20210903_Geotech_Report_CE&G.pdf

785 Ygnacio Valley Road | Walnut Creek | CA94596 6455 Almaden Expwy., Suite 100| San José | CA 95120

23785 Cabot Blvd., Suite 321 | Hayward | CA 94545 www.caleng.com

Pragmatic Expertise™

GEOTECHNICAL DESIGN REPORT

FAIRFIELD JUSTICE CAMPUS ASSET PROTECTION PROJECT

CE&G DOCUMENT NO.: 190841.001

3 SEPTEMBER 2021

Prepared for:

Mead & Hunt, Inc.

Oakland, California

Rocio Briseño, PG Christopher R. Nardi, PE, GE Project Geologist Principal Engineer

Reviewed by:

Dan Peluso, PE, GE Sr. Principal Engineer

Geotechnical Design Report Page ii Fairfield Justice Campus Asset Protection Project 3 September 2021

TABLE `OF CONTENTS

1. Introduction

1.1 General

1.2 Project Description

1.3 Purpose and Scope of Services

2. Site Description

3. Geologic Conditions

3.1 Regional Setting

3.2 Site Geology

3.3 Surficial Soils

3.4 Active Faults and Seismicity

4. Field Investigation

4.1 Subsurface Exploration

4.2 Geotechnical Laboratory Testing

5. Discussion and Conclusions

5.1 Expansive Soils

5.2 Corrosion

5.3 Liquefaction Potential

6. Design and Construction Considerations

6.1 Design Groundwater Level

6.2 Earthwork

6.3 Foundations

6.4 Lateral Earth Pressures

6.5 Seismic Design Parameters

6.6 Appurtenant Slabs

6.7 Pavements

6.8 Technical Review and Construction Observation

7. Limitations

8. References

Geotechnical Design Report Page iii

TABLES

Table 3-1. Distances to Selected Active Fault Traces Table 5-1. Corrosion Testing Results Table 6-1. Lateral Earth Pressures Table 6-2. 2019 CBC Design Parameters Table 6-3. Backfill Parameters

FIGURES

Figure 1. Site Location Map Figure 2. Site Plan Figure 3. Regional Geology Map Figure 4. NRCS Soils Map Figure 5. Fault Activity Map

APPENDICES

Appendix A. Boring Logs Appendix B. Laboratory Testing

Geotechnical Design Report Page 1

Pragmatic Expertise™

1. INTRODUCTION

1.1 GENERAL

Cal Engineering & Geology (CE&G) is providing geotechnical engineering services to Mead & Hunt in support of the Solano County Justice Center Asset Protection Project. The project, located in the City of Fairfield, consists of providing flood protection to the Solano County Justice Center and is currently in Phase III. Our scope of services included a field investigation, laboratory testing, geotechnical analysis, and design recommendations.

1.2 PROJECT DESCRIPTION

The project, located in the City of Fairfield, consists of providing flood protection to the Solano County Justice Center by constructing concrete floodwalls, flood gates, and berms to protect the center from flood events. Figure 1 shows the approximate location of the project site and vicinity. Flood control structures are planned to be founded on spread footings and/or pier foundations. A pump station sump is also planned at the southeastern corner of Clay Street. The proposed locations of these flood protection improvements are shown on Figure 2, Site Plan.

1.3 PURPOSE AND SCOPE OF SERVICES

The geotechnical investigation completed by CE&G was undertaken to assess the existing surface and subsurface conditions in the vicinity of the project area and to develop geotechnical design recommendations for the proposed flood control infrastructure.

The scope of work completed for the geotechnical investigation and report included:

• Completion of an office study to identify and evaluate relevant geologic and geotechnical information available for the site, including published geologic maps, and previous geotechnical reports completed by others.

• Site reconnaissance to observe current site conditions and to mark the proposed boring locations for USA (Underground Service Alert).

• Completion of a subsurface exploration program using a truck-mounted drill rig, in accordance with Solano County Department of Resource Management (SCDRM) requirements.

• Laboratory testing to determine key engineering index properties of selected earth materials.

Geotechnical Design Report Page 2

Pragmatic Expertise™

• Engineering analyses to develop recommendations for foundations, lateral earth pressures, shoring, earthwork, and pavement sections.

• Preparation of this geotechnical design report.

Geotechnical Design Report Page 3

Pragmatic Expertise™

2. SITE DESCRIPTION

The Solano County Justice Center is located along the southern edge of Fairfield, CA, north of Highway 12. The Solano County Justice Center includes the sheriff's office, the coroner's office, the county superior court, the criminal court, and the county jail. The flat, paved area is bordered to the east and partially to the south by an earth levee. Suisun Marsh lies south of Highway 12.

The Justice Center buildings occupy the southern and western portions of the site, and a large at-grade paved parking lot is in the northeastern portion of the site. The site is bounded on the north by Texas Street, on the east by Clay Street, on the west by Union Street, and on the south by Delaware Street. There is a rail line located near the southeastern side of the Justice Center. The site elevations range from about 9 feet above sea level (asl) in the southern portion of the site to 14 feet asl in the northern portion of the site.

There is an unnamed drainage near the southeastern side of the site that drains to Suisun Slough.

Geotechnical Design Report Page 4

Pragmatic Expertise™

3. GEOLOGIC CONDITIONS

3.1 REGIONAL SETTING

The project site lies within the Coast Ranges geomorphic province, which is a series of discontinuous northwest-trending mountain ranges, ridges, and intervening valleys characterized by complex folding and faulting. The greater San Francisco Bay was one of the intervening valleys within the province. The general geologic framework of the San Francisco Bay Area is illustrated in studies by Schlocker (1971), Wagner et al. (1991), Ellen and Wentworth (1999), and Graymer et al. (2006). Fairfield is located in the North Bay portion of the San Francisco Bay, directly north of the Suisun Bay and Suisun Marsh.

3.2 SITE GEOLOGY

The general vicinity of the project site has been mapped several times, with geologic mapping having different emphases: Bezore and others (1998) (Fairfield South Quadrangle); Wiegers and others (2006) (Fairfield North Quadrangle); Wentworth and others (1997); Knudsen and others (2000); Graymer and others (2006); and Witter and others (2006) (Figure 3, Regional Geologic Map).

These geologists are in general agreement that the site is underlain by Holocene alluvial deposits. Wentworth describes these materials as sand, gravel, silt, and mud. Wiegers describes these materials as predominately clay and silt.

3.3 SURFICIAL SOILS

The surficial soils at the project site have been mapped by the USDA National Resource Conservation Service and USDA Soil Conservation Service as the Capay clay for 0 percent slopes (NRCS, 2021). (Figure 4). The Capay clay is described as a moderately well-drained, flood basin silty and clayey alluvium, derived from metamorphic and sedimentary rock over fan alluvium (also derived from metamorphic and sedimentary rock).

The Capay clay is classified as a fat clay in the upper 40 inches with a plasticity index ranging from 29 to 44 percent and a liquid limit ranging from 53 to 72. From 40 inches to 81 inches below ground surface, it is classified as a lean clay with a plasticity index ranging from 25 to 33 percent and a liquid limit ranging from 45 to 56 percent.

Geotechnical Design Report Page 5

Pragmatic Expertise™

3.4 ACTIVE FAULTS AND SEISMICITY

The project site is located within the North Bay, part of the greater San Francisco Bay Area, which is recognized as one of the more seismically active regions of California. The right-lateral strike-slip San Andreas fault system controls the northwest-southeast structural grain of the Coast Ranges and the Bay Area. The fault system marks the major boundary between two of earth’s major tectonic plates, the Pacific Plate to the west and the North American Plate to the east. The Pacific Plate is moving north relative to the North American plate at approximately 40 mm/yr in the Bay Area (WGCEP, 2014).

The transform boundary between these two plates has resulted in a broad zone of multiple, subparallel faults within the North American Plate, along which right-lateral strike-slip faulting predominates. In this broad transform boundary, the San Andreas Fault accommodates less than half of the average total relative plate motion. Much of the remainder of the plate motion in the greater San Francisco Bay Area is distributed across other faults such as the Hayward, Calaveras, Concord-Green Valley, Greenville, Rodgers Creek, and West Napa fault zones. Figure 5, Fault Activity Map, shows active faults in the vicinity of the Fairfield area.

An active fault is generally defined as experiencing fault offset in Holocene time (last approximately 11,000 years). According to the US Geological Survey Quaternary Fault and Fold Database (2006), no active faults are mapped as crossing through the site.

According to the 2018 California Department of Transportation ARS website, the closest mapped fault to the project site, the Great Valley Fault (Pittsburg-Kirby Hills section), can produce a magnitude 6.6 earthquake.

Since the project site is located in seismically active California, it will likely experience strong ground shaking from a large (Moment Magnitude [Mw] 6.7) or greater earthquake along one or more of the nearby active faults during the design lifetime of the project (WGCEP, 2014). Table 3-1 shows the approximate distances between the project site and various major surface fault traces within approximately 50 km of the site (Caltrans, 2018).

Geotechnical Design Report Page 6

Pragmatic Expertise™

Table 3-1. Distances to Selected Active Fault Traces

Fault Name

Approximate Distance and Direction from Site to Surface

Fault Traces Great Valley (Pittsburg Kirby Hills) 6.4 km northeast Cordelia 8.6 west Great Valley (Gordon Valley) 9.0 km northeast Vaca 9.1 km northeast Great Valley 10.9 west Los Medanos 20.0 km southwest West Napa 22.7 km northwest Concord 23.5 km southwest Great Valley (Trout Creek) 32.3 km northeast Clayton 32.6 km south/southeast Great Valley (Midland) 35.5 km southeast Rogers Creek 37.0 km southwest Hayward (North) 39.2 km southwest Great Valley (Dunnigan Hills) 44.8 km northeast Calaveras 46.2 km southwest

A large magnitude earthquake on any of these faults or other active fault systems in the greater Bay Area has the potential to cause significant ground shaking at the site. The intensity of ground shaking that is likely to occur at the property is generally dependent upon the magnitude of the earthquake and the distance to the epicenter.

3.4.1 Liquefaction and Seismic Densification

Earthquake-induced soil liquefaction can be described as a significant loss of soil strength and stiffness caused by an increase in pore water pressure resulting from cyclic loading during shaking. The primary factors affecting soil liquefaction include: 1) intensity and duration of seismic shaking; 2) soil type and relative density; 3) overburden pressure; and

4) depth to groundwater. Liquefaction is associated primarily with loose (low density), saturated, fine- to medium-grained, cohesionless soils below the groundwater table, but can also occur in non-plastic to low-plasticity finer-grained soils. The potential consequences of liquefaction to engineered structures include loss of bearing capacity, buoyancy forces on underground structures, ground oscillations, or “cyclic mobility”, increased lateral earth pressures on retaining walls, liquefaction settlement, and lateral spreading or “flow failures” in slopes.

Geotechnical Design Report Page 7

Pragmatic Expertise™

According to a liquefaction susceptibility study by Witter and others (2006), the project site is in an area mapped as having medium liquefaction susceptibility.

CE&G assessed the soil and groundwater conditions encountered in the exploratory borings. Based on subsurface information collected from our borings during this investigation, we judge the potential for liquefaction at the project site to be low to nil due to the presence of generally stiff cohesive soils with no significant loose natural granular soils.

Seismic densification is the densification of unsaturated, loose to medium dense granular soils due to strong vibration such as that resulting from earthquake shaking. We judge the potential for seismic densification at the sites to be low due to the encountered soils above the groundwater table being primarily cohesive.

Geotechnical Design Report Page 8

Pragmatic Expertise™

4. FIELD INVESTIGATION

4.1 SUBSURFACE EXPLORATION

4.1.1 Exploratory Borings

Five geotechnical borings were drilled as part of our investigation at the approximate locations shown in Figure 2. The borings were advanced 16.5 to 21.5 feet below the ground surface (bgs).

The geotechnical borings were drilled by Taber Drilling, Inc., on May 28, 2021, using a truck-mounted Diedrich D120 drill rig equipped with 4-inch-diameter solid-flight augers and an automatic hammer. Surface conditions at the boring locations consisted of asphalt pavement and gravel base.

Upon completion, the borings were backfilled with cement grout per SCDRM requirements.

Drilling spoils were drummed and transported offsite.

4.1.2 Logging and Sampling

The materials encountered in the borings were logged in the field by a CE&G geologist. The soils were visually classified in the field, office, and laboratory according to the Unified Soil Classification System (USCS) in general accordance with ASTM D2487 and D2488.

During the drilling operations, soil samples were obtained using the following sampling methods:

• California Modified (CM) Sampler; 3.0-inch outer diameter (O.D.), 2.5-inch inner diameter (I.D.) (ASTM D1586)

• Standard Penetration Test (SPT) Split Spoon Sampler; 2.0-inch O.D., 1.375-inch I.D.

(ASTM D1586)

The CM and SPT samplers were driven 18 inches (unless otherwise noted on the boring logs) with a 140-pound hammer using an automatic trip hammer, dropping 30 inches. The number of blows required to drive the samplers through each 6-inch interval was recorded for each sample. The results are included on the boring logs in Appendix A. The blow counts included on the boring logs represent the field values and are uncorrected.

Soil samples obtained from the borings were packaged and sealed in the field to reduce the potential for moisture loss and disturbance. The samples were taken to CE&G’s local office for further analysis and storage.

Geotechnical Design Report Page 9

Pragmatic Expertise™

4.1.3 Soil Conditions Encountered

The soils encountered in the borings generally consisted of artificial fill within the upper

2.5 to 5 feet, and alluvial soils to depths explored. The artificial fill was mostly comprised of fat clay. Concrete debris was encountered in the artificial fill in borings B-02 and B-03.

The alluvial material was generally comprised of mixtures of fat clay, lean clay, and silt of soft to firm consistencies. Poorly-graded gravel with clay and sand was encountered at the bottom 3 feet of boring B-03. The fine-grained materials were generally stiffer with depth.

For a more detailed description of the soils encountered in the borings, please see the boring logs and laboratory test results included in Appendix A.

4.1.4 Groundwater Conditions Encountered

Groundwater was encountered in the borings at depths ranging from 4.5 to 13 feet bgs.

Approximate groundwater levels are included in the boring logs in Appendix A. Note that fluctuations in rainfall, tides, and other factors not apparent at the time of exploration, can influence groundwater levels.

4.2 GEOTECHNICAL LABORATORY TESTING

Testing was performed to obtain information concerning the qualitative and quantitative physical properties of the samples recovered during the subsurface exploration program.

Tests were performed by Cooper Testing Laboratory in Palo Alto, California, and the CE&G Testing Laboratory in Hayward, California, in general conformance with applicable ASTM standards. The following tests were performed:

• Moisture Content and Dry Unit Weight (ASTM D2216)

• Atterberg Limits (ASTM D4318; dry method)

• Particle Size Analysis (ASTM D6913 and ASTM D1140)

• Unconsolidated Undrained Triaxial Compression (ASTM D2850)

• Resistivity (Minimum) (Caltrans 643)

• pH (Caltrans 643)

• Sulfate Content (Caltrans 417-mod)

• Chloride Content (Caltrans 422-mod)

The results of the laboratory testing program are presented in Appendix B and are summarized below.

Geotechnical Design Report Page 10

Pragmatic Expertise™

4.2.1 Index Tests

Moisture and density tests were performed on select samples at various depths from the borings. The soil and bedrock samples tested had moisture contents between 9 and 27 percent with dry densities between 96 and 116 pcf.

4.2.2 Atterberg Limits

Atterberg Limits testing was performed on three samples to determine the plasticity of fine-grained materials. Liquid limits were 26, 37, and 44, with plasticity indices of 7, 15, and 24 percent. A figure plotting liquid limit versus plasticity index is presented in Appendix B.

4.2.3 Shear Strength Testing

Shear strength testing was performed on three clay soil samples for unconsolidated-undrained triaxial strength with back-pressure saturation. Strength testing produced reasonable shear strengths for the soils encountered. The results including the shear stress plots are presented in Appendix B.

Geotechnical Design Report Page 11

Pragmatic Expertise™

5. DISCUSSION AND CONCLUSIONS

The proposed flood protection improvements for the Fairfield Justice Center campus include:

• Raised roadway berms/ramps with retaining walls

• Flood barrier walls

• Passive flood gates

• Interior drainage improvements, including a new pump station

Except for the proposed sumps, the proposed structures will be supported on the existing artificial fill and/or native soils that are potentially expansive. The proposed sumps are deep enough that they will be supported on native soils. The soils encountered consist of fine-grained colluvium and residual soils. It is our opinion that the site can support the proposed improvements provided the design recommendations presented below are incorporated into the design.

5.1 EXPANSIVE SOILS

Near-surface soils are moderately to highly expansive. The shrink/swell effects of expansive soils are most common on pavements and lightly loaded slabs, as opposed to more heavily loaded foundations or mats. The impacts of expansive soils can be mitigated/reduced by proper moisture conditioning during site preparation and grading, and by placing non-expansive fill over the potentially expansive soils.

5.2 CORROSION

Corrosion testing was performed on two samples of near-surface soils and one sample of deeper soil from the project site within the expected depth of work in general accordance with Caltrans methods. Test results are presented below:

Geotechnical Design Report Page 12

Pragmatic Expertise™

Table 5-1. Corrosion Testing Results

Boring (depth in feet)

Minimum Resistivity (Ohm-cm)

Chloride (mg/kg)

Sulfate (mg/kg) pH

B-03 (6.5) 1,367 9 64

(0.0064%) 9.0

B-04 (5.5) 2,844 10 36

(0.0036) 9.1

B-05 (11.0) 1,266 27 95

(0.0095) 8.8

Caltrans Corrosion Guidelines, January 2015, identifies a site as being corrosive for structural elements if one or more of the following conditions exist:

• Chloride concentration is 500 ppm or greater;

• Sulfate concentration is 2000 ppm or greater;

• pH is 5.5 or less.

A minimum resistivity value for soil and/or water less than 1000 ohm-cm indicates the presence of high quantities of soluble salts and a higher propensity for corrosion. Based on the results of the laboratory testing performed, the soil samples tested had values for Chloride, Sulfate, pH that do not meet the Caltrans criteria for a corrosive site. The minimum resistivity of the tested soil samples was above the 1000 ohm-cm threshold defined.

According to ACI 318 Section 4.3, Table 4.3.1:

• Sulfate concentration below 0.10 percent by weight (1,000 ppm) is negligible (no restrictions on concrete type)

• The water-soluble chloride content of less than 500 ppm is generally considered non-corrosive to concrete.

Geotechnical Design Report Page 13

Pragmatic Expertise™

Based on the results of the laboratory testing performed, the soil samples tested had values for Sulfate and Chloride that do not meet ACI criteria and are considered non-corrosive to concrete.

Corrosion results are to be considered preliminary and are an indicator of potential soil corrosivity for the sample tested. Other soils found on site may be more, less, or of similar corrosive nature. Our scope of services does not include corrosion engineering; therefore, a detailed analysis of the corrosion tests is not included.

5.3 LIQUEFACTION POTENTIAL

CE&G performed a qualitative assessment of the liquefaction potential of the soils encountered beneath the project area. The liquefaction assessment was performed by reviewing the soil types encountered, SPT blow counts, and fines content from our boring logs and laboratory testing. Based on the apparent absence of granular materials in the borings, the liquefaction potential is deemed to be low to nil due to the presence of generally stiff cohesive soils.

Geotechnical Design Report Page 14

Pragmatic Expertise™

6. DESIGN AND CONSTRUCTION CONSIDERATIONS

Detailed recommendations for the geotechnical aspects of the proposed flood protection improvements are presented in subsequent sections of this report. Our evaluations and recommendations are based upon the previously discussed information collected for this investigation. The following recommendations may need to be modified if there are any changes in the proposed improvements, their layout or location, or the proposed grading.

6.1 DESIGN GROUNDWATER LEVEL

Groundwater was encountered in t our borings at depths between 4.5 and 13 feet below ground surface (approximately El. 1 to 6.5 feet). Groundwater may fluctuate depending on rainfall, groundwater pumping, proximity to adjacent ditches/channels, and landscape irrigation or other activities.

6.2 EARTHWORK

6.2.1 Clearing

Clearing will include selective removal of existing pavements and flatwork within the project areas along with existing landscaping to facilitate the new construction. Existing and abandoned underground utilities that may be present in areas of the planned improvements will also require removal or relocation.

Site clearing should also include removal of deleterious materials, debris, and obstructions that are designated for removal. Depressions, voids, and holes that extend below the proposed finish grades should be cleaned and backfilled with engineered fill compacted to the recommendations in this report.

6.2.2 Excavations

Excavations for this project will include excavation for subgrade preparation, removing existing underground facilities designated for removal, trenching for underground utilities, excavations for sump structures, and foundation excavations.

Excavations should be constructed in accordance with the current CAL-OSHA safety standards and local jurisdiction. The stability and safety of excavations, braced or unbraced, are the responsibility of the contractor.

Geotechnical Design Report Page 15

Pragmatic Expertise™

Trench excavations adjacent to existing or proposed foundations should be above an imaginary plane having an inclination of 1½:1 (horizontal to vertical) extending down from the bottom edge of the foundations.

6.2.3 Site Preparation

As discussed in Section 5.1, the site is underlain by expansive soils to depths of up to about 4 to 5 feet below the existing grade. Expansive soils beneath slabs or flatwork should be removed to one foot below subgrade and replaced with non-expansive engineered fill and base rock.

After site preparation and before placement of compacted fills, the excavation bottom should be observed and approved by the geotechnical engineer or their representative.

After approval, the subgrade should be scarified to a minimum depth of 8 inches, moisture conditioned to at least 3 percent of optimum moisture content, and compacted to between 88 and 92 percent of the maximum dry unit weight as measured by ASTM D1557. to the recommendations given under Section 6.2.5, Engineered Fill Placement and Compaction.

Prepared soil subgrades should be non-yielding when proof-rolled by a fully-loaded water truck or equipment of similar weight. Moisture conditioning of subgrade soils should consist of adding water if the soils are too dry and allowing the soils to dry if the soils are too wet. After the subgrades have been prepared, the areas may be raised to design grades by the placement of engineered fill.

If unstable, wet, or soft soil is encountered, the soil will require processing before compaction can be achieved. When the construction schedule does not allow for air-drying, other means such as lime or cement treatment, over-excavation, and replacement, geotextile fabrics, etc. may be considered to help stabilize the subgrade. The method to be used should be determined at the time of construction based on the actual site conditions.

We recommend obtaining unit prices for subgrade stabilization during the construction bid process.

6.2.4 Material for Engineered Fill

In general, on-site soils with an organic content of less than 3 percent by weight, free of any hazardous or deleterious materials, and meeting the gradation requirements below may be used as general engineered fill to achieve project grades, except when special material (such as aggregate base or subbase material) is required.

In general, engineered fill material should not contain rocks or lumps larger than 3 inches in greatest dimension, should not contain more than 15 percent of the material larger than

Geotechnical Design Report Page 16

Pragmatic Expertise™

1½ inches, and should contain at least 20 percent passing the No. 200 sieve. In addition to these requirements, import fill should have a low expansion potential as indicated by a Plasticity Index of 15 or less, or an Expansion Index of less than 20. Structure backfill material should not contain rocks or lumps larger than 2 inches in greatest dimension, should not contain more than 15 percent of the material larger than 1½ inches, and should contain at least 30 percent passing the No. 200 sieve, a Liquid Limit less than 30, and a Plasticity Index between 8 and 15.

Engineered fill materials for the proposed flood berms not constrained between retaining walls should not contain rocks or lumps larger than 2 inches in greatest dimension, should not contain more than 10 percent of the material larger than 1½ inches, and should contain at least 30 percent passing the No. 200 sieve. In addition to these requirements, import fill should have a Liquid Limit less than 40 and a Plasticity Index between 8 and 30.

All import fills must be approved by the project geotechnical engineer, before delivery to the site, by providing representative samples of proposed import fills to the engineer for evaluation.

6.2.5 Engineered Fill Placement and Compaction

Engineered fill should be placed on soil subgrades that are prepared as recommended in this report. Engineered fill should be placed in horizontal lifts each not exceeding 8 inches in thickness and mechanically compacted to the recommendations below at the recommended moisture content. Relative compaction or compaction is defined as the in-place dry density of the compacted soil divided by the laboratory maximum dry density as determined by ASTM Test Method D1557, latest edition, expressed as a percentage.

Moisture conditioning of soils should consist of adding water to the soils if they are too dry and allowing the soils to dry if they are too wet.

Engineered fills consisting of on-site soils and imported soils should be compacted to a minimum of 90 percent relative compaction with moisture content at least 2 percent above the laboratory optimum value. In pavement areas, the upper 12 inches of subgrade soil and the full section of aggregate base should be compacted to a minimum of 95 percent relative compaction with moisture content slightly above the optimum value. Aggregate base in vehicle pavement areas should be compacted at slightly above the optimum moisture content to a minimum of 95 percent relative compaction.

Geotechnical Design Report Page 17

Pragmatic Expertise™

6.2.6 Utility Trench Excavation and Backfill

Utility trenches less than 5 feet in depth in the near-surface soil materials should be able to stand near vertical with minimal bracing. Sandy soils, where encountered, may need bracing to prevent the caving of the granular soils. We estimate that excavations should be able to be accomplished with conventional excavating equipment, such as backhoes and excavators. Excavations should be constructed in accordance with the current CAL-OSHA safety standards and local jurisdiction. The stability and safety of excavations, braced or unbraced, are the responsibility of the contractor.

Pipe zone backfill, extending from the bottom of the trench to about 1 foot above the top of the pipe, should consist of free-draining sand (at least 90% passing a No. 4 sieve and less than 5% passing a No. 200 sieve) compacted to a minimum of 90 percent relative compaction unless concrete or cement slurry is specified, or City of Fairfield or Solano County standard specifications dictate otherwise.

Above the pipe zone, underground utility trenches may be backfilled with free-draining sand, on-site soil, or imported soil that is free of deleterious and hazardous material. The trench backfill should be compacted to the requirements given in Section 6.2.5, “Engineered Fill Placement and Compaction.” Trench backfill should be capped with at least 12 inches of compacted, on-site soil similar to that of the adjoining subgrade. The upper 12 inches of trench backfill in areas to be paved should be compacted to a minimum of 95 percent relative compaction. Compaction should be performed by mechanical means only. Water jetting or flooding to attain compaction of backfill should not be permitted.

Trench excavations that extend below an imaginary plane inclined at 1½:1 (h:v) below the bottom edge of foundations should be properly shored to maintain support of the existing facilities. Trenches that run parallel to the proposed foundations should not be excavated within the imaginary plane inclined at 1½:1 (h:v) below the bottom of the footing.

6.2.7 Wet Weather Construction

If site grading and construction are to be performed during the winter rainy months, the owner and contractors should be fully aware of the potential impact of wet weather.

Rainstorms can cause delays to construction and damage to previously completed work by saturating compacted pads or subgrades, or flooding excavations.

Earthwork during rainy months will require extra effort and caution by the contractors.

The grading contractor should be responsible to protect his work to avoid damage by rainwater. Standing pools of water should be pumped out immediately. Construction

Geotechnical Design Report Page 18

Pragmatic Expertise™ during wet weather conditions should be addressed in the project construction bid documents and/or specifications. We recommend the grading contractor submit a wet weather construction plan outlining procedures they will employ to protect their work and to minimize damage to their work by rainstorms.

6.3 FOUNDATIONS

Based on our understanding of the project components, they can be supported on spread footings. We expect that the imposed loads for the various project components will generally be light to moderate. Recommendations for conventional spread footing foundations are presented in the following paragraphs.

Continuous and isolated footings may be designed to impose a maximum allowable bearing pressure of 2,000 pounds per square foot on foundation soils when considering dead loads and 3,000 psf when considering dead plus normal live loading. This allowable foundation soil pressure may be increased by one-third when considering short-term wind or seismic loading. We recommend that spread footings have a minimum width of 15 inches and be embedded a minimum of 24 inches below the lowest adjacent finished grade.

Soil resistance to lateral loads will be provided by a combination of frictional resistance between the bottom of the footings and underlying soils and by passive pressures acting against the embedded sides of the footings. For frictional resistance, an ultimate coefficient of friction of 0.32 may be used for design. In addition, an allowable passive lateral bearing pressure equal to an equivalent fluid pressure of 300 psf/ft may be used provided the footings are poured tight against undisturbed native or compacted soils. These values may be used in combination without reduction.

Concrete should be placed only in excavations that are clean and free of loose soils or debris. Foundation excavations should be maintained in a moist condition before the placement of concrete. A member of our staff should observe foundation excavations to verify that adequate foundation bearing soils have been reached. The project structural engineer should determine the foundation reinforcement.

Settlements are expected to be primarily elastic with most of the settlement occurring immediately upon application of load. Long-term settlement of the foundation system is anticipated to be less than ¾ inch with differential settlements on the order of ¼ inch or less for a distance of 25 feet.

We request the opportunity to review the foundation plans and to provide supplemental recommendations as necessary.

Geotechnical Design Report Page 19

Pragmatic Expertise™

6.4 LATERAL EARTH PRESSURES

Static lateral earth pressure will be imposed on retaining walls and shored excavations.

Table 6 summarizes the lateral earth pressures recommended for use in the design of retaining walls and unbraced temporary shoring. Active pressure should be assumed for conditions where the top of the wall is free to deflect up to ½ inch. Passive pressure should be ignored for a depth of 18 inches in unpaved areas and may be utilized to resist overturning and sliding. Where structures will be located below groundwater, hydrostatic pressures are already considered in the passive lateral earth pressure values shown in Table 6-.

Table 6-1 – Lateral Earth Pressures

Pressure Type Above Groundwater

Level (Equiv. Fluid Pressure)

Below Groundwater Level (Equiv. Fluid Pressure +

Hydrostatic) Active 45 pcf 84 pcf At-Rest 65 pcf 94 pcf Passive 300 pcf 200 pcf

The design team should also consider designing retaining wall systems for a temporary increase in lateral earth pressures due to dynamic earth pressures during seismic events.

For seismic design considerations, the active earth pressure equivalent fluid weight of 45 pcf may be used in conjunction with a seismic pressure increment based on an equivalent fluid weight of 38 pcf. All pressure distributions may be taken as triangular, and all resultant forces may be assumed to apply at a distance of H/3 above the bottom of the wall (where H=wall height). The walls should be designed for the maximum of either the active plus seismic pressures, or the at-rest pressure (of 65 pcf).

Retaining walls should also be designed to resist an additional uniform surcharge pressure acting within the upper 5 feet of the wall, equivalent to one-half of any surcharge pressure applied at the surface. For light traffic loads (e.g., passenger vehicles) applied within 2 feet of the walls, an additional design load of 100 pounds per linear foot applied 1 foot below top of wall, should be added to earth pressures.

The above pressures are based on the assumption that sufficient drainage will be provided behind the walls to prevent the build-up of hydrostatic pressures from surface and subsurface water infiltration. Adequate drainage can be provided by a back drain system consisting of a 4-inch diameter perforated pipe bedded in a 12-inch-wide zone of 3/4-inch clean, open-graded rock at the base of the wall. The entire rock/pipe unit should be wrapped in filter fabric. The rock and fabric placed behind the wall should be at least one

Geotechnical Design Report Page 20

Pragmatic Expertise™ foot in width and should extend to within one foot of finished grade. The upper one foot of backfill should consist of on-site, compacted soils. Alternatively, prefabricated drainage panels can be used instead of drain rock, with the drainage panels connected to a 4-inch-diameter perforated pipe at the base of the wall. The back drainpipe should be sloped to drain by gravity and be connected to a system of closed pipes that lead to suitable discharge facilities. In addition, the "high" end and all 90-degree bends of the subdrain pipe should be connected to a riser which extends to the surface and acts as a cleanout. Where migration of moisture through retaining walls would be detrimental or undesirable, retaining walls should be waterproofed.

As noted previously, the design of unbraced shoring will likely be controlled by deflections and the shoring is anticipated to require bracing.

If the temporary shoring will be braced, a rectangular or trapezoidal loading diagram such as those recommended by Terzaghi & Peck, Tschebortarioff, and others (Caltrans Trenching and Shoring Manual and FHWA GEC No. 4) should be used. These methods generally correlate the earth pressure load to a percentage of the unit weight of the soil times the height of the excavation. The method and loading should be determined by the contractor and provided to the Engineer for review.

If a rectangular or trapezoidal loading is used, the native sandy deposits can be assumed to have a uniform lateral load of 30H psf for the full height (H) (in feet) of the excavation shoring plus a lateral fluid pressure of 62.4 pcf (unit weight of water) starting at the design groundwater elevation to account for groundwater. It is recommended that the contractor’s shoring design engineer evaluate high and low groundwater cases to confirm which case governs the design. Shoring for excavations that penetrate below an imaginary plane having an inclination of 1½:1 (horizontal to vertical) extending down from the bottom edge of the foundations must be designed for foundation surcharge pressures and must limit deflections to less than ½ inch. Surcharge loading from traffic on the adjacent areas and construction equipment adjacent to excavations should be considered in shoring design.

6.5 SEISMIC DESIGN PARAMETERS

Due to the proximity of the site to the numerous active fault systems which traverse the greater San Francisco Bay Area, the project site will likely be subjected to the effects of a major earthquake during the design life of the proposed improvements. The effects are likely to consist of significant ground shaking and accelerations. These ground type movements may cause damage to the proposed improvements. We, therefore, recommend that at a minimum the structural systems for the proposed improvements be designed

Geotechnical Design Report Page 21

Pragmatic Expertise™ under the requirements of Chapter 16 of the 2019 California Building Code (CBC) and ASCE 7-16 for Site Class D type soils. The CBC seismic design parameters for the site are included in Table 6-1. The design parameters utilize a PGA of 0.657g.

Table 6-2. 2019 CBC Design Parameters

Item Design Value Source

Site Soil Class Definition D Table 1613.5.2

PGA 0.657

PGAM 0.723

0.2 Second Spectral Response Acceleration, Ss

1.608 Figure 1613.5(3)

1.0 Second Spectral Response Acceleration, S1

0.563 Figure 1613.5(4)

Values of Site Coefficient, Fa 1.0 Table 1613.5.3(1) Value of Site Coefficient, Fv 1.7 Table 1613.5.3(2) Designed Spectral Response Acceleration for Short Periods, SDS

1.072 Equation 16-38

(SDS=2/3(Fa Ss)

Designed Spectral Response Acceleration for 1-Sec Periods, SD1

0.638 Equation 16-39

(SDS=2/3(Fv S1)

Long-period transition period, TL, sec 8 Note: The above parameters assume structures are not seismically isolated and do not incorporate a damping system. If this is not the case, a ground motion hazard analysis may be required. Reference:

https://asce7hazardtool.online/ and https://hazards.atcouncil.org/#/

In place of the seismic lateral earth pressure parameters provided in Section 6.4, the following parameters may be used for a more rigorous design assuming a design lateral acceleration, kh = 0.5xPGA, and the backfill parameters in Table 6-3 below.

T Table 6-3. Backfill Parameters

Parameter Native Soil Aggregate Base

Unit Weight, pcf 125 129 Saturated Unit Weight, pcf 128 132 Angel of Internal Friction, degrees 30 38

6.6 APPURTENANT SLABS

The use of concrete slab-on-grade construction is anticipated for exterior flatwork, walkways, and similar items. We recommend that a minimum of 12 inches of non-expansive engineered fill be placed beneath exterior concrete slabs-on-grade, prepared as recommended in the Site Preparation Section of this report. Soil subgrade should be https://asce7hazardtool.online/ https://hazards.atcouncil.org/#/

Geotechnical Design Report Page 22

Pragmatic Expertise™ maintained in a moist condition before pouring the concrete slab. If expansive soil has not been over-excavated, moisture-conditioned, and engineered, then the non-expansive fill or base rock should be underlain by a woven geotextile such as Mirafi 500x, or equal.

To reduce the potential for cracking of the appurtenant concrete slabs, we recommend that the slabs be a minimum of 5 inches thick. The slabs should include minimum reinforcement of #3 bars in both directions at 12-inch centers or #4 bars in both directions at 18-inch centers. The steel should be placed in the middle of the slab and should be held in place by dobie blocks or other suitable means. Actual dimensions and reinforcement should be determined by the project Structural Engineer.

Even with the steel reinforcement and base rock, it should be recognized that some cracking and differential movement of the slabs will likely occur and should be expected.

Exterior concrete slabs-on-grade should be cast free from adjacent footings or other non-heaving edge restraints. This may be accomplished by using a strip of 1/2-inch asphalt-impregnated felt divider material between the slab edges and the adjacent structure.

Construction and/or control joints should be provided in concrete slabs.

In the areas where flatwork and other improvements are to be made, we recommend that any underlying loose soil be removed and re-compacted as engineered fill to provide a more stable base for support of the structures. These soils should be compacted to the relative compaction specified in Section 6.2.5 as determined by the ASTM D-1557 test procedure. Due to the high expansion potential of the site soils, removal and re-compaction of the loose soils will reduce, but not eliminate, differential movement of shallow-founded structures and some cracking and heaving of the slabs may occur and should be expected.

To reduce the swell potential of the subgrade soils where slab-on-grade construction is planned, a near-saturation condition of the subgrade soil should be attained before the concrete slab is placed. This should be done to the satisfaction of the engineer or geologist from Cal Engineering & Geology, Inc. observing the work.

Pavements should slope to appropriate stormwater facilities.

6.7 PAVEMENTS

New and replacement pavements will be constructed as part of this project. We understand a design Traffic Indices (TIs) have not yet been established for the subject site, but we estimate that they will be in the range of between 4 and 6. We have developed alternate pavement sections for these TIs. The actual pavement sections for the proposed improvements should be determined by the project Civil Engineer. Soils at the site have an

Geotechnical Design Report Page 23

Pragmatic Expertise™ assumed R-value of 5. Based on this R-value, the following alternative pavement sections were developed:

Table 6-3: Flexible Pavement Section Design for R-Value = 5

Traffic Index Asphalt Concrete, inches

Class 2 Aggregate Base, inches

Total Pavement, inches

4.0 2.5 7.5 10.0

5.0 2.5 11.0 13.5

6.0 3.0 13.5 16.5

Pavement sections should be placed on soil surfaces that have been prepared as outlined in the Grading section of this report. The full section of aggregate base should be compacted to a minimum of 95 percent relative compaction (ASTM D1557, latest edition).

Asphalt concrete should meet the requirements for ½- or ¾-inch maximum, medium Type A Hot Mix Asphalt (asphalt concrete), Section 39, Caltrans Standard Specifications, latest edition. The Class 2 aggregate base material should conform to Section 26 of the Caltrans Standard Specifications.

6.8 TECHNICAL REVIEW AND CONSTRUCTION OBSERVATION

Before construction, the geotechnical engineer should review the project plans and specifications for conformance with the intent of the recommendations presented in this report. The geotechnical engineer should be contacted a minimum of 48 hours in advance of excavation operations to observe the subsurface conditions.

Geotechnical Design Report Page 24

Pragmatic Expertise™

7. LIMITATIONS

The conclusions and recommendations presented in this report are based on the information provided regarding the proposed project, and the results of the site reconnaissance, subsurface exploration, and laboratory testing, combined with interpolation of the subsurface conditions between boring locations. Site conditions described in the text of this report are those existing at the time of our last field reconnaissance and are not necessarily representative of the site conditions at other times or locations. This information notwithstanding, the nature and extent of subsurface variations between borings may not become evident until construction. If variations are encountered during construction, Cal Engineering & Geology, Inc. should be notified promptly so that conditions can be reviewed, and recommendations reconsidered, as appropriate.

It is the Owner’s/Client’s responsibility to ensure that recommendations contained in this report are carried out during the construction phases of the project. This report was prepared based on preliminary design information provided which is subject to change during the design process.

The findings of this report should be considered valid for three years unless the conditions of the site change. After three years, CE&G should be contacted to review the site conditions and prepare a letter regarding the applicability of this report.

This report presents the results of a geotechnical and geologic investigation only and should not be construed as an environmental audit or study.

The conclusions and recommendations contained in this report are valid only for the project described in this report. We have employed accepted geotechnical engineering procedures, and our professional opinions and conclusions are made in accordance with generally accepted geotechnical engineering principles and practices. This standard is in lieu of all other warranties, either expressed or implied.

Geotechnical Design Report Page 25

Pragmatic Expertise™

8. REFERENCES

ASTM International, 2017, Volume 04.08 Soil and Rock (I): D421-D5876.

California Department of Transportation, (2018), Caltrans fault database and Caltrans ARS online reports and data, http://dap3.dot.ca.gov/ARS_Online/technical.php

Graymer, R.W., and 5 others, 2006, Geologic Map of the San Francisco Bay Region. U.S.

Geological Survey, Scientific Investigations Map 2918.

Knudsen, K.L., Sowers, J.M., Witter, R.C., Wentworth, C.M., Helley, E. J., Nicholson, R. S., Wright, H. M. and Brown, K.H., (2000). Preliminary maps of Quaternary deposits and liquefaction susceptibility, nine-county San Francisco Bay region, California; a digital database: U.S. Geological Survey Open-File Report 00-444, 1 :24,000.

Schlocker, J., 1971, Generalized geologic map of the San Francisco Bay region, California, Association of Bay Area Governments, Basic Data Contribution 8, Map Scale 1:500,000.

U.S. Department of Agriculture National Resource Conservation Service, Accessed June 2021, Web Soil Survey, https://websoilsurvey.nrcs.usda.gov/app/

U.S. Geological Survey and California Geological Survey, Quaternary fault and fold database for the United States, accessed October 2020, at: https://www.usgs.gov/natural-hazards/earthquake-hazards/faults

Wagner, J.R., 1978, Late Cenozoic history of the Coast Ranges East of San Francisco Bay, Ph.D.

Dissertation, University of California at Berkeley, 161 pp.

Wentworth, C.M. and others, 1999, Preliminary Geologic Map and Description of the San Jose 30 x 60 Quadrangle, California: U.S. Geological Survey Open-File Report 98-795.

Wiegers, M. O., and 2 others, 2006, Geologic Map of the Fairfield North 7.5 Quadrangle, Solano and Napa Counties, California: A Digital Database. California Geological Survey, Preliminary Geologic Maps

Witter, R. C., and 6 others, 2006, Maps of Quaternary Deposits and Liquefaction Susceptibility in the Central San Francisco Bay Region, California: U.S. Geological Survey Open-File Report No. 2006-1037.

http://dap3.dot.ca.gov/ARS_Online/technical.php https://websoilsurvey.nrcs.usda.gov/app/ https://www.usgs.gov/natural-hazards/earthquake-hazards/faults https://www.usgs.gov/natural-hazards/earthquake-hazards/faults

Geotechnical Design Report Page 26

Pragmatic Expertise™

Working Group on California Earthquake Probabilities (WGCEP), 2003, Earthquake Probabilities in the San Francisco Bay Region: 2002-2031: U.S. Geological Survey Open-File Report 2003-214.

Geotechnical Design Report

Figures

§̈¦80

·|}þ12

Texas St

Un ion

Av e

Pe nn sy lva nia Av e

Travis Blvd

Su ns et Av e

Be ck

Av e

Railroad Ave

Woolner Ave

Railroad Ave

Canvasback Dr

Pintail Dr

E Tabor Ave

FAIRFIELD

M :\2

9\

-M ea dH un t_

Fa irf ie ld

Ju st ic e_

Ph as e3 \G

IS

\A rc G

IS

\F ig 1-

S ite

-L oc at io n.

m xd ; 9

/2 /2

1;

k dr oz yn sk a

190841

SITE LOCATION MAP

SEPTEMBER 2021 FIGURE 1

Site Location

0 0.5 10.25

MILES

Vallejo

Dixon

Vacaville

Oakland

Berkeley

AntiochConcordRichmond

San Francisco

Davis

MAP

LOCATION

785 Ygnacio Valley Rd.

Walnut Creek, CA, 94596 www.caleng.com

Phone: (925) 935-9771

BASEMAP REFERENCE

1.

2.

STREET CENTERLINES…