ora-polk-mitchell1_GR11-21 Final Geotech Report OR Polk 2016(1) Mitchell St to Black Rock.pdf

Mitchell Street and Black Rock Road Reconstruction June 2021

Table of Contents 1-i

Geotechnical Report No. GR 11-21

Mitchell Street and Black Rock Road Reconstruction

FINAL GEOTECHNICAL REPORT

OR POLK 2016(1)

Polk County, Oregon

June 2021

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Table of Contents i

TABLE OF CONTENTS

1 INTRODUCTION

1.1 Project Description

1.2 Purpose and Scope

2 GEOLOGY

2.1 Geologic Hazards

2.1.1 Design Ground Motions

2.1.2 Liquefaction

2.1.3 Surface Rupture

2.1.4 Lateral Spreading

3 SITE CONDITIONS

3.1 Average Temperatures and Precipitation

3.2 Scour

4 SUBSURFACE CONDITIONS

4.1 Borings

4.2 Test Pits

4.3 Subgrade Borings

4.4 Groundwater

4.5 Topsoil Thicknesses

5 ANALYSIS AND DESIGN RECOMMENDATIONS

5.1 Rock Cut Slope Stability Analysis

5.2 Cut Slope Recommendations

5.3 Fill-Side Recommendations

5.3.1 Soil Embankment Slopes

5.3.2 In-Stream Rockeries

5.3.3 Modular Block Retaining Walls

6 CONSTRUCTION CONSIDERATIONS

6.1 Excavations

6.2 Dewatering

6.3 Aggregate Sources

7 LIMITATIONS

8 REFERENCES

List of Figures ii

LIST OF FIGURES

Figure 1-1 Project Location Map

Figure 1-2 Vicinity Topographic Map

Figure 2-1 Excerpt from the Geologic Map of Oregon (Walker and MacLeod, 1991)

Figure 2-2 5-Percent-Damped-Design Response Spectrum

Figure 2-3 Excerpt from USGS Quaternary Faults website

LIST OF TABLES

Table 2-1 Summary of Seismic Parameters

Table 3-1 Climate Data Summary

Table 4-1 Groundwater Summary

Table 4-2 Summary of Topsoil Thicknesses at Test Locations

Table 5-1 Recommended Cut Slope Angles

Table 5-2 Fill Slopes Design Recommendations

Table 5-3 In-Stream Rockery Design Parameters

Table 5-4 Base Width for Various Rockery Heights

Appendices iii

APPENDICES

APPENDIX A

Project Photographs

APPENDIX B

Figures B-1 to B-6 Boring Locations Map

APPENDIX C

Descriptive Terminology for Boring Logs Boring Log Legend Boring Logs BH19-01 to BH19-02 Rock Core Photos

APPENDIX D

Laboratory Test Results

APPENDIX E

Contoured Pole Plot: Discontinuity Orientations with Average Set Orientations Contoured Pole Plot: Flexural Toppling Failure Mode Contoured Pole Plot: Direct Toppling Failure Mode

APPENDIX F

In-Stream Rockery Calculations Modular Block Gravity Retaining Wall Calculations

4 Introduction

1 INTRODUCTION

This report provides geotechnical investigation results, analyses, and final geotechnical recommendations for the Mitchell Street and Black Rock Road Reconstruction Project. The project is located approximately 1 mile west-northwest of Falls City, Idaho along Black Rock Road in Polk County, Oregon.

1.1 PROJECT DESCRIPTION

The project begins on Black Rock Road in Polk County, Oregon jurisdiction west of Falls City, Oregon as shown in Figure 1-1. At the city limits near Dutch Creek crossing, the road becomes Mitchell Street. The project length is 0.95 miles from the west end to the east end near the intersection with Park Street. The existing single-lane gravel roadway is proposed to be widened to a paved, two-lane roadway 22 to 26 feet in width within County limits and up to 24 feet within City limits.

The existing County Bridge, which spans Little Luckiamute River, will be replaced with a two-lane bridge as part of the project. Other improvements will include an in-stream rockery and a precast concrete block retaining wall.

1.2 PURPOSE AND SCOPE

The purpose of this report is to provide geotechnical recommendations for roadway design and construction. The geotechnical scope of work included a visual reconnaissance along the proposed alignment, a subsurface investigation program consisting of six geotechnical test borings, four roadway subgrade borings, three test pits, laboratory soil and rock testing, preparation of interim memorandums, and preparation of this report. The geotechnical borings and laboratory tests were completed by Western Federal Lands Highway Division (WFLHD) and a drilling subcontractor under the direction of WFLHD. Geotechnical recommendations for the bridge replacement over Little Luckiamute River are discussed in a separate foundation report.

The following documents were previously prepared by WFLHD regarding the bridge replacement and are available by request:

• Preliminary Geotechnical Investigation Results Memorandum, GM 35-19, dated November 15, 2019

• In-Stream Rockeries and Revised Cut Slopes Memo, GM 13-20, dated July 7, 2020

• Mitchell Street and Black Rock Road Reconstruction, Little Luckiamute Bridge

Replacement, Final Foundation Report, GR 7-20, dated June 2021

This report supersedes these previous memorandums for this project and supplements the Final

Foundation Report.

Introduction 5

Figure 1-1 Project Location Map

Figure 1-2 Vicinity Topographic Map

Little

Luckiamute

Bridge

6 Geology

2 GEOLOGY

As shown in Figure 1-2, the project site is located in a relatively narrow valley within the Central Oregon Coast Range. Little Luckiamute River flows southeast through the project along the north side of the roadway until the bridge replacement area and then along the south side of the roadway. Based on the Geologic Map of Oregon (Walker and MacLeod, 1991) displayed in Figure 2-1, the eastern half of the project area is mapped as Ty, Yamhill Formation (upper and middle Eocene) and Qal, alluvium (Holocene). The Yamhill Formation is described as massive to thin-bedded marine siltstone and thin interbeds of arkosic, glauconitic, and basaltic sandstone. The alluvium is described as sand gravel, and silt forming flood plains; includes soils containing organic material and thin peat beds. unconsolidated sediments (Walker and MacLeod, 1991).

The western half of the site is mapped as Ti, mafic intrusions (Oligocene). The mafic intrusions are described as sheets, sills, and dikes of massive ferrogabbro; some bodies may be strongly differentiated (Walker and MacLeod, 1991).

As discussed in the Subsurface Conditions section below, silty sand, sandy silt, well-graded sand with silt and gravel interpreted to be alluvium, and rock interpreted to be gabbro, were encountered in the borings and are consistent with the geologic mapping.

Figure 2-1 Excerpt from the Geologic Map of Oregon (Walker and MacLeod, 1991)

Little

Luckiamute

Geology 7

2.1 GEOLOGIC HAZARDS

Geologic hazards can include surface rupture, liquefaction and related phenomenon, soft or subsiding ground, strong ground shaking, and landsliding, among other natural hazards. Our literature search did not discover a published geologic hazard map for the vicinity. Based on the geologic mapping and the subsurface conditions encountered in our borings (discussed later in this report), it is our opinion that the relevant geologic hazards in the site vicinity could include:

surface rupture, liquefaction/lateral spreading, strong ground shaking, and landsliding. These hazards are discussed further in the Site Seismicity section below.

2.1.1 DESIGN GROUND MOTIONS

The procedures outlined in Section 3.10 of the 8th Edition of the AASHTO LRFD Bridge Design Specifications (BDS) were followed to characterize the seismic hazard. The seismic hazard at the site and the importance of the proposed bridge inform the level of seismic analysis that is appropriate for the design. The seismic hazard principles within the AASHTO BDS are that bridges shall be designed to have a low probability of collapse but may suffer significant damage and disruption to service when subjected to earthquake ground motions that have a seven percent probability of exceedance in 75 years. Partial or complete replacement may be required

(AASHTO, 2017).

Based on the AASHTO procedures and the N60 standard penetration resistance of the soil profile at the project site, we recommend a Site Class of “B” for the site. The U.S. Seismic Design Maps website (USGS, 2020) was used to determine the base peak ground acceleration (PGA) and spectral accelerations at the project site for basement rock. The spectral accelerations were then scaled to the Site Class using the site factors recommended by AASHTO to represent the site modified seismic accelerations at the ground surface. The result of this analysis is presented in Table 2-1.

8 Geology

Table 2-1 Summary of Seismic Parameters

Site Latitude 44.8698° Site Longitude -123.4509°

Site Class at Project Site (by Soil Type and Penetration Resistance in Top 100 feet of Soil

Profile) B

Horizontal Peak Ground Acceleration on Site Class B Rock (PGA) 0.281 g Horizontal Response Spectral Acceleration at a

Period of 0.2 sec on Site Class B Rock (Ss)

0.667 g

Horizontal Response Spectral Acceleration at a Period of 1.0 sec on Site Class B Rock (S1)

0.303 g

Mapped Peak Ground Acceleration Coefficient, FPGA 1.00 Mapped Short-Period Spectral Acceleration Coefficient, Fa 1.00 Mapped 1-sec Period Spectral Acceleration Coefficient, Fv 1.00

Scaled Peak Ground Acceleration for Project Site, Site Class B (As)

0.281 g

Scaled Horizontal Response Spectral Acceleration at a Period of 0.2 sec for Project Site, Site Class B (SDS)

0.667 g

Scaled Horizontal Response Spectral Acceleration at a Period of 1.0 sec for Project Site, Site Class B (SD1)

0.303 g

AASHTO Seismic Zone 3

The values in Table 2-1 were used to develop the 5-percent-damped-design response spectrum shown in Figure 2-2 below for the project site.

Figure 2-2 5-Percent-Damped-Design Response Spectrum

Geology 9

2.1.2 LIQUEFACTION

Sol liquefaction typically refers to the condition by which vibration or shaking of the ground, usually from earthquake forces, results in the collapse of the soil structure, the development of excess pore pressures in saturated soils, and a subsequent reduction in soil shear strength. Evaluation of liquefaction potential is dependent on numerous parameters, including soil type, grain-size distribution, soil density, depth to groundwater, in-situ isotropic and/or anisotropic static ground stresses, earthquake magnitude, PGA, and associated earthquake-induced ground stresses. In general, soils that are susceptible to liquefaction include very loose to medium dense, clean to silty sands and very soft to stiff non-plastic silts that are below the water table.

Following the AASHTO procedures, this site was determined to be in Seismic Zone 3. A site in Seismic Zone 3 requires a liquefaction analysis if saturated, loose or very loose cohesionless soils are present, and the Scaled Peak Ground Acceleration for Project Site (As) is 0.15 g or greater. Because the majority of saturated subsurface conditions were in bedrock, the risk of liquefaction is low and a liquefaction analysis was not necessary at the site.

2.1.3 SURFACE RUPTURE

Information from the USGS Quaternary fault and fold database for the United States website, displayed in Figure 2-3 was used to locate nearby active faults. The closest mapped quaternary fault is the Salem-Eola Hills homocline, which is located approximately 15 miles to the east. Due to the relatively large distance to the fault, the risk of surface rupture from active faults is considered low.

Figure 2-3 Excerpt from USGS Quaternary Faults website

Little Luckiamute

Salem-Eola

Hills Homocline

10 Geology

2.1.4 LATERAL SPREADING

Lateral spreading related to seismic activity typically involves lateral displacement of large, surficial blocks of non-liquefied soil when a layer of underlying soil loses strength during seismic shaking. Lateral spreading usually develops in areas where sloping ground or large grade changes are present. Based on the proposed improvements, site topography, and liquefaction potential explained above, the risk of lateral spreading impacting the site is low.

Site Conditions 11

3 SITE CONDITIONS

The Little Luckiamute River flows within a valley at the eastern edge of the Central Oregon Coastal Mountain Range. The project starts in west Polk County and the ends to the east in Falls City.

The river generally flows eastward towards a waterfall at Falls City.

The project begins on the river right bank with a steep slope system on the right that extends beyond the bridge location where the Little Luckiamute River crosses under the roadway. From approximate Stations 22+00 to 25+00, the river bends sharply to the left and the river right bank is lined with riprap. The right cut is steep and forested with numerous fallen trees. Some slope areas from Station 23+00 to 25+00 have exposed soils and are sparsely vegetated with leaning trees, suggesting it is currently over steepened at slopes up to 65 degrees from horizontal; some small zones were vertical. The materials on the slopes appear to be highly weathered to completely weathered Gabbro (R0-R1) on the surface grading to competent Gabbro rock (R3- R4). Some areas are mantled with colluvium, which consists primarily of silty sand with gravel supporting cobbles and boulders up to approximately 4 feet in dimension. Natural gabbro bedrock outcrops can be observed near the slope brow at the top approximately 150 feet above the road.

The majority of the slope appears to be a natural slope, possibly failed from river scour at the bend, except for an obvious vertical roadway cut from approximately Stations 22+80 to 23+50.

No seeps or groundwater were observed during the site visits.

The roadway crosses the Little Luckiamute River near Station 40+00 at an existing approximately 85-foot long single-span, single-lane bridge. The area along the left river bank is relatively flat adjacent to the roadway, with a large steep slope further to the left behind heavily vegetated areas. Additional information on the bridge replacement site, including photos of the existing Little Luckiamute Bridge approaches and stream bank, can be found in the Final Foundation Report.

A second stream crossing includes a culvert over Dutch Creek that was recently replaced with a bridge by the County. After the roadway crosses Dutch Creek, the road rises moderately with a moderately steep slope system on the left and a residential property on top of the roughly 80-foot tall hill. The upper portion of this hill ranges from an angle of approximately 1V:2H to 1V:1.5H and is mantled by loose silty sand. The lower portion of this hill appears cut at approximately 1V:1H to 1V:0.7H into highly weathered to completely weathered gabbro (R0-R4) that is highly fractured with vertical and horizontal joint sets.

The left side slope system reduces in height as the road increases in elevation toward the east with limited height slopes that appear to consist of residual soils to completely weathered bedrock, although more competent bedrock is likely below the surface soils.

12 Site Conditions

3.1 AVERAGE TEMPERATURES AND PRECIPITATION

Table 3-1 shows the average maximum and minimum temperatures, average monthly precipitation, and average monthly snowfall data for the period from 1986 to 2016 for the Fall City, Oregon (352800) weather station (Western Region Climate Center, 2020), located approximately 1 mile from the project site.

Table 3-1 Climate Data Summary

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

Average Max.

Temperature (F)

44.6 49.2 53.9 60.4 66.6 71.7 80.4 80.4 74.3 63.3 52.2 46.2 61.9

Average Min.

Temperature

(F)

31.5 33.3 35.3 37.9 41.8 45.9 48.4 48.2 46.4 42.0 36.2 33.4 40.0

Average Total

Precipitation (in.)

12.5 10.2 9.16 4.36 2.65 1.50 0.30 0.56 1.90 5.22 11.8 13.6 73.8

Average Total

SnowFall (in.)

7.8 2.8 2.3 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.5 2.4 16.0

Average Snow Depth

(in.)

2 1 0 0 0 0 0 0 0 0 0 0 0

Note:

1) Period of Record : 08/01/1896 to 04/30/2016.

2) Percent of possible observations for period of record: Max. Temp.: 99.7% Min. Temp.: 99.9%

Precipitation: 100% Snowfall: 99.9% Snow Depth: 99.9%.

3.2 SCOUR

WFLHD Hydraulics Engineer, Sven Leon, prepared a memorandum titled, “Little Luckiamute River Bank Stabilization Hydraulics Memo”, dated August 31, 2020. Because the widening for the project will shift the roadway to the north at the sharp bend in the river near Stations 23+00 to 24+00, the memorandum recommends stabilization of the river with an in-stream rockery bearing directly on the bedrock near the channel bottom. A more vertically battered rockery was proposed to replace the existing low angle riprap to reduce the horizontal impact of the road widening into the river channel. The rockeries were recommended from Station 22+60 to 25+60 and Station 27+90 to 31+10. No scour was predicted if the rockeries were founded on bedrock, but where bedrock is not present the rockery bottom should be 3 feet below the channel thalweg.

Subsurface Conditions 13

4 SUBSURFACE CONDITIONS

A subsurface investigation program was completed at the site from September 5 to 8, 2019 and consisted of one boring at each abutment, two borings near the proposed in-stream rockeries, two borings near the proposed concrete block retaining wall, three test pits in anticipated rock cut areas, and four subgrade borings throughout the alignment. The explorations were performed at the approximate locations shown on attached Boring Locations Maps, Figures B-1 to B-6. The borings were drilled from the existing roadway with a CME 75 truck-mounted drill rig. Test pits were performed using a Caterpillar 325F (25-metric-ton) tracked excavator. The boring logs and laboratory testing results are included in the appendix of this report.

4.1 BORINGS

Near the bridge replacement location, boring BH19-01 encountered loose to medium dense, moist, silty sand and sandy silt with gravel in the upper 13 feet, which we interpreted to be alluvium. Very dense, moist, silty sand and extremely weak, residual rock was encountered from approximately 13 to 24.5 feet, which we interpreted to be weathered gabbro. At approximately

24.5 feet to the bottom of the boring at 40 feet, rock coring encountered medium strong to strong rock, which we interpret to be intact gabbro. The rock cores had closely spaced discontinuities and joint/fracture ranges from 25 to 85 degrees from horizontal.

Subsurface conditions in boring BH19-02 consisted of medium dense, moist to dry, silty sand well-graded sand with silt and gravel in the upper 12 feet, which we interpreted to be alluvium and residual soil to completely weathered gabbro rock. At approximately 12 feet to the bottom of the boring at 40 feet, rock coring encountered medium strong to strong rock, which we interpret to be intact gabbro. The rock cores had closely spaced discontinuities in good condition and joint/fracture ranges from 0 to 85 degrees from horizontal.

Near the sharp bend in Little Luckiamute River, borings BH19-03 and BH19-04 encountered medium dense silty gravel with sand and silty sand with gravel in the upper approximately 7.5 to 8 feet. A very soft silt layer was encountered from approximately 7.5 to 8.8 feet in boring BH19- 03, and a layer of soft lean clay was encountered from approximately 8 to 9 feet in boring BH19-

04. Medium dense silty sand with gravel, cobbles, boulders was encountered from approximately

8.8 to 17.5 feet in boring BH19-03 and gabbro cobbles and boulders from approximately 9 to 18.5 feet and were interpreted to be residual soil to completely weathered gabbro bedrock with fresh to slightly weathered cobbles and boulders. Intact, very weak to medium strong gabbro was encountered from approximately 17.5 feet to the bottom of the boring at 30 feet and 31 feet for borings BH19-03 and BH19-04, respectively. The rock cores had closely spaced discontinuities in fair condition and joint/fracture ranges from 0 to 85 degrees from horizontal.

Borings BH19-05 and BH19-06 were advanced near the steep right hillside east of Dutch Creek, where a fill-side wall was anticipated to reduce large cuts on the left slope. The borings encountered a loose to medium dense silty sand below the surface gravel down to approximately 19 feet in BH19-05 and 7.5 feet in BH19-06. A zone of very dense silty sand interpreted to be extremely weak to very weak, completely weathered gabbro was encountered in BH19-06 from

14 Subsurface Conditions approximately 7.5 to 12.5 feet, which was still able to be augered. From 19 feet in BH19-05 and

12.5 feet in BH19-06, the borings switched to rock coring and encountered weak to very strong, slightly to moderately weathered gabbro. The rock cores had closely spaced discontinuities in fair to good condition and joint/fracture ranges from 5 to 85 degrees from horizontal. A layer of silt infill between the rock was encountered in boring BH19-06 at approximately 36.25 to 36.75 feet.

4.2 TEST PITS

The test pits were located in the anticipate cut area on the left side of the roadway east of Dutch Creek where portions of the existing cuts exposed bedrock. The test pits encountered loose silty sand surficial soils (topsoil) in the upper approximate 1 to 1.5 feet. Underlying the topsoil, test pits TP19-01 and TP19-02 encountered loose silty sand with cobbles and boulders interpreted to be completely weathered gabbro down to 7 feet and 4 feet, respectively, from the top of the excavation in the slope. Test pit TP19-03 encountered a hard laminated silt from 1 to 9.5 feet, which could be a siltstone layer above the gabbro. Slightly to moderately weathered, strong to very strong, gabbro with numerous joints and fractures was encountered in the lower portions of the test pits to near the base of the existing slope.

4.3 SUBGRADE BORINGS

The subgrade borings encountered dense, silty gravel with sand surface materials down to 1 foot.

Boring SG19-02 encountered a thin one-half inch layer of asphaltic surfacing. Below the surface materials, the borings encountered dense silty sand with gravel to silty clayey gravel to sandy elastic silt with gravel to the bottom of the borings at approximately 5 feet.

4.4 GROUNDWATER

Groundwater was encountered in several of the explorations. Table 4-1 below summarizes the groundwater depths encountered at the time of drilling or excavating. Piezometers were not installed in the borings. It should be noted that groundwater and soil moisture conditions should be expected to fluctuate with adjacent river levels, precipitation, changes in season, and other factors.

Table 4-1 Groundwater Summary

Exploration GW Depth (ft) GW Elevation (ft)2

BH19-01 17.9 422±

BH19-02 NE1 -

BH19-03 12.7 447½±

BH19-04 NE1 -

BH19-05 NE1 -

Subsurface Conditions 15

Exploration GW Depth (ft) GW Elevation (ft)2

BH19-06 NE1 -

TP19-01 NE -

TP19-02 NE -

TP19-03 NE -

SG19-01 NE -

SG19-02 NE -

SG19-03 NE -

SG19-04 NE -

1 Not encountered prior to adding fluid for rock coring.

2 Estimated elevations from the topographic plan. Not surveyed elevations.

4.5 TOPSOIL THICKNESSES

While on-site, we used a shovel to hand excavate in areas to determine topsoil thicknesses. The approximate topsoil depths encountered are listed in Table 1 below. A map showing locations of these sites is presented in Figures B-1 to B-6.

Table 4-2 Summary of Topsoil Thicknesses at Test Locations.

Exploration No. Station Topsoil Thickness (in)

TS-101 33+15 3

TS-104 35+00 3

TS-106 38+80 2

TS-108 41+80 3

TS-109 45+80 4

TP19-02 50+60 6

TS-110 61+80 4

16 Analysis and Design Recommendations

5 ANALYSIS AND DESIGN RECOMMENDATIONS

The above information was analyzed to determine design recommendations for fill-side retaining structures and cut slope angles. The following sections provide our recommendations for roadway design.

5.1 ROCK CUT SLOPE STABILITY ANALYSIS

A rock cut slope stability assessment and kinematic analyses were performed for the proposed rock cut slope from approximately Station 23+50 to 24+60. Our assessment consisted of mapping surficial rock discontinuities and plotting them with a stereonet to determine orientation of discontinuities in the rock mass in relation to the proposed cut slope. Markland’s method of kinematic analyses was used to determine kinematic stability of the rock slope for planar, wedge, and toppling failure modes. It should be noted that only surficial discontinuities were considered in these analyses, which may differ from bedrock at depth due to differing degrees of weathering and possible impacts from the original cut slope construction, such as blasting, at the surface.

Kinematic analyses of data from on-site surficial discontinuity mapping of existing road cut (from approximately Station 23+50 to 24+60) were performed using the Rocscience software program, Dips (v 7.0). This analysis helped determine statistically relevant rock discontinuity sets and mean orientations to estimate the kinematic potential for rock blocks to fail out of proposed cut slopes.

Based on the stereonet analysis of available data, we identified five discontinuity sets that range in dip angle from 4 to 89 degrees. Two joint sets were sub-horizontal and three joint sets were near vertical. The dominant rock structure appears to be columnar jointing. Plots of discontinuity data are presented in Figure E-1.

The rock discontinuity friction angle was assumed to be 18 to 30 degrees based on geomechanical information collected in the field, experience with similar rock types, and guidance from Rock Slope Engineering: Fourth Edition (Wyllie and Mah, 2004).

Based on data collected, kinematic analyses of surficial observations indicate a low potential for an oblique toppling failure mode and a moderate potential for a flexural toppling failure mode from a proposed cut slope. Figures E-2 and E-3 display stereonet plots of the discontinuity kinematic analyses of Direct and Flexural toppling failure mode.

5.2 CUT SLOPE RECOMMENDATIONS

Based on the bedrock type encountered, weathering of the bedrock/soil, Markland’s method of kinematic analysis, and our engineering judgement on the stability of the existing soil slopes, Table 5-1 provides our cut slope recommendations at selected locations.

Analysis and Design Recommendations 17

Table 5-1 Recommended Cut Slope Angles

95% Design Stations

Recommended Cut Slope / Side

Anticipated Soil/Rock Conditions Comments

15+00 – 26+00 1V:1.5H, LT Alluvium, residual Soils, existing embankment

15+00 – 18+00 1V:1.5H, RT Residual Soils

15+00 – 26+00 2V:1H, RT

Limited to ~6’ in cut height

Highly weathered to completely weathered

Gabbro (R0-R1) on surface to competent Gabbro rock (R3-R4).

Some areas of colluvium.

Large high cuts not recommended. Material excavated from this area is likely to be poor quality rock and should not be planned for use where durable stone is needed.

26+00 – 45+00 1V:1H, RT Residual soils

44+00 – 48+00 1V:1.5H, LT Residual soils

48+00 – 55+00 2V:1H, LT for 0 – 10’ high in rock;

1V:1.5H, LT over 10’ high in soil

Rock: Silty sandstone overlying (highly weathered to completely weathered) gabbro (R0-R4), highly fractured with vertical and horizontal joint sets.

Compound cut with rock at base and flatten out at soil contact ~10’ above roadway.

55+00 – 55+25 1V:1H, LT Residual soils Transition from 2:1 to 1:1.5

55+50 – 66+00 1V:1.5H, LT Residual soils

61+75 – 66+00 1V:1.25H, LT Residual soils Tight spot due to houses on RT. Match existing steep cut slope at 1V:1.25H.

18 Analysis and Design Recommendations

5.3 FILL-SIDE RECOMMENDATIONS

A summary of the proposed fill-side slopes and structure locations are shown in the table below.

Table 5-2 Fill Slopes Design Recommendations

95% Design Stations

Recommended Structure

Max.

Face Angle Comments

15+00 – 22+00 Embankment 1V:1.5H

23+20 – 26+00 Rockery, in-stream, riprap backfill

4V:1H Limits recommended by Hydraulics

26+00 – 28+50 Embankment 1V:2H Slope recommended by Hydraulics

28+50 – 31+30 Rockery, in-stream, riprap backfill

4V:1H Limits recommended by Hydraulics

31+30 – 66+00 Embankment or Concrete

Block Retaining Wall

1V:1.5H embank.

8V:1H walls

For embankment heights >10 ft, add 8-ft long reinforcement geotextiles at 3-ft vertical spacing, no facing wrap

5.3.1 SOIL EMBANKMENT SLOPES

Soil embankment slopes consisting of roadway excavation material should be generally limited to maximum slopes of 1V:1.5H (Vertical:Horizontal) for limited heights of less than approximately 10 feet. Slopes over 10 feet in vertical height should be designed for maximum slopes of 1V:2H, or be reinforced with geosynthetics. Slopes should be designed for both temporary and permanent erosion control, such as the use of a hydromulch with tackifier and/or the use of rolled erosion control products (RECP). Roadway excavation materials are anticipated to consist of silty sand and silt with gravel generated from the cut side of the roadway. Excavated rock materials will likely be highly weathered and should not be planned to be reused for riprap or as durable stone material, except for the slightly weathered strong gabbro observed in the lower portion of the cuts near Stations 48+00 to 55+00.

Analysis and Design Recommendations 19

5.3.2 IN-STREAM ROCKERIES

To accommodate widening adjacent to the existing stream, a retaining wall is recommended to reduce the horizontal extent of the roadway fill and limit impacts on the riparian system. Use of a natural facing element, such as large stones, is also recommended to resist potential hydraulic shear forces and provide a more natural aesthetic for habitat.

Initially, we had recommended reinforced fill rockeries. The primary purposes of the reinforced backfill was to allow the use of smaller rock and provide a redundant failure system; should the rock facing fail, the reinforced wrapping behind the facing should stand intact and prevent damage to the roadway. However, the length of geosynthetic reinforcement and expected 1V:1.5H temporary cut slope behind the reinforced zone extended too far towards the sheer cliff near the highest point of the rockery. Therefore, one lane of temporary roadway for traffic could not be maintained.

We have revised the design to use Class 1 riprap as backfill to reduce the horizontal impacts of the reinforced zone and the temporary cut slope in order to maintain at least one lane of traffic during construction. It should be noted that the risk of using unreinforced backfill is that if the rockery facing should fail, a portion of the outside lane could also be damaged. Since this is likely to only happen during high stream flow and extreme flood events, the return period for this type of event is relatively low, and the inside lane should still be passable. The County should inspect the rockery after major high flow or flood events and be prepared to close the westbound lane in case of storm-related damage. One advantage of the use of riprap backfill is the ease of backfilling operations in the event of a repair.

Design parameters for the in-stream rockery are provided in Table 5-3. The backfill directly behind the rockery should consist of Class 1 riprap. A class 1 type A non-woven filter fabric should be placed over the temporary cut slope prior to placing the riprap backfill, and then back over the riprap to provide separation from finer materials placed above it.

Table 5-3 In-Stream Rockery Design Parameters

Parameter Value Units Comments

Rockery Batter 4:1 V:H Front of face batter

Rockery Facing Width Variable FT See Table 5-4

Embedment Depth 3 FT Backfill toe with riprap per Hydraulics

Rockery Backslope 1:2 V:H Assumed length ~4 feet

Rockery Foreslope 1:2 V:H Assumed 1:2 max slope in channel

Excavation. Add’l Width 0 FT Cut to back of rockery

Wall Excavation Slope 1:1.5 V:H Assumed stable OSHA temp. exc. slope

20 Analysis and Design Recommendations

Leveling Pad Thickness 0 FT None, bear on competent bedrock

Foundation Add’l Width 0 FT None, bear on competent bedrock

A new pay item will be used for in-stream rockeries, 25210-1000 Rockery, in-stream.

Measurement will be by the square foot of face (parallel plane to rockery face). Filter fabric and backfill are typically incidental to the rockery pay item. No underdrain system is required.

The base width of the rockery is a function of the rockery height as shown in Table 5-4 below.

The minimum rockery base width should be 4 feet. Calculations for an in-stream rockeries are included in Appendix F.

Table 5-4 Base Width for Various Rockery Heights

Rockery Total Height, H (ft) Base Width, B (ft)

15 6

12 5

9 or less 4

5.3.3 MODULAR BLOCK RETAINING WALLS

To accommodate widening in the area near Stations 48+00 to 55+00, a retaining wall is recommended on the roadway right fill side. Based on input from Polk County representatives, a modular wall constructed from precast concrete blocks is preferred over gabion basket or other metal faced walls due to past experience with corrosion.

Most precast concrete block products intended for modular retaining walls are manufactured from proprietary molds and use return concrete from ready-mix concrete trucks. Although the concrete is a reduced strength, the primary resisting factor is the large mass of the blocks designed as a gravity retaining structure. Special, proprietary shear key designs and block shapes that allow turning or wall radius typically distinguish one manufacturer from another. A slight batter from vertical can add more resistance as the blocks are leaning back toward the retained backfill and the base is angled upward adding additional resistance to base sliding.

We have developed our design assuming a block size with a 2.5-foot base width by a 2.5-foot height by a 5-foot face length, which should be locally or regionally available. Other products may be able to be substituted, based on the final contract requirements. Typically, a 5-foot tall concrete block wall will be adequate with a 2.5-foot base width without the use of geogrid reinforcements or rotation of the base block for extra width. Wall heights in this section are anticipated to be relatively low and not exceed approximately 5 feet in total height. Calculations for a modular gravity block wall using precast concrete units are included in Appendix F.

Construction Considerations 21

6 CONSTRUCTION CONSIDERATIONS

6.1 EXCAVATIONS

Borings BH19-03 and BH19-04 were located on the north edge of the roadway near the in-stream rockery area planned between Stations 23+00 and 25+50. The stream in this area bends sharply on the north side of the roadway embankment. Some riprap or large boulders are currently in place along the stream bank, and these boulders were likely encountered in boring BH19-04.

Medium dense silty sand with gravel was encountered in the upper 7.5 to 8 feet in these borings.

Below this surface material, the borings encountered a silt and lean clay layer(s) to approximately 9 feet bgs. Below the silt and lean clay layers, the borings encountered completely to slightly weathered gabbro consisting of silty sand to cobbles/boulders in particle size. The bearing strata for the in-stream rockeries should be planned for approximate elevations 450 feet and 455 feet near borings BH19-03 and BH19-04, respectively. Additional foundation preparation may be necessary based on recommendations from the Hydraulics team and actual rock conditions encountered. A WFL geotechnical engineer/geologist and a hydraulics team representative should be onsite during initial foundation excavation to assist the contractor and construction team in determining adequate bearing conditions and foundation preparation needs.

We anticipate that the rockery and wall foundation excavations can be performed using temporary slope cuts, which should be the responsibility of the contractor based on site conditions, including soil and rock conditions and surface and ground water conditions. If necessary, shoring should be the responsibility of the contractor to design and monitor.

Slope cuts will likely encounter competent rock below the surficial soils and weathered rock zones.

The contractor should be responsible for reviewing the subsurface information in this report, viewing the site conditions, and determining the means and methods of excavation within cobbles and boulders and competent bedrock at the site. Excavation methods will likely require large excavators with rock ripping teeth, hydraulic breaker bars, and other rock breaking/reducing methods. Blasting may or may not be allowed per the environmental permits.

6.2 DEWATERING

Groundwater is expected at or near the elevation of the stream. Bottom of foundation elevations are anticipated to be at elevation 450 feet and 455 feet rockeries near borings BH19-03 and BH19-04, respectively. Ordinary high water is estimated at elevation 457.5 feet. Therefore, some seepage and/or wet conditions may be encountered at the time of construction. The contractor should be prepared to divert the river and temporarily dewater the excavations, if necessary.

The placement of riprap and other in-stream work may require channel diversions, cofferdams, or other flow barriers. These should be the responsibility of the contractor to design and monitor during construction. We understand that there may be an in-water work window due to certain fish species in the creek. This in-water work window would be listed in the contract documents under environmental requirements.

22 Construction Considerations

6.3 AGGREGATE SOURCES

No government provided material sources were identified for this project. As discussed above, some areas of competent rock may be encountered and used for durable rock materials, provided they pass the material specifications. Material sorting and screening by the contractor will likely be necessary. It should be noted that certain basalt and other rock sources in the region are known from past WFLHD experience to be clay-rich and prone to premature degradation once crushed and wetted. All aggregate sources (including pavement materials, base aggregate, rip rap, and special rock embankment) for the project should be tested for durability using the standard WFLHD Method of Test for Accelerated Weathering of Aggregate by Use of Dimethyl

Sulfoxide (DMSO), available upon request.

Limitations 23

7 LIMITATIONS

This report was prepared to provide subsurface investigation results and geotechnical recommendations for the proposed Mitchell Street to Black Rock Road Reconstruction project.

Professional judgments and recommendations are presented in this report based partly on evaluation of the technical information gathered and partly on the Geotechnical Team’s general experience with similar projects. It should be noted the borings might not represent potentially unfavorable subsurface conditions away from the borings. The observed water levels and/or conditions indicated on the logs are as recorded at the time of the exploration. The water levels and/or conditions may vary considerably, with season/time, changes in climate, rainfall, and other factors and are otherwise dependent on the duration of, and methods used in, and interpretations made from the exploration program.

If conditions are encountered during construction that vary from those discussed in this report, or if design loads, project stationing, and/or structure locations change, the WFLHD Geotechnical Team should be notified immediately in order to evaluate what effects, if any, the changes have on the recommendations contained in this report. The recommendations presented in this report are applicable only to this specific site. These data are not to be used for other purposes.

Questions regarding this report should be addressed to Eric Lim, WFLHD Senior Geotechnical Engineer, at (360) 619-7826, or eric.lim@dot.gov.

mailto:eric.lim@dot.gov

24 Limitations

GEOTECHNICAL REPORT NO. 11-21

FINAL GEOTECHNICAL REPORT

Mitchell Street and Black Rock Road Reconstruction

OR POLK 2016(1)

Polk County, Oregon

June 2021

Prepared by: _________________________________________ Sean Hayter, Geotechnical Engineer

Orion George, Engineering Geologist

Reviewed by: _________________________________________ Eric Lim, Senior Geotechnical Engineer

Approved by: _________________________________________ Douglas A. Anderson, Geotechnical Functional Manager

References 25

8 REFERENCES

AASHTO, AASHTO LRFD Bridge Design Specifications, 8th Edition, 2017, American Association of State Highway and Transportation Officials, Washington, D.C.

Miller, Matthew, 2018, Project Plan, Mitchell Street and Black Rock Road Reconstruction, Western Federal Lands Highway Division, Vancouver, Washington.

USGS, 2017, USGS Earthquake Hazards Program website, U.S. Seismic Design Maps, http://earthquake.usgs.gov/designmaps/us/application.php, accessed June 9, 2020, U.S.

Geological Survey.

USGS, 2018a, Quaternary Fault and Fold Database of the United States Interactive Fault Map website, https://earthquake.usgs.gov/hazards/qfaults/, accessed June 9, 2020.

USGS, 2018b, Unified Hazard Tool website, https://earthquake.usgs.gov/hazards/interactive/index.php, accessed June 9, 2020, U.S.

Geological Survey.

Walker, G.W. and MacLeod, N.N., 1991, Geologic Map of Oregon, U.S. Department of the Interior, U.S. Geological Survey.

Western Regional Climate Center (2020), Cooperative Climatological Data Summaries, Falls City, Oregon (352800): https://wrcc.dri.edu/cgi-bin/cliMAIN.pl?or2800, accessed June 8, 2020.

Wyllie, D. and Mah, C., 2004, Rock Slope Engineering: Fourth Edition

Yeats, R.S., Graven, E.P., Werner, K.S., Goldfinger, C., and Popowski, T.A., 1996, Geologic map of the Central and Southern Willamette Valley, Benton, Lane, Linn, Marion, and Polk Counties, Oregon, in Assessing earthquake hazards and reducing risk in the Pacific Northwest; Volume 2.

http://earthquake.usgs.gov/designmaps/us/application.php https://earthquake.usgs.gov/hazards/qfaults/ https://earthquake.usgs.gov/hazards/interactive/index.php https://wrcc.dri.edu/cgi-bin/cliMAIN.pl?or2800%20

APPENDIX A

Project Photographs

Photo A-1, View of roadway and Little Luckiamute River near STA 23+00 to 24+00 facing SW.

Photo A-2, View of river near STA 23+00 to 24+00 facing west.

Photo A-3, View of existing slope cut near STA 24+00 facing south.

Photo A-4, View of existing slope cuts near STA 24+00 facing SW.

Photo A-5, View of existing vertical slope cut near STA 23+00 facing west.

Photo A-6, View of existing roadway and river near STA 23+00 to 24+00 facing east.

Photo A-7, View of existing riprap lined river near STA 24+00 facing west.

Photo A-8, View of river and existing riprap near STA 24+00 facing NE.

Photo A-10, Photo of truck drilling subgrade boring SG19-03 facing SW.

Photo A-9, Photo of truck drilling boring BH19-04 facing NE.

Photo A-12, Photo of test pit TP19-01 after excavation.

Photo A-11, Photo of excavator at test pit TP19-01 facing west.

Photo A-14, Photo of test pit TP19-02 after excavation.

Photo A-13, Photo of excavator at test pit TP19-02 facing west.

Photo A-16, Close up photo of rock in test pit TP19-03.

Photo A-15, Photo of at test pit TP19-03 facing east.

APPENDIX B

Site Plans

Figures B-1 to B-6, Boring Locations Maps

Figure B-1: Boring Locations Maps

Note: Boring Locations Maps are based off the 95% plan set and are not for construction.

BH19-03

BH19-04

SG19-01

Figure B-2: Boring Locations Maps off the 95% plan set and are not for construction.

SG19-02

TS-101

TS-104

Figure B-3: Boring Locations Maps off the 95% plan set and are not for construction.

BH19-02

BH19-01

TS-106

TS-108

Figure B-4: Boring Locations Maps off the 95% plan set and are not for construction.

TS-109

Figure B-5: Boring Locations Maps off the 95% plan set and are not for construction.

BH19-05

BH19-06

TP19-01

TP19-02

TP19-03

SG19-03

Figure B-6: Boring Locations Maps off the 95% plan set and are not for construction.

SG19-04

TS-110

APPENDIX C

Exploration Logs

Descriptive Terminology for Boring Logs Boring Log Legend

Boring Logs BH19-01 to BH19-06 Test Pit Logs TP19-01 to TP19-03

Subgrade Boring Logs SG19-01 to SG19-04 Rock Core Photos

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.