Attachment F-1 Geotechnical Investigation Report.pdf

Geotechnical Investigation Smith Rock State Park Improvements

Welcome Center and Visitor Upgrade Project Terrebonne, Oregon

Prepared For GreenWorks, P. C.

110 SE Main St. Suite 100 Portland, Oregon 97214

April 11, 2025 Project No. 00-232128-0

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TABLE OF CONTENTS

1.0 PROJECT INFORMATION

1.1 Site Description

1.2 Project Overview

2.0 SUBSURFACE EXPLORATION AND CONDITIONS

2.1 Overview and Data Presentations

2.2 Field Explorations

2.3 Laboratory Testing

2.4 Subsurface Conditions

3.0 SEISMIC DESIGN PARAMETERS

4.0 GEOLOGIC HAZARDS

4.1 Site Geology

4.2 Mapped Geologic Hazards

4.3 Rockfall Hazards

4.4 Geologic Hazard Discussion and Recommendations

5.0 GEOTECHNICAL RECOMMENDATIONS

5.1 Site Preparation and General Earthwork

5.2 Structural Fill

5.3 Trench Backfill

5.4 Gravel Paved Trail Section Recommendations

5.5 Sidewalk Section Recommendations

5.6 Excavations

5.7 Protection of Exposed Ground

5.8 Shallow Foundation Recommendations

5.9 Concrete Floor Slabs-On-Grade

5.10 Engineered Retaining Wall Recommendations

5.11 Pavement Recommendations

5.12 Wet Weather Earthwork

5.13 Temporary Site Drainage Section

6.0 ADDITIONAL SERVICES

6.1 Construction Monitoring

6.2 Plan Review

7.0 LIMITATIONS

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TABLE OF CONTENTS

(continued)

LIST OF FIGURES

Figure 1 Vicinity Map

Figure 2 Project Overview Map

Figure 3 Exploration Location Map

Figure 4 Rockfall Hazard Map

Figure 5 Surcharge-Induced Lateral Earth Pressures for Wall Design

APPENDICES

Appendix A Exploration Logs

Appendix B Laboratory Test Results

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1.0 PROJECT INFORMATION

1.1 Site Description

Smith Rock State Park is a renowned state park located in central Oregon's High Desert, near Terrebonne, in Deschutes County, Oregon. The park is situated approximately three miles east of Highway 97 and spans approximately 690 acres, as noted in the Smith Rock State Park Master Plan adopted by the Oregon Parks and Recreation Commission on June 14th, 2023.

The park is bordered by several private residential lots on the south side of the river canyon, with access provided by Wilcox Road and Crooked River Drive. It features extensive recreational opportunities, including over 22 miles of hiking trails, a Bivouac Campground, and a main park administration building. Two primary day-use areas serve visitors: North Point and the Welcome Center at the Chute Trailhead. Each location provides both on-street parking and an off-street overflow parking lot to accommodate the high volume of visitors.

The park’s landscape is a mixture of mild wooded areas and open grass fields. The terrain gently slopes downward in multiple areas within the site. While much of the site features gentle inclines, some sections present steeper slopes. The park’s location is shown on the Vicinity Map, Figure 1.

1.2 Project Overview

The Smith Rock State Park Welcome Center and Visitor Upgrade Project includes a series of improvements across various areas within the state park along NE Crooked River Drive, extending onto the cliff slopes north of the main parking area.

The project consists of several key enhancements aimed at improving visitor experience and accessibility. These include the construction of a new Welcome Center and restroom, expansion of the day-use parking lot, development of accessible plazas and gathering spaces, and improved trail connections around the Welcome Center. Additionally, a new trail into the canyon will be built, the Rim Trail will be extended to North Point, and a new overflow parking lot will be constructed near the park offices.

As part of these upgrades, the existing Welcome Center parking lot will be expanded to accommodate 116 stalls, an increase from the current 92 stalls. The new overflow parking lot, located directly east of Crooked River Road and south of North Point, is designed to provide 106 total stalls.

According to the architectural drawings prepared by GreenWorks (Sheet G2.00, dated 03/04/2025), the project is divided into four main components:

• Project 1: Welcome Center Area & Parking Lot Improvements

• Project 2: New Canyon Access Trail

• Project 3: Rim Trail Extension and Renovation

• Project 4: Overflow Parking Area

The Project Overview Map is shown in Figure 2.

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2.0 SUBSURFACE EXPLORATION AND CONDITIONS

2.1 Overview and Data Presentations

NGI’s site investigation, field studies, and laboratory testing program were developed to provide a comprehensive understanding of the subsurface conditions within the project area. The objectives of this investigation include:

• Identifying subsurface soil types and characteristics,

• Establishing native soil classifications and geologic formation contacts,

• Developing geotechnical subsurface models to assess seismic design parameters, and

• Conducting sufficient laboratory and in-situ testing to determine key geotechnical parameters

To support these objectives, NGI conducted an on-site subsurface exploration, including test pit excavations, hand auger borings, field tests, and laboratory analyses. The results of these investigations are documented in detailed exploration logs, which are provided in Appendix A.

The locations of all exploration activities included in this report are illustrated in the Exploration Location Plan (Figure 3). This plan has been divided into six sheets, presented sequentially from south to northeast across the project area.

2.2 Field Explorations

The geotechnical investigation for the proposed improvements involved a two-day test pit excavation and hand auger effort, including a total of 24 locations in accordance with the agreed exploration plan.

On the first day, excavation was conducted at 16 test pit locations, designated as TP-1 through TP-18, except for TP-15 and TP-16. These two locations were switched to hand auger borings due to difficult terrain, the presence of boulders, and inaccessibility for the backhoe excavator.

The second day was dedicated to performing 8 hand auger borings, designated as HA-1 through HA-6, HA-15, and HA-16. The distribution of the test pits and hand auger borings across the project areas was as follows:

• TP-1 through TP-10 were excavated at the Welcome Center Area & Parking Lot Improvements.

• TP-11 through TP-14 and HA-1 through HA-6 were excavated and drilled at the New Canyon Access Trail.

• HA-15, HA-16, and TP-17 were drilled and excavated at the Rim Trail Extension and Renovation.

• TP-18 was excavated at the Overflow Parking Area.

The test pits were excavated using a DEER backhoe, owned and operated by SMAF Construction LLC. The depths of the test pits varied, reaching practical bucket refusal, which ranged from approximately 1.5 feet to 10.5 feet below the existing ground surface (bgs). Hand auger borings

- 3 -were advanced until practical auger refusal, which occurred at depths between approximately

0.5 feet to 2 feet bgs.

Each exploration location was logged by a staff geologist from our office, who visually classified the subsurface soils in accordance with ASTM D2487. Soil samples were collected from each stratum and sent to our laboratory for further refinement of field classifications and laboratory testing.

The exploration logs for all locations are provided in Appendix A as Figures A-1 through A-24.

Additionally, the exploration plan is presented in Figure 3.

2.3 Laboratory Testing

Representative subsurface soil samples were collected and returned to our laboratory for further analysis and classification refinement. The laboratory testing program focused on determining the following parameters:

• Moisture content

• Fines content

• Atterberg limits

The results of these tests are documented in the associated boring logs and are presented in Appendix B.

2.4 Subsurface Conditions

The subsurface conditions at the site generally consisted of approximately 6 to 12 inches of sod and topsoil, which is underlain by Smith Rock Tuff deposits. These deposits are typically dense to crumbly, light tan tephra derived from the Crooked River Caldera.

Refusal during testing from the test pits and hand auger borings primarily occurred within the tuff deposits and basalt, indicating their strength and bedrock characteristics. The Smith Rock Tuff (SRT) and basalt of the Crooked River Gorge are considered to possess high strength, which contributes to the stability of the site.

At the bottom of the slope near TP-14, a sand deposit was observed, extending to depths of approximately 10.5 feet bgs. The stratigraphic units from the surface down are described in detail in the exploration logs and summarized below.

Silty Sand – Beneath the topsoil at all exploration locations, a layer of silty sand deposits was observed, with thicknesses generally ranging from approximately 1.5 to 3 feet. However, in test pits TP-14, TP-17, and TP-18, this layer extended up to approximately 6 feet bgs. In sloped areas, the silty sand layer was observed at a shallower depth, extending to approximately 0.5 feet bgs.

Laboratory testing indicated that the moisture content of this layer varied between 10% and 33%, while a sample taken from TP-10 exhibited a plasticity index of 17.

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Clayey Sand – Beneath the topsoil at certain exploration locations, particularly along the slope face and surface face, a layer of clayey sand deposits was observed. The thickness of this layer varies from approximately 0.5 to 3 feet. Laboratory testing indicated that the moisture content of this layer ranges from 10% to 33%. The plasticity index, based on samples taken from TP-7 and HA-5, was found to be between 20 and 34.

Sand – At the slope toe in TP-14, a layer of fine sand was encountered below the silty sand layer, extending to a depth of approximately 10.5 feet bgs with a thickness of approximately 8 feet. The moisture content of this sand layer was found to range from 10% to 13%.

Bedrock – The Smith Rock Tuff was encountered in all exploration locations, with depths ranging from approximately 0.5 to 10.5 feet bgs.

Groundwater was not encountered during our subsurface explorations conducted in March. It is important to note that groundwater levels may be influenced by various factors, including changes in land use, surface water runoff, storm surge conditions, and other environmental variables.

We understand that continuous measurements of groundwater levels are conducted by the State Park at monitoring wells distributed throughout the area. Based on this information, it appears that the static groundwater table is deeper than 100 feet. Therefore, groundwater levels are not considered a geotechnical design factor for this project.

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3.0 SEISMIC DESIGN PARAMETERS

During our investigation, we generally encountered bedrock at depths ranging from approximately

0.5 to 10.5 feet bgs. Additionally, we did not observe groundwater within the depths of our explorations.

In accordance with the 2022 Oregon Structural Specialty Code, which is an amendment to the 2021 International Building Code, and ASCE 7-16, we recommend a Site Class C (very dense soil and soft rock) when considering the average of the upper 100 feet of bearing material beneath the foundations. Inputting our recommended Site Class as well as the site latitude and longitude into the Seismic Design Maps (SEAOC/OSHPD) website (http://seismicmaps.org), we obtained the seismic design parameters shown in Table 1 below.

Table 1: Seismic Design Parameter Recommendations (ASCE 7-16)

PARAMETER RECOMMENDATION

Site Class C Ss 0.359g S1 0.185g Fa 1.300 Fv 1.500

SMS (=Ss x Fa) 0.467g SM1 (=S1 x Fv) 0.277g

SDS (=2/3 x Ss x Fa) 0.311g SD1 (=2/3 x S1 x FV) 0.185g

MCEG PGA 0.164g

FPGA 1.236

PGAM (=MCEG PGA * FPGA) 0.202g Design PGA (=2/3 * PGAM) 0.135g

Note: Site latitude = 44.364677, longitude = -121.138983 http://seismicmaps.org/

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4.0 GEOLOGIC HAZARDS

4.1 Site Geology

Smith Rock State Park in central Oregon features striking cliffs composed primarily of the John Day Formation from the Oligocene Epoch (Paleogene Period), specifically the Smith Rock Tuff, a rhyolitic to dacitic ash-flow tuff deposited approximately 29 to 30 million years ago during large volcanic eruptions associated with the formation of the Crooked River Caldera. This tuff is extensively welded, creating the steep rock faces that define the park.

Overlying the Smith Rock Tuff in some areas are younger basalt flows from the Columbia River Basalt Group, which erupted during the Miocene Epoch (Neogene Period) approximately 15 million years ago. These flood basalts display columnar jointing, a feature formed by contraction as the lava cooled.

Geologically, the park is influenced by Basin and Range extensional tectonics, leading to significant normal faulting and jointing, which have shaped the cliffs and spires. While bedding is often indistinct in the tuff due to welding, some weaker sections reveal stratification. The area continues to evolve through erosion by the Crooked River and ongoing weathering processes.

The area is interpreted to be underlain by up to one foot of vegetative topsoil, which is in turn underlain by the Smith Rock Tuff along the slope face. Depending on the specific area tested, the site also underlain by basalt at the surface. In instances where neither the tuff nor the basalt is present, the underlying material consists of sand and/or alluvial deposits at the slope toe.

According to the Geology of the North Half of the Lower Crooked River Basin, Crook, Deschutes, Jefferson, and Wheeler Counties, Oregon (Bulletin 108, published in 2021), the geology of the project site is mapped as (Qbnc), representing the basalt of Crooked River Gorge (middle Pleistocene). The area north of the proposed Welcome Center and part of the slope face is mapped as (Tjtsi), which denotes the tuff of Smith Rock (lower Oligocene). Additionally, the slope toe geology is mapped as (Qa), indicating alluvium. Refer to Map 1 for the site geology.

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Map 1: Geology Map.

4.2 Mapped Geologic Hazards

The Oregon Department of Geology and Mineral Resources (DOGAMI) “Oregon HazVu:

Statewide Geohazards Viewer” was reviewed for geologic hazard maps pertinent to the project site, including flood hazards, earthquake ground shaking, liquefaction, and landslides. These maps are available through DOGAMI's online Geographic Information Systems (GIS) database, commonly referred to as Oregon’s Statewide GeoHazards Viewer (HazVu).

The maps available in HazVu indicate that the geologic hazards associated with the development of this region include the following:

• Moderate Cascadia earthquake shaking is expected within the planned project site, with Very Strong shaking mapped at the slope toe parallel to the Crooked River.

• Strong earthquake shaking is also anticipated, with Violent shaking indicated at the slope toe parallel to the Crooked River.

• The slope toe, parallel to the Crooked River, is mapped within the 100-year floodplain.

• The area is classified as having no liquefaction hazard, but it is mapped as Very High at the slope toe parallel to the Crooked River.

• There is a Moderate landslide hazard identified in the proposed Welcome Center Area, with a moderate to high hazard mapped at the slope toe parallel to the Crooked River.

• Mapped fan deposits are located to the north and east of the project site, but they are not expected to impact the current improvements.

Additionally, we reviewed the Oregon Department of Geology and Mineral Resources (DOGAMI) Statewide Landslide Information Layer for Oregon (SLIDO) for information on historic and ancient

N

Planned Welcome Center

Qbnc

Qa

- 8 -landslides in the area. Similar to the landslide inventory map produced by HazVu, SLIDO indicates that the project site is not mapped with historical landslides; however, fan deposits have been identified. Maps 2 through 7 illustrate the mapped hazards present at the project site and its surrounding areas.

Map 2: Cascadia Earthquake Hazard Map.

Map 3: Instrument Intensity Map.

N

Planned Welcome Center

N

Planned Welcome Center

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Map 4: Flood Hazard Map.

Map 5: Liquefaction Hazard Map.

Planned Welcome Center

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Map 6: Landslide Hazard Map.

Map 7: Fan Deposits Map.

Planned Welcome Center

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4.3 Rockfall Hazards

A comprehensive site reconnaissance was conducted on March 24, 2025, by our senior geologist, Ken Andrieu, R.G. The focus of the reconnaissance was on the north slope leading to the existing Smith Rock parking area, where the planned New Canyon Access Trail is to be developed.

The inspection included detailed observations of slope geometry, vegetation, surface drainage patterns, exposed soils, rock conditions, and the overall topography. Specific attention was given to identifying any indicators of instability, such as cracks, tension fissures, soil creep, or evidence of recent rockfall or landslide activity.

The proposed trail traverses a slope below a basaltic rim rock formation. The area exhibits steep, fractured rock faces with evidence of past rockfall activity. The slope surface primarily consists of sandy gravel soils with a thin topsoil layer and vegetation cover consisting mainly of high desert grasses and shrubs with a few sparsely spaced trees, mostly along the upper portion of the slope, which may influence slope stability and rockfall dynamics.

While onsite, we observed the following site conditions relating to the rockfall hazard. Areas of concern are documented in Photos 1 through 5. The identified areas have been mapped on Map 8 for reference, with further details and enlargements presented in Figure 4

• Basaltic Rim Rock: The basaltic rim rock is the most prominent source for rockfall hazards along the new path alignment. The basalt is generally slightly weathered with blocky, columnar jointing. The condition of the rim rock varies somewhat from middle portion of the slope that lies above most of the path alignment, and the east and west portions located above the margins of the path alignment. Bedding planes are near horizontal, with columnar jointing near vertical. In the east and west sections, the columnar jointing of the formation is blocky with vertically oriented joints spaced roughly 4 to 6 feet apart, and horizontally oriented joints and discontinuities generally spaced 7 or more feet apart. In the middle portion, the rim rock, where present, has vertical joints spaced approximately 18 inches to 3 feet apart and horizontal joints are spaced approximately 2 to 4 feet apart.

Vertical cliff faces along the east and west portions are generally 7 to 15 feet tall. Cliff faces are often stepped along horizontal joints, with slopes ranging from level to 20 degrees in between that contain scattered boulders up to about 4 feet in size. In the middle portion, the total height of the rimrock is about 15 feet, with vertical faces generally less than 7 feet, and an overall slope of approximately 40 degrees. There are more dislodged boulders up to about 2 feet in size in this area between the intact cliff faces.

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Photo 1: Viewing along upper eastern rim rock section.

Photo 2: Eastern rim rock section from below.

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Photo 3: Middle rim rock section with rhyolitic tuff exposed in slope below. The slope of the exposed tuff was 17 degrees.

• Existing Rockfall Evidence: The rim rock in the east and west sections has many large 4-to 8-foot boulders on the upper portion of the slope below cliff. Further to the east and west, large boulder fields were observed that extend all the way down the slope. The western most, and lowest portion of the proposed alignment crosses through one of these boulder fields. The majority of the proposed path lies between these areas. In this middle portion, there are relatively few boulders. The boulders that are present are generally less than approximately 18 inches in size. We did not observe evidence of recent large scale rock slides in this area.

Anecdotally, the current park manager informed us that he was not aware of any significant rockfall incidents in the park’s history from rocks dislodging from the rim rock.

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Photo 4: Looking west over natural drainage swale area towards western rim rock area with scattered boulders beneath the rim rock and a few trees and boulders in the swale.

• Slope Conditions: The slope below the rim rock generally appears stable with an average slope of about 20 degrees. The slopes are relatively even, and we did not observe signs of deep-seated slope movement. The trees on the slopes are generally straight-trunked suggesting generally stable slope conditions. As stated previously, large boulder fields are present in the east and west portions of the slope. The steepest slopes within the proposed path alignment are approximately 2 Horizontal to 1 Vertical (2H:1V) and lie within a natural drainage swale in the western portion of the slope, near the western boulder field. The thin surface soils along the slope are underlain by rhyolitic tuff, helping to maintain the stability of the slope.

Swale Area

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Photo 5: Looking southwest over the area for the proposed path.

4.4 Geologic Hazard Discussion and Recommendations

Based on our site reconnaissance, subsurface explorations, and office research, we consider the site to have the following geologic hazards:

• Rockfall Sources: The primary rockfall sources are the rim rock and isolated boulders along the slope. Larger boulders are more likely to be expected from the east and west sections of the rim rock. In particular, rocks that have been partially dislodged from freeze-thaw processes or physical disturbance. By increasing foot traffic in this area, the likelihood of rocks being disturbed or dislodged is increased.

• Potential Rockfall Trajectories: Rocks will generally travel straight down the hillside. In the vicinity of the natural swale, rocks will tend to move toward the middle of the swale before traveling down the central trough. The upper portion of the slope tends to have more trees, boulders, and slightly more level areas which may help slow down the rocks before they gather much momentum. Additionally, the proposed path crisscrossing the slope may provide breaks in the slope that could potentially slow or halt the progress of a rolling boulder.

• Impact Zones: The areas where we foresee the greatest impact are below the east and west rim rock sections. Should rocks dislodge from these areas, they would be larger and capable of significantly more damage to people and property. The presence of these boulder fields indicates a greater occurrence of rockfalls in these areas, with boulders potentially travelling all the way to the bottom of the slope. In our opinion, the section with

- 16 -the greatest potential impact are the sections of the trail that travel through the natural swale, and lower portion of the trail that runs through and immediately below the western boulder field

• Risk to Trail Users: While there is a risk to trail users, based our observations of the proposed path area and the relatively few historic rockfall events in the park’s history, we consider the risk of rock falls to users of the trail to be relatively low. We believe the risk would be no greater than the risk to users of the existing trails below the same slope.

It is well-known that Western Oregon is at risk of a major Cascadia Subduction Zone earthquake.

A Cascadia Subduction Zone earthquake is predicted to be as high as magnitude 8 or 9 and is expected to occur sometime within the next 400 years. Should this earthquake strike, there is risk that it could cause a landslide (rock destabilization, or rockfall) to occur at the slope on the subject site.

While we observed a generally stable slope under current conditions, localized areas of concern were identified. In numerous locations, we observed portions of the basalt rock outcrops with open joints and boulders that appeared to have the potential to dislodge from the outcrop. Other limited concerns included drainage patterns directing water toward the slope, and minor soil slumping.

Although we did not observe immediate signs of large-scale instability, the steepness of the slope and the presence of loose material highlight the potential for future instability, particularly under adverse conditions such as heavy rainfall or seismic activity.

To mitigate risks and enhance slope stability, we recommend implementing one or more of the following strategies in the future, as appropriate:

1. Slope Scaling - Remove loose or unstable rocks from the slope and cliff face to reduce the potential for future rockfall.

2. Catchment Structures - Install catchment barriers, fences, or berms at the slope's base to intercept and contain any falling rock.

3. Drainage Improvements – Implement measures to redirect water away from the slope and reduce saturation-induced instability.

4. Signage and Monitoring – Frequent and conspicuous signage to make users aware of the rockfall hazard and to report potential dangers. Frequent monitoring by park officials to identify and mitigate potential hazards.

5. Trail Alignment Adjustments – Consider relocating the sections of the trail that cross though and below the western boulder field.

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Map 8: Aerial image from Google earth showing path alignment and potential hazard areas.

West Rim Rock

East Rim Rock West Boulder Field

East Boulder Field

Swale

Proposed Path Alignment

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5.0 GEOTECHNICAL RECOMMENDATIONS

5.1 Site Preparation and General Earthwork

Before grading, the site should be cleared of debris and any buried obstructions that may be encountered. Water wells, if discovered, should be capped below the finished site grades and abandoned in accordance with local and State guidelines.

We recommend stripping the topsoil, vegetation, roots, compressible soils, manmade debris, and any other deleterious materials from beneath the proposed improvement areas to expose the underlying native soils. Overall stripping depths are anticipated to range from approximately 6 to 12 inches. In the vicinity of TP-18, where the planned new overflow car park will be constructed, stripping should extend beneath the existing base layer until a firm subgrade is encountered, as recommended by the Geotechnical Engineer of Record or their representative. However, localized increases in stripping depth may be necessary for areas with extensive root zones or where fill or disturbed soil is encountered during grading. A representative of the Geotechnical Engineer should determine the appropriate depth of removal during construction.

The subgrade beneath the proposed improvement areas should be evaluated by a representative of the Geotechnical Engineer after preparation. Soils deemed unsuitable by the Geotechnical Engineer’s representative should be over-excavated and replaced with properly compacted structural fill.

Any utilities present beneath the proposed construction should be located and rerouted as necessary. Additionally, any abandoned pipes or utility conduits should be removed to prevent potential subsurface erosion. Utility trench excavations must be backfilled with properly compacted structural fill in accordance with Sections 5.2 and 5.3 below.

5.2 Structural Fill

All structural fill should be free of organics and other deleterious materials, with a maximum particle size of less than 3 inches. The material should be well-graded, with a liquid limit of less than 45 and a plasticity index of less than 25. Based on our professional assessment, the on-site soils appear suitable for use as structural fill, provided they are later tested and verified by our Geotechnical Engineer during construction. Alternatively, if preferred or required, the use of imported well-graded crushed rock or gravel is also recommended for structural fill placement.

Structural fill should be placed in uniform, horizontal lifts on the prepared subgrade, with approval from the Geotechnical Engineer or their representative. If loose soils are present, they should be re-compacted before fill placement. Each lift should be approximately 1 foot thick, though the maximum lift thickness will ultimately depend on the type of compaction equipment used.

All structural fill should be compacted to at least 95 percent of the maximum dry density, as determined by ASTM D1557. Each compacted lift must be tested and approved by a Geotechnical Engineer representative before placing subsequent lifts.

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To ensure structural integrity, each lift of compacted engineered fill must be tested and approved by a Geotechnical Engineer representative before additional lifts are placed.

5.3 Trench Backfill

We anticipate that new underground utilities will be installed for the project. Trench backfill should consist of a clean crushed aggregate (or other suitable granular material) containing less than 8 percent fine materials passing the No. 200 sieve. Appropriate bedding materials should be placed beneath pipes to ensure no point or concentrated loading. All granular trench backfill above the pipe zone and within structural areas should be compacted by mechanical means to at least 95 percent of the maximum dry density determined in accordance with ASTM D1557 (Modified Proctor). Trench backfill should be tested by the Geotechnical Engineer’s representative during construction.

5.4 Gravel Paved Trail Section Recommendations

We have not been provided with the typical section for the trail; however, based on the planned improvements, we provide the following recommendations for the construction of the new Canyon Access Trail and Rim Trail Extension. These trails are designed to be gravel-paved and should be constructed with consideration for drainage management.

We recommend that the subgrade be prepared as outlined in Section 5.1 and evaluated by a geotechnical representative before gravel placement. Any loose, soft, or unsuitable materials should be removed and replaced with compacted structural fill as necessary. The gravel surface does not require compaction but should be knuckled into place using an excavator.

In areas where surface water is expected to cross the trails, a drainage system should be incorporated to prevent erosion and maintain trail usability. A minimum of 10 inches of open-graded drain rock (e.g., 1½-inch to ¾-inch) should be placed beneath the gravel layer to facilitate water movement. This drainage layer should extend at least 2 feet beyond the trail width on either side to allow for periodic excavation and replacement of clogged drain rock.

5.5 Sidewalk Section Recommendations

For sidewalk sections, we recommend constructing a stable base to support the concrete slab. A minimum of 4 inches of ¾-inch crushed aggregate base rock should be placed atop an undisturbed subgrade, a compacted approved subgrade, or compacted engineered fill, as outlined in Section 5.1.

The base rock should extend beyond the curb, if a curb is designed, sloping downward at 1.5H:1V.

The aggregate base should be compacted to at least 92 percent of the maximum dry density, as determined by ASTM D1557.

Surface water management is essential to prevent water accumulation and potential damage to the concrete slab. Drainage should be designed and constructed in accordance with the recommendations outlined in Section 5.4

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5.6 Excavations

It is anticipated that excavations at the site can be completed using conventional heavy earth moving equipment such as excavators. In Federal Register, Volume 54, No. 209 (October 1989), the United States Department of Labor, Occupational Safety and Health Administration (OSHA) amended its "Construction Standards for Excavations, 29 CFR, part 1926, Subpart P". This document and subsequent updates were issued to better ensure the safety of construction workers entering trenches or excavations. It is mandated by this federal regulation that excavations, whether they be utility trenches, basement excavations, or footing excavations, be constructed in accordance with the new OSHA guidelines. It is our understanding that these regulations are being strictly enforced and if they are not closely followed, the owner and the contractor could be liable for substantial penalties.

The contractor is solely responsible for designing and constructing stable, temporary excavations and should shore, slope, or bench the sides of the excavations as required to maintain stability of both the excavation sides and bottom. The contractor's "responsible person", as defined in 29 CFR Part 1926, should evaluate the soil exposed in the excavations as part of the contractor's safety procedures. In no case should slope height, slope inclination, or excavation depth, including utility trench excavation depth, exceed those specified in local, state, and federal safety regulations.

We are providing this information solely as a service to our client. NGI does not assume responsibility for construction site safety or the contractor’s compliance with local, state, and federal safety or other regulations.

5.7 Protection of Exposed Ground

Excavation and construction operations will expose the near-surface moisture sensitive soils to inclement weather conditions. The exposed soils will likely rapidly deteriorate due to precipitation and/or the action of repetitive heavy construction equipment. Accordingly, in areas where aggregate base sections are planned, such as trails, slabs, and structures, the aggregate base should be placed and immediately compacted following subgrade acceptance to protect prepared grades. If the contractor wishes to stage in areas, we recommend that planned aggregate base sections be temporarily over-filled at least 4 inches to allow for added support and removal of surface contamination. Depending on the type and weight of the construction equipment, additional temporary gravel thickness may be necessary to protect subgrade during construction.

5.8 Shallow Foundation Recommendations

Based on the available information, we understand that the project includes the construction of a new Welcome Center and restroom. While no structural details have been provided for the planned construction, we anticipate that the structures will be supported on shallow foundations.

From a geotechnical standpoint, it is acceptable for the structures to be designed with shallow footings, provided they adhere to the recommendations outlined in this section.

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Structures with individual shallow footings and/or continuous spread footings bearing on undisturbed or re-prepared native soils or compacted structural fill may be designed for an allowable bearing capacity of 2,000 pounds per square foot (psf) for dead plus live loads. A one-third increase may be applied to this bearing capacity for short-term wind or seismic loads. If a higher bearing capacity is required, the native silty sand/clayey sand layer must be removed.

Based on subsurface conditions encountered in TP-5 through TP-8, this layer is expected to range from approximately 1.5 feet to 3 feet in thickness. Footings should then bear directly on bedrock, allowing for an allowable bearing capacity of up to 6,000 psf for dead plus live loads. A one-third increase may also be applied to this bearing capacity for short-term wind or seismic loads. All footings for structures should be embedded at least 18 inches below the adjacent exterior grade.

If the decision is made not to bear the footing directly on bedrock, continuous and/or individual spread footings should have a minimum width of 24 inches in the least dimension. To mitigate the risk of soil deterioration due to seepage or wet weather conditions, a 2- to 3-inch-thick layer of clean, crushed aggregate should be placed at the base of the footing excavation and properly seated using mechanical means.

Lateral loads may be resisted by passive pressures acting against footings and by frictional resistance between foundation elements and supporting soils. A passive resistance of 250 pounds per square foot per foot of embedment depth and a friction factor of 0.30 may be used for design. For foundations bearing on the moisture conditioned and compacted structural fill or bedrock a coefficient of friction of 0.40 may be used for design. Passive pressures should be neglected in the upper 1 foot below finished grades unless covered by pavement or slabs-on-grade. Exterior footings should be set back at least 3 feet from the slope crest to reduce the risk of potential undermining.

Total and differential settlement as a result of static loading is expected to be within typical construction tolerances of 1 inch total settlement and 0.5 inches of differential settlement. Where unusual loading patterns result in large differential loads, combined footings, eccentrically loaded footings, or other special foundation requirements, this office should be consulted.

The Geotechnical Engineer or their representative (NGI) must observe the completed foundation subgrades to ensure that the encountered soils and prepared subgrades are consistent with site explorations, existing conditions, and our recommendations. If unsuitable or soft soils are encountered, we may recommend select over-excavation and replacement with properly compacted structural fill.

5.9 Concrete Floor Slabs-On-Grade

We anticipate that a concrete floor slab-on-grade may be incorporated into the project design.

After the slab subgrade is prepared in accordance with Section 5.1 above, we recommend the soil subgrade be visually evaluated by a representative of the Geotechnical Engineer by soil probing. If fill is required, the structural fill should be placed on the prepared subgrade after it has been approved by the Geotechnical Engineer.

For the purposes of designing the slab-on-grade, we recommend the use of a Modulus of Subgrade Reaction (k) of 150 pci when supported by a 6-inch-thick free draining granular material

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(i.e. no more than 5 percent passing the #200 sieve). This subgrade modulus value represents an anticipated value which would be obtained in a standard in-situ plate test with a 1-foot square plate. Use of this subgrade modulus for design of other on-grade structural elements should include appropriate modification based on dimensions as necessary. To reduce the risk of cracking resulting from any differential movement and shrinkage, the slab should incorporate an adequate number of expansion joints.

The 6-inch-thick crushed rock structural fill should provide a capillary break to limit migration of moisture through the slab. If additional protection against moisture vapor is desired (such as in the planned restroom structure), a moisture vapor retarding membrane may also be incorporated into the design. Factors such as cost, special considerations for construction, and the floor coverings suggest that decisions on the use of vapor retarding membranes be made by the project design team, the contractor and the owner.

5.10 Engineered Retaining Wall Recommendations

We anticipate that an engineered retaining wall may be incorporated into the project design, along with underground structures and seat walls. To ensure stability, durability, and performance, we provide the following recommendations.

Retaining walls should be designed to withstand lateral earth pressures and any adjacent surcharge loads. Retaining wall foundations should be designed in accordance with the recommendations presented in Section 5.9 above. Lateral earth pressures on walls, which are not restrained at the top, may be calculated on the basis of an “active” equivalent fluid pressure of 40 pcf for level backfill, and 62 pcf for sloping backfill with a maximum 2H:1V slope. Lateral earth pressures on walls that are restrained from yielding at the top (i.e. basement walls) may be calculated on the basis of an “at-rest” equivalent fluid pressure of 59 pcf for level backfill, and 80 pcf for sloping backfill with a maximum 2H:1V slope. The stated equivalent fluid pressures do not include surcharge loads, such as foundation, vehicle, equipment, etc., adjacent to walls, hydrostatic pressure buildup, or earthquake loading.

The stated equivalent fluid pressures do not include surcharge loads, such as foundation, vehicle, equipment, etc., adjacent to walls, hydrostatic pressure buildup, or earthquake loading. Surcharge loads on walls should be calculated based on the attached calculations/formulas shown in Figure 5.

We recommend that retaining walls be designed for an earth pressure determined using the Mononobe-Okabe method to mitigate future seismic forces. Our calculations were based on the Design Peak Ground Acceleration (PGA) value of 0.135g. We have assumed that the retained soil will have a minimum friction angle of 29 degrees and a total unit weight of about 115 pounds per cubic foot. For seismic loading on retaining walls with level backfill, newer research indicates that the seismic load is to be applied at 1/3 H of the wall instead of 2/3 H, where H is the height of the wall1. We recommend that a Mononobe-Okabe earthquake thrust per linear foot of 5 psf x H2 for active conditions and 10 psf x H2 for at-rest conditions be applied at 1/3 H, where H is the height of

1 Lew, M., et al (2010). “Seismic Earth Pressures on Depp Building Basements,” SEAOC 2010 Convention Proceedings, Indian Wells, CA.

- 23 -the wall measured in feet. Note that the recommended earthquake thrust value is appropriate for slopes behind the retaining wall of up to 10 degrees. For sloping backfill conditions up to 2H:1V, the retaining walls will need to be analyzed on a case-by-case basis.

All backfill for retaining walls should be select granular material, such as sand or crushed rock with a maximum particle size between ¾ and 1 ½ inches, having less than 5 percent material passing the No. 200 sieve. Based on the cohesive nature of the on-site soils, the native soils do not meet this requirement and it will be necessary to import material to the project for structural backfill.

All backfill behind retaining walls should be moisture conditioned to near the optimum moisture content and compacted to between 90 and 95 percent of the material's maximum dry density as determined in accordance with ASTM D1557. Over-compaction of the retaining wall backfill should be avoided. Fill materials should be placed in layers that, when compacted, do not exceed about 8 inches. Care in the placement and compaction of fill behind retaining walls must be taken in order to ensure that undue lateral loads are not placed on the walls.

An adequate subsurface drain system will need to be designed and installed behind retaining walls to prevent hydrostatic buildup. A waterproofing system should be designed for any below grade walls where moisture intrusion is not desirable. A means of monitoring the exterior groundwater elevations is presumed to be incorporated into the project plans.

5.11 Pavement Recommendations

We understand that a flexible pavement is planned for the expansion of the day-use parking lot and overflow parking area, designed to accommodate a total traffic volume of 116 vehicles and provide 106 parking stalls for both day-use and overflow parking, respectively. The parking lots will primarily serve passenger vehicles, with a limited number of accessible and RV parking spaces.

Based on the information provided, we recommend that the flexible pavement section consist of 3 inches of asphaltic concrete (AC) over 8 inches of compacted aggregate base course. The subgrade should be prepared as outlined in Section 5.1 and must be observed and tested by the geotechnical engineer representative. If soft or unstable subgrade conditions are encountered during construction, over-excavation may be necessary. In such cases, additional aggregate base course material should be placed and compacted. Additionally, proper drainage considerations should be implemented to prevent water accumulation, which can lead to subgrade weakening and pavement deterioration. Regular maintenance, including periodic resurfacing and crack sealing, will help extend the lifespan of the pavement.

The crushed aggregate base course should consist of well-graded crushed rock with a maximum particle size no greater than 2 inches. Aggregate base course materials should be free of organics or other deleterious materials, be relatively clean (i.e. less than 5 percent soil passing the U.S.

#200 sieve), well graded. The base course should be moisture conditioned to within 2 percent of optimum and compacted to a minimum of 95 percent of a Modified Proctor as outlined in Section

5.2 of this report. When placed, the lift base course thickness should generally not exceed 12 inches prior to compacting. The type of compaction equipment used will ultimately determine the

- 24 -maximum lift thickness. In addition, we recommend that the structural fill be placed within +/- 2 percent of the optimum moisture for that material.

5.12 Wet Weather Earthwork

Wet weather grading and earthwork are not recommended for this project. The upper soils encountered at the site are expected to be sensitive to disturbances caused by construction traffic and to changes in moisture content. During wet weather periods, increases in the moisture content of the soil can cause significant reduction in the soil strength and support capabilities. In addition, soils that become wet may be slow to dry and thus significantly retard the progress of grading and compaction activities. Therefore, it will be advantageous to perform earthwork and foundation construction activities during dry weather. In the event it is unavoidable, we will provide recommendations at the time of the work and based on the requirements of the facility and conditions encountered in the field.

5.13 Temporary Site Drainage Section

Water should not be allowed to collect in the foundation excavations or on prepared subgrades during construction. Surface water should be diverted from excavations by means of temporary drainage facilities. Excavations should be de-watered as necessary by pumping or other suitable methods. Ponding of surface water in structural areas should also be prevented to the extent practical utilizing temporary drainage facilities.

The site grading plan should be developed to provide rapid drainage of surface water away from the building areas and to inhibit infiltration of surface water around the perimeter of the building and beneath slabs-on-grade.

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6.0 ADDITIONAL SERVICES

6.1 Construction Monitoring

Because of the judgmental character of the art of soil and foundation engineering, as well as the potential for adverse circumstances arising from construction activity, observations during site preparation, mass earthwork, excavation, and foundation construction will need to be carried out by the geotechnical engineer or their representative. These observations then may serve as a basis for confirmation and/or alteration of geotechnical recommendations or design guidelines presented herein to the benefit of the project.

6.2 Plan Review

A geotechnical review of final plans and specifications is necessary to determine whether our recommendations have been properly interpreted and incorporated in the design and construction documents.

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7.0 LIMITATIONS

Within the limitations of scope, schedule, and budget, our services have been completed in accordance with geotechnical practices in this area at the time this report was prepared. No warranty is expressed or implied. This report was prepared for the exclusive use of NGI’s client for the specific project and NGI does not authorize the segmented use of the advice herein or the reliance upon the report by third parties without written authorization of NGI. The exploration logs and related information depict generalized subsurface conditions only at these specific locations and at the particular time the subsurface exploration was completed. Soil and groundwater conditions at other locations may differ. Also, the passage of time may result in a change in the soil and groundwater conditions at the site. This report pertains to the subject site only and is not applicable to other locations within the park or adjacent sites, nor is it valid for types of development other than that to which it refers. Geologic conditions, including materials, processes and rates can change with time and therefore a review of the site and/or this report may be necessary as time passes to assure its accuracy and adequacy. This report may only be copied in its entirety.

As is standard practice in the geotechnical industry, the conclusions contained in our report are considered preliminary because they are based on assumptions made about the soil, rock, and groundwater conditions exposed at the site during our subsurface investigation. A more complete extent of the actual subsurface conditions can only be identified when they are exposed during construction. Therefore, NGI should be retained as your consultant during construction to observe the actual conditions and to provide our final conclusions. If a different geotechnical consultant is retained to perform geotechnical inspection during construction, then they should be relied upon to provide final design conclusions and recommendations and should assume the role of geotechnical engineer of record, as is the typical procedure required by the governing jurisdiction.

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We appreciate the opportunity to perform this geotechnical study and look forward to continued participation during the design and construction phases of this project. If you have any questions pertaining to this report, or if we may be of further service, please contact our office.

Respectfully Submitted, NORTHWEST GEOTECH, INC.

Expires: 6/30/2025

Wail Hamid Ken Andrieu, R.G. Nicholas G. Kam, P.E.

Project Manager Senior Geologist Principal Geotechnical Engineer

Copies: (1) Addressee (E-mail)

FIGURES

TP-1

TP-2

TP-3

TP-4

TP-5

TP-7

TP-8

TP-6

TP-9

SEE INSET ABOVE

FOR TP-10 LOCATION

SMITH ROCK STATE PARK

IMPROVEMENTS

PROJECT NO. 00-232128-0 FIGURE NO. 3-1

TERREBONNE, OREGON

APPROXIMATE SCALE (FEET)

0 25 50 100

SMITH ROCK STATE PARK

IMPROVEMENTS

PROJECT NO. 00-232128-0 FIGURE NO. 3-2

TERREBONNE, OREGON

PROJECT NO. 00-232128-0 FIGURE NO. 3-3

TERREBONNE, OREGON

PROJECT NO. 00-232128-0 FIGURE NO.3-4

TERREBONNE, OREGON

0 15 30 60

PROJECT NO. 00-232128-0 FIGURE NO. 3-5

TERREBONNE, OREGON

0 20 40 80

TP-10

IMPROVEMENTS

PROJECT NO. 00-232128-0 FIGURE NO. 3-6

TERREBONNE,…