Phone - Californiagmw.consrv.ca.gov/SHP/APSI_SiteInvestigation... · 6/6/1990 · -i-r"state of...

~ •· / / -i-r"STATE Of CALIFORNIA ~.THE RESOURCES AGENCY c PETE ~ILSON, Governor

DEPARTMENT Of CONSERVATION DIVISION OF MINES AND GEOLOGY BAY AREA REGIONAL OFFICE 185 Berry Street, Suite 3600 San Francisco, CA 94107 Phone C415l 904-7707

ATSS 539·7707 Fax (415) 904-7715

James O .. Berkland County Geologist

October 18, 1993

Santa Clara County Planning Dept. Government Center, East Wing, 4th Floor 70 West Hedding Street San Jose, CA 95110

Dear Jim:

This is to acknowledge the numerous reports you loaned us from the 1989 earthquake damage file. We copied 11 of these reports for our AP file (marked "AP" on attached list). In addition, 20 reports for sites outside the sszs were considered to be useful because of fault or ground-fissuring data and were copied for our informal consulting file (marked with a "C" on the attached list). The remaining reports from this set were not copied. All of the original reports from the damage file were returned to you by Perry Wong on 10/13/93.

The set of duplicate reports provided to Perry about two weeks ago have not yet been processed. We will provide you with a list of the reports filed when this work is finished. ·

Thanks for making these reports available to us.

EWH:ra Enclosed list/ cc: A-P file v

Sincerely,

f£c!,f-EARL W. HART Senior Geologist &

Program Manager

r '-·

OCTOBER 1989 EARTHQUAKE

DAMAGE INSPECTIONS

LOS GATOS & SANTA CRUZ MOUNTAINS

ADDRESS

Mountain Way 19221 LG

Mountain Way 19267 LG

Mountain Charlie Rd. LG

DAMAGE DESCRIPTION

no visible damage externally - VIOLATION FILE

chimney down; minor

house burned to the ground; severe

Mountain Charlie Rd. 17515 LG fireplace down; minor

Mountain Charlie 17615 LG structure & chimney damage; moderate

Mountain Charlie 17950 LG cracks in adobe veneer; minor

Mountain Charlie 17955 LG house damaged; flp & ·chimney down; severe (from mod

Mountain Charlie 17959 LG fndn cracks; half of fpl down; geologic probs.; moderate

Mountain View 18096 LG broken cabinets, flooring; minor

Mountain View 18113 LG minor damage

Mountain View 18121 LG carport collapsed; chimney down; ret wall damaged; moderate

Mountain View Ct.18151 LG house collapsed; severe

Mountain View Ct 18176 LG stairs damaged; !pier missing; minor --- -....

<!~~ntain View Ct 18180 LG\ . chimney & deck damage; posts leaning; moderate -- -Mountain View Ct.-last home LG house damaged, severe

M~zelle 17876 LG moderate damage

Mozelle 17897 LG cracks in fnd, flp, settlement; moderate

Mozelle 17898 LG fndn, roof damage; moderate

Mozelle 17903 LG slight damage; minor

Mozelle Ct. 17905 LG deck, sheetrock damaged; moderate

Mozelle 17906 LG walls buckling; severe

Mozelle 17909 LG garage collapsed; moderate - VIOLATION FILE

16

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Ms. Betty Farley P. 0. Box569

IJCPI GEOLOGISTS•ENGINEERS

10650 Bubb Road • Cupertino, CA 95014 • (408) 446-4427

FAX (408) 446-0821

June 6, 1990

Project No. JCP-2941

Redwood Estates, CA 95044

Re: Letter Geologic and Soil & Foundation Services for Earthquake Damaged Property at 18180 Mountain View Court, Redwood Estates, Santa Clara County, California APN: 544-46-060

J. Cear Nls . . = a.;iey:

-- . '·

In accordance with your authorization of April 2, 1990, we have performed a Geologic and Soil & Foundation Study for the subject property. The site consists of one parcel of land located in the Redwood Estates area of Santa Clara County, California. The subject structure and property was damaged during the October 17, 1989 Loma Prieta Earthquake. The accompanying report presents the results of our

· field studies, exploration, laboratory tests, office studies and related analyses. The geologic, seismic and soil foundation conditions are discussed and appropriate conclusions and recommendations are presented. An Executive Summary of this report follows this letter for review purposes.

Based on our studies as described and detailed herein, we are of the opinion that . the site is suitable for the proposed repairs or reconstruction if the recommendations contained in this report are adhered to. All conclusions and recommendations presented in this report are contingent upon JCP-Engineers & Geologists, Inc. being retained to review the building location, foundation plans and any grading plans prior to construction. In addition, we should observe and test any grading (earthwork) operations and observe all foundation excavations at the site. Most jurisdictions now mandate inspection and review letters to document the construction process. It is the owners responsibility to schedule inspections for documentation purposes.

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We refer you to the text of this report for a detailed discussion of our findings. If you have any questions concerning this report, please telephone.

INC.

CW/JCP/rb

Copies: Addressee (7)

I

LETTER GEOLOGIC AND SOIL & FOUNDATION STUD.Y

FOR:

EARTHQUAKE DAMAGED PROPERTY AT

18180 MOUNTAIN VIEW COURT, REDWOOD

ESTATES, SANTA CLARA COUNTY, CALIFORNIA

TO:

MS. BETTY FARLEY

P. 0. BOX569

REDWOOD ESTATES, CALIFORNIA 95044

JUNE 1990

TABLE OF CONTENTS

EXECUTIVE SUMMARY ......•......................................................... 1

INTRODUCTION .............................................................................. 2

PROPOSED CONSTRUCTION ....... : ................................... .' ......... 2

SCOPE OF WORK ........................................................................... 2

SITE STUDIES AND EXPLORATION .......................................... 2

OFFICE STUDIES ........................................................................... 3

SITE DESCRIPTION ....................................................................... 3

Surface Conditions .................................................................... 3

Subsurface Conditions ............................................................. 4

ENGINEERING GEOLOGY .•.......................................................... 4

SITE SEISMICITY DISCUSSION ................................................. 5

Seismic Magnitude .................................................................... 6 Seismic Acceleration ................................................................ 6

Calculation Methods Used ....................................................... 6

C.JNCLUSIC1\iS AND nC:CCilliiv1ENDATIONS .......................... 8

Causation of Distress ................................................................ 8 Geologic Conclusion ................................................................. 8 Soil and Foundation Conclusions .......................................... 9

SITE PREPARATION AND EARTHWORK. .................................. 10

DRAINAGE PROVISIONS .............................................................. 11

UNDERPINNING PIER FOUNDATIONS ..................................... 11

PIER FOUNDATIONS ..................................................................... 13

SLABS-ON-GRADE ........................................................................ 15

RETAINING WALLS ........................................................................ 16

LATERAL LOADS ............................................................................ 17

LIMITATIONS .................................................................................... 1 8

BIBLIOGRAPHY

·~

l •

TABLE OF CONTENTS Continued

LIST OF FIGURES

Figure 1. Site Location Map

Figure 2. Area Geology Map ·

Figure 3. Area Surficial Deposits Map

Figure 4. Site Plan and Geologic Map

Figure 5. Cross Section

Figure 6. Seismic Acceleration Table

APPENDICES

A. Exploration Appendix·

6. S2ismic Appendix

C. Drainage Appendix

I i ! .

EXECUTIVE SUMMARY

Page 1 Project No. JCP-2941

The subject property consists of a steeply sloping site with an existing single-family, wood-frame structure located within the Redwood Estates area of Santa Clara County. The existing structure is approximately 60 years old. Appurtenant structures consisting of two retaining walls and an exterior wood staircase are also present. A portion of the existing building pad and large portions of the existing driveway and parking area are underlain by questionable quality fill. The existing structure's foundation consists of continuous and isolated footings. The existing structure was significantly damaged when it partially fell off its foundation during the October 17, 1989 Loma Prieta Earthquake.

The steeply sloping site is underlain by relatively thick accumulations of colluvium fill and ancient landslide deposits. Evidence of active, recent or historic landsliding on the subject property was not observed.

It is our opinion that the existing structure may be repaired by underpinning the existing foundation if the existing foundation is found suitable for use as a grade beam by a registered civil engineer. As an alternative, the existing structure or a new structure can be supported using a new pier and grade beam type foundation. The existing damaged and/or distressed retaining walls at the subject property should also be replaced.

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INTRODUCTION In this report, we present the results of our studies performed for the earthquake damaged property located at 18180 Mountain View Court, Redwood Estates, Santa Clara County, California, as shown on the attached Location Map, Figure 1. The purpose of these studies was to:

PRoposep CONSTRUCTION '

SCOPE OF WORK

: SITE STU PIES AN..12 EXPLORATION

. • Evaluate and define any natural limitations (geologic hazards)

that may exist on the site and to recommend appropriate mitigating measures.

• Evaluate the subsurface soil conditions at the site and to provide foundation engineering recommendations for the reconstruction or repair of the existing house.

• Provide geotechnical recommendations for reconstruction of the structure.

In this report, recommendations for either repair of the existing foundation or construction of a new foundation are presented.

It is our understanding that the existing single-family, wood-frame structure will be repaired or reconstructed by the owner.

Earthwork associated with the construction may consist of minor grading. In general, the grading will be kept to a minimum. The proposed building will utilize the existing septic system.

The scope of our work was partially outlined in our proposal letter dated March 23, 1990 and later expanded after telephone conversations with Betty Farley. The purpose of our studies was to evaluate the geologic and geotechnical conditions of the property relative to the proposed reconstruction and to provide appropriate remedial and design recommendations.

A geologic field study of the site and the engineering exploration were performed on April 9 & 13, 1990 by this engineer and geologist. A detailed Geologic Map of the site was prepared and the geology of adjacent parcels was evaluated with respect to the subject parcel.

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·. Page 3 Project No. JCP-2941

Three exploratory borings, with appropriate sampling of soils, were drilled at the subject property and the locations are shown on Figure 4. Details of this exploration are presented in Appendix A.

OFFICE STUDIES Our detailed in-office studies included:

SlliE DESCRIPTION

Surface Conditions

• A detailed review of existing geologic literature • Stereoscopic examination and interpretation of aerial

photographs • Geologic and engineering analyses of the field and laboratory

data • The preparation of this report.

A bibliography of the reference materials used to assist in our office studies is included at the end of this report.

The irregularly shaped subject property is located adjacent to and west of the western end of Mountain View Court in the Redwood Estates area of Santa Clara Countv as shown .,r •hi;> attached ... 0ca11cn Mc.;:, rigure 1. ·j·;~e SUOjec:; ;ircpe11y is :.;.=.re:.c:erized by steep north-facing slopes which have slope inclinations ranging from 1. 75:1 (horizontal to vertical) at the southern portion of the property to approximately 2:1 (horizontal to vertical) at the northern portions of the property.

The site is currently developed with an approximately 60 year old earthquake damaged house. Damage to the house appears to be confined to failure of substandard foundations and related structural deformation of the house. Two retaining walls are located adjacent to the house; one upslope and the other downslope. Both these retaining walls are in a distressed condition. Portions of the wood stair case leading from the driveway area to the front entrance of the house are also in a distressed condition. The property is heavily vegetated with large redwood trees, fur trees and native underibrush. Portions of the existing access driveway and parking area appear to be underlain by fill.

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Subsurface Conditions

ENGINEERING G;EQLOGY


Three exploratory borings were drilled on the property in the vicinity of the proposed reconstruction. The borings, approximately located as shown on Figure 4, were drilled on April 13, 1990 utilizing hand operated augering equipment. The purpose of the borings was to (1} define the types and depths of the subsurface materials in the vicinity of the existing structure, and (2) collect samples for laboratory testing.

Surficial material encountered in the exploratory borings consisted of 3 to 6-1/2 feet of yellowish brown to dark brown, loose to medium dense pebbly, organic rich sand and sandstone rock fragments (topsoil, fill and colluvium). Underlying the surficial material, a tan to tan brown, medium dense to very dense fractured sandstone and sand with minor organics was encountered to the depths explored.

Free groundwater was not encountered in the exploratory borings at the time of our studies, but probably exists at depths significantly greater than those explored to. It must be noted that isolated seepages could occur at relatively shallow depths, particularly after periods of prolonged rainfall. Fluctuations in groundwater conditions may occur due to factors not evident at the time of our studies.

The attached boring and trench logs depict subsurface conditions only at the specific locations shown on the Site Plan and Geologic Map and on the date of the exploration. Subsurface conditions at other loi::ations mav differ somewhat from coricmion~ ~cc,.minq at our c:;; .. t:JiGi'Cii'.c(/ .;ci· .. ·;~ ..;!· ~taric:::,-:.; lc~at1ons.

The area studied lies within the San Francisco Bay Region, which is bordered by the mountain ranges of the Coast Range Geomorphic Province. The site and vicinity has been previously mapped by Dibblee and Brabb (1978) at a scale of 1 :24,000 (1 inch = 2,000 feet) and Rogers (1971) at a scale of 1 :12,000 (1 inch = 1,000 feet). Dibblee and Brabb mapped the site and vicinity as being underlain by sandstone of the Tertiary Vaqueros Formation. Rogers maps the site and vicinity as being underlain by ancient landslide deposits. We interpret the highly fractured sandstone and sand encountered in · our exploratory borings as being representative of ancient landslide debris with Vaqueros Formation sandstone as its source. Evidence of. active, recent or historic landsliding in the immediate vicinity of the subject structure which would affect the proposed reconstruction was not observed.

The subject structure is located on a steep slope. Evidence of soil creep such as bowed tree growth, failing retaining walls and thick colluvial soils exists on the site.

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SITE SEISMICITY DISCUSSION


Evidence of faults traversing the subject property was not observed during our site reconnaissance nor by aerial photo-interpretation. The closest known active fault to the subject property is the 1906 trace of the San Andreas Fault which is located approximately 1 mile northeast of the subject property. The potentially active Butane Fault is located about 4,000 feet southwest of the subject property.

The San Francisco Bay area is recognized by geologists and seismologists as being one of the most active seismic regions in the United States. The active San Andreas, Hayward and Calaveras Faults as well as many potentially active secondary faults are all located within the Bay area. A large earthquake on one of these faults would affect the subject property by causing severe shaking.

During the earthquake of October 17, 1989, "ridge spreading" phenomena consisting of cracks which paralleled bedding in the Santa Cruz Mountains was a common occurrence. Several "ridge spreading" cracks or fissures have been documented to have occurred in the Redwood Estates area during the October 17, 1989 earthquake. The immediate area of the subject property was studied in an effort to detect ridge spreading that could pose a problem for the proposed reconstructed house. One possible ridge spreading crack was observed to traverse Mountain View Court adjacent to the east of the subject property. Evidence that ridge cracks traverse the existinq building oad was not observed.

On the basis of the historical record, the exploration of the subject property and current technology, it is reasonable to assume that the subject site will be subjected to one or more severe earthquakes during the life. of the subject structure. Such intense earthquake shaking can severely test any structures at the subject site. In addition, future strong earthquakes occurring nearby on the San Andreas Fault could result in ridge spreading cracks which could damage buildings on the subject property.

Although the following discussion on site seismicity is somewhat technical in nature; this summary is presented for non-technical persons. The magnitude of an earthquake is a relative measure of the size of an earthquake using the energy generated during the earthquake as the ruler of measurement. The maximum credible magnitude is the largest possible earthquake on a fault. The maximum probable earthquake. is the largest earthquake expected over a 100-year time period. The acceleration is the speed or intensity of the earthquake shaking as related to the earth's gravity.

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Seismic Magnitude

Seismic Acceleration

Calculation Methods Used


The table attached, Figure 6, entitled "Acceleration Table" relates the subject property relative to nearby fault traces, the magnitudes described above, shaking duration, acceleration and distance to faults. The purpose of this section and discussion is to provide the review geologist and the design engineer with the anticipated magnitudes and accelerations for the subject property for purposes of design and evaluation.

The magnitude of an earthquake is defined as the measure of the strength of an earthquake or the energy released by it as determined by seismograph readings. In theory, there is only one magnitude associated with any one earthquake and its value is not affected by distance or local geologic conditions. Currently, earthquake magnitude is measured using the surface-wave magnitude scale (M5). Other magnitude scales include the better known Richter magnitude (ML) and lesser known moment magnitude (Mw) scales.

Earthquake magnitude measures earthquake strength on a logarithmic scale. This means that an increase of one unit on this scale (e.g. magnitude 5.0 to 6.0) represents an increase of 10 in the amplitude (size) of the wave recorded on a seismograph. In addition, an increase of one unit on this scale represents an increase in enemy released of about 31.5 times for magnitude 5.0 to 6.0.

Tb~ ~~xi"'!i•.1 ,.... ~:"t?':Fb1~ """"~'J,..:"' .. ·r-= :'? ~._,e larg~~~ --:-~-: .... ·~':l ea~:ilquaKe tr.at a parncular fc.u1t a.::;pears capable o; ~i"-:..:ucing. It is a rational and believable event that is in accord with all known geologic and seismologic facts. The maximum probable earthquake is the maximum earthquake that is likely to occur during a 100-year time interval. It is to be regarded as only a probable occurrence, not as an assured event that will occur in a specific time frame. The maximum probable earthquake magnitude is less than the maximum credible magnitude unless the recurrence interval is 100 years or less in which case the two magnitudes may approach each other.

An acceleration value for a site represents a measure of the degree of ground shaking the site may experience as a result of an earthquake. These values can be used by engineers to determine the forces acting on a structure or foundation during earthquake shaking. Accelerations are measured in "g's" or ynjts of acceleration of gravity. For example, if the vertical acceleration was greater than 1 g during an earthquake, objects would be thrown into the air. However, site accelerations discussed here represent horizontal accelerations only.

We estimate that the maximum credible earthquakes produced by rupture along the faults nearest the site are those given in Figure 6. The value of M 8.5 for the San Andreas Fauit was calculated from the

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equations of Wesson and others (1975) for strike-slip faults and assumes half of the total known fault length ruptures during the earthquake. All the other maximum credible magnitudes are calculated using the regression equations of Bonilla and others (1984) for either strike-slip faults or reverse faults depending on which fault type applies.

The master variable in all the equations used to determine maximum credible magnitude is the maximum fault rupture length. The assumed values of this length for each fault affecting the site are shown in Figure 6. As noted by many who have determined maximum magnitudes empirically, including Wesson and others (1975), considerable uncertainty exists in estimating this rupture length especially for faults tacking historic ruptures. As a result of this uncertainty, the values for maximum credible earthquakes shown in Figure 6 should only be considered reasonable approximations. This is also true for the values of ground accelerations based upon them.

Maximum probable earthquake magnitudes were calculated from the maximum credible values assuming a linear relationship between the two magnitudes and the log of the recurrence interval /100. See Figure 6. Maximum probable magnitudes determined this way are conservative estimates compared with values determined by Ridley and Wagner (1986) for faults in Contra Costa County and using values of recurrence intervals from U.S. Geological Survey (1988).

,\ilax1rnum grcund shaking durations were estimated from Greensfelder (1974). Mean site acceleration values were determined using magnitude values from Figure 6 and equation (3) of Campbell (1981 }. These values were in turn multiplied by 1.13 to obtain peak acceleration values. In keeping with Campbell's assertion that an average amplification factor between 1.17 and 1.28 should be applied to values derived from reverse faulting, acceleration values for reverse faults were multiplied by 1.20. See Figure 6.

Values of site accelerations shown in Figure 6 are probably conservative values for bedrock sites. For alluvial sites (Borcherdt and others, 1975) or sites at the top of steep i'idges near the rupturing fault (Campbell, 1981 ), these accelerations may need to be multiplied by a factor of 1.5 to 2.0 or more depending on the type of soil or alluvium that exists at the subject site.

An inherent assumption in determining site accelerations from the distance between the site and the fault is that the epicenter is on the fault at the closest point to the site, This assumption will almost never be exactly correct and causes acceleration values to be larger than would actually be measured at the site which might be close to the rupturing fault but far from the epicenter. An example would be the

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Town Hall of Woodside during the October 17, 1989 Loma Prieta Earthquake.

Methods for determining site accelerations from distances to faults will be revised as strong motion data from the October 17, 1989 Loma Prieta earthquake is incorporated into similar acceleration models. We have compared strong motion data from that earthqua~e (Shakal and others, 1989) with accelerations calculated as above and find reasonable agreement considering the assumptions and limitations discussed. Bedrock accelerations we estimate as above are slightly conservative and accelerations at ridge tops or alluvial sites need to be multiplied by a factor determined by site characteristics (Borchardt and others, 1975).

Refer to Appendix B for more information relative to the SEISMICITY of this area.

CONCLUSIONS ANQ BECOMMENQAT!ONS

Causation of Distress

Geologic Conclusion

All conclusions and recommendations presented in this report are contingent upon JCP-Engineers & Geologists, Inc. being retained to (1) review the building locations and foundation plans prior to construction 2' observe any ear!hworl< ooerations ,,+ !h9 site and 3) observe the foundation excavations tor the repair or reconstruction cf the residence.

It is our opinion that the primary cause of the damaged and distress observed at the subject property is from movement of the existing house relative to the inadequate and substandard foundation. The earthquake shaking of October 17, 1989 was a triggering mechanism.

Based on our studies, we conclude that, from an engineering geologic standpoint, the subject property is suitable for the proposed repair or reconstruction if the limitations inherent to the subject property are fully understood and accepted by the present owner and all future owners. The subject structure is constructed on a steep hillside in an area underlain by relatively thick accumulations of colluvium and ancient landslide debris. There is a higher potential for reactivation of landsliding at the subject property than at other areas in the Santa Cruz Mountains which are underlain by competent bedrock. In addition, ridge cracks were observed adjacent to the subject property and in the general vicinity of the subject property. The possibility of ridge cracks opening on the

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·Soil and . Foundation · Conclusions


subject property in the event of a large earthquake centered nearby on the San Andreas Fault is relatively high.

Because of the known geologic hazards at the subject site, had the property not been previously developed, it would be a marginal site, at best, for new construction. The present owner and all future owners must fully understand and accept these limitations ..

The subject site and vicinity is underlain by an unknown thickness of ancient landslide debris. Evidence of active, recent or historic landsliding on the subject property was not observed. Slope stability analysis to evaluate the possibility of major landsliding or reactivation of landsliding that would involve the entire site and vicinity (tens of acres in area) were beyond the scope of this report. However, we feel that the possibility of such major landsliding is low.

Possible ridge cracks were observed to traverse Mountain View Court adjacent to the subject property. Evidence that ridge spreading cracks or fissures traverse the subject building pad were not observed. Several months and significant amounts of rainy weather may have obscured any evidence of ridge cracks between the time of the earthquake and the time of our site reconnaissance in April, 1990. If ridge spreading cracks and/or fissures did traverse the subject property, then there is a high potential for re-occurrence of ridge spreading cracks during future strong earthquakes on nearby faults. If the soil and foundation recommendations m 0 o;"!nted in suosequem sec•1ons 01 ihis report are adhered to, damage irom future ridge spreading cracks can be minimized.

The site is expected to be subjected to strong seismic shaking at least once during the design life of the planned structure. This seismic shaking is one that is shared by all structures in the San Francisco Bay Area and is normally considered in the structural design of the building. In our opinion, design according to the seismic criteria contained in the 1989 Uniform Building Code is sufficient for the structure.

Based on our field and office studies and considering the soil conditions and geology of the site as discussed above, it is our opinion that the existing earthquake damaged structure can be supported by hand excavated or drilled underpinning piers designed in accordance with the recommendations presented in the "UNDERPINNING PIER FOUNDATIONS" section of this report. Portions, if not all, of the existing foundation, may not be suitable for use as grade beams. The existing foundation should be evaluated· by a registered civil engineer to determine which portions may be utilized as a grade beam. If it is determined that the existing

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SITE :pREfARATION •AIDl EARTHWORK


foundation is not adequate for re-use as a grade beam and the owner chooses to construct a new foundation for the existing structure or a new structure, then the recommendations presented in the "PIER FOUNDATIONS" sections of this report should be utilized in the design of the new foundation.

Two wood-retaining walls exist at the subject site. The ent(re length of the retaining wall located upslope and adjacent to the existing structure is in a distressed condition and should be replaced with an engineered retaining wall. Portions of the wood-retaining wall located downslope of the existing structure and adjacent to the driveway and parking area is also in a distressed condition. It is our opinion that the entire length of this retaining wall should also be replaced with a wall engineered in accordance with the recommendations presented in the "RETAINING WALLS" section of this report.

The existing driveway and parking area appears to be partially underlain by non-engineered fill. The driveway and parking area may be excavated and recompacted in accordance with the recommendations presented in the "SITE PREPARATION AND EARTHWORK" section of this report. Any new retaining walls which may be required in the construction of the driveway and parking areas may be designed in accordance with the recommendations presented in the "RETAINING WALLS" section of this report.

Applicable areas of the site should be cleared and stripped to sufficient depth to remove selected trees, any obstructions and surface vegetation. These materials should be removed from the site. Any natural slopes outside the structure which are stripped should be replanted to minimize future surface sloughing and erosion.

After the site is cleared and stripped, any excavation and/or filling operations required for the access driveways and building areas can be made. All on-site soils below the stripped layer having an organic content of less than 3 percent by volume are suitable for use as fill. Any imported fill material at the site should be a non-expansive material with a plasticity index of 12 or less. All fills on slopes steeper than 6:1 (horizontal to vertical) should be keyed and benched into the slope such that the benches slope into the hillside a minimum of 2 percent. The keying and benching should be performed according to the recommendations of the soil engineer in the field. If sources of groundwater are encountered, subsurface drains may be required by the engineer.

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DRAINAGE :PROVISIONS

.UNDERPINNING PIER FOUNDATIONS


Any fill placed at the site should not contain rocks or lumps greater than 6 inches in dimension with not more than 15 percent larger than 2.5 inches. Fill should be compacted to at least 90 percent relative compaction by mechanical means only as determined by ASTM Test Designation D 1557-78. Fill should be placed in lifts not exceeding 8 inches in uncompacted thickness.

Any utility trenches should be bacldilled and compacted according to the recommendations provided above. Fill placed in trenches, however, should not exceed 6 inches per lift in uncompacted thickness. Compaction of sand should not be attained by water jetting methods. In trenches located within the building envelope or in pavement areas, the upper 18 inches of backfill should be compacted to at least 95 percent relative compaction as determined by ASTM Test Designation D1557-78.

Based on the subsurface materials encountered at the subject property we recommend that any cut and fill slopes made at the site have a maximum inclination of 2:1, horizontal to vertical. At this inclination, the exposed slopes may be subject to minor surface erosion and sloughing thus requiring periodic maintenance of the slopes. We recommend that all cut and fill slopes be planted with appropriate native vegetation to minimize erosion.

Positive surface crainage snoi.;10 be provided acjacent ta the building so as to direct surface water away from the foundations of the building into closed pipes that lead to suitable discharge facilities. Rainwater collected on the roofs of the building should be transported through gutters, downspouts and closed pipes, if necessary, to suitable discharge facilities.

Surface water should not be permitted to pond or flow adjacent to the structure's foundation. One way to alleviate this condition is to grade the ground surface adjacent to the proposed structure such that water flows away from the foundation. In addition, surface interceptor drains may be provided to carry off all excess waters.

It is very important that drainage systems be properly maintained by all future occupants. Refer to Appendix C for additional suggestions relative to drainage .

We recommend that the distressed or settled portions of the existing structure be underpinned utilizing either drilled cast-in-place straightshaft or hand excavated piers that are designed to develop their load bearing capacity through friction between the sides of the pier and

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surrounding subsurface materials. The proposed underpinning piers must be attached to the existing spread footing foundations in a positive manner (designed by a civil engineer).

House wall Ground surface Floe, r Backfilled w~h soil

.::b~=±l::;::;::i:;IJ_ and compacted

Underpinning pier (see report for design)

Depth

r'f--..,_J./~Ground surface

Loose soils -- --Dense soils

l._____. Typical reinforcina steel

NOTTO SCALE

NOTTO SCALE

loose soils

dense soils

Drilled concrete underpinning pier

NOTE: This figure is a schematic diagram only.

Schematic diagram of typical underpinning pier.

Schematic diagram of typical drilled underpinning pier.

The friction piers should have a minimum diameter of 12 inches and they should have a minimum clear spacing of 4 shaft diameters. The actual lengths of the piers can be determined using an allowable skin friction value of 500 pounds per square foot for dead loads plus live loads with a one-third increase for all loads including wind or seismic. The supporting capacity for the upper 3-1/2 feet of

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:PIER 'FOUNDATIONS


material should be neglected in determining the allowable pier bearing capacities.

Resistance to lateral loads can be developed from the piers usirig an equivalent fluid pressure of 350 pounds per cubic foot. This value can be assumed to be acting against 1.5 times the diameter of the individual pier shafts starting 2 feet below rough grade.

Even though the piers will be designed to develop their capacity through friction, their bottoms should be dry and reasonably free of loose cuttings and fall-in prior to installing reinforcing steel and placing concrete.

If the piers are installed in accordance with the recommendations presented in this report, additional differential settlements of the building walls should be minimized. However, we should note the interior of the structure may not be underpinned and may be subjected to some settlements and adjustments in the future. These underpinning pier recommendations are made assuming that positive surface drainage control measures are implemented at the subject property and are maintained in working order in the future.

If the underpinning piers are installed in .accordance with the recommendations made in this report, additional settlements and movements of the building walls should be minimized. However, we shculd "Ote ttiat •tie iriterior cf t'"'e st!"'Jcture will r:~· ':'e underpinned anc may be subject tc minor settlements and adjusimar:rs in ihe future.

If it is determined that the existing .foundation is inadequate for use as a grade beam and that a new foundation will be required for a new structure, then we recommend that the structure be supported on drilled, cast-in-place, straight-shaft piers that are designed to develop their load carrying capacity through friction between the sides of the pier and the surrounding subsurface materials. Friction piers should have a minimum diameter of 12 inches and they should have a minimum clear spacing of 4 shaft diameters. The piers should extend through any loose soils or non-engineered fills to a minimum penetration of 8 feet into dense materials approved by this engineer in the field.

IJCPI

•

Page 14

Friction Pier

Concrete grade beam

"' Reinforcing Steel

:.;·.'·;.:..;.·~· -! .. T..·' ..

Concrete Pier

~ .:~·(~ ·,,~ .. ·)~~.

'\:~cads L/ trans!erred to r soil (friction)


Typical grade beam and attached friction pier.

The actual lengths of the piers can be determined using an allowable skin friction value of 500 pounds per square foot for dead plus live loads with a one-third increase for all loads including wind or seismic. Since the upper portions of the soils at the site have only nominal strengths and since there is evidence of "creep" type movements at the site, we recommend that the upper 3 feet of material be neglected in determining the allowable oier caoacities.

To account ior the presence of colluvial soils and the "creep" type movements that are occurring in these materials, we recommend that the piers be designed to resist a uniform lateral pressure of 600 pounds per foot acting against the projected diameter of the pier to a depth of 3 feet below the bottom of the grade beams.

Lateral loads on the piers may be resisted by passive pressures acting against the sides of the piers. We recommend a passive pressure equal to an equivalent fluid weighing 350 pounds per square foot per foot of depth to a maximum value of 3,600 pounds per square foot. This value can be assumed to be acting against 1-1 /2 times the projected diameter of the individual pier shafts starting 3 feet below the bottom of the grade beams.

All piers supporting bearing walls should be tied together with grade beams that extend up and down the slope between the piers. The grade beams should be designed to span between the piers in accordance with structural requirements. The steel from the piers should extend a sufficient distance into the grade beams to develop its full strength in bond.

IJCPI

•.-

SLABS-ONG RADE


Even though the piers will be designed to develop their capacity through friction, their bottoms should be dry and reasonably free of loose cuttings and fill-in prior to installing reinforcing steel and placing concrete. The concrete should be placed carefully in the pier holes such that overpouring of the piers does not occur. It is important that "mushrooming" of the pier tops be avoided.

If the piers are installed in accordance with the recommendations presented in this report, total settlements should not exceed 3/4 inch and post-construction differential settlement should not exceed one inch.

Slabs-on-grade for a garage must be supported on a minimum of 6 inches of non-expansive materials. Prior to final construction of the slabs, the subgrade surface should be smoothed and compacted to provide a firm surface for slab support. In addition, slab reinforcing should be provided in accordance with the anticipated use and loading of the slabs. However, we recommend that the slabs be reinforced with a minimum of #3 rebar on 18 inch centers.

Typlcal Slab

, Ty~ical

fcc~:;-:g

...........

Reinforcing steel grid w~hin concrete

Sand

~~~~~ji'--Visquene layer

Pea gravel and sand

Typical slab-on.grade showing detail of concrete slab and underlying support.

In any slab area where minor floor wetness would be undesirable, 4 inches of 3/8-inch pea gravel should be placed beneath the floor slab to serve as a capillary barrier between the subgrade soils and the slab. To minimize vapor transmission, an impermeable membrane should be placed over the gravel, and the membrane

IJCPI

I RETAINING WALLS


should be covered with 2 inches of sand to protect it during construction. The sand should be lightly moistened just prior to placing the concrete. The sand, membrane and gravel can be used in lieu of the upper 6 inches of required non-expansive material beneath slabs.

Any isolated exterior retaining walls and any building retaining walls must be designed to resist lateral earth pressures plus additional lateral pressures that may be caused by surcharge loads applied at the ground surface behind the walls.

We recommend that unrestrained walls, 12 feet in height or less, that have a level surface or a sloping surface flatter than 4:1 be designed to resist an equivalent fluid pressure of 40 pounds per cubic foot. Where the sloping surface is at an inclination of 2:1, the walls should be designed to resist an equivalent fluid pressure of 60 pounds per cubic foot. For walls having a sloping surface between 4:1 and 2:1, a straight line interpretation between 40 and 60 pounds per cubic foot may be used. The walls should also be designed to resist an additional uniform pressure equivalent to one-third the maximum anticipated surcharge load applied at the surface behind the walls.

We recommend that any restrained walls be designed for the applic~ljJc::? q~uh181'?~t f\1 .. ·lr:I ~r~ss:~'"e:? ... h,c::?n above fer~~~ ·..:rir1?.s~rained 'NC.:;s 1:c.v1r:~ ;e-..1·:1 z.;;C ::;;cpir:g .;:..;.~aces . • -iowe·'le-r . ... es~rained walls must be designed for (1) an additional uniform lateral pressure of SH pounds per square foot where H = height of backfill above the top of wall footing in feet and (2) one-half of the maximum anticipated surcharge loads applied at the surface behind the walls.

IJ>E11-- Reinforcing steel wdhin retaining wall

~ion t t bearing resistance

S'minimum

Typical retaining wall demonstrating lateral load.

IJCPI

I .. Page 17 Project No. JCP-2941

The above pressures assume that sufficient drainage will be provided behind the walls to prevent the build-up of hydrostatic pressures from surface and subsurface water infiltration. Adequate drainage may be provided by a subdrain system consisting of a 4-inch diameter perforated pipe (ABS slotted) bedded of Class II permeable material or an acceptable substitute such as 1.5 inch diameter drain rock wrapped effectively with filter fabric to a height of at least twothirds the height of the wall. The remaining portion of the walls should be backfilled with on-site or imported fill materials that are compacted to at least 90 percent relative compaction. Any building retaining walls should be water-proofed by a positive method such as hotmopping.

The above recommendations may be utilized for construction of unrestrained wood retaining walls. Wood retaining walls should be . designed for a design life of at least 50 years. At a very minimum, all wood posts should be chemically treated with a preservative in order to obtain a 50 or more years design life and it is strongly recommended that lagging or horizontal members also be constructed of chemically treated wood.

The walls can be supported on foundations as designed in accordance with the recommendations presented previously under "PIER FOUNDATIONS". If the base of the retaining walls are to be t?XC?.'Ja+~~ int~•!--~ dense n~_tj,;c ~'?":!rock, the""" ~~'? -·:~''-::- _,~·''be

:.--.·:- ~ :~ ·_..: ~ ~ . .:::;·::.~ ".:.:~:.~,; ;:_ -· _...:. .. ~1lS. T;"""i~ ;..; . .;:; ;, ... ..:..;;..~.JC designed for an allowable bearing pressure of 2,000 pounds per square foot for dead plus live loads with a one-third increase for seismic loadings. Any footings located adjacent to utility trenches must have their bearing surfaces below an imaginary 1 :1 (horizontal to vertical) plane projected upward from the edge of the bottom of the adjacent trench. Lateral load resistance for the walls can be developed in accordance with the recommendations presented immediately below under "LATERAL LOADS".

·LATERAL LOADS Lateral loads may be resisted by:

• Friction between the foundation bottoms and the supporting materials and

• Passive pressures acting against the sides of the footings.

On slopes, the footings must have at least 5 feet of 'dense' soils between the slope face and the footing bottom to develop the recommended passive pressure. We recommend a coefficient of

IJCPI

LIMITATIONS

i l I


friction of 0.35 and a passive pressure equal to an equivalent fluid weighing (350) pounds per cubic foot starting at the top of the adjacent finished grade for footings and at the top of the footing for retaining walls. In addition, passive pressure acting against retaining wall footings should not begin to act until at least 12 inches below the ground surtace. Disturbance of this material may occur during the life of the structure.

To develop adequate resistance to lateral loads, it may be necessary to construct a key at the bottom of some of the retaining wall footings. If a key is used to develop sliding resistance, it should extend below the footing a minimum of one foot and concrete should be poured neat against the excavated materials. We recommend that the factor of safety against sliding for retaining walls be at least 1.5.

Our services consist of professional opinions, conclusions and recommendations made in accordance with generally accepted geo-

. technical engineering principles and practices. This warranty is in lieu of all other warranties either expressed or implied.

If you have any questions regarding this report, please call us. We would appreciate at least 48 hours notice for our required observations during construction .

.. :_._ ... -·- ~·-= -'i-·-· .: - -,; .

JCP-ENGINEERS & GEOLOGISTS, INC.

IJCPI

..

+ -N-

0 112 1

APPROXIMATE SCALE (MILES)

2

Source: Barclay Maps Locaide, Santa Clara County

LOCATION MAP

IJCPI DATE PROJECT NO. FIGURE 1

.

--N-

APPROXIMATZ SOU:

;,,,,==<===========e:=1.t======3::==:E'=:c=:::i0=:=:=:=:,,,,;=:=:=:=:=:=:=:=:=:=:=:~IM•U. - .I t(XS:Xl-::::EC>3o======100D:E;=:=:~'°"'i2:=====JOOO"""====iii::=====5000'5;=:=:i600)3:::====:'.:'i700Q.fEfl

"===3::::E:·i::·========::E0=::;:;:;:;=:=:=:=:=:=:==:1 KILOMETER

"

. I~

UNITS. sou n;wesr oF SAN ANDREA:.

FAULT

~] l•nci~..ic 4tar" I-

lt.-ii."" F'w ... l'>Orl

M.M1nc. unJsron.

l.tlCONFCRMHY

QC] U.111Dur? Shilt ,..,... ,.,,; . """"" ....

[5J Vk\"tna ~t11n1. Mar;ne .itiouc: Wll0..t1n

[±i1 00 Sift l.onnzo ~icft ~ T=,1,..-..{i~si~-

d&'t'fonc. Tu, :tcu .\tw\f7>t.

........... ~I;>~ U14~ti~~;

~'*s 31.iuun.hc. ~ st0nt .r ,.,q l'ltM" buc;, Zcm~r,..,, • AJi.,;;.,. s"l ..

r,r, T"'°tw !l'llk ...... beQM.ci <lllCIC!t0'4 ihU& wi c.lii~tcnc; Hui:ti.-5..,_

SE Butuio S•n•1to-a

Mannt ...-ko~ie. iMftlM and raic.a .. (OJ~ ~ Nlt'iiiin :lt~6-n.,_ ,......,.,......, ...._ Tb , .. - • ......, .....

UNITS NORTHEAST OF . SAN ANDREAS

FAULT

I aoa I CWtr i!luftllla Graotti, ~ • d.a:J

UNCOllFORMITY

I QTs I *r;ii Oan Foreatian '411.y ~ .. ~. d>y o>d ......

UNCONFORMITY

~ Monru'Y 91.i• MW'lll• ~hcC"OUS !NM

~ nu.:r:-·:-.:•a':;e 7.Q iv..i~

I Tr I r .... ~.cnr.......;. Mant1•~~

UNCONFORMITY

. jr..@ s.o .... "'J.... . Tu.-...i.Tll,lime~

~ Mo< ......... "? ..... ~.•iucaous.-.,

tni..r tru. ~ ...... l'K!,, .,.i.o.;, ~~

!K;11 >§ /¥4v111• SOIOllll•nfM]' l"QUa

XU'I, mic..Ktou.i. sk&I. Kc.g. ~bbl· tong-.....

I vb I vd I M1ii< 1~nc0vl nxlt.t., fll"":r .ltu.d to ~ vb, bu..if LJ .a. ru. gt.1in.d c1!0""

. I sp I lC I Ultram.;~ l'Qtj.,a

sp, Mlll&nt1non. SC, silica. 4~ i.i., mu'!Ci~\~~

f., f9 i,. fc fl f,,

fnnc.i1,~n r11t.i.1 r.,.., • .Mv.U1 ,1iuru1 ......,

~<dstadinwrtary and 'ICbnir. ,,.~1rc r •. _, __ ,...., , .. , •u. r, • I"'"''-• oft.... . ,. • ,-')"'C<M -r.. ~.i...t

. ! ft I liwiafe-fsr. -..,.. tont. ,. .._

. " ...... , .....

. - __ CQl'1?~T

(~sil«i ...hcre :r•~t1or..i r:. ~'cly loartd )

----·-- F.a.ulr"· c~ ,.n.,. .. iftturtd or .-.i&t-.. \IAC.~; dotted ... ~led;~ parall1al ur;M iMtic.ar1 1trik.~ ~•P mo,-r)

-{-- --~-inhdi•c .,.Ci••

AM~ ot fc»ii

.._,,,. -~

" .. ,. - _,,,,,.. Strikt .... dip,,

bedd1n9

7' ""lo00 ............ 0i-

iaod&lido .,.;..t.d Ii halJ. .......

S~urce! Dibblee & Br2.bb G~V3}

A-reA Geo 10 '<;.. Y'(\o.

/JCPj . ~e..s cleV"tc.e..- rYlou~~ View Coor-I ~A<!JECT NO CATE

:l c.f'~ ;).. 'i 4 I :l°"\.l'\-C l~iO Figure i

..._ . . --...

'

QI . ~ .A

, IVL

APPROXIMATE SCALE

1000 0 1000 2000

" feet

'

EXPLANATION OF MAP SY B 0 LS

(.).

r;Z_ QI •H•·>o

...

v

I ~rn lan~Udit: indicated by (XJ vhere ~- d.iimnaiat qenerally

less than 100 feet; landsli~ scar and l..,dslld• debris generally

::;:~~,:~;.~'." ... ' bo~ .. ,r ~•ro"~"'Y '°"""•· ..,,"•• I

Modern land::ll.i~: {vinter of 196914 70) resultinq f?'c11t hO\me p.ad cut into potentially •mst.1.ble sldpe; sout."tvest slooe of su-ns

, Cmk '1ooq s~ l\ndreM ''"') :on •.

l"odi!m l.!ndslide ~iqgered ~y 1906. San rrancisco EarUlquake: bo1.r1d.ui.es approxiJ11.1.~ly located. dashed where inferred; questioned where data unC'l!!r~n. l

Area of iltlundant :nodem lands lideJ, found alcng ::iarqins of steepsi.ded s~rea:n channels eroded into potenti.tlly unstable bedrodc or 'lurf:!.cial deuosits; landslide\ scar and land.slide debris commcxily visible. ·r

11·

Prcbaole old_ landslide or landsli~1

ccmplex: location of boundaries 1-la!r~n; no visible fresh ljsn~lide scars: may be partly e~'.!.: hurim::>dcy ta;>oqr~hy; oft.en c1ta.i.ns thi.clc deposits o! eolluvilllll {landslide debris!: qenerally on steeper sl~es .

. -

Possible old landslide or landslid* ~lex: locatiat of bowida.ries uncertain: irregular tcpoqrap~y, qenerally on steeper slopes: id~tifi•d mly fro• topoqr.prc .... . -

Probable ancient landslide or lan~Ude eoq>!J!xi location o! boundar;es very imcer<:.a.in; no visi,!,le fresh landslide scars; ext.ensi~ly .. ·--· ·~ ·-·~: r ....... ,.-· ... . . '"

. :::'.::: ~r.::;:; = :::· 1-:'"':::.;;::."'~· '"'..:·~ Older allunua1 qravel, sand, silt, and c:lay: fo.md in Santa Clara

Valley ne•r San Tcm&s Creek: lifte.d and so.!Wh.11t •roded by• modern stre-: .ay be equiv.11 nt in age to stn• deposits (QalJ "ithin Sant• Cnz Mo\m\t&ins.

Thidtness of c:olluviumi {+) dic:ates :ain;tlllllll tm.dcne:ss where bue o! eolluvium not v.i.:sibl.e

Colluvi~!illed ancient strek C:.~els.

sbcwa. by <JJ J. _

ProO,.le resU>red _.,,,. of l .. t l~dsli .. .,_iu &lmq Skyllna

Bl'ld.-SUDm.t 1'Dad n.dqe: r.oesible re:atant of l'Madara.U scup

SOURCE: ROGERS .(1971)

/JCP/ Figure

DATE

J"C.f? - :l."I 'I' 3

i i !

l J . I j

' \

•

--~ /

--- -- i-'

i

\'~'/ /

frlOJ·rr6 .. ,~

v.e..v.J c:...o ..,,,-r

.......___.., '4,;v ~· cra.~s

,--.,_ ~co.ve""e"''t

IS- 1iz 0

Tv<;'\

d '

]" f; II j cla.., t., eel ui h~" E:.

o. e \\"'o 'I( ·,,.,,ct H 'I I o c. o. t eol

f//I slope.

er f - f(Hn_ t/{e..:i Ct.

" ·-

0

1f

0

APPROXIMATE SCALE

I 1)'._ ,5

(f~d)

__I 30

fopGoi. I n.ncl C.0 ~I IA'1i \lWI

VERTICAL AND HORIZONTAL IJCP!

N b

Dec.~

'I.- s~c..f 10V'1 A-A'

A'

. ~~ 1-w w u. a:; z 0 i==

]O~ ..J w

QI >

rQ.<"l•i Prof>ec-t1 - rrH- ~ \} te.v.J 12.o.,.,.t

P4'o • ec:.-l No. Do..te. ~i'3. s :) c.~- :J.''I '4 I -s..,~e. l't'l 0

-PEAK AND REPEATJ\• LE ACCELERATION VALUES

Assumed Maximum A£ 1ned Maximum Maximum Peak (Max.) Repeatable Distance to Rupture Length Fault Credible Rer 'ence Probable Duration Horizontal Horizontal Site

Fault (Miles) Type Magnitude lntei I ( r) Magnitude (seconds) Acceleration Acceleration Miles)

S. Andreas (1906 event) 252 SS 8.50 3· 00 8.00 >30 0.87 g 0.80 g 1.00

S. Andreas (1989 event) 26 SS 7.24 1- 00 7.10 27 0.69 g 0.67 g 1.00

Butano 17 SS 7.13 30 :.00 5.65 26 0.70 g 0.49 g 0.75

NOTE: Values determined using models of Wt on and others (1975), Campbell (1981) and Bonilla and others {191 stent with U.S. Geological Survey (1988) d Joyner and Boore (1981)

Recurrence Intervals are estimates co1 , Other references: Greensfelder (1974) < ,_

ss = strike-slip fault reverse = rev. · >e fault

Site Seismicity -~ EQ Damaged Property Mtn. View Court

Date I Prolect No. Figure: 6 Jun 19901 JCP-2941 -

T

1

,,

•

APPENDIX A

FIELD AND LABORATORY STUDIES

FIELD STUDY

Our field study consisted of a detailed reconnaissance of the site and surrounding properties by our engineer on April 13, 1990. Three exploratory borings were drilled at the locations shown on the Site Plan and Geologic Map, Figure 4. The borings, which were 3 inches in diameter, were drilled with hand drilling equipment.

The materials encountered in the borings were continuously logged in the field by our engineer. Logs of the borings as well as a key for the classification of the soils encountered in the borings (Table A-1) are included as part of this appendix.

Representative soil samples were obtained from the exploratory borings at selected depths appropriate to our studies. All samples were transported to our laboratory for evaluation and appropriate testing.

Two different samplers are normally used to obtain soil samples; a Modified California Sampler with liners to obtain relatively undisturbed samples, and split-spoon Terzaghi standard penetrometer sampler. The samplers are driven 18 inches using a 75-pound drop hammer. The number of blows required for each 6 inches of penetration is recorded. When necessarv due to differino samoler size and droo hammer weiqht, the

The boring logs show our interpretation of the subsurface conditions at the locations and on the date indicated, and it is not warranted that they are representative of subsurface conditions at other locations and times. The stratification lines on the logs represent the approximate boundaries between material types; the actual transitions may be gradual.

LABORATORY STUDY

Our laboratory testing program was directed toward a qualitative and quantitative evaluation of the physical and mechanical properties of the materials underlying the site.

The natural water content was determined for 19 samples of the materials recovered from the boring~; these water contents are recorded on the boring logs at the appropriate sample depths.

Dry densities were determined for 3 samples recovered from the exploratory borings; the results are re(:orded on the boring logs at the appropriate sample depth.

IJCPI

I/) .... 0 I/)

c w z :ct a: c w I/) a: < 0 (J

I/) .... 0 I/)

Q w z :ct a: 0 w z ii:

'

PRIMARY DIVISION

GRAVELS CLEAN ...J GRAVELS < (LESS THAN iI: MORE THAN WO HALF OF 5% FINES) 1-0 < "' COARSE :ii .

FRACTION IS GRAVEL u.~ LARGER THAN WITH OzW NO. 4SIEVE FINES LI.<!:::!

...J :c (/) < 1-W :ca:> SANDS

CLEAN Zww SANDS < (!) Cii (LESS THAN :c a: MORE THAN I-<

HALF OF 5% FINES) w ...J

a: (/) COARSE 0-FRACTION IS SANDS :ii

SMALLER THAN WITH

NO. 4SIEVE FINES

w SIL TS AND CLAYS U. C: N ow-...J (/) u. ...J w LIOU ID LIMIT IS --'<>

LESS THAN 50% < :;w :c (/) Cii z (/) 0 <-o i= ...J "' SIL TS AND CLAYS < . wiI:O a:wZ

LIQUID LIMIT IS 01-Z :ii < < GREATER THAN 50% :ii :c I-

HIGHLY ORGANIC SOILS

. SANOS ANO GRAVELS BLOWS t FOOT t

VERY LOOSE 0-4

LOOSE 4- 10,

MEDIUM DENSE 10-30

DENSE 30-50

VERY DENSE OVER. 50

RELATIVE DENSITY

GROUP SYMBOL

GW

GP

GM

GC

SW

SP

SM

SC

ML

CL

OL

MH

CH

OH

Pt

SECONDARY DIVISION

Well graded gravels, gravel-sand mixtures, litt:Se or no fines.

Pooriy graded gravels or gravel-sand mixtures. littSe or no fines.

Silty gravels, grave~sand-day mixllJres. non-plastic fines.

Clayey gravels, gravel-sand-day mixtures, plasttc fines.

Well graded sands, gravelly sands. little or no fines. ·.

Poorly graded sands or gravelly sands. fitUe or no fines.

Silty sands, sand-silt mixtures, non-plastic fines.

Clayey sands, sand-day mixllJres, plastic fines.

Inorganic silts and very fine sands, rock Hour, silty or clayey fine sands or clayey silts with slight plasticity.

Inorganic clays of low ID medium plasticity, gravelly clays, sandy clays, silly clays, lean clays.

Organic silts and organic silty clays of low plasticity.

Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts.

Inorganic clays of high plasticity, fat clays.

Organic clays of medium ID high plasticity, organic silts.

Peat and other highly organic soils.

SILTS AND CLAYS STRENGTH" BLOWS/FOOTt

VERY SOFT 0 - 1/4 0-2

SOFT 1/4. 112 2-4

FIRM 1/2. 1 •-8 STIFF 1 • 2 a - 1s

VERY STIFF 2-• 16·32

HARD OVER• OVER32

CONSISTENCY

' Number of blows of 140 pound hammer falling 30 inches co drtw a 2 inch OD. (1-318 inch 1.0.) split spoon (ASTM 0-1586).

tt Unconfined compressiw strength in IDnSlsq. ft. as determined by laboratory tasting or approxomated by the standard penelnlDon test {ASTM 0-1586), pocket penetrameter, tDMtne, or visual observation.

NOlES: 1.) · Hol•- dry at Iha lime of drilling and was backfilled Immediately (See mxr of repon for disaJssion of grou-ater).

2.) Sampler blows were adjusted lD correspond tD standard penetration rast crerzaorn1.

)JCPI KEY TO EXPLORATORY BORING LOGS Unified Soil Classification System (ASTM D-2487)

TABLE A-1

• f

,.

. J7 p-~rinineers & r.eolonists. Inc. ~l'ICllNf: LOC:: REPnRT Borino Size: 3"

r.;roUndwater Wag not encounterd Lonned bv: cw Orjll HIO'. Hand l"fril'"" uate uni•"": .Anril 13 1990

Dry Descriotion and Classification ::.011 Depth Sample Density Other

l'lescr1nt1on and Remar1<s Color <~ns1st. Tvno (feet) Tvne N W(O/o) IPCFl Tests loose to BAG - .

SAND, pebbly, organics dar1< medium -'- 1 . BAG

(TOPSOIUFILL) brown dense SM _ . BAG 21

-" 2 BAG MC - .

3 MC 9 15 -- . MC - .

4 MG -- . '- . MC 14 13

brown to -'-5 MC

yellowish '- . MG Dis-Decreasing Organics brown . '- 6 . MC 12 15 turtled

'- 7 . MC Sample MG - '- . MC 25 10 '- .

. '- 8 MC SANDSTONE rock fragments and medium '- . SPI sand, minor organics (COLLUVIUM/ dense to

SM-'" 9 . SPT 20 14

LANDSLIDE DEBRIS) brown dense . SPT '-

. '- 10 - ;:ii' I SPT 19 14

'- 11 . SPT . '-SPI '- -Increasing sandstone rock . ,_ 12 • SPT 19 12

fragments ratio ,_ 3 • SPT 1 . Sl'I _,_

SPT 38 13 I ' I

,_ -: I SPT ! I ' . ' - - Bottom oi Boring = 14 i-eet

W = Water Content (%) S = Penetrometer Shear Strength (KSF) LL a Liquid Limit Pl = Plastic Index N = Penetration Resistance (BlowS/Ft). SPT - Standard Sampler MC = Modified California Sampler NOTE: Sampler blows are adjusted to correspond to standard penetration test (SPT, Terzaghi) RX=Rock

• ,. A•IJHl RnR1 LOG

JCP Ennlneers & Geolottlsts. Inc. 18180 Mtn. View Court I Boring No: J'";;' Job Number I l"iate Redwood Estates, California 1

2941 I -Jun-90

JvP t:nameers & ueoloo1sts, Inc. BvRJNL• LOG REPORT Borino :>ize: 3-Groundwater Was not encountered 1..0naed by: i..;W

Drill Ria: Hand Drilled Date Drilled: Anril 13 1990 lJry

Descriotion and Classification =ii uepth Sample Density Other Descrirnion and Hemarks volar Gons1st. Tvne I feet) Tvoe N VV!o/o) IPCFl Tests

BAG - 1 -ORGANICS AND SAND, debris dark very BAG (ORGANIC DEBRIS ROCKS) brown loose sc- - BAG 74 - 2 - BAG -- .

BAG - 3 - Mu 28/6 20 MC - 4 - MC 42 5 96

SAND and sandstone rock tan to med. to -- - MC fragments, minor organics - 5 -tan very :)!-' I

brown dense SM-- - SPT 16 20 6 - SPT -- ::>t' I - 7 - SPT 26 12 -- .

SPT · - . 8 ::>t' I -- .

SPT 53 11 - 9 .

SPT 1:1ottom o 1:1ormg = ~ 1 eet

W =Water Content (%) S = Penetrometer Shear Strength (KSF) LL = Liquid Limit Pl = Plastic Index N = Penetration Resistance (BlowstFt) SPT = Standard Sampler MC = Modified California Sampler NOTE: Sampler blows are adjusted to correspond to standard penetration test (SPT, Terzaghi) RX=Rock

.. xPLuRATuRY BoRJNu LOu

JCP Enolneers & Geoloalsts, Inc. 18180 Mtn. View Court t:1onng No: JCP Job Number I uate Redwood Estates, California 2

2941 I Jun-90

6 I

. avnln11. .. Luu~, J• : .... t:noineers & Gealoo1sts, Inc. l"'\orina v1Ze: 3"

uroundwater Was not encounter= Loaned bv: [.jW urill Rio: Hand urilled 1ate nrilled: Acril 13 1990

Dry Descriction and Classification ::Soil uepth Sample Density Other

Descriction and Hemarks (jOior Consist. Tvce (feet) Tvce N vv1%) IPCFi Tests

SAND and sandstone rock " 1 BAu - BAG

fragments, organics (TOPSOIL ·- - BAG 19 AND COLLUVIUM) brown locse SM " 2

. BAG _,_ - BAG - .

3 Mv - MC 26 13 105 " 4 - MC

SANDSTONE (cobble to boulder medium ....

Mv size) and sand (LANDSLIDE to very - 5

. MC 57 10 108

DEBRIS?) tan dense SM·- . MC - 6

. :>PT ·- - SPT 53 13

Bottom o 1::1onng = 6.5 •eet

W = Water Content(%) S = Penetrometer Shear Strength (KSF) LL = Liquid Limit Pl = Plastic Index N = Penetration Resistance (Blows/A) SPT = Standard Sampler MC = Modified California Sampler NOTE: Sampler blows are adjusted to correspond to standard penetration test (SPT, Terzaghi) RX=Rock

'

"-APLvRA uRY BuwlNG Luu

JCP Enaineers & Geoloalsts. Inc. 18180 Mnt. View Court 1::1onng No: JGP Job Number I Date Redwood Estates, California 3

2941 I Jun-90

•

JCP-2941

Bibliography

Aerial Photo Pair, WAC 895SFO 2-255, 256 (Color), Flown 11/9/89

Bonilla, M.G., Mark, R.K., and Lienkaemper, J.J. (1984) Statistical relations among earthquake magnitude, surface rupture length, and surface fault displacement: Bulletin of the Seismological Society of America, v. 74, no. 6, p. 2379-2411

Borcherdt, R.D., Gibbs, J.F., and Lajoie, K.R. (1975) Prediction of maximum earthquake intensity in the San Francisco Bay Region, CalHornia, for large earthquakes on the San Andreas and Hayward Faults: U.S. Geological Survey, Miscellaneous Field Studies Map 709, p. 1-11

CalHornia Division of Mines & Geology (1975) Recommended guidelines for determining the maximum credible and the maximum probable earthquakes: CalHomia Division of Mines and Geology Note 43

Campbell, K. W. (1981) Near-source attenuation of peak horizontal acceleration: Bulletin of the Seismological Society of America, v. 71, no. 6, p. 2039-2070

Clark, J.C. (1981) Stratigraphy, paleontology and geology of the central Santa Cruz Mountains, CalHornia Coast Ranges: U.S. Geological Survey, Professional Paper 1168, 51 p.

Dibblee, T.W., Jr. and Brabb, E.E. (1978) Preliminary geologic maps of the Chittenden, Los Gatos. and Watsonville East quadrangles, CalHornia: U.S. Geological Survey, Open-File Report 78-453, scale 1 :24,000

Greensfelder, R. w. (1974) Maximum Credible Rock Acceleration from Earthquakes in CalHomia: CalHomia Division of Mines and Geology, Map Sheet 23

Hall, N.T., Sama-Wojcicki, A.M., and Dupre, W.R. (1974) Faults and their potential hazards in Santa Cruz County, CalHornia: U.S. Geological Survey, Miscellaneous Field Studies Map MF-626, scale 1 :62,500

Lawson, A.C., and others (1908) The California earthquake of April 18, 1906: Report of the CalHornia State Earthquake Investigation Commission: Washington, Carnegie Institution, Publication 87, 451 p ..

Rogers, T.H.: Santa Cruz Mountains Study - Surficial Deposits Map, CDMG 1973

Schlocker, J. (1971) Generalized geologic map of the San Francisco Bay region, CalHornia: HUD Publications, Basic Data Contribution #8

U.S. Geological Survey (1988) Probabilities of large earthquakes occurring in Calffomia on the San Andreas fault: U.S. Geological Survey, Open-File Report 88-398, 62 p.

U.S. Geological Survey (1989) Preliminary map of fractures formed in the Summit Road-Skyland Ridge area during the Loma Prieta, CalHornia earthquake of October 17, 1989: U.S. Geological Survey, Open-File Report 89-686, scale 1 :12,000

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