B08_Attachment_4_Final_Geotech_Eval_.pdf

GEOTECHNICAL EVALUATION

VISTA POINT MULTI-USE TRAIL

GOLDEN GATE NATIONAL RECREATION AREA

MARIN COUNTY, CALIFORNIA

PREPARED FOR:

Otak, Inc.

10230 NE Points Drive, Suite 400 Kirkland, Washington 98033

National Park Service Golden Gate National Recreation Area

Building 201 Fort Mason San Francisco, California 94123

PREPARED BY:

Ninyo & Moore

Geotechnical and Environmental Sciences Consultants 1956 Webster Street, Suite 400

Oakland, California 94612

May 27, 2016 Project No. 402412001

May 27, 2016 Project No. 402412001

Mr. Keith Bates Otak, Inc.

10230 NE Points Drive, Suite 400 Kirkland, Washington, 98033

Subject: Geotechnical Evaluation Vista Point Multi-Use Trail Golden Gate National Recreation Area Marin County, California

Dear Mr. Bates:

In accordance with your authorization, we have performed a geotechnical evaluation for the proposed Vista Point Multi-Use Trail located near U.S. Highway 101 and Fort Baker in the Golden Gate National Recreation Area, California. This report presents our geotechnical findings, conclusions, and recommendations regarding the proposed project.

As an integral part of our role as the geotechnical engineer-of-record, we request the opportunity to review the construction plans once available and to provide observation and testing services during construction.

We appreciate the opportunity to be of service on this project.

Sincerely, NINYO & MOORE

Peter C. Connolly, P.E., G.E.

Principal Engineer

DWM/PCC/vmp

Distribution: (1) Addressee

Vista Point Multi-Use Trail May 27, 2016 Marin County, California Project No. 402412001

402412001 R- Final Geo Eval i

TABLE OF CONTENTS

Page

1. INTRODUCTION

2. SCOPE OF SERVICES

3. SITE DESCRIPTION

4. PROJECT DESCRIPTION

5. STEREOSCOPIC AERIAL PHOTOGRAPH REVIEW

6. FIELD EXPLORATION AND LABORATORY TESTING

7. GEOLOGY AND SUBSURFACE CONDITIONS

7.1. Regional Geologic Setting

7.2. Site Geology

7.3. Subsurface Conditions

7.3.1. Fill

7.3.2. Colluvium

7.3.3. Residual Soil

7.3.4. Franciscan Formation Greenstone

7.3.5. Franciscan Formation Sandstone

7.3.6. Franciscan Formation Radiolarian Chert and Shale

7.4. Groundwater

8. DISCUSSION

8.1. Seismic Hazards

8.1.1. Faulting and Ground Surface Rupture

8.1.2. Strong Ground Motion

8.1.3. Liquefaction

8.1.4. Dynamic Settlement

8.2. Landsliding and Slope Stability

8.3. Static Settlement

8.4. Excavation Characteristics

8.5. Corrosive/Deleterious Soils

8.6. Expansive Soils

9. CONCLUSIONS

9.1. Earthwork

9.1.1. Site Preparation

9.1.2. Observation and Removals

9.1.3. Excavation Stabilization and Temporary Slopes

9.1.4. Constructed Slopes

9.1.5. Construction Dewatering

9.1.6. Material Requirements

9.1.7. Subgrade Preparation

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9.1.8. Fill Placement and Compaction

9.1.9. Rainy Weather Considerations

9.2. Retaining Walls

9.2.1. Concrete Cantilever Gravity Wall

9.2.2. Soldier Pile and Lagging Wall

9.3. Preliminary Pavement Design

9.4. Concrete

9.5. Drainage and Site Maintenance

9.6. Review of Construction Plans

9.7. Pre-Construction Conference

9.8. Construction Observation and Testing

10. LIMITATIONS

11. REFERENCES

Tables Table 1 – Criteria for Deleterious Soils on Concrete Table 2 – Recommended OSHA Material Classifications and Allowable Slopes Table 3 – Recommended Gradients for Constructed Slopes Table 4 – Recommended Material Requirements Table 5 – Subgrade Preparation Recommendations Table 6 – Recommended Compaction Requirements Table 7 – Bearing Resistance for Wall Footings Table 8 – Cantilever Gravity Wall Design Parameters Table 9 – Soldier-Pile-and-Lagging Wall Design Parameters Table 10 –Asphalt Concrete Pavement Sections

Figures Figure 1 – Site Location Figure 2 – Boring Location Figure 3 – Site Geology Figure 4 – Fault Locations Figure 5 – Regional Geology Figure 6 – Retaining Wall Drainage Detail

Appendices Appendix A – Boring Logs Appendix B – Laboratory Testing Appendix C – Ambient Air Monitoring Analytical Results

402412001 R- Final Geo Eval 1

1. INTRODUCTION

In accordance with your request, we have performed a geotechnical evaluation for the proposed

Vista Point Multi-Use Trail located on the existing Vista Point Road near U.S. Highway 101 and

Fort Baker in the Golden Gate National Recreation Area, California (Figure 1). The purpose of our evaluation was to provide subsurface information and geotechnical design parameters for the proposed improvements. This report presents our findings and conclusions regarding the geotechnical conditions at the subject site, and our recommendations for the design and construction of this project.

2. SCOPE OF SERVICES

Ninyo & Moore’s scope of services included review of pertinent geologic and geotechnical background data, geologic site reconnaissance, subsurface evaluation, geotechnical laboratory testing, and preparation of recommendations for the project. Specifically, we performed the following tasks:

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

Conducted geologic site reconnaissance to observe the surficial geologic conditions, check for indications of slope instability, and mark the proposed locations for subsurface exploration.

Coordinated with Underground Service Alert and the Golden Gate Bridge Highway & Transportation District to locate underground utilities in the vicinity of the proposed borings.

Preparation of a site health and safety plan including a lead compliance plan for field exploration activities to address the potential for lead impacted soil (issued under separate cover). The results of our testing for air lead monitoring are presented in Appendix C.

Performed a subsurface exploration consisting of ten (10) solid-stem auger borings. Boring locations (Figure 2) were coordinated with NPS staff and the design team. The borings were advanced to depths between approximately 10 and 26 feet below the existing grade. A representative of Ninyo & Moore logged the subsurface conditions exposed in the borings, and collected bulk and relatively undisturbed samples for laboratory testing. The borings were backfilled with grout.

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Performed laboratory tests on selected soil samples to evaluate in-situ moisture content and density, Atterberg limits, percentage of soil particle finer than the No. 200 sieve, expansion index, unconfined compressive strength, R-value, and soil corrosivity.

Compilation and engineering analysis of the field and laboratory data from our subsurface evaluation and the findings from our geologic review.

Preparation of this geotechnical report presenting the findings and conclusions from our evaluation, and our geotechnical recommendations for design and construction of the proposed improvements.

3. SITE DESCRIPTION

The existing Vista Point Road is located in southern Marin County, west of San Francisco Bay and north of the Golden Gate (Figure 1). The road wraps around the eastern portion of the hill (from south to northwest) below the H. Dana Bowers Memorial Vista Point. The lower (southern) end of the road within the project limits is located under the northern end of the Golden Gate Bridge at an elevation of approximately 115 feet above Mean Sea Level (MSL) (US Department of the

Interior, 2014). The road extends uphill at an average grade of about 8 percent for approximately

1,700 feet around the Vista Point. The upper (northwestern) end of the road terminates at an elevation of approximately 250 feet above MSL on the eastern side of US Highway 101 at the freeway entrance from the Vista Point.

The existing Vista Point Road is cut into the hillside below Vista Point and is predominantly unpaved, with minor exceptions. The lower end of the road underneath the Golden Gate Bridge is gated off and is currently in use as a staging area for overhead construction work on the bridge.

The cut slope behind the bridge abutment features several shotcrete wall segments with rock anchors. As the road continues uphill from the area underneath the bridge, the cut slope on the western side of the road is predominantly unsupported and varies in inclination between approximately 40 degrees and vertical. The slope below the road varies between approximately 18 degrees and 45 degrees. Additional features observed along the existing road alignment include several drain pipes and outlets, a cluster of three circular tanks, a steel crib wall approximately 10 feet long and 10 feet high, and a sanitary sewer manhole in front of the crib wall. The tanks are located approximately 10 to 15 feet above the road with diameters ranging from approximately 20

402412001 R- Final Geo Eval 3 to 35 feet. Approximately 200 feet of the road uphill from the tank cluster is partially paved with asphalt concrete.

4. PROJECT DESCRIPTION

The proposed project will consist of improvements to the existing Vista Point Road to create a single-surface multi-use trail with a design capacity and circulation for light duty service vehicles and pedestrians, rest area/pull offs, drainage improvements, handrails, bridge security elements, and associated way-finding. Retaining walls and slope cuts are proposed in order to achieve the designed trail grades and widths.

Based on our review of the draft design plans by the National Park Service (2014), we understand that two cuts into the uphill side of the trail are proposed to move the toe of the existing cut slope inward up to approximately 7 feet from the current location. The approximate locations of these cuts are from station 17+50 to 21+00 and from station 23+50 to 25+20 (Figure 3). We also understand that the project will construct three retaining walls along the trail alignment. The proposed wall from approximate station 10+50 to 14+50 will support up to about 6 feet of fill. The proposed wall from approximate station 26+65 to 27+15 will support up to about 4 feet of cut. The proposed wall from approximate station 21+60 to 21+80 will support up to about 6 feet of fill. The proposed finish grade of the asphalt concrete trail pavement will be within approximately 3 feet below the existing grade and 7 feet above the existing grade.

5. STEREOSCOPIC AERIAL PHOTOGRAPH REVIEW

Selected stereoscopic aerial photos from 1936 to 2000 with coverage of the site vicinity were reviewed. The Vista Point Trail was visible in a configuration similar to the current configuration in a photograph from 1936. Several automobiles were visible on the trail in the photograph and the current US Highway 101 was under construction. The Vista Point Overlook area, which currently sits atop the hill, was not yet developed. A pair of landslide scars is visible in the 1936 photograph below the eastern side of the trail. The more weathered and vegetated (and therefore older) landslide scar of the pair is present immediately below the trail, with its uppermost extent approximately coincident with the trail. The other (younger) landslide scar is steeper, less

402412001 R- Final Geo Eval 4 weathered, and is located further down slope with its lowermost extent approximately coincident with Moore Road along the San Francisco Bay. The uppermost section of Vista Point Trail continued onward to cross the future alignment of US Highway 101 while roadway grading operations were underway. The cluster of three tanks adjacent to the trail area also visible in the

1936 photograph.

Photographs from 1955 indicate that the Vista Point hill top was undergoing grading which involved a significant removal of material, on the order of 50 vertical feet, to flatten the hill top.

The limits of grading were approximately coincident with the present parking lot and sidewalk of the Vista Point Overlook area. No grading activity was visible on site in a photograph from 1958.

Several improvements at the Vista Point Overlook Area were visible in photographs from 1969 including a paved parking area, a perimeter sidewalk, and a small building on the northwestern edge of the lot.

Little observable change to the Vista Point Trail and adjacent improvements were visible in photographs from 1986, 1992, and 1995. Photographs from 2000 indicate that the area beneath the northern end of the Golden Gate Bridge, including the southern end of the project trail alignment, was in use as a construction staging area.

6. FIELD EXPLORATION AND LABORATORY TESTING

Our field exploration included a geologic reconnaissance and subsurface exploration of the project alignment. Our observations concerning the surficial geologic conditions from our site reconnaissance are presented on Figure 3. The subsurface exploration was conducted on November

24 and 25, 2014 and consisted of ten (10) exploratory borings. The locations of the borings are presented on Figure 2. The borings were drilled to depths between approximately 10 and 26 feet using a CME-55 crawler rig equipped with solid-stem augers.

A representative of Ninyo & Moore logged the subsurface conditions exposed in the borings and collected representative soil and rock samples at selected intervals for laboratory testing. The borings were backfilled with bentonite grout shortly after completion. Detailed logs of the borings are presented in Appendix A.

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The laboratory testing program for this study included tests to evaluate the in-place moisture content and dry density, percentage of soil particles finer than the No. 200 sieve, Atterberg Limits, expansion index, soil corrosivity, R-Value, and unconfined compressive strength. The results of the in-place moisture content and dry density tests are shown at the corresponding sample depths on the boring log in Appendix A. The results of the other laboratory tests performed are presented in Appendix B.

Due to health and safety concerns regarding known lead contamination along the trail alignment, Ninyo & Moore personnel collected two ambient air samples during the drilling activities. The purpose of the ambient air sampling was to monitor potential airborne lead exposure resulting from ground disturbance during drilling. Micro Analytical Laboratories, Inc., a California state-certified analytical laboratory, analyzed these air samples. Copies of the analytical reports and chain-of-custody documentation for the air samples are provided in Appendix C. The air sampling analytical data indicates that the ambient air samples (1124-01 = <1.9 g/m3 and 1125-02 = <3.2 g/m3) were below permissible exposure limits as prescribed by OSHA (Lead in Air Action Limit

= 30 g/m3, Permissible Exposure Limit for Lead in Air = 50 g/m3).

7. GEOLOGY AND SUBSURFACE CONDITIONS

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

7.1. Regional Geologic Setting

The site is located west of San Francisco Bay, north of the Golden Gate, in the Coast Ranges geomorphic province of California. The Coast Ranges are comprised of several 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. Occasional Tertiary volcanic events have also deposited extrusive igneous rocks over portions of the Coast Ranges. The San Francisco Bay

Area has several mountain ranges that trend northwest, parallel to major strike-slip faults such

402412001 R- Final Geo Eval 6 as the San Andreas, Hayward, Calaveras, and Green Valley. Major tectonic activity associated with these and other faults within this regional tectonic framework consists primarily of right-lateral, strike-slip movement. Further discussion of faulting relative to the site is provided in

Section 8.1.1 of this report. A map of regional fault locations is presented as Figure 4.

7.2. Site Geology

The project alignment is mapped as being underlain by older pre-Tertiary rocks of the

Franciscan group and younger Quaternary deposits (Schlocker, 1958; Schlocker et. al 1958).

The Franciscan group rocks mapped along the project alignment include Jurassic and

Cretaceous sandstone, radiolarian chert and shale, and greenstone. The Quaternary deposits mapped along the project alignment include slope debris and ravine fill. Additionally, younger and older Quaternary landslide deposits are mapped downhill of the trail alignment to the east

(Schlocker, 1958; Schlocker et. al 1958). A portion of the geologic map prepared by Schlocker

(1958) illustrating the geologic conditions in the vicinity of the site is presented in Figure 5.

Schlocker (1958) describes the Franciscan group sandstone as thick-bedded and massive greywacke sandstone interbedded with thin layers of shale and sandstone with some thick conglomerate lenses. Schlocker et. al (1958) reports that the thickest measured sections of sandstone in the San Francisco North Quadrangle are up to 1,300 feet thick but the sandstone generally occurs in beds about 10 feet (or more) thick separated by thin lenses of shale. The fresh sandstone is a hard, tough medium gray rock. Moderately weathered Franciscan sandstone is light brown and fairly brittle but tough. Highly weathered sandstone is pale orange, very friable, and crumbles readily under finger pressure. The average depth of weathering is less than 30 feet although in a few places the weathered zone extends down 60 or 70 feet.

Schlocker (1958) describes the Franciscan group radiolarian chert and shale as reddish brown with alternating beds of hard brittle chert, 1 to 5 inches thick, and firm brittle shale ⅛- to ½-inch thick. The unit can include bodies of massive chert. Schlocker et. al (1958) reports the habit of bedding is generally consistent, even in sections several hundred feet thick. Pinching and swelling of individual beds is common and small scale intricate contortions are typical.

402412001 R- Final Geo Eval 7

Generally, the chert is grayish red although green and orange chert can be found. Chert is generally resistant to weathering and mild hydrothermal alteration. This unit is found throughout the San Francisco North Quadrangle and forms prominent hills and ridges.

Schlocker (1958) describes the Franciscan group greenstone as greenish gray aphanitic to medium-grained altered volcanic rocks. The primary altered volcanic rock type is basalt but flows, agglomerate, tuff, and pillow lavas are also common. Less commonly this unit is massive or interbedded with radiolarian chert and sandstone. In most exposures the unit is highly altered, soft, crumbly, and reddish brown. Moderately altered greenstone is closely fractured so that it breaks into pieces about a quarter of an inch. Relatively unaltered greenstone is generally found only in deep excavations. The fresh rock is hard, tough, and dark gray or greenish gray. Highly weathered greenstones are almost entirely converted to clay minerals.

Schlocker (1958) describes the Quaternary slope debris and ravine fill as angular rock fragments in a sand, silt, and clay matrix that is generally light yellow to reddish brown.

Schlocker et. al (1958) reports that many slopes are mantled by unconsolidated debris derived from the weathering of the bedrock and these deposits are mapped where they are known to be more than 5 feet thick. Slope debris and ravine fill are generally very poorly sorted and consist of a mixture of angular rock fragments in a matrix of sand, silt, and clay. Where the hills are underlain by radiolarian chert or pillow greenstone, the debris contains a relatively high percentage of coarse gravel and cobble-size fragments.

Schlocker (1958) describes the Quaternary landslide deposits as generally unstratified mixtures of bedrock, sand, silt, and clay in varying proportions. The deposits are derived from landslides in unconsolidated surficial deposits and frequently in weathered or sheared bedrock. Landslide deposits can be categorized as older or younger Quaternary landslide deposits based on recency of movement.

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7.3. Subsurface Conditions

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

7.3.1. Fill

Fill was generally encountered in the borings from the ground surface to depths ranging from about 1 foot (Boring B-6) to 11½ feet (the depth explored in Boring B-2). The fill material consisted of reddish brown to brown, dry to moist, stiff clay and medium dense to very dense clayey sand, silty sand, and clayey gravel. Fill was not encountered in

Borings B-5 or B-7.

7.3.2. Colluvium

Colluvium was encountered in Borings B-5 and B-7 from the ground surface to the depth explored (approximately 11½ feet) and in Boring B-3 from below the fill to a depth of approximately 15 feet. The colluvium generally consisted of reddish brown, moist, medium dense to very dense clayey gravel, clayey sand, and silty, clayey sand.

7.3.3. Residual Soil

Residual soil was encountered in Boring B-9 below the fill from a depth of approximately

4½ feet to a depth of approximately 7½ feet below the existing ground surface. The residual soil generally consisted of reddish brown to dark gray, moist, dense, angular gravel, composed of chert, with sand.

7.3.4. Franciscan Formation Greenstone

Franciscan Formation Greenstone was encountered was encountered in Boring B-6 from below the fill to the depth explored. The greenstone generally consisted of reddish brown to black, moist, weathered to decomposed metavolcanic rock.

7.3.5. Franciscan Formation Sandstone

Franciscan Formation Sandstone was encountered was encountered in Boring B-10 from below the fill to a depth of approximately 10 feet. The sandstone generally consisted of

402412001 R- Final Geo Eval 9 light yellowish brown, dry, weakly cemented sandstone. An approximately 8-inch thick layer of thinly bedded chert and shale was encountered within the Franciscan Formation

Sandstone in Boring B-10.

7.3.6. Franciscan Formation Radiolarian Chert and Shale

Franciscan Formation Radiolarian Chert and Shale was encountered in several of our borings from below the fill in Borings B-4 and B-8; from below the colluvium in Boring

B-3, from below the residual soil in Boring B-9, and from below the Franciscan

Formation Sandstone in Boring B-10. The radiolarian chert and shale generally consisted of gray, reddish brown, and yellowish brown, moist, alternating thinly-bedded chert and shale lamina which varied in hardness from very soft to hard. The chert beds were generally moderately to strongly cemented and the shale lamina were generally weakly to moderately cemented.

7.4. Groundwater

Groundwater was not encountered within our borings. Fluctuations in the groundwater level and perched groundwater conditions may occur because of seasonal variations in precipitation or variations in ground surface topography, subsurface geologic conditions and structure, irrigation, groundwater pumping and other factors not evident at the time of our evaluation.

8. DISCUSSION

This study considered a number of design issues relevant to the proposed construction, including seismic hazards, slope stability, settlement of compressible soil layers from static loading, excavation characteristics, and corrosive and expansive soils. These issues are discussed in the following subsections.

8.1. Seismic Hazards

The project site is located in an area considered to be seismically active. The seismic hazards considered in this study include the potential for ground rupture, ground shaking due to seis-mic activity, seismically induced liquefaction, and dynamic settlement. These potential hazards are discussed in the following subsections.

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8.1.1. Faulting and Ground Surface Rupture

The numerous faults in northern California include active, potentially active, and inactive faults. As defined by the California Geological Survey, active faults are faults that have ruptured within Holocene time, or within approximately the last 11,000 years. Potentially active faults are those that show evidence of movement during Quaternary time

(approximately the last 1.6 million years) but for which evidence of Holocene movement has not been established. Inactive faults do not show evidence of movement within

Quaternary time.

The site is located within an area which does not contain fault evaluation reports for

Alquist-Priolo Fault-Rupture Hazard Zones established by the state geologist to delineate regions of potential ground surface rupture adjacent to active faults. However, the site is located in a seismically active area, as is the majority of northern California, and the potential for strong ground motion in the project area is considered significant during the design life of the proposed structure. The closest fault considered to be active with published evidence of Holocene displacement is the San Andreas Fault Zone. The nearest segment of the San Andreas Fault to the project site is concealed beneath the Pacific

Ocean approximately 6.5 miles away from the site to the west. The offshore of the San

Andreas Fault to the west of the site is currently thought to be the epicenter of the 1906

Great San Francisco Earthquake. The sense of displacement along the San Andreas Fault

Zone is thought to be right lateral with a component of vertical displacement (Stoffer, 2005). The approximate locations of major faults in the region and their geographic relationship to the project vicinity are shown on Figure 4.

Based on our review of the referenced geologic maps, no active faults are known to cross the project site. Therefore, the probability of damage from surface fault rupture is considered to be low. However, lurching or cracking of the ground surface as a result of nearby seismic events is possible.

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8.1.2. Strong Ground Motion

The United States Geological Survey (USGS, 2013) seismic design tool was used to calculate the horizontal peak ground acceleration (PGA) incorporating site class effects with a 7 percent probability of exceedance in 75 years in accordance with the 2009

American Association of State Highway and Transportation Officials (AASHTO) Guide

Specifications for (Load and Resistance Factor Design (LRFD) Seismic Bridge Design.

The PGA with a 7 percent probability of exceedance in 75 years, adjusted for site class C, was calculated to be 0.664g based on a mapped PGA of 0.664g from the AASHTO Guide

Specifications.

8.1.3. Liquefaction

The strong vibratory motions generated by earthquakes can trigger a rapid loss of shear strength in saturated, loose, granular soils of low plasticity (liquefaction) or in wet, sensitive, cohesive soils (strain softening). Liquefaction and strain softening 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 subsidence at the ground surface.

The materials encountered during our subsurface exploration at the location for the proposed trail improvements generally consisted of stiff clay and medium dense to very dense clayey sand, silty sand, and clayey gravel over Franciscan group rocks.

Groundwater was not encountered during our investigation. These materials are generally not susceptible to liquefaction and are not known to be particularly sensitive. In addition, a regional study of liquefaction susceptibility (Witter et al, 2006) indicates that the site is in an area where the liquefaction susceptibility is considered very low. Consequently we do not regard liquefaction, strain softening behavior, or related hazards to be design considerations for the proposed project.

8.1.4. Dynamic Settlement

The strong vibratory motion associated with earthquakes can induce settlements in relatively loose to medium dense granular soils. Dynamic settlement is not limited to the

402412001 R- Final Geo Eval 12 near-surface environment and may occur in both dry and saturated sand and silt. Cohesive soils are not typically susceptible to dynamic settlement.

During our subsurface exploration, we generally encountered deposits of stiff clay and medium dense to very dense clayey sand, silty sand, and clayey gravel over Franciscan group rocks. These materials are not particularly susceptible to dynamic settlement.

Consequently, we anticipate that the dynamic settlement at the site following a significant earthquake will be negligible with respect to the proposed improvements.

8.2. Landsliding and Slope Stability

The Vista Point Trail is located on a steeply sloping hillside in a seismically active area.

Quaternary younger and older landslide deposits are mapped downhill to the southeast of the project alignment (Schlocker, 1958; Schlocker et. al 1958). The published locations of these landslides are presented on Figure 5. These landslides were visible during our review of stereoscopic aerial photographs. As noted previously, the uphill extent of the older landslide is coincident with a portion of the trail alignment. Our interpretation of the approximate location of the upper portion of the older landslide location is shown on Figure 3. Further evaluation of these landslides or the overall stability of the slope is beyond the scope of this study.

Based on our review of the draft design plans (National Park Service, 2014), grading for the currently proposed trail improvements includes the addition of fills to raise the grade up to approximately 6 feet utilizing retaining walls to support the new fill. Given the existing landslides on the slope below the trail, the impact of the new fills on the stability of the slope below the trail is a design consideration. To reduce the impact of the proposed grading on the stability of the slope below the trail, the new fill can be supported by soldier-pile-and-lagging walls. Alternatively, the new fill can be supported by concrete cantilever gravity walls provided that lightweight fill is utilized, in conjunction with overexcavation, so that the net surcharge from the proposed grade increase is negligible. Recommendations for the soldier-pile-and-lagging walls and concrete cantilever gravity walls with lightweight fill are provided.

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Additionally, the proposed trail improvements as shown on the draft design plans include cutting into two existing cut slopes on the hillside above the trail. The cut proposed between

(approximately) station 17+50 and station 21+00 will shift the toe of the existing cut up to

6 feet inward. The geologic unit observed over this interval in the existing cut included greenstone consisting of weathered to decomposed metavolcanic rock. Over this interval, the cut slope is between 10 and 15 feet tall and is generally inclined between 50 degrees and

75 degrees above horizontal at present. At a few locations over this interval the cut is near vertical. The slope above the cut continues toward the Vista Point Overlook area at an inclination of approximately 27 degrees (2:1 horizontal to vertical). Based on our review of stereoscopic aerial photographs review, the side slopes along the trail have maintained an inclination consistent with the present condition since 1936. However, during our field investigation we observed fragments of weathered greenstone up to 8 inches in diameter on the surface of the existing trail indicating that the cut slope is gradually raveling. Additional raveling of the cut slope should be anticipated, particularly under seismic conditions.

Recommendations for slope inclinations to reduce the potential for future raveling or sloughing of the proposed cut slopes are provided in Section 9.1.4. Alternatively retaining walls consistent with the recommendations in Section 9.2 or shotcrete facades with rock bolts or soil nails may be constructed to support the cut slope.

The other proposed cut, between (approximately) station 23+50 and station 25+20, will shift the toe of the existing cut up to 7 feet inward. The geologic units observed over this interval in the existing cut included a section of weakly to strongly cemented radiolarian chert and shale which varied in hardness from very soft to hard and a section of colluvium consisting of medium dense to very dense clayey gravel, clayey sand, and silty clayey sand. Over this interval, the cut slope is between 5 and 15 feet tall and is generally inclined, at present, between 40 degrees and vertical. The slope above the proposed cut continues toward the Vista

Point Overlook area at an inclination of approximately 27 degrees (2:1 horizontal to vertical).

The bedding orientation of the radiolarian chert and shale along this proposed cut ranges between N20°W and N40°W dipping 55 degrees to 70 degrees. These bedding orientations are favorable for the stability of the proposed south-facing cut. However, small scale intricate

402412001 R- Final Geo Eval 14 contortions of the radiolarian chert and shale bedding orientation are typical (Schlocker et. al, 1958) so excavations into this material should be re-evaluated during construction to check for favorable bedding. Along the portion of the proposed cut in colluvium, the inclination approaches approximately 40 degrees. While on site we observed several minor surficial sloughs in the colluvium exposed on the cut slope. Additional sloughing of the slope should be anticipated, particularly under seismic conditions. Recommendations for slope inclinations to reduce the potential for future raveling or sloughing of the proposed cut slopes are provided in Section 9.1.4. Alternatively, retaining walls consistent with the recommendations in

Section 9.2 or shotcrete facades with rock bolts or soil nails may be constructed to support the cut slope.

8.3. Static Settlement

During our subsurface exploration, we generally encountered deposits of stiff clay and medium dense to very dense clayey sand, silty sand, and clayey gravel over Franciscan group rocks. The proposed retaining walls and grade changes are relatively low. We anticipate that static settlement due to the proposed improvements will be tolerable provided that the foundations are designed in accordance with our recommendations.

8.4. Excavation Characteristics

We understand that the proposed project may involve excavations of up to a few feet in depth to reduce the grade of the trail or to construct footings for retaining walls. Deeper pier excavations may be needed for soldier-pile-and-lagging walls. In addition, excavations to shift the alignment of the trail up to 7 feet laterally into the hillside are proposed.

The units encountered during our subsurface exploration in the vicinity of these proposed excavations included fill material consisting of stiff clay and medium to very dense sand and gravel; residual soil consisting of dense sandy gravel; colluvium consisting of medium to very dense clayey sand and gravel; greenstone consisting of weathered to decomposed metavolcanic rock; weakly cemented sandstone; and weakly to strongly cemented radiolarian chert and shale which varied in hardness from very soft to hard.

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We anticipate that heavy earthmoving equipment may encounter difficulty when excavating and cutting the greenstone, sandstone, and radiolarian chert and shale material. Rock-breaking equipment attached to backhoes or excavators may be needed. Seismic refraction surveys can be performed to evaluate the rippability of on-site materials. Blocks of resistant rock may be encountered during pier drilling presenting hard drilling conditions. Core barrels, drop chisels, or boulder rooting bits may be needed for pier drilling in addition to conventional earth/rock augers and buckets.

Excavations may encounter seeping seasonal perched groundwater, or relatively cohesionless soil. Sloughing or collapse of steep cuts or drilled holes may occur. Recommendations for stabilization of temporary excavations and drilled holes are presented in Sections 9.1.3, and

9.2.2, respectively.

8.5. Corrosive/Deleterious Soils

An evaluation of the corrosivity of the on-site materials was conducted to assess the impact to concrete and metals. The corrosion impact was evaluated using the results of limited laboratory testing on samples obtained during our subsurface study. Laboratory testing to quantify pH, resistivity, chloride, and soluble sulfate contents was performed on a sample of the fill. The results of the corrosivity tests are presented in Appendix B. The criteria used to evaluate the deleterious nature of soil on concrete are listed in Table 1. Based on our test results and these criteria, the sulfate exposure to concrete is negligible. The California

Department of Transportation (Caltrans) defines a corrosive environment as an area within

1,000 feet of brackish water or where the soil contains more than 500 parts per million (ppm) of chlorides, sulfates of 0.2 (2,000 ppm) percent or more, or pH of 5.5 or less (Caltrans, 2012).

Caltrans considers an area within 1,000 feet of brackish water as a marine environment.

Portions of the site are located within approximately 230 feet of the San Francisco Bay. Our laboratory testing indicates that the site does not meet the definition of a corrosive environment but the site would be considered a marine environment based on referenced criteria. Corrosion of ferrous metals, particularly exposed ferrous metals, should be anticipated. Recommended corrosion mitigation measures are presented in Section 9.4.

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Table 1 – Criteria for Deleterious Soils on Concrete

Sulfate Content Percent by Weight

Sulfate Exposure

0.0 to 0.1 Negligible

0.1 to 0.2 Moderate

0.2 to 2.0 Severe

> 2.0 Very Severe

Reference: American Concrete Institute (ACI) Committee 318 Table 4.3.1 (ACI, 2012)

8.6. Expansive Soils

Some clay minerals undergo volume changes upon wetting or drying. Unsaturated soil 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 performed on a sample of the near-surface soil to evaluate the expansion index.

The test was performed in general accordance with the American Society of Testing and

Materials (ASTM) Standard D 4829 (Expansion Index). The results of our laboratory test indicate that the expansion index of the sample tested was 25. This result is indicative of a low expansion characteristic. Based upon these results, special mitigation measures for expansive soil are not needed.

9. CONCLUSIONS

Based on our review of the referenced background data, our geologic field reconnaissance, subsurface evaluation, and laboratory testing, it is our opinion that the proposed improvements to the Vista Point Multi-Use Trail are feasible from a geotechnical standpoint, provided that the recommendations presented in this report are incorporated into the design and construction of the subject project. Geotechnical considerations for the project include the following:

Our subsurface evaluation indicated that the project site is underlain by fill, residual soil, colluvium, and Franciscan group rocks including sandstone, greenstone, radiolarian chert and shale.

The site could experience a relatively large degree of ground shaking due to a significant earthquake event on the nearby San Andreas Fault.

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Mapped landslides are present on the slope below the trail. The proposed improvements include placing additional fill to raise the grade for re-alignment of the trail. The impact of the new fill on the stability of the slope below the trail is a design consideration. To reduce the impact of the proposed grading on the stability of the slope below the trail, the new fill can be supported by soldier-pile-and-lagging walls. Alternatively, the new fill can be supported by concrete cantilever gravity walls provided that lightweight fill is utilized, in conjunction with overexcavation, so that the net surcharge from the proposed grade increase is negligible.

Recommendations for the soldier-pile-and-lagging walls and concrete cantilever gravity walls with lightweight fill are provided.

The proposed trail improvements include cutting into two existing cut slopes to shift the toe of the cut slope up to approximately 7 feet inward. During our geologic reconnaissance we observed minor surficial slouging in the colluvium exposed in the existing cut and cobbles of raveled rock on the trail. Recommendations for slope inclinations to reduce the potential for future ravelling or sloughing of the proposed cut slopes are provided. Alternatively, retaining walls consistent with the recommendations in this report or shotcrete facades with rock bolts or soil nails may be constructed to support the cut slope.

The Franciscan group rock encountered during our subsurface exploration and geologic reconnaissance of existing cuts included weakly cemented sandstone, weathered to decomposed greenstone, and weakly to strongly cemented radiolarian chert and shale that varied in hardness from very soft to hard. We anticipate that heavy earthmoving equipment may encounter difficulty when excavating these materials and hard drilling conditions may be encountered for pier excavations. Rock-breaking equipment, core barrels, drop chisels, or boulder rooting bits may be needed.

Excavations may encounter seeping seasonal perched groundwater, or relatively cohesionless soil. Sloughing or collapse of steep cuts or drilled holes may occur. Recommendations for stabilization of temporary excavations and drilled holes are presented.

The site does not meet the definition of a corrosive environment (Caltrans, 2012) but is within a marine environment. Recommended measures to mitigate corrosion of reinforcing steel are presented.

RECOMMENDATIONS

The following guidelines should be used in the preparation of the construction plans. The project plans and specifications should be reviewed by Ninyo & Moore prior to construction bidding to check for consistency with these recommendations.

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9.1. 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.

9.1.1. Site Preparation

Prior to performing earthwork operations, the site should be cleared of vegetation, surface soils containing roots or other organic matter, surface obstructions (e.g., pavements, aggregate base, etc.), rubble and debris, abandoned utilities, and other deleterious materials. Existing utilities within the project limits, if any, should be re-routed or protected from damage by construction activities. Trees or other obstructions that extend below finish grade, if any, should be removed and the resulting holes filled with compacted soils. Materials generated from the clearing operations should be removed from the project site and disposed of at a legal dumpsite. Soils containing roots or other organic matter may be stockpiled for later use as landscaping fill, as authorized by the owner’s representative.

9.1.2. Observation and Removals

Prior to placement of fill or the erection of forms, the client should request an evaluation of the exposed subgrade by the geotechnical consultant. Materials that are considered unsuitable shall be excavated under the observation of the geotechnical consultant in accordance with the recommendations in this section, or the field recommendations of the geotechnical engineer.

Unsuitable materials include, but may not be limited to dry, loose, soft, wet, expansive, organic, or compressible natural soils; fractured, weathered, or soft bedrock; and undocumented or otherwise deleterious fill materials. Unsuitable materials should be removed in accordance with the field recommendations of the geotechnical engineer.

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9.1.3. Excavation Stabilization and Temporary Slopes

Excavations should be stabilized in accordance with the Excavation Rules and

Regulations (29 Code of Federal Regulations, Part 1926) stipulated by the Occupational

Safety and Health Administration. Stabilization should consist of shoring sidewalls or laying slopes back. Table 2 lists the OSHA material type classifications and corresponding allowable temporary slope layback inclinations for soil deposits that may be encountered onsite. 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. Shoring system criteria for excavations up to

20 feet in depth are listed in the OSHA Excavation Rules and Regulations (29 CFR Part

1926). The lateral earth pressures listed in Table 2 may be used to design or select the shoring system. The recommendations listed in this table 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.

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Table 2 – Recommended OSHA Material Classifications and Allowable Slopes

Formation

OSHA

Classification Allowable

Temporary Slope1,2,3 Lateral Earth Pressure on Shoring4, (psf)

Franciscan Rock Vertical (90) N/A

Colluvium Type A ¾h:1v (53) 25D + 72

Fill (clay, clayey sand) Type B 1h:1v (45) 45D + 72

Fill (silty sand) or Residual Soil

Type C 1½h:1v (34) 80D + 72

1 Excavation sidewalls in cohesive soils 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 soils, no layer shall 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.

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.

We understand that the proposed excavations will not be in close proximity to existing structures. Excavations made in close proximity to existing structures may undermine the foundation of those structures and/or cause soil movement related distress to the existing structures. Stabilization techniques for excavations in close proximity to existing structures will need to account for the additional loads imposed on the shoring system and appropriate setback distances for temporary slopes. The geotechnical engineer should be consulted for additional recommendations if the proposed excavations cross below a plane extending down and away from the foundation bearing surfaces of the adjacent structure at an angle of 1:1 (horizontal to vertical).

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9.1.4. Constructed Slopes

To reduce the potential for future ravelling or sloughing, the proposed cut slopes may be constructed at the gradients listed in Table 3. Ninyo & Moore should observe the excavations for cut slopes during construction to check that favorable bedding conditions are present. Swales should be constructed to divert runoff away from slopes. Cut slopes in soil should be vegetated with drought-tolerant, deep-rooted plants to reduce the potential for erosion. Irrigation pipes should be anchored to the slope face rather than placed in trenches. Slope irrigation should be maintained at a level just sufficient to support plant growth. Leaking pipes should be promptly repaired.

Table 3 – Recommended Gradients for Constructed Slopes

Location (Reference by Station)

Slope Type Formation Gradient

17+50 to 21+00 Cut Greenstone 2h:1v (27)

23+50 to 25+20 Cut Chert & Shale ¾h:1v (53)

23+50 to 25+20 Cut Colluvium 2h:1v (27)

9.1.5. Construction Dewatering

Groundwater was not encountered during our subsurface exploration. However, significant fluctuations in the groundwater level may occur as a result of variations in seasonal precipitation and other factors. The depth to groundwater may be different at locations other than those explored due to perched groundwater conditions, an inclined groundwater table, or variations in topography. Water intrusion into the excavations may occur as a result of groundwater seepage or surface runoff. The contractor should be prepared to take appropriate dewatering measures in the event that water enters into the excavations. Considerations for construction dewatering should include anticipated drawdown, volume of pumping, potential for settlement, and groundwater discharge.

Disposal of groundwater should be performed in accordance with the guidelines of the

Regional Water Quality Control Board.

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9.1.6. Material Requirements

Materials used during earthwork, grading, and paving operations should comply with the guidelines listed in Table 4.

Table 4 – Recommended Material Requirements

Material and Use Source Requirements1,2

Select Fill for retaining wall backfill5

Import

Sand Equivalent 20 or more 100 percent passing 3” sieve

35 to 100 percent passing No.