Foundation Assessment Report

PRELIMINARY FOUNDATION REPORT

MAIN POST TUNNELS

DOYLE DRIVE REPLACEMENT PROJECT

April 15, 2008

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TABLE OF CONTENTS

Page

1.0 INTRODUCTION............................................................................................................... 1

2.0 SITE GEOLOGY AND SUBSURFACE CONDITIONS...................................................... 1 2.1 GENERAL SITE GEOLOGY ................................................................................. 1 2.2 SITE SPECIFIC SUBSURFACE CONDITIONS.................................................... 1 2.3 GROUNDWATER ................................................................................................. 2

3.0 SCOUR ............................................................................................................................. 3

4.0 CORROSIVITY ................................................................................................................. 3

5.0 SEISMICITY/LIQUEFACTION .......................................................................................... 3

6.0 FOUNDATION TYPE RECOMMENDATIONS.................................................................. 4

7.0 EARTH PRESSURES....................................................................................................... 4 7.1 VERTICAL EARTH PRESSURES......................................................................... 4 7.2 LATERAL EARTH PRESSURES .......................................................................... 4

8.0 CONSTRUCTION CONCERNS........................................................................................ 5

9.0 ADDITIONAL FIELD WORK AND LABORATORY TESTING........................................... 5

LIST OF FIGURES Figure 1 Locations of Geotechnical Explorations Main Post Tunnels Figure 2 Subsurface Stratigraphy Along North Bound Main Post Tunnel

LIST OF APPENDICES Appendix A Shearwave Velocity Results for Boreholes MPTSB-R3 and MPTNB-R6 Appendix B Caltrans Memorandum, dated February 5, 2008, Doyle Drive Replacement

Retaining Structures Preliminary Foundation Report

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1.0 INTRODUCTION

This preliminary foundation report provides preliminary foundation recommendations for the Main Post Tunnel structures, cut-and-cover tunnels located in San Francisco, California. The proposed structure is part of the Doyle Drive Replacement project which will replace the freeway which stretches from the west end of Lombard Street, through the Presidio of San Francisco, ending at the Golden Gate Bridge toll plaza. The cut-and-cover tunnel will be landscaped above to restore some of the natural beauty of the Presidio, which is now part of the U.S. National Park System.

2.0 SITE GEOLOGY AND SUBSURFACE CONDITIONS

2.1 GENERAL SITE GEOLOGY

The bedrock at the site of the Main Post Tunnels is generally the Franciscan Formation. This formation consists of a variety of heavily folded and sheared assemblages of Greywacke sandstone, siltstone, chert, serpentinite, and Mlange Matrix (a plastic bedrock unit which is similar to a gravelly clay). The hardness of these bedrock units typically range from very soft to hard and may vary in short distances.

Overburden soils consist of mostly marine deposited sediments. Common sediment sequences found elsewhere along the San Francisco Bay fringes are found at this site. The typical sediments begin with Bay Mud at the surface underlain with marine deposits followed by Colma Formation, which is a dense sand. Beneath the Colma Formation is typically Old Bay Clay, a stiff overconsolidated clay. Another deeper sequence of sand and clay are sometimes encountered.

The footprint of the project extending from the Battery Bluff to Lombard Street was originally tidal marsh. This land was reclaimed in 1912 using hydraulic filling method. Up to 12 feet of fill consisting of sandy clays and silts and clayey sands can be found in this portion of the site.

2.2 SITE SPECIFIC SUBSURFACE CONDITIONS

A site map of the Main Post Tunnels area is presented in Figure 1. This map shows the location of the borings, as well as the proposed roadway and tunnel structure. Figure 2 presents a generalized subsurface cross section, showing the soils encountered underneath the northbound tunnel.

The general subsurface condition at the site of the Main Post Tunnels consists of a very deep valley in the bedrock surface that has been filled by sediments. Little is know about the actual contours of the bedrock in this segment of the project. Currently, only two boreholes extended


deep enough to encounter bedrock. In boring MPTNB-R5, bedrock was encountered at a depth of 175 feet below existing ground surface. In boring MPTNB-R6, bedrock was encountered at a depth of 232 feet below existing ground surface.

The surficial geologic unit in this area of the project is fill which extends from 0 to 12 feet. The fill in this area was placed hydraulically to reclaim tidal marsh land in 1912. The fill is primarily very loose sand with some silt and clay. The upper two to three feet has considerably more fines and gravel.

Below the artificial fill is two to four feet of Bay Mud, a common young clay deposit found along the coast of the San Francisco Bay. The Bay Mud is soft with undrained shear strength of approximately 400 psf. Typically, it is high plasticity and relatively free of sand, however because of the thin layer encountered at the site, most of the Bay Mud is sandy and some appears to have liquid limits which may define the sediments as lean clay. Plant material is typically found at the top of this unit confirming that the site was previously a marsh land.

Below the Bay Mud is typically a three to six feet layer of a marine deposited sand followed by a marine deposited clay and then another marine deposited sand. The marine sand layers typically contain from 5 to 35 percent fines and are in a medium dense condition. The marine clay is very similar to the Bay Mud with low undrained shear strengths.

Below the marine deposits lies the Colma Formation unit. This unit is typically 50 to 60 feet thick. The upper portion of the unit is referred to as Colma Sand, which is dense to very dense with typically a trace of fines. There are pockets in the unit which may contain from 15 to 45 percent fines, however, these pockets account for less than one-fifth of the unit. The bottom 10 feet of this unit typically becomes clayey as the unit transitions into the Old Bay Clay.

Old Bay Clay is found underneath the Colma Formation. This unit is a low to high plasticity very stiff clay with infrequent sandy lenses. The thickness of this unit is not well defined because of depth of the boreholes within the Main Post Tunnels area. In the boreholes where it was encountered, its thickness ranges from at least 30 feet at the west end of the tunnel to 89 feet encountered in boring MPTNB-R6. At boring MPTNB-R6, the Old Bay Clay deposit is separated by a 45-foot-thick layer of marine sand.

2.3 GROUNDWATER

There are a several groundwater zones found at the site, which are separated by aquatards. Perched groundwater is found in the fill layer about three to five feet below the ground surface. This water is most likely connected to the bay and fluctuates with the tides. Below the Bay Mud in the sandy marine deposit there is an upper immediate groundwater zone where water levels appear to be at the same elevation as the groundwater in the fill above the Bay Mud.


In the Colma Sand, below the marine clays, a lower immediate groundwater zone is present. The piezometric level of this groundwater zone is under artesian conditions with the water level ranging from 1.9 to 3.8 feet below the existing site grade.

Finally, in the deeper marine sands encountered below the first and the second Old Bay Clay units, a lower groundwater zone appears to be pressurized (i.e., artesian condition) with an equivalent static head of about 11.5 feet above the existing ground surface.

3.0 SCOUR

Scour will not be a concern on this portion of the project. However, surface runoff will cause erosion if engineered fill and native soils are not covered with vegetation.

4.0 CORROSIVITY

This site is anticipated to be corrosive due to its proximity to the San Francisco Bay and the past existence of a tidal marsh. Corrosion samples have been collected from the site during the field investigation and will be tested for their corrosivity properties. Sampling and testing of the site soils and groundwater shall be in conformance with the Corrosion Guidelines for Foundation Investigations (Caltrans, 2003).

5.0 SEISMICITY/LIQUEFACTION

Structural design criteria has been proposed in the report titled Structural Design Criteria: Cut-and-Cover Tunnels and Nonstandard Retaining Walls, Version 2, dated January 23, 2008. This document summarizes the seismic demand placed on the structure. In addition, Geomatrix in conjunction with Dr. Norm Abrahamson will be retained to perform a probabilistic seismic hazard analysis, select time histories, and develop site specific rock motion spectra for the Main Post Tunnels.

Liquefaction is anticipated at the Main Post Tunnels site. The hydraulic fill will most likely liquefy in even a moderate seismic event similar to that of the recent Loma Prieta earthquake. During this earthquake, damage caused by liquefaction was observed extensively in the neighboring Marina district, which was hydraulically filled in the same reclamation project. In addition to the fill, the upper marine sands, which are interbedded with marine clays, have a high potential to liquefy in the event of a large magnitude earthquake. In the event of liquefaction at the site, there is a potential for lateral spreading caused by soil movement towards the bay.

Suspension P-S velocity logging was performed in boreholes MPTSB-R3 and MPTNB-R6. The velocity results for these two boreholes are presented in Appendix A of this preliminary foundation report.


6.0 FOUNDATION TYPE RECOMMENDATIONS

The Main Post Tunnel structures should be founded on Cost-in-Drilled-Hole (CIDH) piles founded in the Colma Sand soil unit. This thick deposit of dense to very dense sand is ideal for supporting the CIDH piles. The soils above the Colma Sand should not be considered to provide any support to the piles due to the liquefaction potential of the sand layers and the clay layers may in fact cause downdrag.

Regarding other pile types, driven piles may not be a viable option due to vibration and noise concerns. If the driven piling option would be selected, a vibration and noise study and monitoring will be required by the Presidio Trust. Also, small diameter displacement piles may not be appropriate when considering the lateral load placed on these piles caused by lateral spreading.

The most appropriate pile type is anticipated to be a fully cased CIDH large diameter piles (about 5 to 8 feet in diameter). Steel reinforcement cages should be cast in place and rebar should extend above the pile to tie-in with spanned bents of the tunnels. Based on previous experience, it is estimated that an allowable skin friction of 1,400 psf can be achieved in the dense Colma Sand.

7.0 EARTH PRESSURES

7.1 VERTICAL EARTH PRESSURES

Landscaping over the top of the tunnel structure will exert a distributed vertical load on the roof of the structure. This dead load pressure from landscaping fill will be equal to 140 pcf times the depth of the fill. For example, if there is 5 feet of fill material placed over the structure, the applied distributed load from this material will be 700 psf.

There will be other vertical loads imposed on the structure from other sources such as traffic; however, these loads are not considered in the scope of this preliminary foundation report.

7.2 LATERAL EARTH PRESSURES

Active lateral earth pressures will develop on the retaining walls at the ends of the tunnels. For preliminary estimation of the active earth pressure at a particular depth, z in feet, the following equation, pa (lbs/ft2) = 45 * z, should be used for Retaining Walls 9 and 10, located at the east end of the Main Post Tunnels. For Retaining Wall 8 (Lincoln Retaining Wall Structure 34-016M), located at the west end of the south bound tunnel, Caltrans has prepared a memorandum: Preliminary Foundation Report, dated February 5, 2008. The Caltrans


memorandum presents preliminary soil design parameters for lateral earth pressures. A copy of the Caltrans memorandum is attached under Appendix B.

At-rest lateral earth pressures will develop on the side walls of the tunnels with the placement of the backfill. For preliminary estimation of the at-rest earth pressure at a particular depth, z in feet, the following equation can be used. pat-rest (lbs/ft2) = 65 * z.

It is assumed that drainage behind retaining walls and the tunnel sidewalls will be provided to relief hydrostatic pressure. Surcharges from traffic loading should be added where appropriate.

Seismic lateral earth pressures will be developed as part of a detailed seismic analysis and are not included in this preliminary report.

The equations to determine preliminary pressures are given for an individual point. Over the side of the tunnel walls, the earth pressures will be a trapezoidal distributed force. Resultants may be easily determined using standard methods.

8.0 CONSTRUCTION CONCERNS

Difficult CIDH pile installation should be anticipated. The CIDH piles should be installed with temporary full casing to ensure a quality pile. A drilling polymer may be required while installing the piles in order to counter balance the artesian groundwater conditions.

Although not historical, there is a building at about Station 62+00 adjacent to the proposed south bound Main Post Tunnel that may require underpinning due to slope excavation required for the new tunnel. The bluff excavation most likely will require some type of top bottom shoring system.

Soft and unstable subgrade conditions will likely be encountered and may require stabilization to operate construction equipment.

The relatively high perched groundwater and artesian conditions in the Colma Sand will require construction dewatering.

9.0 ADDITIONAL FIELD WORK AND LABORATORY TESTING

Two additional exploratory boreholes located between MPTSB-R1 and MPTSB-R2, on top of the bluff are recommended. This bluff will require a temporary shoring wall to install the tunnel structures and the adjacent building may require underpinning.


The laboratory testing program for the 2007/2008 exploration work has not been executed as of the issuing of this preliminary foundation report. The program will consist of laboratory tests that will determine the geotechnical engineering properties of the major soil units.

The recommendations contained in this preliminary foundation report are not final. Any questions regarding the above preliminary recommendations should be directed to the attention of Francis R. Greguras at 415-946-0255.

WEST EAST

DOYLE DRIVE PREFERRED ALTERNATIVE

Subsurface Stratigraphy, Section A-A'Along North Bound Main Post Tunnel

Figure 2

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400Horizontal Distance, ft

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MPTN

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MPTN

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(25' O

ffset,

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MPTN

B-R3

(50' O

ffset,

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th)

MPTN

B-R4

MPTN

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(60' O

ffset,

South

)MP

TNB-

R5

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DNB-

R1

FILLBAY MUD

MARINE DEPOSITS

COLMA SAND

TRANSITION SANDS

OLD BAY CLAY

DEEPSANDS

COLLUVIUMBEDROCK

OLDBAY

CLAY

Proposed Gradeof Roadway

Proposed Gradeof Tunnel Invert

A A'

APPENDIX A

Shearwave Velocity Results Boreholes MPTSB-R3 and MPTNB-R6

SAN FRANCISCO DOYLE DRIVE BORING MPTSB-R3

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VELOCITY (FEET/SECOND)

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VELOCITY (METERS/SECOND)

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R1-R2 Vp

SAN FRANCISCO DOYLE DRIVE BORING MPTNB-R6

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VELOCITY (METERS/SECOND)

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R1-R2 Vs

R1-R2 Vp

APPENDIX B

Copy of Caltrans Memorandum Dated February 5, 2008

Doyle Drive Replacement Retaining Structures Preliminary Foundation Report

Foundation Assessment Report

Documents

Transcript of Foundation Assessment Report