waa-jeffer134309-2_GM 1024.pdf

158377846

REVIEW MEMO

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661

DATE: March 18, 2024

TO: John Davis Project Manager

FROM: Todd Hansen, P.E.

Geological Engineer

SUBJECT: Geotechnical Memo 1024 Technical Review & Culvert Recommendations

WA JEFF 134309(1)

Oil City Rd-Two Trout Jefferson County, Washington

INTRODUCTION

This memorandum summarizes geotechnical recommendations for the culvert replacement based on our review of the following memo and report delivered by others:

Final Geotechnical Engineering Report. Oil City Road Bridge, Project No. 200333.

December 22, 2020. Aspect Consulting.

Two Trout Creek TS&L Concept Memo. Two Trout Creek, WA FLAP JEFFER 134309(2). September 14, 2023. WFLHD HYD.

Todd Hansen, WFLHD Geological Engineer, has not visited this site and all recommendations provided herein are based on a desktop study and review of available engineering documents.

Both the memo and report are attached for reference. Geotechnical recommendations are based entirely on the Aspect Consulting report. No analysis or evaluation have been performed by WFLHD Geotechnical Division.

Refer to the Aspect Consulting Report for information related to the geology, site conditions, subsurface conditions including boring logs, and geotechnical analysis results.

Refer to the WFLHD HYD Memo for information related to recent ground cracking and LiDAR identified unstable slopes.

Previously this project has evaluated a single-span bridge to replace the failed box culvert crossing. The Aspect Consulting report (Dec 2020) has information regarding the proposed bridge structure including deep foundation recommendations. Field visits to the site in the summer of 2023 by other WFLHD staff observed pavement cracking and indicators of continued slope movement in the immediate vicinity and surrounding area. This cracking coincides with LiDAR verified landslide terrain, which indicates the existing unstable slopes continue to incrementally move.

The bridge design would have spanned the western lateral extent of the landslide. After reviewing the proposed bridge design, a combination of risk to the operational life of the structure and the limited resources for Jefferson County to repair bridge deficiencies led to the decision to construct a culvert. A culvert can be more easily constructed and will tolerate more landslide movement than a bridge structure while still remaining serviceable.

CULVERT DESIGN RECOMMENDATIONS

Detailed recommendations for lateral earth pressure are presented in the Aspect Consulting Report, Section 4.2 Bridge Abutments and Wingwalls (page 13 of report). Lateral earth pressure for backfill against the culvert should assume at-rest conditions. The following table from the Aspect Consulting report summarizes the recommended conditions for culvert placement.

Bridge wingwall bearing resistance is presented in Figure 4 of the Aspect Consulting Report, Figures. Based on the provided unfactored bearing resistance the culvert should be designed for an averaged nominal bearing resistance of 3,500 pounds per square foot (psf).

CONSTRUCTION CONSIDERATIONS

The existing detour around the failure allows construction to occur without needing to build additional detour access roads.

Saturated subgrade bearing soils should be removed and replaced with free-draining rock or backfill materials.

CLOSING

Please contact Todd Hansen at (360) 619-7903 or todd.hansen@dot.gov with any questions regarding this memorandum.

INITIALS

CC: PM and CFT Members Geotechnical File

Attachments:

Final Geotechnical Engineering Report. Oil City Road Bridge, Project No. 200333.

December 22, 2020. Aspect Consulting.

Two Trout Creek TS&L Concept Memo. Two Trout Creek, WA FLAP JEFFER 134309(2). September 14, 2023. WFLHD HYD.

GEOTECHNICAL ENGINEERING

REPORT

Oil City Road Bridge Prepared for: KPFF Consulting Engineers Project No. 200333 December 22, 2020 FINAL

GEOTECHNICAL ENGINEERING

REPORT

Oil City Road Bridge Prepared for: KPFF Consulting Engineers

Project No. 200333 December 22, 2020 FINAL

Aspect Consulting, LLC

Andrew J. Holmson, PE Associate Geotechnical Engineer aholmson@aspectconsulting.com

V:\200333 Coast Salmon Fndtn Fish Passage Barrier Rplcmnt Dsgn\Deliverables\Oil City Road Bridge\Final Geotechnical Report_Oil City Road Bridge_20201222.docx

12/22/2020

ASPECT CONSULTING

PROJECT NO. 200333 DECEMBER 22, 2020 FINAL i

Contents

List of Appendices .............................................................................................. ii

1 Introduction

1.1 Project Description

2 Site Conditions

2.1 Surface Conditions

2.2 General Geology

2.3 Subsurface Conditions

2.3.1 Laboratory Testing

2.3.2 Stratigraphy

2.3.3 Groundwater

3 Earthquake Engineering

3.1 Oil City Road Bridge

3.2 Ground Response

3.3 Surficial Ground Rupture

3.4 Liquefaction

4 Geotechnical Engineering Conclusions and Recommendations

4.1 Bridge Foundations

4.1.1 Driven Pile Axial Resistance

4.1.2 Minimum Pile Penetration

4.1.3 Driveability

4.2 Bridge Abutments and Wingwalls

4.2.1 Lateral Earth Pressures

4.2.2 Bridge Wingwall Bearing Resistance

4.2.3 Bridge Wingwall Sliding Resistance

4.2.4 Bridge Abutment and Wingwall Drainage

4.3 Global Stability

5 Construction Considerations

5.1 Earthwork

5.1.1 Temporary Excavation

5.1.2 Construction Dewatering

5.1.3 Subgrade Preparation

5.1.4 Structural Fill

5.2 Driven Pile Installation

5.2.1 Geotechnical Monitoring of Driven Piles

5.2.2 Site Subsurface Variation and Pile Lengths

5.3 Embankment and Pavement Construction

ii FINAL PROJECT NO. 200333 DECEMBER 22, 2020

6 Recommendations for Continuing Geotechnical Services

6.1 Additional Design and Consultation Services

6.2 Additional Construction Services

7 References

8 Limitations

List of Tables 1 Existing Culvert Summary

2 Preliminary Structure Concepts

3 Seismic Design Parameters

4 Resistance Factors for Driven Pile Design

5 Lateral Earth Pressure Parameters

6 LRFD Resistance Factors for Wingwall Bearing Resistance

7 LRFD Resistance Factors for Sliding Resistance

List of Figures 1 Site Location Map

2 Site and Exploration Plan – Oil City Road Bridge

3 HP 14X89 Driven H-Pile Axial Resistance

4 Bridge Wingwall Bearing Resistance

List of Appendices A Subsurface Explorations

B Geotechnical Laboratory Testing Results

C Report Limitations and Guidelines for Use

PROJECT NO. 200333 DECEMBER 22, 2020 FINAL 1

1 Introduction This report presents the results of a geotechnical engineering study performed by Aspect Consulting, LLC (Aspect) in support of a culvert replacement at Mile Post (MP) 0.51 on Oil City Road in Jefferson County (Project).

Our services were provided in support of a preliminary engineering study led by KPFF Consulting Engineers (KPFF) for the Coast Salmon Foundation in accordance with the scope of work outlined in our subconsultant agreement dated September 21, 2020.

This report summarizes the results of our completed field explorations and presents our geotechnical engineering conclusions and recommendations for the culvert replacement.

1.1 Project Description

The Project involves replacing the existing culvert at the location shown in Table 1 and on Figure 1, Site Location Map. The primary purpose of the culvert replacement is to provide improved fish passage and habitat while also providing a long-term, reliable, and maintainable structure for supporting the overlying roadway. The preliminary concept developed by KPFF, as shown in the 30 percent drawings (KPFF, 2020) for the new structure, are shown in Table 2 and on Figure 2.

Table 1. Existing Culvert Summary

Culvert Site Existing Culvert County

Oil City Road MP 0.51 Twin 48-inch CMP Culverts Jefferson County

Table 2. Preliminary Structure Concepts

Culvert Site Proposed Structure

Oil City Road MP 0.51 Single-Span Concrete Bridge

80-Feet-Long Pile-Supported

The bulk of the construction activities will primarily occur south of and adjacent to the existing road alignment allowing for traffic to be maintained through construction. We anticipate favorable conditions for sequencing construction to construct the new bridge first and then remove the existing culvert and roadway embankment, and to construct the stream channel improvements and bridge scour protection elements. Cast-in-place concrete wingwalls will be used to transition grades and support the roadway embankment along the south side of the bridge at both abutments.

We assume that design and construction of the bridge and related improvements will be in accordance with the current American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications (BDS)

2 FINAL PROJECT NO. 200333 DECEMBER 22, 2020

(AASHTO, 2020) and/or the Washington State Department of Transportation Bridge Design Manual (WSDOT, 2020). The Project vertical datum is NAVD881 and the basis for all references to elevations (also Elevation) contained herein.

Our scope of work included a geologic reconnaissance and subsurface explorations at the culvert crossing, geotechnical laboratory testing, coordination with the design team, geotechnical analyses to develop design criteria for the culvert replacement structure, and the preparation of this report.

1 All elevations in this report are relative to the North American Vertical Datum of 1988 (NAVD88).

PROJECT NO. 200333 DECEMBER 22, 2020 FINAL 3

2 Site Conditions This section presents description of the surface conditions, geologic setting, and subsurface conditions at the culvert crossing. This information provides context for the discussion of types and distribution of geologic soil units and a basis for our geotechnical engineering recommendations.

The layout of the site, topography, select site features, locations of our completed subsurface explorations, and the conceptual culvert replacement structure are shown on Figure 2.

2.1 Surface Conditions

Oil City road is a rural, two-lane, part asphalt-paved and part gravel-surfaced roadway in Jefferson County located south of Forks, Washington. At MP 0.51, the roadway narrows to a single, gravel-surfaced lane. We understand the roadway at MP 0.51 was washed-out due to a culvert failure in the early 2000s and a temporary repair was constructed by Jefferson County consisting of diverting the roadway north of its original alignment and installing the current, twin 48-inch-diameter CMP culverts.

The roadway at MP 0.51 is oriented in a west-to-east direction, approximately 15-feet-wide, and has limited grass shoulders. Vegetation in the vicinity of the culvert crossing includes riparian plants near the streams, relatively dense forest ground cover, and a mix of mature evergreen and deciduous trees on either side of the roadway.

The road crosses over Two Trout Creek that is conveyed through the roadway embankment in two 48-inch-diameter CMP culverts. The stream continues south to the Hoh River. The road surface is at Elevation 259 feet and the roadway embankment is approximately 9 feet above the thalweg of the stream with embankment side slopes that are inclined at about 0.5H:1V with isolated areas that are near vertical and partially surfaced with riprap (Photo 1). The road surface does not exhibit extensive cracking, subsidence, or other signs of active distress, but is located at a lower Elevation than the pre-culvert wash-out road profile.

Photo 1. Existing Culverts at MP 0.51 at Oil City Road (Looking North)

4 FINAL PROJECT NO. 200333 DECEMBER 22, 2020

Downstream of the culvert crossing, we observed evidence of large-scale, active erosion occurring along the stream channels including fresh soil exposures of the stream channel side slopes over 30 feet tall, recent landslide activity, and dozens of toppled mature evergreens (Photos 2 and 3).

Photos 2 and 3. Stream Channel Side Slopes Downstream of MP 0.51 at Oil City Road

2.2 General Geology

The site is located in the northwestern portion of the Olympic Mountains physiographic province, which is generally characterized by uplift of oceanic crust at the margin of the Cascadia Subduction Zone (CSZ) and subsequent glaciation and erosion processes.

During middle Tertiary subduction, lower Tertiary marine sediments were thrust beneath the Crescent Formation and caused uplift that formed the Olympic Mountain Range.

During the subsequent Pleistocene, alpine glaciation carved the Olympic Mountain peaks and flooded the coastal lowland with meltwater (Lasmanis, 1991). The surficial geology consists of Pleistocene sediments deposited by alpine glacial processes, including outwash trains, moraines, and drift from multiple pre-Fraser glaciations originating in the Olympic Mountains. Holocene alluvium exists within river, creek, and stream channels.

The Olympic Mountains receive a significant amount of precipitation every year that has continuously contributed to erosive processes in more recent times.

The Oil City Road culvert is located within the greater Hoh River Valley, a coastal lowland area flanked by the Olympic Mountains to the east and the Pacific Ocean to the west. Current geologic mapping (Gerstel, 2000) indicates the Oil City Road crossing is underlain by alpine glacial outwash (Qao). In the vicinity, there are also mapped deposits of alpine glacial drift (Qad). The Qao unit is described as generally unconsolidated to weakly consolidated, weakly to well-stratified sequences of cobble and pebble gravel in a tan to buff sandy matrix with occasional boulders greater than 1 meter in diameter. The Qad unit is described as sediments that may include outwash, till, and lacustrine that

PROJECT NO. 200333 DECEMBER 22, 2020 FINAL 5

underlies much of the valley bottom adjacent to the Hoh River and commonly present in near-vertical exposures, as along stream banks.

The geologic conditions observed at the Oil City Road culvert crossing are in general accordance with the geologic mapping. Although not shown on the geologic map, we also encountered alluvium deposited by the unnamed stream, and embankment fill associated with the construction of the roadway embankment.

2.3 Subsurface Conditions

We completed two soil borings at the Oil City Road culvert crossing on October 29, 2020, designated AB-03 and AB-04 to depths of 18 and 35.5 feet bgs, respectively.

Boring AB-03 encountered refusal conditions on a boulder or series of cobbles and AB- 04 was advanced adjacent to AB-03 to achieve target drilling depths. The borings were advanced using mud rotary drilling techniques with samples collected at select depth intervals following the Standard Penetration Test (SPT)2 standards. The locations of the borings are shown on Figure 2.

Soils were described per the Unified Soil Classification System (USCS) in general accordance with the ASTM International (ASTM) Method D2488 Standard Practice of Description and Identification of Soils (ASTM, 2018). A key to the symbols and terms used on the logs is provided on the Exploration Log Key (Appendix A). The depths on the logs where conditions changed may represent gradational variations between soil types, and actual transitions may be more gradual.

Subsurface conditions at the culvert crossing sites were inferred from the completed field investigation, a review of applicable geologic literature, local geologic experience, and geotechnical laboratory testing. A more detailed description of the field exploration methods and exploration logs are presented in Appendix A.

2.3.1 Laboratory Testing

Laboratory tests were conducted on selected soil samples to characterize certain engineering (physical) properties of the soils. Laboratory testing included determination of moisture content, fines content, grain-size distribution, and plasticity (Atterberg Limits). The laboratory tests were conducted in general accordance with appropriate ASTM International (ASTM) test methods. Detailed descriptions of the tests and results are presented in Appendix B, and the results were incorporated into the exploration logs shown in Appendix A.

2.3.2 Stratigraphy

The primary soil units observed at the Oil City Road crossing, presented in stratigraphic order from top to bottom, were embankment fill, alluvium, and alpine glacial drift. These units are described in more detail below.

2 SPT blow count refers to standard penetration test (SPT) N-values, in accordance with ASTM D1586.

6 FINAL PROJECT NO. 200333 DECEMBER 22, 2020

Embankment Fill We encountered embankment fill at the ground surface consisting of very loose to medium dense, moist gray and brown with iron oxide staining, sandy silt (ML)3, silty sand with gravel (SM), and gravel with silt and sand (GP-GM). Throughout the embankment fill, we encountered small organics and wood debris. At a depth of 7.5 feet bgs, we observed drilling mud loss suggesting open voids within the embankment fill at that depth. The embankment extended to a depth of 12.5 feet bgs. Based on the observed relative density, moisture content, fines content, and presence of organics in the embankment fill, it is likely that the existing roadway embankment was constructed without material, moisture, and compaction control.

The embankment fill can be expected to exhibit low to moderate shear strength characteristics, moderate permeability, moderate compressibility, and moderate to high moisture sensitivity.

Alluvium Underlying the embankment fill at a depth of 12.5 feet bgs, we encountered alluvium that typically consisted of loose to dense, very moist, gray to dark gray gravel with silt and sand (GP-GM) and gravel (GP). The alluvium extended to a depth of 24 feet bgs. At a depth of 18 feet bgs in AB-03, we observed caving conditions, drilling mud loss, and inferred drilling on a boulder or series of cobbles.

The alluvium can be expected to exhibit moderate shear strength characteristics, moderate to high permeability, low compressibility, and low to moderate moisture sensitivity.

Alpine Glacial Drift Underlying the alluvium, at a depth of 24 feet bgs, we encountered alpine glacial drift.

The alpine glacial drift consisted of medium dense to very dense, moist, gray, clayey sand (SC) with low plasticity clay to non-plastic sandy silt with gravel (ML). The alpine glacial drift extended to the bottom of boring AB-04 and we observed the same geologic unit at the base of the stream channel downstream of the crossing.

The glacial drift can be expected to exhibit relatively high shear strength characteristics, low permeability, low compressibility, and moderate to high moisture sensitivity.

2.3.3 Groundwater

Due to drilling using mud rotary methods, we were not able to directly observe the groundwater conditions during drilling; however, we encountered soils with visible water (very moist to wet) at depths corresponding to the adjacent stream level; therefore, we expect that groundwater levels at the culvert crossing will generally correspond with levels in the adjacent stream. Groundwater levels will fluctuate seasonally with precipitation, as well as with changes in site and near-site usage.

3 Soils are classified per the Unified Soil Classification System (USCS) in general accordance with the ASTM International (ASTM) Method D2488 Standard Practice of Description and Identification of Soils.

PROJECT NO. 200333 DECEMBER 22, 2020 FINAL 7

3 Earthquake Engineering The culvert crossing is located within the Olympic Mountains physiographic province, an area of active seismicity that is subject to surficial faults, deep intraslab earthquakes, and the Cascadia Subduction Zone (CSZ).

The culvert site lies within the zone of strong ground shaking from earthquakes associated with the CSZ. Subduction zone earthquakes occur due to rupture between the subducting oceanic plate and the overlying continental plate. The CSZ can produce earthquakes up to magnitude 9.3 and the recurrence interval is thought to be on the order of about 500 years. A recent study estimates the most recent subduction zone earthquake occurred around 1700 (Atwater et al., 2015).

Deep intraslab earthquakes, which occur from tensional rupture of the sinking oceanic plate, are also associated with the CSZ. An example of this type of seismicity is the 2001 Nisqually earthquake. Deep intraslab earthquakes typically are magnitude 7.5 or less and occur approximately every 10 to 30 years.

Seismic design of the proposed bridge will be in accordance with AASHTO and WSDOT design methodologies. The following sections present descriptions of seismic design considerations for the Project.

3.1 Oil City Road Bridge

Per AASHTO Article 4.7.4.2, for structural analysis, single-span bridges need not be analyzed for seismic loads, but the minimum requirements for superstructure connections and support lengths as specified in AASHTO Articles 4.7.4.4 and 3.10.9 apply. For single-span bridges, AASHTO prescribes a minimum design connection force effect in the restrained direction between the superstructure and the substructure equal to the product of the acceleration coefficient (As) and the tributary permanent load (AASHTO, 2020). Additionally, liquefaction design requirements per AASHTO Article 10.5.4.2 apply.

3.2 Ground Response

The AASHTO seismic design is based on an earthquake with a 5 percent probability of exceedance (PE) in 50 years (1000-year return period; AASHTO, 2020). The U.S.

Geological Survey (USGS) provides a unified hazard tool (USGS, 2020a) to look up key design parameters using probabilistic ground motion studies and maps for Washington prepared by the USGS for various design events (return periods). The AASHTO amplification factors were used to adjust the parameters for the representative site class at the Site (design seismic parameters). Based on the subsurface conditions encountered in our explorations and following the guidelines in AASHTO Table C.3.10.3.1-1 (AASHTO, 2020), we recommend the seismic design be completed with the specific ground motion parameters listed in Table 3 below.

8 FINAL PROJECT NO. 200333 DECEMBER 22, 2020

Table 3. Seismic Design Parameters

Design Parameter Oil City Road

Bridge(1)

Site Class D – Stiff Soil

Peak Ground Acceleration (PGA) 0.434g(2)

Short Period Spectral Acceleration (Ss) 1.015g

1-Second Period Spectral Acceleration (S1) 0.468g

Site Coefficient (FPGA) 1.066

Site Coefficient (Fa) 1.094

Site Coefficient (Fv) 1.532

Design Peak Ground Acceleration (AS) 0.463g

Design Short Period Spectral Acceleration (SDS) 1.110g

Design 1-Second Period Spectral Acceleration (SD1) 0.717g

Design Earthquake Magnitude (MW) 9.07

Site Effective Shear Modulus (Gm) 140,000 kPa(3)

Notes:

1. Based on the latitude and longitude: 47.803525°N, 124.262518°W.

2. g = gravitational force

3. kPa = kilopascals

3.3 Surficial Ground Rupture

Surficial faults are present in the vicinity of Oil City Road with known or suspected Holocene-age activity. Approximately 13 miles southwest of the Oil City Road culvert is an unnamed, offshore fault trace that is part of the Cascadia fold and fault belt. This fault trends northwest and is mapped as moderately constrained (USGS, 2020b). Due to the suspected long recurrence intervals associated with the mapped fault traces surrounding the site, the potential for surficial ground rupture at the site is considered low during the expected life of the replacement structure.

3.4 Liquefaction

Liquefaction occurs when loose, saturated, and relatively cohesionless soil deposits temporarily lose strength from dynamic loading (e.g., seismic shaking, construction vibrations, etc.). The primary factors controlling the onset of liquefaction during a seismic event include intensity and duration of strong ground motion, characteristics of subsurface soil, in situ stress conditions, and the depth to groundwater. The Washington Department of Natural Resources (DNR) maps the Oil City Road area as having very low to low liquefaction susceptibility, however several of the major drainages in the vicinity of the sites are mapped as having moderate to high liquefaction susceptibility (i.e., Hoh River), indicating the increased susceptibility of local alluvium deposits (Palmer et. al, 2004). Potential effects of soil liquefaction include temporary loss of bearing capacity

PROJECT NO. 200333 DECEMBER 22, 2020 FINAL 9

and lateral resistance, liquefaction-induced settlement, and sand boils, any of which could result in structural damage to the replacement structures.

Liquefaction analyses based on the AASHTO design ground motions were conducted at the Oil City Road bridge with the aid of WSliq, a liquefaction analysis software program that was created as part of an extended research project supported by WSDOT and authored by Kramer (2008). Liquefaction requires soil saturation, and for the purposes of our analyses we assumed a groundwater level at 10 feet bgs that is consistent with the stream level. Liquefaction analyses were completed based on the specific subsurface data collected and lab testing results of samples from borings AB-03 and AB-04 for Oil City Road.

Based on the results of the analyses, less dense zones of the alluvium deposits are susceptible to minor and likely discontinuous liquefaction under certain conditions. The deeper, denser alluvium and alpine glacial drift are not susceptible to liquefaction. For the AASHTO design ground motions, liquefaction is expected to initiate between approximate depths of 12.5 to 17.5 feet bgs, or between approximate Elevation 245 and

250. In our opinion, the use of deep foundations extending sufficiently below Elevation 245 will mitigate this minor liquefaction hazard to the proposed bridge structure. Using the WSliq program, seismically induced settlements resulting from the design seismic event are on the order of 1 to 2 inches.

10 FINAL PROJECT NO. 200333 DECEMBER 22, 2020

4 Geotechnical Engineering Conclusions and Recommendations

Based on the results of our explorations, lab testing, and geotechnical engineering analyses, the culvert replacement and proposed structure are feasible. We recommend the proposed single-span concrete bridge be supported by driven steel H-piles. A summary of key Project geotechnical conclusions and recommendations are listed below and described in more detail in the following sections.

• The dense glacial drift deposits that underlie the crossing location are not susceptible to liquefaction and will provide suitable support for driven piles.

• At Oil City Road the driven piles should consist of steel H-piles that can also provide a level of redundancy and resiliency for long-term mitigation of the destabilizing effects of scour in the relatively dynamic stream channel.

• Excavations below stream levels during periods of very low creek flow and with a creek diversion through the site in-place, are not expected to encounter a static groundwater table or high seepage inflows. If encountered, seepage inflows can likely be handled by the Contractor with sumps and pumps.

• The existing embankment fill soil at the site typically contains a significant percentage of fines (soil particles passing No. 200 sieve) and has moisture contents above the optimum content for compaction. This indicates the embankment fill is moisture sensitive and may be unsuitable for use as structural fill behind bridge abutments, behind wingwalls, and below the new road surfaces.

However, the embankment fill may be suitable for re-use as structural fill provided it meets the requirements for Gravel Borrow, WSDOT Standard Specification 9-03.14(1) (WSDOT, 20204), can be moisture conditioned for compaction, and pending approval by Aspect at the time of construction.

Imported fill will be required for the bearing pads below wingwall foundations, drainage material behind walls, and pavement base course.

4.1 Bridge Foundations

We recommend driven piles for the proposed Oil City bridge to provide reliable and long-term support of the bridge and aid in long-term mitigation of the destabilizing effects of scour at the site.

In general, the subsurface conditions at the site consist of relatively loose and moisture sensitive embankment fill overlying a thin layer of relatively coarse (cobbles and boulders) alluvium which is underlain by alpine glacial drift. The alpine glacial drift is relatively resistant to erosion and scour and will provide adequate support for driven piles. Driven piles will also improve constructability by reducing the need for excavations below the stream level.

4 Note that all material specifications used in this report are per the 2020 WSDOT Standard Specifications for Road, Bridge and Municipal Construction, Document M 41-10.

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The primary considerations affecting feasibility of different pile types at the Site are the required depth of driving and the coarse nature of the alluvium. Based on our experience, we recommend relatively stout, HP14X89 H-piles for the bridge foundations. Given the rural Project location, damage to adjacent facilities from pile driving vibrations does not pose a significant concern.

A typical cost-effective configuration for single span bridges is to use H-piles spaced in a single row at a center-to-center spacing of 2.5 to 3 pile diameters to support each bridge abutment.

4.1.1 Driven Pile Axial Resistance

We completed axial pile resistance analyses for driven, HP14X89 H-piles in accordance with AASHTO guidelines using the soil conditions observed in borings AB-03 and

AB-04.

The results of our axial resistance analyses are nominal (ultimate) resistances for both bearing (compression) and uplift (tension) resistance for a single pile or pile group with a minimum center-to-center spacing of 2.5B, where B is the pile diameter. The estimated nominal resistances (the sum of the frictional resistance along the side of the pile and the end resistance) are shown graphically on Figure 3. It should be noted that the axial resistance of these piles embedded in the fine-grained alpine glacial drift may change after the end of pile driving. We expect that during driving the silt and clay will be remolded by the disturbance and excess porewater pressures will develop. The axial capacity of the piles will increase as these excess porewater pressures dissipate. If it becomes necessary to verify the axial capacity after this ‘setup’, a re-strike using signal matching analysis of dynamic pile measurements can be completed a minimum of 14 days after driving.

Based on our experience with similar glacially overridden soils, and provided that the piles extend to the minimum recommended tip elevations, we estimate pile settlements will be less than 1 inch under normal service conditions.

The applicable resistance factors corresponding to the service, strength, and extreme limit states are shown in Table 4 and can be used in conjunction with Figure 3 to determine the estimated strength, service, and extreme geotechnical resistances at various pile embedment depths. It is important to recognize that the nominal resistances shown on Figure 3 are estimates based on static analysis methods. It is possible that soil conditions may vary locally at pile locations and should be confirmed by field observations made during driving.

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Table 4. Resistance Factors for Driven Pile Design

Limit State Resistance Factor, ϕ

Bearing Resistance, ϕstat(1) Bearing Resistance, ϕdyn(2) Uplift, ϕup

Service 1.0 1.0 1.0

Strength 0.30 0.55(3) 0.25

Extreme 1.0 1.0 1.0 Notes:

1. Applies to nominal resistance as determined by static analysis methods.

2. Applies to nominal resistance as determined by dynamic analysis methods during pile driving.

3. Assumes the WSDOT driving formula will be used as the basis for the dynamic analysis and pile driving construction control.

4.1.2 Minimum Pile Penetration

Based on the inferred subsurface conditions at the site, we recommend the driven piles be advanced approximately 3 to 5 pile diameters into the bearing stratum to ensure reliable capacity. Piles may need to be driven deeper than the minimum pile tip elevations to develop the required geotechnical resistance. Actual pile depths will need to be evaluated in the field through field observations. This would include evaluation per the WSDOT pile driving formula as specified in Section 6-05.3 of the WSDOT Standard Specifications to confirm the design capacity of the piles has been achieved.

We recommend the driven piles be advanced a minimum of 7 feet into the alpine glacial drift. We estimate this will occur at approximately Elevation 130. The ultimate axial capacity may be achieved while driving through the alluvium, but we recommend terminating the pile in the underlying alpine glacial drift to limit future settlements and better mitigate against scour concerns.

4.1.3 Driveability

In our opinion, the site is suitable for pile driving. Due to the nature of the underlying soil conditions and the size of the recommended piles, the Contractor should anticipate requiring a diesel impact hammer to achieve the minimum required tip elevation, but may also consider using a vibratory hammer to begin initial pile installation. In order to ensure the proposed pile driving system doesn’t damage the piles during driving, the Contractor should submit a wave equation analysis verifying the proposed equipment will not exceed the pile yield stress in accordance with Section 6-05.3 of the WSDOT Standard Specifications.

4.2 Bridge Abutments and Wingwalls

We assume bridge abutments will be constructed with cast-in-place concrete walls on foundations supported by driven piles, but that wingwalls will be supported on shallow spread footings. Due to the anticipated stream channel and approach embankment slopes (2H:1V), the front face of the walls should be embedded a minimum distance below the surrounding finish grade equal to H/7 where H is the exposed wall height, per AASHTO Table C11.10.2.2-1 (AASHTO, 2020).

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4.2.1 Lateral Earth Pressures

The lateral earth pressures acting on the bridge abutment and wingwalls for active, at-rest, and seismic conditions are shown below in Table 5. These values assume properly compacted structural fill (as described in Section 5.1.4) behind the walls.

To invoke active earth pressure conditions, a wall must be capable of yielding laterally at least 0.001 to 0.002H, where H is the exposed height of the wall; otherwise, at-rest conditions should be assumed.

Table 5. Lateral Earth Pressure Parameters

4.2.2 Bridge Wingwall Bearing Resistance

We understand that the wingwalls will be founded on shallow spread footings. We recommend constructing the wingwall foundations atop a 12-inch-thick crushed rock bearing pad (bearing pad) comprised of Crushed Surfacing Base Course (CSBC) per WSDOT Standard Specification 9-03.9(3). The compacted CSBC pad should be placed over a relatively firm and unyielding subgrade.

Earth Pressure Condition

Slope Condition

Earth Pressure Coefficient

Equivalent Fluid Density3

(pcf)1

Uniform Lateral Surcharge Pressure4

(psf)1

Active2 Level Backslope 0.31 36 0.31S

At-Rest Level Backslope 0.47 56 0.47S

Active Seismic

Level Backslope - - 10H

At-Rest Seismic

Level Backslope - - 17H

Passive5 Level Foreslope 3.25 390 -

Passive 2H:1V Foreslope 1.40 170 -

Notes:

1. psf = pounds per square foot; pcf = pounds per cubic foot.

2. To invoke active earth pressure condition, the wall must be capable of yielding laterally at least 0.001 to 0.002H, where H is the exposed height of the wall.

3. The equivalent fluid densities provided above are distributed triangularly along the exposed height of the wall. The uniform lateral surcharge pressures are distributed uniformly (rectangularly) along the exposed height of the wall.

4. S is the vertical surcharge pressure at the ground surface immediately above/behind the wall. The vertical surcharge pressure causes a lateral earth pressure to act on the wall, which is calculated as the product of S and the appropriate lateral earth pressure coefficient. We recommend a traffic live load surcharge of 250 psf be used for wall design. The resultant uniform rectangular lateral pressure should be applied to the full height of the wall.

5. Nominal passive pressures are presented; a Strength Limit State resistance factor (ϕep) of 0.50 should be applied for design. Passive resistance within a depth of 2 feet of the ground surface in front of the walls, or within scour depth in front of the walls, should be ignored (whichever depth is greater).

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The bearing pad will provide relatively uniform subgrade conditions directly beneath the wingwalls and “bridge” non-uniform conditions below the foundations from the embankment fill. The 12-inch-thick bearing pad should extend at least 2 feet in all directions beyond the edges of the foundation.

• Figure 3 presents the Nominal (unfactored) Bearing Resistances for use in evaluating wall foundations with a length equal to 10 feet and widths ranging between 2 to 10 feet. These values assume the wall footings will be embedded at least 2 feet below the adjacent finished grade with average groundwater levels at least 5 feet below the base of the footings.

• The Strength Limit State is dominated by the height and angle of the adjacent slopes and the walls’ proximity to the slopes. The values shown assume the wall footings are setback a minimum of 2 feet from a 10-foot tall slope with a maximum inclination of 2H:1V or otherwise appropriately embedded per AASHTO Table C11.10.2.2-1 (AASHTO, 2020).

• The recommended LRFD resistance factors required to calculate Strength and Extreme Limit State Bearing Resistances from the provided Nominal Bearing Resistances (shown on Figure 3) are provided in Table 6 below.

• Due to the proximity of slopes to the wingwalls, we anticipate the Service Limit State will not be controlling and estimate total wingwall foundation settlement will be less than 1 inch. Differential settlement over the wall lengths are estimated to be about half the total settlement. Settlement is anticipated to occur as loads are applied during construction.

Table 6. LRFD Resistance Factors for Wingwall Bearing Resistance

Limit State Bearing Resistance, φb Strength 0.45

Extreme 0.9

4.2.3 Bridge Wingwall Sliding Resistance

Sliding resistance is developed from the friction occurring between the bottom of the wall footing and the subgrade soil, and the passive resistance developed from the soil around the footing. The frictional and passive resistance values presented assume the footings bear on the materials described in Section 4.2.2.

For passive resistance against the sides of the footings, the nominal passive values provided in Table 5 above may be used for design. For frictional resistance along the bottoms of footings, an unfactored coefficient of 0.6 may be used. LRFD Resistance Factors for determining limit state sliding and passive resistance are provided in Table 7.

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Table 7. LRFD Resistance Factors for Sliding Resistance

Limit State

Shear Resistance to

Sliding, φτ

Passive Pressure Resistance to Sliding, φep Service - -

Strength 0.8 0.5

Extreme 0.9 0.9

4.2.4 Bridge Abutment and Wingwall Drainage

Surface runoff from the paved roadway surface should be diverted or sloped away from bridge abutments and wingwalls to the extent possible. We recommend that structural fill directly behind the walls consist of at least a 12-inch-thick (measured laterally from back of abutment/wall) gravel curtain drain to convey water down to a perforated drainpipe at or slightly below the base of the wall. The curtain drain material should meet the requirements of Gravel Backfill for Walls, WSDOT Standard Specifications 9.03.12(2).

A woven geotextile separator meeting the requirements of Section 9-33.2(1), Table 3 of the WSDOT Standard Specifications should be included at the interface between the embankment fill and the gravel curtain drain behind the walls. Alternatively, a prefabricated composite drain mat may be used for drainage behind the walls.

4.3 Global Stability

We used limit equilibrium methods within Slide (Rocscience, 2018) to evaluate global stability of the preliminary bridge abutment, walls, and creek/river channel grading schemes under static and seismic conditions. The Slide program performs slope stability computations based on the modeled conditions and calculates a factor of safety against slope failure, which is defined as the ratio of resisting forces to driving forces. A factor of safety of 1.0 indicates a “just-stable” condition, and a factor of safety less than 1.0 would indicate unstable conditions. For global stability, we recommend a minimum factor of safety of 1.5 in the static and post-earthquake conditions, and minimum seismic factor of safety of 1.1 when considering the design seismic event (AASHTO 1,000-year return period earthquake ground motions).

Because the final configuration of the structures and channel has not yet been determined, the following assumptions were made for the purposes of the stability analyses:

• The creek/river channel side slopes are graded at a minimum of 2H:1V and the total height from the road surface to the stream level will not exceed 22 feet.

• Scour protection will be incorporated to prevent lateral migration of the stream channel towards the abutments and to prevent future conditions from exceeding a total embankment height of 22 feet.

• The groundwater level is at the stream level.

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• The driven piles penetrate a minimum of 5 feet into the alpine glacial drift underlying the alluvium and support an approximately 6-foot-tall concrete pile cap.

• Shallow spread footings for wingwalls are a minimum of 2-feet from the edge of the slope with the appropriate embedment.

• The analyses include a surface surcharge load of 250 psf at the top of the approach embankments.

Based on the analyses results, the minimum factor of safety for potential failure surfaces extending through the approach embankment exceed our recommended minimum factors of safety. However, we did identify potential for relatively shallow failures of the creek/river channel slopes during the seismic event. These potential failure surfaces did not extend past the pile cap and therefore pose minimal risk to the bridge structure, but could require future repairs following a seismic event.

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5 Construction Considerations We recommend that all foundation subgrade preparation, pile installations, final embankment grading, pavement subgrade preparation, and backfill and structural fill placement be evaluated by Aspect and completed in accordance with the WSDOT Standard Specifications (2020).

5.1 Earthwork

Based on the materials encountered in the explorations, and our understanding of the Project, we anticipate earthwork can be completed with standard construction equipment such as tracked excavators, dozers, and mobile cranes. The equipment should be capable of excavating oversize material such as large cobbles, stumps, and boulders. The existing road embankment is composed of fill and may also include oversize material such as boulders or debris like wood, refuse, or chunks of concrete and asphalt.

Appropriate erosion and sedimentation control measures should be implemented in accordance with local best management practices (BMPs) prior to beginning earthwork activities.

5.1.1 Temporary Excavation

Maintenance of safe working conditions, including temporary excavation stability, is the responsibility of the Contractor. All temporary cuts in excess of 4 feet in height that are not protected by trench boxes or otherwise shored, should be sloped in accordance with Part N of Washington Administrative Code 296-155 for worker safety (WAC, 2016).

For planning purposes and using guidance provided by the WAC and our site observations, we classify the embankment fill as Type C with a maximum allowable temporary slope inclination of 1.5H:1V, up to a maximum height of 20 feet. Near the base of the excavations, alluvium may be encountered and classifies as Type B with a maximum allowable temporary slope inclination of 1H:1V. When saturated, the maximum temporary cut slopes are anticipated to be 3H:1V. The cut slope inclinations estimated above are for planning purposes only.

With time and the presence of seepage and/or precipitation, the stability of temporary unsupported cut slopes can be significantly reduced. Therefore, the Contractor should protect all temporary slopes from erosion. The Contractor should monitor the stability of the temporary cut slopes and adjust the construction schedule and slope inclination accordingly. Vibrations created by traffic and construction equipment may cause caving and raveling of the temporary slopes.

5.1.2 Construction Dewatering

In our opinion, groundwater seepage and/or flow into the excavations is largely dependent on the water levels in the stream and the details of the stream diversion. Based on our explorations and observations and provided the streams are diverted during key construction activities, groundwater seepage into the excavations should be relatively minor. Excavations could encounter zones of seepage emanating from high-permeability layers of soil.

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The Contractor should be required to adequately dewater the excavations, so that the foundation subgrade preparation and structural fill placement can be completed in dry conditions. We anticipate that strategically placed sumps and pumps will sufficiently control water from inside the excavations. Sumps are often constructed by placing a short section of perforated corrugated steel pipe (or surplus 8- to 12-inch-diameter well screen) in a small hole excavated below the trench bottom elevation. The annular space around the pipe is backfilled with drain rock, with several inches placed inside the casing to help control the pumping of fines. Submersible pumps (trash pumps) are then placed inside the casing and connected to a central discharge pipe.

The Contractor should be responsible for design, implementation, and any necessary permits associated with any construction dewatering system used for the Project.

5.1.3 Subgrade Preparation

Subgrade preparation for footings, foundations, pavements, and areas receiving structural fill and new embankments should consist of the grubbing and removal of all existing non-structural fills, and other deleterious materials including demolition or construction debris. Existing road pavement should be removed or pulverized so it does not create a surface that might perch water. Exposed bearing surfaces should be evaluated by Aspect.

Any soft areas or area of unsuitable soils should be overexcavated to firm, undisturbed soil and backfilled with structural fill as defined below.

5.1.4 Structural Fill

Soils placed beneath or around foundations, behind walls, on slope faces, and below paved areas should be considered structural fill and conform to the material and compaction criteria described in the subsections below.

Use of On-Site Soils Due to the fines content, moisture content, and presence of trace organic debris in the existing embankment fill, we anticipate that most of this material will not be suitable for re-use as structural fill and will require excavation and offsite disposal. However, portions of the on-Site material may be considered for use as structural fill for backfill behind the abutments and wingwalls (outside of 12 inches from walls) if properly moisture conditioned and deleterious materials are removed such that the material meets the requirements of Gravel Borrow as specified in Section 9-03.14(1) of the WSDOT Standard Specifications.

Wet Weather Considerations Placement and compaction of structural fill during dry summer months is preferred, but the following considerations should be incorporated into the project requirements in the case that work is completed during wet weather.

• Earthwork should be performed in small areas to minimize exposure to wet weather.

• Excavation or the removal of unsuitable soils should be followed promptly by the placement and compaction of clean structural fill.

• The size and type of construction equipment used may have to be limited to prevent soil disturbance.

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• The ground surface within the construction area should be graded to promote runoff of surface water and to prevent the ponding of water.

• The ground surface within the construction area should be sealed by a smooth-drum vibratory roller, or equivalent, and under no circumstances should be left uncompacted and exposed to moisture. Soils that become too wet for compaction should be removed and replaced with clean granular materials.

• Excavation and placement of fill should be observed by Aspect, the geotechnical engineer, to verify that all unsuitable materials are removed and suitable compaction and Site drainage is achieved.

• Appropriate erosion and sedimentation best management practices (BMPs) should be strategically implemented in accordance with Washington State Department of Ecology (Ecology) and WSDOT recommendations.

Beneath Wingwalls Structural fill for the bearing pad beneath the proposed wingwalls should consist of a minimum of 12-inches of crushed rock consisting of Quarry Spalls as specified in Section 9 13.6 and/or Crushed Surfacing Base Course (CSBC) as specified in Section 9-03.9(3) of WSDOT Standard Specification.

Behind Walls Structural fill placed directly behind walls should consist of a 12-inch-thick curtain drain composed of Gravel Backfill for Walls, WSDOT Standard Specifications 9.03.12(2).

Beneath Pavement We recommend structural fill beneath new asphalt pavement consist of CSBC or crushed surfacing top course (CSTC) for the pavement base course. CSTC may be used over the CSBC for the upper 2 to 3 inches of the base course section or CSTC may be used for the full base course section. CSBC and CSTC material is specified in Section 9-03.9(3) of the WSDOT Standard Specifications.

Embankments Structural fill used for construction of the permanent embankments and extended backfill around culverts (beyond 12 inches from the walls) should meet the requirements for Gravel Borrow per the WSDOT Standard Specification…