ATTACHMENT 6 - Geotechnical Report.pdf

Geotechnical Engineering Report

DESIGN MEDICAL SPECIALTY CLINIC

WKA No. 13141.01

May 26, 2021

Prepared for:

Apogee Consulting Group, P.A.

1151 Kildaire Farm Road

Cary, North Carolina 27511

Mather, California

TABLE OF CONTENTS

INTRODUCTION

Scope of Services

Supplemental Information

Figures and Attachments

Proposed Development

FINDINGS

Site Description

Area History

General Site Geology

Subsurface Soil Conditions

Groundwater Conditions

Faulting and Seismicity

CONCLUSIONS

Seismic Design Criteria

Seismic and Geologic Hazards

Foundation Support

Expansive Soil

Excavation Conditions

Fill Material Suitability

Pavement Subgrade Quality

Soil Corrosion Potential

Groundwater and Seasonal Moisture

RECOMMENDATIONS

Site Clearing

Subgrade Preparation

Engineered Fill

Utility Trench Backfill

Shallow Spread Foundations

Interior Floor Slabs

Moisture Penetration Resistance

Exterior Flatwork

Pavement Design

Site Drainage

Drought Considerations

Geotechnical Engineering Construction Observation Services

LIMITATIONS

Mather, California

TABLE OF CONTENTS (continued)

FIGURES

Vicinity Map .......................................................................................................... Figure 1

Site Plan .............................................................................................................. Figure 2

Logs of Borings .................................................................................. Figures 3 through 5

Unified Soil Classification System ......................................................................... Figure 6

APPENDIX A – General Project Information, Laboratory Testing and Results

Atterberg Limits Test Result .............................................................................. Figure A1

Resistance Value Test Result ............................................................................ Figure A2

Corrosion Test Results ........................................................................ Figures A3 and A4

APPENDIX B – Logs of Previous Test Pits (Project Nos. 2862.01, 2862.36, and 12447.01)

APPENDIX C – Report for Refraction Microtremor Investigation

10535 Hospital Way Mather, California WKA No. 13141.01 May 26, 2021

INTRODUCTION

We have completed a geotechnical engineering study for the planned Design Medical Specialty Clinic to be constructed at the Sacramento VA Medical Center in Mather, California. The purpose of our study has been to explore the site, soil and groundwater conditions across the property, and to provide geotechnical engineering conclusions and recommendations regarding the proposed residential improvements. This report presents the results of our study.

Scope of Services

Our scope of work for this project included the following:

1. Site reconnaissance;

2. Review of historic topographic maps, aerial photographs, and available geologic maps that include the project site;

3. Review of previous geotechnical engineering work performed by Wallace-Kuhl and

Associates (WKA) in the vicinity of the site;

4. Subsurface exploration, including the drilling and sampling of three borings to a maximum depth of approximately 17 feet below the existing ground surface;

5. Laboratory testing of selected soil samples;

6. Engineering and geologic analyses; and,

7. Preparation of this report.

Supplemental Information

To assist with the preparation of this report, we have reviewed the following reports:

Wallace-Kuhl & Associates, Inc., 1995, Geotechnical Engineering Report prepared for the Capital Center Projects A and B, WKA No. 2862.01, dated January 31, 1995;

Wallace-Kuhl & Associates, Inc., 1998, Geotechnical Engineering Report prepared for the Mather VA Hospital, WKA No. 3746.01, dated May 15, 1998;

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• Wallace-Kuhl & Associates, Inc., 1998, Engineering Geologic Report prepared for the

Mather VA Hospital, WKA No. 3746.02, dated May 15, 1998;

• Wallace-Kuhl & Associates, Inc., 1999, Geotechnical Engineering Report prepared for the Mather VA Medical Center Expansion, WKA No. 3746.04, dated June 9, 1999;

• Wallace-Kuhl & Associates, Inc., 1999, Geotechnical Engineering Report prepared for the Capital Center Lots 77 – 80, WKA No. 2862.36, dated June 23, 1999; and,

• Wallace-Kuhl & Associates, Inc., 2002, Geotechnical Engineering Report prepared for the Rancho Cordova Radiation Therapy Building, WKA No. 5332.01, dated December

27, 2002.

• Wallace-Kuhl & Associates, Inc., 2019, Pavement Design Report prepared for the

Mather VA Hospital Parking Lots, WKA No. 12447.01, dated August 28, 2019.

Figures and Attachments

This report contains a Vicinity Map as Figure 1; a Site Plan showing the boring locations and the location of previous test pits as Figure 2; and Logs of Borings as Figures 3 through 5. An explanation of the symbols and classification system used on the logs is contained in Figure 6.

Appendix A contains general information regarding project concepts, exploratory methods used during our field exploration, and laboratory test results that are not included on the boring logs.

Appendix B contains the logs of test pits previously performed within or near the project site.

Appendix C contains results of a refraction Microtremor investigation performed by Gasch

Geophysical Services, Inc.

Proposed Development

We anticipate the proposed clinic will be a single-story, wood or light steel frame building measuring about 13,000 square feet in plan area with a concrete slab-on-grade floor system.

Structural foundation loads are anticipated to be relatively light, typical for a single-story office or commercial building. Associated improvements are anticipated to include underground utilities, irrigated landscaping, various concrete flatwork, and paved parking and drive aisles.

Grading plans for the proposed project were not currently provided. Given the relatively level topography of the site, we anticipate maximum cuts and fills to level the site during earthwork should be less than two feet.

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FINDINGS

Site Description

The proposed clinic will be constructed in the northeast portion of the medical center, southwest of the intersection of International Drive and Femoyer Street. In general, the project site was bound to the north by vacant property covered by gravel, cobble and a moderate growth of weeds; to the south by a hospital maintenance facility and both gravel and asphalt-concrete covered parking lots; to the east by a construction laydown yard and elevated water tower; and to the west by a gravel covered parking lot and what appears to be a stormwater pump station.

According to the U.S. Geological Survey’s topographic map of the Carmichael Quadrangle, California, 7.5-minute series, dated 2018, the ground surface elevation at the site is about

100 feet (NAVD88).

At the time of our field reconnaissance, the project site was occupied by a construction laydown yard with a relatively large stockpile of soil and debris located within most of the proposed building area. Several construction trailers, construction debris and gravel covered parking enclosed by a chain-link fence was located in the eastern portion of the project site. Storage containers, various construction debris and a gravel covered parking lot occupied the western portion of the site. Aside from the stockpiled soil and debris, topographically the project site was essentially level.

Area History

The project site is located within the southwest edge of the Los Americanos dredge fields, an area that was extensively dredged for gold recovery during the early 1900's with large floating dredges. The dredges excavated vast quantities of river deposited sediments as they moved across the land. Gravel and cobbles were screened off and deposited with sand in long ridges as the dredges worked. Clay and silt were washed overboard and were deposited in low-lying areas between the ridges. The ridges are commonly referred to as "tailings windrows" and the clay and silt deposits are referred to as "slickens". Mass grading operations after the completion of dredging created relatively flat terrain across the property.

General Site Geology

The project site is located in the east portion of Sacramento, which in turn is located in the south-eastern portion of the Sacramento Valley. Physiographically, the project site is located along the eastern margin of the Central Valley, which encompasses the San Joaquin and

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Sacramento Valleys and is generally bounded by the Sierra Nevada Mountains to the east, the Coast Ranges to the west, the Mojave Desert and Transverse Ranges to the south, and the Klamath Mountains to the north. The Sacramento Valley extends roughly from the

Stockton-Tracy area on the south to the Klamath Mountains on the north. The Central Valley is a large north westward trending, asymmetric structural trough with a long, gently sloping eastern shelf underlain by the subsurface extension of the Sierran granitic rocks and a shorter more steeply sloping western margin where the basin sediments have been upturned and dip eastward back toward the valley axis. The Central Valley has been filled with more than

30,000 feet of sediment in the San Joaquin Valley and more than 50,000 feet of sediment in the Sacramento Valley (Bertoldi and others, 1991; Harwood and Helley, 1985) derived primarily from erosion of the adjacent Sierra Nevada and Coast Range Mountains. The sediment ranges in age from the Jurassic to recent (approximately 210 to 1.6 million years old) while bedrock underlying the sediment are predominantly marine deposits of siltstone, claystone and sandstone.

The surficial or near-surface soils in the project area consists of dredge tailings that consist of gravel, cobble, and clay. These materials, and the remainder of the hospital facility to the south, are underlain by large broad alluvial fans of Pleistocene age (10,000 to 1.8 million year old) referred to as the Riverbank Formation. This formation is composed of semi-consolidated silt, clay, sand and gravel derived from the western slopes of the Sierra Nevada that buries Miocene to Eocene age (approximately 5 to 58 million year old) alluvial fans of the Turlock Lake, Laguna and Mehrten Formations (Dupras and others, 1988; Harwood and Helley, 1985).

Subsurface Soil Conditions

The subsurface soil conditions within and near the project site were initially explored by WKA in

1994 by excavating test pits (test pit logs T1, T2, T12 and T13) using a track-mounted excavator to depths of about 10 to 15 feet bgs. The subsurface soil conditions were further explored by

WKA in 1999 by excavating additional test pits (test pit logs TP17, TP18, and TP19) using an excavator to depths ranging from about 10 to 12 feet bgs. Additional test pits were excavated in

2019 (test pit logs TP2 though TP5) using a tire-mounted backhoe to depths ranging from about

5 to 11½ feet bgs. Finally, the subsurface conditions within the proposed building area were explored on April 23, 2021, by drilling three borings (borings D1 through D3) to depths of about

8 to 17 feet bgs. Auger refusal in resistant gravel and cobble was encountered at each boring.

The approximate test pit locations are presented as Figure 2.

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The soil conditions encountered appear to be generally consistent with the mapped geology described above and with the soil conditions encountered in the previous referenced studies performed by our firm in the local area. In general the surface and near-surface conditions throughout a majority of the site appear to consist of dredge tailings that vary from sandy to clayey gravel and cobbles. Medium plastic, lean clay with variable concentrations of gravel and cobble where encountered within the proposed building area to depths ranging from about four to 17 feet bgs, the maximum depth explored. Most of these deposits didn’t appear to be

“slickens” associated with previous dredging activities. However, relatively wet and weak clay was encountered between depths of about 6½ and nine feet at boring D3 that appears to be slicken material.

The subsurface conditions described above are a generalized interpretation of the conditions encountered. For specific information regarding the subsurface conditions encountered at each boring location, refer to the exploration logs presented as Figures 3 through 5. The logs of test pits from previous explorations within the project area are presented in Appendix B.

Groundwater Conditions

We did not encounter permanent groundwater within our current borings and previous test pits during our field explorations. To supplement the on-site groundwater data, we reviewed available groundwater information at the California Department of Water Resources (DWR, 2018) website. The DWR periodically monitors groundwater levels (typically once in the spring and again in the fall) in wells across the state. Their website shows one monitored well located less than one mile north of the site. The well is identified as State Well No. 09N06E34R001M and has a ground surface elevation of about 98.8 feet (NGVD 88), similar to the project site.

Groundwater data for the well was recorded from 1962 to at least 2013. The well data suggests that the groundwater level has steadily declined during the monitoring period, with the highest recorded groundwater elevation during the past 20 years of about 21½ feet (NGVD 88) or about

77.3 feet bgs at the well location. This geotechnical evaluation assumes that high groundwater at the project site will not exceed this elevation.

Faulting and Seismicity

Compared with other areas of California, Mather has had moderately low seismic activity based on historical records. The San Andreas fault (SAF), located about 88 miles to the southwest, is considered to dominate the structure and seismicity of the region. To the west of the SAF is the

Pacific plate which moves north relative to the North American plate located east of the fault. In the San Francisco Bay area, movement across this plate boundary is concentrated on the SAF;

Geotechnical Engineering Report Page 6 however, it is also distributed, to a lesser extent, across a number of faults which include the

Calaveras, Concord-Green Valley, Greenville, Rodgers Creek, and Hayward faults among others. Together, these faults are referred to as the SAF system. Some significant events attributed to the SAF include the 1906 (M8+) San Francisco earthquake, the 1838 and 1865

(M7) San Francisco earthquakes, and the 1989 (M6.9) Loma Prieta earthquake. Seismic events attributed to some of the secondary faults located in the east San Francisco Bay area include the 1861 (M5.7) San Ramon Valley earthquake on the Calaveras fault, the 1868 (M6.8)

Hayward fault earthquake, the 1889 (M6.3) Antioch earthquake, the 1955 (M5.4) Concord fault earthquake, the 1980 (M5.8) Livermore earthquake on the Greenville fault, and most recently the 2014 (M6.0) Napa Valley earthquake on the West Napa fault.

In recent years, a series of predominately blind thrust faulting, reverse faults and folds that extend for several hundred miles from the southern San Joaquin Valley to the northern

Sacramento Valley have been used in seismic source characterization. This system is commonly referred to as the Great Valley fault system. Regional studies have suggested that the Great Valley fault system is comprised of between 18 and 25 segments ranging from 7 to

35 miles in length on the basis of the seismo-tectonic environment of the area. Fault segments with the closest approach to the project site are known as the Great Valley 3, 4, and 5 fault zones (Jennings and Bryant, 2010). The Great Valley 4 fault zone, about 38 miles west of the project site, is believed to be the source of the 1892 (M6.4) Winters-Vacaville earthquake

(Toppozada, et. al, 1981). Other earthquakes attributed to the Great Valley fault system include the 1983 (M6.3) Coalinga earthquake and the 1985 (M6.1) Kettlemen Hills earthquake.

Using the USGS and CGS, 2010, Quaternary fault and fold database for the United States, accessed on July 7, 2020, from USGS web site: http://earthquake.usgs.gov/hazards/qfaults/ and the Revised 2002 California Probabilistic Seismic Maps (Cao, et al, 2003), Table 1 shows known faults and fault systems within 100 kilometers (about 62 miles) of the site that are considered capable of producing earthquakes with a moment magnitude (MW) of 6.5 or greater.

http://earthquake.usgs.gov/hazards/qfaults/

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Table 1

Fault Name Distance Maximum

Magnitude (Mw) Miles Kilometers

Great Valley 4a, Trout Creek 38.4 61.8 6.6

Great Valley 4b, Gordon Valley 40.4 65.0 6.7

Great Valley 3, Mysterious Ridge 41.7 67.1 7.1

Great Valley 5, Pittsburg Kirby Hills 42.2 67.9 6.7

Hunting Creek-Berryessa 49.8 80.1 7.0

Green Valley Connected 49.9 80.3 6.8

Greenville Connected 56.0 90.1 7.0

West Napa 58.8 94.6 6.7

Great Valley 7 59.3 95.4 6.9

North San Andreas 88.2 141.9 8.0

CONCLUSIONS

In our opinion, the project is feasible from a geotechnical standpoint, provided that the conclusions and recommendations presented in this report are incorporated into the project design and specifications. The principal geotechnical considerations are discussed in the following subsections.

Seismic Design Criteria

The 2019 edition of the California Building Code (CBC) references the American Society of Civil

Engineers (ASCE) Standard 7-16 for seismic design. Using the latitude and longitude for the approximate center of the project site, Table 2 provides the 2019 seismic design parameters developed using a web interface developed by the Structural Engineers Association of

California (SEAOC) and the California Office of Statewide Health Planning and Development

(OSHPD) (https://seismicmaps.org). Since S1 is greater than 0.2g, the 2019 CBC coefficient values Fv, SM1, and SD1 presented are valid for seismic design, provided the requirements in

Exception Note No. 2 in Section 11.4.8 of ASCE 7-16 apply, specifically if T ≤ 1.5TS. If not, a site-specific ground motion hazard analysis is required.

https://seismicmaps.org/

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Table 2

Latitude: 38.8557° N

Longitude: 121.2861° W

ASCE 7-16

Table/Figure

CBC Table/Figure

Factor/

Coefficient Value

Short-Period MCE at

0.2 seconds Figure 22-1 Figure 1613.2.1(1) SS 0.458

1.0 second Period MCE Figure 22-2 Figure 1613.2.1(2) S1 0.225

Soil Class Table 20.3-1 Section 1613.2.2 Site Class C

Site Coefficient Table 11.4-1 Table 1613.2.3(1) Fa 1.3

Site Coefficient Table 11.4-2 Table 1613.2.3(2) Fv 1.5

Adjusted MCE Spectral

Response Parameters

Equation 11.4-1 Equation 16-36 SMS 0.596

Equation 11.4-2 Equation 16-37 SM1 0.337

Design Spectral

Acceleration Parameters

Equation 11.4-3 Equation 16-38 SDS 0.397

Equation 11.4-4 Equation 16-39 SD1 0.225

Seismic Design Category

Table 11.6-1 Table 1613.2.5(1)

Risk Category

I to III C

Risk Category

IV

D

Table 11.6-2 Table 1613.2.5(2) Risk Category

I to IV D

Notes: MCE = Maximum Considered Earthquake; g = gravity

A refraction Microtremor investigation was performed by Gasch Geophysical Services, Inc. to determine the average one-dimensional shear wave (S-wave) velocity value for upper 100 feet of subgrade soils. The results of the evaluation estimated an average shear wave velocity

(Vs100) of 1,220 feet per second. In accordance with Table 20.3.1 of ASCE 7-16, the Vs100 profile falls within a site “C” classification. The results of the refraction Microtremor investigation is provided in Appendix C.

Seismic and Geologic Hazards

The general project area is characterized by recurring seismic activity. During the design life of the proposed development, it is probable that at least one earthquake will cause moderate to strong ground shaking in the vicinity of the project. In comparison with other areas of California, however, the project area is seismically quiescent. No active faults which displace valley alluvium are known to exist at or near the project site nor does the site lie with an Earthquake

Alquist-Priolo Fault Zones (2018).

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Given the lack of groundwater (both encountered and historical) in the upper 50 feet of subgrade and that the soils encountered were the generally high in clay content and/or to stiff/dense to be susceptible to liquefaction, the potential for liquefaction or liquefaction induced settlement at the project site is considered remote. Furthermore, it is our professional opinion that the potential for other seismic or geologic hazards, such as dynamic settlement, slope instability, and regional subsidence, is low to negligible.

Foundation Support

Based on our findings, the proposed project site appears to be underlain by old dredge tailing consisting of sandy and clayey gravel and cobble and medium plastic clay that are variably-dense and pose a risk for excessive differential movement. In our opinion, this material, in its current condition, will not be suitable for support of the proposed structure. The most direct method to improve the subgrade conditions would be to support the structure on a uniform layer of overexcavated and compacted materials. This procedure would serve to provide a uniform, stable subgrade immediately below the structure, thus reducing differential settlement.

The clay slickens encountered at boring D3 are very weak, highly compressible, and not suitable for support of structures or pavements. To reduce the risk for adverse differential settlement, potholes should be performed once the building site has been overexcavated to verify the presence or absence of slickens. If encountered, these soils should be removed or mixed with on-site gravel and cobbles until a stable fill condition is achieved.

Expansive Soil

The near-surface gravel and cobble that appear to blanket most of the project site are non- to low-plastic and should exhibit low expansion (shrink/swell) characteristics. Accordingly, measures to resist or control potential soil expansion pressures are not considered necessary where improvements are supported on these materials. The medium plastic clay, however, may exhibit a moderate expansion potential and poses a risk for future heave and cracking of concrete slabs, as well as lightly loaded foundations and pavements. Accordingly, the clay, where encountered during grading, should not be used as engineered fill or remain within the upper two feet of final subgrade in the improvement areas. Where encountered, the clay should be overexcavated and replaced with native sand, gravel or import soils.

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Excavation Conditions

The surface and near-surface soils at the site should be readily excavated using conventional earthmoving and trenching equipment. Shallow excavations (less than 5-feet deep) in the silt and clay encountered should stand vertically for a period long enough for typical foundation and utility, unless they become wet or are disturbed. The gravel and cobble encountered, however, is cohesionless and will likely cave and/or slough soon after it is exposed in the excavation. The contractor should be prepared to brace or shore the excavations, as necessary, where these materials are encountered.

Temporarily sloped excavations and shored excavations less than 20 feet in depth should be constructed in accordance with federal, local and OSHA standards (29 CFR Part 1926) under the guidance of the Contractors qualified “competent person.” For preliminary evaluation, the silts and clays encountered would classify as Cal-OSHA Type B soil, while the gravel and cobble would classify as Type C soils. In no case should the information provided be interpreted to mean that Wallace-Kuhl & Associates is assuming responsibility for site safety or the Contractor’s activities.

Excavated materials should not be stockpiled directly adjacent to an open excavation to prevent surcharge loading of the excavation sidewalls. Heavy or frequent truck and equipment traffic should also be avoided near excavations. If material is stored or heavy equipment is stationed and/or operated near an excavation, a shoring system must be designed to resist the additional pressure due to the superimposed loads.

Fill Material Suitability

The on-site soils would generally be suitable for use as engineered fill. The clay deposits should not be used as fill unless they are at a workable moisture content, are thoroughly mixed with granular soil, and used at least two feet below final building pad elevation. Aggregate base recovered from existing parking areas may be reused as general engineered fill.

Pavement Subgrade Quality

The results of previous laboratory Resistance ("R") values (California Test 301) tests indicate the near-surface clay, silt and sand will provide marginal support characteristics for pavements as represented by R-values ranging from 15 and 25. Conversely, the gravel and cobble dredge tails should provide good support for pavements with an R-value of at least 40. Given the anticipated grading and mixing of soils during earthwork construction, in our opinion it is

Geotechnical Engineering Report Page 11 reasonable to use an R-value of 25 for pavement evaluations. Clay, where encountered within two feet of final soil subgrade, is considered poor quality materials for support of asphalt pavements and will require removal and replacement with granular soils, or other mitigation.

Soil Corrosion Potential

One sample of the near-surface soil were submitted to Sunland Analytical Lab of Rancho

Cordova, California, for testing to determine pH, chloride and sulfate concentrations, and minimum resistivity to help evaluate the potential for corrosive attack upon buried concrete. The results of the corrosivity testing are summarized in Table 3. A copy of the test report is presented on Figures A3 and A4.

Table 3

SOIL CORROSIVITY TESTING

Analyte Test Method

Sample

Identification

D1 (0'-3) pH CA DOT 643 Modified* 7.0

Minimum Resistivity CA DOT 643 Modified* 1,580 -cm

Chloride CA DOT 422 3.1 ppm

Sulfate CA DOT 417 44.7 ppm

Sulfate – SO4 ASTM D-516 42.6 mg/kg

Notes: * = Small cell method, -cm = Ohm-centimeters, ppm = Parts per million, mg/kg= milligrams per kilogram

The California Department of Transportation (Caltrans) 2018 Corrosion Guidelines

(Version 3.0), considers a site to be corrosive to foundation elements if one or more of the following conditions exists for the representative soil sample taken: the soil has a chloride concentration greater than or equal to 500 ppm, sulfate concentration greater than or equal to

2,000 ppm, or the pH is 5.5 or less. Based on this criterion, the on-site soils tested are not considered corrosive to concrete or steel reinforcement properly embedded within Portland cement concrete (PCC).

The California Amendments to Section 10.7.5 of the American Association of State Highway and Transportation Officials (AASHTO) bridge design specifications, 6th Edition (AASHTO 2012) considers soils to be corrosive to buried metals if the minimum resistivity is 1,000 ohm-cm or

Geotechnical Engineering Report Page 12 less. Based on this criterion, the on-site soil tested is also not considered corrosive to buried metal.

Table 19.3.1.1 – Exposure Categories and Classes, of American Concrete Institute (ACI)

318-14, Section 19.3 – Concrete Design and Durability Requirements, as referenced in

Section 1904.1 of the 2016 CBC, indicates the severity of sulfate exposure for the sample tested is Exposure Class S0 (water-soluble sulfate concentration in contact with concrete is low and injurious sulfate attack is not a concern). The project structural engineer should evaluate the requirements of ACI 318-14 and determine their applicability to the site.

Wallace-Kuhl & Associates are not corrosion engineers. Therefore, if it is desired to further define the soil corrosion potential at the site, a corrosion engineer should be consulted.

Groundwater and Seasonal Moisture

Previous referenced exploration data, near-by well data, and our current findings suggests that groundwater levels should not encroach near-surface or impede grading operations at the site.

However, if site grading is performed during or following extended periods of rainfall (winter and spring months), the moisture content of the near-surface soils may be significantly above optimum and unstable.

Typical remedial measures include discing and aerating the soils during dry weather, mixing the soils with dryer materials, removing and replacing the soils with an approved fill material, stabilization with a geotextile fabric or grid, or mixing the soils with an approved hydrating agent such as a lime or cement product. Our firm should be consulted prior to implementing any remedial measure to observe the unstable subgrade condition and provide site-specific recommendations.

RECOMMENDATIONS

The recommendations presented below are appropriate for typical construction in the late spring through fall months. The on-site soils likely will be saturated by rainfall in the winter and early spring months, and will not be compactable without drying by aeration, chemical treatment to dry the soils, or geogrid stabilization. Should the construction schedule require work during wet conditions, additional recommendations can be provided, as conditions dictate.

The Geotechnical Engineer of record referenced herein should be considered the Geotechnical

Engineer that is retained to provide Geotechnical Engineering observation and testing services during construction. References to Geotechnical Engineer should be understood to be the

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Geotechnical Engineer of Record, or his or her designated on-site representative. A representative of the Geotechnical Engineer should be present during all earthwork operations to evaluate compliance with the recommendations and the guide earthwork specifications included in this report, and to verify the conditions encountered.

Site Clearing

Construction areas should be cleared of any existing fill, debris, or other surface and subsurface structures to expose firm and stable soils, as determined by the Geotechnical Engineer’s representative. The area to be cleared should extend at least five feet beyond the edge of all exterior foundations and at least five feet beyond any exterior flatwork or pavements, where practical.

Any existing underground utilities designated to be removed or relocated should include all trench backfill and bedding materials. The resulting excavations should be restored with engineered fill placed and compacted in accordance with the recommendations included in this report. On-site wells, septic systems, or below-grade tanks were not noted at the site during the time our field exploration was performed; however, if any of these items are discovered, they should be properly abandoned in accordance with State and local requirements.

Existing surface vegetation/organics and organically laden soil within construction areas should be stripped from the site. Debris from the stripping should not be used as general fill within structure, concrete slab or pavement areas. With prior approval from the Geotechnical

Engineer, strippings may be used in proposed landscape areas, provided they are kept at least five feet from the building footprint, pavements, concrete slabs and other surface improvements.

Any trees, bushes and other vegetation designated for removal should include the entire root-ball and roots larger than ½-inch in diameter. Adequate removal of debris and roots may require laborers and handpicking to clear the subgrade soils to the satisfaction of the

Geotechnical Engineer’s representative.

Depressions resulting from site clearing operations, as well as any loose, soft, disturbed, wet, or organically contaminated soils, as identified by the Geotechnical Engineer’s representative, should be cleaned out to firm, undisturbed soils and backfilled with engineered fill placed and compacted in accordance with the recommendations in this report. It is important that the

Geotechnical Engineer’s representative be present during site clearing operations to verify adequate removal of the surface and subsurface items, as well as the proper backfilling of resulting excavations.

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Subgrade Preparation

The entire building pad should be overexcavated to a depth of at least three feet below the bottom of the footings and the material that is removed should be compacted as engineered fill.

Clay encountered within the upper two feet of final soil subgrade should be removed and replaced with gravel, cobble, import fill or other non-expansive material. Prior to compaction, potholes should be performed to verify the presence or absence of clay slickens deposits within the building pad. If encountered, these soils should be entirely removed and replaced with gravel and cobble fill or thoroughly mixed with gravel and cobble so that it doesn’t exceed 30 percent by volume. The gravel or mixed fill should be compacted as engineered fill. The clay slickens appeared to extend to a depth of about nine feet beneath the existing ground surface at boring D3. Accordingly, construction bid documents should contain a unit price (price per cubic foot) for additional excavation due to unsuitable materials and replacement with engineered fill.

The building pad should be excavated to a distance of at least five feet beyond the outside edge of the perimeter footings.

All areas that will support exterior concrete slabs, engineered fill or pavement, should be thoroughly scarified to a depth of at least 12-inches, brought to a uniform moisture content above the optimum moisture content, and compacted to not less than 90 percent of the maximum dry density per ASTM D1557 specifications. In pavement areas, the relative compaction of the upper 6 inches of final soil subgrade should be increased to 95 percent of the maximum dry density. Clay encountered in the upper two feet of final soil subgrade in pavement and exterior flatwork areas should be removed, and the resulting depression backfilled with a properly compacted mixture of sand, gravel and cobbles, or other suitable granular material.

The performance of pavement is critically dependent upon uniform and adequate compaction of the soil subgrade, as well as all engineered fill and utility trench backfill within the limits of the pavements. Final pavement subgrade preparation (i.e. scarification, moisture conditioning and compaction) should be performed after underground utility construction is completed and just prior to aggregate base placement.

Pavement subgrades should be stable and unyielding under heavy wheel loads of construction equipment. To help identify unstable subgrades within the pavement limits, a proof-roll should be performed with a fully-loaded, water truck on the exposed subgrades prior to placement of aggregate base. The proof-roll should be observed by the Geotechnical Engineer’s representative.

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The prepared subgrade soils should be protected from disturbance until covered by capillary break material or aggregate base. Disturbed subgrade soils may require additional processing and recompaction just prior to construction of these improvements, depending on the level of disturbance.

All subgrade preparation must be performed in the presence of the Geotechnical Engineer’s representative who will evaluate the performance of the subgrade under compaction loads and identify any loose or unstable soil conditions that could require remediation.

Engineered Fill

On-site soil and gravel or cobble (rock) materials may be used as engineered fill if they do not contain debris, organics or other deleterious materials. Excessive concentrations of rock should be avoided. Rock should be spread and thoroughly mixed with soils to reduce the chances of voids being created within fill. A reasonable effort should be made to break down rock larger than 12 inches into smaller pieces during earthwork for use as engineered fill material.

Although it may not be possible to eliminate all rock larger than 12 inches from fill material without screening the fill material prior to placement, rock exceeding 12 inches in diameter be avoided in the upper two feet of fill. Rock between 12 and 30 inches in diameter that cannot be reasonably broken down to less than 12-inch size particles may be selectively placed within the deeper portions of thick engineered fills if approved by the Geotechnical Engineer. Large rock used as engineered fill material should be separated to allow proper compaction around and over the boulders. Soils placed around large rock should be uniformly moisture conditioned and compacted to the satisfaction of the Geotechnical Engineer in accordance with the recommendations presented in the following paragraphs. Large rock should be avoided in areas where later excavation is likely, such as for underground utilities. Rock exceeding 30 inches in diameter should be broken into smaller pieces prior to use as fill, or removed from the construction area.

Engineered fill should be placed in horizontal lifts not exceeding six inches in compacted thickness. Each layer should be compacted at a uniform moisture content of at least the optimum moisture to not less than 90 percent of the maximum dry density (ASTM D1557).

The upper six inches of engineered fill placed in pavement areas should be uniformly compacted to at least 95 percent relative compaction at a moisture content of at least the optimum moisture content.

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If the gravel or cobble do not break down to a gradation compatible with in-place density testing, then compactive effort should be applied using track equipment weighing at least 20 tons

(Caterpillar 815 or larger) until there is no perceptible increase in fragmentation of the particles or observable consolidation of the fill during repeated passes of the compaction equipment.

Import fill materials, if required, should be granular in nature, with a Plasticity Index not exceeding 15, and a maximum particle size of three inches with larger particle sizes potentially approved at the discretion of the geotechnical engineer depending on application. Import fill material used in the upper two feet of street subgrade should possess an R-value of at least 30.

Import fill material must be approved by the Geotechnical Engineer prior to transporting the material to the project.

The Geotechnical Engineer’s representative be present on a regular basis during all earthwork operations to observe and test the engineered fill and to verify compliance with the recommendations of this report and the project plans and specifications.

Utility Trench Backfill

Bedding and initial backfill should conform to the requirements of the pipe manufacturer. In general, utility trench backfill materials could consist of on-site soils or approved granular import material. Rock over three inches in diameter should be excluded from intermediate trench backfill to a height of one foot above the initial backfill. Rock over 12 inches may be used in the upper portions of utility excavations if the excavations are wide enough to allow access with a large sheepsfoot compactor and the rocky material is placed and compacted as engineered fill in accordance with the recommendations included in the Engineered Fill section of this report.

The upper foot of trench backfill in pavement area should consist of material similar to the adjacent subgrade soils to provide uniform support of the pavement section. Utility trench backfill should be continuously observed during construction.

Although sand may be used for bedding, shading, and initial backfill material to a height of

12 inches above utility pipes, native soils or approved imported materials should be used as backfill materials in the zone from 12 inches above the pipe to subgrade. Properly compacted native soils would provide adequate protection of utilities and uniform subgrade support for pavements.

Utility trench backfill should be compacted as engineered fill. Jetting of trench backfill for compaction should not be permitted.

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Underground utility trenches that are aligned nearly parallel with shallow foundations should be at least three feet from the outer edge of foundations, wherever possible. As a general rule, trenches should not encroach into the zone extending outward at one horizontal to one vertical

(1(h):1(v)) inclination below the bottom of shallow foundations. Additionally, trenches parallel to shallow foundations should not remain open longer than 72 hours. The intent of these recommendations is to prevent loss of both lateral and vertical support of shallow foundations, resulting in possible settlement.

Shallow Spread Foundations

The proposed building may be supported upon continuous and isolated spread foundations that extend at least 12 inches below lowest adjacent soil grade. Lowest adjacent soil grade is defined as the grade upon which the capillary break material is placed or exterior soil grade, whichever is lower. Continuous foundations should maintain a minimum width of 12 inches, while isolated spread foundations should be at least 24 inches in plan dimension. Foundations should be continuous around the perimeter of the building to reduce moisture variations beneath the structures.

Foundations bearing on compacted native soils, engineered fill, or a combination of those materials may be sized for maximum allowable “net” soil bearing pressure of 3,000 pounds per square foot (psf) for dead plus live load. A one-third increase in the allowable bearing pressure may be applied when considering short-term loading due to wind or seismic forces. The weight of the foundation concrete extending below lowest adjacent soil grade may be disregarded in sizing computations.

Total settlement of an individual foundation will vary depending on the plan dimensions of the foundation and the actual load supported. Based on the foundation criteria discussed above and the assumed foundation loads, foundations are anticipated to experience a maximum total static settlement on the order of about ¾ inch, and differential settlement on the order of about

½ inch for 50 lineal feet or the shortest distance of the structure, whichever is less.

All foundations be adequately reinforced to provide structural continuity, mitigate cracking and permit spanning of local soil irregularities. The structural engineer should determine final foundation reinforcing requirements.

Because of the gravel and cobble conditions at the site, removal of large rock during foundation excavation may cause footing trenches to be wider and deeper than shown on the foundation plans. The foundation contractor should anticipate this condition.

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Resistance to lateral foundation displacement may be computed using an allowable friction factor of 0.35, which may be multiplied by the effective vertical load on each foundation.

Additional lateral resistance may be computed using an allowable passive earth pressure equivalent to a fluid pressure of 300 psf per foot of depth, acting against the vertical projection of the foundation. These two modes of resistance should not be added unless the passive earth pressure component is reduced by 50 percent. The upper 12 inches of passive earth pressure resistance should be neglected unless the surface in front of the foundation is paved or covered with flatwork.

Interior Floor Slabs

Interior concrete slab-on-grade floors should be supported by the soil subgrade prepared in accordance with the recommendations contained in the Subgrade Preparation and Engineered

Fill sections.

The interior concrete slabs should be at least four inches thick, however, the project structural or civil engineer should determine final floor slab thickness, reinforcement and joint spacing.

Temporary loads exerted during construction from vehicle traffic, cranes, forklifts, other construction equipment, storage of palletized construction materials, etc. should be considered in the design of the thickness and reinforcement of the interior concrete slabs-on-grade.

Moisture Penetration Resistance

It is likely that floor slab subgrade soils will become very moist or wet at some time during the life of the structure. This is a certainty when slabs are constructed during the wet season or when constantly wet ground or poor drainage conditions exist adjacent to structures. For this reason, it should be assumed that interior slabs with moisture-sensitive floor coverings or coatings will require protection against moisture or moisture vapor penetration through the slabs.

Interior floor slabs for the planned building should, as a minimum, be underlain by a layer of free-draining crushed rock/gravel, serving as a deterrent to migration of capillary moisture. The crushed rock/gravel layer should be between four- and six-inches-thick and graded such that

100 percent passes a one-inch sieve and less than five percent passes a No. 4 sieve.

Additional moisture protection may be provided by placing a vapor retarder membrane (at least

10-mils thick) directly over the crushed rock/gravel. The water vapor retarder membrane should meet or exceed the minimum specifications as outlined in ASTM E1745 and be installed in strict conformance with the manufacturer’s recommendations. For portions of the interior floor slabs

Geotechnical Engineering Report Page 19 that are designated to support vehicular traffic, the vapor retarder membrane should be placed directly over compacted aggregate base.

Floor slab construction practice over the past 30 years or more has included placement of a thin layer of dry sand or pea gravel over the vapor retarder membrane. The intent of the sand/pea gravel is to aid in the proper curing of the slab concrete. However, during the wet seasons moisture can become trapped in the sand or pea gravel, which can lead to excessive moisture vapor emissions from floor slabs. As a consequence, we consider use of the sand/pea gravel layer as optional. The concrete curing benefits should be weighed against efforts to reduce slab moisture vapor transmission.

It is emphasized that the crushed rock/grave and the vapor retarder membrane suggested above provides only a limited, first line of defense against soil-related moisture issues and will not "moisture proof" the slab. Nor do these measures provide an assurance that slab moisture transmission levels will tolerable levels to prevent damage to floor coverings or other building components. If increased protection against moisture vapor penetration is desired, a concrete moisture protection specialist should be consulted. The design team should consider all available measures for slab moisture protection. It is commonly accepted that maintaining the lowest practical water-cement ratio in the slab concrete is one of the most effective ways to reduce future moisture vapor penetration of the completed slabs.

Exterior Flatwork

The final subgrade for exterior concrete flatwork (i.e., sidewalks, patios, etc.) should be prepared and constructed in accordance with recommendation provided in the Subgrade

Preparation and Engineered Fill sections. Exterior flatwork should be underlain by at least four inches of aggregate base compacted to at least 95 percent relative compaction to provide stability during slab construction and to protect the soils from disturbance during construction.

Exterior flatwork concrete should be at least four inches thick. Consideration should be given to thickening the edges of the slabs at least twice the slab thickness where wheel traffic is expected over the slabs. Expansion joints should be provided to allow for minor vertical movement of the flatwork. Exterior flatwork should be constructed independent of other structural elements by the placement of a layer of felt material between the flatwork and the structural element. The slab designer should determine the final thickness, strength and joint spacing of exterior slab-on-grade concrete. The slab designer should also determine if slab reinforcement for crack control is required and determine final slab reinforcing requirements.

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Practices recommended by the Portland Cement Association (PCA) for proper placement, curing, joint depth and spacing, construction, and placement of concrete should be followed during exterior concrete flatwork construction.

Pavement Design

The subgrade soils in pavement areas should be prepared and constructed in accordance with recommendations provided in the Subgrade Preparation and Engineered Fill sections.

All aggregate base should be compacted to at least 95 relative compaction.

Based on previous laboratory testing results and nearby experience, an R-value of 25 was used for design of pavements supported on compacted dredge tailings. The pavement sections presented in Table 4 have been calculated using traffic indices assumed to be appropriate for the project using pavement design criteria outlined in Chapters 600 to 670 of the California

Highway Design Manual (Caltrans, 2019). The project civil engineer should determine the appropriate traffic index and pavement section based on anticipated traffic conditions. If needed, we can provide alternative pavement sections for different traffic indices.

Table 4

Traffic Index

(TI)

Typical

Pavement Use

Maximum

Equivalent

18-kip Axle

Load

(EAL)

Number of Loaded 5-

Axle Trucks per Week (20-year Design)

Untreated Subgrade R-value = 25

Type A Asphalt

Concrete (inch)

Class 2 Aggregate

Base (inch)

Portland Cement

Concrete (inch)

4.5 Automobile

Parking Only 4,710 1

2½ 6 --

3* 5 --

-- 4 4

5.0 Automobile Drive

Lanes 10,900 3

2½ 8 --

3* 7 --

-- 4 4

6.0 Automobile, Light

Delivery Truck and Fire Lanes

47,300 13

2½ 11 --

3½* 9 --

-- 5 4½

7.0

Trash Enclosures and Moderate Delivery Truck

Traffic

164,000 47

3 13 --

4* 11 --

-- 6 5

Notes: * = Asphalt concrete thickness contains the Caltrans safety factor.

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All pavement materials and construction methods of structural pavement sections should conform to the applicable…