STATE OF CAllFORNIA-- lHE AGfNCY ---·· -- ···=--;:_· __ . ,,__;_ -···-· _. ;_,._::::.--·-.:::.......;.:..""':""""'7::'.:.:::. __ .:· ·---:::--==-..:.:=:,...___.-' --·-· DIVISION OF MINES AND GEOLOGY SAN 'RANCISCO DISTRICT mFICE FERRY BUILOI NG SAN fr?ANCISCO, CA 941 l l (Ph{lne 41 !i .. 557 06331 November 7, 1379 Jim Berkland County of Santa Clara County Government Center, East Wing 70 West Hedding Street San Jose, California 95110 Dear Jim: This is to acknowledge receipt of the geological reports submitted with your I etter of November I, 1979. Those reports approved in accordance with the AJquist-Priolo Special Studies Zones Act will be placed on file as required. The other reports will be filed informally. The fault investigation reports outside the SSZ'c are appreciated. EWH/fnl cc: A-P Bill Bryant EARL W. HART Senior Geologist CEG 935
Transcript
STATE OF CAllFORNIA-- lHE RE~OURCES AGfNCY ---·· -- ···=--;:_· __ . ,,__;_ -···-· _. ;_,._::::.--·-.:::.......;.:..""':""""'7::'.:.:::. __ .:· ·---:::--==-..:.:=:,...___.-' --·-·
DIVISION OF MINES AND GEOLOGY SAN 'RANCISCO DISTRICT mFICE FERRY BUILOI NG
SAN fr?ANCISCO, CA 941 l l
(Ph{lne 41 !i .. 557 06331
November 7, 1379
Jim Berkland County of Santa Clara County Government Center, East Wing 70 West Hedding Street San Jose, California 95110
Dear Jim:
This is to acknowledge receipt of the geological reports submitted with your I etter of November I, 1979. Those reports approved in accordance with the AJquist-Priolo Special Studies Zones Act will be placed on file as required. The other reports will be filed informally.
The fault investigation reports outside the SSZ'c are appreciated.
EWH/fnl
cc: A-P file~,
Bill Bryant
EARL W. HART Senior Geologist CEG 935
County of Santa Clara California
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Land Development Engineering- and SurieylnSI County Gove~nment Center, East Wing
70 West HAdding Street San Jose, Callfornia 95110
299·2a71 Area Code 408
November 1, 1979
State Geologist c/o Earl Hart California Division of Mines & Geology Ferry Building San Francisco, CA 94111
Dear Earl:
Enclosed are ten copies of geological reports submitted to Jim Berkland, Santa Clara County Geologist, under the provisions of the Alquist-Priolo Act and the Santa Clara County Geologic Hazards Ordinance. The reports are listed on an attached sheet.
#Reports in areas not in Alquist-Priolo Zones but in County-designated Hazard Zones and are included for your files.
*Reports for a single-building site in an Alquist-Priolo Zone which is exempted from the Alquist-Priolo Act.
Sincerely, ·.·.· :-:'-.''
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cu~ <J1,J~Q_. •. ' ..... "/'
.,,,...,.,,.~, '~'jj" '"i.~ , •. "1fl.
JB:ac
tJ.sim Berkland CEG 58 Santa Clara County Geologist
*Alum Rock Avenue '" Car 1 Murrell 764-4 7- 73-79S
New Avenue Stephen Malone 22 S 76.14
Bannister Avenue Green Acres Sales Co. 22S75.14
#Hecker Pass Hwy.,,- Big Creek Lumber 806-63-15-79P
*Comanche Trail' Fay Petroff 766-28-42-79B
#Alram Road Jack w. Collie
#Pharmer Road"~ Edward A. Lazzarini
625-46-43- 79A 626-46-43-79A 627-46-43-79A
647-66-21-798
Date Of Report
Date Approved
10-11-79
9-4-79 9-25-79
11-15-78 1-12-79
J-78 2-8-78
3-797 9-13-79
8-30-79 9-17-79
7-10-79 10-15-79
9-27-79 10-19-79
#Edmundson Avenue / Joseph c. and Paula c. Barbara
793-66-34-79B-79G 9-28-79 10-23-79
*Crothers Road ·- Paul Hough 17S78.12 5-1-79 10-12-79
Geologist
Rogers Johnson CEG 101
J.C. Prendergast CEG 9
Ed Danehy CEG 8
Ed Danehy CEG 8
James Slosson CEG 22
J .c. Prendergast CEG 9
J.C. Prendergast CEG 9
Michael Kern CEG 996
Curtis Messenger CEG 6
Jeremy Wire CEG 71
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S7AfF 6~-.i<l21f1'~
'•1'1
EDWARD A. DANEHY
Professional Geologist
737 Pride Street
San Jose, CA 95127
tS A-"' rJ l~T fA: Au""'" " (oRNew Ave) --
1.. 'l. $ 7S' ''1 ( hAfH} ""7!..fJ '
,~fl::f'lllf.I (,"•M fjf'vl.'MAL!l;("'.f 1-J~-·)$ .. --~ t:ng1neering Geology Unit ' County of Santa Ciara l~an~. ~?vGlopment fnuineering ~o ~.e,, Hodd111g Stroet &.:~, Jose, CA 85110 (299-2871)
CONCLUDING FEASIBILITY GEOLOGIC REPORT of ROLLING HILLS ESTATES, Units 1 and 2,
Santa Clara County, California
For
GREllN ACRES SALES CO. 15585 Monterey Street Morgan Hill, CA 95037
January 1978
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Rolling Hills/1-78
Introduction
This concluding feasibility geologic report is prepared for the 193 acres of Rolling Hills Estates, Units 1 and 2. Figure 1 shows the site location and Figure 2 shows the site. It is proposed that 62 lots of J-acre size be developed together with arterial roads and a water system. A septic system will serve each lot as part of the future homesite construction. These and other aspects of the proposed development are discussed in the Master Environmental Impact Report (Terra-Sol Ltd, September 1977) .
Conclusions
The following conclusions are reached regarding this site1 o it is geologically suitable for the proposed development provided
that the foundations and other improvements are designed for expansive soil conditions and erosion control measures are used
o there is a low hazard for landsliding provided the recommendations below are incorporated into homesite development
• o there is a low hazard for ground rupture due to faulting based on
the findings discussed later in this report o there is a moderate to high hazard for ground shaking due to an
earthquake and appropriate design of structures is needed as is discussed in the recommendations below •
Recommendations
This report provides the geologic setting and conditions of the site based on the scope of work indicated. The design and construction of site improvements should be based on engineering criteria and the design recommendations should be provided by a registered engineer. Soil and foundation recommendations for this development have been provided by Reynolds & Associates (Soil Investigation for Rolling Hills Units No. 1 & 2, September 1977)•
Shrink-swell Damage swell) characteristic. 16 and 17 of Unit 1.
The site soils have an expansive (shrinkThis is especially the case for lots 14, 15,
Progressive damage to foundations, concrete slabs, etc. may occur if corrective design is not used. Refer to recommendations by Reynolds & Associates •
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Rolling Hills/1-78 2 •
Soil Erosion The site soils have the potential of being eroded readily. Where runoff water from paved areas, roofs, etc. is collected it should be discharged with provision to prevent erosion. Graded areas or places devoid of vegetation should be restored with new seeding or plantings •
Landslide Damage The potential for soil slippage or landsliding (slope instability) is enhanced by an increase in the water content of soils and by increasing the slope of the ground. Therefore, discharge of surface water should be designed with respect to nearby improvements. Septic system leach fields increase the water in the ground. It is recommended that leach fields not be placed immediately downslope of a homesite or improvements if the gradient of the ground exceeds 3•1 (horizontal to vertical). Grading should follow the recommendations of Reynolds & Associates,
Earthquake Damage The site conditions relative to seismic (earthquake) shaking and ground rupture are discussed later in t~is report. It is possible that the intensity of shaking may exceed the seismic factor of the 1976 edition of the Uniform Building Code (UBC). The design standards of that code should be used as a minimum basis for the seismic design of buildings. Additionally, roadways and stream crossings that are founded on alluvium or weak soils should be adequately designed (see conditions under Site Geology),
Scope of Work
The initial work consisted of ground and aerial reconnaissance and aerial photographic analysis of the site and vicinity which is
• presented in my Feasibility Geologic Report, July 20, 1976.
Subsequently, several field examinations of the site and vicinity were made with James o. Berkland, Santa Clara County Geologist, and
e one included Charles Armstrong, California Division of Mines and Geology geologist. The subject site is within the map area of the
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Rolling Hills/1-78
Environmental Geologic Analysis of the Diablo Range Study Area, Southern Santa Clara County, California by Charles F. Armstrong
J.
and David L. Wagner, Calif, Div. of Mines and Geology, 1976 (issued by Santa Clara County, 1977),
Detailed field investigations relative to this site included work done on adjacent property to the east by permission of the owner, William Blaettler (see Fig. 2). The work consisted of 4 reversed, refraction seismic traverses totalling 450 lineal feet (see Figs • 4 and 7) and J exploratory trenches totalling 100 lineal feet (see Figs. 4, 5 and 6), Additionally, 2 test pits dug for inspection by the Santa Clara County Health Department were logged (see Figs. 2 and 6). A total of Ji mandays were spent in the field during the reconnaissance and detailed phases of this project by the author of this report.
Site Geology
The topography and geology of the site are shown on Fig. 4. Based on stream bank exposures, test pits, trenches and borings done on or near the site, the Santa Clara Formation underlies topsoil and local occurences of surficial material. The descriptions on Figs. 5 and 6 are representative. The Santa Clara Formation is generally yellowish to reddish brown in color and ranges from silty, gravelly clay to silty, clayey sand or gravel. From an engineering standpoint it is a medium dense to dense soil (see report of Reynolds & Associates).
No slope failures or landslides were observed within the site . However, development of the property will cause changes in surface runoff, soil moisture due to septic leach fields and ground surface gradients. Where slopes are steeper than J11 (horizontal to vertical) caution is warranted. Areas of moderate to severe erosion exist in parts of the site and care is needed to control and minimize erosion •
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Rolling Hills/1-78 4.
Alluvium occurs in the lower flood plain of the stream in Unit 1 (see Fig. J). Most of the alluvial area falls within the corridor dedicated to the Santa Clara Valley Water District. However, roadways will cross this alluvium and other water courses. Weaker or water saturated soils may exist in such places •
Topsoil derived from the Santa Clara Formation and thick is moderately well developed over the site.
·it is poorly developed or absent •
1t to 2! feet In some places
In the northeastern part of the site and east of the property there is a distinct very dark brown soil which is shown as unit "b" on Fig. 3, This soil has a greater expansive character than the other soils on the site. Also, it has a moderately abundant surface accumulation of basalt rock which is not present in the subsurface of the soil. The subrounded basalt rock is cobble to boulder in size and is very abundant on the steeper slopes of the adjoining property to the east. Test pit P-1 and trenches T-2 and T-J (see Figs. J, 4 and 6) suggest a relationship between the dark brown soil and the underlying Santa Clara Formation as clasts of rock typical of the Santa Clara Formation are found in it. However, the association of the basalt with the dark brown soil also indicates a relationship with the volcanic rocks found in the hills east of the site •
Of the 43 borings drilled by Reynolds & Associates, only one (B-14 near pit P-1, Fig. J) encountered water. That was at a depth of 14 feet on 2/10/77, In March, 1976 water was observed in a shallow pool at the junction of the creeks near the east end of the alluvium in Unit 1. Except for this occurrence, all of-the water courses on the site have been dry. There is no indication that ground water will be an unusual problem in site development .
Faults and Earthquakes
The site lies near but not within a Special Studies Zone (see Fig. 2),
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Rolling Hills/1-78 5,
Several postulated faults were mapped by Armstrong and Wagner (1976) as crossing this site. No evidence was found either on or off the site to support the presence of these faults. In particular, a fault postulated near the eastern property line was specifically investigated. Several observational features were used to locallize the field work which initially consisted of seismic traverses (see Figs. 4 and 7). Anomalous readings were further checked by trenching and logging (see Fig. 5). It was found that the location of the anomalous readings corresponded exactly with a lateral change in soil type within the Santa Clara Formation. Careful examination, including observation by James O. Berkland, indicated that the lateral change was a normal sedimentary relationship and not a fault. The lack of anomalous readings in traverse L-3 (see Fig. 7) further supports the lack of a fault in the area investigated. No evidence of active faults exists on the site.
A potentially active fault lies about 1/2 mile east of the site (see Fig. 1) and the active Calaveras fault lies approximately 1 mile east of the site. The active San Andreas fault zone lies 12 miles southwest of the site and has been the major source of earthquake activity in the historical past. A major earthquake centered on either the Calaveras or San Andreas faults could cause moderate to severe ground shaking at this site. The major effects would be potential damage to man-made improvements including buildings, roadways, bridges, embankments, etc •
Areas of weak soils, water saturated soils or steep slopes should be given care in any grading or construction of fills, embankments or structures. The several existing reservoir embankments on the site should be considered relative to their stability and potential risk. Buildings and other homesite construction are recommended to conform to the 1976 edition of the Uniform Building Code as a minimum. If site conditions are suspect or unusual structures are planned, it is recommended that special consultation be obtained from an appropriate professional •
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Rolling Hills/1-78 6.
Limitations
The conclusions and recommendations of this report have been derived in accordance with presently accepted geologic principles and practices and are based upon data obtained from field observations and explorations made at the locations indicated in this report .
Conditions or features encountered during site development or homesite construction which relate to the geology or conclusions of this report should be brought to the attention of the undersigned .
Edward A. Danehy R.G. No. 20, C.E.G. No. 8
Illustrations attached:
Figure 1 - Index Map 2 - Site Map J - Site Geology 4 - Detailed Geology 5 - Log of Trench T-1 6 - Trench and Pit Logs 7 - Seismic Traverses
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Portion of USGS Gilroy 7~ minute quadrangle, 1955 (photorevised 1968)
A Topsoil1 Silty CLAY, some scattered sandstone & basalt rock fragments, dark brown, dry.
A' Faint development of topsoil, medium brown.
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:=_._ ·-·-·-· - ·-_ ·-. - . ::'.:.Q :ia 0 ' 0
B Santa Clara Formation1 Clayey, sandy GRAVEL to clayey, silty SAND with gravel & cobbles of sandstone (no basalt noted), medium reddish brown, slightly damp, hard.
C Santa Clara Fm.1 Sandy, silty CLAY with gravel & rock fragments (mostly sandstone, one gravel-size piece of basalt), light yellowish brown, damp, hard •
C' Grades more sandy & gravelly C" Clay layer, 1/8-1/4" thick, medium
gray, damp.
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Trench T-2 (see Fig. 4)
0 - 4 ft Unit "b"1 Silty, gravelly CLAY, very dark brown to black, dry to slightly damp, hard; abundant subrounded basalt cobbles and boulders on the ground surface but not within the soil, gravel in the clay consists of rock derived from the underlying Santa Clara Formation.
4 - 5 Santa Clara Fm.; Gravelly, silty CLAY, reddish brown, slightly damp, hard1 gravel mostly sandstone, tuff and siltstone.
Trench T-3
O - 3 ft Unit "b"1 as T-2 3 - 4 Santa Clara Fm.1 as T-2
Pit P-1 (see Fig. 3)
O - 4 ft Unit "b"; Silty CLAY, very dark brown 0-3 ft, medium brown with sandstone rock fragments 3-4 ft, dry to slightly damp, hard; scattered subrounded basalt cobbles on the ground surface but not within the soil; gradational contact with Santa Clara Fm. •
4 - 7 Santa Clara Fm.; Gravelly, silty CLAY, sand to cobble-size sandstone and some basalt fragments, light to medium yellow brown, orange brown in part, slightly damp, hard .
Pit P-2
0 - 1 ft Topsoil1 Silty CLAY, some scattered fragments of sandstone and basalt, dark brown, dry.
1 - 6 Santa Clara Fm.1 reddish brown gravel with clay layers
All trenching was done on 7/22/77 using a rubber-tired tractor mounted with a backhoe .
Rolling Hills Trench and Pit Logs Fig. 6
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Traverses L-1, L-2 and L-2a
The location of the anomalous readings coincided on both the forward and reverse traverses of each line and are shown on Fig. 4. Trench T-1 encountered an increase in elastic content of a clay layer and an abrupt lateral change to gravel below a depth of 6 feet (see Fig. 5), These changes coincide with a slight surface furrow and the anomalous readings of L-2 and L-2a.
Traverse L-3
All traverses were run 3/30/77 using a Nimbus ES-100 Signal Enhancement Seismograph. The Rolling Hills Unit 2 property line lies 10 feet west of station o.o and station 100.0 lies at the edge of soil unit "b" (see Fig. 4) •
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iw-·J. SEP 2 1 1979 v~ M. Engineering Geology Unit County of Santa Clara Lnnd Development Engineering tu Wost Hedding Street S:m Jose, CA 95110 (299-2871}
Subject: Summary of Geologically Suitable Building Sites, Golden Hills, Unit 1
Dear Mr. Malone:
Because of lot number and lot line changes, prior maps showing geologically suitable building sites in Unit 1 are no longer valid. For purposes of prospective purchasers, the attached map is provided which shows the approximate location of building sites. Should a purchaser wish to locate a house elsewhere, a geologic inspection and report for approval by the Santa Clara County Geologist would be needed. Note that only the eastern part of Lots 17 and 18 are within a Special Study Zone of the Alquist-Priolo Act.
All of the locations on the attached map are suitable for development provided that the recommendations of the soil and foundation engineering report by Reynolds and Associates are incorporated into the design and construction of each parcel.
Where slopes are steeper than 3•1 (horizontal to vertical), it ~s especially important to control runoff water and drain fields should be located away and not directly downslope from house sites. This is particularly true of Lots 10, 11 and 12 which have areas of unstable slopes near the building sites. It is suggested that on Lot 12 the house be sited closer to Lot 8 than the steep slopes to the southeast. For such reasons it is important for all of the lots in Unit 1 that the specific location of buildings, drain fields and any excavations over 5 feet in depth be within the areas on the Final Map of Unit 1 approved by me.
Sincerely,
~a ,,-:/ .·!''
Edward A. Danehy
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Summar,Y~of Geologically Suitable Building Sites
GOLDEN HILIS, Unit 1
Approximate location of geologically suitable building site (supercedes prior maps of Unit 1)
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