. -~ k

Planning Department Contra Costa County County Administration Building. North Wing

P.O. Box 951 Martinez, California 94553 -0095

Anthony A. Dehaesus Director of Planning

Phone: 372-2024

November 5, 1981

Mr. Earl Hart Calif. Division of Mines and Geology Ferry Building, Room 1003 San Francisco, CA 94111

Dear Earl:

In regard to the PG&E draft report (Perry Wong called me to say this is the report you referred to in item 2 of your 10/30 letter), the report has not and will not be formally reviewed, and is offered for general information only.

Thanks for the copy of PG&E's report of January 1981.

TN:dg

Sincerely,

Anthony A. Dehaesus Director of Planning

Todd Nelson Senior Planning Geologist

October 30, 1981

Mr. Todd Mel son Senior Planning Geologist Contra Costa County Planning

Depart111ent P. O. Box 951 Martinez, California 94553

Dear Todd:

~-

The following reports were received with your letter of October 29, 1981:

Kaldveer and Associates, October 17, 1978, Geotechnlcal Investi­gation, trow Canyon Executive Park, San ~mon, CA.

v" Pacific Gas and Elnctrlc Company, October 3, 1979, Preliminary Report, San Ramon Valley Fault Pick-up, August 31, 1979 (draft).

The Kaldveer report will be filed Informally pending official review by the County. If this has already been approved, please Indicate who reviewed It and send review C011111ents so we can place this report In our A-P file.

f1G; F(/fv,fi,.d/h~~~) The PG&E report will be placed In the file with AP 1280, a;Keld1ee1 -

.a..d..J;_11e'n'ts Investigation of January 1981. A copy of the latter re-port Is enclosed as requested.

EWH/clz

Enclosure

cc: A-P fl le (2)

Sincerely,

Earl W. Hart Senior Geologist

t' +&a·&210'1REv. s.701/ . ' ' , ·po~E .. ~------·------

M­

FOR INTRA-COMPANY USES

DIVISION OR DEPARTMENT ENGINEER! NG RESEARCH FILE NO. 4QQ RE LETTER OF

sueJEcT San Ramon Valley Fault - Preliminary Report

October 10, 1979

MR. S. H . SMYTH :

The attached report outlining our subsurface investigation of the San Ramon Va 11 ey Fault is presented in draft form for your review prior to release.

This work was initiated primarily to develop a field operational guide for DER personnel use and operation of the Company's MD-5 Seismograph. This scope of work, however, was extended due to the recent inquiry by Contra Costa Geologist, Mr. Baker, regarding the requested geological trenching at DER's San Ramon facility and survey.

The data obtained from our seismic refraction studies and related information obtained from others indicate that a fault or shear zone exists on the eastern side of the San Ramon Valley, near the base of the Dougherty Hills. The exact location, however, will be established by the County Geologist from the trenches which are presently being excavated on DER property.

Please review and comment on this report, and determine status relative to general release.

K-11- t?-eeot<­- Utt !71''!~

R Smyth, Jr. • t uzQ40

' Beale S • " 94106 in Francisco, CA

,.,.,,_. ·;;_ ; ' .

JaJreS B. Baker TO Mr· artnent

Plar;r1ing-::g~ North Wing AdJtl:ln. B streets Pine.& EsC~4553 Martmez, ..

' ·'

. . "", Report Issued: Report 400-79-10

61 .... 250- 7/73

PACIFIC GAS AND ELECTRIC COMPANY DEPARTMENT OF ENGINEERING RESEARCH

3400 Crow Canyon Road San Ramon, California 94583

LABORATORY TEST REPORT

SUBJECT: PRELIMINARY REPORT, SAN RAMON VALLEY FAULT

PICK-UP, August 31, 1979

This report presents the ts of a subsurface investigation per-:t .,,.; '

formed at two sites located 09/Jf!t:!"!!§,:~;· ~~=-·-~f the San Ramon Valley,

Alameda-Contra Costa County, ifR~:;.t•'.;J:Q,e0~s'ill.._~} the work included the

preparation of an applicabl~ld operation manual for the MD-5 Seismograph;

testing of selected areas t/}_~~~~:(~-~on unit fo~v~sibl~ faults and dis­

continuities in the subsurfa~~nment,_,and the:reconna1ssance of ob--~.~-·-"'"'

servab 1 e surface express ions. Pre 1 imi nar~~tl~i~ maps and reports, pro-~

'. ·. ·.· ... 0 vided by the United StaJe!i:fir~ati:::~~;.,w~ used. Included in this

report are possible co~~-l~~~?~?J'~~;Th1 ~ent status and location ,., .. . !_! (::) . '

of the Eastern Branch of the San Ramon~;JJY ~llJ:lt .

. ·~c· . ,--.:: .. - ~ .. "': ,., . :..-,,~< &.::..1 . "' .- 'I • """ ..,, . ~

Figure 1 and 7 1 sti1{.tf;~~1'.~ca=~t~two field sites and a general-

; zed profile of the surroundiii'g :..topqgraphy """'The seismic wave velocities for

site 1 are calculated a;:· ~d~~~d~~~:;;ggr~ (1-2), while the seismic wave

velocities for site·'~._;;~~~t~ted and ;~tted in diagrams (3-5). A table .. .'' /~-···:r··' ~~:-·· ·_.,~~ 0

of representatiye s~{siiilc:'·Vel.9~i'n:es· is presented in Appendix A.

~c,:~·f'.~~:~~ __ j/.' . . "<i

..

. ;

" . .

2 Report 400-79-10

The two major field sites (fig. 1 & 2) are located on the eastern param­

eter of the San Ramon Valley, at the western base of the Dougherty Hills.

Site 1 is located 500 feet northeast of Pacific Gas and Electric Co.'s Depart­

ment of Engineering Research building. The location for field site 1 was

chosen due to its accessibility, and because of the probability that the

San Ramon fault runs in a northwesterly direction under the Research building,

and several adjoining structures.

Site 2 was located 5 miles south of site 1, on several western-facing

slopes, adjacent to the junction between Alcosta Blvd., and Old Ranch Road,

San Ramon. The location for site 2 was chosen due to its accessibility and

its· association with several east-west trending trenches, excavated by a local

land developer firm last June. The trenches have since been filled in, but

are still clearly marked with survey stakes to indicate the location of a

shear zone, trending N25W. In all, there are twelve such exploratory trenches

located a 1 ong the base of the Dougherty Hi 11 s. A survey 1 i ne through severa 1

of these trenches shows that the overall fault is orientated at N3rn.

..

•' . ·.'

3 Report 400-79-10

Field site 1 (Diagram 1) presented the problem of having no previous or

current data pertaining to the subsurface environment. The general topo­

graphic expression had long since been changed by the introduction of over

100 cubic feet of fill material, introduced from nearby construction.

A single traverse (west-east) was made at-field site 1 with the MD-5

Seismograph, 1500 feet north of the Department of Engineering Research build­

ing (Diagram 1). The seismic profile can be seen in fig. 1, while representa­

tive soil velocities can be seen in fig. 2. By using the seismic velocity

chart in Appendix A, changes in seismic wave velocities can be categorized

into representative soil types, giving a greater detailed analysis of the

subsurface conditions. A general explanation of the seismic profile (fig. 1)

would be that a large amount of hard topsoil exists over a second layer of

softer, yet quite similar soil. This confirms any initial observations con­

cerning the introduction of fill material to the site. Again, using the

profile, there also appears to be a large vertical discontinuity at a depth

of 15 - 20' (data from Field Site 2 will confirm such a claim, as well as a

general orientation).

Field Site 2 provided not only a good test location for the operation

of the MD-5 Seismograph, but also provided a precedent for confirmation on

the location and history of the San Ramon Valley Fault. A local firm has

..

• . ' ,'

4 Report 400-79-10

recently completed work on this section of the San Ramon Valley, yet any re­

sults are made unavailable to public view. However, the original exploratory

trenches that have since been filled in, are marked with survey stakes and

flags, giving direct evidence to the location of a shear zone. According to

the position of these trenches, a major shear zone runs N28E through the

eastern side of the San Ramon Valley, According to one survey stake, there

was a vertical displacement of more than 15', in a right lateral, high step

thrust fashion.

The general soil types present at Field Site 2 consisted mainly of a

hard, dry clay-rich topsoil, extending to a depth of 4'. Below this was a

layer of soft, sandy clay-rich soil, extending to a depth of 15 - 20'. Below

this was a harder base rock consisting of a unit of fossiliferous sandstone

and marine-deposited chert pebbles.

A traverse was made on several trenches (Dia. 2) trending in an east-west

fashion, represented by a seismic profile (fig. 3), and a determination of

soil types (fig. 4) and their relative depth (fig. 5).

The seismic profile chart shows a similar soil profile to Site 1, again

characterized by a major change in wave velocity at a depth of 12'. It appears

that this is the location for a major shear zone or discontinuity.

"

5 Report 400-79-10

Conclusions

The MD-5 Seismograph has been proven effective for determining subsurface

structures, such as faults and soil types, under various field conditions.

The revised field operation manual provided supplementary operational data,

as well as a good foundation for using the refraction unit, along with the

main operation manual. Both were used as helpful keys in determining seismic

profiles.

According to evidence recorded in the main body of this report, a possi-

ble shear zone or subsurface discontinuity exists along the base of the

Dougherty Hills, trending in a N31W direction through the San Ramon Valley.

This shear zone appears to pass through a parking lot on the eastern corner

of the Department of Engineering Research building and continues across an -

empty field, passing next to the Aerotest building, and intersecting Moore

Co.'s new addition to their main building. From the data represented in this

report, it would also appear possible that this discontinuity represents the

··-eastern branch of the San Ramon Valley Fault system, also known as the Ridge

Front Fault.

The seismic profile charts (Dia. 1 & 3) recorded at the two field sites

show that the discontinuity represents a right step-high angle reverse fault,

..

•' ..

6 Report 400-79-10

with a vertical displacement of >20'. The faulting occurs in the Orinda

formation and appears to have had recent movement in the last 100 years.

In Summary

It has been shown that a major fault runs up the eastern side of the

San Ramon Valley, near the Dougherty Hills. The location of the fault to

several large structures suggests that further work be initiated to determine

the fault's actual safety margin.* The entire investigation was done through

the work of the MD-5 Seismograph and the results indicate that the refraction

unit is a highly valuable tool when investigating subsurface structures.

Recommendations

- A local seismograph located near the fault zone adjacent to the Aerotest

building could indicate the fault's recent movement. An investigation through

the U.S.G.S. in Menlo Park, could possibly add information on potential fault

activity.

*Recent trenching has begun (9-24-79) and should confirm, substantiate or

refute the findings in this report.

. ' .<,

DER.

LEGEND --con four

-------- supplemenfary confour \.\\\'£shear zone i fau/f -frace ////fill material

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C. ESTIMATING COMPOSITION OF EARTH MATERIALS FROM SEISMIC

38

VELOCITIES. After analyzing the seismic graphs, and determining the velocities and depths, proceed to estimate the composition of the subsurface materials from the table in Figure Til·6.

NOTE: Before attempting to estimate composition from the computed velocities, the seismic analyst should familiarize himself with the general nature of the terrain under study. For instance, he should know generally where the area water table is, whether ·the overburden is a product of weathering of underlying bed~ rock, whether it is mostly glacial drift over limestone, etc. Velocities alone are not a positive indication of material composition.

\Vhen estimating composition, there are several gener::i.l rules which the analyst should keep in mind as he studies the velocity chart. These rules are:

1.) Velocity is roughly proportional to degree of consolidation, or hardness, of the rock or soil.

2.) In unconsolidated materials, velocity increases somewhat with water content.

3.) Weathering of a rock will greatly reduce its velocity.

4.) A particular rock type will include a range of velocities, and these ranges mav overlap for different rock tvpes.

5:) Correlation of velocity with the type of earth material will depend to a great extent on the overall geological characteristics of the area under study (see NOTE above).

6.) Velocity measurements are very sensitive to dip of the interface. If high-precision measure­ments of velocity are required (for such pur­poses as estimating rippability under borderline conditions), always assume that a dip exists and follow the procedure for "Dipping Discon­tinuity" in Section X.

It is entirely possible to use the seismic velocity data to determine such rock properties as rippability and bearing capacity. The velocity requirements for

such properties may differ with each user, depending on the particular field conditions and type of e·arth­moving equipment used. Figure lY-7 shows published data on rippability with a Caterpillar D-9 with ~ 9 Ripper. The data were developed by Caterpillar Trac­tor Company with an MD-1 Seismograph. Velocity data for use with MD-1 will be supplied upon request by the various equipment manufacturers.

FIGURE Ill-6

Table of Representative Velocity Values

(Note: Occasional formations may yield velocities which lie outside of these ranges)

Unconsolidated Materials Most unconsolidated materials

Soil - normal - hard packed

Water Loose sand - above v.•ater table

- below water table Loose mixed sand and gravel, wet Loose gravel, wet

Consolidated l\1aterials Most hard rocks

Coal Clay Shale - soft

- hard Sandstone - soft

- hard Limestone - weathered

- hard Basalt Granite and unwe3.thered gneiss Compacted glacial tills,

hardpan, cemented gravels Frozen soil Pure ice

Below 3,000 feet/second

800 to 1,500 1,500 to 2,000 5,000

800 to 2,000 1,500 to 4,000 1,500 to 3,500 1,500 to 3,000

Above 8,000 feet/second 3,000 to 5,000 3,000 to 6,000 4,000 to 7,000 I 6,000 to 10,000 ! 5,ooo to 1 ,ooo I 6,000 to 10,000 !

As low as 4,000 j 8,000 to 18,000 ! 8,000 to 13,000 !

10,000 to 20,000 J

I 4,ooo to 1 .ooo 1

.

4,000 to 7,000 10,000 to 12,000 I

. . Appendix A

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