ora-umatill582-1_Final_Revised Geotechnical Report.pdf

Geotechnical Investigation OR UMATIL 582(1), Mill Creek Road

Umatilla County, Oregon

July 24, 2024

Prepared for

David Evans and Associates, Inc.

2100 S River Parkway Portland, OR 97201

Prepared by

16520 SW Upper Boones Ferry Road, Suite 100 Tigard, OR 97224-7661

(503) 641-3478 | www.gri.com

GR08-24

GRI 6737-TO6 – Mill Creek Road Geotechnical Investigation Report July 24, 2024

TABLE OF CONTENTS

1 INTRODUCTION

2 PROJECT DESCRIPTION

3 BACKGROUND INFORMATION

4 SITE DESCRIPTION

4.1 Geology

4.2 Site Conditions/Field Reconnaissance

5 SUBSURFACE CONDITIONS

5.1 General

5.2 Soils

5.3 Groundwater

6 FINDINGS AND RECOMMENDATIONS

6.1 General

6.2 Retaining Wall/Slope Reinforcement Recommendations

6.3 Seismic Considerations

6.4 Construction Considerations

7 DESIGN REVIEW AND CONSTRUCTION SERVICES

8 LIMITATIONS

9 REFERENCES

TABLES

Table 6-1: Correction Factors for Ruvolum® Software Table 6-2: Soil Properties for Anchored Wired-Mesh Analysis Table 6-3: Design Allowable Load Transfer Table 6-4: Recommended Length of Nails Table 6-5: Spectral Values and Site Parameters (1,000-Year Hazard Level)

FIGURES

Figure 1: Vicinity Map Figures 2 – 5: Site Plans

APPENDICES

Appendix A: Field Explorations and Laboratory Testing Appendix B: Site Photographs Appendix C: Geologic Cross-Sections Appendix D: RUVOLUM Output Files Appendix E: Slope-Stability Figures

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1 INTRODUCTION

As a subconsultant to David Evans and Associates, Inc. (DEA), GRI completed a geotechnical investigation for replacement of nine retaining walls for OR UMATIL 582(1), Mill Creek Road, in Umatilla County, Oregon. A vicinity map of the project area is included as Vicinity Map, Figure 1. This project is part of a 2016 Federal Lands Access Program (FLAP) application submitted by Umatilla County. This work continues from the pavement-investigation work completed by GRI for the alignment and provided in our September 30, 2020, report titled “Pavement Evaluation, OR UMATIL 582(1), Mill Creek Road, Umatilla County, Oregon,” prepared for DEA. As part of the pavement upgrade, the roadway surface of Mill Creek Road will be surfaced with a minimum of 4 inches of aggregate base and chip seal.

This report was previously issued as a final report on January 13, 2023. Based on correspondence with DEA, Western Federal Lands Highway Division (WFLHD) geotechnical engineers, and the WFLHD project manager between April 30, 2024, and June 24, 2024, the January 2023 final geotechnical report was revised to include additional analysis and design modifications requested by the project team.

2 PROJECT DESCRIPTION

Mill Creek Road is located approximately 13 miles southeast of Walla Walla, Washington, in Umatilla County, Oregon. The project limits extend from the Oregon-Washington state line to Tiger Creek Road (Forest Road 65). Mill Creek Road is a 2.91-mile-long, aggregate-surfaced, two-lane, two-way, minor collector. The roadway has variable widths, and in a few locations, only one vehicle can pass at a time. Mill Creek Road serves as the primary access to the Umatilla National Forest, Wenaha-Tucannon Wilderness Area, Jubilee Lake, and private homes. This section of road includes nine tied-back timber retaining walls. The Federal Highway Administration (FHWA) and Umatilla County (County) are jointly working to improve this section of road to address continued maintenance and safety concerns.

We understand FHWA and the County plan to leave the existing timber retaining walls in place and construct a new anchored wired-mesh retaining system over the walls due to deterioration of the existing timber piles. During a previous phase of the project, two wall alternatives were evaluated by the design team for replacement of the nine walls: an anchored soldier pile wall and an anchored wired mesh system. After discussions with the project team and review of the geotechnical information collected as part of this project, the preferred wall option is an anchored wired-mesh-reinforced slope. We further understand that the existing walls will be left in place to allow continued use of the roadway during construction. This report summarizes our findings and wall-design recommendations.

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3 BACKGROUND INFORMATION

During scoping for the pavement evaluation portion of the project, nine tied-back timber soldier pile walls were identified. The existing walls were unknown to the County, and as-built information for the walls is unavailable. DEA completed a structural inspection of the nine existing retaining walls to assess their structural competency and possible remaining design life and provide maintenance recommendations. The results of DEA’s inspection were provided in their July 2020 report to FHWA titled, “Wall Condition Report, Mill Creek Road Project, Task Order No. DTFH7015D00002, Project No. OR UMATIL 582(1), Mill Creek Road.” Based on their inspection, the timber walls have a remaining service life of 1 year to 10 years. Wall defects were observed in some of the piles where severe deterioration was observed. For continued long-term, safe functioning of the walls, DEA indicated the walls would require significant repair or replacement. FHWA and the County have selected the replacement option for all nine walls.

During our site reconnaissance, a private utility locator was able to locate conductible cables that appear to run to a deadman anchor system in the roadway section behind the walls.

4 SITE DESCRIPTION

4.1 Geology

The site is located in the northern Blue Mountains geologic province of Umatilla County in northeastern Oregon, as shown on Figure 1. The nine retaining walls included in the project are located along an approximately 0.6-mile section of Mill Creek Road located approximately 14 miles southeast of the town of Walla Walla, Washington, and about

1.4 miles west of the access gate to the Mill Creek watershed, an important drinking water source for the region. Mill Creek Road is located on the northern side of Mill Creek, a tributary to the Walla Walla River that flows into the Columbia River. Surficial sediments, including Holocene-age alluvium, are mapped near the project site and correspond generally to the alignment of Mill Creek; nearby upland areas, including the project area, are mapped as Miocene-age Grande Ronde Basalt, a component of the Columbia River Basalt Group (Madin and Geitgey, 2007).

No mapped or historic landslides were identified within the limits of the project site on the Oregon Department of Geology and Mineral Industries (DOGAMI) statewide landslide hazard database (SLIDO Version 4.0). DOGAMI is the state agency responsible for geologic hazard mapping in Oregon. Regional landslide susceptibility at the site is mapped as moderate to high (Franczky et al., 2019). Areas of greater landslide susceptibility generally correspond to areas of greater relief.

The nearest mapped fault to the existing retaining walls listed in the U.S. Geological Survey (USGS, 2020) Quaternary Fault and Fold Database is the Kooskooskie section of the Hite

GRI 6737-TO6 – Mill Creek Road Geotechnical Investigation Report Page 3 fault system, located approximately 0.8 miles to the west of Wall 1. The Kooskooskie section is depicted as a north-south-oriented, left-lateral fault and does not have documented historical seismicity, although it is considered to be potentially Quaternary active with the interpreted age of most recent prehistoric deformation listed as middle and late Quaternary (Personius et al., 2002).

Groundwater depth throughout the project area is estimated to be about the elevation of nearby Mill Creek, with groundwater flow generally to the south toward Mill Creek. Local subsurface geologic and manmade features can affect groundwater flow, and localized perched conditions may occur at shallower depths within the surficial soil (where present) during periods of prolonged or intense precipitation. Therefore, this groundwater depth and flow interpretation is only an estimate.

4.2 Site Conditions/Field Reconnaissance

4.2.1 General

An Oregon-registered geologist and Oregon-registered professional geotechnical engineer from GRI conducted a walking reconnaissance of the project corridor near the locations of the nine existing walls on September 29 and 30, 2020. The roadway section is located on sloping topography adjacent to Mill Creek, and portions of the road appear to have been constructed with a cut into the uphill side of the alignment. GRI conducted the reconnaissance to evaluate surface conditions near the nine walls and to look for indications of slope instability, other geologic conditions, and potential hazards relative to the site. Additionally, only minimal topographic survey data was available below the walls at the time of our reconnaissance, and the reconnaissance included an approximation of slope angles and distances below the wall to assist in our preliminary slope-stability analyses.

In June 2021, DEA completed an additional survey of the existing slopes below the wall.

During their survey work, notes on the location of the top of rock elevation were recorded when observed along the steep slopes below the walls.

Site photographs from our September 2020 geologic reconnaissance are provided in Appendix B. The following sections summarize observations from this reconnaissance and review of additional survey data from June 2021 for each of the project retaining walls.

4.2.2 Wall 1: Station 93+42.55 to Station 93+87.72

Measuring about 44 feet long and up to about 4 feet tall, Wall 1 is the westernmost wall in the project alignment. The tied-back timber pile wall includes eight piles. Downslope of the base of the wall, the hillside slopes at approximately 33° for an estimated length of about 11 feet, beyond which the slope steepens to greater than 50° to the active channel

GRI 6737-TO6 – Mill Creek Road Geotechnical Investigation Report Page 4 of Mill Creek, about 27 feet below the elevation of Mill Creek Road. A DEA survey identified the top of basalt for one location at about elevation 2,226 feet (North American Vertical Datum of 1988 [NAVD-88]) along the slope below the east side of the wall. GRI did not observe evidence that high-flow flood events in Mill Creek rose to the elevation of the roadway. The slopes below the wall and on either side were vegetated with grass and shrubs at the time of our reconnaissance. An area of surficial erosion was observed on the hillside below the wall, based on the slope shape and sparser vegetative cover compared to the adjacent downslope areas. Adjacent to the western edge of the retaining wall, there is a recent landslide approximately 25 feet wide that has toe material deposited into the creek below. On the cut slope upslope of Mill Creek Road, gravel and cobbles with sand and silt were visible at the time of our reconnaissance.

4.2.3 Wall 2: Station 94+38.71 to Station 94+75.61

Wall 2 is approximately 36 feet long and up to about 5 feet tall. The tied-back timber wall includes five piles. Downslope from the base of the wall, the hillside slopes at approximately 40° for a length of about 12 feet before increasing in steepness to greater than 50° to Mill Creek, which is about 35 feet below the elevation of Mill Creek Road. A DEA survey identified the top of basalt elevation to be 2,230 feet for one location along the slope below the wall. The slope is well vegetated with trees, shrubs, and grass. An area of surficial erosion was observed in the area downslope of the wall. Soils observed in the partially vegetated cut slope on the uphill side of the road consist of subrounded to rounded cobbles and gravel with sand and silt. Several small areas of shallow, surficial soil failures were also observed in the cut slope. Soil slumps blocking the inboard drainage ditch and drainage culverts may result in runoff flowing over the roadway, causing erosion of the outboard embankment. Snowmelt and heavy precipitation events may exacerbate this mechanism of fill slope erosion.

4.2.4 Wall 3: Station 95+61.48 to Station 95+92.22

Wall 3 is approximately 30 feet long and up to about 5 feet tall. The tied-back timber wall includes six piles. Downslope from the base of the wall, the hillside slopes at approximately 40° for a length of about 8 feet before increasing in steepness to greater than 45° to Mill Creek, which is about 36 feet below the elevation of Mill Creek Road. A DEA survey identified the top of basalt at elevation 2,234 feet for one location along the slope below the wall. The slope is heavily vegetated with grass, trees, and shrubs. An area of surficial erosion was observed in the area downslope of the wall. The cut slope on the uphill side of the road is composed of silt and sand with scattered gravel and cobbles. An approximately 25-foot-long by 20-foot-tall, shallow, surficial soil failure was observed in the cut slope and encroaches into the inboard drainage ditch.

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4.2.5 Wall 4: Station 97+24.15 to Station 97+97.04

Wall 4 is approximately 73 feet long and up to about 6.5 feet tall. The tied-back timber wall includes 12 piles. Downslope from the base of the wall, the hillside slopes at approximately 40° for a length of about 20 feet before flattening to a slope of about 30° for a length of about 6 feet, beyond which the slope is about 42° to the active channel of Mill Creek, estimated at about 34 feet below the existing roadway elevation of Mill Creek Road. A DEA survey identified top of basalt at elevations 2,238 feet, 2,234 feet, and 2,240 feet for locations below the west, center, and east sides of the wall, respectively. The slope is heavily vegetated with trees, grass, and shrubs. An area of surficial erosion was observed in the area downslope of the wall about 125 feet to the east of the wall. The cut slope on the uphill side of the road is heavily vegetated with grass and shrubs. No obvious rock outcrops were visible in the cut slope.

4.2.6 Wall 5: Station 103+38.01 to Station 103+54.25

Wall 5 is about 16 feet long and up to about 3 feet tall. The tied-back timber wall includes three piles. Downslope from the base of the wall, the hillside slopes at approximately 26° for a length of about 9 feet. Below this area, the slope is about 47° for a length of about 17 feet before flattening to a slope of about 30° for a length of about 16 feet above an approximately 60° slope to Mill Creek, which is about 42 feet below the roadway elevation of Mill Creek Road. A DEA survey identified the top of basalt at elevation 2,236 feet for one location along the slope west of the wall. The slope below the wall is heavily vegetated with grass and shrubs. An area of surficial erosion was observed in the area downslope of the wall. Mature fir trees with bent trunks were observed east and west of the wall, which indicate slope instability or soil creep during the life of the trees. The cut slope on the uphill side of the road is composed of silt and sand soils. An approximately 15-foot-long by 15-foot-tall shallow surficial soil failure was observed in the cut slope and encroaches into the inboard drainage ditch. Horsetail vegetation observed in the cut slope indicates the year-round presence of groundwater that may have contributed to the surficial slope failure.

4.2.7 Wall 6: 103+97.91 to Station 104+69.10

Wall 6 is approximately 72 feet long and up to about 5.5 feet tall. The tied-back timber wall includes 12 piles. Downslope from the base of the wall, the hillside slopes at about 50° to 60° for a length of about 11 feet. Below this area, the slope is about 32° for a length of about 11 feet, then about 42° for a length of about 14 feet above a steep, near-vertical slope to Mill Creek, estimated at about 45 feet below the elevation of Mill Creek Road. A DEA survey identified the top of basalt at elevation 2,249 feet at locations along the slope below the west, central, and east ends of the wall. An 18-inch culvert drains the inboard ditch through the wall about 15 feet west of the eastern edge of the wall. Mature fir trees with bent trunk areas were observed both below and above the roadway, which indicate

GRI 6737-TO6 – Mill Creek Road Geotechnical Investigation Report Page 6 slope instability or soil creep during the life of the trees. The cut slope on the uphill side of the road is composed of silt and sand soils, and a shallow surficial soil failure was observed in the cut slope and encroaches into the inboard drainage ditch. Horsetail vegetation observed in the cut slope indicates the year-round presence of groundwater that may have contributed to the surficial slope failure.

4.2.8 Wall 7: Station 104+96.27 to Station 105+16.62

Wall 7 is about 20 feet long and up to about 6 feet tall. The tied-back timber wall includes four piles. Downslope from the base of the wall, the hillside slopes at about 37° for a length of about 20 feet before steepening to about 50° to Mill Creek, estimated at about 26 feet below the existing roadway elevation of Mill Creek Road. A DEA survey identified the top of basalt at elevations of 2,250 feet and 2,252 feet at locations along the slope west and east of the ends of the wall, respectively. An area of surficial erosion was observed in the area downslope and east of the wall. The cut slope on the uphill side of the road is composed of silt and sand soils, and a shallow surficial soil failure was observed in the cut slope and encroaches into the inboard drainage ditch. Horsetail vegetation observed in the cut slope indicates year-round presence of groundwater that may have contributed to the surficial slope failure.

4.2.9 Wall 8: Station 121+59.93 to Station 122+08.86

Wall 8 is approximately 48 feet long and up to about 7 feet tall. The tied-back timber wall includes nine piles. Downslope from the base of the wall, the hillside slopes at approximately 43° for a length of about 14 feet before flattening to about 10° for a length of about 4 feet. The slope then steepens to about 48° or steeper to Mill Creek, about 38 feet below the elevation of Mill Creek Road. A DEA survey identified the top of basalt at about elevation 2,279 feet at locations along the slope west of the end of the wall and below the east end of the wall. A DEA survey identified a rock outcrop at elevation 2,266 feet along the slope below the center portion of the wall. A recent outboard fill failure approximately 10 feet wide was observed at the west edge of the wall. The cut slope on the uphill side of the road was composed of colluvial silt, sand, and rounded cobbles.

Basalt rock outcrops were also observed above the road elevation in the central and western portions of the cut slope. The exposed basalt is non-vesicular, dark gray, fresh, moderately hard (R3) with moderately to closely spaced jointing, and is mapped as Grande Ronde Basalt, which is part of the Columbia River Basalt Group. In steeper-cut areas, several surficial soil failures were observed that encroach into the inboard drainage ditch.

4.2.10 Wall 9: Station 123+16.01 to Station 123+66.89

Wall 9 is approximately 49 feet long and up to about 6 feet tall. The tied-back timber wall includes eight piles. Downslope from the base of the wall, the hillside slopes at approximately 48° for a length of about 23 feet, flattening to a slope of about 27° to Mill

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Creek, which is estimated to be about 31 feet below the elevation of Mill Creek Road.

Basalt rock was observed downslope to the east and west of the wall, within about 15 feet of the outboard edge of the roadway. A DEA survey identified a rock outcrop at about elevation 2,266 feet along the slope east of the wall. The cut slope on the uphill side of the road is composed of colluvial silt, sand, and rounded cobbles. In steeper-cut areas, several surficial soil failures were observed that encroach into the inboard drainage ditch. A private driveway was observed on the uphill side of the road, east of the wall. A culvert with fill was observed near the driveway.

5 SUBSURFACE CONDITIONS

5.1 General

Subsurface materials and characteristics at the site were investigated between October 12 and 14, 2020, by GRI with five borings, designated B-1 through B-5. The borings were completed to depths ranging from about 16 feet to 30 feet. The general locations of the borings are shown on the Site Map, Figure 2, with more specific locations of the borings shown on the Site Plans, Figures 3 through 5. The field-exploration and laboratory-testing programs are described in Appendix A. Logs of the borings are provided on Figures 1A through 5A. The terms and symbols used to describe the materials encountered in the borings are defined in Tables 1A and 2A and the attached legend. Rock-core photographs are provided on Figures 6A through 8A. Geologic cross-sections at the nine wall locations based on the available survey data and the results of our recent borings are provided in Appendix C. The subsurface profiles at each wall location are primarily based on boring B- 1 for Walls 1 through 3, boring B-2 for Wall 4, boring B-3 for Walls 5 through 7, boring B- 4 for Wall 8, and boring B-5 for Wall 9. Slight modifications of basalt elevation were made based on survey notes by DEA survey crew. Borings B-1 through B-5 were completed at approximate roadway Stations 95+65, 97+65, 104+35, 121+80, and 123+40, respectively.

5.2 Soils

For the purpose of discussion, the materials disclosed by the borings have been grouped into the following units based on their physical characteristics and engineering properties and listed as they were encountered from the ground surface downward:

a. Silty GRAVEL (Fill)

b. SILT (Possible Fill)

c. Silty GRAVEL to Silty GRAVEL and COBBLES (Colluvium)

d. Gravelly SILT (Colluvium/Possible Fill)

e. BASALT (Grande Ronde Basalt)

The following paragraphs provide a description of the materials encountered and a discussion of the groundwater conditions at the site.

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a. Silty GRAVEL (Fill) The borings were advanced through the gravel-surfaced roadway section of Mill Creek Road and encountered silty gravel fill to depths ranging from about 2.5 feet to 5 feet in all the borings. The silty gravel fill is brown and contains variable percentages of fine- to coarse-grained sand, ranging from some sand to sandy. The gravel fill is subangular to subrounded. A trace of clay was encountered in the sandy gravel fill below a depth of about 2.5 feet in boring B-5. Based on Standard Penetration Resistance (SPT) N-values and observations of the action of the drill rig, the relative density of the silty gravel fill ranges from dense to very dense.

b. SILT (Possible Fill) Possible silt fill was encountered below the silty gravel fill in borings B-2 and B-5 and extends to depths ranging from about 5 feet to 7 feet. The silt is gray-brown to light brown and contains variable percentages of fine- to coarse-grained sand, subangular gravel, and clay, ranging from some sand to sandy, trace to some gravel, and up to a trace of clay. A grain-size distribution plot for the possible silt fill is provided on Figure 9A. Scattered organics were observed in the possible silt fill in boring B-2. Based on SPT N-values, the relative consistency of the silt is very stiff.

c. Silty GRAVEL to Silty GRAVEL and COBBLES (Colluvium) Colluvium consisting primarily of silty gravel or silty gravel and cobbles was encountered below the fill in all borings and extends to depths ranging from about 9 feet to 20 feet. A boulder was encountered in the silty gravel and cobbles at a depth of about 17 feet in boring B-5. Based on our observations of exposures of this unit along the road and creek, this unit contains boulders. The silty gravel and gravel and cobbles unit contains varying percentages of fine- to coarse-grained sand, ranging from trace to some sand. A trace of clay was encountered in the gravel in boring B-5, and trace to some clay was encountered in the gravel from a depth of about 2.5 feet to 7.5 feet in boring B-4. Trace to some silt was encountered in the gravel below a depth of about 7.5 feet in boring B-3. Grain-size distribution plots for the colluvium are provided on Figures 9A and 10A. Scattered organics were encountered in the gravel in borings B-1 through B-3. The silty gravel and cobbles in boring B-5 are possible fill. Based on SPT N-values, the relative density of the silty gravel to silty gravel and cobbles ranges from dense to very dense.

d. Gravelly SILT (Colluvium/Possible Fill) Colluvium/possible fill consisting primarily of gravelly silt was encountered below the silty gravel and cobbles in boring B-5 and extends to a depth of about 23 feet. The gravelly silt is brown and contains some fine- to coarse-grained sand, a trace of clay, and cobbles.

Based on our observations of exposures of the colluvium along the road and creek, the colluvium contains boulders. A grain-size distribution plot for the gravelly silt is provided on Figure 10A. Based on an SPT N-value, the relative consistency of the gravelly silt is hard.

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e. BASALT (Grande Ronde Basalt) Grande Ronde Basalt was encountered below the colluvium in all borings and extends to depths ranging from about 16 feet to 30 feet, the maximum depth explored in all borings.

The basalt is typically medium hard (R3) to very hard (R5); red, gray, brown, or red-brown;

and slightly to moderately weathered with very close, randomly oriented joints and fractures. In boring B-3, extremely soft (R0) basalt decomposed to the consistency of silty sand was encountered from about 10 feet to 12.5 feet, and very soft (R1) to medium-hard (R3), slightly to moderately weathered basalt was encountered from a depth of about

12.5 feet to 15 feet. The basalt is moderately weathered to predominantly decomposed from a depth of about 16 feet to 17 feet in boring B-4. The unconfined compressive strengths of two rock samples are provided on the boring logs, Figures 1A and 4A.

5.3 Groundwater

Groundwater was not encountered in the borings at the time of drilling. We anticipate groundwater closely reflects water levels in the nearby Mill Creek. However, shallow perched-groundwater conditions may develop during periods of prolonged precipitation.

6 FINDINGS AND RECOMMENDATIONS

6.1 General

Subsurface explorations for this investigation indicate the roadway alignment at the location of the retaining walls is mantled with a variable thickness of fill underlain by colluvium consisting primarily of dense to very dense silty gravel with cobbles and boulders or hard, gravelly silt containing cobbles. Beneath the colluvium, the borings encountered generally medium-hard (R3) to very hard (R5) basalt. We anticipate groundwater levels are influenced by the structure of underlying basalt and typically approach the level of nearby Mill Creek; however, shallow, perched groundwater may develop during periods of prolonged precipitation.

Based on conversations with DEA, it is our understanding that the County requires the road to remain open to traffic during construction of the new retaining walls. Due to the steepness of the slope below the retaining walls and the limited construction work area to allow the road to remain open during construction, an anchored, wired-mesh slope-reinforcement system was selected as the preferred wall option. The following sections of this report provide our preliminary conclusions and recommendations for design and construction of the proposed improvements.

6.2 Retaining Wall/Slope Reinforcement Recommendations

6.2.1 General

To replace the existing deteriorating tied-back timber soldier pile walls currently providing roadway support for Mill Creek Road, several retaining wall options were considered, including an anchored steel soldier pile wall and an anchored wired mesh slope

GRI 6737-TO6 – Mill Creek Road Geotechnical Investigation Report Page 10 reinforcement system. Per discussion with the design team, an anchored wired mesh system was the preferred reinforcement option to move toward final design. The existing tied-back timber walls will be left in place to provide continued access and shoring during construction. Our recommendations for design and construction of the anchored, wired-mesh reinforcement option are provided below.

Control of surface water runoff is an important construction consideration. The roadway should be graded to prevent surface water runoff from the roadway section from running over the face of the anchored wired-mesh slope to reduce the risk of slope erosion.

6.2.2 Anchored Wired Mesh

Based on our observations and review of recent survey data, slopes consisting of colluvium as steep as about 60° are located below the existing tied-back timber walls to the top of the intact basalt. Although the existing timber walls will remain in place, our analysis assumes the existing walls will deteriorate and not contribute any structural or facial support for slope stability for the long term condition. In addition to long-term deterioration of the walls, there is some risk of long-term stability issues for the oversteepened slopes below the wall and new anchored slope system. In general, these steeper slopes appear to be performing well over the lifetime of the road. However, during our geologic reconnaissance, signs of slope instability were observed near the walls that appeared to have been caused by uncontrolled stormwater channelization off the roadway. The proposed project includes stormwater improvements to control surface water in ditches on the upslope side of the roadway. In this regard, it is our opinion that the risk of slope instability below the walls is relatively low.

Based on review of these slope stability considerations for the oversteepened slopes below the walls, WFLHD geotechnical engineers indicated this risk is acceptable to WFLHD. In our opinion, suitable long-term support for the walls can be provided by an anchored wire mesh system. The anchored wired-mesh system functions similarly to a soil nail wall / slope system with a high-strength wired-mesh fascia instead of conventional shotcrete fascia. The anchored wired-mesh fascia system is tensioned over the slope face and existing retaining wall face by plates bolted to anchor nails placed on a gridded pattern.

To evaluate the use of anchored wired mesh, the TECCO® product manufactured by Geobrugg AG was evaluated for the fascia design of the mesh and nails. This proprietary system uses high-strength wired mesh held in place by soil or rock anchors consisting of steel bars (nails) installed on a gridded pattern to retain the active zone of soil at the face of the cut. The manufacturer has developed an analysis methodology in cooperation with universities and consulting engineers in Switzerland, Germany, and Poland. This methodology is summarized in a 2012 book by Cala et al. and in the November 3, 2014, RUVOLUM® Software Manual published by Geobrugg AG.

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The RUVOLUM® software was primarily used to evaluate spacing of grouted nails based on the loads that can be transferred from the soil to the mesh. The calculations used TECCO® G65 3 mm mesh (G65/3), with a nominal punching resistance of 40.5 kips and nominal resistance to shear perpendicular to the nail of 20.2 kips. A “correction factor” was applied to the mesh resistance against anchor-plate punching, and the anchor-nail bar strength in both tension and shear. The resistance and load correction factors applied to individual elements of the system are summarized in the table below. It should be noted that a “model uncertainty correction value” of 1.3 or 1.5 was used to apply an overall factor of safety to the calculations for static evaluation and a correction value of 1.1 was applied for seismic evaluation.

Table 6-1: CORRECTION FACTORS FOR RUVOLUM® SOFTWARE

Model Element Resistance or Load

Correction Correction

Factor Notes

Global Stability

1.3 or 1.5

(static)

1.1 (seismic)

Factor of safety, “model uncertainty correction factor”

Soil Properties

Angle of Internal Friction 1.0

Cohesion 1.0

Unit Weight 1.0

Mesh

Shear Resistance at Upslope Edge of Spike Plates

Resistance Reduction, /1.5

Slope-Parallel Load Transfer to Mesh

Load Increase, 1.5x

Punching Resistance Resistance Reduction, /1.5 Plate punching through mesh

Anchor Nail Bar

Positive Influence of Pretension Load

Load Reduction, 0.8x Plate punching through mesh and soil layer sliding

Negative Influence of Pretension Load

Load Increase, 1.0x Plate punching through mesh and soil layer sliding

Bar Strength – Tension Resistance Reduction, /1.5 Resistance to loads from mesh

Bar Strength – Shear Resistance Reduction, /1.0 Resistance to combined soil and mesh loading

The anchored wired-mesh system was evaluated in the RUVOLUM® program for a vertical slope (wall) supporting fill materials and a 32° (Wall 1) and 60° (remaining walls) colluvium slope for both static and seismic loading conditions. For seismic loading conditions, a coefficient of horizontal acceleration due to earthquake loading of 0.07 g was applied.

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Based on our evaluation of the anchored wired mesh system, a #8 (1.00 inches) threaded 75-kips-per-square-inch (ksi) steel bar or Dywidag bar with equivalent strength properties will be required for the nails with TECCO P33 spike plates or equivalent. The RUVOLUM software generates estimates of anchor-nail fascia loads; however, for this application, nail loads were primarily evaluated using a limit equilibrium analysis. Outputs from the RUVOLUM software are provided in Appendix D.

Additional analysis of nail loading, nail length and spacing, and global stability was completed with the assistance of the limit-equilibrium slope-stability SLIDE2 software developed by Rocscience, Inc., of Toronto, Ontario, Canada, to further refine the results obtained from RUVOLUM.

The soil properties used in the models were based on the information obtained from the borings and laboratory testing completed as part of our 2020 investigation and our experience with similar projects in similar soils. The soil parameters used in the SLIDE2 global stability models are summarized in Table 6-2.

Table 6-2: SOIL PROPERTIES FOR ANCHORED WIRED-MESH ANALYSIS

Static/Seismic Analysis Ultimate Bond

Strength, psi

Horizontal Acceleration Coefficient, g

(Static)

Horizontal Acceleration Coefficient, g

(Seismic) Soil Unit γ’, pcf φ’ C’, psf kh kh

Fill 125 36° 0 36 0 0.07

Colluvium 130 38° 0 36 0 0.07

Decomposed Basalt

130 38° 0 36 0 0.07

Basalt 170 Infinite Strength 0 0.07

The global slope-stability analyses were completed for both static and seismic conditions for the anchored, wired-mesh, slope-reinforcement alternative. A discussion of the findings is included in the following sections. For static conditions, a live-load vertical surcharge of 250 psf was used to model construction traffic on the roadway. A horizontal pseudo-static coefficient (kh) of 0.07, equal to about half of the peak ground acceleration (PGA), was used to represent seismic loading for the 1,000-year return-interval earthquake.

The design allowable load transfers provided in Table 6-3 were used in the models, assuming a grouted, 6-inch-diameter hole with a bond zone assumed in the fill or colluvium. The allowable load transfer values were based on a factor of safety of 2.0.

GRI 6737-TO6 – Mill Creek Road Geotechnical Investigation Report Page 13

Table 6-3: DESIGN ALLOWABLE LOAD TRANSFER

Wall Number

Allowable Load Transfer, Type I (a) Anchor Nails, kips/ft

Allowable Load Transfer, Type II (b) Anchor Nails, kips/ft

1 0.55 3

2 0.80 NA

3 0.50 NA

4 0.70 NA

5 0.50 NA

6 0.95 3

7 0.80 NA

8 0.70 NA

9 0.90 3

Notes:

(a)Type I anchor nails are nails planned through (1) the existing wall sections, (2) along existing slope within 2 feet of bottom or sides of wall, and (3) the top two rows of nails along the slope beyond the ends of the existing walls.

(b)Type II anchor nails are all additional nails along the slope not identified as Type I anchor nails.

The thickness of the soil layers varies based on the conditions disclosed in the borings and observations of exposed basalt along the slope, which are provided on the SLIDE2 slope stability figures provided in Appendix E for the nine wall locations.

A minimum of four nails should be installed between adjacent timber soldier piles for the existing walls. The nails should be installed within 12 inches of the sides of existing soldier piles. The first row of nails should be installed approximately 1 foot below the grade behind the wall, at the approximate center of a lagging board, and the second row of nails within the center of the lowermost lagging board. If the horizontal spacing between adjacent nails between soldier piles exceeds 5 feet, a fifth nail should be installed within the middle of the lagged section between adjacent soldier piles, approximately equidistant from the other four anchor nails. Additional nails should be installed between the first and last timber piles for the wall and the ends of the boundary of the wire mesh system. Nails should also be installed within 2 feet of the bottom or ends of the existing wall along the existing slope. Additional anchor nails on the colluvium slope beyond the ends of the walls should be installed in a diamond pattern with a maximum horizontal and vertical anchor row spacing of 5 feet as measured along the contour of the existing slope for the first two rows of anchors planned adjacent to the existing wall below the roadway. All additional anchor nails on the slope, if required, should be installed in a diamond pattern with

GRI 6737-TO6 – Mill Creek Road Geotechnical Investigation Report Page 14 maximum horizontal and vertical anchor row spacing of 7 feet as measured along the contour of the slope.

Factors of safety against global instability were computed as part of this analysis for static (final grading) and seismic conditions (final grading). The computed factor of safety against instability is defined as the ratio of the forces (or moments) tending to resist failure to the forces (or moments) tending to cause failure. Factors of safety of less than 1.0 represent potentially unstable conditions. Per discussions with WFLHD geotechnical engineers, the walls and slopes will typically be designed for minimum factors of safety of

1.3 and 1.1 for static and seismic conditions, respectively. However, portions of the slope near Wall 1 were identified as a former landslide, and isolated portions of the slope near Wall 6 are as steep as 60°. These areas of the slope will be designed for minimum factors of safety of 1.5 and 1.1 under static and seismic conditions, respectively.

No wall properties were included for the existing timber walls, assuming long-term degradation of the existing walls will occur. The nails were modeled with a 15° angle from horizontal and spacings of 4 feet horizontal and 3 feet vertical along the height and length of the existing wall section and 7 feet horizontally and vertically along the colluvial slope below the wall sections measured along the length of the slope. The top row of nails was assumed to be located near the top of the wall, the second row of nails near the bottom of the wall, and the third row of nails about 2 feet below the bottom of the wall along the length of the slope. Based on our stability modeling, the nail lengths shown in Table 6-4 below meet the minimum factors of safety required for both static and seismic loading conditions.

GRI 6737-TO6 – Mill Creek Road Geotechnical Investigation Report Page 15

Table 6-4: RECOMMENDED LENGTH OF NAILS

Wall Number Recommended Nail Length, ft

Type I (a, c) Anchor Nails Recommended Nail Length, ft

Type II (b, c) Anchor Nails

1 7.5 12

2 10 NA

3 12 NA

4 13 NA

5 9 NA

6 11.5 12

7 11 NA

8 13 NA

9 15 16

Notes:

(a)Type I anchor nails are nails planned through existing wall, nails planned within 2 feet of bottom of wall along slope, and the first two rows below existing roadway adjacent to walls.

(b)Type II anchor nails are all additional nails along the slope not identified as Type I anchor nails.

(c)The recommended nail lengths are either the lengths listed in the table above or 5 feet into intact basalt, whichever occurs first.

The Type I anchor nails were selected to reduce the required load transfer on the shallow upper rows of nails to the extent practical without creating conflicts with the existing waterline. The nail lengths were designed to maintain at least 3 feet of clearance between the tip of the nail and the nearest side of the water pipe based on preliminary water-pipe locations.

The design should include supplemental anchor nails to compress the mesh down into low spots and other surface irregularities, including between the soldier piles of the existing wall. We recommend the supplemental nails have the same length as the other nails. Additional supplemental nails and boundary ropes should be installed at the top, bottom, and sides of the anchored mesh to properly align the mesh.

6.3 Seismic Considerations

6.3.1 General

We understand the walls will be seismically designed in accordance with current American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design Bridge Design Specifications (LRFD BDS). The AASHTO LRFD BDS requires retaining walls to be designed for a 1,000-year return-period (Life Safety design criteria)

GRI 6737-TO6 – Mill Creek Road Geotechnical Investigation Report Page 16 earthquake (7% probability of exceedance in 75 years). The ground-motion parameters for the 1,000-year return-period design earthquake are based on the 2014 USGS National Seismic Hazard Mapping Project to evaluate earthquake-induced peak bedrock acceleration. In addition, the AASHTO LRFD BDS methodology develops the design-level spectra for seismic analysis using three points at spectral periods of 0.0 second, 0.2 second, and 1.0 second (PGA, Ss, and S1) for Site Class B/C boundary conditions. Site coefficients FPGA, Fa, and Fv are then used with a site-class designation to adjust the bedrock spectral accelerations to ground surface spectral accelerations. These parameters for the 1,000-year interval hazard levels were obtained for the site coordinates of 45.9888° N latitude and 118.0906° W longitude and 45.9871° N latitude and 118.0796° W longitude using the ODOT design spectrum spreadsheet tool (1,000 years). The PGA, Ss, and S1 spectral values corresponding to the 1,000-year hazard levels for the site are tabulated below. For design of the nine retaining walls for the project, we recommend conservatively assuming Site Class C soil conditions for all retaining wall structures. The table below provides site parameters for Site Class C soil conditions.

Table 6-5: SPECTRAL VALUES AND SITE PARAMETERS (1,000-YEAR HAZARD LEVEL)

Bedrock Spectral Values

Site

Class

Site Class C Site Parameters

PGA, g Ss, g S1, g FPGA Fa Fv

0.11 0.24 0.07 C 1.29 1.30 1.5

Based on the depth to groundwater and our subsurface explorations, we anticipate the risk of liquefaction and earthquake-induced lateral spreading is very low. Based on our review of the 2014 USGS Quaternary Fault and Fold Database (the most current available dataset) and as previously discussed in the Geology section of this report, the Kooskooskie section of the Hite fault system crosses Mill Creek Road within the project alignment and is mapped approximately 0.8 miles to the west of the western edge of the retaining walls (USGS, 2020). The Kooskooskie section is depicted as a north-south-oriented, left-lateral fault and does not have documented historical seismicity, although it is considered to be potentially Quaternary active with the interpreted age of the most recent prehistoric deformation listed as middle and late Quaternary (Personius et al., 2002). Based on the age and location of known mapped faults in the area, we anticipate the potential for fault rupture or displacement of the walls is low unless occurring on a previously unknown or unmapped fault. The risk of tsunami and seiches at the site is absent.

6.4 Construction Considerations

6.4.1 General

A large waterline and a telecommunications line run parallel to or within Mill Creek Road.

In addition, the existing timber walls appear to be tied back to a deadman anchor system

GRI 6737-TO6 – Mill Creek Road Geotechnical Investigation Report Page 17 installed within the roadway. The waterline, other existing utilities, the existing deadman anchor, and the system connecting the deadman to the existing walls should be located.

Plans should be prepared to avoid impacting the existing utilities and deadman during construction of the new walls, including installation of nails.

6.4.2 Anchored Wired Mesh Construction Considerations

Tree stumps and shrubs should be flush-cut to the ground surface below the wall within the area where anchored wired mesh will be installed. The potential for root entanglement during toppling of trees increases the risk of damage to the anchored-mesh system.

Therefore, we anticipate most maintenance of the anchored-mesh system will primarily prevent re-establishment of trees and brush in the mesh-covered areas. Low-lying, small vegetation can be allowed to re-establish through the mesh.

Very hard (R5) basalt was encountered at the site.

Due to the presence of coarse-grained granular soil within the foundation depth, temporary casing and/or drillable and groutable anchors should be considered to prevent caving of the nail holes during nail installation. The contractor should have temporary casing available during construction if necessary. Cobbles and boulders were identified in the borings and should be anticipated during nail installation. The contractor should mobilize equipment and tooling to the site capable of drilling in this material.

The nails and attachment hardware should be galvanized, or the nail thickness should be increased to include sacrificial steel to accommodate long-term corrosion. The nails should be inspected for damage, and any damaged areas identified prior to installation should be repaired with materials and methods approved by the manufacturer.

One proof test should be completed at each wall on a production nail with a bond zone established in the colluvium. The proof test should be completed at the locations shown on the plans and/or as approved by the designer. Proof nail tests should be conducted to at least 150% of the design nail test load and may require a larger bar size to accommodate the loading. The results of the tests will be used to review and revise, if necessary, the nail design criteria. All production nails should be tensioned to the design nail load and tested by holding the load for 10 minutes with no loss of load. The production-tested nails can be tested with a calibrated torque wrench.

To reduce the risk of loss of material through the mesh openings, we recommend the installation of rolled erosion control product, Type 5.C, on the existing slope face prior to placement of the wired mesh.

GRI 6737-TO6 – Mill Creek Road Geotechnical Investigation Report Page 18

A recessed pocket must be created at each nail-head location in relation to adjacent ground surface elevations for nails installed in the slope below or adjacent to the walls to allow pre-tensioning of the wired mesh. The nails should be locked off at the anchored mesh manufacturer’s recommended tensioning load or anchor design load, whichever is lower. The wired mesh in the areas of existing wall should be tensioned by pulling tight against adjacent soldier piles.

The wired mesh is designed to allow some elongation of the mesh as a load is applied to the mesh. It should be understood that some movement of the mesh will occur over time as the existing timber wall deteriorates and additional load is transferred to the wired mesh. The nails may need to be retensioned as the timber deteriorates. In addition, anchor nails require some soil deformation to mobilize their design resistance. The County should anticipate roadway repair of the chip seal, including repair of tension cracks, due to long-term movement of the wired mesh as the soldier pile wall deteriorates and additional load is transferred to the wired mesh.

The first row of nails will be installed approximately 1 foot below the grade behind the wall at the approximate center of a lagging board for the existing wall, which is as high as the anchor and mesh can be installed due to proximity of the road to the back of the wall.

Prior to installation of the nails, the top row of lagging for the existing wall could be replaced with new pressure-treated lumber to delay deterioration of the top row of lagging that creates the retained roadway edge, thereby lengthening the time before maintenance of the roadway shoulder will be required. As the top lagging board deteriorates, the uppermost backfill behind the existing wall will start to slough because it is not retained by the anchored mesh. Maintenance may include replacing the deteriorated lagging board with some other system to retain the soil above the anchored-mesh system.

The nails should extend a minimum of 5 feet beyond the ends of the existing walls or areas of existing instability beyond the ends of the walls.

We anticipate nails installed through the wall and along the upper portion of the slope can be installed using a drill connected to a large excavator positioned along the outside edge of Mill Creek Road. Nails farther down the slope that cannot be reached from the surface of Mill Creek Road using an excavator will likely require the use of a wagon wheel drill rig or spyder rig to complete nail installation along the slope. Temporary anchors installed along the top of the slope or within Mill Creek Road may be required to support the wagon drill rig.

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