133/sim3358.

Post Tensioning Institute (PTI) (2014), PTI Recommendations for Prestressed Rock and Soil Anchors.

http://dx.doi.org/10.3133/sim3358

Rocscience, 2018, Slide2 8.020 Analysis Program. Build date November 27, 2023.

Washington State Department of Natural Resources (DNR), 2022, Geologic provinces of Washington (summary of physiographic provinces), accessed December 1, 2023 from DNR website: https://www.dnr.wa.gov/programs-and-services/geology/explore-popular-geology/geologic-provinces-washington.

Washington State Department of Transportation (WSDOT), 2022, Geotechnical Design Manual, M 46-03.13, dated February 2022.

Washington State Department of Transportation, 2018, Geotechnical Report, SR 14/Wind River Road – Intersection Improvements, XL-4995; C.S. 300136; Vicinity MP 47.44, dated August 2018.

CLOSING

Please contact Mark Swank at (360) 210-6640 or Mark.Swank@dot.gov with any questions regarding this memorandum.

Attachments: Figures: Figure 1. Site Vicinity Map

Figure 2. Active and Prehistoric Landslide Figure 3. Exploration and Landslide Features Map

Appendix A: Subsurface Boring Logs Appendix B: Geotechnical Laboratory Tests Appendix C: Slope Inclinometer and Vibrating-Wire Piezometer Graphs Appendix D: Slope Stability Analyses https://www.dnr.wa.gov/programs-and-services/geology/explore-popular-geology/geologic-provinces-washington https://www.dnr.wa.gov/programs-and-services/geology/explore-popular-geology/geologic-provinces-washington mailto:Mark.Swank@dot.gov i

ATTACHMENTS

FIGURES

Brooks Rd

CG

1380 ft

Carson Creek

Cars on DepotRd

CG 2200A Rd

CG

2200B

Carson

Col um bi a

Ri ve r H w y

Wye th Rd

B u c k M o u n t a i n

State Hwy 14 14 te M zg er

Rd

Sm ith

B ec kon

Rd

M onagha dR n ry eP oll za

Rd

I

St Martins Hot Springs

582 ft 668 ft

G rays

Creek

Wyeth Rd

SITE VICINITY MAP

WIND RIVER ROAD SLIDE RECONSTRUCTION

WIND RIVER ROAD

SKAMANIA COUNTY, WASHINGTON

Project No.

Scale:

File Name:

Date:

FIGURE

Western Federal Lands Highway Division 610 East Fifth Street Vancouver, WA 98661

1517535921351

1 inch = 2,000 feet

DECEMBER 2023

W A S H I N G T O N

Bellingham

Olympia

Port Angeles

Seattle Spokane

Tacoma Wenatchee

Carson

Yakima

PROJECT LOCATION

Sources: Esri, Airbus DS, USGS, NGA, NASA, CGIAR, N Robinson, NCEAS, NLS, OS, NMA, Geodatastyrelsen,Rijkswaterstaat, GSA, Geoland, FEMA, Intermap and the GIS user community, Oregon Metro, Oregon State Parks, State of Oregon GEO, WA State Parks GIS, Esri, HERE, Garmin, SafeGraph, GeoTechnologies, Inc, METI/NASA, USGS, Bureau of Land Management, EPA, NPS, US Census Bureau, USDA

Basemap Layer Credits || Esri, HERE, Garmin, (c) OpenStreetMap contributors, and the GIS user community

PROJECT LOCATION

0 2,000 4,000

Feet

BH23-01

BH23-04

BH23-03

BH23-02

A’

A

0 1,200 2,400

Feet

0 200 400

Feet

PROJECT

LOCATION

ACTIVE PROJECT LANDSLIDE PREHISTORIC LANDSLIDE COMPLEX

Landslide deposit: A mass of rock, earth, and (or) debris that has moved downslope.

Scarp and flank: The headscarp or uppermost scarp, which in many cases exposes the primary failure plane (surface of rupture), and �anks and shear zones.

SLIP landslide: Landslide deposit and scarp and �ank area mapped with limited attributes collected.

Fan deposit: Cone-shaped deposits at the mouth of a drainage. Sediment on the fan is deposited by streams, �oods, and (or) debris �ows and are typically sourced from a single channel. Fans not mapped in all studies.

SLIP fan: Fan deposit mapped with limited attributes collected.

Rock fall: A rapid downslope movement of individual rocks and rock masses. The rocks sometimes accumulate as talus at the base of a steep cli�.

Scarp: Uppermost extent of the headscarp and internal scarps within the body of the landslide.

High Low

Moderate

Basemap Layer Credits || Source: Google Earth Pro

ACTIVE AND PREHISTORIC LANDSLIDE MAP

Project No.

Scale:

File Name:

Date:

FIGURE

Western Federal Lands Highway Division 610 East Fifth Street Vancouver, WA 98661

WIND RIVER ROAD SLIDE RECONSTRUCTION

WIND RIVER ROAD

SKAMANIA COUNTY, WASHINGTON

1517535921351

JANUARY 2024

Legend

Landslide Classification

Confidence

Boring BH23-01

SSA Pro�le LineA A’

Headscarp, Arrow indicates Movement Direction

Landslide Toe, Teeth on Overriding Block

Basemap Layer Credits || Source: Google Earth Pro

EXPLORATION AND LANDSLIDE FEATURES MAP

Project No.

Scale:

File Name:

Date:

FIGURE

Western Federal Lands Highway Division 610 East Fifth Street Vancouver, WA 98661

WIND RIVER ROAD SLIDE RECONSTRUCTION

WIND RIVER ROAD

SKAMANIA COUNTY, WASHINGTON

1517535921351

1 inch = 80 feet

DECEMBER 2023

0 65 130

Feet

Legend

Boring BH23-01

SSA Pro�le Line

Appr. Retaining Wall Location

A A’

Headscarp (Arrow Indicates General Movement Direction)

Landslide Toe, Teeth on Overriding Block

BH23-01

BH23-04

BH23-03

BH23-02

A’

A

APPENDIX A

Subsurface Explorations

10/04/2018

SOIL CLASSIFICATION CHART

Criteria for Assigning Group Symbols and Group Name Using Laboratory TestsA Group

Symbol Group NameB

COARSE-GRAINED

SOILS

More than 50% retained on No. 200 Sieve

GRAVELS

More than 50% of coarse fraction retained on No. 4 Sieve

Clean GRAVELS Less than 5% fines E

Cu ≥ 4 and 1 ≤ Cc ≤ 3C GW Well-graded GRAVEL D

Cu < 4 and/or 1 > Cc > 3 C GP Poorly-graded GRAVEL D

GRAVELS with fines More than 12% fines E

Fines classify as ML or MH GM Silty GRAVEL D,F Fines classify as CL or CH GC Clayey GRAVEL D,F

SANDS

Less than 50% retained on No. 4 Sieve

Clean SANDS Less than 5% fines I

Cu ≥ 6 and 1 ≤ Cc ≤ 3 C SW Well-graded SAND H

Cu < 6 and/or 1 > Cc > 3 C SP Poorly-graded SAND H

SANDS with fines More than 12% fines I

Fines classify as ML or MH SM Silty SAND F,H Fines classify as CL or CH SC Clayey SAND F,H

FINE-GRAINED

SOILS

50% or more passes the No. 200 Sieve

SILTS and CLAYS Liquid limit less than 50

Inorganic PI > 7 and plots on or above the “A” line J CL Lean CLAY K,L,M PI < 4 or plots below “A” line J ML SILT K,L,M

Organic Liquid limit – oven dried Liquid limit – not dried < 0.75 OL Organic CLAY K,L,M,N

Organic SILT K,L,M,O

SILTY and CLAYS Liquid limit 50 or more

Inorganic PI plots on or above “A” line CH Fat CLAY K,L,M PI plots below “A” line MH Elastic SILT K,L,M

Organic Liquid limit – oven dried Liquid limit – not dried < 0.75 OH Organic CLAY K,L,M,P

Organic SILT K,L,M,Q

HIGHLY ORGANIC

SOILS Primarily organic matter, dark in color, and organic odor PT PEAT A Based on material passing the 3-inch sieve.

B If field sample contains cobbles or boulders, add “with cobbles” or “boulders” or both, to group name.

C Cu = D60/D10 Cc = (D30)2 / (D10 x D60) D If soil contains ≥15% sand, add “with sand” to group name.

E Gravels with 5 to 12% fines require dual symbols:

GW-GM well-graded GRAVEL with silt GW-GC well-graded GRAVEL with clay GP-GM poorly-graded GRAVEL with silt GP-GC poorly-graded GRAVEL with clay

F If fines classify as CL-ML, use dual symbol GC-GM or SC-SM G If fines are organic, add “with organic fines” to group name.

H If soil contains ≥15% gravel, add “with gravel” to group name.

I Sand with 5 to 12% fines require dual symbols:

SW-SM well-graded SAND with silt SW-SC well-graded SAND with clay SP-SM poorly-graded SAND with silt SP-SC poorly-graded SAND with clay J If Atterberg limits plot in hatched area, soil is a CL-ML, silty

CLAY.

K If soil contains 15 to 29% plus No. 200, add “with sand” or “with gravel”, whichever is predominant.

L If soil contains ≥ 30% plus No. 200, predominantly sand, add “sandy” to beginning of group name.

M If soil contains ≥ 30% plus No. 200, predominantly gravel, add “gravelly” to beginning of group name.

N PI ≥ 4 and plots on or above “A” line.

O PI < 4 or plots below “A” line.

P PI plots on or above “A” line.

Q PI plots below “A” line.

APPARENT DENSITY OF COARSE-

GRAINED SOIL

SPT N-value (blows per foot) Apparent Density

0 to 4 Very Loose

5 to 10 Loose

11 to 30 Medium Dense

31 to 50 Dense

> 50 Very Dense

CONSISTENCY OF

FINE-GRAINED SOIL

SPT N-value (blows per foot) Consistency

0 to 1 Very Soft

2 to 4 Soft

5 to 8 Firm

9 to 15 Stiff

16 to 30 Very Stiff

> 30 Hard

SOIL STRUCTURE TERMS

Stratified Alternating layers of varying material or color with layers > 1/4 inch (6 mm), note thickness and inclination.

Laminated(1) Alternating layers of varying material or color with layers < 1/4 inch (6 mm), note thickness and inclination.

Fissured(1) Breaks along definite planes of fracture with little resistance to fracturing.

Slickensided(1) Fracture planes appear polished or glossy, sometimes striated.

Blocky(1) Cohesive soil that can be broken down into smaller angular lumps which resists further breakdown.

Disrupted Soil structure is broken and mixed. Infers that material has moved substantially -landslide debris.

Homogeneous Same color and appearance throughout.

Lensed Inclusion of small pockets of different soil, such as small lenses of sand scattered through a mass of clay; < 1/4 inch (6 mm) note thickness.

(1) Do not use laminated, fissured, slickensided, or blocky for coarse-grained soils.

Descriptive Terminology for Boring Logs

Field descriptions of borings are based on the FLH Soil and Rock Description and Identification Guidelines that generally follow the Visual-Manual Procedure (ASTM D 2488). The soil classifications shown on the boring logs are based on laboratory tests (ASTM D 2487) when the two-letter group symbol follows the group name in parenthesis.

Standard Penetration Test (SPT): the SPT consists of driving a 2 in (50 mm) O.D. split barrel sampler a depth of 18 in (450 mm) or 24 in (600 mm) using a 140 lb (63.6 kg) hammer with a 30 in (750 mm) drop. The blow count is the number of blows recorded for each 6 inch (50 mm) increment.

The N-value is the total number of blows for the second and third increments. Note that the N-values shown on the boring logs do not include any corrections for non-standard sampler size, hammers, drill rods, etc.

ANGULARITY OF

COARSE-GRAINED SOILS

Angular

Sharp edges and relatively plane sides with unpolished surfaces

Subangular

Similar to angular description, but with rounded edges

Subrounded

Nearly plane sides, but will have well-rounded corners and edges.

Rounded

Smoothly curved sides and no edges

PARTICLE SIZE OF COARSE-GRAINED SOILS

Component Grain Size Limits

Boulders > 12” (> 300 mm)

Cobbles 3 – 12” (75 – 300 mm)

Coarse Gravel 3/4 - 3" (19 - 75 mm)

Fine Gravel #4 Sieve - 3/4" (4.75 - 19 mm)

Coarse Sand #10 - #4 Sieve (2.00 - 4.75 mm)

Medium Sand #40 - #10 Sieve (0.425 - 2.00 mm)

Fine Sand #200 - #40 Sieve (0.075 - 0.425 mm)

10/04/2018

GRAIN/CRYSTAL SIZE FOR ROCKS (MODIFIED AFTER WENTWORTH, 1972)

Grain Size Description Criteria Less than 0.003 inches (<0.075 mm)

Very fine grained Cannot be distinguished by unaided eye. Few to no mineral grains are visible with a hand lens.

0.003 to 0.02 inches (0.075 to 0.425 mm) Fine grained

Few grain/crystal boundaries are visible; grains can be distinguished with difficulty by the unaided eye but can be somewhat distinguished by hand lens.

0.02 to 0.08 inches (0.425 to 2 mm) Medium grained Most grain/crystal boundaries are visible; grains distinguishable by eye and with the aid of a hand lens.

0.08 to 0.2 inches

(2 to 4.75 mm) Coarse grained Grain/crystal boundaries are visible; grains distinguishable with the naked eye and hand lens.

Greater than 0.2 inches

(>4.75 mm)

Very coarse grained

Grain/crystal boundaries are Clearly visible; grains are distinguishable with the naked eye.

GRAIN SHAPE (FOR SEDIMENTARY ROCKS)

Description Characteristic

Angular Showing very little evidence of wear. Grain edges and corners are sharp. Secondary corners are numerous and sharp.

Subangular

Showing definite effects of wear. Grain edges and corners are slightly rounded off. Secondary corners are slightly less numerous and slightly less sharp than in angular grains.

Subrounded Showing considerable wear. Grain edges and corners are rounded to smooth curves. Secondary corners are reduced greatly in number and highly rounded.

Rounded Showing extreme wear. Grain edges and corners are smoothed off the broad curves. Secondary corner are few in number and rounded.

Well-rounded Completely worn. Grain edges or corners are not present. No secondary edges or corners are present. DEGREE OF WEATHERING

Term Description Grade

Fresh No visible sign of rock material weathering; slight discoloration on major discontinuity surface is possible. I

Slightly Weathered

Discoloration indicates weathering of rock material and discontinuity surfaces. All rock material may be discolored by weathering and the external surface may be somewhat weaker than in its fresh condition.

II

Moderately Weathered

Less than half the rock material is decomposed and/or disintegrated to a soil.

Fresh or discolored rock is present either as a discontinuous framework or as corestones. A minimum 2-inch (50 mm) diameter sample cannot be broken readily by hand across the rock fabric.

III

Highly Weathered

More than half of the rock is decomposed and/or disintegrated to soil. Fresh or discolored rock is present either as a discontinuous framework or as corestones.

A minimum 2-inch (50 mm) diameter sample can be broken readily by hand across the rock fabric.

IV

Completely Weathered

All rock material is decomposed and/or disintegrated to soil. The original mass structure is still largely intact. Material can be granulated by hand. If rock is considered to be completely weathered, use FLH Soil Description and Identification Guidelines to describe the residual soil material.

V

Residual Soil

All rock material is converted to soil. The mass structure and material fabric are destroyed but the apparent structure remains intact. There may be a large change in volume, but the soil has not been significantly transported. Material can be easily broken-down by hand. If rock is considered to be completely weathered, use FLH Soil Description and Identification Guidelines to describe the residual soil material.

VI

RELATIVE STRENGTH OF SOIL INFILLING (ISRM, 1978 & 1981)

Grade Description Field Identification Approximate

Uniaxial Compressive

Strength

S1 Very Soft Easily penetrated several inches by fist

<3.5 psi (<25 kPa)

S2 Soft Easily penetrated severl inches by thumb

3.5 - 7 psi (25 - 50 kPa)

S3 Firm

Can be penetrated several inches by thumb with moderate effort

7 - 14.5 psi (50 - 100 kPa)

S4 Stiff

Readily indented by thumb but penetrated only with great effort

14.5 - 36 psi (100 - 250 kPa)

S5 Very Stiff Readily indented by thumbnail

36 - 72.5 psi (250 - 500 kPa)

S6 Hard Indented with difficulty by thumbnail

>72 psi (>500 kPa)

RELATIVE STRENGTH OF INTACT ROCK SPECIMENS (ISRM, 1978 &1981)

Grade Description Field Indentificaiton Approximate Uniaxial Compressive Strength

R0 Extremely Weak Rock Specimen can be indented by thumbnail 35 - 150 psi

(250 - 1,000 kPa)

R1 Very Weak Rock

Specimen crumbles under sharp blow with point of geological hammer, and can be peeled with a pocket knife.

150 - 725 psi (1,000 - 5,000 kPa)

R2 Weak Rock Shallow cuts or scrapes can be made in a specimen with a pocket knife. A firm blow with a geological hammer point creates shallow indents.

725 - 3,500 psi (5,000 - 25,000 kPa)

R3 Medium Strong Rock

Specimen cannot be scraped or cut with a pocket knife.

Specimen can be fractured with a single firm blow with a geologic hammer point.

3,500 - 7,250 psi (25,000 - 50,000 kPa)

R4 Strong Rock Specimen requires more than one firm blow of the geologic hammer point to fracture.

7,250 - 14,500 psi (50,000 - 100,000 kPa)

R5 Very Strong Rock

Specimen requires many firm blows from the hammer end of the geologic hammer to fracture.

14,500 - 36,250 psi (100,000 - 250,000 kPa)

R6 Extremely Strong Rock

Specimen can only be chipped with firm blows from the hammer end of the geologic hammer.

>36,250 psi (>250,000 kPa)

JRC RANGES (ISRM, 1978 &1981)

DISCONTINUITY CONDITION…