department of conservation division of mines and …...jun 14, 1978 · state of california • the...

-~..,

STATE OF CALIFORNIA • THE RESOURCES AGENCY PETE UJLSON, Governor

DEPARTMENT Of CONSERVATION DIVISION OF MINES AND GEOLOGY BAY AREA REGIONAL OFFICE 185 Berry Street, Suite 3600 San Francisco, CA 94107 Phone (415) 904-7707

ATSS 539-7707 Fax (415) 904-7715

November 3, 1993

James.O. Berkland, County Geologist s.anta Clara County Planning Dept. Government Center, East Wing, 7th Floor 70 West Hedding Street San Jose,. CA 95110

Dear Jim:

This is to acknowledge receipt of the reports submitted to us for file a month or so ago. Eleven of the reports (list enclosed) are for projects in Special Studies Zones and are accepted for the AP-file. An additional 34 reports (list enclosed) are fault or fissure investigations for sites outside the SSZs and will be added to our c-file collection-

Also, receipt is acknowledged for the 6 geologic and geotechnical reports prepared by consultants for the Planning Bepartment following the Loma Prieta earthquake for the Mary Alice Way landslide (Assoc. Terra Consultants), Redwood Drive landslide (Assoc. Terra Consultants), Alder Heights Rd. - Laurel Rd. (Wm. Cotton), Helen Way area, Redwood Estates (Assoc. Terra ~onsultants), and Goebel Court Landslide (Assoc. Terra ~onsultants). These reports will be placed in our C-file.

. The remaining geologic and soils reports (perhaps 100 or more) are for sites outside SSZs and do not provide significant information on faults or fissures. It is our intention to discard these or give them away to a consultant. If you want these reports returned to your file or object to our giving them away, please let me or Perry Wong know by November 17.

Thanks for your help and cooperation.

EWH:ra Enclosures cc: Perry Wong/A-Pi"

Sincerely,

EARL W. HART, Senior Geologist &

Program Manager

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I I I I I I I I. I I I I I I I I I I I.

PLANNI~!"' C:PT. COPY.

GEOLOGIC EVALUATION LANDS OF HALL

SANTA CLARA COUNTY, CALIFORNIA

For

Wil 1 iam Hal 1 P.O. Box 606

Gilroy, California

By

TERRATECH, I NC. 8505 Church Street G i1 ray, Ca 1 i fa rn i a

June, 1978

(&({) Uva., Rd ?-</. '7 71.iJ

Cl d.J. <' .f-t ·~ C'f'i

vec'<l.JUN 1 2. 1979 e'A? . Engineering Geology Unit County of Santa Clara Land Development Engineering 70 West Hedding Street San Jose, CA 95110 (299-2871)

c{ /CJ-2-7- 7i

FIGURE I

Base map ref. U.S.G.S., 1973 1 Mt. Madonna, 71/21

Quadrangle.

Fau Its mapped by Dibblee REGIONAL MAP

---- Fault located in LANDS OF HALL field by TERRATECH ·

SANTA CLARA COUNTY, CALIFORNIA fg Green stone

Scale: i"= 2000' fs Sandstone

ROJECT 7244 TERRATECH JUNE I 1978

I I I I I I I I I I I I I I I I I :1

••

GEOLOGIC EVALUATION LANDS OF HALL

SANTA CLARA COUNTY, CALIFORNIA

INTRODUCTION

Lands of Hall, consisting of approximately 319 acres, is located west of Uvas Reservoir and Uvas Road, extending into Eastman Canyon, in Santa Clara County, California. It is proposed to subdivide the property to provide approximately twenty-five (25) individual parcels of minimum 10 acre size. Three parcels will. contain existing structures, and a single family residential structure will be constructed on each of the remaining twenty two (22) parcels. The primary objective of this evaluation was to identify geologic hazards present on the property with particular emphasis on an evaluation of potential building sites.

INFORMATION PROVIDED

A 1" = 100 1 scale map, entitled "Tentative Map, Lands of Hall," (undated) prepared by MH Engineering Company, project civil engineers, was provided, a reduced copy of which was used to prepare our Site Geologic Map, Figure 2.

SCOPE OF WORK

Our geologic evaluation of this property was limited to the following work:

1.

2.

3.

4.

s.

A review of available geologic maps and literature pertaining to the subject property and its environs.

A review of stereoscopic aerial photographs of the property and surrounding area.

Field mapping of pertinent surface geologic features.

Magnetometer surveys across zones of suspected faulting.

Preparation of this report presenting:

a. A regional Map, showing the location of the property, (Fig.1)

b. A Site Geologic Map and a Geologic Cross Section (Figs. 2 and 3 respectively) presenting pertinent geologic features and subsurface geologic structure.

c. A discussion of existing geologic hazards.

d. Our findings and recommendations concerning the suitability of the site to accommodate the development as currently planned, including a discussion of the proposed roadway alignments.

•

I I I I I I I I I· I I I I I I I I I I

June 14,-l978 Project 7244 - 2 -

FINDINGS

Si~e description

Lands of Hall is situated within the western foothills of the Santa Cruz Mountains west of Uvas Reservoir (see Figure 1, Regional Map). The site consists of moderately steep ridges and narrow valleys associated with and including the Eastman Canyon Drainage. The ridges north of the creek surrounding Eastman Canyon are generally covered with low brush and wild grasses. However, some of the ridge flanks and valleys support a very luxuriant assortment of California live oak, bay, various conifers, madrone trees and ground cover of large manzanita and other varieties of dense brush.

The creek flowing through Eastman Canyon occupies a valley ranging in width from approximately 400 feet in the eastern portion fo the site to about 30' in the west. The creek channel is predominantely straight, with only a few slight meanders, and no apparent flood plain or levee development noted on the property. The creek bed itself is lined with coarse grained elastic sediments, ranging from sand to cobble size. The hills south of this creek are also moderately steep and covered with very dense vegetation in the form of oak, madrone, ?ines and redwoods and a dense ground cover of various types of heavy brush.

Several homes existing on the site are to remain as part of site development. Their locations are shown on the Site Geologic Map (Figure 2). At least two large areas have been used in the past as dump sites, and many smaller areas on the site bave debris in lesser amounts.

Underground utilities (primarily water lines) may be present within several locations on the site. A wel 1 and pump is currently in use belQ~ the pond on the northern portion of the site.

Regional geology

This site is located in the steep· foothi 1 ls of the northwest trending Santa Cruz Mountains, directly west of Uvas Reservoir (see Flg. 1). Bedrock in this area consists predominately of the Jurassic to Cretaceous Age Franciscan Formation with scattered outcrops of younger Serpentine and Tertiary Age elastic sedinentary rocks (ref. 3). Past tectonic activity along the numerous faults in the vicinity of this site have_ created complex structural and stratigraphic relationships between these various rock units. Many of those units are highly folded and sheared and are somewhat prone to landsliding. Major faults in the area include the active San Andreas Fault, located approximately 4 miles southwest of the property, the potentially active Sargent Fault Zone about 1.5 miles southwest of the site and the active Calaveras Fault located approximately 8 miles to the northeast (ref. 1).

I I I I I I I I I I I I I I I I I I I

June 14, 1978 Project 7244 - 3 -

Mr. Thomas W. Dibblee, Jr. of the United States Geological Survey (ref.3) has mapped numerous small faults including a fault within and paralleling the southern property boundary (see Figure I). Al I of these faults have a general trend to the west, may have demonstrated both horizontal and vertical displacement in the past and are be! ieved to be inactive. The fault mapped by Dibblee near the southern property line was located by field mapping, and its proper location is shown on both Figures I and 2.

Site geology

The subject property consists of moderate to steep slopes and accompanying narrow incised valleys associated with the easternmost part of the Eastman Canyon drainage system into Uvas Reservoir. Elevations within the site range from approximately 1000 feet on the north boundary to about 480 feet along Eastman Canyon Floor. The dominant rock type found on the property is greenstone (an altered volcanic rock) of the Jurassic to Cretaceous Age Franciscan Formation. Scattered pockets of tan, silty, shaley claystone with locally interbedded large (diameter to 6 inches), black chert. nodules (Monterey Formation? of Miocene Age) and a cream to buff, dolomitic limestone (Calera Ls? of Jurassic to Cretaceous Age) were noted on the property as· surface float material, probably existing insitu at depth (see Fig. 2) A thin accumulation of colluvial soil, generally less than one (I) foot thick has developed on the steeper slopes and ridge crests. This surface soil. cover thickens rapidly in a downslope direction converging with thick alluvium (Qal) along the drainage of Eastman Canyon. Several large, moderately old, deep seated landslides were observed occupying the moderately steep slopes along the south property boundary (see Fig. 2). These landslide areas are characterized by a chaotic mixture of Franciscan greenstone and Franciscan sandstone debris, hummocky topography and erosional remnants of scarps and toes. The exact dimensions of these slide masses were difficult to de! ineate due to dense brush and forest canopy covering the slope. A dip slip (vertical displacement) fault separating Franciscan greenstone from Franciscan sandstone has been mapped (ref. 3) as generally extending through these slide masses within the property boundaries. Our study and interpretation of aerial photographs as well as field mapping accomplished in this area indicate that this fault is located immediately south of and off the subject property. Movement along this fault may, in part, be responsible for the extensive lands I iding in this portion of the property.

Several old landslides have been noted on the property.north of the Eastman Canyon drainage (see Figure 2). These slide masses are very large, and appear to have been stable for some time.


June 14, 1978 Project 7244 - 4 -

We detected no.evidence of faulting on the property, however, by our .study of aerial photographs and subsequent detailed field examinations, we have detected a series of linear features extending through several saddles in that portion of the property north of the Eastman Canyon drainage. These linear features suggest the possibility of shear zones within the Franciscan greenstone bedrock of this area (see Fig. 2). Two active springs coincide with this lineation, further indicating a contact or possible zone of weakness. We therefore think it prudent and recommend discretion in locating building sites relative to I ineation features as shown on Figure 2.

The creek which flows through Eastman Canyon and ultimately through the southern half of the property is fed from the water shed at higher elevation in the Santa Cruz Mountains and adjacent foothills.

As previously stated, the creek channel is basically straight, meandering only slightly in the wider portions of Eastman Canyon, crossing the unpaved Eastman Canyon access road in several locations. The creek bottom is paved with predominantly coarse grained sediments ranging in size from sand to cobble size clasts consisting predominantly of greenstone but pebbles and cobbles of chert, greenstone breccia, and sandstone were also noted. Both clast size and distribution, as well as geometry of the channel would indicate this creek is subject to periods of high velocity water flow. In certain areas, the creek bank has experienced minor erosion due to stream cutting and may ultimately result in shallow seated slope failures.

GEOLOGIC HAZARDS

Landsliding

Several large, deep seated (40 to 60(±) feet), moderately old landslides exist on the slopes along the southern property boundary (see Fig, 2). Although these slides are relatively old, there is the possibility that the slides have not reached full equilibrium and if a major seismic event were to occur, they could be reactivated. Several old landslides also exist within the northern portion of the site.

Most of the lands I ides in this area appear to be old and stable; and the topographic features that generally typify recent sliding have been altered or obliterated by erosion.

Seismic conditions

Seismic hazards constitute one of the most significant hazards to the property. The nearest known active fault is the San Andreas Fault, located approximately 4 miles southwest of the property. the active Calaveras Fault system is approximately 8t miles to the northeast and the potentially active Sargent Fault Zone lies about 1.5 miles southwest.


June 14, 1978 Project 7244 - 5 -

Earthquakes caused by movement along any of these faults could affect the site. The major seismic hazards and their potential effects of the site are described below:

I .

2.

J.

Ground Rupture - Since there are no known active faults crossing the property, we feel the like] ihood of ground rupturing due to faulting is remote, however, the presence of several suggestive Jineation features on the property dictates discretion in building site selection.

Soil Liquefaction - This hazard is primarily associated with loose, saturated, clean fine ·sands, which we do not be! ieve to be present on the property.

Ground Shaking - This property, like many other coastal zone properties in Northern California, lies within one of the more active seismic regions of the United States. During the next few years it is probable that the site will be shaken severely by an earthquake with an epicentral location on or near one of the active faults within a few miles of the property. This hazard is common to all properties in and around the Santa Clara Valley. It is probable that prolonged severe ground shaking during a seismic event could reactivate steeper portions of the existing lands! ide masses on the property, or possibly even initiate new sliding.

Flooding

Although we feel the possibility of flooding along Eastman Canyon Creek to be remote it must be considered as none the less a possibility. To afford maximum protection from this particular hazard we recommend that building sites be located as distant as practicable from the existing creek channel.

CONCLUSIONS AND RECOMMENDATIONS

It is our opinion that the site is suitable for the proposed residential use, provided that the recommendations presented in this report are carefully followed.

Building sites

Twenty two (22) building sites chosen by MH Engineering Company, as shown on our Site Geologic Map, are suitable for single-family residential structures, provided care is exercised in the construction of cuts and fills of both building pads and access roads.

Cuts and fills

All grading operations should be inspected by an Engineering Geologist or Soils Engineer. Where approved, cut slopes in competent bedrock materials can be established at an inclination of H:l (horizontal to


June 14, 1978 Project 7244 - 6 -

vertical). of all fill (hor i zonta I

All excavation in soil material as well as the construction slopes, should be established at no steeper than 2:1 to vertical).

Leachfields

Leachfields generally should not be constructed on slopes steeper than 3.3:1 (30%). Where possible, leachfields should be located as far away from the residential structures as possible, and preferably not immediately downslope of the house.

These portions of the site underlain by greenstone are likely to exhibit only low to moderate permeability.

Access roads

The ma in access road through the site, and the roads to ind iv i dua I houses shown on the Tentative Site Plan prepared by MH Engineering, are feasible from a geologic. standpoint. It is important, however that all cuts and fills along roads be inspected (and tested, as necessary) by an Engineering Geologist and/or a Soils Engineer during construction. Determination of appropriate cut slopes can be made only in the field after inspection of the exposed bedrock.

Further investigation

I. Physical percolation testing should be performed upon representative site soi ls to properly assess permeabi I ity characteristics and to determine leachfield requirements.

2. A Geotechnical Investigation should be performed upon each selected building site to establish the physical and mechanical properties of the site soils as well as to provide information relative to foundation design and retaining wall parameters.

LIMITATIONS

Modification in the planned development could render our initial conclusions invalid unless such changes are reviewed by our staff and our specific conclusions modified accordingly.

Our findings and conclusions,although preli~inary in nature, have been made in accordance with the geologic and geotechnical engineering principles and practices generally employed by the geotechnical profession. They have been based upon data obtained from review of published I iterature,from direct field observations and from discussions with officials of pub! ic agencies.


June 14, 1978 Project 7244 - 7 -

The contents of this report must be considered preliminary and should be supplemented by a detailed field investigation of site geotechnical conditions. Any person concerned with this project who has knowledge of conditions or features of the site or its surrounding areas that are significantly different from those described in this report should report them immediately to this office for evaluation;

Report prepared by:

JDM:kd

Reviewed by:

'-144<-d~a~ Mi chae 1 A. Kern CEG 996


REFERENCES

1. Brown, Robert D., 1970, Faults that are Historically Active or Show Evidence of Geologically Young Surface Displacement, San Francisco Bya Region: U.S. Geological Survey, Misc. Field Studies Map, MF-331.

2. California Division of Mines and Geology, 1976, Special Studies Zone Map of the Mt. Madonna Quadrangle, State of California.

3. Dibblee, Thomas, W. Jr., 1973, "Preliminary Geologic Map of the Mt. Madonna Quadrangle, Santa Clara and Santa Cruz Counties, California: U.S. Geological Survey Open File Map.

4. Rogers, Thomas H., and Williams, John W., 1974, Potential Seismic Hazards in Santa Clara County, California: California Division of Mines and Geology, Special Report 107.

5. U.S. Geological Survey, 1973, Topographic map of the Mt. Madonna 7!' Quadrangle: U.S. Department of the Interior.

6. Aerial Photographs in stereo pairs:

\

~ Series Photo No. Type Scale

USGS VCMI 1-145 b/w l :80,000 1-147

Santa Clara 771021-A 2-1 to 2-6 b/w f1:5,658 Aerial 3-1 to 3-3

Date Flown

4/24/70

2/16/78

l

\

- r,

8505 CHURCH STREET

Hr. James 0. Berkland, County Geologist County of Santa Clara Land Development and Engineering 70 West Hedding Street San Jose, California

Subject: Geologic Report Lands of Ha 11 No. 24S77.21

GILROY, CALIFORNIA 95020

June 8, 1979 Project 7244

Santa Clara County, California

Dear Jim:

We have just received a copy of your May 31st, 1979 Memorandum to Hr. Dick Hall, Senior Planner, concerning the Lands of Hall.

In accordance with our recent discussions and to expedite matters somewhat, we have printed additional copies of all of our previous reports and are submitting them.

(408) 842 023~

We would be pleased to respond to any questions or comments you might have.

Very truly yours,

(. { Q ., / T'.ERRATECH,~lC. r .. (~/ . {' / ~~eph N. Gabriel

\~G:jb ,'

cc: HH 'Engineering, Attn: Hr. Bill HcClintock Hr. Di ck Ha 11

,,

-(

8505 CHURCH STREET

MH Engineering Company 15685 Monterey Road Morgan Hill, California

Attention: Mr. William McClintock

Subject: Proposed Homesites Lands of Hall


March 7, 1979 Project 7244


Gentlemen:

(408) 842-0236

This letter presents our geologic evaluation of the proposed homesites shown on your 1"=100' scale map entitled "Tentative Map, Lands of Hall, ••. " undated (Revision #3, February 2, 1979).

A 11 of the homes i tes shown on that map, with the exception of, the one shown on Lot 21, are geologically acceptable. The homesite shown on Lot 21 is not considered.feasible because of the steepness of the slope.

Those portions of the leachfields shown on Lots 18 and 19 that lie downslope of the homesites, should be reserved for expansion.

As previously discussed, a geotechnical investigation should be performed on each proposed homesite prior to finalizing detailed development ·plans. ThiS'"iS necessary to determine the geotechnical suitability of the individual homesites, as well as to delineate any special design and construction considerations that might be required.

We hope the information presented in this letter satisfies your immediate, project needs.

Very truly yours,

TERRATECH, I NC.

Michael A. Kern

NAK:jb

cc: Mr. Darrel Shuck Mr. William Hall

I

8505 CHURCH STREET

Terra-Sol Ltd. P.O. Box 1824 Gilroy, CA 95020

Attention: Mr. Darrel Shuck

Subject: Lands of Hal_l


September 8, 1978 Project 7244


Dear Mr. Shuck:

·.

(408) 842-0236

You recently requested information concerning the effect that a rapid drawdown of Uvas Reservoir might have on the proposed residential development. At full capacity there- is a minor filling of the creek channel on the south of Lots 2 through 9. The spillway crest at elevation 487.7 feet would limit backwater elevation to no more than about 490(±) feet along this frontage, if there are no other obstructions or control features in the channel. Rapid withdrawal of water from this channel would not significantly affect the stability of Lots 2-9 to the north of the channel.

A slightly different effect might be experienced on Lot 1, the easterly portion of which has been mapped as an ancient landslide. The slide is ·comprised of a large rock mass, as evidenced by distinct bedrock exposures in the reservoir below and to the east of Uvas Road. Rapid drawdown of the reservoir could result in the buildup of hydrostatic forces on this slide mass, which when combined with seismic forces, could lead to reactivation of the slide.

Should you have any further questions regarding this subject, please contact us.

Very truly yours,

TERRATECH, I NC.

Michael A. Kern

MAK:jb

• j

. '

8505 CHURCH STREET

MH Engineering Company 15685 Monterey Road Morgan Hill, California

Attention: Mr. William McClintock


August 30, 1978 Project 7244

Subject: Proposed House ·Locations - Lots 4, 18, 21 and 24 Lands of Ha 11 Santa Clara County, California

Gentlemen:

(408) 842-0236

This letter presents our geologic evaluation of the revised homesite locations proposed for development on Lots 4, 18 and 24, and the homesite to be added on Lot 21. These four changes are shown on the map entitled "Tentative Map, Lands of Hal 1", prepared by MH Engineering Company, revised May 1978.

From a geologic standpoint all four.homesites appear to be suitable for the intended residential use. We must emphasize, however, that a geotechnical investigation should be performed on each proposed homesite prior to construction.

Should you have any further questions concerning this matter, please contact us.

Very truly yours,

TERRATECH, INC.

Mi chae 1 A. Kern

MAK:jb

•

• . '

Mr. William Hall P.O. Box 606 Gilroy, CA 95020

8505 CHURCH STREET

Subject: Geologic Evaluation Lands of Hal l Santa Clara County, California

Dear Mr. Hal l :


February 2, 1978 Project 7244

..

(408) 842-0236

A major portion of the work required for the Geologic Evaluation of your property has been completed. Prior to performing any additional work on this project however, we must await the topographic map and aerial photographs currently being prepared by both Santa Clara Aerial Surveys and MH Engineering Company.

Discussions with representatives of these two firms indicate that the photos will be available in about one week and the topographic map about three to four weeks, thereafter.

We plan to use .both the topographic map and photos to more accurately locate geologic features observed during our field mapping program. The map will also serve as the base for numerous exhibits to be contained within our formal geologic report, which we anticipate being available about two to three weeks after receipt of these documents.

We sha l1 appraise you of any change in this revised schedule, as appropriate.

Very truly yours,

TERRATECH, I NC.

Michael A. Kern

MAK:kd

department of conservation division of mines and …...jun 14, 1978 · state of california • the...

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