Walker_Ph3-Geotech_Report-FINAL.pdf

REPORT OF GEOTECHNICAL ENGINEERING SERVICES

NW Walker Road – NW 173rd to NW 185th Washington County, Oregon

For 3J Consulting Inc.

March 15, 2021

Project: WashCo-73-01

9450 SW Commerce Circle, Suite 300 l Wilsonville, OR 97070 l 503.968.8787 www.geodesigninc.com

March 15, 2021

3J Consulting Inc.

9600 SW Nimbus Avenue, Suite 100 Beaverton, OR 97008

Attention: Jake Johnston

Report of Geotechnical Engineering Services NW Walker Road – NW 173rd to NW 185th

Washington County, Oregon Project: WashCo-73-01

GeoDesign, Inc., DBA NV5 (GeoDesign) is pleased to submit this report of geotechnical engineering services for NW Walker Road between NW 173rd and NW 185th Avenues in Washington County, Oregon. Please contact us if you have questions regarding this report.

Sincerely, GeoDesign, Inc., DBA NV5

Nick Paveglio, P.E. Jeffery D. Tucker, P.E., G.E.

Principal Engineer Principal Engineer cc: Pete Slocum, DOWL (via email only)

NNP:JDT:kt

Attachments

One copy submitted (via email only)

Document ID: WashCo-73-01-031521-geor.docx

© 2021 GeoDesign, Inc., DBA NV5 All rights reserved.

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EXECUTIVE SUMMARY

Based on information provided by Washington County and the results of our explorations, laboratory testing, and analyses, it is our opinion that the project is feasible. The primary geotechnical considerations for the project are summarized as follows:

A gravity sewer system with zero settlement tolerance is located near the Willow Creek culvert replacement. In order to achieve zero settlement, the net increase in pressure above and near the system as a result of filling must be zero and geofoam or lightweight concrete fill will be required within 25 feet of the gravity sewer system. If lightweight concrete is used, the existing subgrade must be excavated and replaced with lightweight concrete to achieve a zero net increase in pressure.

Lightweight or conventional fill can be used in areas more than 25 feet from the gravity sewer system or other settlement-sensitive elements. Utility installation and paving should be delayed until primary consolidation settlement is complete in areas where 2 feet or more of fill is placed (delay will not be required in zero net pressure increase locations).

Due to the surficial soft soil and proximity to the gravity sewer system, conventional spread footings are not appropriate and driven piles should support the Willow Creek culvert. Due to the proximity of the gravity sewer system, driven pile locations should be pre-drilled and cased with sacrificial tubes a minimum of 5 feet below the bottom of gravity sewer system.

The near-surface soil along the project alignment contains fine-grained soil that is easily disturbed during wet weather. The contractor should protect the subgrade areas from disturbance during construction.

Liquefaction and lateral spreading are not design considerations for the project.

WashCo-73-01:031521

TABLE OF CONTENTS PAGE NO.

ACRONYMS AND ABBREVIATIONS

1.0 INTRODUCTION 1

2.0 PURPOSE AND SCOPE 1

3.0 SITE CONDITIONS 2

3.1 Geologic Setting 2

3.2 Surface Conditions 3

3.3 Subsurface Conditions 3

4.0 DESIGN 5

4.1 Permanent Slopes 5

4.2 Fills 5

4.3 Seismic Considerations 6

4.4 Culvert Foundation Support 9

4.5 Drainage 11

4.6 Retaining Structures 11

4.7 Traffic Control and Light Pole Foundation Recommendations 16

5.0 CONSTRUCTION 17

5.1 Erosion Control 17

5.2 Site Preparation 17

5.3 Wet Weather Considerations 18

5.4 Excavation 19

5.5 Materials 20

6.0 OBSERVATION OF CONSTRUCTION 23

7.0 LIMITATIONS 24

REFERENCES 26

FIGURES

Vicinity Map Figure 1 Site Plan Figure 2

APPENDICES

Appendix A Field Explorations A-1 Laboratory Testing A-2 Exploration Key Table A-1 Soil Classification System Table A-2 Boring Logs Figures A-1 – A-11 Atterberg Limits Test Results Figure A-12 Consolidation Test Results Figure A-13 Summary of Laboratory Data Figure A-14 SPT Hammer Calibration

TABLE OF CONTENTS PAGE NO.

APPENDICES (continued) Appendix B Driven Pile Axial Resistance B-1 Driven Pile Axial Resistance – West Abutment (B-1) Figure B-1 Driven Pile Axial Resistance – East Abutment (B-2) Figure B-2 Appendix C LPILE Parameters C-1 LPILE 6.0 Soil Parameters – West Abutment (B-1) Table C-1 LPILE 6.0 Soil Parameters – East Abutment (B-2) Table C-2

ACRONYMS AND ABBREVIATIONS

AASHTO American Association of State Highway and Transportation Officials AC asphalt concrete ASTM American Society for Testing and Materials BGS below ground surface CSZ Cascadia Subduction Zone CSZE Cascadia Subduction Zone Earthquake g gravitational acceleration (32.2 feet/second2) GDM Geotechnical Design Manual H:V horizontal to vertical km kilometers ksf kips per square foot LRFD Load and Resistance Factor Design MSE mechanically stabilized earth ODOT Oregon Department of Transportation OSHA Occupational Safety and Health Administration OSSC Oregon Standard Specifications for Construction (2021) pcf pounds per cubic foot PDA Pile Driving Analyzer® PGA peak ground acceleration psf pounds per square foot psi pounds per square inch SPT standard penetration test USGS U.S. Geological Survey

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1.0 INTRODUCTION

This report presents the results of GeoDesign’s geotechnical engineering evaluation for NW Walker Road between NW 173rd and NW 185th Avenues in Washington County, Oregon. The site relative to surrounding features is shown on Figure 1. The project layout is shown on Figure 2. Acronyms and abbreviations used herein are defined above, immediately following the Table of Contents.

The approximately 0.75-mile-long project includes widening NW Walker Road to two travel lanes in each direction with a center turn lane. The project includes replacement of the culvert at Willow Creek, continuous sidewalks and bike lanes along the alignment, stormwater upgrades (including a new pond), a new traffic signal at NW 178th Avenue/NW Cambray Street, and construction of sound walls.

2.0 PURPOSE AND SCOPE

The purpose of our geotechnical engineering services was to explore the subsurface conditions near the proposed improvements and develop geotechnical engineering recommendations for design and construction of the project. Our scope included the following:

Coordinated and managed the field explorations, including locating utilities, access preparation, and scheduling contractors and GeoDesign staff.

Prepared traffic control plans and obtained right-of-way permits from Washington County.

Completed the following explorations:

Two deep drilled borings to depths between 51.5 and 101.5 feet BGS for culvert foundation design and adjacent retaining walls

Six shallow drilled borings for pavement design, retaining/sound wall design, and earthwork recommendations

Eleven hand auger borings for retaining/sound wall design and earthwork recommendations

Maintained documentation of each exploration, visually classified the soil encountered, and collected soil samples as appropriate for the soil conditions encountered.

Conducted the following laboratory tests on soil samples from the explorations:

Twenty-one moisture content determinations in general conformance with ASTM D2216 Four particle-size analyses in general conformance with ASTM D1140 Three consolidation tests in general conformance with ASTM D2435 Four Atterberg limit tests in general conformance with ASTM D4318

Provided geotechnical engineering construction recommendations for site preparation, structural fill compaction criteria, and wet/dry weather earthwork procedures.

Provided geotechnical engineering recommendations for design and construction of retaining walls, including foundation type, allowable bearing capacity, and lateral earth pressures.

Provided geotechnical engineering recommendations for design and construction of traffic signals and light poles.

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Provided recommendations for design and construction of culvert foundations, including nominal and factored driven pile resistances and seismic design parameters.

Provided this geotechnical report presenting the results of our field explorations and our geotechnical engineering recommendations.

3.0 SITE CONDITIONS

3.1 GEOLOGIC SETTING

3.1.1 General

The site is located in the Tualatin Basin of the Puget Sound-Willamette Valley physiographic province, a tectonically active lowland located along the convergent Cascadia margin (Orr and Orr, 1999). The Tualatin Basin is formed by a gentle syncline between the uplifted Coast Ranges to the west, the Chehalem Mountains to the south, and the Tualatin Mountains to the north and east. The Tualatin Mountains have been uplifted along northwesterly oriented faults, including the steeply dipping Portland Hills fault located along the eastern flank of the mountains.

The near-surface geologic unit mapped at the site is the fine-grained facies of the Quaternary flood deposits (Madin, 1990). The unit consists of unconsolidated silt and sand deposited by catastrophic floods associated with the sudden release of waters from glacial Lake Missoula during the late Pleistocene Age. Many dozens of these Missoula Floods occurred between approximately 15,500 and 12,500 years ago (and perhaps during earlier glaciations). Flood waters several hundred feet deep swept out of the Columbia Gorge and over the lowlands of the Portland area. Thickness of the flood deposits in the site vicinity is approximately 40 to 50 feet.

Underlying the Quaternary flood deposits are a thick accumulation of moderately to poorly lithified mudstone and sandstone mapped as the Sandy River Mudstone equivalent (Madin, 1990). These were deposited in a large delta where the ancient Columbia and Willamette rivers converged, probably during the middle Miocene to Pleistocene (approximately 6 million to 1 million years ago). The thickness of the unit in the site vicinity is approximately 250 feet.

Basement rocks underlying the Sandy River Mudstone equivalent in the site vicinity consist of the Miocene Columbia River Basalts emplaced approximately 17 million to 6 million years ago in the Portland area (Madin, 1990). The Columbia River Basalts are exposed in the Tualatin Basin in the highlands surrounding the valley and in a group of mountains south of the site, which include Cooper Mountain and Bull Mountain. The Columbia River Basalts consist of thick flows of basalt erupted from fissures in eastern Oregon, Washington, and western Idaho that traveled down the ancient Columbia River Gorge to fill the lowland areas around Portland.

3.1.2 Seismicity

Three scenario earthquakes are possible within the local seismic setting. Two of the possible earthquake sources are associated with the CSZ, and the third event is a shallow, local crustal earthquake that could occur in the North American Plate. The three earthquake scenarios are discussed below.

The CSZ is the region where the Juan de Fuca Plate is being subducted beneath the North American Plate. This subduction is occurring in the coastal region between Vancouver Island and

3 WashCo-73-01:031521 northern California. Evidence has accumulated suggesting that this subduction zone has generated eight great earthquakes in the last 4,000 years, with the most recent event occurring approximately 300 years ago (Weaver and Shedlock, 1991). The fault trace is mapped approximately 50 to 120 km off the Washington Coast.

Two types of subduction zone earthquakes are possible:

1. An interface event earthquake on the seismogenic part of the interface between the Juan de Fuca Plate and the North American Plate on the CSZ. This source is reportedly capable of generating earthquakes with a moment magnitude of between 8.5 and 9.0.

2. A deep intraplate earthquake on the seismogenic part of the subducting Juan de Fuca Plate. These events typically occur at depths of between 30 and 60 km. This source is capable of generating an event with a moment magnitude of up to 7.5.

A significant earthquake could occur on a local fault near the site within the design life of the culvert. Such an event would cause ground shaking at the site that could be more intense than the CSZ events, although the duration would be shorter. The closest mapped faults to the site are the Beaverton fault zone (2.9 miles south), Helvetia (3.4 miles northwest), Oatfield (3.6 miles northeast), and the Portland Hills (7.0 miles east).

3.2 SURFACE CONDITIONS

The site is located on NW Walker Road between NW 173rd and NW 185th Avenues. NW Walker Road is predominately a two-lane road with occasional left turn lanes. Near the intersections of NW 173rd and NW 185th Avenues NW Walker Road widens to four lanes. Development along the project alignment primarily consists of residential houses with some retail near NW 185th Avenue.

Topography in the west portion of the site is relatively flat and slopes gently downward from west to east in the east portion. Sidewalks are present on one or both sides of NW Walker Road along the project alignment.

Willow Creek is located between NW Cambray Street and NW 174th Terrace. The existing creek crossing is a culvert with a small pedestrian bridge just south of NW Walker Road. Metal guardrails are present on both sides of the creek crossing, and the creek has low to moderate flow depending on season and rainfall. Vegetation primarily consists of small to large trees and brush along the shoulders of the road near the creek.

3.3 SUBSURFACE CONDITIONS

3.3.1 General

We explored the subsurface conditions along the project alignment by drilling 2 deep borings (B-1 and B-2) to depths between 51.5 and 101.5 feet BGS, 6 shallow borings (C-1 through C-6) to depths between 3 and 8.5 feet BGS, and 11 hand auger borings (HA-1 through HA-11) to depths between 1 foot and 10 feet BGS. The deep borings were completed to provide culvert foundation, retaining/sound wall, and earthwork recommendations. The shallow borings and hand auger borings were completed for retaining/sound wall and earthwork recommendations.

The approximate locations of the explorations are shown on Figure 2. Exploration logs and laboratory test results are presented in Appendix A.

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3.3.2 Willow Creek Crossing

3.3.2.1 Pavement Section

We encountered a pavement section in both borings completed near the proposed culvert. The pavement section consisted of 7.75 to 8 inches of AC underlain by approximately 8 to

10.3 inches of aggregate base.

3.3.2.2 Fill

Undocumented fill consisting of soft silt with organics was encountered in both borings. Some clay was also observed in boring B-1 and minor gravel was observed in boring B-2. The fill is present directly below the pavement section and extends to a depth of approximately 5 feet BGS.

We anticipate the fill was associated with construction of the existing culvert.

3.3.2.3 Alluvium

Recent alluvium was observed below the fill in the borings. The alluvium consists of soft to stiff silt with variable sand content. The alluvium extends to depths of approximately 30 feet BGS.

Laboratory testing indicates that the fine-grained alluvium has low plasticity. The moisture content of the alluvium at the time of our explorations varied between 33 and 42 percent.

3.3.2.4 Sandy River Mudstone Equivalent

Sandy River Mudstone Equivalent consisting of stiff to very stiff silt and clay is present below the alluvium. The silt and clay contains variable sand and ranges from low to medium plasticity. It is typically gray-brown-orange and moist. The Sandy River Mudstone Equivalent has low settlement potential. The moisture content of the Sandy River Mudstone Equivalent at the time of our explorations varied between 31 and 38 percent.

3.3.2.5 Groundwater

Groundwater could not be directly measured in the deep borings due to the presence of drilling fluid, but increased soil moisture content was observed within 5 to 10 feet of the existing ground surface in the borings. The depth to groundwater may fluctuate in response to seasonal weather and changes in surface topography, and sites within close proximity to the creek will likely experience groundwater levels consistent with Willow Creek water levels.

3.3.3 Areas Away from Willow Creek Crossing

3.3.3.1 Pavement Section

A pavement section consisting of 4.8 to 9.0 inches of AC on top of 4.5 to 19.2 inches of aggregate base was observed in borings C-1 through C-6.

3.3.3.2 Fill

Undocumented fill was observed in approximately half of the shallow borings along NW Walker Road and a majority of the hand auger borings. The fill in the shallow borings consists of medium stiff to stiff silt or medium dense gravel with variable proportions of sand. The fill is moist and varies from 0.5 foot to 1.4 feet in thickness. The fill in the shallow borings extends to a maximum depth of 2.5 feet BGS.

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The fill in hand auger borings consists of a variable mixture of medium stiff clay, loose gravel, and medium dense sand. The fill is moist and extends to depths up to 2.5 feet BGS. All fill is likely associated with construction along NW Walker Road.

3.3.3.3 Alluvium

Recent alluvium was observed below pavement section or fill in the shallow borings and hand auger borings. The alluvium generally consists of medium stiff silt and clay with variable proportions of sand. The alluvium extends to the maximum depths explored, where encountered, in the shallow borings and hand auger borings.

3.3.3.4 Groundwater

Groundwater was not encountered in the shallow borings or hand auger borings. Based on experience in the area, groundwater will be between 10 and 15 feet BGS. Perched water may accumulate at shallower depths during the wet season.

4.0 DESIGN

4.1 PERMANENT SLOPES

Permanent fill slopes on the site should not exceed a gradient of 2H:1V, unless specifically evaluated for stability. Slopes should be planted with appropriate vegetation or armored to provide protection against erosion as soon as possible after grading. Surface water runoff should be collected and directed away from slopes to prevent water from running down the face of the slope.

4.2 FILLS

4.2.1 General

Approach fills will be required for the Willow Creek crossing. Fill will cause consolidation of the underlying fine-grained fill and alluvium that overlie the stiff to very stiff silt and clay. Total post-construction settlement will be dependent on the fill thicknesses required.

According to Clean Water Services, the sewer system near Willow Creek consists of a concrete gravity line with zero settlement tolerance. In order to achieve zero settlement, the net increase in pressure above and near the system as a result of filling must be zero. A zero net pressure increase can be achieved by using lightweight geofoam for the culvert approach fills. Geofoam has a unit weight between approximately 1 and 6 pcf, which is significantly less than the weight of compacted fill (115 to 135 pcf). Geofoam can be degraded by petroleum-based products and should be encased if used for the project.

Lightweight concrete can be used as an alternative to geofoam. Lightweight concrete typically has a unit weight of 20 to 30 pcf, which is approximately one-quarter to one-third of conventional fills. To achieve zero settlement near the sewer system, the existing subgrade must be excavated and replaced with lightweight concrete to achieve a zero net increase in pressure. A unit weight of 110 pcf should be assumed for unsaturated soil when calculating over-excavation depths. The unit weight should be reduced 48 pcf for saturated soil.

Groundwater for saturated conditions should be based on the ordinary water elevation of Willow

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Creek. Due to the variability of the existing ground surface, the thickness of over-excavation and replacement required will vary across the zero net increase area.

To mitigate the potential for settlement, the net zero pressure condition should extend at least 25 feet from settlement-sensitive elements. Lightweight or conventional fill is suitable in areas more than 25 feet from settlement-sensitive elements. In addition to the use of lightweight fill, foundations for the culvert should be supported by driven piles as described in the “Culvert Foundation Support” section to mitigate settlement.

The fine-grained fill and native soil has potential for relatively long consolidation times. We anticipate that settlement will be essentially complete within approximately two to four months after completion of embankment construction and that the majority of the settlement will occur within the first eight weeks.

Utility installation and paving should be delayed until primary consolidation settlement is complete in areas where the equivalent pressure of 2 feet or more of fill is placed (delay will not be required in zero net pressure increase locations).

4.2.2 Settlement Monitoring Plates

Settlement monitoring should be provided at several locations under the embankment fills to determine the settlement of subsurface soil as a result of fill. Settlement monitoring points can be installed at the finished surface immediately after fills are complete. Settlement monitoring points should consist of fixed, ridged elements that can be protected from disturbance and easily surveyed. We recommend that the location of the settlement monitoring points be established by GeoDesign based on coordination with the contractors relative to construction sequencing.

The points should be surveyed at least twice a week for one month and then weekly thereafter until the survey data indicates that consolidation of the soil is complete. Settlement monitoring points should be read using survey equipment with an accuracy of 1/100th of a foot and referenced to a stationary datum established at least 500 feet from fill placement. The survey data should be supplied to the appropriate personnel at Washington County within three days of the survey. Once the survey data indicates that primary settlement is essentially complete, settlement-sensitive structural elements can be erected on the fills.

4.3 SEISMIC CONSIDERATIONS

4.3.1 Geologic Hazards

4.3.1.1 General

In addition to ground shaking, site-specific geologic conditions can influence the potential for earthquake damage. Deep deposits of loose or soft alluvium can amplify ground motions, resulting in increased seismic loads on structures. Other geologic hazards are related to soil failure and permanent ground deformation. Permanent ground deformation could result from liquefaction, lateral spreading, landsliding, and fault rupture. The following sections provide discussion regarding potential seismic hazards.

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4.3.1.2 Fault Surface Rupture

Fault rupture is a seismic hazard that includes abrupt, large, differential ground movements that can cause damage to structures located on a known fault. Based on USGS (2014), the closest mapped fault is the Beaverton fault zone, which is located approximately 3 miles south of the site. Due to the distance of the fault, the risk of fault rupture is low along the project alignment.

4.3.1.3 Liquefaction

Liquefaction can be defined as the sudden loss of shear strength in a soil due to an excessive buildup of pore water pressure. Liquefied soil layers generally follow a path of least resistance to dissipate pore pressures, often resulting in sudden surface settlement, sand boils or ejections, and/or lateral spreading in extreme cases. Clean, loose, uniform or silty, fine-grained, saturated sand is particularly susceptible to liquefaction.

We performed liquefaction analysis using the procedures recommended in Section 6.5.2.2 of the ODOT GDM (2018). Based on analysis, liquefaction settlement will be negligible along the project alignment.

4.3.1.4 Lateral Spreading

Lateral spreading is a liquefaction-related seismic hazard. Areas subject to lateral spreading are typically gently sloping or flat sites underlain by liquefiable sediment adjacent to an open face (such as a riverbank or slough). Liquefied soil adjacent to open faces may “flow” in that direction, resulting in surface cracking and lateral displacement towards the open face. The magnitude of lateral spreading decreases with distance from the open face. Due to the lack of liquefaction, lateral spreading is not anticipated along the project alignment.

4.3.2 Seismic Design Parameters

Earthquake ground motions at the site were determined using seismic data from the 2016 ODOT Seismic Design maps. Peak bedrock accelerations for return periods of 500 years and 1,000 years for the “Life Safety” criteria are reported in Table 1. Based on geologic mapping and our explorations, the recommended seismic design Site Class is “D.” The “Life Safety” seismic design parameters for Site Class D soil are provided in Table 2.

Table 1. ODOT 2016 Seismic Design Parameters for Bedrock “Life Safety” Criteria

Parameter Value

Site Class (Bedrock) B

Return Period 500 years 1,000 years

PGA 0.18 g 0.28 g

Ss 0.39 g 0.61 g

S1 0.14 g 0.23 g

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Table 2. ODOT 2016 Seismic Design Parameters for Soil “Life Safety” Criteria

Site Class (Soil) D

Return Period 500 years 1,000 years

FPGA 1.44 1.32

Fa 1.488 1.268

Fv 2.32 2.140

SDS 0.580 g 0.773 g

SD1 0.325 g 0.492 g

As 0.259 0.370

Peak bedrock accelerations for the ”Operational” criteria are reported in Table 3. Based on geologic mapping and our explorations, the recommended seismic design Site Class is “D.” The “Operational” seismic design parameters for Site Class D soil are provided in Table 4.

Table 3. ODOT 2016 Seismic Design Parameters for Bedrock “Operational” Criteria

Site Class (Bedrock) B

Return Period CSZE

PGA 0.16 g

Ss 0.32 g

S1 0.17 g

Table 4. ODOT 2016 Seismic Design Parameters for Soil “Operational” Criteria

Site Class (Soil) D

Return Period CSZE

FPGA 1.48

Fa 1.554

Fv 2.260

SDS 0.494 g

SD1 0.384 g

As 0.240

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4.4 CULVERT FOUNDATION SUPPORT

4.4.1 General

The near-surface soil at the Willow Creek crossing consists soft, compressible, fine-grained material and that is not suitable to support the culvert. Due to the soft soil and proximity to the Clean Water Services gravity sewer system, conventional spread footings are not appropriate and deep foundations should support the culvert.

Based on the subsurface soil conditions and the anticipated loads, driven piles will be the most cost-effective deep foundation for the project. Driven piles can be installed relatively quickly, will produce no spoils, and will generally be the cleanest of the deep foundation alternatives. In addition, testing of the piles can be completed to achieve the lowest factor of safety versus other deep foundation alternatives. Based on our experience with Washington County, 16-inch-diameter, closed-end driven pipe piles are the preferred deep foundation type to support culverts and bridges.

Due to the proximity of the gravity sewer system, driven pile locations should be pre-drilled and cased with sacrificial tubes a minimum of 5 feet below the bottom of the Clean Water Services system. The pre-drilled holes should be a minimum of 24 inches in diameter and left open during driving. Upon approval from Washington County, the void between the driven pile and the tube can be backfilled to the surface with sand or ¾-inch-minus crushed rock.

4.4.2 Driven Pile Resistances

Sixteen-inch-diameter, closed-end pipe piles are suitable to support the proposed culvert.

Preliminary AASHTO LRFD Strength 1 loads for individual piles provided by DOWL are approximately 100 kips. We completed an analysis to determine factored axial resistances for individual piles using methods described in AASHTO LRFD (2017). Our analysis is based on the following design and construction assumptions:

An approximate top of pile elevation of 156.22 feet per DOWL.

The piles are pre-bored to an elevation of 152.0 feet (5 feet below gravity sewer system bottom).

The factored pile resistances are based on a resistance factor of 0.65 associated with the use of dynamic pile testing (PDA) and a calibrated wave equation analysis during construction.

This corresponds to an approximate factor of safety of 1.54 for soil resistance on the piles.

Without dynamic pile testing, AASHTO LRFD (2017) requires that a significantly lower resistance factor of 0.4 be used (an equivalent factor of safety of approximately 2.5), which will result in longer piles.

The axial resistance factors of the piles should meet the requirements of Section 6.5.4.2 of

AASHTO LRFD (2017).

The structural engineer will determine the wall thickness required for the pipe piles.

Geofoam or lightweight fill will be used to create a zero net pressure increase near the driven piles. If the design methodology changes and the conventional fills are placed near the piles, we should be contacted to determine if consolidation-induced downdrag loads act on the piles.

On-site soil is not liquefiable and does not impart downdrag forces on the piles.

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The analysis does not account for scour effects.

The minimum pile spacing, clearance, and embedment into the cap as described in this section are adhered to.

Pile resistance plots for the west and east abutments of the proposed culvert are presented in Appendix B.

4.4.3 Minimum Pile Spacing, Clearance, and Embedment into Cap The efficiency of pile groups should be evaluated when piles are closely spaced and overlapping stresses from adjacent piles are present or when the pile cap is not in contact with the ground.

Driven pile spacing should not be less than 3B (diameter) or 30 inches, center to center, to negate pile group effects to the factored resistances. We anticipate the pile caps will be constructed in firm contact with the ground. If pile caps are not constructed in firm contact with the ground, a reduction from group efficiency effects will be required for determining the capacity.

The distance from the side of any pile to the nearest edge of the pile cap shall not be less than

9.0 inches. The top of piles should extend at least 12.0 inches into the pile cap after all damaged material has been removed. If the pile is attached to the cap by embedded bars or strands, the pile shall extend no less than 6.0 inches into the cap.

Where a reinforced concrete beam is cast in place and used as a bent cap supported by piles, the concrete cover on the side of the pile shall not be less than 6.0 inches, plus an allowance for permissible pile misalignment. Where pile reinforcement is anchored in the cap satisfying Section 5.12.4.1 of AASHTO LRFD (2017), the concrete cover may be less than 6.0 inches.

4.4.4 Wave Equation Analysis

According to AASHTO LRFD (2017), wave equation analysis should be calibrated using PDA data and performed for determination of pile driving criteria for all new piles without dynamic testing performed. Wave equation analysis will be performed using the computer program WEAP 2010 based on information provided by PDA testing. The wave equation parameters necessary for hammer selection are presented in the following sections. PDA testing will be completed by the agency’s representative (GeoDesign, Inc.) on a minimum of one pile per bent.

4.4.5 Deep Foundation Lateral Analysis Parameters

Lateral analysis parameters for analyzing the driven pile foundations with the LPILE computer software program are presented in Tables C-1 and C-2 of Appendix C.

In addition to the lateral resistance provided by the foundation elements, lateral forces can be resisted by passive pressure of the abutment walls. Provided the abutment walls are at least 2 feet BGS, an available passive earth pressure of 300 psf can be used for loading away from Willow Creek and where the passive pressures do not exert forces on any planned retaining wall face. We recommend that the available passive earth pressure be reduced to 250 psf where the forces will act on the retaining walls. If abutment walls are located below the water table, the available passive earth pressure should be reduced to 100 psf. This additional pressure will need to be accounted for in retaining wall design if it is within the influence zone of retaining walls.

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4.4.6 Deep Foundation Settlement

Settlement of foundations meeting the requirements above and embedded in the stiff to very stiff clay or silt will be negligible beyond the elastic compression of the foundations. For design purposes, a maximum settlement of 1 inch should be estimated for driven steel pipe piles designed and constructed in accordance with the recommendations in this report.

4.4.7 Impact Panels

Impact panels should be constructed on embankment structural fill compacted to at least 95 percent of the maximum dry density, as determined by AASHTO T 99. Total post-construction settlement is expected to be less than 1 inch, and differential settlement between parallel edges of the impact panels should be less than 1 inch given the anticipated loads. This assumes that the full duration of the embankment settlement will occur prior to construction of the impact panels. It is anticipated that the permanent contact pressures imposed on the impact panels will be low enough such that resulting additional settlement will not be significant.

4.5 DRAINAGE

During grading, the contractor should be responsible for temporary drainage of surface water as necessary to prevent standing water and/or erosion at the working surface. Surface runoff should not be allowed to flow over the face of the exposed soil.

4.6 RETAINING STRUCTURES

4.6.1 General

Retaining walls could consist of gravity and MSE walls near the Willow Creek crossing, gravity walls for the Cambray pond, permanent shoring walls near the Cambray pond, and sound and retaining walls along NW Walker Road. Sections of the sound walls may also act as retaining walls along portions of the alignment.

4.6.2 Design Pressure Recommendations

4.6.2.1 Willow Creek Crossing Wall Design Parameters

Soil parameters in Table 5 can be used for design of the Willow Creek crossing retaining walls.

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Table 5. Wall Design Parameters

Soil Type Unit Weight

(pcf)

Friction Angle, ϕ (degrees)

Cohesion (psf)

Retained Soil – Granular Backfill 1

135 35 0

Retained Soil – Native Clay or Silt Soil, Existing or New Fill2

105 28 0

Lightweight Foam 1 to 4 Contact GeoDesign – dependent on slope of adjacent backfill and geofoam grade

Lightweight Concrete Varies (like 20 to 30 pcf)

Contact GeoDesign – fluid pressure should be considered during backfilling

Foundation Soil – Native Clay or Silt Soil, Existing or New Fill3

105 26 0

1. Wall backfill for wall should meet the specifications provided OSSC 02630.10 – Dense-Graded Aggregate.

2. Retained soil should meet the specifications provided in OSSC 00330.12 – Borrow Material.

3. Foundation soil should be firm native or fill soil.

Seismic, traffic, and retained slope surcharges should be included where appropriate as described in the “Wall Surcharges” section. Foundations for the walls should be designed as described in the “Wall Foundations” section.

4.6.2.2 Cambray Pond Walls

Gravity walls for the pond can be designed using an equivalent fluid pressure of 35 pcf. The design pressures assume drainage is provided behind the walls to prevent hydrostatic pressures from developing. If the walls are not drained, they should be designed assuming an equivalent fluid pressure of 70 pcf.

Seismic, traffic, and retained slope surcharges should be included where appropriate as described in the “Wall Surcharges” section. Foundations for the walls should be designed as described in the “Wall Foundations” section.

4.6.2.3 Permanent Shoring Wall at the Cambray Pond

Permanent solider or sheet pile walls could be required near the Cambray pond. Walls above groundwater can be designed using an equivalent fluid pressure of 35 pcf. Below groundwater the active pressure should be increased to 70 pcf. These pressures assume the walls can rotate slightly at the base. The active pressure should act over 1 times the pile width for soldier piles and along the entirety of sheet pile walls below the ground surface.

An equivalent passive pressure of 300 pcf can be used use for soil above groundwater and 130 pcf below groundwater. Passive pressure should be neglected 2 feet below the bottom of the wall or where scour is possible. Passive pressure acts over 2 times the pile width for soldier

13 WashCo-73-01:031521 piles and the entirety of the sheet pile walls. Design pressures described above do not include factors of safety. Seismic, traffic, and retained slope surcharges should be included where appropriate as described in the “Wall Surcharges” section.

We anticipate that lagging will consist of pressure-treated lumber or concrete. To maintain the integrity of the excavation, prompt and careful installation of lagging, particularly in areas of seepage and loose soil, is recommended. All voids behind the lagging should be backfilled after installation. Lagging should be designed to resist hydrostatic pressures below groundwater.

4.6.2.4 Sound and Combination Sound/Retaining Walls

Because sound walls may support soil, non-standard design of foundations is required. The subsurface conditions at the sound/retaining walls consist of predominately fine-grained soil and design of foundations using Broms’ method in accordance with AASHTO guidelines is appropriate. The soil parameters in Table 6 be used for design of the sound/retaining walls along NW Walker Road.

Table 6. Wall Design Parameters

Soil Type Unit

Weight (pcf)

Friction Angle, ϕ (degrees)

Cohesion (psf)

Passive Earth Pressure

Coefficient, Kp

Retained Soil – Granular Backfill 1

135 35 0 --

Retained Soil – Native Clay or Silt Soil, Existing or New Fill2 115 0 75 --

Foundation Soil – Native Clay or Silt Soil, Existing or New Fill3 115 (534) 0 750 2.76

1. Wall backfill should meet the specifications provided OSSC 02630.10 – Dense-Graded Aggregate.

2. Retained soil should meet the specifications provided in OSSC 00330.12 – Borrow Material.

3. Foundation soil should be firm native or fill soil.

4. Saturated unit weight (groundwater should be assumed to be 10 feet BGS)

Seismic, traffic, and retained slope surcharges should be included where appropriate as described in the “Wall Surcharges” section. Foundations for the walls should be designed as described in the “Wall Foundations” section.

4.6.3 Wall Surcharges

Where traffic loads are located within a horizontal distance from the top of the wall equal to one-half the wall height, the lateral earth pressure shall be increased by a surcharge load using the following equation using Tables 7 and 8:

14 WashCo-73-01:031521 p = kgsheq where, p = constant horizontal earth pressure due to live load surcharge (psf) k = coefficient of lateral earth pressure (should be taken as 0.28) s = total unit weight of soil (pcf); should be assumed as 125 pcf heq = equivalent height of soil for vehicular load (per Tables 7 and 8)

Table 7. Equivalent Height of Soil for Vehicular Loading on Abutments Perpendicular to Traffic

Bent Height

(feet) heq

(feet)

5 4

10 3

>20 2

Table 8. Equivalent Height of Soil for Vehicular Loading on Abutments Parallel to Traffic

Bent Height (feet) heq (feet) Distance from Wall Back Face to Edge of Traffic

0.0 Feet 1.0 Foot or Further

5 5 2

10 3.5 2

>20 2 2

The design equivalent fluid pressure should also be increased for walls that retain sloping soil.

We recommend the following lateral earth pressures be increased using the following factors (Table 9) when designing walls that retain sloping soil.

Table 9. Lateral Earth Pressure Increase Factors for Sloping Soil Backfill

Slope of Retained Soil

(degrees) Lateral Earth Pressure

Increase Factor

0 1.00

5 1.06

10 1.12

20 1.33

25 1.52

30 2.27

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Seismic forces should be modeled based on the pseudo-static approach developed by the Mononobe-Okobe method. As described in AASHTO LRFD (2017), a seismic coefficient of one-half of the PGA should be used when analyzing internal stability. PGAs are provided in the “Seismic Design Parameters” section.

4.6.4 Temporary Cuts

Temporary cuts may be required to construct the proposed retaining walls. Excavations into the slopes need to be carefully planned so as not to destabilize the slopes. Cuts less than 4 feet should stand vertical. Deeper excavations should initially be cut back at an inclination 1½H:1V or flatter or be shored. Depending on the depth of cuts and the soil encountered, excavation inclinations may be able to be steepened to 1.25H:1V or even 1H:1V. GeoDesign should be contacted at the time of excavation to observe the soil and determine if the steeper slope excavations are feasible. Because of the adverse effects that water can have on slope stability, we anticipate that this process will not be feasible for wintertime construction and wall construction should occur in dry summer months or temporary shoring used.

4.6.5 Wall Foundations

4.6.5.1 General

Non-headwall wall foundations may be supported on 1-foot-thick granular pads on firm native soil or organic-free fill soil, provided they are evaluated as described in the “Site Preparation” section. If soft soil is present at time of construction, it may be necessary to increase the thickness of the granular pads.

Granular pads should extend 6 inches beyond the margins of the footings for every foot excavated below the footing’s base grade. The granular pads should consist of imported crushed rock compacted to not less than 95 percent of the maximum dry density, as determined by ASTM D1557, or until well keyed, as determined by one of our geotechnical staff. A geotextile separation sound be placed between the interface of the granular pads and existing soil. Separation fabric should meet the requirements in the “Materials” section

A factored bearing resistance at the strength limit design state (qR) of 2,000 psf can be used for design of spread footings as prepared above. The factored bearing resistance can be increased to 6,000 psf for the extreme limit state.

Continuous footings constructed on or above slopes should be at least 18 inches wide and at least 24 inches below the lowest adjacent final grade. A minimum horizontal distance of at least 5 feet from the top of the footing shall be provided between the near face of the footing and the nearest face of any finished slope.

4.6.5.2 Resistance to Sliding

Lateral loads on footings can be resisted by passive earth pressure on the sides of the structures and by friction on the base of the footings. Our analysis indicates that the available passive earth pressure for footings above groundwater is 250 pcf. If footings are installed below groundwater the passive earth pressure should be reduced to 110 pcf. Adjacent footing elements, pavement, or the upper 12-inch depth of adjacent, unpaved areas should not be considered when calculating passive resistance.

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A coefficient of friction equal to 0.30 may be used when calculating resistance to sliding for footings placed directly on silt or clay. A coefficient of friction equal to 0.40 may be used when calculating resistance to sliding for footings placed directly on gravel or crushed rock.

4.6.5.3 Wall Backfill and Drains

A minimum 12-inch-wide by 12-inch-tall zone of drain rock should be placed at the heel of all conventional segmental block retaining walls. Perforated collector pipes should be embedded at the base of the drain rock. The drain rock should meet the requirements provided in the “Materials” section. The perforated collector pipes should discharge at an appropriate location away from the base of the wall. The discharge pipe(s) should not be tied directly into stormwater drain systems unless measures are taken to prevent backflow into the wall’s drainage system.

The wall backfill should be compacted as recommended in the “Materials” section. Backfill placed within 3 feet of the wall should be compacted in lifts less than 6 inches thick using hand-operated tamping equipment (such as a jumping jack or vibratory plate compactor).

4.6.5.4 Construction Considerations

All footing subgrades should be evaluated by qualified personnel to confirm suitable bearing conditions. Observations should also confirm that all loose or soft material, organic material, unsuitable fill, prior topsoil zones, and softened subgrades (if present) have been removed.

Localized deepening of footing excavations may be required to penetrate deleterious material.

4.7 TRAFFIC CONTROL AND LIGHT POLE FOUNDATION DESIGN RECOMMENDATIONS

4.7.1 Traffic Control Structures

Signal poles will be installed at the intersection of NW Walker Road and NW 178th Avenue/ NW Cambray Street. Design of the signal pole foundations will be in accordance with Detail #6922 “Traffic Signal Supports Foundation Standard (Round).” Based on explorations, the subsurface conditions at the intersection meet the minimum design parameters in Note 5 and the foundations can be designed in shown on Detail #6922.

4.7.2 Non-Standard Luminaire Supports

Non-standard luminaire supports may be designed in accordance with the simplified method outlined in Section 13.10 – Embedment of Lightly Loaded Small Poles and Posts of AASHTO (2001). An allowable soil bearing pressure (S1) of 1.5 ksf may be assumed.

4.7.3 Construction Considerations

We recommend that careful observation of the drilled shaft foundation installation be conducted by qualified personnel to verify that subsurface soil conditions are as anticipated. The excavation sidewalls may experience sloughing if groundwater seepage is encountered or if the excavations are dry but left open for extended periods of time (more than a few hours).

Temporary casing will likely be required, particularly if groundwater is present (as discussed in the “Excavation” section). The signal pole foundation system should be constructed in conformance with the specifications provided in OSSC 00963 – Signal Support Drilled Shafts.

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5.0 CONSTRUCTION

5.1 EROSION CONTROL

When exposed, the soil at this site can easily be eroded by wind and water; therefore, erosion control measures should be carefully planned and in place before construction begins. Where feasible, existing pavement and aggregate base should be left in place to protect the ground surface. Measures employed to reduce erosion include, but are not limited to, silt fences, hay bales, plastic sheeting, buffer zones of natural growth, and sedimentation ponds.

5.2 SITE PREPARATION

5.2.1 Demolition

The limits of the required demolition should be determined by the project engineer, although they should include all improvements that will impede construction of the improvements.

Demolished material should be transported off site for disposal, except as noted in the “Materials” section.

Within structural areas (new pavement, foundations, and fills), excavations resulting from removing existing foundations, tanks, and other subsurface elements should be backfilled with compacted engineered fill that meets the criteria outlined in the “Materials” section and as described in OSSC 00310 – Removal of Structures and Obstructions. In addition, the bottom of excavations should expose firm subgrade before filling. The sides of excavations should be cut into firm material and sloped at a minimum gradient of 1H:1V to allow for more uniform compaction of backfill. Utility lines abandoned under new structural components should be completely removed or (with written approval) grouted full if left in place. Soft or loose soil encountered in existing utility line excavations should be removed and replaced with structural fill.

5.2.2 Clearing

Stripping and clearing shall be completed in conformance with the specifications provided in OSSC 00320 – Clearing and Grubbing. Limited areas of exposed soil are currently present in the proposed work area. Based on our explorations, we anticipate that topsoil stripping depths will be 4 to 6 inches. However, greater stripping depths may be encountered. The actual stripping depth should be based on field observations at the time of construction.

Trees and shrubs should be removed from all new pavement and improvement areas. In addition, root balls should be grubbed out to the depth of the roots greater than ½-inch diameter, which could exceed 5 feet BGS for some of the larger trees adjacent to the alignment.

Depending on the methods used to remove root balls, considerable disturbance and loosening of the subgrade could occur during site grubbing. We recommend that soil disturbed during grubbing operations be removed to expose firm, undisturbed subgrade. The resulting excavations should be backfilled with structural fill.

Stripped material should be transported off site for disposal or placed as topsoil dressing as directed by the project engineer and Washington County.

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5.2.3 Subgrade Evaluation

Upon completion of stripping and prior to the placement of foundations, fill, or pavement, the exposed subgrade should be evaluated by proof rolling or probing with a foundation probe to identify soft, loose, or unsuitable areas. A proof roll should be completed with a fully loaded dump truck or similar heavy, rubber tire construction equipment. The proof roll should be completed in conformance with the specifications provided in OSSC 00330.43 – Earthwork Compaction Requirements. Qualified personnel should observe proof rolling to evaluate yielding of the ground surface.

If soft or yielding subgrade areas are identified during evaluation, we recommend the subgrade be excavated and replaced with compacted imported granular fill in conformance with stabilization material as described in the “Materials” section and the specifications provided in OSSC 00331 – Subgrade Stabilization. Scarification and compaction of soft subgrade should not be attempted unless the material can be aerated and dried during extended periods of dry weather.

5.3 WET WEATHER CONSIDERATIONS

The fine-grained soil at the site is easily disturbed during the wet season or when the moisture content of the material is more than a few percentage points above optimum. This will likely be throughout the year, except between mid-summer through early fall (typically from early July to mid-October). If not carefully executed, site preparation and construction work can create extensive soft areas and significant subgrade repair costs can result.