Appendix_Geo_Tech_Report.pdf

Environmental and Geotechnical Consultants

GEOTECHNICAL AND GEOLOGIC SERVICES

REPAIR OF CONCRETE WALL

ALONG MCDOWELL ROAD

San Francisco, California

Kennedy/Jenks Consultants

21 October 2008 Project No. 4873.01

ENVIRONMENTAL AND GEOTECHNICAL CONSULTANTS

4030 MOORPARK AVE. SUITE 210 SAN JOSE CALIFORNIA 95117 T 408 551 6700 F 408 551 0344 www.treadwellrollo.com

21 October 2008 Project 4873.01

Mr. Donald Barraza Kennedy/Jenks Consultants 622 Folsom Street San Francisco, California 94107

Subject: Geotechnical and Geologic Services

Repair of Concrete Wall along McDowell Road San Francisco, California

Dear Mr. Barraza:

This letter transmits our geotechnical and geologic report for the repair of a portion of the concrete retaining wall along the downhill side of McDowell Road at Fort Mason in San Francisco, California. Our services were performed as part of the project titled “Repair of Concrete Wall along McDowell Road”, in accordance with our proposal dated 5 June 2008.

McDowell Road was constructed on a hill slope at the north end of Fort Mason, between the Golden Gate National Recreation Area, the San Francisco Maritime Historical Park, and Lower Fort Mason. A concrete retaining wall supports a portion of the downhill side of the roadway. A portion of the retaining wall is cracked, and the adjacent portion of the roadway has deteriorated resulting from erosion of the soil and rock on the sea cliff downhill of the roadway. The proposed project consists of replacing approximately 134 feet of the retaining wall and approximately 264 linear feet of the road pavement.

Our investigation included a review of prior geologic and geotechnical investigations performed for the site, a review of aerial photographs and geologic literature, and a site reconnaissance and mapping by our geologists. The purpose of this study was to provide you with recommendations for the proposed repair, and our professional opinion of the future site performance based upon our evaluation of the surficial site conditions and past site history, and our general knowledge of the area.

In general, we conclude that a repair of the retaining wall and roadway is feasible from a geologic and geotechnical perspective, provided the improvements are designed and built in accordance recommendations provided in this report. We conclude there are two primary options for the repair:

Option A involves removing the distressed portion of the wall and construction of a new pier-supported retaining wall in the same location, and Option B involves constructing a pier supported mat for the roadway and guardrail that can cantilever in lieu of the retaining wall. The reader should refer to the text of the report for detailed findings and conclusions.

We appreciate the opportunity to assist you with this project.

Sincerely yours

TREADWELL & ROLLO, INC.

Christopher R. Hundemer, C.E.G. 2314 John Gouchon, G.E. 2282 Senior Project Geologist Principal Engineer

48730101.LTR

Environmental and Geotechnical Consultants

REPAIR OF CONCRETE WALL

Kennedy/Jenks Consultants

21 October 2008 Project No. 4873.01

TABLE OF CONTENTS

1.0 INTRODUCTION

2.0 SCOPE OF SERVICES

3.0 SEISMICITY AND GEOLOGY

3.1 Regional Geology

3.2 Regional Seismicity

4.0 SITE CONDITIONS

5.0 GEOLOGIC HAZARDS

5.1 Sea Cliff Erosion

5.2 Fault Rupture

5.3 Strong Ground Shaking

5.4 Seismically-Induced Ground Deformations

5.5 Non-Seismic Ground Failures

5.5.1 Landsliding

5.5.2 Expansive Soil

5.5.3 Collapsible Soil

6.0 CONCLUSIONS

6.1 Option A – Replace Existing Retaining Wall

6.2 Option B – Drilled Piers and Cantilever Roadway

7.0 RECOMMENDATIONS

7.1 Drilled Piers (Options A and B)

7.2 Concrete Retaining Wall (Option A)

7.3 Asphalt Pavements

7.4 Concrete Mat (Option B)

7.5 Seismic Design Criteria

7.6 Construction Consideration

8.0 PROPOSED GEOTECHNICAL SERVICES AND MONITORING

9.0 LIMITATIONS

REFERENCES

FIGURES

DISTRIBUTION

LIST OF FIGURES

Figure 1 Site Location Map

Figure 2 Regional Geologic Map

Figure 3 Map of Major Faults and Earthquake Epicenters in the San Francisco Bay Area

Figure 4 Modified Mercalli Intensity Scale

Figure 5 Partial Site Plan and Engineering Geologic Map

Figure 6 Idealized Subsurface Profile A-A’ and B-B’

Figure 7 Schematic Repair Plan for Option B

Figure 8 Schematic Repair Cross-Section Option B

REPAIR OF CONCRETE WALL ALONG MCDOWELL ROAD

San Francisco, California

1.0 INTRODUCTION

This report presents the results of our geotechnical and geologic investigation for the project titled

“Repair of Concrete Wall along McDowell Road” in San Francisco, California. McDowell Road was constructed on a hill slope at the north end of Fort Mason, between the Golden Gate National Recreation

Area, the San Francisco Maritime Historical Park, and Lower Fort Mason, as shown on Figure 1.

A concrete retaining wall supports a portion of the downhill side of the roadway. A portion of the retaining wall is cracked, and the adjacent portion of the roadway has deteriorated. The proposed project consists of replacing approximately 134 feet of the retaining wall and approximately 264 linear feet of the road pavement.

The objective of this study was to provide you with recommendations for the proposed repair, and our professional opinion of the future site performance based upon our evaluation of the surficial site conditions and past site history, and our general knowledge of the area.

2.0 SCOPE OF SERVICES

Our scope of services was performed in accordance with our proposal dated 5 June 2008. As the basis for this investigation, we have reviewed geologic maps, hazard maps, and aerial photographs of the site and vicinity. On 26 August 2008, our geologist conducted a site walk-through with representatives of other firms on the project design team. On 2 September 2008 our geologist returned to the site and conducted geologic mapping of the site including an evaluation of exposed rock on the downhill side of the retaining wall. On 25 September 2008 our geologist visited the site and conducted geologic mapping and prepared two slope profiles through the area of distress. A more detailed geotechnical investigation that would include subsurface exploration and laboratory testing was beyond the scope of this study.

48730101.CRH 21 October 2008

3.0 SEISMICITY AND GEOLOGY

3.1 Regional Geology

According to the Geologic Map of the San Francisco North Quadrangle, San Francisco and Marin Counties, California (Schlocker, et al 1974), the site is underlain by the Jurassic age (approximately 145 to 213 million year old) to Cretaceous age (approximately 65 to 145 million year old) Franciscan sandstone as shown on the Regional Geologic Map, Figure 2. The Franciscan sandstone in general has massive bedding (thickness ranges from 10 to 35 feet), contains angular, fine- to medium-grained sand, and is considered a greywacke. The characteristics of greywacke are poor sorting, 10 percent or more clayey matrix, low porosity, and dark color.

3.2 Regional Seismicity

The major active faults in the area are the Monte Vista, San Andreas, Hayward, and Calaveras Faults.

These and other faults within a 90km radius of the site are shown on Figure 3. For each of the active faults, the distance from the site and estimated mean characteristic Moment magnitude1 [2007 Working

Group on California Earthquake Probabilities (WGCEP) (2007) and Cao et al. (2003)] are summarized in

Table 1.

TABLE 1

Regional Faults and Seismicity

Fault

Approximate

Distance from Site

(km)

Direction From Site

Maximum or Mean

Characteristic Moment

Magnitude San Andreas – 1906 Rupture 12.1 West 7.90 San Andreas – Peninsula 12.1 West 7.15 San Andreas – North Coast South 13.0 West 7.45 North Hayward 16.9 Northeast 6.49 Total Hayward 16.9 Northeast 6.91 Total Hayward-Rodgers Creek 16.9 Northeast 7.26 Northern San Gregorio 17.0 West 7.23 Total San Gregorio 17.0 West 7.44 South Hayward 19.2 East 6.67 Rodgers Creek 31 North 6.98

1 Moment magnitude is an energy-based scale and provides a physically meaningful measure of the size of a faulting event. Moment magnitude is directly related to average slip and fault rupture area.

Approximate

Distance from Site

(km)

Direction From Site

Maximum or Mean

Characteristic Moment

Magnitude Mt. Diablo – MTD 35 East 6.65 Total Calaveras 37 East 6.93 Point Reyes 39 West 6.80 Concord/Green Valley 39 East 6.71 West Napa 43 Northeast 6.50 Monte Vista-Shannon 44 Southeast 6.80 Greenville 53 East 6.94 Hayward – South East Extension 61 Southeast 6.40 Great Valley 6 62 East 6.70 Great Valley 5 67 East 6.50 Great Valley 4 72 Northeast 6.60 Hunting Creek-Berryessa 74 Northeast 6.90 San Andreas – Santa Cruz Mnts. 80 Southeast 7.03 Great Valley 7 80 East 6.70 Sargent 86 Southeast 6.80 Maacama-Garberville 88 North 6.90 Zayante-Vergeles 90 Southeast 6.80

Figure 3 also shows the earthquake epicenters for events with magnitude greater than 5.0 from

January 1800 through December 2000. Since 1800, four major earthquakes have been recorded on the

San Andreas Fault. In 1836 an earthquake with an estimated maximum intensity of VII on the Modified

Mercalli (MM) scale (Figure 4) occurred east of Monterey Bay on the San Andreas Fault (Toppozada and

Borchardt 1998). The estimated Moment magnitude, Mw, for this earthquake is about 6.25. In 1838, an earthquake occurred with an estimated intensity of about VIII-IX (MM), corresponding to a Mw of about

7.5. The San Francisco Earthquake of 1906 caused the most significant damage in the history of the Bay

Area in terms of loss of lives and property damage. This earthquake created a surface rupture along the

San Andreas Fault from Shelter Cove to San Juan Bautista approximately 470 kilometers in length. It had a maximum intensity of XI (MM), a Mw of about 7.9, and was felt 560 kilometers away in Oregon, Nevada, and Los Angeles. The most recent earthquake to affect the Bay Area was the Loma Prieta

Earthquake of 17 October 1989, in the Santa Cruz Mountains with a Mw of 6.9, approximately 97 km from the site.

In 1868 an earthquake with an estimated maximum intensity of X on the MM scale occurred on the southern segment (between San Leandro and Fremont) of the Hayward Fault. The estimated Mw for the earthquake is 7.0. In 1861, an earthquake of unknown magnitude (probably a Mw of about 6.5) was reported on the Calaveras Fault. The most recent significant earthquake on this fault was the 1984

Morgan Hill earthquake (Mw = 6.2).

The 2007 WGCEP at the U.S. Geologic Survey (USGS) predicted a 63 percent chance of a magnitude 6.7 or greater earthquake occurring in the San Francisco Bay Area in 30 years. More specific estimates of the probabilities for different faults in the Bay Area are presented in Table 2.

TABLE 2

WGCEP (2007) Estimates of 30-Year Probability of a Magnitude 6.7 or Greater Earthquake

Probability (percent)

Hayward-Rodgers Creek 31

N. San Andreas 21

Calaveras 7

San Gregorio 6

Concord-Green Valley 3

Greenville 3

Mount Diablo Thrust 1

4.0 SITE CONDITIONS

The distressed portion of McDowell Road and the retaining wall along the downhill side of the road is located on a north-facing sea cliff on the northernmost point of Fort Mason. McDowell Road is presently used as a walkway and bicycle path connecting Fort Mason and the Maritime Historical Park. The road is asphalt-paved and approximately 25 feet wide. An approximately 264 feet long stretch of the road has been distressed, between approximate Elevations2 33 feet and 61 feet (Mean Sea Level). The distress appears to consist of cracks parallel to the road direction, confined to the northern portion of the road.

We anticipate a wedge of fill was placed beneath the roadway to create a level surface.

2 All elevations reference Mean Sea Level Datum.

A cut slope and low (2 foot tall) concrete retaining wall are located on the uphill side of this portion of the road. Above the eastern portion of this section of roadway portions of the cut slope have been supported by terraced, segmented unit retaining walls. The slope has a gradient of between about 1½:1 and 2:1

(horizontal to vertical), as shown on Figure 6, Idealized Subsurface Profile A-A’ and B-B’. The cut slope is covered with dense brush and vegetation.

A poured concrete retaining wall supporting up to approximately 8 to 10 feet of artificial fill beneath

McDowell Road is located along the downhill side of the road. The wall is supported on a shallow continuous footing and it extends about 3 feet above McDowell Road to serve as a guardrail. A portion of the retaining wall measuring approximately 134 feet in length has been severely cracked and is leaning downslope. During our site visit, we observed that significant voids have developed beneath portions of the footing along this stretch of the wall. Observations through cracks in the wall did not reveal evidence of reinforcing steel in the wall.

The slopes of the sea cliff downhill of the retaining wall are very steep to near vertical (see Figure 6).

Two significant erosional scars were observed and mapped on the slope, near the middle of the distressed portion of the wall (see Figure 5). In addition, smaller, localized areas of erosion were noted on the slopes just east and west of the erosional scars. The slopes expose a thin layer of weathered rock and colluvium over sandstone bedrock. Medium hard, fine- to medium-grained sandstone bedrock was observed at a very shallow depth below the foundation of the wall, where voids had developed from erosion of the less competent surficial soil. Vegetation on the sea cliffs varies from dense brush and poison oak to areas that are bare.

Site drainage is characterized by unchannelized sheet-flow down the cut slope onto McDowell Road, which is collected in a v-ditch that flows towards the east, along the southern edge of the road. Runoff below McDowell Road is characterized as sheet-flow down the sea cliff into San Francisco Bay.

5.0 GEOLOGIC HAZARDS

On the basis of the results of our geologic hazard evaluation and site reconnaissance, we conclude the primary geologic hazard affecting the roadway is continued erosion of the sea cliff immediately downslope. The following sections describe this and other, less likely geologic hazards at the site.

5.1 Sea Cliff Erosion

The sea cliff below the retaining wall is experiencing a low amount of active erosion. This erosion appears to result from two mechanisms, 1) rainfall and surface runoff from above eroding the soil and weathered rock on the slope, and 2) wave undercutting at the base of the cliff. It should be anticipated that on-going erosion will continue, but that this erosion should not impact the proposed improvements to the roadway and retaining wall if they are designed and constructed in accordance with the recommendations presented in Section 7.0.

5.2 Fault Rupture

Following California's Alquist-Priolo Special Studies Zone Act of 1972, construction of structures for human occupancy in designated Earthquake Fault Zones is not permitted until a site-specific evaluation of surface fault rupture and fault creep has been performed (CDMG, 1997). Theses zones are established along faults or segments of faults that are judged to be sufficiently active and well-defined as to constitute a potential hazard to structures from surface faulting or fault creep. The site is not located in an Earthquake Fault Rupture Zone and has a low to negligible potential for ground rupture due to faulting.

5.3 Strong Ground Shaking

During a major earthquake on one of the nearby active faults in the general region, the site will experience strong ground shaking similar to other areas of the seismically active San Francisco Bay

Region. The intensity of the earthquake ground motion at the site will depend upon the characteristics of the generating fault, distance to the earthquake epicenter, magnitude and duration of the earthquake, and specific site geologic conditions. During its history, the site has been subjected to strong ground shaking from moderate to large earthquakes on the San Andreas, Hayward, and Calaveras, and future strong to very ground shaking should be expected.

5.4 Seismically-Induced Ground Deformations

During a major earthquake on a segment of one of the nearby faults, strong to very strong shaking is expected to occur at the project site. Strong shaking during an earthquake can result in ground failure such as that associated with soil liquefaction3, cyclic densification4, and lateral spreading5. Because the site is located in an area of very shallow bedrock, we conclude that the potential for liquefaction, cyclic densification, and lateral spreading is negligible.

5.5 Non-Seismic Ground Failures

Potential geologic hazards associated with ground failure not caused by earthquakes include landslides, expansive soil, and collapsible soils. These are discussed in the following subsections.

5.5.1 Landsliding

Our investigation disclosed no evidence of deep-seated landsliding in the vicinity of McDowell Road.

Because of the moderate to steep slopes above and below the road, the occurrence of a new shallow landslide within or adjacent to the subject property cannot be excluded. A new shallow landslide in this area could be triggered by excessive precipitation or strong ground shaking associated with an earthquake. In our opinion, a landslide of this nature should not constitute an immediate threat to the proposed improvements provided they are designed and constructed in accordance with our recommendations.

The long-term stability of many hillside areas is difficult to predict. A hillside will remain stable only as long as the existing slope equilibrium is not disturbed by natural processes or by the acts of man.

Landslides can be activated by a number of natural processes, such as the loss of support at the bottom of a slope by erosion or the reduction of soil strength by an increase in groundwater level from excessive precipitation. Negative effects caused by man may include improper grading activities; or the introduction of excess water through excessive irrigation, improperly designed or constructed leachfields, or poorly controlled surface runoff.

3 Liquefaction is a transformation of soil from a solid to a liquefied state during which saturated soil temporally loses strength resulting from the buildup of excess pore water pressure, especially during earthquake-induced cyclic loading. Soil susceptible to liquefaction includes loose to medium dense sand and gravel, low-plasticity silt, and some low-plasticity clay deposits.

4 Cyclic densification is a phenomenon in which non-saturated, cohesionless soil is densified by earthquake vibrations, resulting in ground surface settlement.

5 Lateral spreading is a phenomenon in which surficial soil displaces along a shear zone that has formed within an underlying liquefied layer. Upon reaching mobilization, the surficial blocks are transported downslope or in the direction of a free face by earthquake and gravitational forces.

5.5.2 Expansive Soil

Expansive soils are those that shrink or swell significantly with changes in moisture content. The clay content and porosity of the soil also influence the change in volume. The shrinking and swelling caused by expansive clay-rich soil often results in damage to overlying structures. Expansive clay was not observed at the site in the area of the retaining wall during our geologic reconnaissance.

5.5.3 Collapsible Soil

Soil collapse is the densification of sediments resulting from significant increases in their moisture content. This process typically results from moisture infiltration into soil that has a weakly cemented structure that can be broken down by the introduction of water. This phenomenon is more prevalent in low-density, silty, sandy soil deposited in semi-arid and arid climates where the soil has not been subjected to saturation. Based on the shallow depth to bedrock, we judge the potential for soil collapse at the site to be negligible.

6.0 CONCLUSIONS

Based upon the results of our site observations, we conclude that the roadway and retaining wall repairs are feasible from a geotechnical and engineering geologic perspective. We conclude the distress to the roadway and retaining wall appear to be caused by erosion of the soil and rock beneath the wall and consolidation or loss of lateral support of the wedge of fill beneath the roadway. On the basis of our discussions with you, we understand you are considering two options for the project. Option A involves removal of the distressed portion of the retaining wall and roadway, replacement of the wall with a new pier-supported wall, and replacement of the asphalt roadway. Option B involves constructing a structural

“deck” for the roadway supported on a grid (two rows) of drilled piers, and a cantilevered guard-rail matching the existing retaining wall at the current wall location. Based upon the results of our site observations, we conclude that the roadway and retaining wall repairs are feasible from a geotechnical and engineering geologic perspective.

6.1 Option A – Replace Existing Retaining Wall

Option A consists of the removal and replacement of the existing distressed portion of the retaining wall shown on Figure 5. We conclude the retaining wall should be supported on drilled piers bearing in the underlying sandstone bedrock and should be designed to retain the existing wedge of fill beneath the downhill edge of the roadway. At the present location, we anticipate that the bottom of the wall would need to extend 2 to 3 feet below the elevation of the current bottom of wall to retain this fill.

Following construction of the new wall, the portion of the distressed pavement to be replaced (see

Figure 5) can be replaced with new pavement. We conclude that the upper 2 feet of existing fill should be over-excavated and replaced as engineered fill in accordance with the Asphalt Pavement recommendations below in Section 7.3.

6.2 Option B – Drilled Piers and Cantilever Roadway

Option B involves drilling a grid of new piers throughout the roadway in the area behind the failed retaining wall, connecting the piers with a structural mat, then repaving over the mat with a new asphalt pavement section. The mat would structurally support the roadway and a cantilevered guardrail. This option would facilitate drilling of the piers and construction of the mat while the existing wall is in place, and would not require the construction of a retaining wall to support the fill beneath the roadway.

Following construction of the piers and structural slab, the existing retaining wall can be removed and replaced with the concrete guard-rail.

7.0 RECOMMENDATIONS

All drilled piers, retaining walls, and pavement should be designed in accordance with the following recommendations.

7.1 Drilled Piers (Options A and B)

We recommend that drilled piers have a minimum diameter of 24 inches and be embedded a minimum of

8 feet into the underlying bedrock measured below the point where there is at least 7 feet of horizontal confinement between the downhill face of the pier and the sea cliff. Total pier depth will vary across the site depending on the thickness of non-supportive fill and proximity to the sea cliff.

Drilled piers should be designed to derive their axial capacity from the underlying sandstone bedrock.

Piers installed in a group should be spaced at least three diameters on center. To compute the axial capacity of drilled piers, we recommend using an allowable skin friction of 1,500 pounds per square foot

(psf) for dead and live loads. For temporary, compressive, total loads, including wind and/or seismic load, the skin friction value can be increased by one third. For temporary uplift loads, we recommend an allowable skin friction of 1,500 psf.

Piers will provide lateral resistance from passive pressure acting on the upper portion of the piers where in contact with the fill beneath the roadway, and from their structural rigidity. Lateral resistance of piers will depend on the pier diameter, pier head condition (restrained or unrestrained), allowable deflection of the pier top, and the bending moment resistance of the piers.

Piers should be designed to resist active loads on the upper portion of the piers from the fill. We recommend an equivalent fluid weight of 40 pcf taken over 2 pier diameters be used for design. The depth of the active pressure will vary across the building site depending on the depth of the non-supportive material. For Option A, we anticipate that the retaining wall atop the piers will support all of the fill, and therefore the active loads on the piers themselves will be negligible. For Option B, the active loads along the uphill row of piers located near the cut-fill contact will be negligible; however along the downhill side of the road the active pressure may extend a depth of 8 feet.

Lateral loads can be resisted using a passive pressure equal to an equivalent fluid weight of 350 pcf to a maximum of 2,000 psf taken over 1½-pier diameters for the length of the piers in the bedrock where there is at least 7 feet of horizontal confinement (below the upper 4 feet). This zone (the upper 4 feet of bedrock may be assumed as active-passive neutral.

The bottoms of the pier excavations should be free of all loose cuttings and slough prior to the installation of the reinforcing steel and the placement of the concrete. Any accumulated water in the excavations should also be removed prior to the placement of the steel and concrete; alternatively if water is present in the shafts, concrete should be placed by the tremie method.

We should be retained to verify that the piers are founded in material of sufficient supporting capacity, are of sufficient depth, and have been properly prepared. We anticipate the shafts will remain open in rock; however casing may be required to prevent caving of the fill.

7.2 Concrete Retaining Wall (Option A)

The proposed retaining wall should be supported on drilled piers designed in accordance with the recommendations above. Prior to installation of the new site retaining wall, the existing wall should be removed. The site may be open cut at a slope or temporarily shored. It is the responsibility of the contractor to determine the excavation slopes; however, they should be no steeper than 1.5:1

(horizontal:vertical). The excavated soil can be reused as backfill material behind the retaining wall.

Backfill should be placed in layers less than 8 inches thick prior to compaction, and compacted to at least

90 percent relative compaction6. Light compaction equipment should be used within 5 feet of the walls.

For cantilever, drained walls retaining level backfill, we recommend designing the walls for active lateral pressures corresponding to an equivalent fluid unit weight of 40 pounds per cubic foot (pcf). Walls that are restrained from rotation at the top should be designed using at-rest pressures corresponding to an equivalent fluid unit weight of 60 pcf. Where traffic is expected within a distance equal to the height of the walls, the walls should be designed for an additional uniform lateral pressure of 100 psf to be applied over the entire height of the wall or 10 feet, whichever is less.

Because the site is in a seismically active area, the design should be checked for seismic condition, in which the wall pressure is determined by adding the earth pressure due to earthquake shaking to the active earth pressure. The incremental seismic pressure is approximated by a uniform pressure, in psf, of

15 times the height of the wall in feet, acting over the entire height of the wall.

The lateral earth pressures recommended for the sections above the water table are applicable to walls that are backdrained to prevent the buildup of hydrostatic pressure. One acceptable method for backdraining the wall is to place a prefabricated drainage panel against the back of the wall. The drainage panel should extend down to a four-inch-diameter perforated PVC collector pipe at the base of the walls. The pipe should be surrounded on all sides by at least four inches of Caltrans Class 2 permeable material (see Caltrans Standard Specifications Section 68-1.025) or wrapped in filter fabric

(Mirafi 140N or equivalent). We should check the manufacturer’s specifications regarding the proposed prefabricated drainage panel material to verify it is appropriate for its intended use. The pipe should be connected to a suitable discharge point. As an alternative to using prefabricated drainage panel, the wall may be drained using Caltrans Class 2 permeable material (Caltrans Standard Specifications Section 68-

6 Relative compaction refers to the in-place dry density of soil expressed as a percentage of the maximum dry density of the same material, as determined by the ASTM D1557-00 laboratory compaction procedure.

1.025) or clean drain rock wrapped in a geotextile filter fabric (Mirafi 140N or equivalent). The gravel drain should be at least 12 inches wide and should extend up the back of the wall to about 2 feet below the ground surface; the upper 2 feet should be covered with a clay cap to reduce infiltration of surface water. A four-inch-diameter perforated PVC collector pipe should be placed within the gravel blanket near the base of the wall to drain the water to a suitable discharge. The pipe should be surrounded on all sides by at least four inches of Caltrans Class 2 permeable material or drain rock.

7.3 Asphalt Pavements

The State of California flexible pavement design method was used to develop the recommended asphalt concrete pavement sections. We expect the final soil subgrade in asphalt-paved areas will generally consist of silty sand with gravel fill generated during original site grading. Based on our experience with

Franciscan sandstone excavated and placed as fill we have assumed an R-value of 25. For our calculations, we assumed a Traffic Index (TI) of 5.5 for roadways and truck-use areas; these TIs should be confirmed by the project civil engineer. Table 4 presents our recommendations for asphalt pavement sections.

TABLE 4

Pavement Section Design

TI

Asphaltic Concrete

(inches)

Class 2 Aggregate Base R = 78

(inches)

5.5 3 8

Pavement components should conform to the current Caltrans Standard Specifications. The upper six inches of the soil subgrade in pavement areas should be moisture-conditioned to above optimum and compacted to at least 95 percent relative compaction and rolled to provide a smooth non-yielding surface. Aggregate base should be compacted to at least 95 percent relative compaction.

7.4 Concrete Mat (Option B)

We understand the concrete mat for Option B will be designed as a free-standing structure capable of spanning between drilled piers and capable of supporting a cantilevered roadway and guardrail up to

8 feet in width. All soil bearing between piers should be neglected for support. The thickness of concrete and reinforcing schedule should be determined by the structural engineer based upon the size and spacing of the piers. The top of the concrete mat should be 3 inches below final roadway grade allow for 3 inches of asphalt pavement to conform with the existing or new asphalt pavement section.

7.5 Seismic Design Criteria

The retaining wall structure should be designed to appropriate seismic codes. For the 2007 California

Building Code (CBC) seismic code, we recommend the following:

• Mapped spectral acceleration values of Ss and S1 equal to 1.50g and 0.637g respectively

• Site Class B

• Site coefficients Fa and Fv equal to 1.0 and 1.0 respectively.

7.6 Construction Consideration

Drilled piers should be installed by a qualified contractor with demonstrated experience in this type of foundation. The bedrock at the site has variable consistency and locally can be very hard. We recommend that a high-powered well-maintained drill rig equipped with rock teeth be used to drill the holes. If drilling refusal is met above the design depth, Treadwell & Rollo, Inc. and the structural engineer should be contacted to revise the design recommendations. Hard cobbles or even small boulders may be encountered during drilling. The contractor should plan for this condition when choosing the appropriate means and methods of drilling. Casing may be required to prevent cave-in of the upper fill wedge.

Concrete placement should start upon completion of the drilling and clean out. Concrete should be placed from the bottom up in a single operation using a tremie and/or a pumper pipe. The tremie pipe should be maintained at least 5 feet below the upper surface of the concrete during casting of the piers.

The concrete should have a slump between 7 and 9 inches. As the concrete is placed, casing used to stabilize the hole can be withdrawn. The bottom of the casing should be maintained at least 3 feet below the surface of the concrete.

8.0 PROPOSED GEOTECHNICAL SERVICES AND MONITORING

Following the design, Treadwell & Rollo, Inc. should review the final plans and specifications for the repair to verify that they conform to the intent of our recommendations. During construction, it is important that our field engineer be present to observe the installation of foundation elements and placement of fill. These observations will allow us to compare actual with anticipated soil/rock conditions, to see and verify that the contractor's work conforms to the geotechnical aspects of the plans and specifications.

9.0 LIMITATIONS

The findings and recommendations presented in this letter report apply only to the design for repairs to the concrete wall and pavement for McDowell Road as we have described, and are the result of limited engineering studies and our interpretations of the existing geotechnical conditions at the time of our field activities. We have prepared this report for the exclusive use of our client in substantial accordance with the generally accepted geotechnical and geologic engineering practice as it exists in the area at the time of our study. No warranty is expressed or implied.

REFERENCES

California Building Standards Commission (2007), 2007 California Building Code, California Code of Regulations Title 24, Part 2, Volume 2 of 2, June 2007.

California Division of Mines and Geology (1996), Probabilistic Seismic Hazard Assessment for the State of California. DMG Open-File Report 96-08.

California Division of Mines and Geology (CDMG), 1996, Probabilistic Seismic Hazard Assessment for the State of California, CDMG Open-File Report 96-08.

California Division of Mines and Geology (CDMG), 1998, Maps of Known Active Faults.

California Geological Survey (1997), Guidelines for Evaluating and Mitigating Seismic Hazards in California. Special Publication 117.

Cao, T., Bryant, W.A., Rowshandel, B., Branum, D., and Willis, C.J., 2003, The Revised 2002 California Probabilistic Seismic Hazard Maps.

Schlocker, J. (1974), Geology of the San Francisco North Quadrangle, U.S. Geological Survey Professional Paper 782.

Townley, S.D. and Allen, M.W. (1939), Descriptive Catalog of Earthquakes of the Pacific Coast of the United States 1769 to 1928; Bulletin of the Seismological Society of America, Vol. 29, No. 1; 1939.

United States Geological Survey (2000), Earthquake Hazards Program, website URL, December 29.

Working Group on California Earthquake Probabilities (WGCEP) (2007). “The Uniform California Earthquake Rupture Forecast, Version 2.” Open File Report 2007-1437.

FIGURES

Project No. FigureDateTreadwell&Rollo 4873.0110/07/08 1

SITE LOCATION MAP

SITE

Base map: The Thomas Guide San Francisco County

0 1/2 Mile

Approximate scale

1/4

REPAIR OF CONCRETE WALL

Project No. FigureDate 4873.0110/09/08 2

SITE

Treadwell&Rollo

REGIONAL GEOLOGIC MAP

EXPLANATION

Base map: Geology Map of the San Francisco North Quad, San Francisco and Marin County, California, Schlocker, 1966.

REPAIR OF CONCRETE WALL

ALONG MCDOWELL ROAD

San Francisco, California

Geologic contact

Qaf Artificial Fill (Quaternary)

Qd Dune sand (Quaternary)

0 2000 Feet

Approximate scale

Qu Surficial deposits (Quaternary)

KJsh Shale (Jurassic and Cretaceous)

KJs Sandstone and Shale (Jurassic and Cretaceous)

Qb Modern beach deposits (Quaternary)

Ql Landslide deposits (Quaternary)

Qc Colma formation (Quaternary) dashed where approximate and dotted where concealed

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Project No. FigureDate

I Not felt by people, except under especially favorable circumstances. However, dizziness or nausea may be experienced.

Sometimes birds and animals are uneasy or disturbed. Trees, structures, liquids, bodies of water may sway gently, and doors may swing very slowly.

II Felt indoors by a few people, especially on upper floors of multi-story buildings, and by sensitive or nervous persons.

As in Grade I, birds and animals are disturbed, and trees, structures, liquids and bodies of water may sway. Hanging objects swing, especially if they are delicately suspended.

III Felt indoors by several people, usually as a rapid vibration that may not be recognized as an earthquake at first. Vibration is similar to that of a light, or lightly loaded trucks, or heavy trucks some distance away. Duration may be estimated in some cases.

Movements may be appreciable on upper levels of tall structures. Standing motor cars may rock slightly.

IV Felt indoors by many, outdoors by a few. Awakens a few individuals, particularly light sleepers, but frightens no one except those apprehensive from previous experience. Vibration like that due to passing of heavy, or heavily loaded trucks. Sensation like a heavy body striking building, or the falling of heavy objects inside.

Dishes, windows and doors rattle; glassware and crockery clink and clash. Walls and house frames creak, especially if intensity is in the upper range of this grade. Hanging objects often swing. Liquids in open vessels are disturbed slightly. Stationary automobiles rock noticeably.

V Felt indoors by practically everyone, outdoors by most people. Direction can often be estimated by those outdoors. Awakens many, or most sleepers. Frightens a few people, with slight excitement; some persons run outdoors.

Buildings tremble throughout. Dishes and glassware break to some extent. Windows crack in some cases, but not generally. Vases and small or unstable objects overturn in many instances, and a few fall. Hanging objects and doors swing generally or considerably.

Pictures knock against walls, or swing out of place. Doors and shutters open or close abruptly. Pendulum clocks stop, or run fast or slow.

Small objects move, and furnishings may shift to a slight extent. Small amounts of liquids spill from well-filled open containers. Trees and bushes shake slightly.

VI Felt by everyone, indoors and outdoors. Awakens all sleepers. Frightens many people; general excitement, and some persons run outdoors.

Persons move unsteadily. Trees and bushes shake slightly to moderately. Liquids are set in strong motion. Small bells in churches and schools ring. Poorly built buildings may be damaged. Plaster falls in small amounts. Other plaster cracks somewhat. Many dishes and glasses, and a few windows break. Knickknacks, books and pictures fall. Furniture overturns in many instances. Heavy furnishings move.

VII Frightens everyone. General alarm, and everyone runs outdoors.

People find it difficult to stand. Persons driving cars notice shaking. Trees and bushes shake moderately to strongly. Waves form on ponds, lakes and streams. Water is muddied. Gravel or sand stream banks cave in. Large church bells ring. Suspended objects quiver.

Damage is negligible in buildings of good design and construction; slight to moderate in well-built ordinary buildings; considerable in poorly built or badly designed buildings, adobe houses, old walls (especially where laid up without mortar), spires, etc. Plaster and some stucco fall. Many windows and some furniture break. Loosened brickwork and tiles shake down. Weak chimneys break at the roofline.

Cornices fall from towers and high buildings. Bricks and stones are dislodged. Heavy furniture overturns. Concrete irrigation ditches are considerably damaged.

VIII General fright, and alarm approaches panic.

Persons driving cars are disturbed. Trees shake strongly, and branches and trunks break off (especially palm trees). Sand and mud erupts in small amounts. Flow of springs and wells is temporarily and sometimes permanently changed. Dry wells renew flow.

Temperatures of spring and well waters varies. Damage slight in brick structures built especially to withstand earthquakes; considerable in ordinary substantial buildings, with some partial collapse; heavy in some wooden houses, with some tumbling down. Panel walls break away in frame structures. Decayed pilings break off. Walls fall. Solid stone walls crack and break seriously. Wet grounds and steep slopes crack to some extent. Chimneys, columns, monuments and factory stacks and towers twist and fall. Very heavy furniture moves conspicuously or overturns.

IX Panic is general.

Ground cracks conspicuously. Damage is considerable in masonry structures built especially to withstand earthquakes; great in other masonry buildings - some collapse in large part. Some wood frame houses built especially to withstand earthquakes are thrown out of plumb, others are shifted wholly off foundations. Reservoirs are seriously damaged and underground pipes sometimes break.

X Panic is general.

Ground, especially when loose and wet, cracks up to widths of several inches; fissures up to a yard in width run parallel to canal and stream banks. Landsliding is considerable from river banks and steep coasts. Sand and mud shifts horizontally on beaches and flat land. Water level changes in wells. Water is thrown on banks of canals, lakes, rivers, etc. Dams, dikes, embankments are seriously damaged. Well-built wooden structures and bridges are severely damaged, and some collapse. Dangerous cracks develop in excellent brick walls. Most masonry and frame structures, and their foundations are destroyed. Railroad rails bend slightly. Pipe lines buried in earth tear apart or are crushed endwise. Open cracks and broad wavy folds open in cement pavements and asphalt road surfaces.

XI Panic is general.

Disturbances in ground are many and widespread, varying with the ground material. Broad fissures, earth slumps, and land slips develop in soft, wet ground. Water charged with sand and mud is ejected in large amounts. Sea waves of significant magnitude may develop. Damage is severe to wood frame structures, especially near shock centers, great to dams, dikes and embankments, even at long distances. Few if any masonry structures remain standing. Supporting piers or pillars of large, well-built bridges are wrecked.

Wooden bridges that "give" are less affected. Railroad rails bend greatly and some thrust endwise. Pipe lines buried in earth are put completely out of service.

XII Panic is general.

Damage is total, and practically all works of construction are damaged greatly or destroyed. Disturbances in the ground are great and varied, and numerous shearing cracks develop. Landslides, rock falls, and slumps in river banks are numerous and extensive. Large rock masses are wrenched loose and torn off. Fault slips develop in firm rock, and horizontal and vertical offset displacements are notable. Water channels, both surface and underground, are disturbed and modified greatly. Lakes are dammed, new waterfalls are produced, rivers are deflected, etc. Surface waves are seen on ground surfaces. Lines of sight and level are distorted. Objects are thrown upward into the air.

4873.0110/07/08 4

MODIFIED MERCALLI INTENSITY SCALE

REPAIR OF CONCRETE WALL

ALONG MCDOWELL ROAD

San Franciso, California

Treadwell&Rollo

DISTRIBUTION

6 copies: Mr. Donald Barraza Kennedy/Jenks Consultants.

622 Folsom Street San Francisco, California 94107