Madonna Getoech Report 6-30-22.pdf

Geotechnical Investigation Madonna Creek

Miramontes Ridge Open Space Preserve San Mateo County, California

Midpeninsula Regional Open Space District 330 Distel Circle | Los Altos, California 94022

June 30, 2022 | Project No. 404258001

Geotechnical | Environmental | Construction Inspection & Testing | Forensic Engineering & Expert Witness

Geophysics | Engineering Geology | Laboratory Testing | Industrial Hygiene | Occupational Safety | Air Quality | GIS syao Text Box Attachment D

2149 O’Toole Avenue, Suite 30 | San Jose, California 95131 | p. 408.435.9000 | www.ninyoandmoore.com

Geotechnical Investigation Madonna Creek Miramontes Ridge Open Space Preserve San Mateo County, California

Mr. David Liefert

Midpeninsula Regional Open Space District 330 Distel Circle | Los Altos, California

June 30, 2022 | Project No. 404258001

Chad Stellern, PG Senior Staff Geologist

CDS/AD/gvr

Anthony Dover, GE, PE Principal Engineer http://www.ninyoandmoore.com/

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CONTENTS

1 INTRODUCTION 1

2 SCOPE OF SERVICES 1

2.1 Task 1 – Geotechnical Evaluation 1

2.2 Task 2 – Approach Assessment 2

3 SITE AND PROJECT DESCRIPTION 2

4 FIELD EXPLORATION AND LABORATORY TESTING 2

5 GEOLOGIC AND SUBSURFACE CONDITIONS 3

5.1 Regional Geologic Setting 3

5.2 Site Geology 3

5.3 Subsurface Conditions 3

5.3.1 Lower Dam and Bridge Approach 4

5.3.2 Upper Dam Approach 4

5.3.3 Upper Dam 4

5.4 Groundwater 5

6 GEOLOGIC HAZARDS AND CONSIDERATIONS 5

6.1 Seismic Hazards 5

6.1.1 Historical Seismicity 5

6.1.2 Faulting and Ground Surface Rupture 6

6.1.3 Strong Ground Motion 6

6.1.4 Liquefaction and Strain Softening 7

6.2 Construction Feasibility 8

6.3 Corrosive/Deleterious Soil 8

6.4 Expansive Soils 8

6.5 Dam Stability Analysis 9

6.6 Public Safety 11

7 CONCLUSIONS 11

8 RECOMMENDATIONS 12

8.1 Earthwork 12

8.1.1 Pre-Construction Conference 13

8.1.2 Site Preparation 13

8.1.3 Subgrade Observations 13

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8.1.4 Remedial Grading for Expansive Soils 14

8.1.5 Material Recommendations 14

8.1.6 Subgrade Preparation 15

8.1.7 Fill Placement and Compaction 16

8.1.8 Excavation Stabilization 18

8.1.9 Rainy Weather Considerations 19

8.2 Pavements and Flatwork 20

8.2.1 Asphalt Pavement 20

8.2.2 Exterior Flatwork 21

8.3 Concrete 22

8.4 Surface Drainage and Site Maintenance 22

8.5 Review of Construction Plans 23

8.6 Construction Observation and Testing 23

9 LIMITATIONS 24

10 REFERENCES 25

TABLES

1 – Significant Historic Earthquakes

2 – California Building Code Seismic Design Criteria

3 – Slope Stability Shear Strength Parameters

4 – Slope Stability Analysis Results

5 – Recommended Material Requirements

6 – Subgrade Preparation Recommendations

7 – Fill Placement and Compaction Recommendations

8 – OSHA Material Classifications and Allowable Slopes

9 – Asphalt Concrete Pavement Sections

FIGURES

1 – Site Location 2 – Lower Exploration Locations 3 – Upper Exploration Locations 4 – Fault Locations and Earthquake Epicenters 5 – Regional Geology 6 – Seismic Hazard Zones

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APPENDICES

A – Boring Logs B – Laboratory Test Results C – Corrosivity Testing (CERCO Analytical) D – Dam Stability Analysis

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1 INTRODUCTION

In accordance with your authorization, we have conducted a geotechnical evaluation for the instream dam features within Madonna Creek, an intermittent stream in Miramontes Ridge Open Space

Preserve in San Mateo County, California (Figure 1). According to the District’s Request for Proposal

(RFP) dated February 16, 2022 and our pre-proposal site visit on March 15, 2022, the results from this investigation will inform the District’s conceptual and technical approach to habitat restoration, bridge replacement, site access and public safety. The purpose of our services is to evaluate the subsurface conditions of the dam features and bridge approaches to assess their stability during nominal, flooding, and seismic events, address the potential effects of seepage on dam stability and provide data and recommendations for the design and construction of the implementable repair or redesign.

2 SCOPE OF SERVICES

Our scope of services included the following:

2.1 Task 1 – Geotechnical Evaluation

• Reviewed readily available geologic and seismic literature pertinent to the project area, including geologic maps and reports, regional fault maps, aerial photographs, and seismic hazard maps.

• Conducted an engineering reconnaissance to observe the general site conditions and to mark the proposed locations for subsurface exploration.

• Coordinated with Underground Service Alert (USA) to locate underground utilities in the vicinity of the proposed exploration.

• Conducted a subsurface exploration consisting of drilling, logging, and sampling seven (7) auger borings using hand auger and truck-mounted equipment. The borings were advanced to depths of up to 26.5 feet below the existing grade to evaluate the subsurface conditions. A representative of Ninyo & Moore logged the subsurface conditions exposed in the borings and collect bulk and relatively undisturbed samples for laboratory testing. The borings were backfilled with grout and soil cuttings upon completion.

• Laboratory testing on selected soil samples to evaluate soil dry density and moisture content, soil gradation, Atterberg limits, expansion index, compressive strength, direct shear, corrosivity and R-value.

• Compilation and engineering analysis of the field and laboratory data, and the findings from our background review.

• Prepare this geotechnical evaluation report presenting our findings and conclusions regarding the subsurface conditions of dam features and bridge approaches during nominal, flooding and seismic events. Provide recommendations for the design and construction of the proposed improvement.

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2.2 Task 2 – Approach Assessment

• Conducted an additional subsurface exploration for the approach to the upper bridge consisting of drilling, logging, and sampling two (2) auger borings using hand auger and truck-mounted equipment. The borings were advanced to depths of up to 16.5 feet below the existing grade to evaluate the subsurface conditions. A representative of Ninyo & Moore logged the subsurface conditions exposed in the borings and collect bulk and relatively undisturbed samples for laboratory testing. The borings were backfilled with grout and soil cuttings upon completion.

3 SITE AND PROJECT DESCRIPTION

The Miramontes Ridge Open Space Preserve is located in San Mateo County near Half Moon

Bay (Figure 1) and is comprised of mixed landcover including shrublands, mature eucalyptus stands, riparian wetlands, and farmland. The subject stream, Madonna Creek, is an intermittent stream that drains approximately 1.7 square miles of undeveloped land running approximately east-west direction.

Madonna Creek is impacted by two earthen dams, approximately 2,000 feet apart, with concrete and earthen spillways adjacent to farmland and is currently being used as an agricultural water source for on-site farming. According to recent surveys, the earthen dams have incised 15 to 25 feet creating an artificial fish barrier. Wooden bridges on top of the earthen dams provide equestrian and pedestrian access to the property.

The stream valley is generally gentle sloping from east to west. According to recent survey provided by the client, the upper dam surface is approximately 189 feet above Mean Sea Level

(MSL) down to approximately 175 feet at the bottom of the spill way. The lower dam surface is at

140 feet above MSL down to approximately 115 feet at the bottom of the spill way.

4 FIELD EXPLORATION AND LABORATORY TESTING

Our subsurface investigation at the site was conducted on May 23 and 24, 2022, and consisted of nine (9) geotechnical borings excavated to a depth of up to 26.5 feet. A representative of Ninyo

& Moore logged the subsurface conditions exposed in the borings and collected bulk and relatively undisturbed soil samples from the borings. The samples were transported to our geotechnical laboratory for testing. The borings were backfilled with grout immediately after excavation. The boring logs are presented in Appendix A. Locations of the geotechnical borings for the lower and upper earthen dams are presented in Figures 2 and Figure 3, respectively.

Laboratory testing of soil samples recovered from the borings included tests to evaluate in-situ soil moisture content and density, grain size distribution, Atterberg limits, expansion index, compressive strength, direct shear, and R-value. A soil sample was submitted to CERCO

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Analytical for a corrosivity evaluation. The results of the in-situ moisture content and dry density tests are presented on the boring logs in Appendix A. The results of the other laboratory tests are presented in Appendix B. The results of the corrosivity tests are shown in Appendix C.

5 GEOLOGIC AND SUBSURFACE CONDITIONS

Our findings regarding regional geologic setting, site geology, subsurface stratigraphy, and groundwater conditions at the subject site are provided in the following sections.

5.1 Regional Geologic Setting

The site is located in the eastern foothills of the Santa Cruz Mountains, which are part of the Coast

Ranges geomorphic province of California. The Coast Ranges are comprised of northwesterly trending mountain ranges and structural valleys formed by tectonic processes commonly found around the Circum-Pacific belt. Basement rocks have been sheared, faulted, metamorphosed, and uplifted, and are separated by thick blankets of Cretaceous and Cenozoic sediments that fill structural valleys and line continental margins. The San Francisco Bay Area has several ranges that trend northwest, parallel to major strike-slip faults such as the San Andreas, Hayward, and

Calaveras (Figure 4). Major tectonic activity associated with these and other faults within this regional tectonic framework consists primarily of right-lateral, strike-slip movement.

5.2 Site Geology

Regional geologic mapping of the area by Brabb et al. (1998) indicates that the site is underlain by Miocene-aged marine sedimentary rocks of the Monterey Formation and lies near a complex zone of faulting including the San Andreas fault zone (Figures 4 and 5). According to Brabb et al.

(1998), the marine sedimentary rocks in this area are primarily fine-grained shale, claystone, and siltstone that is thinly bedded and ranges in color to grayish-brown to very pale orange. The stream valley and adjacent farmland is mapped as Holocene-aged alluvium described as unconsolidated, fine-grained sand, silt and clayey silt deposited in narrow canyons.

5.3 Subsurface Conditions

The following sections provide a generalized description of the materials encountered during our subsurface investigation. More detailed descriptions are presented on the logs in Appendix A.

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5.3.1 Lower Dam and Bridge Approach

Subsurface observations for the lower dam and bridge approach are recorded in borings B-

1 through B-3 presented in Appendix A. Soils observed in boring B-1, located south of the dam bridge, generally consisted of sandy clay found in loose to medium dense and moist conditions of up to 10 feet below present grade (BPG). In boring B-2, advanced just north of the dam to 26.5 feet, we found a brown, moist, loose to medium dense clayey sand with gravel to 7.5 feet BPG. We observed a dark brown, firm to stiff, wet, lean clay with gravel from 7.5 feet to 20 feet BPG overlying a blueish-gray, wet, stiff, fat clay with gravel to 25 feet

BPG over lying a gray, dense clayey gravel to 26.5 feet. Groundwater was observed at 9.0 feet BPG.

The subsurface materials for the approach to the lower dam were logged in Boring B-3.

Surficial soils were found to be a dark brown, very stiff to stiff, moist, sandy clay down to 7.5 feet BPG. Dark brown, loose to medium dense, wet, clayey sand were found below the sandy clay to termination depth, 16.5 feet, BPG. Ground water was encountered at 9.5 feet BPG.

Dam soils are considered undocumented and unsuitable fill materials as their installation and compaction is not documented. Fill soils are difficult to distinguish from native undisturbed soils as the fill was most likely sourced nearby.

5.3.2 Upper Dam Approach

Subsurface observations for the upper dam approach are recorded in borings B-4 through

B-6 presented in Appendix A. We observed primarily brown, medium dense, moist, clayey sand with gravel in all of the borings down to 16.5 feet BPG. Degraded concrete was observed near the surface of B-4 and at 3.5 feet BPG in B-5. Some road base was found in boring B-6. No groundwater was observed.

5.3.3 Upper Dam

Subsurface observations for the upper dam are recorded in borings B-7 through B-9 presented in Appendix A. Subsurface conditions were highly variable but in general were dark brown, loose to medium dense, moist clayey sand to clayey gravel to approximately 10 feet BPG. From approximately 10 to 15 feet BPG, the subsurface consisted of a brown, moist, stiff, sandy clay overlying a blueish- gray, loose, wet, clayey sand to the extent of our borings

26.5 feet BPG. Groundwater was found between 10 and 14 feet BPG.

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5.4 Groundwater

Groundwater was encountered at 9.5 feet at the Lower Dam and between 10 and 14 feet BPG at the Upper Dam. Historical groundwater elevations were not available near the project site locations from standard sources. Fluctuations in the groundwater level across the site and over time may occur due to seasonal precipitation, variations in topography or subsurface hydrogeologic conditions, or as a result of changes to nearby irrigation practices or groundwater pumping. In addition, seeps may be encountered at elevations above groundwater levels due to perched groundwater conditions, leaking pipes, preferential drainage, or other factors not evident at the time of our exploration.

6 GEOLOGIC HAZARDS AND CONSIDERATIONS

This study considered a number of issues relevant to the proposed construction, including seismic hazards, unsuitable materials, settlement of compressible soil layers from static loading, flood hazards, soil corrosivity, expansive soils, and infiltration characteristics. These issues are discussed in the following subsections.

6.1 Seismic Hazards

The seismic hazards considered in this study include the potential for ground rupture due to faulting, seismic ground shaking, liquefaction, dynamic settlement. These potential hazards are discussed in the following subsections.

6.1.1 Historical Seismicity

The site is located in a seismically active region. Figure 4 presents the location of the site relative to the epicenters of historic earthquakes with magnitudes of 5.5 or more from 1800 to 2000. Table 1 summarizes the significant historic earthquakes with a magnitude of 6 or more that have occurred within 100 kilometers (km) of the site since 1900. Records of historic ground effects related to seismic activity (e.g. liquefaction, sand boils, lateral spreading, ground cracking) compiled by Knudsen et al. (2000), indicate that there is evidence of lateral spreading related to historic seismic activity have been reported for the site.

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Table 1 – Significant Historic Earthquakes

Date Place Location Magnitude Depth 1989-10-18 Loma Prieta 37.036°N 121.880°W 6.9 17.2 km 1984-04-24 Northern California 37.310°N 121.679°W 6.2 8.2 km 1926-10-22 Offshore Central California 36.725°N 122.180°W 6.3 10.0 km 1911-07-01 San Francisco Bay Area 37.250°N 121.750°W 6.6 -

1906-04-18 The 1906 San Francisco Earthquake 37.750°N 122.550°W 7.9 11.7

Reference: https://earthquake.usgs.gov/earthquakes/search

6.1.2 Faulting and Ground Surface Rupture

The site is not located within an Alquist-Priolo Earthquake Fault Zone established by the State

Geologist (CGS, 2018) to delineate regions of potential ground surface rupture adjacent to active faults. As defined by the California Geological Survey (CGS), active faults are faults that have caused surface displacement within Holocene time, or within approximately the last

11,700 years (CGS, 2018). The closest fault rupture hazard zone is the one associated with the Pilarcitos Fault, which is approximately 1.5 mile east from the study area.

Based on our review of the referenced geologic maps, known active faults are not mapped on the site and the site is not located within a fault-rupture hazard zone. Therefore, the probability of damage from surface fault rupture is considered to be low.

6.1.3 Strong Ground Motion

Table 2 presents the Risk-Targeted, Maximum Considered Earthquake (MCER) spectral response accelerations consistent with the 2019 California Building Code and corresponding site-adjusted and design level spectral response accelerations based on the USGS seismic design maps (SEAOC/OSHPD, 2022).

https://earthquake.usgs.gov/earthquakes/search

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Table 2 – California Building Code Seismic Design Criteria Seismic Design Parameter

Evaluated for 37.474105º North latitude, 122.404506º West longitude

Value

Site Class D-Default

Site Coefficient, Fa 1.2 Site Coefficient, Fv 1.7 Mapped Spectral Acceleration at 0.2-second Period, Ss 1.819g Mapped Spectral Acceleration at 1.0-second Period, S1 0.740g Spectral Acceleration at 0.2-second Period Adjusted for Site Class, SMS 2.183g Spectral Acceleration at 1.0-second Period Adjusted for Site Class, SM1 1.258 Design Spectral Response Acceleration at 0.2-second Period, SDS 1.455 Design Spectral Response Acceleration at 1.0-second Period, SD1 0.839 Seismic Design Category for Risk Category I, II, or III I

6.1.4 Liquefaction and Strain Softening

The strong vibratory motions generated by earthquakes can trigger a rapid loss of shear strength in saturated, loose, granular, or fine-grained soils of low plasticity (liquefaction).

Liquefaction is generally not a concern at depths of more than 50 feet below ground surface.

Liquefaction can result in a loss of foundation bearing capacity or lateral spreading of sloping or unconfined ground. Liquefaction can also generate sand boils leading to settlement at the ground surface. The site has been assessed by Knudsen et al. (2000) and ABAG (2022) and found to have low to moderate liquefaction potential (Figure 6).

We encountered deposits of granular soils below the groundwater level during our subsurface exploration. We evaluated the potential for liquefaction in accordance with the methods presented by Idriss and Boulanger (2008) using the penetration resistance collected from the borings during the subsurface exploration. Our analysis assumed a design groundwater level of 10 feet below the ground surface, and considered a seismic event producing a PGAM of

0.935g resulting from a Magnitude 7.48 earthquake. The results of our analysis, indicate that a basal layer of clayey gravel soil at Boring B-2 (Lower Dam) at a depth of approximately 25 feet is liquefiable. Additionally, the basal layer of clayey sand at Boring B-7 (Upper Dam) at a depth of 15 feet below the ground surface may liquefy under the considered ground motion based on a computed factor of safety against liquefaction that is less than one. Liquefaction reduces the shear strength of soils and may affect the stability of the dams. This information was used in the development of the seismic scenario for the dam stability analysis in Section

6.5.

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6.2 Construction Feasibility

Fill materials that were not placed and compacted under the observation of a geotechnical engineer, or fill materials lacking documentation of such observation, are considered to be undocumented fill and unsuitable as a bearing material below foundations due to the potential for differential settlement resulting from variable support characteristics or the potential inclusion of deleterious materials. Based on the findings from the limited soil investigation, the earthen dams contain variable depths of uncontrolled fill. Soil containing roots or other organic matter are not suitable as fill or structures. Surficial soil containing roots or other organic matter should be removed as part of clearing and grubbing operations.

Shallow foundations (spread footings), additional fill or structures should not be placed on uncontrolled and undocumented fills. If construction of a vehicle bridge or other structures are being considered, we would recommend deep foundations, or removal, replacement, and amendment of unsuitable materials as recommended in Section 8.

Undocumented and uncontrolled fill material used during dam construction does not imply that the structure of the dam is unsound or poses a public safety risk; please refer to the dam stability analysis (Section 6.5). However, the dam was not constructed using contemporary methods nor does it conform to current code, and new construction must be designed to current codes.

6.3 Corrosive/Deleterious Soil

Corrosivity analysis was performed by CERCO Analytical, Inc. of Concord, California on samples of the near-surface soil. As reported by CERCO Analytical, the samples were determined to be

“moderately corrosive” based on resistivity test results. CERCO Analytical’ s report (see Appendix

C) included the following recommendation: “All buried iron, steel, cast iron, ductile iron, galvanized steel and dielectric coated steel or iron should be properly protected against corrosion depending upon the critical nature of the structure. All buried metallic pressure piping such as ductile iron firewater pipelines should be protected against corrosion.” Please refer to the CERCO Analytical report included in Appendix C for more information regarding their test results and brief evaluation.

6.4 Expansive Soils

Some clay minerals undergo volume changes upon wetting or drying. Unsaturated soils containing those minerals will shrink/swell with the removal/addition of water. The heaving pressures associated with this expansion can damage structures and flatwork. Laboratory testing was conducted on a select sample of the near-surface soil to evaluate the expansion index. The tests were performed in general accordance with the American Society of Testing and Materials

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(ASTM) Standard D 4829 (Expansion Index). The results of our laboratory testing to determine

Expansion Index of soils indicate that the expansion index of the near-surface soil is about 97, which is consistent with a high expansion characteristic (ASTM D 4829).

The Atterberg limits data from our laboratory tests show that the plasticity index for the near surface soil can be as high as 31 (Appendix B). Based on studies by Holtz and Gibbs (1956) and

Chen (1988), the expansion potential of the soil in this location can be classified as high.

To reduce the potential for differential movement and distress to the proposed improvements due to shrink/swell behavior, mitigation should be developed for expansive soils. We anticipate that suitable dam repairs and subgrade preparation can be used to mitigate the expansive soil conditions.

6.5 Dam Stability Analysis

Dam slope stability analyses were conducted for the existing site conditions. We used two water level scenarios, flood stage and nominal stage (groundwater level as observed). The results of our slope stability analyses are presented in Appendix D.

Slope stability analyses were conducted in order to evaluate the stability of the dams. Cross sections A-A’, B-B’, (Appendix D) were prepared to represent the existing site and slope conditions for the Lower and Upper Dams, respectively. The slope stability analysis model is based on our interpretation of the information obtained from our geologic reconnaissance and subsurface data associated with our exploratory borings. Shear strength parameters used in our analysis were selected based on laboratory test results, including grain size, Atterberg limits, and direct shear tests. The parameters used in our analysis are summarized in Table 3.

Table 3 – Slope Stability Shear Strength Parameters

Cross Section Geologic Unit Moist Unit

Weight (pcf)

Friction Angle (degrees)

Cohesion (psf)

Lower Dam Clayey Sand (SC) 110 30 0

Lower Dam Lean Clay (CL) 117 0 3,700

Lower Dam Fat Clay (CH) 120 0 2000

Lower Dam Clayey Gravel (GC) 120 35 0

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Our slope stability analyses were conducted based on the Spencer limit equilibrium method using a two-dimensional stability analysis program, Slope/W (Geo-Slope International Ltd., 2018) for static conditions. Failure surfaces were generated using the “entry and exit” search algorithms.

Iterations using these subroutines yield what we consider to be critical (lowest factor of safety) potential failure surfaces that could impact the access road. Potential shallow failure surfaces, less than about 3 feet in thickness, were not considered in the analyses. Tree roots and animal burrows may have a negative structural effect on the dams through piping, and erosion; however, these biologic factors were not considered in these analyses. The static analyses were conducted for the existing substrate conditions with two water stage scenarios, flood stage and nominal.

Seismic analysis was conducted using nominal groundwater level. The results of our analyses are summarized below in Table 4.

Table 4 – Slope Stability Analysis Results

Cross Section Factor of Safety (Nominal Water Level)

Factor of Safety (Flood Stage)

Factor of Safety (Seismic Event)

Lower Dam 2.76 2.27 1.97

Upper Dam 2.17 1.98 1.52

Factor of Safety is the ratio of the resisting forces over the driving forces for a given dam based on site specific parameters. A calculated Factor of Safety less than 1.0 is an indication that the dam or slope may fail. Given variability and uncertainty in geotechnical data, Factor of Safety of of 1.5 is considered to be the minimum acceptable.

The results of our slope stability evaluation indicate that the existing slopes as shown on cross-sections Lower Dam and Upper Dam, have calculated factors of safety of about 2.0 or more under static loading conditions for both nominal and flood stage (Table 4). Factors of Safety during a seismic event are greater than about 1.5 and indicate that the slopes would be globally stable during a Magnitude 7.48 earthquake with a groundwater level of 10 feet for both the Lower and

Upper Dams. There may be some shallow sloughing of loose soils during the shaking. Please note that basal soil units were identified as liquefiable and some ground surface settlement and cracking should be expected during a seismic event.

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6.6 Public Safety

The earthen dams are located in a rural area where access is limited. We have identified that this area is a Seismic Risk Category I, where the dams were constructed for agricultural purposes and are not designed for human occupancy. According to our dam stability analysis, dam failure

(significant large displacements and loss of dam cross section) is unlikely during a seismic or flooding event (Table 4). The likelihood of a seismic event occurring during a flood stage is infinitesimally small and not accounted for in this analysis.

In our opinion, in the unlikely event that a dam failure occurs, some property damage to agricultural and residential property downstream due to flooding may occur. Temporary loading due to people, horses or vehicles would not significantly increase the risk of failure or pose a public safety risk as long as the bridges are in good working condition and advised weight is not exceeded.

7 CONCLUSIONS

Our geotechnical evaluation included a review of project documents provided by you and readily available background materials, engineering site reconnaissance, excavation of borings, and compilation and engineering analysis of the field and laboratory data. Our main conclusions are as follows:

• The subsurface exploration for this study encountered fill and potential undisturbed native material. The subsurface conditions, as encountered, generally consisted loose to medium dense, clayey sand and firm lean clay. Bedrock was not encountered during our exploration.

• Fill soils, as encountered, were highly variable in composition and density. Compaction efforts, if any, were not documented during construction of the earthen dams or approach. In the current condition, we do not recommend shallow foundation design and construction without soil improvement.

• The on-site sandy soils, including fill soils, generally should be suitable for re-use as backfill once moisture-conditioned to within 2 % of the optimum moisture content. Oversize materials with a diameter of 4 inches or more should be anticipated and should be removed before use as fill. Deleterious materials such as roots, trash or soils with high organic content should not be used as fill.

• Expansion Index and Atterberg limit testing indicates that the near-surface soil / fill on site has a moderate to high expansion potential. Recommendations are provided below for remedial grading, and subgrade preparation to reduce the potential for expansive soil movement below proposed improvements and dam repairs.

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• Earthwork at the site is anticipated to expose some granular soils with little cohesion.

Accordingly, steep excavations will be subject to caving. Temporary construction slopes should be excavated at a slope ratio of approximately 1½:1 (horizontal to vertical).

Alternatively, steeper excavations should be shored. Excavations and shoring should conform to the Occupational Safety and Health Administration (OSHA) standards for Type C soil.

• Groundwater was encountered in borings near the dams’ spill ways (Boring B-2, B-7, B-8, and B-9) found between 9.5 to 14.0 feet below present grade. Fluctuations in the groundwater level across the site and over time may occur due to seasonal precipitation, pond and stream water level, variations in topography or subsurface hydrogeologic conditions, or as a result of changes to nearby irrigation practices or groundwater pumping.

• Laboratory corrosion testing indicates that the near-surface site soils are considered moderately corrosive. A corrosion engineer should be consulted to provide specific guidance on protective measures to mitigate corrosion.

• The site could experience a relatively large degree of ground shaking during a significant earthquake event on a nearby fault.

• Liquefaction and liquefaction-related seismic hazards (e.g., dynamic settlement, ground subsidence, lateral spreading) should be considered for remedial design and efforts for the dams based on the materials encountered and depth to groundwater. Saturated, unconsolidated, coarse grained material at the basal sections of both dams are likely liquefy during a magnitude 7.48 earthquake from a nearby fault causing some settlement. Liquefiable soils were accounted for during our dam stability analysis.

• The dam stability analysis found that both the Lower and Upper Dams are consider stable during the static scenarios (nominal and flood water stage) exceeding the factor of safety threshold of 1.5. According to our analysis the earthen dams are stable during a large seismic event with factor of safety being greater than 1.5; however, some settlement is likely due to liquefiable soils.

8 RECOMMENDATIONS

The following sections present our geotechnical recommendations for the design and construction of the proposed improvements. The project improvements should be designed and constructed in accordance with these recommendations, applicable codes, and appropriate construction practices.

8.1 Earthwork

The earthwork should be conducted in accordance with the relevant grading ordinances having jurisdiction and the following recommendations. The Geotechnical Engineer should observe earthwork operations. Evaluations performed by the Geotechnical Engineer during the course of field operations may result in new recommendations, which could supersede the recommendations in this section.

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8.1.1 Pre-Construction Conference

We recommend that a pre-construction conference be held. The owner and/or their representative, the governing agencies’ representatives, the civil engineer, Ninyo & Moore, and the contractor should be in attendance to discuss the work plan and project schedule and earthwork requirements.

8.1.2 Site Preparation

Site preparation should begin with the removal of vegetation, utility lines, debris and other deleterious materials from areas to be graded. Tree stumps and roots should be removed to such a depth that organic material is generally not present. Clearing and grubbing should extend to the outside of the proposed excavation and fill areas. Rubble and excavated materials that do not meet criteria for use as fill should be disposed of in an appropriate landfill. Existing utilities to be abandoned should be removed, crushed in place, or backfilled with grout.

Excavations resulting from removal of buried utilities, tree stumps, or obstructions should be backfilled with compacted fill in accordance with the recommendations in the following sections.

8.1.3 Subgrade Observations

Prior to placement of fill, the client should request an evaluation of the exposed subgrade by

Ninyo & Moore. Materials that are considered unsuitable should be excavated under the observation of Ninyo & Moore in accordance with the recommendations in this section or supplemental recommendations by the geotechnical engineer.

Unsuitable materials include, but may not be limited to dry, loose, soft, wet, expansive, organic, or compressible natural soil, and undocumented or otherwise deleterious fill materials. Unsuitable materials should be removed from trench bottoms and below bearing surfaces to a depth at which suitable foundation subgrade, as evaluated in the field by Ninyo

& Moore, is exposed. Excavations should be backfilled with controlled low strength material

(CLSM) as per our Material Recommendations section. Undocumented fill that can be processed to meet the general criteria in our Material Recommendations section can be re-used as general fill.

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8.1.4 Remedial Grading for Expansive Soils

Laboratory testing indicated that the near-surface soil on site has a moderate to high expansion characteristic. To reduce the potential for differential movement and distress to the proposed improvements due to shrink/swell behavior, a zone of material with low expansion potential should be created by removing the existing soil, as-needed, and placing fill with low expansion characteristics below slabs-on-grade, flatwork, and pavement. The zone of low expansion fill should consist of select, low-expansion import fill conforming with the Material

Recommendations section.

The lateral limits of over-excavations should extend a distance of 5 feet or more beyond the limits of the slab-on-grade and 2 feet or more beyond the limits of the flatwork or pavement.

The zone of low expansion material should extend to a depth of 18 inches below building slabs-on-grade, and 12 inches below exterior flatwork or pavement. The aggregate base or capillary break gravel under building slabs or exterior flatwork or pavement may be considered as part of the zone of low expansion material. The zone of exclusion/removal or lime treatment should be detailed on the construction plans to reduce the potential that these recommendations are overlooked during construction bidding.

8.1.5 Material Recommendations

Materials used during earthwork operations should comply with the requirements listed in

Table 5. Materials should be evaluated by Ninyo and Moore’s Geotechnical Engineer for suitability prior to use. The contractor should notify the geotechnical consultant 72 hours prior to import of materials or use of on-site materials to permit time for sampling, testing, and evaluation of the proposed materials. On-site materials may need to be dried out before re-use as fill. The contractor should be responsible for the consistency of import material brought to the site.

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Table 5 – Recommended Material Requirements

Material and Use Source Requirements1,2

Select (Low Expansion) Fill:

- below building slabs, pavement, or flatwork

Import

Close-graded with 35 percent or more passing No. 4 sieve and either:

Expansion Index of 50 or less, Plasticity Index of 12 or less, or less than 10 percent, by dry weight, passing No. 200 sieve

General Fill:

- for uses not otherwise specified

Import As per Select (Low Expansion) Fill

On-site borrow No additional requirements1

Controlled Low Strength Material (CLSM) Import CSS4 Section 19-3.02G

Permeable Aggregate

- capillary break gravel Import

Open-graded, clean, compactable crushed rock or angular gravel;

nominal size ¾ inch or less

Aggregate Base Import Class II; CSS4 Section 26-1.02

Asphalt Concrete Import Type A; CSS4 Section 39-2

Pipe/Conduit Bedding and Pipe Zone Material -material below pipe invert to 12 inches above pipe

Import 90 to 100 percent (by mass) should pass No. 4 sieve, and 5 percent or less should pass No. 200 sieve

Trench Backfill

- above bedding material

Import or on-site borrow

As per general fill and excluding rock/lumps retained on 4-inch sieve or 2-inch sieve in top 12 inches Notes:

1 In general, fill should not consist of pea-gravel and should be free of rocks or lumps in excess of 6-inches diameter, trash, debris, roots, vegetation or other deleterious material.

2 In general, import fill should be tested or documented to be non-corrosive3 and free from hazardous materials in concentrations above levels of concern.

3 Non-corrosive as defined by the Corrosion Guidelines (Caltrans, 2018b).

4 CSS is California Standard Specifications (Caltrans, 2018a).

8.1.6 Subgrade Preparation

Subgrade in trenches and below slabs, footings, flatwork, or fill, should be prepared as per the recommendations in Table 6. Prepared subgrade should be maintained in a moist (but not saturated) condition by the periodic sprinkling of water prior to placement of additional overlying fill or construction of footings and slabs.

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Table 6 – Subgrade Preparation Recommendations Subgrade Location Preparation Recommendations

Below Foundations

• Check for unsuitable materials and remove as-needed per Sections 8.1.2 and

8.1.3. Replace overexcavated soil with CLSM or extend footing as-needed.

• Scarify and moisture condition exposed subgrade as-needed to achieve a moisture content 2 points or more above the optimum as evaluated by ASTM D1557. Compact moisture-conditioned subgrade per Section 8.1.7.

• Keep in moist condition by sprinkling water.

Below Slabs and Flatwork

• After clearing and grubbing per Section 8.1.2, check for unsuitable materials as per Section 8.1.3.

• Perform remedial grading as per Section 8.1.4. Scarify 8 inches then moisture condition and compact as per Section 8.1.7 if in-place lime treatment is not performed.

• Keep in moist condition by sprinkling water.

Below Fill • After clearing and grubbing per Section 8.1.2 check for unsuitable materials as per Section 8.1.3.

• Scarify 8 inches then moisture condition and compact as per Section 8.1.7.

• Keep in moist condition by sprinkling water.

Below Pavement

• After clearing and grubbing per Section 8.1.2, check for unsuitable materials as per Section 8.1.3.

• Perform remedial grading as per Section 8.1.4. Scarify 8 inches then moisture condition and compact as per Section 8.1.7 if in-place lime treatment is not performed.

• Proof roll compacted subgrade with loaded water truck under the observation of the geotechnical engineer. Mitigate yielding areas in accordance with the recommendations of the engineer.

• Keep in moist condition by sprinkling water.

Utility Trenches • After clearing per Section 8.1.2, check for unsuitable materials as per

Section 8.1.3.

• Remove or compact loose/soft material.

Subgrade that has been permitted to dry out and loosen or develop desiccation cracking, should be scarified, moisture-conditioned, and recompacted as per the requirements above.

A thin layer (approximately 3 inches) of lean concrete or controlled low strength material

(CLSM) may be poured over prepared subgrade for footings or slabs to maintain the appropriate moisture condition during erections of forms and placement of reinforcing steel.

8.1.7 Fill Placement and Compaction

Fill and backfill should be compacted in horizontal lifts in conformance with the recommendations presented in Table 7. The allowable uncompacted thickness of each lift of fill depends on the type of compaction equipment utilized, but generally should not exceed

8 inches in loose thickness.

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Table 7 – Fill Placement and Compaction Recommendations

Fill Type Location Compacted Density1

Moisture Content2

Subgrade

Below pavement and foundation (within 2 feet of finished grade) 95 percent + 2 percent or above

In locations not already specified 90 percent + 2 percent or above

Trench Backfill

Below pavement and foundation (within 2 feet of finished grade) 95 percent + 2 percent or above

In locations not already specified 90 percent + 2 percent or above

Select or General Fill (not lime-treated)

Below pavement and foundation (within 2 feet of finished grade) 95 percent + 2 percent or above

In locations not already specified 90 percent + 2 percent or above

Lime-or cement-treated subgrade or fill

In locations not already specified 95 percent + 2 percent or above

Aggregate Base Pavement section or below hardscape 95 percent Near Optimum

Asphalt Concrete Pavement section 91 to 97 percent Not Applicable

Notes:

1 Expressed as percent relative compaction or ratio of field density to reference density (typically on a dry density basis for soil and aggregate and on a wet density basis for asphalt concrete and lime treated subgrade). The reference density of soil, lime-treated subgrade, and aggregate should be evaluated by ASTM D 1557. The reference density of asphalt concrete should be evaluated by

ASTM D 2041.

2 Target moisture content at compaction relative to the optimum as evaluated by ASTM D 1557.

Compacted fill should be maintained in a moist (but not saturated) condition by the periodic sprinkling of water prior to placement of additional overlying fill or construction of footings and slabs. Fill that has been permitted to dry out and loosen or develop desiccation cracking, should be scarified, moisture conditioned, and recompacted as per the requirements above.

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8.1.8 Excavation Stabilization

Excavations, including vault and utility excavations, should be stabilized by shoring sidewalls or laying slopes back in accordance with the Excavation Rules and Regulations (29 Code of

Federal Regulations [CFR], Part 1926) stipulated by the Occupational Safety and Health

Administration (OSHA). Table 6 lists the OSHA material type classifications and corresponding allowable temporary slope layback inclinations for soil deposits that may be encountered on site. Alternatively, a shoring system conforming to the OSHA Excavation

Rules and Regulations (29 CFR Part 1926) may be used to stabilize excavation sidewalls during construction. The lateral earth pressures listed in Table 6 may be used to design or select an internally-braced shoring system or trench shield conforming to the OSHA guidelines. Our recommendations for lateral earth pressures and allowable slope gradients are based upon the limited subsurface data provided by our exploratory borings and reflect the influence of the environmental conditions that existed at the time of our exploration.

Excavation stability, material classifications, allowable slopes, and shoring pressures should be re-evaluated and revised, as-needed, during construction. Excavations, shoring systems and the surrounding areas should be evaluated daily by a competent person for indications of possible instability or collapse. Dewatering pits or sumps should be used to depress the groundwater level (if encountered) below the bottom of the excavation.

Table 8 – OSHA Material Classifications and Allowable Slopes

Formation OSHA Classification

Allowable Temporary Slope1,2,3

Lateral Earth Pressure on

Shoring4 (psf)

Cohesive Soil (above groundwater) Type B 1h:1v (45°) 45×D + 72

Granular Soil (above groundwater) Type C 1½ h:1v (34°) 80×D + 72

Notes:

1 Allowable slope for excavations less than 20 feet deep. Excavation sidewalls in cohesive soil may be benched to meet the allowable slope criteria (measured from the bottom edge of the excavation). The allowable bench height is 4 feet. The bench at the bottom of the excavation may protrude above the allowable slope criteria.

2 In layered soil, layers shall not be sloped steeper than the layer below.

3 Temporary excavations less than 5 feet deep may be made with vertical side slopes and remain unshored if judged to be stable by a competent person (29 CFR, Part 1926.650).

4 ‘D’ is depth of excavation for excavations up to 20 feet deep. Includes a surface surcharge equivalent to two feet of soil.

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The shoring system should be designed or selected by a suitably qualified individual or specialty subcontractor. The shoring parameters presented in this report are preliminary design criteria, and the designer should evaluate the adequacy of these parameters and make appropriate modifications for their design. We recommend that the contractor take appropriate measures to protect workers. OSHA requirements pertaining to worker safety should be observed.

The excavation bottoms may become unstable and subject to pumping under heavy equipment loads if the excavation subgrade is exposed to water. The contractor should be prepared to stabilize the bottom of the excavations. In general, unstable bottom conditions may be mitigated by scarifying the subgrade and aerating the soil to achieve a moisture content near the optimum, dewatering to depress groundwater levels below the bottom of the excavation, overexcavating to a suitable depth and replacing the wet material with suitable fill, compacting a layer of crushed rock fill into the subgrade, or using geogrid to stabilize additional fill. Specific recommendations for excavation stabilization will be influenced by the nature of the excavation and the conditions encountered during construction.

8.1.9 Rainy Weather Considerations

We recommend that the construction be performed during the period between approximately

April 15 and October 15 to avoid the rainy season. In the event that grading is performed during the rainy season, the plans for the project should be supplemented to include a stormwater management plan prepared in accordance with the requirements of the relevant agency having jurisdiction. The plan should include details of measures to protect the subject property and adjoining off-site properties from damage by erosion, flooding or the deposition of mud, debris, or construction-related pollutants, which may originate from the site or result from the grading operation. The protective measures should be installed by the commencement of grading, or prior to the start of the rainy season. The protective measures should be maintained in good working order unless the project drainage system is installed by that date and approval has been granted by the building official to remove the temporary devices.

In addition, construction activities performed during rainy weather may impact the stability of excavation subgrade and exposed ground. Temporary swales should be constructed to divert surface runoff away from excavations and slopes. Steep temporary slopes should be covered with plastic sheeting during significant rains. The geotechnical consultant should be consulted for recommendations to stabilize the site as-needed. A thin layer (approximately 3

Ninyo & Moore | Madonna Creek, San Mateo County, California | 404258001 | June 30, 2022 20 inches) of lean concrete or CLSM may be poured over prepared subgrade for footings or slabs to maintain the appropriate moisture condition during erections of forms and placement of reinforcing steel.

8.2 Pavements and Flatwork

Recommendations for asphalt pavement and exterior flatwork are presented in the following sections. The design R-value used for evaluating the pavement sections was selected based on

R-value testing performed on a sample collected during our subsurface exploration. The pavement subgrade should be observed by the geotechnical engineer during grading to check that the exposed materials are consistent with the findings from our subsurface exploration and the support characteristics assumed for pavement design. Additional R-value testing may be needed, based on these observations, with subsequent revision to the pavement sections.

Recommendations for preparation of subgrade are presented in Section 8.1.6.

Pavement sections were evaluated for a range of traffic indexes or loading conditions.