Friar Associates, Incorporated Engineers Soils ...€¦ · 22/1/2004 · Friar Associates,...

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Friar Associates, Incorporated Soils . Foundations . Geology

Engineers Geotechnology

2656 Nicholson Street, San Leandro, CA 94577 Tel: (510) 351-3930 Fax: (510) 351-1020

C.-0C?<Consultants

GEOTECHNICAL INVESTIGATION PROPOSED TWO-LOT RESIDENTIAL DEVELOPMENT

14625 AND 14635 MIDLAND ROAD ALAMEDA COUNTY, CALIFORNIA

PROJECT 1351

Prepared for

VICTOR VAN 14625-14635 MIDLAND ROAD

SAN LEANDRO, CALIFORNIA 94578

Prepared by

FRIAR ASSOCIATES, IN CORPORA TED 2656 Nicholson Street

San Leandro, California 94577

JANUARY 2004

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TABLE OF CONTENTS

l : INTRODUCTION ............................................................. 1

! ; PLANNED CONSTRUCTION ................................................... 1

I 1 INFORMATION PROVIDED .................................................... 1

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SCOPE OF WORK ............................................................ 1 r !

\, FINDINGS ................................................................... 2 Surface Conditions ...................................................... 2 Subsurface Conditions ................................................... 3 Seismic Considerations ................................................... 3

DISCUSSION ................................................................ 4

RECOMMENDATIONS ........................................................ 5 Site Preparation, Grading and Compaction ................................... 5 Building Foundations .................................................... 6 Concrete Slabs-on-Grade ................................................. 7 Retaining Walls ......................................................... 7 Utility Trenches ......................................................... 9 Surface Drainage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 O Follow-up Geotechnical Services .......................................... 10

LIMITATIONS .............................................................. 11

FIGURES Figure 1 - Vicinity Map Figure 2 - Site Plan and Location of Exploration Test Pits

APPENDICES Appendix A - HYDRO-GEO CONSULTANTS GEOLOGIC REPORT

January 22, 2004 Project 1351

1. Reviewed geologic and geotechnical information in our files pertinent to the site and the surrounding area.

2. Reviewed in-house files for projects done in the vicinity of the site.

3. Explored, sampled and classified foundation soils by means of two exploratory trenches both as part of our soil investigation and fault investigation. The exploratory trenches were logged by a certified engineering geologist and an engineer on staff. Also present during the site exploration were an engineer from the Alameda County Public Works agency and a representative of Alameda County Geologic Consultants, who also inspected the materials exposed in the exploratory trenches.

4. Reviewed and analyzed the field data.

5. Based on the findings of items 1 through 4 above, developed geotechnical recommendations for site preparation, grading and compaction; provided geotechnical design parameters for the proposed building foundations, concrete slabs-on-grade and retaining walls; provided recommendations for utility trench backfilling and site drainage.

6. Provided design section for the driveway pavement.

7. Prepared this report summarizing our findings, conclusions and recommendations.

FINDINGS

Surface Conditions

The site is on the west side of midland Road. The ground surface slopes down to the west with moderate to steep gradients. Near the east end, the ground surface slopes down from Midland Road with very steep gradients (about 1.3horizontal:lvertical) for a distance of about 23 feet to the toe of slope. From thence the ground surface is almost level westward for a distance of some 20. There is a grade change near the middle area of the site where there is a mound of old fill. An steep concrete driveway (about 21 degrees inclination) is located at the north end of the property. This driveway presently serves as the access from Midland Road to the property. A masonry retaining wall (about 10 feet high from its low end) separates the western property line from the adjacent property to the west. The slope is relatively smooth and uniform with no irregular features suggestive of slope instability at the site. Vegetation at the site include short grass and a few fruit trees.

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Subsurface

The descriptions given below pertain only to the subsurface conditions found at the site at the time of our subsurface exploration on December 17, and 19, 2003. Subsurface conditions, particularly ground water levels and the consistency of the near-surfac~ soils, will vary with the seasons.

Site investigation was done by excavating two exploration trenches to a maximum depth of 24 feet below the ground surface. A rubber-tire backhoe (CAT Tractor backhoe) with a 24-inch bucket was used to do the field investigation.

Within the depth of exploration, the exploratory trenches encountered fill, topsoil and bedrock. The fill encountered in the exploratory trench is estimated to be about two to three feet in thickness.

No ground water was encountered in any of the exploratory trenches. Full descriptions of the materials encountered are appended geology report by HYDRO-GEO CONSULTANTS.

futlsmic Considerations

This site is located within the seismically active San Francisco Bay region but outside any of the Alquist-Priolo Earthquake Fault Zones (see the Hydro-Geo Consultants report)

Type A and Type B faults close to the site are listed in the following table.

Fault Type Maximum Moment Slip Rate Distance

Magnitude (mm/yr) (miles/km)

San Andreas (1906 Segment) A 7.9 24 18.0/29

Hayward (Total Length) A 7.1 9 0.22/0.36

Calaveras (North of Calaveras B 6.8 6 8.0/13

Reservoir)

Concord-Green Valley B 6.9 6 11.8/19

* California Division Of Mines & Geology Open File Report 96-08

Seismic hazards can be divided into two general categories, hazards due to ground rupture and hazards due to ground shaking. Since no active faults are known to cross this property, the risk of earthquake-induced ground rupture occurring across the project site appears to be remote.

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Should a major earthquake with an epicentral location close to the site occur, ground shaking at site will undoubtedly be severe, as will be for other properties in the general vicinity of the site. Even under the influence of severe ground shaking, the soils that underlie the area proposed for the development are unlikely to liquefy.

The following general site seismic parameters may be used for design in accordance with the 1997 Uniform Building Code.

Seismic Zone Soil Type Seismic Source Type

DISCUSSION

4 Sc: Soft Rock Type A; Hayward Fault, 0.36km; Type B; Calaveras Fault, 13 km Na= 1.5 Nv= 2.0

The County of Alameda has designated the site the immediate area of the site to be within a special earthquake fault zone. A fault/geologic investigation at the property was done to determine whether a strand of the mapped active Hayward Fault traverses the subject property and to satisfy the requirements of the County of Alameda Public Works. The results of the geologic/fault investigation are appended. No strand of the mapped active Hayward Fault was found at the property.

An evaluation of the existing slope indicates that slope the slope is stable under existing conditions. Site grading should be done so as not to undermine the slope. As with all hillside development, poor grading practices and the lack of adequate drainage to collect both surface and subsurface water to suitable collection and discharge facilities can adversely affect slope stability in general. Therefore, proper and adequate drainage (surface and subsurface) system should be incorporated into the planned residential development. Runoff collected from roof drains and area drains as well as discharge from subdrains should not be released on portions of the slope that could be the cause of instability or erosion. Appropriate discharge locations should be provided during site grading.

If planned development will be done in the area where existing is located, we recommend that the fill be removed and replaced as structural fill. The proposed building should be supported on cast-in-place reinforced concrete pier and grade beam system. Details are provided in a section below.

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RECOMMENDATIONS

The following recommendations, which are presented as guidelines to be used by project planners and designers, have been prepared assuming FRIAR ASSOCIATES, IN CORPORA TED will be commissioned to review the grading and foundation plans prior to construction, and to observe and test during site grading and foundation construction. This 'additional opportunity to inspect the project site will allow us to compare subsurface conditions exposed during construction with those that were observed during this investigation.

Site Preparation, Grading and Compaction

Areas of the site that will be built on or paved should be stripped to remove surface vegetation and organics. Soils containing more than two percent by weight of organic matter should be considered organic.

The fill soil in areas that will be paved or built on should be excavated to expose the native grmmd surface. Old utility lines and buried pipes such as, leach lines, sanitary sewers and storm drains that exist at the property should be rerouted away from the proposed building and pavement sites, or dug out and hauled off-site. The resulting voids and cavities should be brought to grade with structural fill.

Trees and shrubs that will be designated on the project plans for removal should be felled and their roots system should be grubbed. The resulting depressions from these operations should be backfilled with structural fill.

Soil surfaces exposed by excavations of loose fills and stripping and removal of surface vegetation should be scarified to a depth of eight inches, conditioned with water (or allowed to dry, as necessary) to produce a soil water content of about two percent above the optimum water content and then compacted to 90 percent relative compaction based on ASTM Test D1557-91.

Structural fill may then be placed up to design grades in the proposed building and pavement areas. Structural fill using on-site inorganic soil, or approved import, should be placed in layers, each not exceeding eight inches thick (before compaction), conditioned with water (or allowed to dry, as necessary) to produce a soil water content of about two percent above the optimum water content and then compacted to 90 percent relative compaction based of ASTM Test D1557-91. The upper eight inches of pavement subgrades should be compacted to about 95 percent relative compaction based on ASTM Test Dl557-91.

Structural fill placed on sloping ground should be keyed in accordance with the CAL TRANS STANDARD SPECIFICATIONS, latest edition. The following excerpt from subsection 19-6.01 of those specifications is pertinent:

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"When embankment is to be made and compacted on hillsides .... the slopes of original hillsides .... shall be cut into a minimum of 6 feet horizontally as the work is brought up in layers. Material thus cut out shall be compacted along with the new embankment material.. ... "

The toe key for structural fill placed on sloping ground should be at least 8 feet wide with its base horizontal or gently sloping back into the hillside.

Permanent cut and fill slopes should be constructed no steeper than 2: 1 (horizontal to vertical).

On-site soils proposed for use as structural fill should be inorganic, free from deleterious materials, and should contain no more than 15% by weight of rocks larger than three inches (largest dimension) and no rocks larger than six inches. The suitability of existing fill soil for reuse as a structural fill should be determined by a member of our staff at the time of grading. We expect that most of the existing fill soil will be suitable for reuse as structural fill. If import is required for use as structural fill, it should be inorganic, should preferably have a low expansion potential and should be free from clods or rocks larger than four inches in largest dimension. Prior to delivery to the site, proposed import should be tested in our laboratory to verify its suitability for use as structural fill and, if found to be suitable, further tested to estimate the water content and density at which it should be placed.

Building FoundatiQ.ns

We recommend that the proposed structures be supported on reinforced concrete "pier and beam" foundations with the piers deriving their vertical support from "skin friction" or adhesion between the shaft of the pier and the surrounding competent soil/bedrock material. The piers should be a minimum of 16 inches in diameter and should penetrate at least eight feet into the bedrock material.

Piers should be spaced at least three diameters apart (center to center) but no more than 10 feet apart. The allowable load-carrying capacity (dead plus normal live loads) of each pier may be calculated assuming "skin friction" or adhesion of 500 psf between the shaft of the pier and the adjacent competent material, but ignoring the upper two feet of embedment of the pier below the lowest adjacent grade. No adhesion should be assumed in any fill. The piers should be designed by the project structural engineer based on soil parameters given above but actual depths should be determined in the field based on soil conditions during foundation construction.

Reinforced concrete piers should be designed to resist lateral loads resulting from potential creep of the near-surficial layer of loose soil and or fill in areas where the ground surface gradient is 5horizontal: 1 vertical or steeper. A lateral soil pressure of at least 55 pounds per cubic foot may be assumed to act on the top three feet over 2 Y2 diameters of the piers. The allowable lateral

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bearing pressure of the ground in front of the piers may be taken as 300 pounds per square foot (psf) per foot of depth to a maximum value of 3000 psf in the weathered bedrock material.

The allowable foundation pressures given previously may be increased by one-third when considering additional short-term wind or seismic loading.

Concrete Slabs-On-Grade

Concrete floor slabs should be constructed on compacted soil subgrades prepared as described in the section on Site Preparation, Grading and Compaction.

To minimize floor dampness, a section of capillary break material at least five inches thick and covered with a membrane vapor barrier should be placed between the floor slab and the compacted soil subgrade. The capillary break should be a free-draining material, such as 3/8" pea gravel or a permeable aggregate complying with CAL TRANS Standard Specifications, Section 68, Class 1, Type A or Type B. The material proposed for use as a capillary break should be tested in our laboratory to verify its effectiveness as a capillary break. The membrane vapor barrier should be a high quality membrane. A protective cushion of sand or capillary break material at least two inches thick should be placed between the membrane vapor barrier and the floor slab.

If floor dampness is not objectionable, concrete slabs may be constructed directly on a minimum six-inch thick compacted aggregate base over the water-conditioned and compacted soil subgrade. The aggregate base material should be compacted to at least 93 percent relative compaction (ASTM D1557-91).

Retaining Walls

Retaining walls that will be part of proposed building should be supported on piers that should be designed to resist sliding, overturning as well as vertical loading. Retaining walls located outside of the proposed building may be supported on shallow footing-type foundations bearing on competent native undisturbed soil or compacted structural fill.

The following may be used in the design calculations for reinforced concrete retaining walls. In addition to the parameters below, retaining walls should be designed to withstand lateral loads resulting from all anticipated surcharge loads that will be located adjacent to the walls. An additional lateral design load equal to 33 percent of the adjacent vertical surcharge loading may be assumed for the design of the walls.

1. The average bulk density of material placed on the backfill side of the wall will be 120

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pcf.

2. The vertical plane extending down from the ground surface to the bottom of the heel of the wall will be subject to pressure that increases linearly with depth as follows.

Condition Slope Behind Wall (degrees) Design Pressure

Active, drained 0 40pcf

Active, drained 3H:1V 45 pcf

Active, drained 2H:1V 55 pcf

The above values are for non-seismic conditions.

3. The effects of earthquakes may be simulated by applying a horizontal line load surcharge to the stem of the wall at a rate of27H2 lb/horizontal foot of wall, where His the height of the surface of the backfill above the base of the wall. This surcharge should be applied at a height of 0.6H above the base of the wall.

4. A coefficient of "friction" of 0.35 may be used to calculate the ultimate resistance to horizontal sliding of the wall base over the ground beneath the base.

5. An equivalent fluid pressure of 350 psf/ft may be used to calculate the ultimate passive resistance to lateral movement of the ground in front of the toe of the wall and in front of any "key" beneath the toe or stem of the wall. If both friction and passive resistance are combined to resist lateral loading, the lower value should be reduced by 50 percent.

6. 2500 psf may be used as the maximum allowable bearing pressure for the ground beneath the toe of the wall. This value is for non-seismic conditions and may be increased to 3325 psfwhen considering additional loads on the wall resulting from earthquakes.

A zone of drainage material at least 12 inches wide should be plac.ed on the backfill side of walls designed for drained condition. This zone should extend up the back of the wall to about 18 inches down from the proposed ground surface above. The upper 12 inches or so of material above the drainage material should consist of native, clayey soil.

The drainage material and the clayey soil cap should be placed in layers about six inches thick and moderately compacted by hand-operated equipment to eliminate voids and to minimize post-construction settlement. Heavy compaction should not be applied; otherwise, the design

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pressure on the wall may be exceeded.

The drainage material should consist of either Class 2 Permeable Material complying with Section 68 of the CAL TRANS Standard Specifications, latest edition, or 3/4 to 1 Y:z inch clean, durable coarse aggregate. If the coarse aggregate is chosen as the drainage material, it should be separated from all adjacent soil by a filter fabric approved by the project Engineer.

Any water that may accumulate in the drainage material should be collected and discharged by a four-inch-diameter, perforated pipe placed "holes down" near the bottom of the drainage material. The perforated pipe should have holes no larger that 1/4-inch diameter.

Utility Trenches

The attention of contractors, particularly the underground contractor, should be drawn to the requirements of California Code of Regulations, Title 8, Construction Code Section 1540 regarding Safety Orders for "Excavations, Trenches, Earthwork".

For purposes of this section of the report, bedding is defined as material placed in a trench up to one foot above a utility pipe and backfill is all material placed in the trench above the bedding.

Unless concrete bedding is required around utility pipes, free-draining sand should be used as bedding. Sand proposed for use in bedding should be tested in our laboratory to verify its suitability and to measure its compaction characteristics. Sand bedding should be compacted by mechanical means to achieve at least 90 percent compaction density based on ASTM Tests 01557-91.

Approved, on-site, inorganic soil, or imported material may be used as utility trench backfill. Proper compaction of trench backfill will be necessary under and adjacent to structural fill, building foundations, concrete slabs and vehicle pavements. In these areas, backfill should be conditioned with water (or allowed to dry) to produce a soil-water content of about five percent above the optimum value and placed in horizontal layers not exceeding six inches in thickness (before compaction). Each layer should be compacted to 85-90 percent relative compaction based of ASTM Test 01557-91. The upper eight inches of pavement subgrades should be compacted to about 90 percent relative compaction based on ASTM Test 01557-91.

Where any trench crosses the perimeter foundation line of any building, the trench should be completely plugged and sealed with compacted clay soil for a horizontal distance of at least two feet on either side of the foundation.

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Surface Drainage

Surface drainage gradients should be planned to prevent ponding and to promote drainage of surface water away from top of slopes, building foundations, slabs, edges of pavements and sidewalks, and towards suitable collection and discharge facilities. Thy proposed building should be provided with downspouts that should be connected to non-perforated pipes. The nonperforated pipes should discharge to suitable drainage facilities located away from the proposed building site.

To minimize the potential for erosion of surface soils that could be caused by surface water runnoff, provisions should be made to collect and control surface runoff. Paved ditches with catch basins are recommended on the backfill side of all retaining walls. Water collected in these catch basins should be conveyed by pipes to suitable discharge points downslope and away from critical areas of the project site.

Water seepage or the spread of extensive root systems into the soil subgrades of foundations, slabs, or pavements, could cause differential movements and consequent distress in these structural elements. This potential risk should be given due consideration in the design and construction of landscaping.

Follow-up Geotechnical Services

Our recommendations are based on the assumption that FRIAR ASSOCIATES, IN CORPORA TED will be commissioned to perform the following services.

1. Review final grading and foundation plans prior to construction.

2. Observe, test and advise during grading, excavation and placement of structural fill.

3. Test proposed capillary break material that will be used beneath concrete slabs-on-grade and advise on suitability.

4. Observe and advise during foundation and retaining walls construction.

6. Observe, test and advise during utility trench backfilling.

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LIMITATIONS

The reconunendations contained in this report are based on certain plans, information and data that have been provided to us. Any change in those plans, information and data will render our reconunendations invalid unless we are commissioned to review the change and to make any necessary modifications and/or additions to our reconunendations.

Subsurface exploration of any site is necessarily confined to selected locations. Conditions may, and often do, vary between and around such locations. Should conditions different from those encountered in our explorations come to light during project development, additional exploration, testing and analysis may be necessary; changes in project design and construction may also be necessary.

Our reconunendations have been made in accordance with the principles and practices generally employed by the geotechnical engineering profession. This is in lieu of all other warranties, express or implied.

Should conditions different from those assumed in this report come to light during project development, additional exploration, testing and analysis may be necessary; changes in project design and construction may also be necessary.

All earthwork and associated construction should be observed by our field representative, and tested where necessary, to compare the generalized site conditions assumed in this report with those found at the site at the time of construction, and to verify that construction complies with the intent of our recommendations.

Report prepared by:

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Base Map Source: 2003 The Thomas Guide, Bay Area Metro; Scale: 1"= 1900'

FRIAR ASSOCIATES, INCORPORATED

Jan 2004

VICINITY MAP

PROPOSED 2-LOT RESIDENTIAL DEV. 14625 AND 14635 MIDLAND ROAD ALAMEDA COUNTY, CALIFORNIA

FIGURE 1

PROJECT 1351

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Base Map Source: Cistran Group; Scale: 1"=20'

FRIAR ASSOCIATES, INCORPORATED

Jan 2004

SITE PLAN AND APPROXIMATE LOCATION OF EXPLORATORY TRENCHES

PROPOSED 2-LOT RESIDENTIAL DEV. 14625AND14635 MIDLAND ROAD ALAMEDA COUNTY, CALIFORNIA

FIGURE 2

PROJECT 1351

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APPENDIX A

HYDRO-GEO CONSULTANTS GEOLOGIC INVESTIGATION REPORT

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GEOLOGIC ASSESSMENT OF ON-SITE FAULT POTENTIAL PROPOSED TWO-LOT RESIDENTIAL DEVELOPMENT

14625 and 14635 MIDLAND ROAD ALAMEDA COUNTY, CALIFORNIA

Project 5389-H

Pr~pared for

Friar Associates, Inc. 2656 Nicholson Street San Leandro, California 94577

By

HYDRO-GEO CONSULTANTS, INC. P.O. Box 4353 Mountain View, California 94040

JANUARY, 2004

P.O. Box 4353, Mountain View, CA 94040-4353

HYDRO-GEO c9~JlYa~A~~5i~~·

Project 5389-H

Friar Associates, Inc. 2656 Nicholson Street San Leandro, CA 94577

Attention: Mr. John Friar

Subject: Geological Assessment of On-Site Fault Potential Proposed Two-Lot Residential Development 14625 and 14635 Midland Road Alameda County, California

(650) 428-0588 • FAX (650) 428-0589

Presented herein is our assessment of the existing geologic conditions at the captioned site that are based on a review of the available geotechnical data listed in the references at the end of this report, observations from a site reconnaissance and information extrapolated from two exploratory trenches. You on verbally authorized this assessment December 18, 2003, acting as the agent for the property owner, in accordance with our "Terms and Conditions of Agreement."

A two-lot subdivision is proposed for this currently undeveloped, rectangular (~0.34 acre) parcel located in Alameda County; see Plate 1, Earthquake Seismic Zones Map. Each lot will have a 50 foot frontage on Midland Road. A singlefamily residence is to be constructed on each of these lots .

. GEOLOGIC CONDITIONS

The subject property has been roughly graded resulting in a near vertical, 10-foot high cut adjacent to the easterly property line. A concrete driveway slopes downward into the existing parcel at approximately 21 degrees from Midland Road, as shown on Plate 2, Plot Plan. Non-engineered fill, several feet thick, has been placed on the western half of the site.

Robinson, in his 1956 map of the Hayward Quadrangle, shows the subject property as being underlain by gabbro and serpentine that were intruded, sill-like,

Friar Associates, Inc. January 13, 2004

Project 53 89-H Page 2

roughly parallel to the regional strike, into the Upper Jurassic rocks in this area. His map indicates the Hayward Fault trends near the subject property as a normal fault with the ground on the west side of the fault dropping downward. Subsequent geologic mapping by Radbruch-Hall (1974) and Dibblee (1980) showed this strand as "doubtful" and mapped other strands of this active fault zone east of the

·site, as shown on Plate 3, Regional Geologic Map.

In 1978, Alameda County authorized Woodward-Clyde Consultants to investigate these eastern strands of the Hayward Fault that pass through the Fairmont Hospital and Juvenile Hall in this area; see Plate 1. Their detailed investigation, which included exploratory trenching and borings, identified two main strands of the active Hayward passing through these facilities .. The western strand of this identified active fault zone is well defined within a 10 to 25 foot wide zone. This is the area where the ground ruptured during the 1868 earthquake with offsets of 3 feet horizontally and 1 foot vertically, and on-going tectonic creep is occurring along this portion of the Hayward Fault "Surface rupture beyond the relatively narrow fault zones defined by the west~rn and eastern traces is considered unlikely and would represent only a small fraction of the displacement that would be expected along the main fault traces; the amount of displacement would be expected to decrease with distance from the mainfault trace" (Taylor, et al, 1982).

In view of the above findings, the California Division of Mines and Geology reviewed this segment of the active Hayward fault zone iri 1981, and

·recommended extensive revisions under the Alquist-Priolo Special Studies Act, including deletion of the questionable branch fault in the vicinity of the subject property (Hart, 1981). The current location of the active traces of the Hayward Fault in this area, designated by the California Geological Survey, is shown within the special studies boundary lines shown on Plate 1.

There are no known landslides at this site and the potential for slope instability in this area is considered to be moderate site (Nilsen, 197 5).

HYDRO-GEO CONSULTANTS, INC.

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Project 5389-H Page 3

EXPLORATORY TRENCH AND SUBSURFACE CONDITIONS

Two exploratory trenches, 130 and 25 feet in length, were excavated to a depth of 3 to 24 feet on December 17 and 19, 2003, using a backhoe with a 24-inch bucket, at the location shown on Plate 2. An engineering geologist logged the trenches, and the data is presented on Plates 4A through 4C.

The exploratory trenches, which were excavated perpendicular to the regional trend of the active Hayward Fault Zone, which is north 40 degrees west. Enstatite- rich serpentine (bastite) was encountered in the exploratory trenches overlain by a zone of iron and manganese stained serpentine and serpentine (antigorite). Approximately 3 feet of caliche-cemented serpentine, 2.5 feet of topsoil and up to 2 feet of artificial fill overly the serpentine in the western portion of the site. We did not encounter any asbestos containing serpentine ( chrysotile) or free water in the exploratory . trenches to the depths excavated.

Changes in the site's subsurface material may occur during the passage of time, due to natural processes or the works of man. Therefore, the exploratory trench log shows our interpretation of the subsurface conditions on the date and at the location indicated. It is not warranted that it is representative of subsurface conditions at other locations or times.

FINDINGS AND RECOMMENDATIONS

The confirmed main trace of active the Hayward Fault Zone is located approximately 1,200 feet northeast of the subject property; it is capable of generating a 6.9 Magnitude earthquake, with maximum credible bedrock accelerations on the order of 0.6 gravity (Petersen, et al, 1996; Mualchin and Jones, 1992). Major earthquakes occurred along this fault zone in 1836 and 1868; both of these earthquakes had an estimated intensity of X on a

. maximum scale of XII. The 1868 earthquake is known to have ruptured the ground surface in this area.


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Frair Associates, Inc. January 13, 2004

Project 5389-H Page4

Topographic lineations or other indications of active faulting in the vicinity of the site were not observed on the aerial photographs. We also did not observe any gouge zones, groundwater barriers, offsets, or significant shearing of the rocks exposed in the exploratory trenches that would be indicative of active faulting. Therefore, it is our opinion that an active strand of the Hayward Fault Zone does not pass through the captioned site.

Seismically induced ground failure from liquefaction is minimal at the proposed building sites because of the nature of the overlying soil and the presence of .bedrock near the ground surface.

The potential for landsliding at this site is low. However, site grading and the oversteepening of cut slopes could cause minor slumping of the unstable surface material and possible damage to Midland Road.

The proposed two residences are to be designed in accordance with the applicable provisions set forth in the current edition of the Uniform Building Code (UBC). Design of the proposed structures should consider the potential for severe ground shaking that could result from the maximum probable earthquake generated by the active San Andreas Fault at the captioned site. The structural engineer is to design the proposed residences in accordance With Chapter 16 of the 1997 Uniform Building Code for a Type "A" Seismic Source, with an "Sc" Soil Profile and a Seismic Zone Factor (z) of 0.4, using the following Near Source Factors: Na= 1.5 and Nv= 2.0.

LIMITATIONS

This report is intended for the exclusive use of our client. Any use or reliance of this evaluation, or any of the information herein, by a third party shall be at such party's sole risk.

Our services consist of professional opinions and recommendations undertaken within the agreed to requirements and limitations stated in our "Terms and Conditions of Agreement." Our opinions are made in


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accordance with generally accepted engineering geology principles and ·practices and are limited to the specific scope of this investigation. This warranty is in lieu of all other warranties, either expressed or implied.

It should be understood that geology is not an exact science, and a complete determination of the geotechnical risk at this site cannot be made without more detailed investigations by our firm, extending beyond the boundaries of the subject property.

The opinions and recommendations presented herein apply only to on-site conditions at the time of our study. They cannot apply to conditions or changes of which we are not aware nor.have had an opportunity to review and evaluate. Grading, modification of drainage or other conditions created by developments on this and adjacent properties may result in slope instability or other adverse conditions at the subject site.

As with most of the San Francisco Bay Area, the subject property is in a region of high risk of seismic activity. Because of the site's close proximity to the active Hayward Fault Zone, very strong ground shaking is to be expected at the subject

·site during a major earthquake along this active fault zone. Such strong ground shaking can cause ground failure and structural damage. However, damage can be mitigated by the use of good construction techniques and by building according to the standards established in the current Uniform Building Code for this seismically active area.

The findings of this report are valid at this present time. However, the passing of time may change the conditions of the existing property, due to natural processes or the works of man. In addition, legislation or the broadening of knowledge may require modifications or further recommendations. Accordingly, the finpings of this report may be invalidated, wholly or in part, by changes beyond our control. Therefore, this report should not be relied upon after a period of three years without the review of an engineering geologist.


.Friar Associates, Inc. January 13, 2004

CLOSURE


We have enjoyed working with you on this project. If you have any questions, please do not hesitate to call our office. The following list of references and plates are attached and complete this report.

Plates: 1. Earthquake Fault Zones Map 2. Plot Plan 3. Regional Geologic Map 4A. Log of Exploratory Trench ET-1 4B. Log of Exploratory Trench ET-2 4C. Explanation For Exploratory Trench Logs

Ve1)' truly yours, HYDRO-GEO CONSULT ANTS, INC.

JO 'R:jod 5 copies submitted

JLQ_TO~ Yo~ ;_~~'Rourke Engineering Geologist 419



REFERNCES


Cistran Group, 2003, Topographic Map APN # 79-4-11-1, Owner Steve Sferdian, 1465 Midland, Oakland, CA 94678: Unpublished map (Project No. 031032-G.

Davis, J.L., 1982, Earthquake Fault Zones, Hayward 7.5' Quadrangle, California {Alquist-Priolo Special Studies Zones Act): California Geological Survey.

Dibblee, Jr., T.W., 1980, Preliminary Geologic Map of the Hayward Quadrangle, Alameda and Contra Costa Counties, California: U.S. Geological Survey, OpenFile Report 80-540.

Hart, E.W., 1981, Smmnary Report: Fault Evaluation Program, 1979-1980 Area -South San Francisco Bay Region [And Other Areas]; Alameda, Contra Costa, Mendocino, Mono, San Benito, San Francisco, San Mateo, Santa Clara, and Santa Cruz Counties, California: California Division of Mines and Geology, Open-File Report 81-3.

Herd, D., 1978, Map of Quaternary Faulting Along the Northern Hayward Fault Zone: U.S. Geological Survey, Open-File Report 78-308 (Sheet 7 of 8).

International Conference of Building Officials, 1997, Division IV - Earthquake Design: Uniform Building Code, Volume 2 .

. Lienkaemper, J.J., 1992, Map of Recently Active Traces of the Hayward Fault, Alameda and Contra Costa Counties, California: U.S. Geological Survey

Mualchin, L. and Jones, A.L., 1992, Peak Acceleration From Maximum Credible Earthquakes in California (Rock and Stiff-Soil Sites): California Division of Mines and Geology, Open-File Report 92-1.

Nilsen, T.H., 1975, Preliminary Photointerpretation Map of Landslide and Other Surficial Deposits of the Hayward 7.5' Quadrangle, Alameda County, California: U.S. Geological Survey, Open File Map 75-277-19.

Petersen, M.D., et al, 1996, Probabilistic Seismic Hazard Assessment for the State of California: U.S. Geological Survey, Open-File Report 96-706.


SOURCE: Davis, 1982

EARTHQUAKE FAULT ZONES MAP SCALE: 1" == ~ 1,500'

PLATEl HYDRO-GEO CONSULTANTS, INC.

SOURCE· C' . . · 1stran Group, 2003

\ \

fLOTPLAN -\

PLATE2

"' ,·

REGIONAL GEOLOLGIC MAP SCALE: l" == 2,000'

EXPLANATION: Qa Alluvium rh Rhyolite K- Panoche Formation (s, sandstone; c, conglomerate; p, clay shale) JKK Knoxville Formation sandstone & shale 1:. Franciscan Assemblage (s, sandstone; g, greenstone; sp, serpentine)

-------:--- Contact between geologic units ~.Y Strike and dip of beds

--? Fault (dashed where inferred; queried where existence doubtful)

SOURCE: Dibblee, Jr., 1980

PLATE3


GROUND SURFACE~

01

5

10

15

/

/ /

/ ./

.. 20·.....------.-----·--,---

0 5 10

/

15

9 / /

/ /

2

/ /

7

. ... CALICH_.S, .,..

,.,.., .....

-·-------·--, 20 25

LOG OF EXPLORATORY TRENCH ET-2 HORIZONTAL AND VER TIC AL SCALE IN FEET

PLATE4B

SEE PLATE 4C FOR EXPLANATION


'i i I

'

EXPLANATION FOR EXPLORATORY TRENCH LOGS

1. Dusky yellow (5GY5/2) to grayish olive green (5GY3/2) serpentine (bastite) with an abundance of altered enstatite, highly weathered, fractured, manganese stained, some minor caliche; this rock is hard, however, it readily breaks into small piece. --- ----- ----

2. Pale yellow (5Y7/3) serpentine, highly weathered with caliche, friable; soft.

3. Light gray (5Y7/1) limestone, fractured; hard.

4. Dusky yellow green (505/2) to grayish yellow green (5GY7/2) serpentine ( antigorite) with manganese staining, weathered and fractured; hard.

5. Moderate olive brown (5Y4/4) serpentine (antigorite) highly sheared, manganese stained, readily breaks along internal shear planes; hard.

6. Sandy to silty clay fill with small rock fragments and some organic material (Artificial Fill).

7. Mottled yellowish brown (10YR5/6) to very dark greenish brown (10YR3/2) serpentine, highly weathered and fractured, cemented with iron and manganese, minor caliche; hard.

8. Light gray (2.5Y7/2) to pale brown (10YR6/3) caliche-cemented, angular serpentine fragments; hard, friable.

9. Black (N2) to dark brown (7.5YR4/2; in the lower 6 inches) silty clay (CH)** with minor caliche and organics, plastic, subject to shrink-swell conditions; stiff ("Topsoil").

* Munsell Soil Color Chart Number ** Unified Soil Classification System Designation Exploratory trenches excavated on December 17 and 19, 2003. Excavated with a backhoe with an extend-a-hoe and 24" bucket Free water encountered was not encountered to the depth excavated Trench backfilled was compacted.

PLATE4C


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