B09_Amend_02_Attach_3B_20376009-Rev0-Otak_Tuolumne_Meadows_CG_Geotech-08032021_0002.pdf

REPORT

Geotechnical Exploration and Recommendations Report Tuolumne Meadows Campground Improvements

Submitted to:

Otak, Inc.

11241 Willows Rd NE, Suite 200

Redmond, WA 98052

Submitted by:

Golder Associates Inc.

7 Corporate Park, Suite 260, Irvine, California, USA 92606

+1 714 508-4400

20376009

August 3, 2021

August 3, 2021 20376009 i

Distribution List

Keith J. Bates, ASLA, LEED ap ii

Table of Contents

1.0 INTRODUCTION

1.1 Project Understanding

1.2 Existing Site Conditions

1.3 Previous Geotechnical Reports

1.4 Objective and Scope

2.0 SITE SETTING

2.1 Site Geology

3.0 GEOTECHNICAL EXPLORATION

3.1 Pre-Field Activities

3.2 Hand Augering and Rock Coring

3.2.1 Laboratory Testing

3.2.2 Subsurface Conditions Encountered at the Boring Locations

3.3 Geophysical Survey

3.3.1 Seismic Refraction Survey

3.3.1.1 Accuracy of Seismic Refraction

3.3.1.2 Seismic Refraction Survey Design

3.3.1.3 Data Processing and Interpretation

3.3.2 Ground Penetrating Radar

3.3.2.1 Data Acquisition and Interpretation

3.3.3 Surficial Soils and Depth to Bedrock

3.3.4 Rippability of Bedrock

3.4 Site Subsurface Conditions

3.5 Geologic Hazards

3.5.1 Strong Ground Motions

3.5.2 Liquefaction Potential

4.0 ENGINEERING RECOMMENDATIONS

4.1 Seismic Design Parameters

iii

4.1.1 Site Class

4.1.2 Ground Motion Parameters

4.2 Bearing Capacity and Foundations

4.3 Permanent Wall Design Criteria

4.4 Structural Pavement Section

4.5 Soil Corrosivity

5.0 CONSTRUCTION RECOMMENDATIONS

5.1 Subgrade and Foundation Preparation

5.2 Fill Placement and Compaction

5.3 Temporary Excavations

5.4 Pipe Bedding and Trench Backfill

6.0 ADDITIONAL SERVICES

7.0 CLOSING

TABLES

Table 3-1: Boring Location Summary

Table 3-2: Seismic Refraction Acquisition Parameters

Table 3-3: GPR Observation Summary Near Proposed Manhole Vault locations

Table 4-1: 2019 California Building Code (CBC) Design Parameters

Table 4-2: Geotechnical Design Parameters for Permanent Walls

Table 4-3: Summary of Structural Pavement Calculations

FIGURES

Figure 1: Geophysical Investigation Locations

Figure 2: Regional Geologic Map

Figure 3: Seismic Refraction Lines 1 and 2 Velocity Models

Figure 4: Seismic Refraction Lines 3 and 4 Velocity Models

Figure 5: Seismic Refraction Line 5 Velocity Model

Figure 6: Example GPR Record from Proposed Road Alignment iv

Figure 7: Example GPR Record from Near MH 120 with MH 120 Profile

APPENDICES

Appendix A

Boring Logs

Appendix B

Laboratory Results

Appendix C

Important Information About Your Geotechnical Report

Aekj

No table of figures entr ies found .

No table of figures entr ies found .

No table of figures entr ies found .

1.0 INTRODUCTION

Golder Associates Inc. (Golder) submits this report to Otak, Inc. (Otak) to present the results of a subsurface exploration at the Tuolumne Meadows Campground (the site) and to provide guidance on relevant geotechnical aspects that are applicable to the proposed campsite improvement features. The site is located on the eastern side of Yosemite National Park, California, as shown in Figure 1.

This report has been prepared in accordance with our approved and agreed upon scope of work presented in our proposal 20376009, revised December 15, 2020, and Task Order 140P2019F0372, and Project Number

33166.E00.

1.1 Project Understanding

Golder understands that the National Park Service (NPS) plans to improve the conditions of many areas within the campground. The purpose of the proposed improvements is to make the campground more accessible and upgrade the site facilities. Based on the Preliminary Design Development Drawings plan set prepared by Otak and dated March 5, 2021 (reference PMIS/PKG NO. 229677), Golder understands that Otak proposes to implement the following primary improvements:

Regrade and construct new paved access roads, walkways and parking lots including hot-mixed asphalt, concrete cement pavement and compacted gravel.

Replace existing and construct new utilities (i.e., water and sewer) in trenches at least 3 but up to 11 feet below existing grades and includes the installation of manholes.

Construct two new restroom buildings that will be 1-story lightweight constructed on slab-on-grade foundations with thickened edges and strip footings to support the proposed building walls.

Replace the foundations of 8 existing restrooms with adjacent parking lots.

Sawcut existing pavement and construct culvert crossings.

The drawings provided indicate maximum cut depths and fill heights of roughly 11 feet will be required to install manholes MH-110 and MH-110. Per the Grading Plan section of the Preliminary Design Development

Drawings, areas surrounding culvert crossings and proposed road alignments generally propose cuts and fills on the order of 5 feet or less.

1.2 Existing Site Conditions

The site consists of a densely vegetated (mature trees) campground with approximately 311 existing campsites. It can be accessed via Tioga Pass Road (Highway 120) near the town of Lee Vining, California. With exception of existing restroom buildings (with running water and sewer lines) and a park ranger gate house, there are no permanent structures on the site. A network of paved loop roads connects the campsite clusters and provides vehicle access to the campsites during the summer months when the campground is open to the public. There are few telecommunications utilities running through the site.

1.3 Previous Geotechnical Reports

Previous geotechnical explorations and pavement distress surveys were performed in the vicinity of the site including:

Central Federal Lands Highway Division CA PRA YOSE TIOGA ROAD PAVEMENTS Report (CFLHD 2011)

Shannon & Wilson, Inc. CA NPS YOE 17(4) & 941(1), Tuolumne Meadows Parking (S&W 2020)

These reports were reviewed by Golder and summarized in a desktop study dated February 23, 2021 (Golder

2021). The information provided in these reports was taken into consideration prior to the execution and completion of our scope described in Section 1.4.

1.4 Objective and Scope

The purpose of this document is to provide Otak with a summary of our site-specific subsurface exploration and to provide guidance on geotechnical aspects applicable to support Otak’s design and budgeting of the proposed campground improvements. The geotechnical recommendations provided herein pertain to our understanding of the proposed improvements as described in Section 1.1. If the design of the proposed improvements is modified, Golder should be provided with an updated set of plans for review. Depending on the nature of the design modifications, the recommendations of this report may need to be updated to reflect the modified design.

The proposed scope of work was broken into a three-phase approach to perform pre-field activities, a geotechnical and geophysical exploration, and a written report. As agreed upon, our scope of work, and therefore this report, do not include a pavement distress survey, recommendations for frost depth, or any engineering analyses (i.e., settlement, slope stability, liquefaction potential, etc.).

2.0 SITE SETTING

2.1 Site Geology

Published geologic mapping of the Tuolumne Meadows Quadrangle from the United States Geological Survey

(USGS) (Bateman 1983) shows that the site is primarily underlain by Quaternary glacial deposits. Figure 2 shows a map of the regional geology surrounding the Tuolumne Meadows Campground. The USGS map does not elaborate further on the specific geology of the glacial deposits, however, the deposits are described as unconsolidated. The Tuolumne Meadows Quadrangle geologic map (Bateman 1983) also indicates that the surrounding areas of the site also include quaternary alluvial deposits as well as formations of the Johnson

Granite Porphyry and Cathedral Peak Granodiorite. Johnson Granite Porphyry is a fine-grained and equigranular granite that contains K-feldspar phenocrysts. The Cathedral Peak Granodiorite is a medium-grained, weakly foliated hornblende-biotite granodiorite containing blocky K-feldspar megacrysts.

3.0 GEOTECHNICAL EXPLORATION

Golder performed a site-specific geotechnical exploration to support Otak’s design of the proposed campground improvements. The geotechnical exploration program consisted of the following components:

Pre-field Activities

Hand augering and rock coring

Geophysical Survey

The following sections describe our geotechnical exploration in detail.

3.1 Pre-Field Activities

Golder coordinated with Otak to confirm the most effective and accessible locations for borings and geophysical surveys. Golder also coordinated with Otak and the drilling subcontractor to schedule and plan the field work at the site. In preparation for drilling activities, Golder prepared a site-specific health, safety, and environment plan for use by field staff during the field work.

On May 18, 2021, a Golder field engineer visited the site to perform a visual reconnaissance of the site and mark out proposed boring locations. Golder notified Underground Service Alert to identify underground utilities in the areas of the geotechnical field exploration. Additionally, Golder subcontracted a private utility locator to locate subsurface utilities to assure the proposed boring locations were clear of underground utilities on May 24, 2021 prior to commencing the subsurface exploration. The boring locations were cleared of underground utilities by

Underground Services Alert (USA) before breaking ground, as required by law. Golder subcontracted Exaro

Technologies of Fresno, California to use portable ground-penetrating radar (GPR) to identify underground utilities near each boring location.

Additionally, since the geotechnical exploration was conducted on federally owned property, it was not required to obtain drilling, encroachment, or similar permits from Tuolumne County.

3.2 Hand Augering and Rock Coring

Golder performed a site-specific geotechnical field exploration at the site from May 25 to May 26, 2021 which consisted of six borings at the locations shown in Figure 1. Due to equipment issues, Golder and Otak agreed after working on May 25 (and rock coring at boring B-A2) that the drilling subcontractor would hand auger each of the borings to refusal and perform rock coring, moving forward, only at boring B-GC-1. During the geotechnical exploration, bulk samples were collected from each boring.

The six borings (B-A1, B-A2, B-D1, B-GC1, B-ND3, and B-ND5) were advanced to refusal using a hand auger.

Upon refusal, two borings (B-A2 and B-GC1) were advanced through rock using the coring attachment on the

CME-75 limited access rig. The borings were advanced by Gulfshore Construction Services of Rancho Cordova, California, under subcontract to Golder. All borings were advanced on unpaved areas at the site, and their approximate elevations were recorded by Golder using an EMLID Reach RS+ satellite receiver. Table 3-1 and

Figure 1 provide a summary of boring locations and depths.

Table 3-1: Boring Location Summary

Boring

ID

Latitude

(WGS84

degrees)

Longitude

(WGS84

degrees)

Approximate Ground Surface Elevation

(ft MSL)

Hand Auger Depth (ft)

Core Depth (ft)

B-A1 37.8733478 -119.3522034 8595.92 2.6 -

B-A2 37.872693 -119.351243 8602.66 1.4 2.0

B-D1 37.87204541 -119.3639773 8574.61 1.5 -

B-GC1 37.872105 -119.356298 8615.98 5.3 -

B-ND3 37.871671 -119.353194 8627.49 2.8 16.5

B-ND5 37.87137267 -119.3561626 8638.42 1.5 -

Note: Boring locations surveyed with mapping grade EMLID Reach RS+ satellite receiver with estimated accuracy of +/- 3 feet horizontal and elevations were estimated from the topographic surface XC Topo.dwg provided by Otak.

A Golder engineer logged the borings and obtained representative samples of the soils encountered. Visual classifications of the encountered soils were made in general accordance with the Unified Soil Classification

System (USCS) and American Society for Testing and Materials (ASTM D 2488) classification procedures. The logs of the borings are presented in Appendix A. The soils were described in general accordance with ASTM D

2487 (Unified Soil Classification System). Rock cores were classified using the Standard Practice for Rock Core

Drilling and Sampling of Rock for Site Exploration (ASTM D2113). The boundaries between different soil/rock types shown on the logs are approximate; the actual transition between layers may be gradual.

Rock coring at B-GC1 and B-A2 used an HQ diamond core bit which drills through the rock, attached to a core barrel that collects the core during drilling. The core barrel is 5 feet long with an outer diameter (OD) of 3.8-inches and an inner diameter (ID) of 2.5-inches and is used to collect and retrieve the rock core from the boring. Each core run is the length drilled (no greater than the 5-foot length of the barrel) to collect each individual core sample.

The rock is inspected based on several geotechnical factors including the Rock Quality Designation (RQD), ISRM, and weathering index. RQD is a rough measure used to define the degree of fracture or jointing in rock mass (i.e., a measure of weathering). RQD is calculated as the summation of core pieces greater than 10 millimeters (mm) divided by the total core length. RQD of 0% is entirely fractured, and RQD of 100% is intact. Rock masses with poor quality may not have favorable engineering parameters (bearing capacity, sliding, settlement) for foundations but could increase rippability. The ISRM rock strength index rates the rock from extremely weak rock (R0, can be indented with a thumbnail) to extremely strong rock (R6, can only be chipped with a geologic hammer).

At the completion of each boring, the entire depth was backfilled with neat cement grout. All soil cuttings were drummed and taken off-site. Environmental laboratory testing (e.g. volatile organic compounds, CAM 17 metals, total petroleum hydrocarbons) was not required for the project, and hence was not conducted.

3.2.1 Laboratory Testing

The bulk samples of soil collected at each boring primarily consisted of silty sand with gravel and significant amounts of organic material. Representative soil samples collected during the drilling program were transported to

Hushmand Associates Inc. of Irvine California for the following geotechnical laboratory tests:

Particle Size analysis (ASTM D6913)

Atterberg limits (ASTM D4318)

R-Value analysis (CA Test Method 301)

Soil chemistry for corrosion potential (CA Methods 643, 417, and 422)

All results of the geotechnical laboratory testing were reviewed by Golder and are presented in Appendix B.

Sieve analysis confirmed the correct soil classification per the Unified Soil Classification System is shown on the logs in Appendix A. Atterberg limits testing results indicate the tested samples are non-plastic.

The samples’ corrosive potential is described in Section 4.5.

3.2.2 Subsurface Conditions Encountered at the Boring Locations

Subsurface conditions encountered at each boring are summarized below followed by a summary table with approximate depths and drilling methods.

B-A1

0 to 2.6 feet-below ground surface (bgs): topsoil and silty sand with significant amounts of organics

2.6 feet-bgs: hand auger refusal

Groundwater was not encountered at the time of drilling

B-A2

0 to 1.3 feet-bgs: topsoil and clayey sand with organics/rootlets and cobbles up to 4-inches in diameter

1.3 feet-bgs: hand auger refusal

1.3 to 2 feet-bgs: rock coring through granitic rock, slightly weathered, strong to very strong (ISRM R4-R5), RQD between 40 and 50%

Groundwater was not encountered at the time of drilling

B-D1

0 to 1.6 feet-bgs: topsoil with silty sand with organics and cobbles ranging in diameter from 2-inches to

4-inches

1.6 feet-bgs: hand auger refusal

Groundwater was not encountered at the time of drilling

B-GC1

0 to 2.4 feet-bgs: well-graded silty sand

2.4 to 7.4 feet-bgs: rock coring yielded slightly weathered granite cobbles and silty sand, only about 7-inches of recovery

7.4 to 12 feet-bgs: rock coring yielded slightly weathered granitic bedrock, RQD between 60 and 70%, strong to very strong (ISRM R4-R5)

12 to 16.5 feet-bgs: very strong (ISRM R5) fresh granitic bedrock, RQD between 90 and 100%

16.5 feet-bgs: completed drilling

Groundwater was not encountered at the time of drilling

B-ND3

0 to 1.5 feet-bgs: topsoil with clayey sand and organics, gravel present up to 3-inches in diameter

1.5 feet-bgs: hand auger refusal

Groundwater was not encountered at the time of drilling

B-ND5

0 to 5.3 feet-bgs: well graded silty sand with gravel, trace organics, gravel up to 3-inches in diameter

5.3 feet-bgs: hand auger refusal

Groundwater was not encountered at the time of drilling

Groundwater was not encountered in any of the six borings that were advanced as part of our geotechnical exploration. Based on the time of year of our geotechnical exploration, groundwater levels are generally near the seasonally highest level; however, potential seasonal fluctuation of the groundwater levels, if any, could not be measured as part of the current study. Golder could not locate any readily available data regarding potential seasonal groundwater fluctuations at the site.

3.3 Geophysical Survey

Between June 3 and June 5, 2021, Golder performed both seismic refraction and GPR explorations in select portions of the campground to obtain subsurface information using non-intrusive methods. The following bulleted list summarizes the geophysical field efforts and details are described in Section 3.3.1.2.

Approximately 6,400 linear feet of GPR data was collected over the project area to assist in determining depth to rock. Tracklines of GPR data are shown in Figure 1.

Significantly more GPR data was collected than was scoped to assess the consistency of results and obtain

GPR data adjacent to all hand auger locations to calibrate/ground truth the results.

Five seismic refraction lines were collected and their approximate locations are shown by the red lines in

Figure 1. Each line coincides with a boring/auger location.

3.3.1 Seismic Refraction Survey

Seismic refraction is the traditional method for mapping the thickness of overburdened soils and depth to bedrock using a controlled energy source (e.g., hammer, blank shotgun shells, chemical explosives) to generate a seismic signal into the earth. The seismic signals are received by an array of geophones (24, for example) that are connected to a seismic cable laid on the ground in a linear manner. The geophones are placed several inches into the ground and spaced approximately 5 to 15 feet apart along the seismic cable.

The seismic energy source is discharged at several places along the array and off both ends of the array. The seismic wave front travels through the earth to the geophones. The geophones transfer the acoustic energy in the ground to an electric signal in the geophone cable. This process is illustrated in Graphic 1. The seismograph detects the arriving electric signals with respect to time and stores the records for future data processing. The seismic data is processed to determine the seismic velocity of the earth material through which the energy has traveled and to model the subsurface geology. The geophysical model depicts the earth in cross-sections showing the velocity and thickness of the subsurface layers below the seismic line.

Graphic 1: Seismic Refraction Survey Process

3.3.1.1 Accuracy of Seismic Refraction

Seismic refraction often works well for characterizing subsurface conditions along a continuous profile when used in conjunction with other types of exploration methods. This method is well-suited for mapping both weathered bedrock and competent bedrock. In cases where no competent bedrock is detected, the seismic refraction method still provides valuable geotechnical information such as the variation in velocity structure, which is often a proxy for variation in geologic structure, or to document that no competent bedrock exists within the construction depths of the project. The depths to interpreted subsurface boundaries are generally accepted as accurate to within 15 percent of the true depths to the boundaries. In some cases where there are large velocity contrasts at sharp geologic boundaries and boring test data are available for model calibration, it may be possible for seismic refraction interpretations to match geotechnical borings to within 5 to 10 percent. In cases where bedrock is very shallow, the expected resolution is related to the geophone spacing and is often limited to between 1 and 2 geophone intervals.

Overall, there is a decrease in the accuracy of the seismic refraction method with depth. This is often due to the natural variability within the shallow geologic layers. This effect can be minimized by using a combination of closely spaced geophones, long survey lines, and large numbers of shot points. In practice, this results in a survey design where the length of the geophone array is approximately 5 times longer than the expected depth to bedrock and the geophone spacing is 1/3 of this depth. By using seismic refraction, complemented by traditional test borings, a more accurate subsurface characterization can be performed for a reduced cost, with far less uncertainty, and with a lower impact to the environment than drilling alone.

3.3.1.2 Seismic Refraction Survey Design

For this exploration, the Golder geophysicists utilized a Geometrics Geode seismograph, a 24-channel geophone cable with twenty-four 4.5 Hertz (Hz) geophones, hammer switch, and a 10-pound hammer source. Geophone spacing of 5 feet was used for all seismic lines, resulting in a line length of 115 feet. Additional seismic survey parameters are shown in Table 3-2.

Table 3-2: Seismic Refraction Acquisition Parameters

Parameter Setting

Geophone interval 5 feet

Shot interval 20 feet, based on field data

Off-end Shots 20 and 40 feet off each end

No. of Geophones 24

Geophone frequency 4.5 Hz

Record length 200 milliseconds

Sample interval 0.03125 milliseconds

Pre-amp gain 36 dB

Acquisition filters none

File format SEG2

Golder geophysicists located each seismic line where boring (hand auger or core log) information was available to provide verification and calibration to the seismic data. Figure 1 shows the approximate seismic line locations completed for current exploration.

Otak provided Golder with site-specific electronic topographic data in file XC topo.dwg, updated October 5, 2020, which was used to cut topographic profiles of each refraction line and incorporate a common vertical dataset into the seismic refraction data interpretation. For the purpose of this report, Golder considers this survey to be recent and applicable.

3.3.1.3 Data Processing and Interpretation

Seismic refraction data were processed using industry standard, commercially available SeisImager 2D PRO software from Geometrics Inc. The following processing steps were performed to produce the 2-dimensional velocity models presented in Figures 3, 4, and 5.

Edit source and geophone locations

Pick first arrival times

Export file of first arrival times and geophone locations

Generate initial velocity model (10 layers, 50-feet thick, constrained to 900 to 13,000 feet per second [fps])

2-dimensional inversion to determine velocity model

Export model results as three-column data files containing x, z, and velocity

Plot model results

Seismic refraction models are interpreted with respect to geologic conditions and based primarily on the seismic velocity structure. A typical range of seismic velocity values for weathered bedrock is 5,000 to 8,000 fps, while for competent bedrock the range is accepted as 8,000 to 16,000 fps. Weathered and fractured bedrock seismic velocity is often significantly lower than competent bedrock of the same type. These distinctions are the basis of our interpretations for soil/colluvium/overburden, hard colluvium or weathered bedrock, and more competent bedrock for each of the seismic refraction models. The seismic refraction results are presented in Figures 3, 4, and 5.

3.3.2 Ground Penetrating Radar

GPR data is collected by administering electromagnetic (radar) pulses into the ground from an antenna.

Reflections of these pulses from subsurface features are produced where there is a contrast between the electrical properties of subsurface objects (e.g., the surrounding soil, buried utilities). Reflected electromagnetic pulses are received by the antenna, converted into an electric signal, and recorded by the GPR unit. The GPR unit compiles these pulses to produce an interpretive profile image of the subsurface beneath the path of the antenna.

The penetration depth of the GPR signal is a function of the antenna frequency and the conductivity of the subsurface material. As the frequency of the GPR antenna increases, the resolution (ability to detect small objects) increases, but the depth of subsurface penetration decreases. A lower frequency antenna is capable of greater subsurface penetration, but with reduced resolution. Materials that are electrically conductive, such as clay, tend to attenuate the GPR signal, resulting in a decrease in subsurface penetration.

3.3.2.1 Data Acquisition and Interpretation

Golder collected GPR data along a total of 39 lines, shown in Figure 1 in green, ranging from approximately 90- to

415-feet in length. GPR data were collected along each of the seismic refraction survey lines, nearly all of the proposed roadway alignments, as well as other selected locations accessible using the four-wheel GPR cart.

GPR data were collected using a mid-frequency, 350-megahertz (MHz) GSSI UtilityScan radar system and processed on a desktop computer using RADAN 7 software. GPR data was collected to image depths up to approximately 12 feet-bgs. An example of GPR results is presented in Figure 6 to illustrate a small sample of subsurface conditions at the site.

The preliminary improvement plan drawings for the campground improvements provided to Golder indicate which utility trench excavations will range from 3 to 11 feet below existing grades and are deepest at the planned manhole vault locations. Using the provided drawings, we extracted the locations of manholes and compared the

GPR response along lines closest to each manhole vault where GPR data were available to provide the observations summarized in Table 3-3. An example of the GPR data closest to MH 120 is shown in Figure 7 along with a scaled graphic of the MH 120 profile. This example provides an indication of where the presence of boulders in the subsurface may require additional efforts to trench in the HDPE sewer line and sewer vault.

Table 3-3: GPR Observation Summary Near Proposed Manhole Vault locations

Feature Name

Distance from nearest GPR record (feet)

Observations from GPR Nearby Record(s)

MH 120 23

Soil layer boundary at 3 feet, rock appears 11 to 13 feet bgs, but with boulder layer 7 to 8 feet bgs

MH 130 26 Few cobbles but no obvious rock

MH 140 18

Cobbles at 4.5 to 6 feet rock dipping to the east 7.5 feet to 12+ feet bgs

MH 150 7

The existing trench is about 5 feet bgs, may be wet at the base of the trench

MH 160 5 Layering at 3 feet bgs, not clear

MH 180 35

GPR line end shorts of MH180, but rock appears less than 10 feet bgs

MH 200 15

Sandy with frequent cobbles from near the surface to 4 feet, 1 to 2 feet of boulders below cobbles; bedrock depth is uncertain

MH 210 25

Sandy with frequent cobbles from near the surface to 4 feet, 1 to 2 feet of boulders below cobbles; bedrock depth is uncertain

MH 220 25

Sandy with frequent cobbles from near the surface to 4 feet, 1 to 2 feet of boulders below cobbles; bedrock depth is uncertain

MH 221 18

Sandy with frequent cobbles from near the surface to 4 feet; either broken bedrock or boulders at 5 feet bgs

MH 230 6

Frequent cobbles from near the surface to 3 feet, 1 to 2 feet of boulders below cobbles; bedrock depth uncertain

MH 240 7 Occasional cobbles, no obvious bedrock to 9 feet bgs

MH 250 18 Occasional cobbles, possible bedrock at 8 to 8.5 feet bgs

MH 260 8 Occasional cobbles, possible bedrock at 8 to 8.5 feet bgs

MH 281 17

Sandy at the surface, frequent cobbles below 2 feet; possible broken bedrock or boulders below 3 feet bgs

MH 282 6

Occasional cobbles from near the surface to 3.5 feet; either broken bedrock or boulders at 3.5 feet bgs

MH 290 4

Frequent cobbles from near the surface to 4 feet; either broken bedrock or boulders at 4 feet bgs

MH 350 5

GPR evidence of rock at approximately 5 feet bgs, cobbles above rock

Feature Name

Distance from nearest GPR record (feet)

Observations from GPR Nearby Record(s)

MH 360 8 GPR evidence of rock at approximately 6 feet bgs

MH 370 40 Cobbles 3 to 6 feet bgs, appears wet to saturated below 8 feet

MH 380 38 GPR evidence of large cobbles/boulders from 3 to 6.5 feet bgs

MH 390 76 GPR evidence of large cobbles/boulders from 3 to 6.5 feet bgs

3.3.3 Surficial Soils and Depth to Bedrock

Subsurface material properties can be interpreted based on compressional seismic velocity; material descriptions

(i.e. lithology) are based solely on velocity structure. Seismic refraction results for each line of data collected are shown in Figures 3, 4, and 5. For each line, the velocity models are plotted using colored contours between

900 and 13,000 fps (feet per second) with a contour interval of 300 fps. Cool colors (e.g., blue and green) represent relatively low seismic velocity (colluvium, alluvium, and highly weathered bedrock) while hot colors

(e.g., orange and red) represent relatively high seismic velocity (bedrock).

There are three distinct layers of subsurface material interpreted in the models presented here with a further distinction provided relative to rippability. These layers are:

Soil/soft colluvium that is easily rippable; less than 5,000 fps

Hard colluvium/weathered rock that is easily rippable; between 5,000 and 7,800 fps

Fresh to weathered rock that is marginally to non-rippable; greater than 8,000 fps

A velocity contrast at 5,000 fps is used to delineate softer and harder colluvium which grades into weathered rock.

Ability to excavate or “rip” the material is highly dependent on equipment specifications. Groundwater also has an approximate velocity of 5,000 fps; so, while seismic refraction cannot directly detect groundwater, it can be assumed there is no groundwater present where the refraction model indicates velocities less than 5,000 fps.

3.3.4 Rippability of Bedrock

From the perspective of planning and cost assessment for excavations, rippability is a critical element in excavation costs. Graphic 2 below depicts an example rippability chart from the Caterpillar Performance

Version 48 (Caterpillar 2018) for a Caterpillar D9R/D9T dozer with ripper attachment for various materials. As shown in the chart, maximum rippable velocity is variable, based on the type of rock. Based on its rippability chart, a Caterpillar D9R or D9T can easily rip igneous materials, such as the granite underlying the site, with seismic velocities up to approximately 6,800 fps.

Graphic 2: Caterpillar Rippability Chart

Based on our understanding of the proposed improvements within the campground and the results of our subsurface investigation, we expect it will be common for the excavation and placement of foundation elements to encounter layers that are difficult to penetrate or rip within the upper 10 feet. Borings, GPR, and seismic refraction data all indicate the depth to competent, difficult-to-rip rock is highly variable, ranging from 0 (rock at the surface) to up to 20 feet-bgs.

Significant difficulties excavating to the subgrade elevation for new water and sewer infrastructure are not anticipated along most of the proposed alignments given the typical utility installation depth is 4 to 7 feet-bgs and as the mean interpreted depth to non-rippable material rock is approximately 10 feet-bgs, while the average excavation depth for the improvement infrastructure is much less. At select locations where proposed improvements are deeper than depths to non-rippable material, alternative excavation methods need to be considered during construction to avoid significant construction difficulties.

3.4 Site Subsurface Conditions

The subsurface conditions encountered at the locations of our exploration are highly variable. Surficial soils of varying depths consist of topsoil and silty sand with gravel and organics underlain by weathered bedrock and ultimately by fresh, hard bedrock. Comparison of seismic refraction lines with hand augering data indicates the following:

Boring B-GC1 encountered hand auger refusal at 2.8 feet-bgs whereas the seismic velocity profile along

SL-1 indicates soil/colluvium depths to a depth of approximately 8 feet-bgs at its location.

Boring B-A1 encountered hand auger refusal at 2.6 feet-bgs whereas the seismic velocity profile along SL-2 indicates soil/colluvium to a depth of approximately 8 feet-bgs at its location.

Boring B-A2 encountered hand auger refusal at 1.4 feet bgs which corresponds well with the seismic velocity profile along SL-3 at its location.

Both Boring B-ND5 and Seismic Line SL-4 encountered hard colluvium/weathered rock deeper than other exploration locations. However, B-ND5 encountered hand auger refusal at 5.3 feet-bgs whereas the seismic velocity profile along SL-4 indicates soil/colluvium to a depth of approximately 9.5 feet-bgs at its location.

Boring B-D1 encountered refusal at 1.5 feet-bgs whereas the seismic velocity profile along SL-5 indicates soil/colluvium to a depth of approximately 8 feet-bgs at its location.

In most cases, the interpreted depth to rock from the seismic velocity profiles does not match the stratigraphy and conditions encountered at the boring locations. However, the disparity is likely because borings were advanced using a 3-inch OD hand auger, rather than a drill rig, which could not deliver sufficient energy to push through or displace the cobbles and/or gravels that were encountered and, subsequently, resulted in shallow refusal depths.

It is our opinion that the seismic velocity profiles provide a more representative interpretation of the depth to marginal/non-rippable material than the depths to hand auger refusal.

3.5 Geologic Hazards

3.5.1 Strong Ground Motions

The site is located in an area of moderate seismicity and is subject to strong ground shaking. Ground motion parameters, as described in Section 4.1, indicate the peak ground acceleration (PGA) at the project site, corresponding to the risk-based maximum considered earthquake (MCE) ground motion and Site Class B material, is 0.345 g where g is the acceleration due to gravity. The impact of ground motions on the structural features related to the restroom buildings can be addressed by implementing the design recommendations provided in the latest California Building Code (CBC).

3.5.2 Liquefaction Potential

Liquefaction is the process in which the shear strength of soil decreases as a result of a decrease in the effective, confining stress at depth caused by an increase in excess pore water pressure. Liquefaction can be induced as a result of seismic shaking, and is most commonly associated with loose, saturated, cohesionless soils (i.e. sand and some non-cohesive silt).

The soils encountered during the geotechnical drilling are predominantly non-cohesive sand with varying amounts of non-plastic silt. Typically, soils with a significant clay fraction that exhibit cohesion (Plasticity Index greater than 7) have a low susceptibility to liquefaction, and the susceptibility increases as cohesion and plasticity decrease. Additionally, based on the ability to hand auger through the material, the encountered soils appear to have a loose to compact relative density. Generally, these are unfavorable characteristics in terms of liquefaction potential. However, because the recovered soil samples were dry to moist and groundwater was not encountered or observed in any of the borings, the encountered site soils are not likely to be susceptible to liquefaction.

Moisture of in-situ soils is expected to be caused by recent rain and snowmelt at the site. Although, as noted in

Section 3.2.2, the seasonal fluctuation of the groundwater levels, if any, could not be measured as part of the current study.

The overall risk of liquefaction is considered to be low due to the absence of groundwater and shallow depth to rock. Therefore, based on our understanding of the project, we do not believe a detailed liquefaction assessment is warranted. Furthermore, based on the low likelihood of liquefaction, we consider the potential for liquefaction-induced settlement to be low.

4.0 ENGINEERING RECOMMENDATIONS

4.1 Seismic Design Parameters

The site is located in a moderately active seismic region and could be subject to strong ground shaking. The basis for the 2019 CBC seismic design is the PGA and the 5-percent damped spectral accelerations for 0.2 seconds

(Ss) and 1 second (S1) at a rock site (Site Class B). Site coefficients (FPGA, Fa and Fv) are then used to scale the peak ground and spectral accelerations as a function of Site Class (if different than Site Class B) to develop a site-specific, PGA and 5-percent damped acceleration response spectrum. Section 1613 of the CBC outlines this procedure.

4.1.1 Site Class

Sites are classified according to the average soil profile properties in the upper 100 feet-bgs. The 2019 CBC refers to Chapter 20 of the American Society of Civil Engineers (ASCE), Minimum Design Loads for Buildings and

Other Structures (2016) (ASCE 7-16) to determine Site Class. The borings related to the current study were advanced to a maximum of 16.5 feet-bgs, and the seismic refraction survey was able to collect data to a maximum depth of approximately 50 feet-bgs. Although Golder did not explore to depths of 100 feet-bgs to definitively determine site class per ASCE 7-16, due to the depth of our explorations, the presence of relatively thin amounts of soil/colluvium over fresh granitic bedrock, and our understanding of the local geology, we consider it appropriate to classify the Site as Site Class B for seismic design purposes.

4.1.2 Ground Motion Parameters

Ground motion parameters used for design per the 2019 CBC include the mapped spectral acceleration and site coefficients, which can be found in Section 1613 of the 2019 CBC. The mapped spectral accelerations correspond to Site Class B conditions, and the following design parameters are based on the MCE ground motion. The interpolated probabilistic ground acceleration values in percent gravity (g) were obtained from the Structural

Engineers Association for California (SEA) hazard tool online (https://seismicmaps.org/).

Table 4-1 provides the recommended 2019 CBC seismic design parameters for the site based on Golder’s understanding of the site and Section 1613 of the 2019 CBC.

Table 4-1: 2019 California Building Code (CBC) Design Parameters

2016 CBC Seismic Parameter Value

Site Class B 0.345g(1)

Site Class B, 5-percent damped, 0.2-second spectral acceleration (Ss)

0.808

Site Class B, 5-percent damped, 1-second spectral acceleration (S1)

0.268

2016 CBC Seismic Parameter Value

Site Coefficient Fa 1.0

Site Coefficient Fv 1.0

Notes: (1) g represents the acceleration due to gravity

4.2 Bearing Capacity and Foundations

Based on the results of the current study, footings for the proposed restroom buildings will likely be founded on loose to compact colluvium soil. The seismic line on Figure 3 shows Boring GC-1 planned at a location with over

7 feet of soil/colluvium underlain by an additional 3 feet of easily rippable hard colluvium/ weathered bedrock.

Therefore, we do not consider it likely that foundations will encounter or be keyed into bedrock.

Based on the data collected from the current, we recommend the following regarding the bearing capacity of the native colluvium:

A maximum allowable bearing capacity of 2,500 pounds per square foot (psf) with a factor of safety (FS) of

2.5 for strip footings between 18 inches and 36 inches wide. These dimensions were denoted on the plan set titled Design Development Dawings_050521_Building Plans-Volume 2, Sheet SN1.0 which are shown in

Graphic 3.

The recommended maximum allowable bearing capacity may be increased by one-third for seismic loading.

The recommended maximum allowable bearing pressure will result in less than 1-inch of total settlement and differential settlement on the order of ½-inch.

Colluvium should be over-excavated 2 feet and recompacted unless a representative of Golder is present during construction to verify the in situ relative density of the native material is adequate.

Wall footings should be embedded at least 24 inches below the adjacent finished grades or deeper as required for frost protection.

If loose, soft, wet, frozen or otherwise disturbed soils are encountered at the foundation subgrade, these soils should be removed to expose suitable foundation soils, and the resulting over-excavation backfilled with compacted engineered fill. The base of all excavation should be dry and free of loose materials at the time of concrete placement.

A representative of Golder should be allowed to observe the foundation-bearing soils prior to placement of forms and rebar to verify the foundation-bearing soils are consistent with the soils encountered at the time of this study.

The foundations must resist lateral loads due to earth pressures, wind, and seismic events. For design purposes, these loads can be resisted simultaneously by:

Base Friction: An allowable value of 0.3 can be assumed for base friction between the soil and footings.

This value includes a factor of safety of 1.5. The allowable base friction value may be increased by one-third for the seismic loading.

Passive Resistance on Sides of Footings: For design purposes, we recommend ignoring the upper 2 feet and using a passive pressure corresponding to 150 psf per foot of depth. The allowable passive resistance can be increased by one-third for seismic loading. The recommended allowable passive resistance includes a factor of safety of 2.

Graphic 3: Restroom Foundations Provided by Otak

4.3 Permanent Wall Design Criteria

For backfill placed against permanent walls, we recommend importing clean (i.e. <10% fines) granular fill (sand and gravel). Lateral earth pressure coefficients and soil unit weight recommendations for the design of permanent walls based on imported engineered fill and are presented in Table 4-2. We recommend a triangular pressure distribution to represent the earth pressures. Additional surcharges from traffic, construction, buildings, etc., should be added to these values. We recommend accounting for the construction surcharge on the wall with horizontal pressure equal to 100 psf acting over the wall height. We recommend accounting for vehicle surcharge

(passenger cars and light maintenance vehicles) on the wall with a horizontal surcharge equal to 60 psf acting over the wall height. The earth pressures presented assume a drained condition behind the wall.

Table 4-2: Geotechnical Design Parameters for Permanent Walls

Design Parameter Value

Active Earth Pressure Coefficient (Ka) 0.28

At-Rest Earth Pressure Coefficient (K0) 0.44

Seismic Active Earth Pressure Coefficient (Kae1) 0.40

Seismic Active Earth Pressure Coefficient (Kae2) 0.32

Total Unit Weight 130 pcf

Notes:

1) Use Ka for design of permanent walls free to rotate.

2) Use K0 for design of permanent walls restrained at the top.

3) Use Kae1 for design of permanent walls that cannot deflect during design earthquake. Assumes Kh = PGA

4) Use Kae2 for design of permanent walls where permanent deflections of 1 to 2 inches resulting from the design earthquake are acceptable.

Assumes Kh = 1/2PGA

Additionally, drainage should be provided behind the walls and can consist of a full face geocomposite drainage mat or a minimum of a 2-foot wide zone of clean sand and gravel fill with less than 5-percent passing the No. 200 sieve.

4.4 Structural Pavement Section

Per the laboratory testing results presented in Appendix B, the subgrade soil samples at BA-1 and BND-5 have an

R-value of 70 and 71, respectively. However, Golder calculated a structural pavement thickness for the proposed parking lots and access road configurations using the maximum recommended design R-value of 50 as outlined in chapters 600, 610 and 630 of the California Department of Transportation (CalTrans) Highway Design Manual

(HDM) (Caltrans 2020). In accordance with Table 613.5B of the HDM, a minimum traffic index (TI) of 5.0 is recommended for a 20-year service life for automobile parking areas. We calculated a structural pavement section for the recommended TI of 5.0, as well as for more conservative TI values of 6.0 and 7.0 for comparison.

These results are summarized in Table 4-3.

Table 4-3: Summary of Structural Pavement Calculations

TI Thickness of AC (inches) Thickness of AB (inches)

5.0 3.0 3.0

6.0 3.5 4.0

7.0 4.0 5.0

Golder was not provided with site-specific traffic data, however, based on our experience we believe that TI values of 6.0 and 7.0 are suitable for campground access roads with occasional RV traffic. Golder understands that the park experiences about 140,000 annual visitors and a full capacity of 500 people during the busy season.

We also understand that the campground is open from May/June to October/November and these calculations account for the limited operating season. In areas where heavy traffic (e.g. “vactor” trucks, garbage trucks, buses, etc.) are expected, a thicker section is recommended.

4.5 Soil Corrosivity

Laboratory testing on samples of native soil from BGC-1 and BND-3 indicates characteristics of generally non-corrosive material. The tested samples’ pH, minimum resistivity value, chloride content, and sulfate content are all within the typical ranges of noncorrosive material. Although the pH of both tested samples is on the low end of the generally noncorrosive range, the other attributes are well within the range of generally noncorrosive values.

Golder tested two samples for corrosion potential. Additional testing may be necessary prior to or during construction if proposed buried concrete and/or metal elements are considered sensitive to corrosion. The laboratory testing results are presented in Appendix B.

5.0 CONSTRUCTION RECOMMENDATIONS

Based on our field exploration described in Section 3 and the findings of the desktop study (Golder 2021), we have prepared the following construction recommendations for Otak’s proposed campground improvements.

Careful earthworks planning and subgrade protection by the contractor and implementation of the recommendations presented herein will help minimize unanticipated costs. We recommend that any excavation on the site be sequenced to limit the amount of exposed subgrade particularly if construction starts during the rainy season.

All compaction requirements presented in this report are relative to ASTM D1557. Relative compaction is a percentage that is calculated as the ratio of the in-place measured soil density divided by the same soil’s maximum dry density as determined by the ASTM D1557 laboratory test procedure. Optimum moisture content is the corresponding moisture content of the same soil at its maximum dry density.

Note that earthwork construction during wet weather can significantly increase costs by making placement of fill soils difficult due to over-optimum moisture contents, increasing the cost for off-site disposal of unsuitable excavated soils, increasing the effort to control water, and increasing subgrade disturbance resulting in the need for soil admixtures, geotextiles, or rock working mats. During construction, surface water runoff should be controlled and directed away from any excavation and any temporary cut slopes.

5.1 Subgrade and Foundation Preparation

Based on the results of the current study, and the drawings provided to Golder, the subgrade of the proposed slab on grade foundations is anticipated to consist of native colluvium soil with a loose to compact relative density. The drawings provided to Golder indicate utility trench excavations will range from 3 to 11 feet below existing grades.

Therefore, based on the seismic velocity profiles along the location of our exploration, Golder expects a majority of the excavation will be easily rippable although some portions of marginally to non-rippable rock near the surface should be anticipated. Excavation of hard, non-rippable rock may require the use of a hydraulic hammer

(e.g. “Hoe-Ram”), chemical expanding foam, trench blasting, or alternative methods.

Site construction activities include clearing and grubbing, excavation, subgrade preparation, utility trenching, placement of foundations and placement and compaction of engineered fills. During construction, surface water runoff should be controlled and directed away from any excavation and any temporary cut slopes.

We recommend that the subgrade conditions are observed by a qualified geotechnical engineer prior to the placement of rebar, concrete or engineered fill. Where feasible, we recommend that proof-rolling with a fully loaded dump truck (or other heavy equipment) be observed.

Any soft or excessively yielding areas should be stabilized or removed and replaced. Similarly, any areas that are harder or stiffer than the surrounding area should be re-conditioned. Properly prepared subgrade is the basis for uniform support of a slab on grade, pavement, and minimizing differential settlement between footings.

Native competent subgrade that is wet and unsuitable or that becomes loosened by the contractor’s operation should be properly moisture conditioned and compacted or over-excavated and replaced with a suitable engineered fill. Subgrade that is loosened by the contractor’s operations should be repaired or replaced at the contractor’s expense. Footing excavations should be free of organics, and any loose, soft, disturbed material or water prior to placement of reinforcing bars and concrete.

5.2 Fill Placement and Compaction

The subsurface conditions encountered at the locations of our geotechnical borings indicate the surficial 1 to

3 feet-bgs consist of silty sand with gravel. Based on the amount of organic material present in the native soils encountered, Golder does not recommend reusing the material generated from the shallow cuts to construct the utility trenches as a source of fill material.