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• January 20, 1982
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GEOTECHNICAL INVESTIGATION
CARTER PARK NO. 2 SOUTH SAN FRANCISCO, CALIFORNIA
FOR FRANK CHUNG
Job No. 922-10
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FHANK r.:1::1;:...ooAri I ICNRY I MIN(.;H
I i:-NAHll ~) LCNG 1,;r(JH(:;I- (l Hl::lll
DAVID I H()H(:Hl::l'i'l
January 20, 1982 Joh No. 922-10
Mr. Frank Chung
Berlogar, Long & Associates 44G6 Black Avanue •Suite H, PIHanunton. Califorr1ia Hll566
141 S) 484-0770
San Mateo Development Company c/o Geo rt om, Inc. 507 Vicente San Franci~co, California 94116
Subject: Geotechnical Investigation Carter Park No. 2 Carter Drive South San Francisco, California
Dear Mr. Chung:
~OIL E~GIN1-;:1-1:-;
1- Nli1N1::1::111"Jc. c::ro~ 1")(; 1:;; 1 ~·,
We are pleased to submit herewith 10 copies of our geotechnical investigation of the 2.839-acre subject parcel in South San Francisco, California. This study was performed in accordance with the authorization as formalizeJ in our proposal of .July 27, 1981. Our investigation inJicates that development of this property is geotechnically feasible if the recommendations contained in thi.s report are followed. Our fielJ investigation revealed a secondary trace of the San Andreas fault on the property. We recommend not less than 15-foot setbacks on both sides of the secondary fault for all structures intenJe<l for human occupancy.
Recommendations for site grading and foundation designs are also presented in the attached report.
It ha5 been a pleasure to be of service to you on this project. Please do not hesitate to call us if you have any questions or if we can be of further service .
Respectfully submitted,
Bf.RLOGAR, LONG I\ ASSOCIATES
~--{_Yy,~~ Henry L. Minch Chief Geologist
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GEOTJ::C:IINICAL INVESTTGATION CARTER PARK NO. 2
SOUTH SAN FRANC:TSCO, CALIFORNIA
FOR FRANK CHUNG
INTRODUCTION
Job No, 922··10
This report presents the results of our geotechnical investigation of the parcel situated at the northwest corner of Carter Drive and Callan Bo11levar<l ln South San Francisco, California. The report is organized into three sections. The first section describes the purpose an<l scope of our investigation and presents a site description and our understanding of the proposed development. Second is an interpretation of the stereo paired aerial photography and topographic maps and our evaluation of the geologic conditions at the site. The third part presents our conclusions an<l recommendations regarding the geotechnical aspects of the proposed development .
The location of the site is shown on the attached Vicinity Map, Plate 1. The Geologic Map, Plate 2, shows the surface geology and the locations of the borings, test pits and the exploratory trenches. Plate 3 is the Arca Geologic Map. The Regional Fault and Seismicity Map, Plate 4, shows faults and earthquake epicenters in the vicinity of the project site. The Composite Topographic Map, Plate 5, is a compilation of two topographic maps produced by Theodore V. Tronoff Engineers and dated June 1968 and .July 1977. The Composite Topographic Map shows the site topography before and after grading. Plates 6, 7 and 8 are Logs of Trenches T-1 through T-8 and Table A presents the Test Pit Logs. Plates 9 through 21 are Boring Logs. Plate 22 contains the Shear Strength Test Data and Plates 23, 24 and 25, Consolidation Test Data .
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PURPOSE AND SCOPE
Page Two January 20, 1982 Joh No. 922-10
The purpose of this investigation is to provide geotochnical information and recoimnendations for the development of the site . The geologic factors eval11ated include soii, fill and hedrock and surface conditions, along with regional and local faulting.
Our ii1vestigation consisted of tho following:
1. Review of pertinent puhlished and unpublished geologic literature and reports in our library.
2. Interpretation of topographic maps and stereopaired aerial photographs.
3. Site and vicinity geologic reconnaissance .
~- Drilling and logging of nine borings.
S. Excavation and logging of eleven test pits.
6. Excavation and logging of eight backhoe excavated trenches.
7. Laboratory testing of selected samples taken from the borings.
8. Engineering and geologic analyses of the data obtained during the course of this investigation.
9. Preparation of this written report which contains our findings and geotechnical recommendations for tho proposed development.
• SITE llESCRIPTION
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The site, comprising 2. 8 39 acres, is bounded on the east by Call an Drive, on the west by California State Hi.ghway 35 easement, on tho north by King Drive and on the south hy Carter Drive. Adjacent to the northeast boundary of tho site there is a 7-Hlevon Store and a vacant lot. The site slopes gently to tl1e so11th and drains to Carter Drive. The highest point on the site is at tho northwest corner at ahout elevation 633 (MSL) and the lowest point is at tho southwest corner along Carter Drive at ahout elevation 606 (MSL) for an overall relief of approximately 27 feet .
13erlogar, Lo11g ,"-. Associates
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Page Throe January 20, 1982 Job No. 922-10
At the time of this investigation, the site was vacant and covered with a sparce growth of annual grasses, random trash piles and broken concrete.
PROPOSED DEVELOPMENT
It is 011r understanding that the site is to be developed as a residential subdivision consisting of three- and four-story condominiums and tow1tl1ouses or approximately 120 units with subterranean parking. The proposed subterranean parking garage will cover almost the entire development area and will be constructed below exterior finished grade with the garage floor placed at four levels varying from elevations 610 to 625 (MSL). Columns of this structure will be spaced on the order of 20 to 30 feet. The project architect has informed us that the structural loads are unknown at this time .
INVESTIGATION DETAILS
Stereo-Paired Aerial Ph?_t_o_graphy and To_p_cig:raphic Map In_terr_:rotations
Four sets of black and white and a set of color stereo-paired aerial photographs and two topographic maps were analyzed in detail during tl1e course of this investigation. The topographic maps reproduced as a composite drawing in this report on Plate 5, were produced by Theodore V. Tronoff Engineers at a scale of 1"=200' and 1"=20' with contour intervals of five foot and one foot and dated .lune 1968 and July 1977, respectively. The sources and identification of the aerial photographs and topographic maps are listed in the references of this report.
Analysis of the older stereo-paired air photos and topographic maps indicates that the site had about 65 feet of relief prior to grading. Grading and filling between 1968 and 1977 has rendered the site for the most part flat and nearly level. The 1955 aerial photographs show that a natural pond occupied the central portion or the site.
A comparison of the 1968 topographic map with the 1977 map suggests that the site was mostly design cut revealing cuts as deep as 35 feet in the west corner of the site (see Composite Topographic Map). However, our exploratory trenches T-2, T-7 and T-8, test pits TP-1, TP-2, TP-3, TP-6, TP-10 and borings B-3 and B-8 indicate fill at their respective locations on the site suggesting that the c11t portion of the site was locally over excavated and later backfilled to its present grade .
Rerlogar, Long & Associates
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Page Four January 20, 1982 Job No. 922-10
Geologic interpretation of the aerial photographs indicates a photolineament on the site believed to be associated with a secondary fault within the San Andreas fault zone. The lineament traverses the site along a trend of about N22W beginning at a point 130 feet from the southwest corner of tho site measured along Carter Drive (Geologic M<~P). This interpretation made from analysis of photos predating area grading is based on the presence of a bend in the long axis of tl1e pond (now removed) that lies along trend with a saddle in the ridge (also removed) to the south of the site .
Aerial photographs taken before grading reveal strong geomorphic features about 50 feet west of the site that are indicative of the San Andreas fault trace.
_Fi<;'l_d_ Tn_v_o0_s tiga tion
The field investigation consisted of a geologic reconnaissance, drilli.ng and logging of seven borings, and excavation and examination of eight exploratory trenches (combined linear length of 1192 feet) and eleven test pits that extended to a maximum depth of 19 feet .
The borings were drilled on July 24 and August 12, 1981 to depths of 8!J to 34~ feet. Relatively undisturbed soil samples were retrieved at selected depths for testing in each boring. All borings were logged by our field engineer as presented in the Boring Logs .
In order to f11rther evaluate the subsurface materials and the potential for faulting, we excavated eight trenches and eleven test pits on the site. Excavation and logging of the trenches and test pits were conducted between July 28 and August 31, 1981, The trenches and test pits were backfilled using the excavated materials with little or no compactive effort,
Locations of the trenches and test pits shown on the Geologic Map were determined by a survey conducted by Theodore V. Tronoff, Civil Engineer, dated August 10, 1981. Locations of the nlne borings were measured from surveyed features on the site. The ground surface elevations at the boring locations were determined by interpolating between ground contours presented on a topographic map prepared by Tronoff Engineers dated July 28, 1977. These locations and clevation5 should be considered accurate to the degree implied by the methods used .
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Page Five January 20, 1982 Job No . 9 2 2 -10
Laboratory moisture and Jensity determinations were performed on most samples and the results arc shown on the Boring Logs. UnconfineJ compress-ion tests were performed on three representative soil samples obtained from three different locations and depths and the results are presented on l'late 22, Shear Strength Test Data. Consolidation tests were performeJ on three representative soft organic peat samples; the results are summarized on Plates 23, 24 and 25, titleJ Consolidated Test Data.
GEOLOGIC CONDITIONS
Maximum relief across the site is about 27 feet. Cut slopes in the sandstone beJrock near the 7-Elcven Store, the vacant lot adjacent to the subject property and the fill slope along King Drive typically have a maximum gradient of 2:1 (horizontal to vertical ratio) with Jess than 5 feet of relief. The remainder of the site is almost flat with a gentle gradient that slopes generally to the south towarJ Carter Drive .
Surficial Units
Artificial Fill - Artificial fill is soil, rock and debris placed 6y man. Fill was encountered in tl1e north portion of the site in the area of the old pond and drainage feature visable on the 1955 photographs. The fill may have been placed after over-excavation to remove high organic content or soft materials. The thickness of artificial fill encountered in the test pits and borings ranged up to 14 feet. The fill consists predominantly of fincgrained silty sanJ with cobble-size angular rock fragments. Penetration resistance encountered during sampling of the borings indicates that the fill material is typically medium dense .
Alluvium - The former pond area and drainage feature near the c~iiiei ~f the site have probably been subject to alluvial deposition over the past several thousand years, perhaps since before the Holocene Epoch (more than 10,000 years ago). The alluvium remaining after the former site grading consists of layers of very dark gray-black peat, brown clayey silty sand and light blue fine-grained sand. A 19-foot-deep test pit, TP-11, excavated near trench T-1 at station 1•95 revealeJ a repeating sequence of these three alluvial units from about
llerlog;-1r, Long & ;\-,,rwi<1tes
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Page Six January 20, 1982 Job No. 922-10
three feet below present ground surface to the depth of the test pit. The alluvium encountered in trench T-J and T-6 appears well consolidated with the exception of the peat layers. The peat layers are probably much thinner now than wl1en first deposited due to partial consolidation. The peat within the alluvium is a well developed resi<luum of high organic content with almost compl!"te der,:omp_o_sition of the original parent plant mated al, I' sug-gcsting that di-e -_ii"l_)J~il!!!L)-~ _geolog_~_C:'11 ly o1 d _;J,nd may have been deposited before Holocene time .
Bedrock
The bedrock material is pre<lominantly a fine-grai.ned sandstone with minor beds of clay (weathered claystone) assigned to the Merced Formation of late Pliocene/early Pllestocene age (deposited about two million years ago). The rock is poorly to moderately indurated, well-sorted, light brown to light blue with medium to thick bedding. In the vicinity of the fault exposed in trenches T-1 and T-3 through T-6, beds of folded and very weathered claystone were encountered.
The regional strike of the beds exposed in the vicinity of the site and in excavations on the site is northwest with steep dips to the north and south. Regional maps of the area (Brabb and Pampeyan, 1972; Bonilla, 1971) sl1ow Franciscan greenstone in fault contact with Merced sandstone west of the si.te. Greenstone bedrock was not encountered on site .
Faulting
The site lies within a currently designated State or California Special Studies Zone for the active San An<lreas fault zone (CDMG, 1981). The most recent California Division of Mines and Geology map showing Special Studies Zones shows an ''approximately located" secondary trace of the San An<lreas fault trending about N22W through the site .
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Page Seven January 20, 1982 Job No. 922-10
The attached Regional F•n1l t and Seismici ty Map shows the re1 ationship of the site to known active faults and historic earthquake epicenter locations. The San Andreas fault is a major active fault capable of an earthquake estimated to have a maximum magnitude of 8.S Richter (Wes5on and others, 1975). The fault separates bedrock units of highly different character and ages and is considered by most geologists to be the boundary between the Pacific Plate on the west and the North American Plate on the east. The main trace of the San Andreas fault that produced ground rupture in the 1906 San Francisco earthquake is mapped about 50 feet to the west of the site (Bonilla, 1976 CDMG, 1976; Brabb and Pampcyan, 1972a) where it is overlain by more than 50 feet of fill and alluvium.
During the 1906 earthq11ake, as much as thirteen feet or horizontal displacement is reported to have occurred along the main trace of the San Andreas fault south of tile site (Mr. Jon C. Cummings, written communication - letter to Mr. Louis H. Goss, Director of Public Works, City of South San Francisco, April 15, 1972 and Lawson, 1908). This measurement, however, is considered by some geologists to be overstated. The estimated recurrence interval for the maximum credible earthquake is from 100 to 1000 years (Borcherdt and others, 1975). Smaller earthquakes arc likely to occur more frequently.
Groundwater
Perched groundwater was encountered at a depth of five feet in exploratory trench T-2 near the west property line and at depths of 7\, 9 and 13 feet in test pits l'P-3 and TP-10 and boring B-8, respectively. Groundwater was not encountered in other borings or test pits at the site. Seasonal fluctuations in groundwater levels should be expected.
STTE FAULT INVESTIGATION
Eight exploratory trenches were excavated on the site and logged to explore for and evaluate near surface faulting. Tile following descriptions present the details from the trench exploration .
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Trench T-1
Page Eight January 20, 1982 Job No. 922-10
Exploratory trench T-1 (490 feet in length) was excavated about 35 feet nortl1west of and parallel to Carter Drive. Comparison of topographic map5 made before and after grading indicates that approximately 35 feet of soil and bedrock at station 4+90 and 15 feet of alluvium at station 2<·60 were removed cluring grading some 15 years ago (see Composite Topographic Map). The bedrock exposed at the four foot depth between stations 4+50 and 4+90 was therefore about 39 feet below original ground surface and the alluvium exposed at a nine foot depth at station 2+60 was about 24 feet below original ground surface.
From station 0+00 to 1+40, rock of the Merced Formation was exposed in trench T-1. This exposure contained a three foot wide zone of faulted bedrock between stations 0+63 and 0+66. From station 1+40 to 2+80 a series of horizontal and concave upward, overl:~pping lenses of interbed<led peat and sand alluvium were exposed. East of the alluviated area from station 2+80 to the end of the trench at station 4+90, moderately hard to hard sandstone be<lrock of the Merced Formation was exposed.
Trench T-2
Exploratory trench T-2 (250 feet in length) was excavated across the northern portion of the site in an area of about two to three feet of former cut, to evaluate faulting in that area. However, trench T-2 exposed seven feet of artificial fill overlying one foot of alluvium to the 8 foot depth of the trench west of station 2+00 and did not expose the fault. The fault, exposed in other trenches in bedrock on both sides of trench T-2, docs not cut alluvium (in thi5 trench) indicating that the fault has not produced ground rupture since the deposition of the alluvium. Beginning near station 2+00 and extending to the end of the trench at station 2+50 unfaulted Merced sandstone was exuosed .
Trench T-3
Exploratory trench T-3 (95 feet in length) was positioned and oriented to intersect the fault exposed in trench T-1 for further evaluation of the nature of faulting in the bedrock. This trench revealed that the fault trends N22W overall but has an irregular surface with locally variable strike and dip. Trench T-3 revealed alluvium beginning at station 0+60 and extending to the end of trench at station 0+95. The remainder of the trench revealed Merced bedrock with fault gouge located between stations 0+41 and 0+43 .
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Trench T-4
Page Nine January 20, 1982 .Job No. 922-10
Exploratory trench T-4 (65 feet in length} was excavated to evaluate the fault south of trench T-1. This trench confirmed the N22W strike of the fault. Trench T-4 revealed Merced bedrock its full 1.ength except where fault gouge was exposed between stations 0+07 and 0+09.
Trench T- 5
Exploratory trench T-5 (25 feet in length) was excavated to evaluate the fault north of trench T-1. This trench exposed fault gouge between stations 0+17 and 0+19; otherwise, the trench revealed unfaulted Merced bedrock.
Trench T-6
Exploratory trench T-6 (185 feet in length) exposed Merced sandstone between stations 0+00 and 0+30. A one to two foot wide layer of fault gouge dipping to the west was encountered between stations 0+10 and 0+30, but does not extend to the present ground surface .
A buried channel, filled with very dense alluvium, was encountered between station 0+20 and 0+90. The channel alluvium, consisting of mottled dark brown clayey sand, appears very weathered (pebbles are weathered in place and arc coated with relatively thick clay skins) s11ggosting an age or perhaps pre-Holocene . From stati.on 0+90 to the end of the trench at station 1+85, a continuous sequence of Merced Formation rock was encountered.
Analysis of topographic maps made before and after grading at the site indicates that at the location of the fault in trench T-6 two to three feet of surface material was excavated and removed. However, the fault in the sandstone does not extend upward to the buried cut surface but is insteod folded to the east and pinches out (Log of Exploratory Trench T-6} .
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Trenches T-7 and T-8
Page Ten January 20, 1982 Job No . 9 2 2 -1 0
Exploratory trenches T-7 and T-8 were excavated in order to investigate shearing in two clay beds encountered between stations 0+85 and 1+15 in trench T-6. The clay beds are derormed and folded without fracturing adjacent sandstone beds indicating soft sediment deformation before llthifaction or the sediment. The clay beds do not show evidence of faulting.
CONCLUSIONS AND RECOMMENDATIONS
General
From a geotcchni.cal viewpoint, we conclude that it is technically feasible to develop the site as presently proposed, provided that the conclusions and recommendations presented in this report are incorporated into the development plans. The primary geotechnical items within the proposed development area necessitating special consider<~tion include: potential of ground rupture along the secondary fault encountered during our field investigation (Plate 2); potential for "very violent" ground shiiking rrom a local earthquake on the San Andreas or other ra11lt; and post construction settlement potential of the soft alluvium deposits at the former pond site, and the existing fill present over a large portion of the site.
Seismic Hazards
Ground Rupture - The 1906 ground rupture tr<ice of the San Andreas fault is mapp.cd some SO feet we<;t of the south corner of the site and trends about N40W (Earl Hart, CDMG, personal conummication). It is generally agreed a-1l10n.[( geoiogisi::s ·that surface rupturing and strong earthquake shocks will he experienced along this section of tho San Andreas fault iind that future breaks are most likely to occur along the traces of latest movement.
Bedrock encountered in the exploratory trenches, test pits and borings on the site consists of fine-grained sandstone and weathered claystonc of the Merced Formation which indicates that the fault contact with Franciscan greenstone lies west of the site. This conclusions is supported by analysis of pregrading aerial photographs tl1at gives strong geomorphic evidence that the San Andreas fault lies about 50 feet west of the site .
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Page Eleven January 20, 1982 Job No. 922-lO
The seismic/geologic report prepared by Gasch and Associates (Nov. 1977; Project No. GA704) for the project site states that th(,rc are "two anomalous slow velocity zones beneath the site" (Gasch 1977, Plate 2). It is our pinion that these seismic anomal.ies observed by Gasch and Associates are caused by concentrations of low velocity alluvium, especially the soft peat revealed during our subsurface exploration.
Our field investigation program revealed evidence of a secondary trace of the San Andreas fault in trenches T-1 and T-3 through T-6 trending N22W across the site. Jn tho vicinity of trench T-6, the fault is folded eastward in the direction of an old alluviated channel. The deformation of tho fault toward the old channel suggests that a gravity differential exi.sted east of the fault that allowed slope failure (either by soil creep or landslide) in which the near surface portion of the fault was deformed to its present configuration. Since last movement of the fault and s11bsequent deformation, the old channel has been filled with alluvium. The apparent age of the alluvium that now occupies the channel suggests that the fault !1as not been active since before Holocene time.
Since the secondary fault lies within the active San Andreas fault zone it may move sympathetically with renewed movement of the ma in trace. We recommend minimum setbacks of 15 feet on both sides of the fault for all structures i_ntendecl for human occupancy. These setbacks are recommended beca11se the surveyed fault trace is well located in bedrock exposed in the exploratory trenches and because it cuts rock along only one well defined trace.
The recommended minimum setbacks are shown on the Geologic Map. The area within the setback zone may be 11sed for streets, parking lots, open spaces or other low use functions, but should not be used as areas for construction of buildings intended for human occupancy. However, cantilever struct11res overhanging the recommended setback zone above ground, in our opinion, are acceptable provided that these structures on both sides o[ the fault should not be extended beyond the centerline of the setback zone and are aligned to avoid contact between buildings on opposite Si.dos of the fault .
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Page Twelve January 20, 1982 Job No. 922-10
It is our opinion that no other active faults cross the site. This conclusions is based on our findings that overlapping peat and sandy layers (inferred to he older than H_olocene age) and Merced bedrock have riot been offset~ · · ·
Ground Shaking - The site can he expected to undergo "very violent" shaking from a major earthquake on the San Andreas fault or other nearby active fault (Borcherdt and others, 1975). The maximum credible peak bedrock acceleration from seismic shock at the site and throughout the Bay Area is estimated at O.Sg (Greensfelder, 1974) and possibly due to the proximity of the active San Andreas fault, as high as 0.74g (Schnabel and Seed, 1972). These high accelerations may he felt for several tens of seconds. However, these data should not be considered design level accelerations for the proposed structures. Ground shaking is a hazard which cannot be eliminated, but can be partially mitigated through proper attention to seismic structural design and observance of good construction practices. Structures built on the site should be engineered to withstand "very violent" shaking as well as possible distortions of the ground due to shock induced settlement within the fill and alluvium areas. We recommend that the final project planning of structure orientation, arrangement, and design he reviewed by a structural engineer with recognized experience in earthquake engineering.
Other Seismic Hazards - In our opinion, flooding, lurching and fiqtiefacTio-n ar·e-ncJ"t"problems at the project site due to the location of the site and subsurface conditions revealed during our investigation.
Grading
Most of planned site grading will involve earth cut and export of the excavated materials. Should structural fill be placed the following criteria are recommended. Whenever referred to in this report, "percent compaction" is relative to the maximum dry density determined in accordance with ASTM Test Designation: Dl557-70:
1. All rubble, debris, and other deleterious materials should be removed from the areas to be graded prior to the start of grading operations ..
lkrlogar, Long & Associates
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Page Thirteen January 20, 1982 Job No. 922-10
2. The uppermost two inches of surficial soils, including all vegetation, should be stripped from both cut and fill areas to be graded.
3. Scarify, moisture-condition and compact the upper foot of subgrade in areas to receive fill to not less than 90 percent compaction.
4. Use as fill material from on-site excavations which is free of organic matter and debris. The excavated on-site soil, excluding any deleterious matter, is considered satisfactory for use in these fills.
5. Compact fill material in level layers up to the design grade. All fill should be compacted to not less than 90 percent compaction .
6. Inspection and field tests 5hall be carried out during grading to assist the contractor and obtain the required degree of compaction and propor moisture content. Where compaction is less than required, additional compactive effort should be made with adjustment of the moisture content as necessary until the specified compaction is obtained.
7. The soil engineer should be notified at least 24 hours prior to commencement of grading operations. Procedures and methods of grading may he discussed between the contractor and the soil engineer. This will facilitate the performance of grading operations and minimize possible construction delays .
8. It must be the contractor's responsibility to select equipment and procedures that will accomplish the grading described above. He must also organize his work in such a manner that one of our representatives can test and/or inspect each element of the grading, including preparation of the site, examination of existing materials, compaction of the subgrade and compaction of each fill layer .
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Settlement
Page Fourteen January 20, 1982 .Job No. 922-10
The alluvial deposits contain layers of organic clay and peat of irreg11l.ar thicknesses. In our opinion, these organic clay and peat layers have very large post-construction settlement potential. Based on our preliminary evaluation, tho differential settlement may be on the order of 6 to 7 Inches for bearing pressure of 3000 and GOOD pounds per square foot, respectively. Differential settlement during and earthquake may especially affect Coundations extending between bedrock and artificial fill .
Foundation D9_"._iJ:n
Results of our Ciold investigations reveal that there are tl1ree different and distinct typos of subsurface materials; bedrock, alluvium and artificial fill (see Geologic Map). In areas where bedrock is close to pad grade, spread footings may he used. They should extend not less than 1 foot into competent bedrock and may be des i.gne<l for an al 1 ow able bearing pressure of 10, 000 pounds per square foot for dead plus live loads.
To preclude differential settlement within the alluvium and artificial fill areas, we recommend that al.1 the proposed structures be founded on bedrock.
The drilled piors should be at least 24 inches in dia.meter and should extend at least three feet into sound rock. Drilled piers should be designed for an allowable end bearing o[ 20,000 pounds per sq11are foot. This design criteria applids only to piers with tips placed at least 10 feet below finished grade (garage floor level). The depth to bedrock in the central alluviated area is not well defined and should be determined by borings .
lt may not be economically feasible to support the garage floor 5Jab in fill and alluvial ereas on drilled piers. If the garage slab is structurally separatod and "free f1 oating" from the i"oundation system, we estimate that future settlement of the slab should he loss than 2 inches. While such settlement may cause slab di5tress and cracking, and perhaps even require local remedial work, it would not affect foundation performance of the drilled pier system. This phenomenon will be more obvious in the transition areas of two different types of subgrade soils. In these areas, we recommend use of control joints to allow tho slabs to crack along predetermined lines .
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Page l'ifteen January 20, 1<)82 Job No. 922-10
All foundation excavations or pier dril.Jing should be inspected by personnel from this office to verify that the subsurface conditions encountered are consistent with tl1ose which formed the basis for the ahove foundation design criteria .
Subter,ri:11ean G~_rage Walls
As the propo5ed subterranean garage walls will be partly or completely helow tho outside grades, they should be designed to withstand lateral soil pressures induced hy backfill materials. Utilizing the on-site sandy soils for backfill, the garage walls should be designed for an at-rest earth pressure computed as an equivalent fluid having a density of SO pounds per cuhic foot. The wall backfills should be compacted to not more than 90 percent of the maximum dry density determined in accordance with ASTM Designation Dl557-70. Furthermore, the wall backfill compaction should he achi.eved by light handheld equipment. Subdrains should be provided around the perimeter garage walls to prevent huild-up hydrostatic pressure behind the garage walls. Tho clrains should be suitably outletted to an a<ljacent storm drain.
Utilities
Due to the potential for rcnClwed fault induced ground rupture, utility lines should he designCld to avoid or, as much as possible, minimize crossing the fault zone. Where it is necessary for a utility to cross the fault, it sho11ld he of flexible design and provided with automatic sl1ut-off valves in case of rupture .
Trench Backfill
The excavation for the proposed garage structure will probably remove the uncompacted backfill in our exploratory trenches. Should any of the ur1<.;ompactCld backfill remain after the garage Clxcavation has hecn completed, it should he re-excavated and replaced as compacted engineered fill .
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INVF.STIGATJON LIMITATIONS
Page Sixteen .January 20, 1982 .Job No. 922-10
This gootochnical invesllgatlon was based on a surface reconnaissance, field investigatlon, photogeologic mapping, review of available geologic literature, engineering and geologic analyses and our understanding of the proposed development. The data accumulated during our investigation are believed to be representative of the total area; however, soil and geologic conditions may vary in character between trenches, test pits, horings and other points of cxami.nation. If the plans for development are changed, we should be notified so that we can make appropriate adjustments in our recommendations.
The conclusions and recommendations contained herein arc professional opinions derived in accordance with current standards of professional practice; no other warranty is expressed or implied .
Respectfully suhmitted,
BE:L//' '.11 'Assomrns l«:~4~-__) Project Geologist
~ e_..__ "\\\:__~ Henry L. \11nch Chief Geologist, CEG 757
Peter Lai Project Engineer
Frank Ilerlogar RCE 20383
Attachments: Plate l - Vicinity Map Plate 2 - Geologic Map Plate 3 - Arca Geologic Map Plate 4 - Regional Fault and Seismicity Map Plate 5 - Composite Topographic Map Plates 6 through 8 - Logs of Trenches T-1 thru T-8 Plates 9 through 21 - Boring Logs Plate 22 - Shear Strength Test Data Plates 23 through 25 - Consolidation Test Data Table A - Test Pit Logs
13erlogar, I ong ,«, Associate-.
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SCALEt 1 1'=2,000 1
VICINITY MAP CARTER PARK NO. 2
CARTER DRIVE
SAN FRANCISCO, SOUTH, CALIFORNIA
FOR
FRANK CHUNG
BASE; A PORTION OF u.s.G.S. SAN FRANCISCO SOUTH, CALIFORNIA, 7\-MINUTE TOPOGRAPGIC QUADRANGLE, PHOTOREVISED 1973.
'--------------------------...... -··-~ ... -~-,,·--~·--------------~ PLATE I
GEOLOGIC MAP, PLATE 2, WAS
REVISED AND IS INCLUDED IN
THE ATTACHED SUPPLEMENTAL
GEOLOGIC REPORT
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SCALE: 1"=5,280'
AREA GEOLOGIC MAP
CARTER PARK NO. 2 LEGEND
CARTER DRIVE GEOLOGIC CONTACT
SOUTH SAN FRANCISCO, CALIFORNIA
Oaf
O<!lr
OTm
FAULT TRACE
ARTIFICIAL FILL
SLOPE WASH AND RAVINE FILL
MERCED FORMATION
fg FRANCISCAN GREENSTONE
FOR
FRANK CHUNG
BASE, A PORTION OF PRELIMINARY GEOLOGIC MAP OF SAN MATED COUNTY~ CALIFORNIA, COMPILED BY EARL E. BRABB AND EARL H. PAMPEYAN, DATED 1972.
'------------------·---··~-------------·----------------~ l3erlogJr, Long & Assucidl<'' PLATE 3
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l•.h NA••l!ll!ll ~ll>l•L~ ouiK<!TvnU -.:(11~1[0 '"'" Qltr.Cll:l;HLDl:lt (1117~)
--'11--. rMLT, [1,11,5>11!.p~lll! ..,...,.tl ...... 11,111.11:.-p ""'""' VJf!;l[KUlfl, DOTTED
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r.11.!ll.T l.OG&TllHll lllODWll:ll r111111 l[ljljll!IH t11n~. >!lfllD NICI ll!:U.CY 11tTTJ, llLllD U,77) illlltl l'&lll'LTiAll (llTll.
l.,~,•IU"jr: !(I' fAllTHi;w,iU:ll IN-' 1Hll.(1iAlll 19'1'1 'lt¢11 t.lll:lt-•(, ••lllillt J.1110 110!!1'111 jll74). WITM lll:LlliTll:lllo
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flllall ~l,Tlll, u.-.a.1., {llllllTTlll COllllllUlllCATIDlll, I-,, 1111.lol!l•Ulll CAEDllU lllAQlllTUDCI 111001r110 fllotl
•*f1•.,llD'Ell l!•rotl, "i.Hto.i "'"'ilt (1111( ... (l•H~ ~II MIKIATIDll llf' Sll.1" Alll"' SOV(~11¥fllll'I llHCll
lllllllElt lll.MIC:ll h, !Sit
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REGIONAL FAULT AND SEISMICITY MAP
l3erlogar, l.011g & Associates PLATE 4
OVERSIZED -_ DOCUMENT HAS
BEEN PULLED AND SCANNED WITH THE MAP
FILE.
• BORING LOG B-1
JOB NO' '127-1(1 DATE DRILLED: 7-24-81
JOB NAME: Carter Pnrk No. 2 SURFACE ELEV_: 607.8 EQUIPMENT' DRILLING Flight Auger DATUM: MSL
• SAMPLER TYPE DRIVE WEIGHT- LB HEIGHT OF FALL-IN
2. ~3 I.D. Solit Spoon 1~0 30
• - ., ~ " c - 0 - <f'l I ~ .. - L c ·2 ~ 4- - 0 a. -<= ~-
=> -"' ., () ·u; ·..:
"' ? -~ u Description E a. 0 L "' - ., <f'l "' "
0 "' - "' -., c > .. d. a - " <.> 0 0 ~ "' ::l- ·-<f'l 0 m a. :.;: <.) 0 <.) -• - SM Silty fine sand with sandstone fragments, brown and gray, damp, medium dense
• ....
4-5 24 12.9 106
5 -
•
• 9-10 28 16.5 110
10 -
~
• ~ SC- Clayey sand with rock frJgments, brown, SP with thin clayey silt seams, wet, medium
dcn~e
14-15 2'i
• 15 -
-- OL Organic clayey si 1 t, gray, with some rock
fragments and sand, medium stiff
• -
19-20 12 19. f 106 20 -
• --- . ·- --·----- ····-·--··~,~-
Uerlogar, Long ,'I, Associates PLATE 9
• BORING LOG B-1
JOB NO.: '122-10 SHEET 2 OF 2 --· JOB NAM£: Carter Park No. 2 DEPTH 20 TO ;;~ ,, FT.
• ....: ~ - c: - ., 0
VJ I c:
~ "'.,_; L - ·c - . - 0
"- .c: " - " (ii ::> ..c ""-; .c ., O'Ui -E - 0 -;;; O' 0 - VJ "' " "'- L - "- ., "
., co ., . i5 c: >,·- ci_ "' - " 0 0 L <1>
:=l i3 VJ 0 "'- ::.;: 0 3: 0 -u 20
OL Organic clayey si.lt (continued) -
• -
CL Sanely clay, gray, inoist, hard
• 24-25 37 20.8 107
25 -
--
~·· -- - - ..... ~
• Bedrock, f i11e-graj r1cd .. sandstone, l. i ght
29-30 60 16.6 113 gray-blue, c1amD to rnoist, \lery clense.
30 -
• -
~
34- .'i41- 59/ - -
• 6" ·~·~·""''""
... _ ...... , .... ,~.- .. ~. '"""'"'"' M--- -· ..~.-
35 - Boring tennina ted at 34'-i rcct No wator encOLmtered
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• PLATE 10
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-· _'!' ._ Q_ -"
E Q. 0 "' Cf) 0
w-Jc .,_ 0 L
- ID rn a.
4-4lj 28
9-10 29
14-15 -'3
16.4 lM
14.6 115
18.l 108
19-20 62 13.2 116
-"' "' ., a_ ._
"' 0
L.
Description
Sandy silt, Jjght brown, w.ith little clay, rlamp, very sitff
I ~~ly silt, da;k gray, ,.;j_J:'11 wood fibers, I/ rock fragments and organic matters, damp,
e----4' vc ry st i rr ------ -------·· ·----- -----··-~ 5 _ 1-0::.::I._1------
SM L. Silty .sand, tan, with layer of rock fragments and clay, r!Jmp to moist, dense
10-
15-
20 -
1-- ···-·----"'---- -~·'-~--------------- ------
~ SC-SP Clayey sand with weathered rock fragrncnts, dark broivn, Jrunp to wet, very dense
..._ ______________________ -----····---------------------____, lkrlogJr, Lonr, K A<,)(_Kiates PLATE 11
• BORING LOG B-2
JOB NO.: <)22-10 SHEET 2 OF 2
JOB NAME: Carter Park No. 2 ---····'-~·
DEPTH 20 TO ·rn FT. . ---m~r-
• ...; ~ - " " - Q) Q en I ~ '" -' ~ <: ·c ....... . - 0
a. .c ,.- => :::::> J:: ~ .c "' o·m -E - Q) O' 0 - CJ) "' " "'- 0 ~ "' Q. ID
" ID "' ·;:; <: ~-m ci ID - " 0
0 co a. 0 o :.:: ::::> ~ ·-
"' ::;: <..> Cl <..> -20 r;c<;r Clayey sancl (continued}
~
• ~
24-25 75 13.6 116
• zs-
c-
I-
• ~ . Sandy silt, dark gray, with sandstone
29-30 111 H.9 118 fragments, druno ' ' har<l
~n "' ·-... ·- . ·-. "~' - -
• I- Boring tcTT!linated at 30 feet No water encow1tere<l
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• -1-
I-
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PLATE 12
• BORING LOG B-3
JOB NO' '122-]0 DATE DRILLED: 7-21-81
JOB NAME: CJ rt er Park No. 2 SURFACE ELEV_: 625. 2
EQUIPMENT: DRILLING I'] i 2ht A11ger DATUM: HSL -• Ji.[l.~PLEB...._TYPE DRIVE WEIGHT- LB HEIGHT OF FALL- IN
2.43" TD SJ2 l it Spoon 140 30
• -· "' ~ c: c ._ 0 - (/") I
"' L - 0 .c '" - "' c c ....c-..:
"' U"(f)'.;: CL ,,,- "' :J -~ ti ..c Description E 0. 0 L '" - "' (/") .. " c ,.. "' Ci. a. - :::>E .~ 0 .,
- "' . ., 0 o:;i: "' <.n Cl "' 0. :::;: <.) Cl u-
• ML Silty fine sond, light brown, <l3lllp' dense
F,-2\ 30 12.8 100
• 4-5 25 15.7 113
s---·· ···-··" ··-·- -----• OL Organic sandy clay, black, moist, stiff
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• S~·i-9~"2 21 19.2 108
lo-~
~ • ~
14-15 22 18.9 llO CL Sandy clay, light gray, moist, very stiff
• 15- to harJ
~
• 19-20 33 18.0 113
20-
• . .. ·--··"'""""~'
PLATE 13
• BORING LOG B-3
JOB NO.: 922-10 SHEET 7 OF 2
JOB NAME: Carter Park No. 2 DEPTH 20 TO ZS FT. .. -- -
• "'"' ~ - c - "'
0 c "' "' -' L - c - . - fJl I 0 a. ..:: ,.- " c ::::J-'=~ .c "' u·u; ~ E - 0 - "' "'u - "' fJ) ., " <i L "' - ci. <i -"' ii.'i "' ·a c ,....-
" u " L "' "' fJ) 0 Q. ::;; 0 0 3: ::::>- ·-u 0 u-
LTI ·-CL Sandy clay (continued)
... _,. __
• - 01 Silty c1 a)', gray, with little S8ll<l, moist, very stiff
~
24-25 30 30.2 86 25 • ---·'"'-·'···~-~~···· - .....
... Boring tenninated at 25 leet No water encountered
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- -13erlogar, Long & :\ssoci<-il<» PLATE 14
BORING LOG B-~
JOB No: __ '122-10 DATE DRILLED: 7-2~-81
• Carter Park No. 2 61.3. 6
JOB NAME: SURFACE ELEV_:
EQUIPMENT: DRILLING Flight Au2.er DATUM' MSL
• SAMPLER TYPE DRIYE WEIGHT- LB HEIGHT OF FALL- IN
2.43" TD So lit Spoon ]40 30
• - ~ c c - "' 0 - c/J I "' .,- - ·c: .:c . - 0
Q. "' "'- " c :J -~ ~ .I= "' u·cn ·.;: - ., Description
E "' - - "' {/) "' " a_ 0 ~ c >- "' "- a_ - c<> 0 "' - "' -., 0 Ci~ ., ::> - --
{/) 0 en o. ::;;: <.> 0 <.> -
• SM- Bedrock, sandstone, finc-gniilled, tan, - SP sl i gl1tly drunp, very dense.
2- zi, 50/6' - -
-• -
s~ "-
-• -
8~-_;l 50/la' - -
, .... _ ----~~
···-~··· -• Boring tcnninated at 8\ feet due to auger
10-- re f1LSal No water encoun t crcd
h
~ • -
• --
-• -
• ..
--~
PLATE:;: 15
• BORING LOG Tl - 5
JOB N0° 97'2-Jll DATE ORI LL ED' 8-12-81
Corter Park No. 2 608,'J JOB NAME' SURFACE ELEV.' EQUIPMENT' DRILLING Flil'ht Auver DATUM' MSL
• SAt-1PLER TYPE DRIVE WEIGHT- LB HEIGHT OF FALL- IN --2.43 ID Split Spoon 140 30
• - ,!! c c - "' 0 - V> I ~ ,,,_ ~ c ·c .:c ..,_; - 0
0. -"' '< - " _c "' o·U) ·-i:: - "' :::'.) -~ u "' fJ) "' " Description E 0. 0 ~ "' 'E ,... "' ci CL -0 "' - "' 0 u
0 0 Ci:;=: "' ::J- -~ fJ) D rn o. ::!' u D u-
• ML Sand, fine -grained , light- brown, damp, ~
dense .
....
-• -
5 -r --·-
OH Organic silty clay, dark grciy-black,
• 6-7l;c 14 moist, stiff
15 ·---···-····--·-
S!j-9 SC Clayey sand, light red-hrown and light
- or.av~ moist, medium dense. -· -• --- .•.. , .. - . - -
l 0 _..._ Boring terminated at 9 feet. No water encountered.
....
• ....
f-
....
• 15 _._
f-
....
• ....
20 _,..
• -· .. ,, .......
PLATE 16
• BORING LOG Tl-SA
JOB NO: g72-]Q DATE DRILLED' 8-12-81
JOB NAME: C;-:1.rt.cr Park No. 2 SURFACE ELEV.' (108. 9
EQUIPMENT' DRILLING Flight A11Fer DATUM' MSL
• SAMPLER TYPE DRIVE WEIGHT- LB HEIGHT OF FALL- IN
2.43 TD SJ21it Spoon 140 30
• - ~ c c - "' 0 (/)I ~ .,,- ~ - ·2 E - 0
.c " c '""'
., U'Ui ~ a. :c - - "' ::> "' ._; .c Description E "- 0 ~ "' c - ., (/) "' " :;i-. "4) <i "- - " <.> 0 ., - ., 0 0 cs 3: "' :::>-·-
(/) 0 al a. ::;: u 0 u-
• ~[, Sand, fi nc-grained, light red-hrown, - damp, rncd:irnn dense.
·--··-· _,, ___ .·-·-
CL Silty clay, black, damp to moist, stiff. ~"--
• 2l;-3\ 27 t3C-SP Clayey gravelly sm1d with sandstone fra.grncnts, light brown, damp, dense.
4-5 17 -· -
5 -SC Clayey sm1d, light gn.y-brown with lirnonitc
• 6-6\ 13 staini.ng, moist, meclium dL'IlsC.
---· .-- ··'"'" ._, --- --
• 9-10 18 19.2 110 CL Sanely c:l ay, mediwn green -gray, moist, very stj rr, minor calichc.
10 -
--• Increasing sand and rock fragments f--
-. -- -· 3\-15 36 SC Clayey sand, with greens tone rock fragments,
light to meditnn blue-gray, damp to moist,
• 15 - dense, Jimonite staining.
--
-• 8!1- 20 36
20 -
I
• '-w" ~.,. . .,.~
fkrlogar, Long & Associate> PLATE 17
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BORING LOG B-SA
JOB NO. : _9 f.""2-...,,1~0 __ SHEET ~~~7'--- OF ~-.c"-
JOB NAME: [ar~cr Park No. 2 DEPTH TO FT.
..;
~ -o_ -<;
E "'-" "' U'l 0
231,-2 35
,8'2-29 98
S! ., 0
L -" c - ., '" -~ ,~
c --<; ., - "' c. -"' 0
20 SC Clayc')' sand (continued)
Inc:rca-;ing rock fragments
25 ~
-
-/ Sand.stone hedrock, well imlurated,
1.1---,..,..----r/ light gray, very dense. ~"
30 --
-
-
-
Boring tenninateJ at 29 feet. No water encountered .
PLATE 18
• BORING LOG B-6
JOB NO: 922-10 DATE DRILLED' 8-12-81
JOB NAME: Carter Po.rk 'Jo. 2 SURFACE ELEV.: 6J0.8
EQUIPMENT: DRILLING Fl i1!11t Auger DATUM: MSL
• S4MPLER TYPE DRIVE WEIGHT- LB HEIGHT OF FALL- IN 2.~3 ID Split SJ)OOn 140 30
• - ~ c c .,_ "' 0 - U'J I ~ "' - L - ·c;;E . - 0 CL .<:: '< .,_ " c
:::) -~ ~ .<:: "' u ·w ·.;: Description E - "' - .,
U'J "' " Ci 0 L "' -c ,... "' Ci "- .,_ :=> ~ .~ 0 0) - "' ·c; 0 0 3' .,
U'J 0 rn "- ::;;: u 0 u .,_
• ML Sand, f:inc-grajncd, light hrown, slightly I..
<lamp, dense.
I..
-• ,_,_
~-5 lG - - CL Silty clay with sand lL"TlSC~ light hrown and
5 - gray, moist) stiff.
S':;-(/; 13 26.5 17.o ~-- -- -
• Oil Organjc.: s:i1ty c.:lay, black-brown, damp to
7-8 13 moist. small roots. stiff.
·-···
Sand, f .i.ne-gralned, light green-gray, cbrnp to moist, dense.
~ .. 11 811~ 9!1 15 --···· --··- ... .,~.·~
• CL Sandy day, 1ight gray, with vertical sandy
10-11 17 IO -silt lense, moist, stiff
(gray silty fine sancl lense at 10.5')
~ • I..
,_ . ·-·-·-·-. ..... - ·-
SS Sandstone bedrock, light gray and light green-gray, weathered, damp to 111olst,
• 14-15 46 15 - dcm;e to ve1y dense.
I..
I..
• I.. Increasing density
52+ -·-· -·--· ·-,. ·-·--- ... ·-·' -·---·· ·····- ....... ' -'·--·
20 _ \.. Boring terminated at 19'i feet. Moisture at 9 feet.
• Flc-rlngar. Long & Associates PLATE 19
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BORING LOG B·? JOB N0~~9~"2~.--l~O~-·
JOB NAME: Carter f>Jrk No. 2
EQUIPMENT: DRILLING Flight Auger
- ID ~ - 0
ID .,,- L -;: Q_ -"' '< .,_ =>
"' E -"'- 0 L "' " 0 ID - ID '6 0 (/') 0 rn "- ::;;; u
41;-5 60
26
48 7- ll"J
53
12-13 10
14-15 91
118
SAMPLER TYPE 2.43 TD Split Spoon
c "' - <n I ·c ~.....: - 0
:::> "' . -" ID o·u;-~
" - ... (/')"' 0 ::>-.'~ ci "- -0 3' "' :::>E-~
0 u-
ML ---
s -
-'
~C-SM
SC
10 -
SS
15 -
-- ·--
DATE DRILLED~' _S_-_12_-_8_1 ___ _
SURFACE EL EV. : __ Ci J_4_._s ___ _ DATUM~'--~-IS-~L ________ ~
DRIVE WEIGHT- L 8
140 HEIGHT OF FALL- IN
30
Description
Silty sanLi, fine-grained, light brown, slightly damp, dense to very dense.
Tncreasing density
Thin layers of clayey sand, silty sand and clay, light gray and light red-brown, moist, rncdiwn clense.
Clayey Sa1lli, mccli1nn- to fine-grained, light yellow-hrown, moist, dense.
Sandstone hedrock, well indurat.cd, fine-grnined, J imonite staining along fractures.
,., ,_ •. , Boring tenninated at 20 feet, no water .
PLATE 20
• BORING LOG B-8
JOB NO' __ ')2:'-·10 DATE DRILLED: 8-12-81
JOB NAME: Carter Park No. 2 SURFACE ELEV.: 621. 6
EQUIPMENT' DRILLING Fli2ht J\ug er DATUM' MSL
• SAMPLER TYPE DRIVE WEIGHT- LB HEIGHT OF FALL- IN
z :1 ~ H.l SQI it Spoon 1 4f) .'in
• - ~ - c c:; - "' 0 (/) I ~ .,,- L - ·c .L; .....: - 0 a. .c lt .... " c .c "' u·w·.;:: ., ~-~ti Description E "- 0 L "' - - ID (/) "' 0 c >- " ci. "- ._ oO " " - ., ·c; 0 Ci :;:
., :::::i-·-(/) D rn "'- ::l' '-' 0 '-' -• FILL Sand, light hrown, slightly damp, dense.
h
~
• h Increasing moisture
4-5 13 34.6 87 OH Organic clay, light hrown to black, stiff, moist.
5 -
~ • ~
• SS Sandstone bedrock, medium-grained,
9-10 60 moderately to well indnrated, light gray and. light reJ-brown, damp to moist, very
10 - dense.
• I.fa' Sandstone, mediLun-grained, dark brown,
t .
SS
13'>:-l' 1-., l Oil Sandstone bedrock, dump, hard. SS very
• 15 - Boring terminated at 1414 feet. \Vater at 13 lcet.
h
• h
20 -
• lkrlog,1r, Long ,'Y. Associates PLATE 21
•
• Depth
Ror_!ng _No. (ft.)
• B-SA 9
R-6 6
B-8 4\
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• BFrlngcir, Long & /\ssrxialt"'
SHEAR STRENG1H '11-::~T DATA
Moisture Content Dry Density
(%) (pcf)
19.2 110
172. 7 27
34.6 87
Job No. 922-10
Unconfn1ed Compression Shear Strength
(n~ f) -· ·-·----··-··~ ..:....
3900
2100
4600
Plate 22
•
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z 0
... "' 0
.06
• 5 .08
"' z 0 <..>
• .10
0 '" u ,. If) ....... ,. u
c • j'3
µ,
•
PRESSURE IN ~BS Isa. FT.
100 1000 10 000 100 000
.
- I I I I "1¥-,..__ I I • I N Cl..._ I I
' ..... . - --- '"' .............. " I I
~ I I
I I I I I I I I
-.1 ...... " '·'·'"""""'"'"'~ .. --.- --·" I I
~ I II
I I ' I I ' I I " ! I I
----re" I -. -- ···-- - -- -T 1, ' I
I I I I
I I I
~-J I
' I ' '" ~~~ . ' ' ' I I
I I\ I I
I I I I I I ' 't I l. . -· I I i I I I I
\ I I
I I I I . -,·--·-- --·-~ . -
~ I ' \{
I
I I
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I "-l I I .
·--+- -~ ~''~"~••M•''•• - -~ ., ---' ......... 1\ I I """< I
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I I I I ... .,!, \i ---I I ~ I
I I ,.___ I ~
I I I I I --- , __ ,_ t· I I I I
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-,~.,~-·, -· ~~ --·-·r···· ·-~....i.-- _,. ., ,,.~ . ·~~ --~ " ' ' I I ' I I I I I I I
_J_ ___ I I I I .
_, - ' _ __ , .... ,.,_, __ -· .... , ··- ·--
I I
i
I I
I I I I I I I I
'
NATUA:AL NATURAL EFFECTIVE MAX. EFf. DE8CRIPTIO" MOISTURE DRY OEN__.OVIE.R8URDt.M PAs-r BORING DEPTH SOii..
'~'' ~R!SSUAE PRE~SUAE
l----+----+------1-----------~~.~,- ... ~~,~~ ::.?~~_!.~ (P.C.11;) (P.S.F.) (P.9.F.)
0 ll-3 81 ... 1 ·z Organic sandy clay wit 21. 9 "''--- "'1+ hl--1-
107
CONSOLIDATION TEST DATA
1000 3000
Rerlogar, Long & Associates Plate 23
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OEPTli SOIL DESCRIPTION MOISTURE Ort'T' DENs;ify OVC..RBURDEM f>i&3T PRESSUR! PJll E$!iU,.!
CONTENT'% ( P, C.fl:) I P.S.f.) t P. 8_f_) --·-
71..~• Organic silty clay,
32.3 136 870 2500 .. <lark brown ··--·· . ·-·-
- -···- ·-
. ·----·--~--__ " ___
··~----·-
CONSOLIDATION TEST DATA
P1ate 24
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N.l11.HfAL N .ltURA.L E:FFF,C.flVE MAX. EFF. .. ,. DORI NO DEPTH SOIL DESCRIPTION MOISTURE DRY OE"N:;irn OVER$1,1i;tOEN PA.ST F'FIESSUFl:E F'R ESSURE
COHTE.._T,; (P.C.F.) -~)'•.S.~J '~_s .. !:...L -- -··-··
j "=t~ 0 B6 7'-;; Dark hrown peaty silt 33.4
_, ___ - ___ ,~.~-- ---
-- ------ ---
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CONSOLIDATION TEST DATA
l:lerlog;ir, I ong 8, Ass<>ci<ites Plate 25
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TP-3 0-2
2-8
TABLE A TEST PIT LOGS
Job No. 922-10
Description
Fill, clayey silty sand, rneJiwn brown, damp, dense, many fine roots.
Fill, mixed lnyers snndy clay, clayey sand medium to dark brown, some small 1 eIL~es of very dark gry sanely clay (peat) damp, derLse .
Total depth 14', no groillldwater.
Fill, mixe<l layers of clayey sand with lenses of sanely clay (peat), dark brown to very dark gray, moist, <lense to medium dense.
Total depth 4 ', no groWldwa ter .
Fill, mixed 1 ayers of clayey sand and sandy clay, dark brown to very <lark gray, m:iist, medium derL~e .
Cl;~yey sand, light to Jnrk brown-gray, wet, medium dense to loose, appears in place.
Total depth 8', standing wa.ter at 711' .
l_--------------------------'~n----·-----•-----~
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'11'-4 0-J,,' • J,,-2
2-9 •
• TP-5 0-2
• 2-31,,
• TP-6 0-2J,,
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TABLE A TEST PIT LOGS
Silty sand, fine-grained, medium brown, damp, loose to medium dense, fine roots.
Silty sand with minor clay, fine-grained, medium brown-yellow, damp, medium dense.
Clayey ~ilt, dark brown-gray, damp, slight organic smell, becomes damper toward bottom.
Total depth 9' , no free groundwater .
Fill, clayey silt with fragments of sandstone; silt, dark brown-gray, damp, stiff, sandstone, yellow to white-gray, fine-grained, poor to moderate induration .
Si1ty sandstone, medium yellow-gray, fine- grained, poor to moderate induratfon.
Total depth 31,,' , no free groundwater .
Fill mixed silty clay (dark gray) and sand, fine-grained, yellow-gray, damp, limonite stained .
Silty sandstone, mc..-...-!ium yellow- gray, fine-grained, weathered, poor induration.
Total depth 4', no free groundwater .
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• l -12
• n'-9 0-7
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• 10-14
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TABLE A TEST PIT LOGS
Descriptio_l!
Alternating layers of sandy peat, silty clayey sand and sand, colors range from very dark gray (s<mdy peat) to dark brown (silty clayey sand) and light blue (san<l), damp, loose to dense .
Total depth 14', no free groundwater.
San<l, fine-grained, light brown, slightly damp, dense to very dense, appears well compact .
i\l ternating layers of sandy peat (dark gray), sand (light brown) and sand (light blue) , damp, medium dense to dense .
Total depth 12', no free groundwater.
Alternating layers of sandy peat (dark gray) , sand (light brown) and sand (light hl uc) , damp, medium dense to dense .
Total depth 7', no free groundwater.
Alternating silty sand (light blue) and clay (dark grayish-brown), 3' layer of sandy clay (peat), occasional gravel (sandstone) approximately 9" thick, 7" layer of silty sand (blue) mixed with occasional light brown sand layers .
Highly weathered silty sandstone with clay veins, silty sandstone is light yellow-gray, clay is Umonite mottled.
Total depth 14', seepage at 9-foot depth .
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TP-11 0-1 • 1-2
• 2- 2'1
2'1-3
• 3'1- 7
7-10
• 10-11
11-13
• 13-16
• 16-18
18-19
}
TA13LE A TEST PIT LOGS
Description -···-·•"'·-····
Clayey silty sand, 1ight brown, slightly damp, loose, many fine root;;.
Sand, [inc-grained, light brown, slightly <lamp, mcdi um dense.
Peat, sandy clay, dark gray, damp, stiff, very high in organic matorial.
Sand, fine-grained, brown, damp, medium <lerLse.
Sand, fine-grained, brown interlayers of clayey sand and sandy peat.
About l' al terri;<ting layers of brown, fine-grained sand and light blue finegrained ~and w1derl ai.n by approximately l" of peat .
Alternating layers of peat and brown sand, fine-grained.
Sand, fine-grained, light blue, damp, dense .
Alternating layers of peat and conglomeratic bouldery gravel, gravel is red-black cherty, rounded pebbles.
Peat, very dark gray to black, organic layer.
Sand, light brown, moist.
Total depth 19', no free groundwater.
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SUPPLEMENTAL GEOLOGIC REPORT
CARTER PARK UNIT 12
CARTER DRIVE
SOUTH SAN FRANCISCO, CALIFORNIA
Job No. 92z,10
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rRAN~·. 1-:1-11LOU1l."l HCNRY I MINCH
-I rNAHll Ll. LONCi 1:;rnH(il:: (). nc1c 01\VID I ~IJrl() l!:iH
June 10, 1982 Job No. 922-10
Berlogar, Long & Associates 4466 Black Avenue• Suite H. Pleasunton. California 945fl6
14 i:;1 484-0220
Mr. William Cotton William Cotton & Associates 314 Tait Avenue Los Gatos, California 95030
Subject: Carter Park Unit #2 Carter Drlve So11th San Francisco, California
Dear Mr. Cotton:
:::01L l:NG1Nr1-i-:~: FN(ilNl::l::rnNC.; (iHJLOUISTS
This letter presents supplemental geologic information regarding the subject property. This information is given in response to our discussion with you at the office of William Spangle and Associates on May 28, 1982 regarding the presence of faults and photolineament5 in the vicinity of the site and the potential for future surface rupture along an active fault.
1) We are enclosing a Regional Fa11lt Map (Plate A) and Regional Photolineament Map (Plate B) of the site and surrounding area 5howing mapped faults and aerial photolineaments. The mapped faults are derived from Bonilla (1971), Brown (1972), Brahb an<l Pampeyan (1972), California Division of Mines and Geology, c;pecial Studies Zonc5 (1974 q 1981). 1~e attached References list the published maps and aerial photographs utilized. Tho locations of the faults sl1own on the Regional Fault Map were transferred by scaling from the published smaller scale maps (scale 1:24,000 and 1:62,500). Note that only faults C, F, and J are currently designated as active or potentially active fa11lts on the latest CDMG Special Studies Zone Map5. Fault trace A, previously shown as a zoned fault trace on CDMG Special Studies Zone (1974) has been removed from the 1981 CDMG maps as a zoned \ fault trace because of a "complete l<<ck of evidence of its ,: existence" (E. Hart, CDMG, rersonal communication, 1981). / Fault trace I, shown on the 1974 Special Studies Zone map
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Page Two June 10, 1982 .Job No. 922~ 10
as the 1906? break, has been relocated to the southwest on the 1981 Special Stu<lies Zone map (see fault trace J). Fault trace G, revealed ln exploratory trenches excavated on Carter Park 12, is close to fa11lt trace C, transferre<l from the 1981 Special Stu<lies Zone map. Fault traces A and E (Brown, 1972), which are shown passing through or bordering the site, are not evident as photolincaments based on our review of aerial photography (sec Regional Photolincament Map}.
The Regional Photlineament map presents lineaments (possibly fault-related} from our review of aerial photographs from the years 1955, 1969, 1970, 1974, and 1975. The photolineaments were rated as to the prominence of the linear features (tonal soil changes, scarps, troughs, ground water barriers, sag pon<ls, etc.} by: 1) "strong"(S}, 2) "mo<lerate"(/.1} and 3} "weak"(W}. The southernmost "strong" lineament with the annotation of "fence offset?" probably represents the 1906 surface rupture .
The exploratory trenches excavated on Carter Park #2 were strategically locate<l to traverse the apparent photolineaments. The fault trace Jocate<l by Berlogar, Long and Associates (1982) lies in the vicinity of a strong to weak photolineament that is shown passing through the western portion of the site. A "moderate'' photolineamcnt shown as passing through the eastern portion of the site is bel.ievcd to be a linear feature related to a resistant calcite-cemented fossiliferous bed as revealed In the exploratory trenches on the site.
2) Alluvial materials were encountered in the central portion of the site and an irregularly-shaped pond (modified by man before 1955) is visible on the 1955 aerial photographs. We believe the pond was formed by movement on the main trace of the San Andreas fault (fault trace J on the Regional Fault Map) or the secondary trace (exposed in our trenches on Carter Park #2 and apparent as a lineament on the aerial photographs) with the effect of a shutter ridge or possibly a pressure ridge blocking the preexisting northwest•southeast drainage and ponding water and alluvium behind it. Exploratory trenches T-1 and T-6 revealed depositional contacts along the boundary of the ancient pond and unfaulted alluvium and bedrock (to the <lepths of the exploratory trenches) in the area of the pond. In addition, the presence of interbedded granular materials including bouldery gravels (with
13erlogar, I ong & Associates
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Page Three June 10, 1982 Joh No. 922-10
red chert clasts and well rounded sandstone pebbles) apparent in some of the alluvium supports the notion that some of the alluvial materials consist of channel deposits as opposed to materials ordinarily found in sag ponds. We find no evidence to support the theory that the pond is a sag pond caused by right-stepping en echelon fault traces.
3) We hclieve the age of the unfaulted alluvium is pre-Holocene (greater than 11,000 years B.P.) based on several lines of evi<lence:
a. The peat in the alluvium is composed of completely disintegrated organic matter.
b. The peat exposed in test pit TP-11 contains abundant crysonomads (A<lams, personal communication, USGS, 1981} which in<licates much colder than existing climate and probable Pleistocene age.
c. The unfaul tecl alluvium exposed in exploratory trench T-J was also found to extend to at least a depth of 19 feet below the present ground surface at the location of test pit TP-11. The location of TP-11 is approximately 15 feet below original ground, therefore making the depth of the bottom of TP-11 on the order of 3~ feet below original ground. This thickness of alluvial materials suggests an age greater than Holocene.
4) We have re-examined the location of the fault zone as shown on the Geologic Map (Plate ZR). The apparent slight discrepancy (on the order of one to two feet) between the location or the fault in the trench logs and on the Geologic Map is due to <lifferences in the surveyed locations of the fault trace to the end points of the trenches. Because the fault trace was staked an<l surveyed along with the end points of the trench we are relying on the surveyed locations of the fault trace as the best location of the fault rather than the 5tationing on the trench log which is less accurate. We have adjusted the setback lines from the fault zone to reflect a 15 foot setback from the margins of the fault 7.one .
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Pago Four June 10, 1982 Job No. 922-10
5) To facilitate your review, we have revised tho trench logs to simplify the classification of the geologic units exposed in the trenches.
The other information, regarding design load parameters (ground accelerations) and soil and foundation engineering aspects (stability of excavations during construction, site surface drainage and subdrainage), that you requested will be evaluated f11rther and presented at a later date but prior to issuance of a building pernli t .
We trust that the above information covers the significant noints requiring clarification for your completion of this review, specifically with respect to the presence of faults and the potential for surface rupture.
Respectfully submitted,
l3l:!RLOGAR, LONG a ASSOCIATES
~ c 'e___ Vy._ .;_ ~ Henry L. Minch Chief Geologist, CEG 757
HLM/rw
Attachments: References Plate A - Regional Plato B ~ Regional Plate2R - Geologic Plates 6R, 7R, SR
Copies: Mike Callan (6)
Fault Map Photolineamcnt Map Map - Logs of Exploratory
T-1 thru T-8
Wm. Spangle a Associates (1) Attn: Tom Vlasic
Daniel Christians (1)
Berlc_)gar, Long & As~c>ciates
Trenches
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.lob No, 922-IO
Ll ST OF GEOLOGT CJ\L REFJ;RENCES
Bonilla, M.G., 1971; Preliminary Geologic Map of the San Francisco South Quadrangle and part of Hunters l'oint Quadrangle, California; San Francisco Bay Region Environmental and Resources Planning Study, U.S.G.S. Basic Data Contribution 29.
Brabb, E.E., and Pampeyan, E.H., 1972, Preliminary geologic map of San Mateo County, California, and Ellen, S.D., Wentworth, C.M., Brabb, E.E., and Pampeyan, E.ll., Description of geologic units, San Mateo County, California II. S. Geological Survey Miscellaneous Field Studies Map MF-328, BDC 41.
Brown, R.D., Jr., 1972, Active faults, probable active faults and associated fracture zones, San Mateo County, California, U.S. Geological Survey Miscellaneous Field Studies Map MF-355, RDC 44.
California Division of Mines and Geology, 1974, State of California, Special Studies Zones, San Francisco South 7.5-minute quadrangle .
California Division of Mines and Geology, 1981, State of California, Special Studies Zones, San Francisco South 7.5-minute quadrangle.
U.S.G.S.
U.S.G.S .
U.S.G.S.
Pa<.:ific Aerial Surveys
" " "
Date
4/19/70
4/30/70
6/7/74
5/10/55
5/10/55
10/29/69
4/28/75
AERIAL PHOTOGRAPHY --
Scale
1:80,000 B&W
1:80,000 B&W
1:20,000 Color
1:12,000 Bl;W
1:12,000 B&W
1:12,000 BiiW
1:12,000 Bt1W
F1:_iz.h.~ Photo No.
VCMI 1-21
VCMI 2-183,184
7 26,27
AV-170 4-19,20,21
AV-170 3-23,24,25
AV-933 4-17,18
AV-1188 3-17,18
