STA Siuslaw Amendment 0004 USCG SIUSLAW GEOTECHNICAL REPORT (version 12-07-2023).pdf

Geotechnical Investigation

STA Siuslaw River Covered Moorings and

Correct Shoreline Erosion Florence, Oregon

December 7, 2023

Prepared for

TranSystems Corporation 2000 Center St, Suite 303

Berkeley, CA 94704

Prepared by

16520 SW Upper Boones Ferry Road, Suite 100

Tigard, OR 97224-7661

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

DRAFT

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Rpt December 7, 2023

TABLE OF CONTENTS

1 INTRODUCTION

2 PROJECT DESCRIPTION

3 SITE DESCRIPTION

3.1 Site Conditions

3.2 Geologic Setting

3.3 Seismicity

4 SUBSURFACE CONDITIONS

4.1 General

4.2 Soils

4.3 Groundwater

5 CONCLUSIONS AND RECOMMENDATIONS

5.1 General

5.2 Seismic Considerations

5.3 Foundation Support

5.4 Wall Design

5.5 Earthwork

6 LIMITATIONS

7 REFERENCES

TABLES

Table 5-1: Recommended Seismic Design Parameters for Site Class C and D Conditions Table 5-2: Nominal or Ultimate (i.e., Unfactored) Static and Seismic Compressive Axial Pile

Resistance for Plumb and Battered 16- and 20-Inch Diameter Open-Ended Steel Pipe Piles

Table 5-3: Soil Properties for L-PILE Analysis Table 5-4: Lateral Pile Group Analysis Table 5-5: Recommended Static Earth Pressure Coefficients for New Walls Table 5-6: Recommended Seismic Earth Pressure Coefficients for New Walls for 1,000 Year

Return Period

FIGURES

Figure 1: Vicinity Map Figure 2: Site Plan Figure 3: Surcharge-Induced Lateral Pressure

December 7, 2023

APPENDICES

Appendix A: Field Explorations and Laboratory Testing Appendix B: Previous Explorations by Others Appendix C: Geoprofessional Business Association Guidance Document

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1 INTRODUCTION

At your request, GRI completed a geotechnical investigation for the above-referenced project located near Florence, Oregon. The general project location is shown on the Vicinity Map (Figure 1). Our investigation included a review of available subsurface and geologic information for the project site and surrounding area, upland and in-water subsurface explorations, laboratory testing, and engineering analyses. This report describes the work accomplished and provides our geotechnical conclusions and recommendations for the design and construction of the project.

2 PROJECT DESCRIPTION

The United States Coast Guard (USCG) first established Station Siuslaw River in 1917. The Station’s primary mission is to provide search and rescue to commercial mariners, recreational boaters, and surfers along the Oregon coast between Cape Perpetua and the Siltcoos River. The station also frequently escorts fishing vessels across the Siuslaw River bar and provides marine environmental protection and fisheries conservation enforcement to the area. The station is home to various onshore and offshore facilities, including a pile-supported boathouse and floating dock connected to land via a small bridgeway, and a debris screen located upstream. Many of the existing offshore structures are approaching the end of their useful service life. Onshore, the station contains a main operations and administration building, on-site housing, facilities and equipment maintenance buildings, fuel storage, parking lots, and a water retention facility. Along the shoreline, a steep slope separates the in-water structures from the upland areas of the site. Ongoing attrition of the riverbank and shoreline retreat due to scour from changing river dynamics, long-term bed degradation, lateral channel bank migration, and surface water runoff threaten the aging in-water infrastructure and upland facilities at the station. Scour has significantly reduced embedment of the pile-supported in-water structures, and temporary repairs have been required.

The purpose of the project is to preserve the USCG’s long-term waterfront operations by addressing ongoing degradation of the riverbank, replacing aging in-water structures that threaten the ability of the station to continue to meet its current response standard, and update infrastructure to accommodate new equipment. More specifically, the project includes the demolition of the existing boathouse and construction of a new modernized covered mooring with an interior floating dock designed to accommodate two 47-foot surf-ready motor lifeboats (MLBs), a new exterior floating dock designated to accommodate one 29-foot response boat-small (RB-S II), an access bridgeway, and a new debris screen. The new covered mooring will be approximately 4,700 square feet and slightly larger than the existing boathouse in the same general footprint and orientation.

As currently planned, the new in-water structures will be supported by 16- and 20-inch-diameter steel pipe piles. Structural loading information for the new structures has not

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Report Page 2 December 7, 2023 been provided at this time. To accommodate estimated sea level rise over the design life of the structure, the main deck surface for the new covered mooring will be 1.5 feet higher than the existing boathouse at elevation +16.5 feet (NAVD88).

A design alternatives and analysis report prepared by Reid Middleton in 2018 (Reid Middleton, 2018) looked at a number of different alternatives to address ongoing erosion of the riverbank, concluding the construction of a tubular interlocking pile wall near the shoreline would provide the best engineering design with minimal environmental impact.

Based on conversations with the USCG and the permitting agencies, we understand the installation of additional slope armoring is not feasible at this time. For that reason, riverbank improvements will include a new 400-foot-long tied-back interlocking steel sheetpile retaining wall along the shoreline, an approximately 70-foot-long cantilevered soldier pile retaining wall at top of the riverbank near a parking area in the southwest corner of the property, regrading oversteepened portions of the riverbank slope, and site drainage improvements. The parking lot wall will have a maximum exposed height of about 7 feet. The shoreline wall will have a maximum exposed height of about 18 feet post-construction and will contain 30-foot-long returns up the slope at either end. The interlocking pile retaining wall is planned to be installed just upslope of an existing riprap section. The top of the new shoreline wall will be established between about elevation +19 feet and +30 feet (NAVD88), above the highest measured tide (HMT) and 50-year calculated extreme tide at the site. As discussed further in the groundwater section, the groundwater conditions behind the wall will be influenced tidally and by upslope groundwater. A drainage system outletting through the wall will be installed to provide drainage of water upslope of the wall and reduce the amount of differential head (hydrostatic pressure) on the wall. The shoreline wall and new in-water structures will be designed to accommodate future riverbed and bank scour, estimated by TranSystems over a period of about 75 years. Based on discussions with the design team, we understand the new shoreline wall will be seismically designed based on ground motions associated with a 1,000-year return period (about 7% probability of exceedance in 75 years). We anticipate the new in-water structures will be designed in accordance with American Society of Civil Engineers (ASCE) 61-14, Seismic Design of Piers and Wharves. Designs for both the in-water structures and new shoreline wall will utilize ground motion values based on the 2014 U.S. Geological Survey (USGS) seismic hazard maps, which are incorporated in current code cycles.

Appledore Marine Engineering (AME) is supporting the project with structural design of the new in-water structures, while TranSystems is supporting the structural design of the new retaining walls.

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3 SITE DESCRIPTION

3.1 Site Conditions

Station Siuslaw River is situated along the east bank of the Siuslaw River, approximately 3 miles northwest of the city of Florence, Oregon, and 1.5 miles inland from the Pacific Ocean. The station is surrounded by residential houses to the north, east, and south. The majority of the station’s facilities are located on a bluff that is about 45 feet to 50 feet above the river at about elevation +50 feet (NAVD88). Accessible via a narrow bridgeway located near the waterline, an existing timber and steel pile-supported boathouse, floating dock, and debris screen are located about 80 feet west-southwest of the shoreline. Site surveys completed by others indicate the elevation of the mudline in the vicinity of the in-water structures ranges from about -20 feet to -30 feet (NAVD88). A steep, heavily vegetated slope with average grades approaching about 1.5H:1V (Horizontal to Vertical) or steeper exists between the river and upland portions of the site. The slope is somewhat flatter (2H:1V) in some locations below the waterline. A single-lane road traverses the slope between the bridgeway and the upland areas of the site.

The Siuslaw River is subject to significant tidal fluctuations and relatively strong currents, and scour has been a significant issue at the site for several decades. In addition, previous studies at the site have indicated groundwater seepage and surface water erosion have contributed to slope movements at the site. Examples of slope distress historically observed at the site include surface cracking near the crest of the slope indicative of progressive slope failure, scarps resulting from localized slope instability, sinkholes, and damage to nearby roadway and parking infrastructure. Remedial measures to reduce the ongoing erosion have been taken in recent years, including the installation of riprap at typical tidal levels along portions of the shoreline in 2016 and the construction of a water retention and dewatering facility in 2019. This newer water retention facility is located on the north side of the bridgeway/boathouse access road and consists of a catchment basin built into the hillside. An array of horizontal drains extends northeast to southeast from the facility, and an outfall pipe transverses the slope down toward the river. During visits to the site, station personnel reported that several feet of natural scour/erosion have occurred at the site and no maintenance dredging has been performed to their knowledge.

Recent scour studies completed by TranSystems estimate an average lateral rate of erosion of the submerged riverbank on the order of 0.5 feet per year, and vertical scour in excess of 15 feet is anticipated to occur at the site over the next 75 years.

3.2 Geologic Setting

The site is located at the western edge of the coastal headlands of the central Coast Range and lies along an active convergent tectonic plate boundary, known as the Cascadia Subduction Zone (CSZ). Formation of the Coast Range began around 50 million years ago, when a series of volcanic island chains made up of submarine and subaerial basalt flows

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Report Page 4 December 7, 2023 with sedimentary interbeds collided with the North American crustal plate. Known as the Siletz Terrane, this group of accreted rocks, along with other Cenozoic terranes, forms the basement rock of the Coast Range. As this process occurred, the basin area between the approaching terranes and the coastline, referred to as the Tyee Basin, began accumulating sediment, eventually forming rock of the Tyee Formation as the area transitioned from a subduction basin to a forearc basin. Ongoing collision along the plate boundary that continues to present day has led to faulting, folding, uplift, and rotation of the rocks, bringing the Tyee Formation closer to the ground surface. Convergence of these plates dominates the regional tectonics and results in megathrust earthquakes. Offshore, subduction causes a deformation zone along the western edge of the accretionary wedge complex, strike-slip faulting in the North American plate, and a zone of folding extending from the coast westward. Onshore, the major tectonic elements associated with the subduction zone include a deformed forearc basin (the Coast Range and Willamette Valley), a volcanic arc complex (the Cascade Range), and a backarc.

The underlying bedrock structure at the site is concealed by Quaternary and Pleistocene sedimentary units derived from wind and river depositional processes. Part of the Florence Dune Sheet, the near-surface soils at the site generally consist of unconsolidated dune sand, lightly consolidated sand, and silt and estuarine deposits including clay, silt, and sand (Hampton, 1963; Schlicker et al., 1974). In general, the sand is initially somewhat loose and gradually increases in relative density with increasing depth. The sand may exhibit some localized cementation at depth as well. Paleosol, or buried soil horizons, are present within the sand. The active channel of the Siuslaw River is underlain by Holocene unconsolidated silt, sand, and gravel. No specific record of historical fill placement has been made available at this time; however, we anticipate some fill is likely present as a result of development of the site.

3.3 Seismicity

In general, due to a lack of historical seismicity and limited published information regarding many of the potential seismic sources in the area, a complete characterization of the seismic hazard for this project is not possible without a detailed, in-depth study, which is beyond our current scope. In this regard, the following discussion is intended to provide the design team with a general description of the overall anticipated seismic hazard at the site. Historically, the site is located in a region with relatively low seismic activity compared to neighboring states of Washington and California; however, geologic evidence suggests that large megathrust earthquakes have repeatedly occurred in the region during the Holocene epoch, all but one of which were prior to written historical record (Atwater et al., 1995). This evidence has been presented in the form of paleoseismic investigations in the Pacific Northwest, describing coastal subsidence and turbidities or submerged seismically induced landslides. Additional evidence of seismic activity in the

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Report Page 5 December 7, 2023

Pacific Northwest has been provided as written accounts of inundation of Japanese harbors by an orphan tsunami due to a distant subduction zone earthquake in about the year 1700.

On a regional scale, the site lies on the western shore of the coastal headlands of the central Coast Range, approximately 40 kilometers inland from the surface expression of the CSZ, a broad, eastward-dipping zone of contact between the upper portion of the subducting slabs of the Gorda, Juan de Fuca, and Explorer plates, and the overriding North American Plate (Goldfinger et al., 2012). On a local scale, a complex pattern of strike-slip, shallow crustal faults is present throughout the area. Our review of the USGS Quaternary Fault and Fold Database indicates there are several Class A faults within about 30 miles of the site, including those that are part of the Cascadia fold and fault belt and an unnamed Siuslaw River anticline. Class A faults are defined as those in which geologic evidence has demonstrated their existence. Although the location and extent of several faults have been inferred from stratigraphic and geophysical evidence, clear evidence regarding recent seismic activity is not available, and large uncertainties associated with any estimates of probable magnitude, location, and frequency of occurrence of earthquake that might affect the site currently exist. The information that is available, however, indicates the potential seismic sources that may affect the site can be grouped into three independent categories: CSZ events related to sudden slip between the upper surface of the Juan de Fuca Plate and the lower surface of the North American Plate, subcrustal events related to deformation and volume changes within the deeper subducted mass of the Juan de Fuca Plate, and local crustal events associated with movement on shallow, local faults. All three of these earthquakes contribute to the seismic hazard at the site; however, evaluation of potential liquefaction and lateral spreading hazards in the project area is generally controlled by local crustal and CSZ events.

4 SUBSURFACE CONDITIONS

4.1 General

Subsurface materials and conditions at the site were investigated in two separate mobilizations. The first mobilization included the completion of one upland boring near the northwest corner of the site on April 6, 2023, designated boring B-1. The second mobilization included the completion of five overwater borings in the vicinity of the existing boathouse, floating dock, and debris screen between August 15 and 25, 2023, designated as borings B-2 through B-6. Boring B-1 was advanced to a maximum depth of about 102 feet below existing site grades (elevation -57 feet). Borings B-2 through B-6 were advanced to depths ranging from about 80 feet to about 94 feet below mudline (elevation -104 feet to -123 feet). Approximate locations of the upland and overwater borings are presented on the Site Plan (Figure 2). A log of the materials and conditions disclosed by the borings is provided on Figures 1A through 6A. A discussion of the field-

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Report Page 6 December 7, 2023 exploration program completed for this project is provided in Appendix A. The terms and symbols used to describe the soils and rock, encountered in the borings are defined in Tables 1A and 2A and the attached legend. Logs from previous explorations by GRI and others are provided in Appendix B.

4.2 Soils

The soils disclosed by the borings are generally consistent with those encountered during previous investigations. In general, the site is mantled with sand fill, followed by dune and estuarine sands underlain by sedimentary rock consisting of siltstone.

For the purpose of discussion, the materials disclosed by explorations completed at the project site have been grouped into categories based on their physical characteristics and engineering properties:

a. SAND (Fill)

b. SAND (Dune Sand)

c. SILT (Paleosol)

d. SAND (Estuarine Deposits)

e. SILTSTONE (Tyee Formation)

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

a. SAND (Fill) Sand fill was encountered at the ground surface in boring B-1 and extended to a depth of about 2 feet below existing site grades. The sand is fine grained, poorly-graded, colored light brown, and contains some silt, gravel, and cobbles. The relative density of the sand fill appears to be typically loose to medium dense.

b. SAND (Dune Sand) Dune sand was encountered below the fill in boring B-1 and extended to a depth of about 38 feet below existing site grades (elevation +7 feet). The sand is fine to medium grained, typically contains trace to some silt, and colored light brown to gray. The relative density of the sand steadily increases with depth, from medium dense to very dense.

c. SILT (Paleosol) A relatively impermeable layer of silt, referred to as a Paleosol unit, was encountered in boring B-1 at a depth of about 38 feet below existing site grades. The silt contains trace to some sand, is colored dark gray, has a relative consistency of medium stiff, and contains organics. The Paleosol unit encountered in boring B-1 extended to a depth of about

41.5 feet below existing site grades. The Paleosol unit appears to be present over large portions of the site, as it was encountered in several of the previously completed borings

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Report Page 7 December 7, 2023 and is visible at low tide in some locations along the shoreline. In general, the Paleosol appears to gently slope downward from east to west toward the river.

d. SAND (Estuarine Deposits) Sand was encountered below the Paleosol unit in boring B-1 and below the mudline in borings B-2 through B-6. Although different depositional histories may exist between the sand encountered below the Paleosol in the upland boring and the sand encountered in the in-water borings, they are grouped together into a single unit, referred to as Estuarine Deposits, based on their similar composition, overall estimated relative densities, and anticipated behavior. Sand was encountered in boring B-1 to the maximum depth explored of about 102 feet below existing site grades (elevation -57 feet). The sand in borings B-2 through B-6 was encountered to depths ranging from about 62 feet to 77 feet (elevation -88 feet to -94 feet) below mudline. The sand is typically fine-grained, colored light brown to gray mottled rust, and contains trace to some silt. Zones of weak cementation and higher silt content were encountered in the sand, as were relatively thin interbedded layers of silt with varying amounts of clay and organics. Zones of wood debris and trace amounts of gravel were encountered throughout the sand. The relative density of the sand ranges from medium dense to very dense and is typically dense.

e. SILTSTONE (Tyee Formation) Siltstone of the Tyee Formation was encountered beneath the estuarine sand in each of the in-water borings at depths ranging from about 62 feet to 77 feet below mudline. The siltstone is typically dark gray, decomposed, and extremely soft (R0). Borings B-2 through B-6 were terminated in the siltstone at depths ranging from about 80 feet to 94 feet below mudline (elevation -104 feet to -123 feet).

4.3 Groundwater

The materials encountered at the site are generally associated with relatively high hydraulic conductivity or permeability. Due to the close proximity of the Siuslaw River and the Pacific Ocean, we anticipate groundwater levels at the site will fluctuate primarily in response to changes in the tidally influenced river as well as seasonal precipitation and heavy rainfall events. Tidal elevations in the river typically range from about +0 feet to +10 feet.

Additionally, water has been observed exiting the toe of the slope in the form of springs following periods of heavy rainfall. In general, surface water infiltrating the slope and upland portions of the site is anticipated to collect and flow along the top of the relatively impermeable silt Paleosol before eventually daylighting near the shoreline. Previous studies on site report that hydraulic gradients develop in the sand above the Paleosol, particularly during periods of significant rainfall coupled with lower tidal elevations and contribute to material piping and erosion observed at the site.

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An open-standpipe piezometer was installed in boring B-1, located near the top of the riverbank near the northern boundary of the site. Groundwater measurements following completion of the boring indicated the groundwater level was about 34 feet below existing site grades (elevation +11 feet). Groundwater data previously collected at the site by others indicate the groundwater level ranges from about elevation +8 feet to about +22 feet. During completion of the subsequent overwater explorations, a piezometer with an internal data logger was installed in the open standpipe. Data from this instrument will be downloaded and processed during our next visit to the site.

5 CONCLUSIONS AND RECOMMENDATIONS

5.1 General

The explorations completed for this investigation indicate the upland areas of the project site are mantled with a relatively thin layer of sand fill underlain by dune sand, referred to as Dune Sand, extending to an elevation of about +7 feet (NAVD88). This unit is underlain by a 5-foot-thick, relatively impermeable layer of silt with clay and organics, referred to as a Paleosol. Sand, referred to as Estuarine Deposits, is present beneath the Paleosol and below the mudline in the in-water portions of the site. The sand encountered in the borings is generally dense and contains zones of weak cementation, interbedded layers of silt, and wood debris. Extremely soft (R0) siltstone underlies the sand at about elevation -90 feet.

Groundwater at the site is complex and is influenced tidally by the Siuslaw River, precipitation, and can be perched above the silt Paleosol.

Significant geotechnical considerations for the project include the driven piles, constructability of the shoreline wall and associated tiebacks relative to the existing oversteepened riverbank slope, proximity of existing infrastructure, and managing groundwater conditions. Although the findings of our geotechnical investigation indicate the risk of seismically induced liquefaction at the site is low, the region is associated with high seismicity, and the new walls will need to be designed to accommodate relatively high seismic earth pressures. The following sections of this report provide our geotechnical conclusions and recommendations for design and construction of the project.

5.2 Seismic Considerations

5.2.1 General

We understand the new in-water structures including the new covered mooring, floating dock, and debris screen will be designed in accordance with ASCE 61-14, Seismic Design of Piers and Wharves (ASCE 61-14), and the corresponding “low” design classification. For structures designed for the “low” design classification, ASCE 61-14 defines ground motions for one seismic hazard level: the Design Earthquake (DE). The DE hazard level is defined per ASCE 7-05, Minimum Design Loads for Buildings and Other Structures (ASCE 7-05), which develops the response spectra based on ground motions associated with the

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Maximum Considered Earthquake (MCE), which is generally represented by a probabilistic earthquake with a 2% probability of exceedance in 50 years (i.e., 2,475-year return period).

The design-level response spectrum representing the DE is obtained by taking two-thirds of the MCE-level ground motions and represents a performance level of life safety protection. ASCE 7-05 serves as the reference document for ASCE 61-14. We understand the boathouse structure on top of the dock will be designed in accordance with the methodology outlined in ASCE 7. To facilitate more consistency through design, the more recent mapped ground motions and site coefficients provided in ASCE 7-16 were used to develop the design earthquake values for the dock and boathouse structure. It should be noted that the ASCE 7-16 based values utilize risk and directivity coefficients that were not considered in the ASCE 7-05 approach but the resulting changes are relatively small. We understand the new shoreline wall will be designed based on ground motions associated with a 1,000 year return period in accordance with American Association of State Highway and Transportation Officials (AASHTO) guidelines.

The ASCE and AASHTO methodologies for developing design response spectra use two spectral response parameters, SS and S1, corresponding to periods of about 0.2 second and 1.0 second. The approximate latitude and longitude coordinates for the site are 44.0022° N and 124.1229° W. The SS and S1 parameters for the in-water structures and shoreline wall are 1.43 g and 0.75 g and 0.89 g and 0.39 g, respectively, for Site Class B/C boundary conditions. To establish the ground-surface response spectra, these bedrock spectral parameters are adjusted for soil site class using the short- and long-period site coefficients, Fa and Fv. For the in-water structures, the design-level response spectrum is calculated as two-thirds of the ground-surface MCER spectrum.

The results of the explorations completed for this project indicate that the soil profile in the upland portions of the site is typically representative of Site Class D conditions. The in-water explorations completed for the project indicate the soil profile in the offshore portions of the site is typically representative of Site Class C conditions. The recommended MCER and design spectral response parameters for Site Class C and D conditions are tabulated below.

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Table 5-1: RECOMMENDED SEISMIC DESIGN PARAMETERS FOR SITE CLASS C AND D CONDITIONS

Seismic Variable In-water Structures

(ASCE 7)

Shoreline Wall (1,000 year hazard level

[AASHTO])

Site Class C D

MCER Spectral Response Acceleration Parameter at Short Periods, SMS

1.72 g -

MCER Spectral Response Acceleration Parameter at 1.0-Sec Period, SM1

1.05 g -

Design Spectral Response Acceleration Parameter at Short Periods, SDS

1.15 g 1.01 g

Design Spectral Response Acceleration Parameter at 1.0-Sec Period, SD1

0.70 g 0.74 g

The following sections provide additional details regarding the anticipated seismic behavior of the on-site soils.

5.2.2 Liquefaction

Liquefaction is a process through which loose, saturated, granular materials, such as sand, and non-plastic and lower-plasticity silts temporarily lose strength during and immediately after a seismic event. Liquefaction occurs as seismic shear stresses propagate through saturated soil and distort the soil structure, causing loosely packed groups of particles to contract or collapse. If drainage is impeded and cannot occur quickly, the collapsing soil structure increases the pore-water pressure between the soil grains. If the pore-water pressure increases to a level approaching the weight of the overlying soil, the soil temporarily behaves as a viscous liquid rather than a solid.

The potential for liquefaction is typically evaluated using a simplified method that compares the earthquake-induced cyclic shear stresses within the soil profile (demand) to the cyclic shear strength of the soil available to resist these stresses (resistance). Estimates of seismically induced stresses are based on earthquake moment magnitude (MW) and peak ground acceleration (PGA). The cyclic resistance of soils is dependent on several factors, including the number of loading cycles, relative density or consistency, confining stress, plasticity, natural water content, stress history, age, depositional environment (fabric), and composition. For the sand-like soils encountered on site, the cyclic resistance is typically evaluated using Standard Penetration Test (SPT) N-values or cone penetration test (CPT) probe tip-resistance values normalized for overburden pressures and corrected for factors that influence cyclic resistance, such as fines content.

Our assessment of seismically induced liquefaction at the site was completed in accordance with the DE values based on the ASCE 7-05 methodology and at the 1,000-year

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Report Page 11 December 7, 2023 hazard level. In accordance with the study by Finn and Wightman (2007), the site-specific mean magnitude was utilized with the site-specific probabilistic PGA for initial liquefaction screening. Based on our review of the 2014 USGS disaggregation for the site, the mean Mw associated with the site is 8.8.

The potential for liquefaction at the site was evaluated using the methods recommended by Idriss and Boulanger (2008), with subsequent revisions by Boulanger and Idriss (2014).

For this procedure, the earthquake-induced cyclic shear stresses with the soil profile were estimated using the magnitude-acceleration pair previously discussed. Soil resistance was estimated based on information disclosed by the subsurface explorations and laboratory testing programs completed for this project. Our evaluation indicates the risk of liquefaction at the site is low.

5.2.3 Other Seismic Hazards

5.2.3.1 Fault Rupture

Based on our review of regional geology and the absence of mapped faults near the site, we have not identified a risk of ground surface fault rupture at the site.

5.2.3.2 Coseismic Subsidence

Modeling of megathrust earthquake ruptures on the CSZ indicates sequences of interseismic uplift and coseismic coastal subsidence. The predictions for coastal subsidence are confirmed by geologic evidence following the 1,700 CSZ earthquake, such as submerged trees and buried intertidal marshes. This repeated coastal subsidence pattern has been documented along the length of the CSZ (Atwater et al., 1995).

Subsidence estimates for the area have been updated as part of the recent updated tsunami modeling completed for the Southern Oregon Coast (Witter et al., 2011). This study indicates the maximum subsidence for the most likely scenario considered for this study (M1) is on the order of 2 meters (6.6 feet). ASCE 7-16 also presents profiles of coastal deformations from northern California to southern British Columbia based on geologic information. Based on our review of the online ASCE 7-16 Tsunami Hazard Tool, the estimated 2,475-year probabilistic subsidence at the site is about 5.3 feet.

5.2.3.3 Tsunami Hazard

The west coast of the United States has historically been subject to inundation from tsunamis generated by distant earthquakes in South America, Alaska, and Japan. Kelsey et

al. (2005) noted that tsunamis generated from these distant subduction zone earthquakes have minor inundation effects because of the long diagonal approach of tsunami waves to the West Coast from these sources. Based on this explanation, observations made around the Indian Ocean following the 2004 megathrust Sumatran earthquake, and recent modeling (Oregon Department of Geology and Mineral Industries [DOGAMI], 2013), a

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Report Page 12 December 7, 2023 tsunami generated by a megathrust earthquake on the CSZ will present the greatest tsunami inundation risk at the site.

ASCE 7-16 presents probabilistic 2,475-year hazard maps for tsunami-prone regions and states, including Oregon, Washington, California, Alaska, and Hawaii. Based on our review of this information, there is a risk for tsunami inundation at the site with estimated runup elevations of about 25 feet (NAVD88) and estimated inundation depths along the nearby coastline of about 50 feet to 60 feet.

5.3 Foundation Support

5.3.1 General

As currently planned, the new in-water structures, including the covered mooring, floating dock, and debris screen, will be supported by 16- and 20-inch-diameter concrete-filled steel pipe piles. We understand both plumb and 4V:1H battered piles will be utilized.

Design loads for the new piles have yet to be provided. We anticipate the new piles will be installed open-ended; however, based on discussions with AME, we understand piles with a plate installed partway up the inside of the pile (i.e., a “plug plate”) may also be considered to increase end bearing resistance and facilitate concrete infilling.

5.3.2 Axial Pile Capacity

Our review of the available project plans indicates the main deck surface for the new covered mooring will be at elevation +16.5 feet (NAVD88). Site surveys completed by others indicate the elevation of the mudline in the vicinity of the in-water structures ranges from about -20 feet to -30 feet (NAVD88). The explorations completed in the vicinity of the new covered mooring, floating dock, bridgeway, and debris screen indicate siltstone is present at about elevation -90 feet. We understand the design of all new piles will include an allowance for riverbed scour over the facility’s design life based on the 2023 study prepared by TranSystems. Based on information provided by the design team, we understand up to about 16 feet of scour is anticipated to occur in the vicinity of the new covered mooring and floating dock and about 8 feet of scour is anticipated to occur at the debris screen. Between about 3 feet and 16 feet of scour is anticipated to occur along the new bridgeway depending on proximity to the shoreline. Based on this information, we estimate the amount of sand overburden between the future scoured mudline and the underlying siltstone at the site will be about 45 feet to 50 feet at the new covered mooring and floating dock, and about 55 feet to 65 feet at the debris screen. We anticipate the amount of sand overburden below the anticipated future scour elevation at the bridgeway, piles will range from about 45 feet to greater than 70 feet near the shoreline. Table 5-2 presents the estimated nominal or ultimate (i.e., unfactored) compressive axial pile resistance for plumb and battered 16- and 20-inch-diameter open-ended steel pipe piles as a function of depth below the scoured mudline. As discussed in Section 5.2 of this

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Report Page 13 December 7, 2023 report, we anticipate the risk of liquefaction and soil strength loss at the site is low based on conditions disclosed by the explorations completed at the site; therefore, the estimated static and seismic pile resistances presented below are identical.

Table 5-2: NOMINAL OR ULTIMATE (i.e., UNFACTORED) STATIC AND SEISMIC COMPRESSIVE AXIAL PILE RESISTANCE FOR PLUMB AND BATTERED 16- AND 20-INCH DIAMETER OPEN-ENDED STEEL PIPE PILES

Depth Below Scour Elevation, feet

Estimated Nominal or Ultimate Static and Seismic Compressive Axial Pile Resistance, kips

16-Inch-Diameter Open-Ended Steel Pipe Piles

20-Inch-Diameter Open-Ended Steel Pipe Piles

30 200 250

35 260 320

40 320 400

451,3 380 470

50 430 550

552 460 620

60 490 680

65 520 710

Notes:

1 Estimated depth to rock below scoured mudline for covered mooring and floating dock piles = 45 to 50 feet.

2 Estimated depth to rock below scoured mudline for debris screen piles = 55 to 65 feet.

3 Estimated depth to rock below scoured mudline for bridgeway piles = 45 to 70+ feet.

The values presented in the table above are suitable for open-ended steel pipe piles tipped in dense to very dense sand. Linear interpolation can be used to estimate axial pile resistance at depths between the values shown in the table above. The estimated compressive axial capacities presented above can be increased by 60 kips and 100 kips for 16- and 20-inch-diameter open-end steel pipe piles installed a minimum of 5 feet into the underlying siltstone, respectively. If additional embedment into siltstone is required, GRI can provide additional recommendations for pile capacity. Axial pile resistance in tension (i.e., uplift) can be estimated by applying a factor equal to 0.35 to the resistances provided in the table above. Estimated resistances for piles installed using plug plates can be provided upon request as the design progresses. Closed-end pipe piles could also be considered to achieve larger capacities at a given depth; however, the installation of these piles may be difficult and require significantly more impact driving.

Assuming Pile Driving Analyzer (PDA) testing will be completed on a series of representative piles, we recommend a factor of safety of at least 2 for dead load plus frequently applied live loads and 1.5 for the total of all loads, dead, live, and wind or seismic. Alternatively, piles being designed within the LRFD framework can utilize a

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Report Page 14 December 7, 2023 resistance factor equal to 0.65 for compression and 0.5 for uplift provided PDA testing is completed on at least two production piles. The PDA data can subsequently be used to confirm required pile embedment and terminal driving criteria.

5.3.3 Lateral Pile Capacity

Lateral structural loads can be resisted by the piles in bending. The lateral load behavior of the piles can be analyzed using the computer program L-PILE by Ensoft, Inc. We recommend using the input parameters summarized in the following table to model the soils at the site.

Table 5-3: SOIL PROPERTIES FOR L-PILE ANALYSIS

Soil/Rock Unit

L-PILE

Soil Type Condition

Soil Properties K, pci γ’, pcf φ c, psf ε50

Submerged

Sand Sand (Reese) Static & Seismic 125 63 36 N/A N/A

It should be noted that L-PILE parameters provided above are for single, isolated piles.

Depending on the direction of the loading and orientation of the piles, group effects should be considered for spacing less than five pile diameters. This reduction is often applied as a p-multiplier. L-PILE uses a p-multiplier as a reduction of the kh value for pile spacing less than five pile diameters. The following table provides a summary of p-multipliers for various center to center pile spacing.

Table 5-4: LATERAL PILE GROUP ANALYSIS

Center-to- Center

Pile Spacing

Calculated P- Multipliers for Rows 1, 2, and 3+

3d 0.80, 0.40, 0.30

4d 0.90, 0.65, 0.50

5d 1.0, 0.85, 0.70

5.3.4 Construction Considerations

In our opinion, the primary risks associated with the installation of new in-water piles at the site are vibration-sensitive soils that mantle the nearby oversteepened riverbank slope, damage to nearby existing structures from vibration and/or settlement, and the potential to encounter practical refusal in dense sand prior to reaching the required pile embedment.

We anticipate the preferred method of installation for the piles for the new in-water structures will be vibratory techniques. Our review of installation records from piles

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Report Page 15 December 7, 2023 previously installed during retrofit of the existing boathouse indicates vibratory installation techniques were able to be used to successfully install H-piles into the deeper, dense sands at the site. Based on our review of the available installation records, which indicate a relatively slow rate of pile penetration, and our experience with similar site conditions, we anticipate vibratory installation techniques can be used to install the new steel pipe piles to partial depth; however, some impact driving will likely be required to complete the installation. The amount of impact driving will depend primarily on the required depth of embedment, the size of vibratory hammer, and whether plug plates are utilized. In this regard, it should be understood that the piles will be subject to resistance from soils present above the assumed scoured mudline at the time of installation; this resistance will need to be considered when selecting an appropriate hammer to install the piles. GRI should review the contractor’s proposed hammers prior to mobilization to site.

We understand a passive cathodic protection system and protective coating will be applied to the new in-water piles. In this regard, it should be noted that the estimated pile capacities provided in Table 5-2 contain some uncertainty. Variations in the actual required pile length from the values estimated in Table 5-2 should be expected. In our opinion, the contracting methodologies for the project should have considerations for pre-negotiated splice costs and over-coating for variations in tip elevation on the order of 10 feet to 20 feet. The over-coating allowance should reduce the risk of needing to apply additional coating in the field should practical refusal be encountered at depths shallower than anticipated due to higher soil resistance.

There is some risk of encountering obstructions and shallow practical refusal, particularly for piles installed near existing riprap. To avoid damage to the pile during installation, driving stresses should not exceed 0.9 times the yield strength for steel piles. Utilizing tip protection for the new in-water piles is recommended.

We recommend implementing a vibration monitoring program in the upland areas of the site near sensitive infrastructure and neighboring residential homes to monitor ground motions and assist the contractor with determining when typical thresholds for damage have been exceeded during pile installation. In addition, some settlement of nearby structures and utilities may occur as a result of new pile installation. This may be particularly important for any existing in-water piles that are to remain following construction of the new in-water structures. GRI can assist with evaluating potential settlement of existing structures if necessary.

We anticipate construction of the new shoreline wall and upland regrading efforts will take place prior to construction of the new in-water structures. Regrading to flatten the existing riverbank slope will help reduce the risk of significant slope failures during construction;

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Report Page 16 December 7, 2023 however, some minor sloughing should be anticipated during pile installation. We recommend the contractor monitor the riverbank and immediately upland for any signs of significant slope movement during construction.

5.4 Wall Design

5.4.1 General

As currently planned, the project includes the construction of two new walls: a 400-foot-long tied-back interlocking sheetpile shoreline wall and a shorter 70-foot-long cantilevered soldier pile retaining wall near the crest of the slope in the southwest corner of the station property. The interlocking sheetpile retaining wall will be installed immediately upslope of an existing riprap section. The top of the wall will be established between about elevation +19 feet and +30 feet (NAVD88), above the highest measured tide (HMT) and 50-year calculated extreme tide at the site. We understand finished site grades in front of the shoreline wall will be at about elevation +12 feet; however, like the piles for the new in-water structures, we understand the shoreline wall will be designed for significant future scour corresponding to elevations on the order of -10 feet to -15 feet (NAVD88). The top of the parking lot wall will be established at about elevation +46 feet and have an exposed height of about 7 feet.

5.4.2 Lateral Earth Pressures

The required embedment depths for the new walls are currently not known. Based on conversations with the design team and our review of the preliminary grading plans, we understand the ground surface above the new parking lot retaining wall will be flat. The ground surface downslope of the new parking lot wall and upslope of the new shoreline wall will be regraded to achieve a 2H:1V slope. As previously mentioned, we understand some areas along the riverbank may retain their existing slope angles steeper than a 2H:1V to preserve the existing vegetation. It is our understanding these areas will not be located in the immediate proximity of the new walls. Our review of the scour report prepared by TranSystems indicates the configuration of the slope in front of the new shoreline wall will vary somewhat from about 2H:1V to about 10H:1V. Design lateral earth pressures for the new walls for the static condition for different sloping ground conditions can be evaluated using the lateral earth pressure coefficients provided Table 5-5. Additional loading due to surcharge loads should be added in accordance with the criteria shown on Surcharge- Induced Lateral Pressure (Figure 3).

GRI 6839-A – STA Siuslaw River Covered Moorings and Correct Shoreline Erosion Geotechnical Report Page 17 December 7, 2023

Table 5-5: RECOMMENDED STATIC EARTH PRESSURE COEFFICIENTS FOR NEW WALLS

Soil Unit

Active Earth Pressure Coefficient, ka

Flat Ground 2H:1V Slope

Dune Sand 0.28 0.40

Estuarine Deposits 0.24 -

Soil Unit

Passive Earth Pressure Coefficient, kp Flat

Ground 2H:1V Slope 4H:1V Slope 8H:1V Slope 10H:1V Slope

Dune Sand 6.7 1.6 - - -

Estuarine Deposits 8.4 1.8 4.2 6.6 7.2

The values provided in Table 5-5 should be considered in conjunction with total unit weights equal to 115 pounds per cubic foot (pcf) and 125 pcf for the Dune Sand and Estuarine Deposit units, respectively; buoyant unit weights equal to 53 pcf and 63 pcf should be used for the Dune Sand and Estuarine Deposits, respectively, in locations below the water surface. It is noted that the values in Table 5-5 are unfactored; in our opinion, a factor of safety equal to 2.0 should be applied to the passive earth pressure coefficient, kp, to limit deformations under static loading. We recommend applying a factor of safety equal to 1.5 to passive earth pressure under seismic loading. For a sheet pile wall, the active and passive pressures against the wall should be considered to act over the entire surface area of the wall and embedded elements. For a soldier pile wall, passive pressure acts over two times the diameter of a pile element, assuming a minimum center-to-center pile spacing equal to 3 diameters.

We understand a drainage system will be installed behind the new shoreline wall, and effects from hydrostatic pressure are not included in Table 5-5. Hydrostatic pressures on the front of the shoreline wall should be evaluated, assuming a water elevation that corresponds to low tide. Hydrostatic pressures on the back of the shoreline wall will depend on the final configuration of the drainage system. We recommend assuming a hydrostatic head differential that is at least 5 feet greater on the back of the wall. The layout of the drainage system should consider the location of the relatively impermeable silt, Paleosol, and the potential for some water and therefore hydrostatic pressures to accumulate behind the wall. GRI should review the final drainage configuration and hydrostatic pressure assumptions during final design.

Seismic lateral earth pressures for the new walls can be evaluated using the lateral earth pressure criteria provided in Table 5-6.