ATTACHMENT E-3

GEOTECHNICAL REPORT

Geotechnical Investigation and Geologic Hazard Report

OPRD Cape Lookout Park Upgrades Tillamook County, Oregon

December 4, 2025

(REVISED)

Prepared for

KPFF, Inc.

111 SW 5th Avenue, Suite 2600

Portland, OR 97204

Prepared by

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

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

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report December 4, 2025 (Revised)

TABLE OF CONTENTS

1 INTRODUCTION

2 PROJECT DESCRIPTION

2.1 General

2.2 Concept-Level Improvements (Design Concept 2)

2.3 Reduced Project Scope Improvements (Revised Scope Phase 1)

3 SITE DESCRIPTION

3.1 Site Conditions

3.2 Geology

4 SUBSURFACE CONDITIONS

4.1 General

4.2 Soil

4.3 Existing Well Logs

4.4 Groundwater

5 INFILTRATION TESTING

6 CONCLUSIONS AND RECOMMENDATIONS

6.1 General

6.2 Seismic Considerations and Geologic Hazards

6.3 Site Preparation and Grading

6.4 Foundation Support

6.5 Slab-on-Grade

6.6 Retaining Walls and Embedded Structures

6.7 Utility Installation

6.8 Domestic Water Well Considerations

7 DESIGN REVIEW AND CONSTRUCTION SERVICES

8 LIMITATIONS

9 REFERENCES

TABLES

Table 4-1: Summary of Depth of Groundwater Observed in Explorations Table 5-1: Summary of Infiltration Test Results Table 6-1: Recommended Seismic Design Parameters (2022 OSSC/ASCE 7-16)

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report

FIGURES

Figure 1.1: Vicinity Map Figure 2.1: Concept-Level Site Plan Figure 2.2: Reduced Project Scope Improvements Site Plan Figure 6.1: Footing Overexcavation Detail Figure 6.2: Surcharge-Induced Lateral Pressure

APPENDICES

Appendix A: Field Explorations and Laboratory Testing Appendix B: Geologic Hazard Report Appendix C: Geoprofessional Business Association Guidance Document

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report Page 1

1 INTRODUCTION

As requested, GRI has conducted a geotechnical investigation and geologic hazard evaluation for proposed improvements at Cape Lookout State Park in Tillamook County, Oregon. The general location of the park is shown on the Vicinity Map, Figure 1.1. GRI’s understanding of the proposed project improvements is discussed in Section 2, below.

The purpose of this investigation was to evaluate subsurface materials and conditions at the project site and develop geotechnical design and construction recommendations for support of the conceptual design improvements (Design Concept 2) that were developed in 2023. The investigation included a review of the available information for the site and vicinity, subsurface drilling and test-pit explorations, infiltration testing, laboratory testing, and engineering analyses. GRI completed field investigations in 2023 for Design Concept 4 and provided a draft preliminary geotechnical report to KPFF, Inc. (KPFF) dated June 7, 2023. After the June 2023 report was submitted, modifications to the proposed project layout were made by the Oregon Parks and Recreation Department (OPRD) and KPFF, resulting in a significantly reduced scope of planned improvements (Revised Scope Phase 1). GRI provided a final geotechnical report dated November 20, 2024, that also included a Revised Scope Phase 1. Once the specific locations of the Camp Loop D bathroom/shower facility and the vault toilets were decided upon in 2025, GRI completed additional explorations at these locations, performed additional engineering and seismic analyses, and revised our geotechnical report. This current report describes the work accomplished and provides our conclusions and recommendations for conceptual design of the proposed improvements outlined in our original scope of work for the original concept-level improvements and revised final recommendations for the reduced scope of planned improvements. GRI has also conducted a pavement investigation for the project, which is provided under a separate cover.

The project area is mapped within a tsunami hazard area. The Tillamook County Land Use Ordinance (TCLUO) Section 4.130(1), Geological Hazard Areas, and Section 3.530, Beach and Dune Overlay Zone (BD) require an evaluation of the geologic hazards at the site, which is provided in the Geologic Hazard Report presented in Appendix B. The applicable content requirements of TCLUO Section 4.130(1) and Section 3.530 have been included in this report.

The depths discussed in this report are measured from the ground surface, unless otherwise noted.

2 PROJECT DESCRIPTION

2.1 General

As discussed previously, GRI conducted a preliminary geotechnical investigation for concept-level improvements to the park as presented in our 2023 preliminary geotechnical

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report Page 2 report. The improvements proposed as part of the concept-level plan (Design Concept 2) are shown on the Concept-Level Site Plan, Figure 2.1. Following submission of our report, OPRD and KPFF have developed a reduced scope of improvements for final design (Revised Scope Phase 1) dated September 23, 2024. The reduced scope project improvements are shown on the Reduced Project Scope Improvements Site Plan, Figure 2.2. The concept-level and reduced scope improvements are discussed in detail below.

2.2 Concept-Level Improvements (Design Concept 2)

The initial concept-level project was proposed to address maintenance issues and improvements to better meet current and future demands at the park. The various improvements are annotated with notes by KPFF on Figure 2.1. The overall project improvements consist of new buildings, including a bathroom/shower facility, a park office, a ticket booth, covered storage, and a shop/maintenance building; a new recreation vehicle (RV) lift station; new roadways and parking lots; new campsites; new pedestrian access; culvert replacements; and utilities. Development of a domestic water well was also considered during initial project work to provide an alternative source of potable water for the park. However, after further evaluation by OPRD, the preferred alternative was to connect the park’s water service to an existing water district transmission line located in Netarts Bay Road, about 1 mile north of the park.

If the new bathroom/shower facility, ticket booths, and other buildings for the project are constructed in the future, we anticipate that they may consist of relatively lightly loaded, single-level buildings supported on shallow foundations consisting of spread footings with slab-on-grade floors or a concrete slab-on-grade with thickened edges. We anticipate that permanent cuts and fills associated with general site grading will be no greater than 5 feet to 10 feet and that excavations associated with new utilities will typically be no deeper than about 6 feet to 7 feet, although deeper excavations may be required locally. New stormwater detention/infiltration ponds are anticipated to be no deeper than about 5 feet.

2.3 Reduced Project Scope Improvements (Revised Scope Phase 1) The reduced project scope improvements are shown on Figure 2.2 with annotations made by KPFF providing a description of each improvement. The currently proposed improvements consist of electrical and water service upgrades, installation of two pre-manufactured CXT vault toilets in the day-use area, two new culverts in Camp Loop D, and new pavement in the day-use parking area and Camp Loop D. We anticipate the CXT vault toilets will be supported on precast concrete vaults placed no deeper than 5 feet to 6 feet below the finished ground surface. We anticipate that new water and electrical services will typically be placed no deeper than 4 feet to 5 feet below the ground surface.

We assume that new culverts will be relatively small in diameter.

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report Page 3

A new bathroom/shower facility measuring approximately 50 feet by 26 feet was planned in Camp Loop D as part of the Reduced Project Scope Improvements to replace an aging facility. We understand that the structure has since been removed from the scope.

However, this report still includes discussion of the facility, including geotechnical explorations that were completed specific to the structure and subsequent design recommendations.

3 SITE DESCRIPTION

3.1 Site Conditions

Cape Lookout State Park is at the south end of Netarts Bay, along the westerly shore of the Pacific Ocean (ocean). The town of Netarts Bay is about 5 miles north of the park. The proposed project area south of the entrance road consists of a developed campground that is lightly to heavily vegetated with trees and low brush. A sand dune is present at the western edge of the campground at the head of the sand beach that slopes gently down toward the west, to the ocean. The developed portion of the park includes campgrounds improved for recreational vehicle use, multiple bathroom facilities, an amphitheater, a day-use area, a parking lot, a ranger station/registration kiosk, an RV dump station, and maintenance and administrative facilities. There are several paved roadways throughout the developed portion of the park. West of the entrance road, the site is mostly undeveloped and heavily forested, with the exception of a large, cleared area of forest where the septic drain field for the park is located. A gravel road provides access from the main entrance road southeast to the drain field.

Jackson Creek, in the southern portion of the park, flows to the west-northwest and serves as the current park water supply. Water is diverted to a treatment facility before being stored for potable use in an aboveground, steel, 90,000-gallon reservoir storage tank located off of Cape Lookout Road.

3.2 Geology

The site is located in the Coast Range physiographic province. Approximately 21 miles to the west is the seafloor projection of the Cascadia Subduction Zone (CSZ), an active plate boundary along which remnants of the Farallon plate (the Gorda, Juan de Fuca, and Explorer plates) are being subducted beneath the western edge of the North American plate. The subduction zone is 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. The plates move about 1.5 inches per year but can lock in place, increasing stress over time. A sudden release of the accumulated energy violently generates an earthquake.

Published geologic mapping indicates the site is mantled with Quaternary beach sand consisting of moderately well-sorted, fine- to medium-grained beach sand and well-

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report Page 4 sorted, cross-bedded fine-grained sand comprising active and inactive dune ridges. The sand locally includes basalt gravel and boulder deposits derived from rocky headlands and fine fluvial and lacustrine mud and sand behind coastal dune ridges. Underlying the beach deposits are lower Miocene basalt flows of the Grande Ronde Formation of the Columbia River Basalt Group. The Grande Ronde unit consists of dark gray to light gray, columnar-jointed subaerial flows, submarine pillow basalt, and isolated pillow breccia. It includes interbedded palagonitic hyaloclastite breccias (basalt eruptions into water), commonly cemented by clays, zeolite, or calcite, and locally includes interbeds of basalt conglomerate and micaceous, carbonaceous mudstone and sandstone (Wells et al., 1994).

Locally, hillsides around the project site have formed in basalt rock and include the resistant headlands of Cape Lookout. Northwest-trending normal faults are mapped through the Grande Ronde basalt and underlying early Miocene marine sedimentary rock of the Astoria Formation northeast of the site. Faults have not been mapped across the site. However, a fault mapped in the middle Miocene basalt is generally oriented more westerly toward the entrance of the site. The fault line is not depicted as crossing Quaternary landslide deposits, and this fault is not considered to be seismogenic or a Quaternary fault by the U.S. Geological Survey (USGS). Quaternary faults are those that have been recognized at the surface and exhibit measured displacement in the past 1,600,000 years. Information regarding the continuity and potential activity of these faults is lacking, due largely to the scale at which geologic mapping in the area has been conducted and the lack of detailed seismogenic studies. Other faults may be present within the project area, but clear stratigraphic offset and/or geophysical evidence regarding their location and extent is not presently available.

Several large debris flow deposits originating from upslope sources east of the park boundary were identified within the limits of the project site on the Oregon Department of Geology and Mineral Industries (DOGAMI) Statewide Landslide Hazard Database (SLIDO Version 4.0). DOGAMI is the state agency responsible for geologic hazard mapping in Oregon. The debris flow deposits are indicated to be greater than 150 years old. Additional discussion of landslide hazards and these debris flow deposits is found in Section 6.2.10 of this report and the Geologic Hazard Report provided in Appendix B.

4 SUBSURFACE CONDITIONS

4.1 General

GRI completed explorations to evaluate the subsurface materials and conditions at the project site associated with the concept-level evaluation and the reduced project scope improvements. These explorations were completed by GRI on the following dates:

• Test pit excavations TP-1 through TP-12, February 16 and 17, 2023

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• Pavement borings PB-1 through PB-9, March 8 and 15, 2023

• Geotechnical borings GB-1 and GB-2, March 8, 2023

• Hand-augered borings HA-1 and HA-2, March 15, 2023

• Geoprobe GP-1, March 15, 2023

• Geotechnical borings GB-3 through GB-5, April 3 and 4, 2025

The approximate locations of the explorations are shown on Figures 2.1 and 2.2. The explorations extended to depths ranging between 1.7 feet and 101.5 feet below the ground surface. Details of the subsurface explorations and laboratory testing programs completed for this investigation are provided in Appendix A. Logs of the explorations are provided on Figures 1A through 22A. The terms and symbols used to describe the soils encountered in the explorations are defined in Table 1A and the attached legend.

4.2 Soil

For the purpose of discussion, the materials disclosed by the GRI explorations have been grouped into categories based on their physical characteristics and engineering properties. Listed as they were encountered from the ground surface downward, the major units disclosed by the explorations are as follows:

a. Pavement

b. FILL

c. SILT and SAND

d. GRAVEL and Silty GRAVEL

a. Pavement Asphalt concrete (AC) pavement and aggregate base (AB) were encountered at the ground surface in pavement borings PB-1 and PB-5 through PB-9. The AC thickness ranged from 2 inches to 8 inches, and the AB thickness ranged from 5 inches to greater than 20 inches.

In pavement boring PB-4, a 4.5-inch thickness of AC overlying an 8.5-inch-thick oil mat (layer of oiled crushed rock) was present at the ground surface. Pavement boring PB-8 was terminated in the AB at a depth of about 20 inches.

b. FILL Fill was encountered beneath the pavement section in pavement boring PB-7 and at the ground surface in geotechnical boring GB-3 and test pits TP-1, TP-3, TP-4, TP-5, TP-10, and TP-11. It is possible that some fill is also present at the ground surface in geotechnical boring GB-5. The fill generally consists of sand, silty sand, silt, and gravelly silt to silty gravel. The sand constituent ranges from fine to coarse grained. The color of the fill soils ranges among various shades of brown and gray. Organics consisting of woody debris and/or roots ranging in diameter from 1.5 inches to 6 inches were observed in the fill in

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report Page 6 the test pits. Cobbles were present in the fill in test pits TP-1 and TP-9 through TP-12. The relative density of the sand and gravel fill was estimated to range from loose to medium dense based on Standard Penetration Resistance (SPT N-values) and visual observation.

The relative consistency of the silt, sandy silt, and gravelly silt fill ranged from very soft to medium stiff based on SPT N-values and visual observation. The natural moisture content of the fill ranged from approximately 14% to 50%. Additional information regarding the characteristics of the fill is provided in Appendix A. The fill extended to depths between

1.5 feet (pavement boring PB-7) and 7.5 feet (geotechnical boring GB-3). Test pit TP-4 was terminated in sand fill at a depth of 4 feet below the ground surface.

c. SILT and SAND Layers of silt or sand are present at the ground surface in geotechnical borings GB-1, GB-4, and GB-5; in pavement borings PB-2 and PB-3; in hand-augered borings HA-1 and HA-2; in test pits TP-2, TP-6 through TP-9, and TP-12; and beneath the surface organics in Geoprobe GP-1. Layers of silt or sand are present beneath the pavement section in pavement borings PB-1, PB-4 through PB-7, and PB-9. Layers of silt or sand are present beneath the fill in pavement boring PB-7 and test pits TP-1, TP-3, TP-5, TP-10, and TP-11.

The color of the silt and sand generally consists of various shades of brown, orange, and gray. The silt contains varying amounts of clay, sand, and gravel. The sand contains varying amounts of silt and gravel. Gray to dark gray to dark brown silt and sand with abundant organics were encountered between depths of 3 feet and 4.5 feet (bottom of boring) in pavement boring PB-6, in test pit TP-1 between 3.5 feet and 6.5 feet (bottom of test pit), and in test pit TP-3 between depths of 2 feet and 8 feet. These three explorations were in the northern portion of Camp Loop D, and we anticipate that these silty and sandy soils with abundant organics are likely associated with deposition of sediments at the south end of Netarts Bay that have subsequently been overlain with natural deposits of dune sand and/or fill. Significant organics consisting of wood roots and plant debris were also observed at a depth of approximately 5 feet in geotechnical boring GB-5 at the day-use parking lot.

In addition to these five explorations, organics consisting of woody debris and/or shrub and tree roots ranging from ¼ inch to 6 inches in diameter were observed within some zones of the silt and sand within several other explorations. Cobbles were encountered in the silt in hand-augered borings HA-1 and HA-2 below depths ranging between 2 feet and 3 feet, respectively. Topsoil, heavily rooted zones, and/or surface organics ranging in thickness between about 4 inches and 1.5 feet were observed in hand-augered borings, test pits, geotechnical borings, and geoprobe explorations. The subsurface soil beneath a depth of approximately 25 feet consists primarily of sand. The relative consistency of the silt was estimated to range from very soft to very stiff based on SPT N-values and visual observation. The relative density of the sand was estimated to generally be very loose to

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report Page 7 loose within the depth of the shallow explorations and in deeper geotechnical boring GB-3 between depths of 0 feet and 25 feet, based on SPT N-values and visual observation. In geotechnical boring GB-3, the relative density of the sand was estimated to be medium dense between depths of 25 feet and 50 feet and dense to very dense between depths of 50 feet and 100 feet, based on SPT N-values and visual observation.

The natural moisture content of the silt and sand ranges between approximately 22% and 243%. Additional information regarding the characteristics of the silt and sand encountered in the explorations is provided in Appendix A. Pavement borings PB-4 through PB-7 and PB-9 were terminated in silt at depths ranging between 4.5 feet and 5 feet below the ground surface. Hand-augered borings HA-1 and HA-2 were terminated in silt at a depth of 3 feet below the ground surface. Test pits TP-1 and TP-2, TP-4, TP-5, and TP-11 were terminated in sand or silt at depths ranging between 3 feet and 8.5 feet.

Geotechnical borings GB-4 and GB-5 were terminated in sand or silt at depths ranging between 14 feet and 16.5 feet. Geoprobe GP-1 was terminated in gravelly silt at a depth of 8.5 feet below the ground surface. Geotechnical boring GB-3 was terminated in sand at a depth of 101.5 feet.

d. GRAVEL and Silty GRAVEL Gravel or silty gravel was encountered beneath the silt or sand in geotechnical boring GB-1; pavement borings PB-1, PB-2, and PB-3; and test pits TP-6 through TP-8 and TP-12.

Gravel was encountered in geotechnical boring GB-2 between depths of 5 feet and 7 feet and 10 feet and 11.5 feet (bottom of boring); below the silt and sand containing abundant organics in test pit TP-3 at a depth of 8 feet to 9.5 feet (bottom of test pit); in test pit TP-9 between depths of 4.5 feet and 5 feet and 5.5 feet and 10.5 feet (bottom of test pit); and in test pit TP-12 between depths of 5 feet and 8 feet and 9.5 feet and 10 feet (bottom of test pit). Only minor, scattered gravel was encountered in geotechnical borings GB-3 through GB-5. The gravel and silty gravel contain variable amounts of silt and sand. The color of the gravel ranges among shades and combinations of gray, brown, and orange.

The gravel in test pits TP-3 and TP-6 contains cobbles and was moderately to strongly cemented. The gravel in test pits TP-7 through TP-9 contains cobbles, and the gravel in TP-10 and TP-12 contains cobbles and boulders up to about 16 inches in diameter. The gravel in test pits TP-7 through TP-10 and TP-12 appeared to consist of weathered basalt.

Based on SPT N-values and visual observations, the relative density of the gravel encountered in the explorations ranged from medium dense to very dense. Additional information regarding the characteristics of the gravel is provided in Appendix A.

Geotechnical borings GB-1 and GB-2; pavement borings PB-1 through PB-3; and test pits TP-3, TP-6 through TP-10, and TP-12 were terminated in the gravel at depths ranging between 5 feet and 11.5 feet.

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4.3 Existing Well Logs

One geotechnical hole report (TILL 52028) and one water well report (TILL 716) for borings previously drilled at the park site were obtained from the Oregon Water Resources Department well report query. The logs are attached at the end of Appendix A.

Geotechnical boring B-1, drilled in November 2009, was completed in campsite A-35 and disclosed gravel from the surface to a depth of 5 feet, underlain by sandy silt with gravel and wood to a depth of 21 feet, underlain by sand with gravel to a depth of 26 feet, underlain by sand to the final hole depth of 40 feet. A water well boring was completed in the park in September 1978, although the location of the boring is not known. The well log reports layers of clay, sand, gravel, and boulders to a depth of 130 feet, the maximum depth drilled. Cedar logs were encountered at depths of 26 feet to 33 feet and 66 feet to 72 feet.

4.4 Groundwater

Groundwater was observed in several of the exploration locations at the time of our exploration program. The depths to groundwater observed in explorations are summarized below, in Table 4-1.

Table 4-1: SUMMARY OF DEPTH OF GROUNDWATER OBSERVED IN EXPLORATIONS

Location Depth of

Groundwater, feet

GB-1 5

GB-2 5.5

GB-3 2

GB-4 5

GB-5 4.5

PB-5 1.5

PB-7 2

PB-9 2

TP-1 2

TP-2 1.5

TP-3 2

GP-1 6.5

Obvious groundwater seepage was not observed in the remaining explorations. Based on our explorations and location relative to the ocean and bay, we anticipate the groundwater in the lower westerly portion of the park is relatively shallow and may fluctuate somewhat in response to the rise and fall of the tides. As the ground rises to the east, we anticipate that the groundwater will likely rise with the topography but may be at somewhat greater

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report Page 9 depths overall. However, perched groundwater at shallow depths is likely common within the park and may approach the ground surface, particularly during the wet seasons of the year.

5 INFILTRATION TESTING

Three falling-head infiltration tests, designated IT-1 through IT-3, were completed between February 17 and March 15, 2023, at the locations shown on Figures 2.1 and 2.2 (where applicable) in areas being considered for stormwater infiltration. The infiltration testing was completed in polyvinyl chloride (PVC) casings inserted to the base of the exploration. Water was added, and then the depth of the water with respect to time was measured as the water infiltrated into the soil. The infiltration testing procedure is discussed in more detail in Appendix A. A summary of infiltration test results is provided below, in Table 5-1.

Table 5-1: SUMMARY OF INFILTRATION TEST RESULTS

Location

Depth of Infiltration Test,(a) feet Soil Description

Average Infiltration Rate, inches per hour

IT-1 4.0

SILT, trace to some sand, some clay to clayey 0.25

IT-2 5.0 GRAVEL, some silt and sand <0.25

IT-3 2.5 SILT, trace gravel and sand 3.5

Note:

(a) Depth of infiltration test is measured from below the ground surface.

The infiltration rates measured in the field, as shown in Table 5-1, should be divided by a factor of safety to obtain the design infiltration rate of the stormwater system. In our opinion, a factor of safety of 2.0 is appropriate for design. However, the infiltration rates measured in IT-1 and IT-2 were very low, and we anticipate that infiltration at these locations may not be practical. Groundwater was observed at shallow locations in several explorations, as discussed in Section 4.4, and infiltration in areas with high groundwater may not be feasible.

6 CONCLUSIONS AND RECOMMENDATIONS

6.1 General

The explorations made for this project indicate the project site is mantled with highly variable soil types consisting of layers of silt, sand, and gravel to depths of up to 25 feet.

Below 25 feet, deep geotechnical boring GB-3 disclosed that these deposits are underlain by a layer of silty sand that extends to a depth of at least 101.5 feet, the maximum depth explored. The relative consistency/density of the soil ranges from very soft / loose to very stiff / very dense. The relative consistency/density of the soil is generally very soft to soft

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/ very loose to loose within the upper 25 feet of the profile, becoming medium dense to very dense with increasing depth. Very soft to soft / very loose to loose, sandy silt and silty sand with abundant organics were encountered within about 2 feet to 3 feet of the ground surface in the eastern portion of Camp Loop D. Organics, including wood debris and roots with diameters up to 6 inches, were observed in several of the test pit excavations associated with adjacent shrubs and trees. Very soft silt with abundant organics was also observed at depths of approximately 5 feet in the day-use parking area. Cobbles and boulders were also encountered at shallow depth in several of the explorations.

Groundwater was observed to be relatively shallow, within 1.5 feet to 2 feet of the ground surface in some of the explorations.

We anticipate that the primary geotechnical considerations for the project include the moisture sensitivity of the fine-grained soils and soft and loose soils that are easily disturbed by construction equipment, high groundwater conditions and potential for caving and running soils in excavations, static and seismic settlements, the potential of encountering organics at building footing and vault toilet subgrade elevations, and the potential of encountering cobbles and boulders in foundation and utility excavations.

Provided below are our conclusions and recommendations for development of the project.

It is our opinion that the proposed limited project improvements discussed in this report can be constructed without exacerbating the existing geologic hazards at the site, which are identified and discussed in this report and in the Geologic Hazard Study in Appendix B.

6.2 Seismic Considerations and Geologic Hazards

6.2.1 General

We anticipate the 2022 Oregon Structural Specialty Code (OSSC), which incorporates recommendations for seismic design from the 2016 American Society of Civil Engineers (ASCE) document 7-16, Minimum Design Loads for Building and Other Structures (ASCE 7- 16), will be used to develop seismic parameters for the proposed improvements at Cape Lookout State Park. The ASCE 7-16 seismic hazard levels are based on a Risk-Targeted Maximum Considered Earthquake (MCER). The ground motions associated with the probabilistic MCER represent a targeted risk level of 1% in 50 years probability of collapse in the direction of maximum horizontal response. In general, these risk-targeted ground motions are developed by applying adjustment factors of directivity and risk coefficients to the 2% probability of exceedance in 50 years, or a 2,475-year return-period hazard level.

The risk-targeted probabilistic values are also subject to a deterministic limit.

The ASCE methodology uses two bedrock spectral response mapped acceleration parameters, SS and S1, corresponding to periods around 0.2 seconds and 1.0 seconds, to develop the MCER response spectrum. To establish the ground-surface MCER spectrum, GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report Page 11 these mapped bedrock spectral parameters are adjusted for site class using the short- and long-period site coefficients, Fa and Fv, in accordance with Section 11.4.3 of ASCE 7-16, which includes new seismic site coefficients to adjust the mapped values for soil properties.

6.2.2 Mapped Acceleration Parameters

The ASCE 7-16 SS and S1 mapped spectral response acceleration parameters for the site located at the approximate latitude and longitude coordinates of 45.362677°N and 123.968098°W are 1.29 g and 0.67 g, respectively, for Site Class B/C boundary (bedrock) conditions.

6.2.3 Site Class

The explorations made for our investigation extended to a maximum depth of 101.5 feet.

The Site Class is based on the average shear wave velocity in the upper 100 feet of the subsurface profile, which we estimated using published correlations with SPT blow counts.

We anticipate that areas of the park could be at risk of potentially liquefiable soils from the code-based earthquake, particularly in the westerly portion close to the ocean, where loose and very soft sand and silty soil below groundwater were encountered within the depth of the explorations. In accordance with ASCE 7-16, sites with subsurface conditions identified as vulnerable to failure or collapse, such as liquefied soils, are classified as Site Class F. For Site Class F sites, ASCE 7-16 Section 20.3 requires the completion of a site response analysis unless the structures have a fundamental period of vibration less than or equal to 0.5 seconds. The response spectrum for sites with structures having a fundamental period of less than 0.5 seconds can be derived using the non-liquefied subsurface profile. Based on our understanding of the improvements planned for the site, we anticipate the fundamental period of vibration for the structures will likely be less than

0.5 seconds. Therefore, based on the subsurface conditions encountered and in accordance with Section 20.4 of ASCE 7-16, we recommend using Site Class D, or a stiff soil profile, for calculating the spectral response acceleration parameters for the planned structures.

6.2.4 Site Coefficients

The site coefficients Fa and Fv of 1.2 and 1.7, respectively, were used to develop the Site Class D MCER-level spectrum in accordance with Section 11.4 of ASCE 7-16. The design-level response spectrum is calculated as two-thirds of the ground-surface MCER spectrum.

6.2.5 Recommended Seismic Design Parameters

Because the S1 acceleration parameter is greater than 0.2 g, Section 11.4.8 of ASCE 7-16 requires a ground-motion hazard analysis for Site Class D locations unless the value of the 1-second acceleration parameter, SM1, as determined by ASCE 7-16 Equation 11.4-2, is increased by 50% for all applications per the exception in ASCE 7-16 Supplement 3. In accordance with the exception in ASCE 7-16 Supplement 3, the recommended MCER- and

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report Page 12 design-level spectral response parameters for Site Class D conditions are summarized in Table 6-1, below. According to OSSC Section 1613.2.3, the short-period acceleration parameter, SMS, shall not be taken as less than SM1 except when determining the seismic design category. Therefore, SMS in Table 6-1 was increased to match the value of SM1.

Table 6-1: RECOMMENDED SEISMIC DESIGN PARAMETERS (2022 OSSC / ASCE 7-16)

Seismic Parameter Recommended

Values

Site Class D

MCER 0.2-Second Period Spectral Response Acceleration, SMS 1.73 g(a)

MCER 1.0-Second Period Spectral Response Acceleration, SM1 1.73 g(b)

Design-Level 0.2-Second Period Spectral Response Acceleration, SDS 1.14 g

Design-Level 1.0-Second Period Spectral Response Acceleration, SD1 1.14 g

Abbreviations: OSSC = Oregon Structural Specialty Code; ASCE = American Society of Civil Engineers;

MCER = Risk-Targeted Maximum Considered Earthquake

Notes:

(a) Value increased per the requirements of OSSC Section 1613.2.3 to not be less than SM1.

(b) Value includes a 50% increase relative to ASCE 7-16 Equation 11.4-2.

6.2.6 Liquefaction / Cyclic Softening

Liquefaction is a process by which loose, saturated, granular materials such as sand and, to a somewhat lesser extent, soft to medium-stiff, nonplastic and low 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. After liquefaction is triggered, there is an increased risk of settlement, loss of bearing capacity, lateral spreading, and/or slope instability, particularly along waterfront areas. Liquefaction-induced settlement occurs as the elevated pore-water pressures dissipate and the soil consolidates after the earthquake. Cyclic softening is a progressive reduction in strength of cohesive soils due to earthquake shaking, even though the soil may not liquefy.

The potential for liquefaction is typically estimated using a simplified method that compares the cyclic shear stresses induced by the earthquake (demand) to the cyclic shear

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report Page 13 strength of the soil available to resist these stresses (resistance). Estimates of seismically induced stresses are based on earthquake magnitude (MW) and peak ground acceleration adjusted for site effects (PGAM). The cyclic resistance of soils is dependent on several factors, including the number of loading cycles, relative density, confining stress, plasticity, natural water content, stress history, age, depositional environment (fabric), and composition. For sand-like soils, the cyclic resistance is typically evaluated using SPT N-values or cone penetration test tip-resistance values normalized for overburden pressures and corrected for factors that influence cyclic resistance, such as fines content.

The potential for liquefaction and cyclic softening 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 were obtained using magnitude-acceleration pairs consistent with the 2014 USGS Probabilistic Seismic Hazard Analysis, which characterizes the controlling seismic sources at the site and forms the basis for developing seismic ground motions. The 2014 USGS deaggregation data for the 2,475-year return period indicate the CSZ earthquake controls the seismic hazards at the site. For our liquefaction studies, we considered a mean MW 9.0 earthquake with a Maximum Considered Earthquake Geometric Mean (MCEG) PGAM of

0.71 g. We obtained the peak ground acceleration value from the ASCE (2025) Hazard Tool and the MW value from the USGS (2025) seismic hazard tool. For the purpose of our liquefaction studies, we assumed a groundwater depth of 2 feet below the existing ground surface.

Our evaluation indicates that some liquefaction and cyclic softening of the soils at the project site could occur during a code-level earthquake between depths of 2 feet and 25 feet. Some of the medium plasticity silt layers within this zone are not susceptible to liquefaction, and the relatively dense sand below a depth of 25 feet is also not susceptible to liquefaction. We anticipate the risk of potential liquefaction and cyclic softening in the near-surface soils is greatest west of the entrance road, in the general area of the campground, due to the softer and looser soils observed and high groundwater.

We estimated the magnitude of liquefaction-induced seismic settlement using the procedures developed by Ishihara and Yoshimine (1992), Tokimatsu and Seed (1987), Ishihara et al. (2016), and Cetin et al. (2009). Based on the average results from these methods, we estimate that up to approximately 4 inches to 7 inches of liquefaction-induced settlement could result from a code-level earthquake. We estimate that differential settlement could be up to one-half of the total estimated settlement over a distance of 50 feet, depending on the variability of the subsurface profile. Additional settlement could occur due to lateral spreading, which is discussed in Section 6.2.7 of this

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report Page 14 report. Vertical settlement due to lateral spreading is typically estimated to be one-third to one-half of the lateral displacement.

6.2.7 Lateral Spreading

Lateral spreading is a liquefaction-related seismic hazard and occurs on gently sloping or flat sites underlain by liquefiable sediment adjacent to an open face, such as a riverbank.

Liquefied soil adjacent to an open face can flow toward the open face, resulting in lateral ground displacement. In this case, we anticipate that liquefied soils experiencing lateral spreading would move downslope and west, toward the beach

We estimated the magnitude of lateral spreading using the approach recommended by Youd, Hansen, and Bartlett (2002) that introduces empirical equations developed from a multilinear regression analysis of a large case history database. These equations use input parameters such as earthquake magnitude, distance to fault, cumulative thickness of liquefied soil layers, fines content, grain size, and slope geometry to estimate lateral spreading. We modeled an MW 8 earthquake in our analysis, which is the largest earthquake that is recommended to be used with this method.

The results of our analysis indicate that up to approximately 12 inches of lateral spreading could occur during a code-level earthquake. We note that this is less than the displacement limit of 18 inches allowed by Section 12.13.9.2 of ASCE 7-16 for Risk Category I and II structures supported on shallow foundations.

6.2.8 Tsunami Hazard

DOGAMI recently completed an updated tsunami inundation model for the entire Oregon coastline. This model incorporates recent changes to the CSZ recurrence interval and uses newer hydrodynamic numerical models that provide more accurate estimates of tsunami inundation. The results of this analysis are described by Witter et al. (2011) and presented as a new series of DOGAMI tsunami inundation maps (TIM). The DOGAMI tsunami inundation map indicates that the project site is within the potential tsunami inundation zone; therefore, there is a high risk of tsunami inundation at the site following a CSZ earthquake.

6.2.9 Coseismic Subsidence

Modeling of megathrust earthquake ruptures on the CSZ indicates sequences of interseismic uplift and coseismic coastal subsidence. Leonard et al. (2010), Witter et al.

(2011), and ASCE 7-16 present profiles of coastal deformations from northern California to southern British Columbia based on this geologic information. Based on a review of the ASCE Tsunami Hazard Tool, the earthquake-induced ground subsidence at the site is estimated to be 3.5 feet.

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6.2.10 Fault Rupture

Based on fault mapping conducted by USGS, the weighted location of the CSZ rupture is about 35 kilometers (km) from the site. The nearest mapped crustal fault is the Happy Camp fault, located approximately 8 km north of the site. In our opinion, unless occurring on a previously unmapped or unknown fault, the risk of fault rupture at the site is very low to low.

6.2.11 Landslide Hazards

Landslide deposits have been identified east of the proposed development site. Detailed evaluation of the landslide feature designated Tillamook 1780 and the debris flow hazard associated with the Jackson Creek drainage were completed as part of initial phase work for the park improvements (GRI, 2023; WEST Consultants, 2023). This detailed work was used by the project team to refine conceptual project improvements away from areas with unacceptable landslide or debris flow risk. Based on the supplemental landslide and debris flow evaluations, it is our opinion that the risk of landslide hazard to the Reduced Project Scope Improvements is low.

6.3 Site Preparation and Grading

We anticipate that conventional construction equipment can be used to complete site earthwork. Areas to receive new structural fills, structures, and pavements should be stripped of vegetation and surficial organics, if present. The test pits encountered a heavily rooted zone generally ranging from 4 inches to 8 inches, but up to 18 inches in some areas.

Geoprobe GP-1 encountered an approximately 12-inch thickness of woody debris and roots at the ground surface. Deeper stripping and grubbing will be required to remove roots associated with large, woody vegetation, where present. We recommend removing roots larger than about ½ inch in diameter in areas to receive new structural fill and within the footprint of structures and pavement. Vegetation, roots, and other deleterious materials will be unsuitable for use as structural fill but may be used for landscaping purposes, if desired.

Soil removed from excavations that is free of organics or other deleterious materials should be suitable for use as general structural fill for site grading. Structural fill for general site grading can also consist of imported fine- or coarse-grained materials approved by the geotechnical engineer. However, soil containing more than about 10% to 15% of fines (silt and clay) by weight can only be placed and adequately compacted during dry conditions; moisture conditioning of the fill to dry it out may be required. For construction during wet conditions, fills should be constructed using imported granular materials that are relatively clean. We recommend that structural fill for construction during wet conditions consist of imported granular materials having a maximum particle size of no

GRI 6750-A – OPRD Cape Lookout Park Upgrades Geotechnical Investigation Report Page 16 greater than 1½ inches and less than about 8% passing the No. 200 sieve (washed analysis).

New structural fill placed on ground steeper than 5H:1V (horizontal to vertical) should be placed on minimum 6-foot-wide horizontal benches excavated into firm, undisturbed ground as the height of the fill increases. New structural fills should be placed in maximum 8-inch-thick lifts for lightweight equipment and maximum 12-inch-thick lifts for heavy rollers and compacted to at least 95% of the maximum dry density, as determined by ASTM International (ASTM) D698 (Standard Proctor). Granular fill materials should be compacted with vibratory compaction equipment. Fine-grained soil, if used, should be compacted with segmented pads or sheepsfoot rollers. Structural fills should be compacted within about 3% of the optimum moisture content, as determined by ASTM D698. Compaction of the fill should be evaluated with periodic field moisture-density testing. It should be assumed that wetting of the on-site sand will likely be required to achieve the required compaction, particularly during dry conditions. New structural fill slopes should be constructed 2 feet to 3 feet horizontally beyond finished grades and trimmed back to a firm surface at final grade with a trackhoe equipped with a smooth-edge bucket.

Temporary cut slopes should be sloped no steeper than 1.5H:1V. However, flatter slopes may be necessary to reduce the risk of sloughing if sandy, gravelly, or soft, silty soil and/or groundwater seepage is encountered. If temporary excavation slopes encounter perched groundwater, a blanket of relatively clean, well-graded, crushed rock placed on the slopes may be required to reduce the risk of seepage-induced soil movements. The thickness of the granular blanket should be evaluated based on actual conditions but will likely be in the range of 12 inches to 24 inches.

The on-site soil will be susceptible to erosion from wind and rain, and it may be necessary to cover temporary slopes with geotextile fabric or plastic to reduce the risk of erosion.

Final graded structural fill and cut slopes should be no steeper than 2H:1V. Final graded slopes should be vegetated and/or covered with soil erosion materials to reduce the risk of erosion.

6.4 Foundation Support

New structures initially proposed for the concept-level project included a bathroom/shower facility, a ticket booth, and covered storage. A new at-grade bathroom/shower facility was planned in Camp Loop D but has since been removed from the scope. We understand the Reduced Scope Project will now include CXT toilet structures in the day-use parking area. We anticipate that the new structures will be relatively lightly loaded.

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We anticipate the precast concrete CXT toilet structures will be supported on precast concrete vaults placed no deeper than 5 feet to 6 feet below the finished ground surface.

The bottoms of the bathroom vaults established on firm, undisturbed native soil or structural fill may be designed using an allowable soil-bearing pressure of up to 1,500 pounds per square foot (psf). Footings should be provided with a minimum embedment of 18 inches measured from the bottom of the footing to the lowest adjacent finished grade.

Based on our review of the subsurface conditions, it is our opinion that foundation loads for other lightly loaded structures that may be considered within the project area can be supported on conventional spread footings, such as continuous footings with stem walls, isolated pier/column footings, and the thickened edges of slabs using the design bearing pressure noted above.

Due to the disclosed soft and loose soil in the Camp Loop D and day-use area, we recommend a minimum granular base course thickness of 12 inches be placed below new foundations for the vault toilets or other proposed structures. We recommend the granular fill under foundations consist of well-graded, crushed rock having a maximum particle size of up to 1½ inches and containing less than 8% fines passing the No. 200 sieve (washed analysis) compacted to 95% of the maximum dry density, as determined by ASTM D698 (Standard Proctor). Base course should be placed and compacted in dry conditions. Water that may be present should be removed before placing and compacting the base course. Due to sensitive subgrade soils, the base course should be compacted using hand-operated compaction equipment. To reduce the risk of subgrade soils migrating into the base course during and following construction, we recommend placing a nonwoven geotextile (filter fabric) such as Mirafi 140N on the subgrade prior to placement and compaction of the base course.

As discussed previously, soft and loose silt and sand containing abundant organics were encountered within about 2 feet to 3 feet of the ground surface in the eastern portion of Camp Loop D near the easterly bathroom in test pits TP-1 and TP-3 and in pavement boring PB-6. Loose to very loose sand was also encountered in test pits TP-2 and TP-4 in Camp Loop D at the westerly bathroom. The sand extended to depths of at least 4 feet to 5 feet in test pits TP-2 and TP-4 and to a depth of 7.5 feet in geotechnical boring GB-3, which was underlain by relatively soft and loose soils to a depth of about 25 feet. Based on information from OPRD, we understand that settlement and cracking or shifting of the existing bathroom structures in Camp Loop D have not been observed. However, some potential settlement in the middle of the concrete slab-on-grade floor has been observed.

The settlement could be related to the presence of similar soft/loose organic abundant soil observed in the nearby aforementioned explorations.

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Subsurface conditions at the locations of the planned vault toilets were evaluated in geotechnical borings GB-4 and GB-5 and were found to consist of very loose to loose sand and very soft silt, with significant organics encountered at a depth of approximately 5 feet in GB-5.