B08_Attachment_10_Geotechnical_Report_0005.pdf

Geotechnical Engineering Services 50 Percent Report

National Park Service Bocce Court Retaining Wall Replacement along Van Ness Street San Francisco, California for Davido Consulting Group

December 22, 2022

Geotechnical Engineering Services 50 Percent Report

National Park Service Bocce Court Retaining Wall Replacement along Van Ness Street San Francisco, California for Davido Consulting Group

December 22, 2022

13220 Evening Creek Drive South, Suite 115 San Diego, California 92128 619.314.5043

December 22, 2022 | Page i File No. 0861-043-00

Table of Contents

1.0 INTRODUCTION AND PROJECT UNDERSTANDING

2.0 SCOPE OF SERVICES

3.0 FIELD EXPLORATIONS AND LABORATORY TESTING

3.1. Field Explorations

3.2. Geophysical Evaluation

3.3. Laboratory Testing

3.4. Geology

3.5. Site Conditions

3.5.1. Surface Conditions

3.5.2. Soil Conditions

3.5.1. Groundwater

4.0 GEOLOGIC HAZARDS

4.1. Surface Fault Rupture

4.2. Liquefaction Potential

4.3. Tsunamis and Seiche Evaluation

4.4. Landsliding

4.5. Compressible and Expansive Soils

5.0 CONCLUSIONS AND RECOMMENDATIONS

5.1. Retaining Wall Subgrade Preparation and Earthwork Recommendations

5.1.1. Subgrade Preparation and Evaluation

5.1.2. Excavation

5.1.3. Erosion Control

5.2. Structural Backfill

5.2.1. On-Site Soils

5.2.2. Imported Structural Fill

5.2.3. Aggregate Base

5.3. Fill Placement and Compaction

5.4. Subsurface Wall Drainage

5.5. Temporary Shoring

5.5.1. Upper Wall

5.5.2. Lower Wall

5.5.3. Shoring Wall Performance

5.6. Slopes

5.6.1. Permanent Slopes

5.6.2. Temporary Slopes

5.6.3. Slope Drainage

6.0 STRUCTURAL DESIGN RECOMMENDATIONS

6.1. Earthquake Engineering (2018 IBC /ASCE 7-16 Seismic Design Information)

6.2. Retaining Wall Design

6.2.1. Foundation Support and Minimum Size

6.2.2. Bearing Capacity

December 22, 2022| Page ii File No. 0861-042-00

6.2.3. Footing Bearing Surface Preparation

6.2.4. Foundation Settlement

6.2.5. Lateral Earth Pressures

7.0 DESIGN REVIEW AND CONSTRUCTION SERVICES

8.0 LIMITATIONS

9.0 REFERENCES

LIST OF FIGURES

Figure 1. Vicinity Map Figure 2. Site Plan Figure 3. Regional Seismicity Map

APPENDICES

Appendix A. Field Exploration and Laboratory Testing Figure A-1. Key to Explorations Figures A-2 and A-3. Log of Boring Laboratory Test Results

Appendix B. Geophysical Survey Appendix C. Report Limitations and Guidelines for Use

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1.0 INTRODUCTION AND PROJECT UNDERSTANDING

This report presents the results of GeoEngineers, Inc.’s (GeoEngineers’) geotechnical engineering services in support of the proposed Bocce Court Retaining Wall Replacement along Van Ness Street project located at the San Francisco Maritime National Historical Park in San Francisco, California. The site is shown relative to surrounding physical features on the Vicinity Map, Figure 1 and the Site Plan, Figure 2.

It is our understanding that the project will consist of removal and replacement of an existing 50-foot-long two-tier wooden retaining wall and associated sidewalk and fence behind the upper tier as well as existing shed in front of the bottom tier wall. We understand the existing wall consists of an approximately 4.5-foot-tall lower level, a 1H:1V (horizontal to vertical) slope leading to the higher-level, and an approximately 5-foot-tall higher-level wall. Failure of the upper and lower level retaining walls has resulted in loss of soil and settlement and cracking of the pavement behind the wall. We understand that the new wall is planned to be cantilever concrete walls consisting of a 6.5-foot-tall upper level, a 4-foot-wide flat bench followed by a 2H:1V slope leading to the higher-level, and an approximately 5- to 6-foot-tall higher-level wall.

2.0 SCOPE OF SERVICES

The purpose of our geotechnical engineering services presented in this report is to evaluate geologic and geotechnical conditions, and provide conclusions and recommendations for use in design of the project.

Our specific scope of geotechnical services is summarized below:

■ Review readily available existing soil and groundwater information in the immediate vicinity of the project.

■ Perform a geologic reconnaissance of the site to evaluate current conditions and site access for planning the subsurface exploration program. Coordinate with National Parks Service (NPS) personnel with regard to access and locating the exploration.

■ Explore subsurface conditions by drilling two borings and performing one geophysical survey line behind the retaining wall.

■ Provide geotechnical engineering analysis and recommendations including:

A general description of site topography, geology and subsurface conditions;

A site plan showing the location of the explorations;

The characterization of general soil and groundwater conditions across the site;

Recommendations for seismic design in accordance with American Society of Civil Engineers (ASCE) 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures;

Evaluation of liquefaction potential based on our simplified liquefaction analysis (if required);

Recommended type(s) of retaining wall systems to support the proposed development, and design lateral earth pressures;

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Evaluation of the global stability of the proposed retaining wall system using computer program Slope/W;

Construction considerations and recommendations, including excavation, subgrade preparation, material requirements, and compaction requirements;

Evaluation on suitability of on-site soil for use as fill, including structural fill behind the wall and beneath pavement areas, and provide criteria for imported fill and structural fill placement and compaction;

Recommendations for pavement subgrade preparation and pavement design sections; and

Drainage considerations for construction and design based on the conditions encountered in our explorations and the proposed site grading.

■ Provide a report that provides our geotechnical conclusions and recommendations for the site and the proposed project.

3.0 FIELD EXPLORATIONS AND LABORATORY TESTING

3.1. Field Explorations

The subsurface soil and groundwater conditions at the site were evaluated by completing two geotechnical boring. Boring B-1 was completed to a depth of 16.5 feet below existing site grades (bgs) and boring B-2 was completed to a depth of 21.5 feet bgs. The borings were completed on October 28, 2022 by AEC Drilling using a track-mounted, hollow-stem auger drilling equipment. Drill soil spoils were placed in drums and removed from the site in accordance with applicable laws and regulations.

The location of the explorations completed for this project are presented on the Site Plan, Figure 2. Details of the field exploration program and boring log are presented in Appendix A.

3.2. Geophysical Evaluation

The geophysical evaluation was performed by Norcal Geophysical Consultants, Inc. on November 15, 2022.

The purpose of the evaluation was to determine the average shear wave velocity in the upper 100 feet (Vs100) at the site to establish Seismic Site Class.

The evaluation consisted of developing a one-dimensional (1D) data profile using Multichannel Analysis of Surface Waves (MASW) 1D methodology. The MASW profile was conducted with a 24-channel spread seismograph. MASW data captured shear wave velocity to a depth of 100 feet below ground surface. The seismic traverse was conducted as close as possible to the existing retaining wall, as limited by existing site conditions. Details of the geophysical evaluation and the shear wave velocity profiles for the site are presented in Appendix B.

3.3. Laboratory Testing

Soil samples were obtained during the drilling and taken to a laboratory for further evaluation. Selected samples were tested for the determination of moisture content, fines content (material passing the U.S.

No. 200 sieve), and sieve analysis. The tests were performed in general accordance with test methods of ASTM International (ASTM). A description of the laboratory testing and the test results are presented in Appendix A.

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Field screening completed on soil samples obtained from the borings and no visual evidence of soil contamination was observed.

3.4. Geology

The geology around San Francisco is the result of forces along the Pacific and North American plates. The Pacific Plate is slowly creeping north past the North American Plate, forming the San Andreas fault system.

San Francisco Bay was created by movement on these faults about 650,000 years ago. San Francisco locates in west-central of the San Francisco Bay block. The San Andreas fault zone is offshore to the west.

San Francisco geology is comparatively simple: several fault-bounded northwest trending bands of Mesozoic and Paleogene basement overlain by Quaternary surficial deposits (and artificial fill) and lacking any of the Late Cretaceous and Tertiary overlap sequences found elsewhere in the region, except for a narrowband of Plio–Quaternary Merced Formation outcrops in the far southwest corner of the city.

Mesozoic rocks are exposed in sea cliffs and as resistant knobs and hills throughout the city. Merced Formation is only seen in sea cliffs near Fort Funston. Quaternary surficial deposits are poorly exposed, filling the low-lying areas between hills. Much of the land in the north and east side of the city is artificial fill overlying bay mud deposits.

Artificial Fill (AF) is widely distributed across San Francisco, especially in the downtown and South of Market area. Generally, AF consists of very loose to medium dense sand (SP), silty sand (SM), medium stiff sandy clay (CL). Occasionally, AF contains gravels, and locally contains miscellaneous debris (bricks, wood, metal, concrete, glass, crushed rock, etc.). Along the eastern side of San Francisco where over time the coastline has changed considerably due to fill placement, a wide variety of materials were used to reclaim or expand usable land. In many of these areas filling commenced by placing material below sea level and directly on top of Young Bay Mud and/or dune sand. The former Yerba Buena Cove and Mission Creek areas along the eastern shore were places where ships were converted from vessels to stationary structures. After years of fires, and landfill placement, including construction of the San Francisco seawall, portions of these wooden vessels have now become part of the stratigraphy of modern San Francisco.

3.5. Site Conditions

3.5.1. Surface Conditions

The subject site is located south of the Aquatic Cove in San Francisco, California, as shown on Figure 1.

The site consists of a two-tier retaining wall. The ground surface above the upper level wall is paved with asphalt for approximately 7 feet followed by 36-inch by 36-inch pavement panels. The ground surface at the top of the lower wall consists of fill soils placed at approximately 1H:1V slope. The area in front of the lower wall is graded with decomposed granite (DG). In addition, an existing shed and bocce court are located in front of the lower wall.

3.5.2. Soil Conditions

GeoEngineers’ understanding of subsurface conditions is based on review of two new borings (B-1 and B-2) completed as part of this study. Based on review of the subsurface information, the subsurface soil condition is consistent with the geologic mapping discussed above. The approximate location of the boring is presented in Figure 2.

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Soils encountered at the site within our exploration consist of fill soils. Fill soils generally consist of soft clay and loose to medium dense clayey sand to sandy clay with construction debris fragments. The thickness of fill extended to the depth explored in this study.

3.5.1. Groundwater

Groundwater was not encountered during our explorations and likely present at depths greater than 21 feet bgs. Based on proximity to the Aquatic Cove, it is likely that regional groundwater depth at the project site coincides with the elevation of the cove. Groundwater conditions at the site are expected to vary seasonally due to rainfall events and other factors not observed in our explorations.

4.0 GEOLOGIC HAZARDS

A geologic hazard or geohazard is an adverse geologic condition capable of causing widespread damage or loss of property and life. These hazards are geological and environmental conditions and involve long-term or short-term geological processes. Geologic hazards associated with earthquakes include ground rupture, ground shaking, tsunamis, seiches, seismic-induced flooding, liquefaction, seismic-induced ground settlement, and seismic-induced slope instability. In addition to geologic hazards associated with earthquakes and faulting, there are other potential geologic hazards that may impact the site. These include landslides, expansive soils, collapsible soils and groundwater. It appears that geologic hazards at the site are primarily limited to those caused by shaking from earthquake-generated ground motions. The following comments are provided with respect to these hazards.

4.1. Surface Fault Rupture

Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface, however, not all earthquakes result in surface rupture. Based upon review of referenced literature, the project site is in a seismically active region. Moderate to large earthquakes could be anticipated during the service life of the project. According to the California Geologic Survey (CGS), a fault is considered active if it has offset Holocene sediments less than approximately 11,700 years old. A fault is considered potentially active if fault offsets occurred 11,700 to 2.85 million years ago. The San Andreas, San Gregorio, and Hayward faults are the major faults closest to the site. Site is approximately 8.1 miles away from the San Andreas fault, and approximately 9.9 miles and 10.6 miles away from San Gregorio, and Hayward faults respectively. As such, the geologic risk associated with ground surface rupture at the San Francisco bocce court and surrounding site area considered to be moderate to high. The regional faults are presented on Figure 3.

4.2. Liquefaction Potential

When saturated, cohesionless soil liquefies during a major earthquake, it experiences a temporary loss of shear strength caused by a transient rise in excess pore water pressure generated by strong ground motion.

Flow failure, lateral spreading, differential settlement, loss of bearing, ground fissures, and sand boils are evidence of excess pore pressure generation and liquefaction.

Due to the lack of a shallow groundwater table, and the density of the soils underlying the site at depth, the potential for liquefaction, seismic settlement, lateral spreading, and related effects considered to be low.

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4.3. Tsunamis and Seiche Evaluation

The site is not located in a zone of potential tsunami inundation based on review of emergency planning maps prepared by the California Emergency Management Agency and the CGS. In addition, damage caused by oscillatory waves (seiche) is considered unlikely due to the site elevation relative to the Pacific Ocean.

4.4. Landsliding

According to regional geologic mapping, no landslides are mapped in the site area. In addition, evidence of landslides or landslide potential was not observed during the geologic site reconnaissance. As such, landsliding is not considered to be a significant geologic hazard at the subject site.

4.5. Compressible and Expansive Soils

The subgrade soils encountered during the recent investigation within the vicinity of the site do not appear significantly compressible or subject to hydro-collapse based on the anticipated loading. Surficially exposed fill materials and terrace deposits observed at the site consist of clayey/silty sand of minimal plasticity. As such, it is our opinion that the potential for the existence of expansive soils at the site is low.

5.0 CONCLUSIONS AND RECOMMENDATIONS

5.1. Retaining Wall Subgrade Preparation and Earthwork Recommendations

In general, site preparation and earthwork for retaining wall construction will include:

■ Locating all the existing utilities prior to excavation. The contractor shall support utilities adjacent to the excavation to avoid damage (including but not limited to the electrical line located behind the upper wall).

■ Installing steel piles in front of the existing lower level retaining wall for temporary support.

■ Excavation of a 4-foot-wide trench in front of the lower wall

■ Construction of the proposed lower wall while maintaining the existing lower wall.

■ Installing two steel piles at the upper wall along with property line with the neighbor to the south to support the landscaping of the neighboring property.

■ Excavating behind the existing upper level retaining wall and removing the existing wall. The temporary sloping is not to exceed a 1H:1V slope and may have a maximum 3-foot vertical cut at the top. It needs to be noted that performing vertical cuts can result in potential damage to existing sidewalk behind the excavation.

■ Construction of the proposed upper retaining wall.

■ Backfilling the site to proposed grade.

Demolition of upper existing retaining walls, stripping and grubbing, grading the site and excavating for the two new retaining walls, and may also include removal or support of existing site utilities where present behind the proposed retaining walls. Existing structures, or remnants of structures or other debris should be demolished and removed from the site.

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Existing voids and new depressions created due to removal of existing utilities, or other subsurface elements, should be cleaned of loose soil or debris down to firm soil and backfilled with compacted structural fill. Disturbance to a greater depth should be expected if site preparation and earthwork are conducted during periods of wet weather.

5.1.1. Subgrade Preparation and Evaluation

Exposed subgrades should be probed to identify soft, loose, or unsuitable areas. Probing should be conducted prior to placing fill and should be performed by a representative of GeoEngineers who will evaluate the suitability of the subgrade and identify areas of yielding that are indicative of soft or loose soil.

If soft or loose zones are identified during probing, these areas should be excavated to the extent indicated by our representative and replaced with structural fill.

5.1.2. Excavation

Based on conditions encountered in our subsurface exploration, it is our opinion that conventional earthmoving equipment in proper working condition should be capable of making necessary general excavations in overburden soil.

The earthwork contractor should be responsible for reviewing this report, including the boring logs, providing their own assessments, and providing equipment and methods needed to excavate the site soils, and rock if necessary, while protecting subgrades. Construction debris is likely to be encountered during excavation.

5.1.3. Erosion Control

Erosion control plans are required on all construction projects in City and County of San Francisco.

Measures that can be employed to reduce erosion include the use of silt fences, hay bales, buffer zones of natural growth, sedimentation ponds and granular haul roads.

5.2. Structural Backfill

All structural fill soils should be free of debris, clay balls, roots, organic matter, frozen soil, man-made contaminants, particles with greatest dimension exceeding 4 inches other deleterious materials. The suitability of soil for use as structural fill will depend on the gradation and moisture content of the soil. As the amount of fines in the soil matrix increases, the soil becomes increasingly more sensitive to small changes in moisture content and achieving the required degree of compaction becomes more difficult or impossible. Recommendations for suitable fill material are provided in the following sections. All construction should be done in the dry season. If a rain event were to occur during construction excavations need to be backfilled.

5.2.1. On-Site Soils

On-site near-surface soil consists of sand with silt and clay and is moisture sensitive. On-site soils can be used as structural fill, provided the material is free of debris, can be moisture conditioned and compacted to meet the compaction requirements or amended with cement. An experienced geotechnical engineer from GeoEngineers should determine the suitability of on-site soil encountered during earthwork activities for reuse as structural fill.

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5.2.2. Imported Structural Fill

Select imported granular material may be used as structural fill. The imported material should consist of pit or quarry run rock, crushed rock, or crushed gravel and sand that is fairly well-graded between coarse and fine sizes (approximately 25 to 65 percent passing the U.S. No. 4 sieve). It should have less than 5 percent passing the U.S. No. 200 sieve and have a minimum of 75 percent fractured particles according to American Association of State Highway and Transportation Officials (AASHTO) TP-61.

5.2.3. Aggregate Base

Aggregate base material should consist of imported clean, durable, crushed angular rock. Such rock should be well-graded, have a maximum particle size of 1 inch and have less than 5 percent passing the U.S. No. 200 sieve (3 percent for retaining walls), and meet the gradation requirements in Table 1. In addition, aggregate base shall have a minimum of 75 percent fractured particles according to AASHTO TP-61 and a sand equivalent of not less than 30 percent based on AASHTO T-176.

TABLE 1. RECOMMENDED GRADATION FOR AGGREGATE BASE

Sieve Size Percent Passing

(by weight)

1 inch 100

½ inch 50 to 65

No. 4 40 to 60

No. 40 5 to 15

No. 200 0 to 5

5.3. Fill Placement and Compaction

Structural fill should be compacted at moisture contents that are within 3 percent of the optimum moisture content and compacted to at least 95 percent of the maximum dry density (MDD) as determined by ASTM Test Method D 1557 (Modified Proctor). The optimum moisture content varies with gradation and should be evaluated during construction. Fill material that is not near the optimum moisture content should be moisture conditioned prior to compaction.

Fill and backfill material should be placed in uniform, horizontal lifts, and compacted with appropriate equipment. The appropriate lift thickness will vary depending on the material and compaction equipment used. It is the contractor’s responsibility to select appropriate compaction equipment and place the material in lifts that are thin enough to meet these criteria. However, in no case should the loose lift thickness exceed 18 inches.

A representative from GeoEngineers should evaluate compaction of each lift of fill. Compaction should be evaluated by compaction testing unless other methods are proposed for oversized materials and are approved by GeoEngineers during construction. These other methods typically involve procedural placement and compaction specifications together with verifying requirements such as proof-rolling.

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5.4. Subsurface Wall Drainage

Positive drainage is imperative behind retaining walls, unless they are designed to resist hydrostatic forces.

We estimate permeability of the on-site soils behind the retaining walls to be in the order of 1.6E-04 feet per second. We recommend a zone of free-draining material behind the retaining structure with weepholes to relieve hydrostatic pressures. The site soils encountered in our explorations contain a significant percentage of fines (material passing the U.S. No. 200 sieve). Fine soils are susceptible to particle migration, potentially clogging the drainage. We recommend one of the following options for drainage behind retaining walls:

■ Drainage material consisting of “pervious backfill material” described in Section 19-03.2D of the CALTRANS Standard Specifications. The drainage zone should extend horizontally at least 24 inches from the back of the retaining structure.

■ Drainage material consisting of material similar to “pervious backfill material” described in Section 19-03.2D of the CALTRANS Standard Specifications. The drainage zone should extend horizontally at least 12 inches from the back of the retaining structure. A filter fabric designed for separation should be placed between the gravel backfill and native site soils to prevent soil migration.

A perforated, smooth-walled, rigid polyvinyl chloride (PVC) pipe with a minimum diameter of 4 inches should be placed at the bottom of the drainage zone along the entire length of the retaining structure with the pipe invert at or below the elevation of the base of the footing. The drainpipes should collect water and direct it to a tightline leading to an appropriate disposal system. Cleanouts should be incorporated into the design of the drains in order to provide access for regular maintenance.

5.5. Temporary Shoring

We understand that temporary is required at the upper wall at the property line with the neighboring property to the south. We understand that the temporary shoring system will be supporting the landscaping of the neighboring property and will not experience any traffic or construction surcharge. In addition, temporary shoring is required in front of the lower wall to support the excavation for and placement of the proposed lower level retaining wall. We understand that soldier pile and lagging shoring is the preferred excavation support system in this area.

We anticipate the excavation at the upper wall will extend between 8 to 11 feet bgs and the excavation at the bottom wall will extend approximately 6.5 feet bgs. We recommend temporary shoring walls to be designed to limit deflections to 1 inch or less in order to protect the adjacent improvements against damage form movements of the support of excavation system,

5.5.1. Upper Wall

We recommend that soldier pile walls be designed using the earth pressures provided in Section 6.2.5 of this report (Lateral Earth Pressures). The earth pressures presented in this report are for full-height cantilever soldier pile walls, and the pressures represent the estimated loads that will be applied to the wall system for various wall heights.

Table 2 presents GeoEngineers’ recommended lagging thicknesses (roughcut) as a function of soldier pile clear span and depth.

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TABLE 2. RECOMMENDED TIMBER LAGGING THICKNESS

Depth (feet) Recommended Lagging Thickness (roughcut) for clear spans of:

5 feet 6 feet 7 feet 8 feet 9 feet 10 feet

0 to 25 2 inches 3 inches 3 inches 3 inches 4 inches 4 inches

Lagging should be installed promptly after excavation, especially in areas where perched groundwater is present or where clean sand and gravel soils are present and caving soil conditions are likely. The workmanship associated with lagging installation is important for maintaining the integrity of the excavation.

The space behind the lagging should be filled with soil as soon as practicable. Placement of this material will help reduce the risk of voids developing behind the wall and damage to existing improvements located behind the wall.

5.5.2. Lower Wall

Temporary shoring in front of the lower retaining walls should be installed prior to any excavation efforts. It is our understanding that driven wide-flange piles with timber lagging in the preferred excavation support system in this area. We understand that the lateral load performance of the proposed piles for the temporary shoring will be evaluated using the computer software program LPILE produced by Ensoft, Inc.

Our recommended LPILE soil parameters are presented in the Table 3. Alternatively, the soldier piles can be designed using the earth pressures provided in Section 6.2.5 of this report (Lateral Earth Pressures).

Timber lagging should be installed per recommendations included in section 5.5.1 of this report.

TABLE 3. RECOMMENDED STATIC SOIL PARAMETERS (STATIC CONDITIONS)

Depth (feet)

Soil Unit

USCS

Soil Type

Friction Angle

(degrees) Cohesion

(psf)

Lateral Analysis Parameters

P.Y Curve Model

Total Unit Weight (pcf)

Soil Modulus K (pci)

Strain Factor e50

0 to 16 Upper Fill SC/SM 33 100 Sand

(Reese) 120 90 -

12 to 35 Lower Fill SC/SM 30 50 Sand

(Reese) 120 25 -

Notes:

Depth is from top of the upper wall.

psf – pounds per square foot pcf – pounds per cubic foot pci – pounds per cubic inch

5.5.3. Shoring Wall Performance

A shoring monitoring program should be established prior to installation of the shoring system to monitor the performance of the temporary shoring walls and to provide early detection of deflections that could potentially damage nearby improvements. We recommend that a preconstruction survey of adjacent improvements, such as streets, utilities and buildings, be performed prior to commencing construction.

The preconstruction survey should include a video or photographic survey of the condition of existing improvements to establish the preconstruction condition, with special attention to existing cracks in streets or buildings.

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5.6. Slopes

5.6.1. Permanent Slopes

Permanent fill slopes should not exceed a gradient of 2H:1V. Fill slopes should be overbuilt by at least 12 inches and trimmed back to the required slope to maintain a firm face.

Slopes should be planted with appropriate vegetation to provide protection against erosion as soon as possible after grading. Surface water runoff should be collected and directed away from slopes to prevent water from running down the face of the slope.

5.6.2. Temporary Slopes

All temporary soil cuts associated with site excavations (greater than 4 feet in depth) should be adequately sloped back to prevent sloughing and collapse, in accordance with applicable OSHA and state guidelines.

Temporary cut slopes should not exceed a gradient appropriate for the soil type being excavated. The temporary sloping behind the upper wall should not be steeper than a 1H:1V slope and may have a maximum 3-foot vertical cut at the top. Vertical cuts in general increase the possibility of damage to existing sidewalks and other structures behind them. We understand that parking will not be allowed along the sidewalk behind the upper wall during construction and our recommendations for temporary slopes are provided based on this assumption.

However, because of the variables involved, actual slope angles required for stability in temporary cut areas can only be estimated before construction and might need to be modified based on conditions observed during construction. The stability and safety of cut slopes depend on a number of factors, including:

■ The type and density of the soil;

■ The presence and amount of any seepage;

■ Depth of cut;

■ Proximity and magnitude of the cut to any surcharge loads, such as stockpiled material, traffic loads or structures;

■ Duration of the open excavation; and

■ Care and methods used by the contractor.

We recommend that stability of the temporary slopes used for construction be the responsibility of the contractor, since the contractor is in control of the construction operation and is continuously at the site to observe the nature and condition of the subsurface. If groundwater seepage is encountered within the excavation slopes, the cut slope inclination may have to be flatter than 1H:1V. However, appropriate inclinations will ultimately depend on the actual soil and groundwater seepage conditions exposed in the cuts at the time of construction. It is the responsibility of the contractor to ensure that the excavation is properly sloped or braced for worker protection, in accordance with applicable regulations and guidelines.

To assist with this effort, we make the following recommendations regarding temporary excavation slopes:

■ Protect the slope from erosion with plastic sheeting for the duration of the excavation to minimize surface erosion and raveling.

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■ Limit the maximum duration of the open excavation to 14 days or shorter.

■ Perform earthwork during dry season and backfill the excavated areas in case of rain to avoid failure.

■ Place no surcharge loads (equipment, materials, etc.) within 10 feet of the top of the slope.

More restrictive requirements may apply depending on specific site conditions, which should be continuously assessed by the contractor.

5.6.3. Slope Drainage

If seepage is encountered at the face of permanent or temporary slopes, it will be necessary to flatten the slopes or install a subdrain to collect the water. We should be contacted to evaluate such conditions on a case-by-case basis.

6.0 STRUCTURAL DESIGN RECOMMENDATIONS

6.1. Earthquake Engineering (2018 IBC /ASCE 7-16 Seismic Design Information)

The following presents our recommended seismic design parameters for use in design of the proposed retaining wall per 2018 International Building Code (IBC) and ASCE 7-16. The results of the geophysical testing indicate a Site Class C classification at the site (Vs30=1760 feet per second). Table 3 presents mapped seismic parameters for the site per IBC 2018/ASCE 7-16.

TABLE 3. IBC 2018/ASCE 7-16 MAPPED SEISMIC DESIGN PARAMETERS

ASCE 7-16 Parameter Recommended Value

Site Class C

Short-period mapped MCER spectral response acceleration, SS (g) 1.5

Long-period mapped MCER spectral response acceleration, S1 (g) 0.6

Short-period site coefficient, FA 1.2

Long-period site coefficient, FV 1.4

Design Spectral Acceleration at 0.2 second period (SDS) 1.2

Design Spectral Acceleration at 1.0 second period (SD1) 0.56

Ts = SD1/SDS 0.47

Notes:

Parameters developed based on latitude 37.80620 and longitude -122.425450 using the Applied Technology Council (ATC) Hazards online tool.

6.2. Retaining Wall Design

6.2.1. Foundation Support and Minimum Size

Proposed retaining walls can be satisfactorily founded on continuous wall footings supported on firm native soils, or on structural fill placed over native soils. The footings should be established at least 30 inches below the lowest adjacent grade for the lower level retaining wall. The recommended minimum footing depth is greater than the anticipated frost depth. In addition, the wall footings should have minimum width of 24 inches.

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6.2.2. Bearing Capacity

We recommend that footings founded as recommended be proportioned using an allowable soil bearing pressure of 2,500 pounds per square foot (psf) for footings bearing on the underlying firm native silty clay/sand. The recommended allowable bearing pressure applies to the total of dead and long-term live loads and may be increased by one half when considering total loads, including earthquake or wind loads.

This is a net bearing pressure. The weight of the footing and overlying backfill can be ignored in calculating footing sizes. We recommend the allowable frictional resistance may be computed using a coefficient of friction of 0.4 applied to vertical dead-load forces.

6.2.3. Footing Bearing Surface Preparation

Foundation bearing surfaces should be prepared to a cleaned-out and non-yielding condition. Loose or disturbed materials present at the base of footing excavations should be removed or compacted.

Foundation bearing surfaces should not be exposed to standing water. Should water infiltrate and pool in the excavation, it should be removed before placing structural fill or reinforcing steel.

We recommend that a representative from GeoEngineers to observe foundation excavations before placing reinforcing steel in order to confirm that adequate bearing surfaces have been prepared or provide recommendations for removal of unsuitable soil. Unsuitable bearing materials should be recompacted or removed and replaced with compacted structural fill as recommended by the geotechnical engineer.

6.2.4. Foundation Settlement

We estimate that settlement of footings designed and constructed as recommended will be less than 1 inch, for the assumed retaining wall heights. Differential settlements comparably loaded along the 50 feet of continuous wall section should be less than ½ inch. Settlement is expected to occur rapidly as loads are applied. Settlements could be larger than estimated if footings are placed on loose or disturbed soil.

6.2.5. Lateral Earth Pressures

Static and seismic lateral earth pressures were evaluated for use in design of the project. The evaluation assumed that: (1) backfill placed directly against walls will be free draining and meet the requirements of presented in this report; (2) the backfill above the wall is level; and (3) that backfill placed within 2 feet of the wall is compacted using hand-operated equipment. The following subsections present recommended lateral earth pressures for use in design of conventional gravity retaining walls.

The recommended static lateral earth pressure coefficients and equivalent fluid pressures (triangular pressure distribution) for use in design of conventional gravity walls are presented in Table 4 for both imported structural fill as well as native on-site soil. Active earth pressure conditions assume that walls are not structurally restrained and are free to rotate.

December 22, 2022| Page 13

TABLE 4. STATIC EARTH PRESSURES FOR CONVENTIONAL GRAVITY WALLS

Wall

Earth Pressure Coefficient Equivalent Fluid Pressure (psf)

Active (Ka) Passive (KP) Active (Ka) Passive (KP)

Upper Wall 0.27 1.23 33*H 100*H

Lower Wall 0.27 3.00 33*H 240*H

Notes:

1. The magnitude of lateral earth pressure at a given height of wall is presented in units of pcf per foot of wall height (H), or psf. The wall height is the distance between the ground surface and the base of the wall. Walls should be designed to resist loads from surcharge and adjacent at-grade structures.

2. For drained earth pressures to be used for design, provisions for adequate drainage behind the wall must be included.

3. Compaction within 2 horizontal feet of the walls should be performed with lightweight, hand-operated equipment so that compaction-induced lateral stresses are limited.

For seismic active earth pressures for conventional gravity walls, use an additional dynamic increment force equal to 12*H psf, where H is the wall height, applied over wall height (exposed and unexposed height).

The recommended pressures do not include the effects of surcharges from surface loads. If vehicles will be operated within one-half the height of the wall, a traffic surcharge should be added to the wall pressure.

The traffic surcharge can be approximated by the equivalent weight of an additional 2 feet of backfill behind the wall. Additional surcharge loading conditions should also be considered on a case-by-case basis.

7.0 DESIGN REVIEW AND CONSTRUCTION SERVICES

Recommendations provided in this letter report are based on the assumptions and design information stated herein. We welcome the opportunity to review and discuss construction plans and specifications for this project as they are being developed. In addition, GeoEngineers should be retained to review the geotechnical-related portions of the plans and specifications to evaluate whether they are in conformance with the recommendations provided in this report.

Satisfactory construction and earthwork performance depend to a large degree on quality of construction.

Sufficient monitoring of the contractor’s activities is a key part of determining that the work is completed in accordance with the construction drawings and specifications. Subsurface conditions observed during construction should be compared with those encountered during the subsurface explorations.

Recognition of changed conditions often requires experience; therefore, qualified personnel should visit the site with sufficient frequency to detect whether subsurface conditions change significantly from those anticipated.

In order to continue as geotechnical engineer of record for the project, we recommend that GeoEngineers be retained to observe construction at the site to confirm that subsurface conditions are consistent with the site explorations, and to confirm that the intent of project plans and specifications relating to earthwork, pavement and foundation construction are being met.

December 22, 2022| Page 14

8.0 LIMITATIONS

We have prepared this report for use by the National Park Service, Davido Consulting Group and other members of the design team for the Diablo Lake Boathouse Rehabilitation project at the existing West Ferry Landing on the northern shore of Diablo Lake in the North Cascades National Park in Whatcom County, Washington.

Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted geotechnical practices in this area at the time the report was prepared. No warranty or other conditions, express or implied, should be understood.

Please refer to the Appendix C, Report Limitations and Guidelines for Use, for additional information pertaining to use of this report.

9.0 REFERENCES

ASCE/SEI 7-16, 2016. “Minimum Design Loads for Buildings and Other Structures.”

California Division of Mines and Geology, 2008. “Guidelines for Evaluating and Mitigating Seismic Hazards in California.” Special Publication 117A.

Haugerud and Tabor, 2009. “Geologic Map of North Cascade Range, Washington.” United States Geologic Survey Map.

U.S. Geological Survey, Quaternary Fault and Fold Database of the United States, accessed July 18 3, 2016, http://earthquake.usgs.gov/hazards/qfaults/.

California Department of Transportation, 2021. “Construction Manual.”

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

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Washington St Clay St

Greenwich St

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Union St Filbert St

Vallejo St

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MARINA

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Post St Bush St

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Vicinity Map

Figure 1

National Park Service - Replace Retaining Wall between Bocce Court and Van Ness Street

San Francisco, California

Livermore

San Leandro

San Mateo Fremont

Concord

Vallejo

San Francisco

Data Source: ESRI

Notes:

1. The locations of all features shown are approximate.

2. This drawing is for information purposes. It is intended to assist in showing features discussed in an attached document. GeoEngineers, Inc.

cannot guarantee the accuracy and content of electronic files. The master file is stored by GeoEngineers, Inc. and will serve as the official record of this communication.

Projection: NAD 1983 UTM Zone 10N

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National Park Service - Replace Retaining Wall between Bocce Court and Van Ness Street

San Francisco, California

Figure 2

Notes:

1. The locations of all features shown are approximate.

2. This drawing is for information purposes. It is intended to assist in showing features discussed in an attached document. GeoEngineers, Inc. cannot guarantee the accuracy and content of electronic files. The master file is stored by GeoEngineers, Inc. and will serve as the official record of this communication.

Projection: NAD 1983 StatePlane California VI FIPS 0406 Feet

Legend

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Notes:

1. The locations of all features shown are approximate.

2. This drawing is for information purposes. It is intended to assist in showing features discussed in an attached document.

GeoEngineers, Inc. cannot guarantee the accuracy and content of electronic files. The master file is stored by GeoEngineers, Inc.

and will serve as the official record of this communication.

Projection: NAD 1983 StatePlane California VI FIPS 0406 Feet

Legend

Site Quaternary Faults

Based on time of most recent surface deformation historical (<150 years), well constrained location historical (<150 years), moderately constrained location historical (<150 years), inferred location latest Quaternary (<15,000 years), well constrained location latest Quaternary (<15,000 years), moderately constrained location latest Quaternary (<15,000 years), inferred location late Quaternary (<130,000 years), well constrained location late Quaternary (<130,000 years), moderately constrained location late Quaternary (<130,000 years), inferred location undifferentiated Quaternary(<1.6 million years), well constrained location undifferentiated Quaternary(<1.6 million years), moderately constrained location undifferentiated Quaternary(<1.6 million years), inferred location

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Serra fault zone

Mount Diablo thrust fault

Chabot fault

Franklin fault

M oraga fault

M iller Creek fault

Data Source: USGS. ESRI.

Regional Seismicity Map

National Park Service - Replace Retaining Wall between Bocce Court and Van Ness Street

San Francisco, California

Figure 3

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APPENDIX A

Field Exploration and Laboratory Testing

December 22, 2022| Page A-1

APPENDIX A

FIELD EXPLORATION AND LABORATORY TESTING

Field Explorations

Borings

Borings were completed using truck-mounted, continuous-flight, hollow-stem auger drilling equipment. The boring was continuously monitored by a staff from our firm who examined and classified the soils encountered, obtained representative soil samples, observed groundwater conditions and prepared a detailed log of each exploration.

The soils encountered in the borings were sampled at 2.5-foot vertical intervals in the top 10 feet and generally sampled at 5-foot vertical intervals below that. Samples were obtained using a 2-inch outside diameter split-barrel standard penetration test (SPT) sampler. The disturbed samples were obtained by driving the sampler 18 inches into the soil with a 140-pound automatic hammer free-falling 30 inches. The number of blows required for each 6 inches of penetration was recorded. The blow count ("N-value") of the soil was calculated as the number of blows required for the final 12 inches of penetration. This resistance, or N-value, provides a measure of the relative density of granular soils and the relative consistency of cohesive soils. Where very dense soil conditions precluded driving the full 18 inches, the penetration resistance for the partial penetration was entered on the logs. The blow counts are shown on the boring logs at the respective sample depths.

Soils encountered in the borings were visually classified in general accordance with the classification system described in Figure A-1. A key to the boring log symbols is also presented in Figure A-1. The logs of the borings/monitoring wells are presented in Figures A-2. The boring logs are based on our interpretation of the field and laboratory data and indicate the various types of soils and groundwater conditions encountered. The logs also indicate the depths at which these soils or their characteristics change, although the change may actually be gradual. If the change occurred between samples, it was interpreted. The densities noted in the boring logs are based on the blow count data obtained in the borings and judgment based on the conditions encountered.

Observations of groundwater conditions were made during drilling. The groundwater conditions encountered during drilling are presented in the boring logs. Groundwater conditions observed during drilling represent a short-term condition and may or may not be representative of the long-term groundwater conditions at the site. Groundwater conditions observed during drilling should be considered approximate.

Laboratory Testing

Soil samples obtained from the explorations were transported to our laboratory and examined to confirm or modify field classifications, as well as to evaluate index properties of the soil samples. Representative samples were selected for laboratory testing consisting of the determination of the moisture content, fines content, and grain size distribution (sieve analysis). The tests were performed in general accordance with test methods of the ASTM International (ASTM) or other applicable procedures.

December 22, 2022| Page A-2

Moisture Content Testing

Moisture content tests were completed in general accordance with ASTM D 2216 for representative samples obtained from the explorations. The results of these tests are presented on the exploration logs in Appendix A at the depths at which the samples were obtained.

Percent Passing U.S. No. 200 Sieve (%F)

Selected samples were “washed” through the U.S. No. 200 mesh sieve to estimate the relative percentages of coarse- and fine-grained particles in the soil. The percent passing value represents the percentage by weight of the sample finer than the U.S. No. 200 sieve. These tests were conducted to verify field descriptions and to estimate the fines content for analysis purposes. The tests were conducted in accordance with ASTM D 1140, and the results are shown on the exploration logs in Appendix A at the respective sample depths.

Sieve Analyses

Sieve analyses were performed on selected samples in general accordance with ASTM D 422. The wet sieve analysis method was used to determine the percentage of soil greater than the U.S. No. 200 mesh sieve. The results of the sieve analyses were plotted, and were classified in general accordance with the Unified Soil Classification System and are presented in Appendix A.

It should be noted that the sieve analyses were performed on soils obtained from samplers that have an opening size of 1½ inches, so larger sized…