SUIIMIT DNGINDERING
. House and Vacant Lot lnsoection
. Soils Reports
. Surveying, Parcel Maps, Subdivisions
. Storm Drain Design
. Hazardous Waste Studies
Javier Chavarria, RCE
JC ENGINEERING, ARCHITECTS
225 Rockaway Beech Avenue, Suite 400
Pacifica, CA 94044
March 9th, 20II
RE: Geotechnical Report for the Proposed Residence at
Parce1 55, Bay View Road, Pacifica' California.
Dear Mr. Chavarria i
The attached geoLechnical report is based on a detailed engineering study by the undersigned of the above properEy, where a new single family residence is planned in Lhe future.
We concfude that from a geotechnical standpoint, the land is suitable for the proposed construction, provided that our recommendations are implemented and good building practices are followed.
Special concerns for this site include the potential hazards of ,L.prg., seismic shock and differential foundation movement.
Therefore, we ask that this report be carefully studied and taken into account for Lhe engineering design which is to fol-l-ow.
Upon request, we will review foundation plans, and inspect earthwork construction and foundation install-ation on a regular basis while the work is performed. We will- also discuss construction procedures, and field changes if needed in a Final Report.
n'l ^^aa F^^'1 t- .Hredbe reer -ure€ tO COntaCt uS at anytime in the future if there are questions about this report, or we may be of further service. 5855 Castle Drive
OaXtand, CA94611
Tel: (510) s31-665s
Fax:(510) 482-58/'g
Q i nnoral rr v+..vv3v+J t
GEOTECIINICAL IWESTIGATION
FOR
THE PROPOSED RESIDENCE AT
PARCEI, 55, BAY VIEW ROAD
PAEIFIEA. EA 94044
FOR
,fAVIER CHAVARRIA, RCE
.JC ENGINEERING. ARCHITECTS
225 ROCKAWAY BEACH AVENUE. SUITE 4OO
PACIFICA, CA 94044
SI]MMIT ENGINEERING
5855 CASTLE DRIVE
OAKITAIiID, CAITIFORNIA 946LL
I{ARCH 9, ZOIL
TABIJE OF CONTENTS
Report Summary ii
Introduction ...... 1
Proposed Construction
Qanna 1 DUUPg
Site Topography and Vegetation.. lGanl nrrrr 1
Field Investigation 2
Laboratory Testing... 3
Soil- s 3
Sei sm'i r:i f rz. 3
Concl-usions.. ......5
Recommendations 5
A. Site Preparatj-on and Grading ...... 5
B. Drainage ...... 7
C. Drilled Pier Foundat.ion. . . 8
D. Shallow Foundations.. ...10
E. Concrete Slabs on Grade ......11
F. Pavement ......L2
G. Lateral Pressures.... ...12
H. Lateral Load Resistance... ...I4
Limitations.. ......14
References. ......1-5
Annend'i x .....17
E'i arrrae -1 R
Boring Logrs .....26
REPORT SI'MI4ARY
The presenL geotechnical study can be summarized as foffows :
- The subject siLe is a vacant parcel of land where a new/ twostorey, singl-e-family residence is proposed in the fut.ure. A hammer-head turnaround for fire trucks will also be buift. - The surface d.eposits consist of Franciscan Greenstone (fg) f,rrufuufrry inr-lrrdincr l'ra--1Li uqbct-LLJ-Ur ^ --^r VLJfLArr. ^^.^ic rocks. - Ground waLer was not found during driIIing.
- The nearest acLive fau]E is the San Andreas Fault (T1pe A, the nearest fault trace is 2.7 miles NE of the site) .
- The proposed structure should have all down-spouts collected and extended to discharge in a properly prepared drainage dissipation area.
- The proposed house will rest on drilled piers to be embedded in Hard, weathered clay/gravel conglomerate.
t- t-
INTRODUCTION
This report. compiles the results of an invest.igation of the soif and geologic condit.ions of a vacant parcel located on Bay View Road in the city of Pacifica, San Mateo County, California. The parcel's access is by an easement located between Lhe houses on 125 and 171 Bay View Road. The parcel has 21,853 square feet (0.50 acres) of surface area (Figures 1 and 3).
PROPOSED CONSTRUCTION
The current owner plans to build a new a two-storey, single family residence. The proposed house will rest on drilled piers to be embedded in Hard, weathered clay/gravel conglomeraLe. Grading will al-so include building the access road and fire Lruck turnaround, the new foundation, retaining walls, and to provide drainage and runoff disposal.
SCOPE
The scope of this investiqation included:
A geologic reconnaissance of the surrounding area;
A rewiew of pertinent geologic maps and reports
relevant to the site;
The drilling of three soil borings, and col-Iecting
representative soil samples,' and
4. The testing of subsurface materials to determine
representative index properties and the correlation of
dril1ing resistance with shear strength.
SITE TOPOGRAPHY AND VEGETATION
The site has an irregular polygonal shape. The access road will reach the parcel in the middle portion. This will al-fow a down-slope configuration of the proposed structure. The average slope grade of the building site is 56% (1.79H:1.00V). Site vegretaLion consists of thick, natiwe brushes and some invasive bush species (Figs 1 and 3) .
GEOLOGY
Geologic maps covering the area (Ref. I, Figure 2A) indicate that the parcel l-ies within Franciscan GreensEone (fg) consisting of basaltic, wofcanic rocks incl-udingi f1ows, pillow lavas, breccias, and tuffs ln different proportions; generally un-bedded, but sometimes in- distinctly bedded. Commonly fractured at cl-ose spacing or moderate spacing. where fresh, the rocks are gray to gray-green rn col-or. where fractures are closely spaced, fresh rocks are firm to hard.
Where fractures are moderately spaced, fresh rocks are hard. Weathering commonly reaches 10 to 30 feet, and sometimes to 50 feet. Where weathered rocks are orange-brown or other shades of red and brown.
Where fractures are closely spaced, weathered rocks are firm to soft.
Where fractures are moderately spaced, weathered rocks are firm to hard. This unit includes some chert and limestone to small to be mrnnad
Awai]abl-e l-and stability maps (Ref . 2b, Figure 28) show the site to be moderately to highly sensitive to seismic landsl_ides. The official- San MaLeo County map of geotechnical hazards (Ref. 3, Figure 2c) show the subject site in Zone 5 with indications of 'poor to good' slope stability, 'poor to good' earthquake stability, and ,poor to g:ood' foundation conditions.
SeismiciLy maps (Figure 2D) show the site located 2.7 miles (4.3 Km) SW of the well-known, seismically-active san Andreas Fault. The seismically-act.iwe, NW-trendingr San Gregorio/Seal Cove Faul-t is mapped 4 miles (5 Km) SW of the site. Alt.hough consi-d.ered inactive, a number of geologic faults are mapped in the San Francisco peninsula.
Such are the Pil-arcitos and San Mateo Faults, erc.
There is also a number of active faults on the East Bay. The Hayward and Cal-averas Faults are located 20 miles (32 xm) NE, and 29 mires ENE of the site, respectively; the Concord Faul-t. is located 33 mil-es NNE of the site (Figure 2D).
FIELD I}ryESTIGATION
Field investigation consisted of a detailed site inspeccion and subsurface exploration. During the detailed site inspection on March 7, 20Lr, the site topography and features were examined. No obvious indications of soil instability were noLiced in the proposed building site. The adjacent residences appeared in sound condition. subsurface exploration, performed on March 7, 20rr, consisted of drilling three soil borings at the Locations shown in Figure 3. The borings were drilled by using a portable, power drilling rig (Minuteman) equipped with a 3-inch solid auger. The borings were ended in Hard, native, gravelly clay conglomerate. Boring rogs and details regarding field and laboratory invest.igations are included in the attached Appendix.
LABORATORY TESTING
Samples of materials from the spoon samplers were collected for standard faboratory analyses Lo determine representative soil properties.
Pl-asticity tests were performed as they are usefuf for soil cl-assification and property correlation.
SOILS
The B-1, B-2, and B-3 exploratory borings eneountered a thin layer of gravelly clay colluvium underlain by weathered, gravelly clay conglomerate which becomes harder with depth.
For engineering purposes, the foll-owing layers will be considered:
0-2 feet, black silty c1ay, fill and topsoil, its resistance will be disregarded for design purposes (Creep Depth);
2-6 feet, Stiff to Hard, gravelly clay and weathered gravelly clay nnna] nmaraf a .
reev,
>6 feet, weathered, Hard, gravelly clay conglomerate;
Groundwater was not found in any of the borings during drilling.
Detailed descriptions of the materials encountered in the borings are presented on the boring logs in the Appendix. The attached boring 'r ^-d --^ r^'r aLed ruYD, qrru !Erq information show subsurface conditions at the approximate locations shown on the Site Pl-an in Figure 3.
It should be noted that at the building site, shallow soils are clayey in nature with LL = 46eo, Pl = 20% (classified CL) , have Moderate plasticity, and probably a moderate expansion potential- See Lhe fol-lowing table of commonly used correl-aLions between PI wafues and expansion potential :
PI (%) Expansion Potential-
< t2 Nil
13-15 Low
L6-25 Moderate
26-35 High
> 35 Very High
SEISMICITY
The toL is located in the San Francisco Bay Area which is one of t.he most seismically-active region of the United States. The nearest actiwe fau1t and also the Iikely producer of the controlling seismic event is the San Andreas Fault which is mapped 2.7 miles (4 Km) NE of the site. The seismically-active San Gregorio/Seal Cowe Fault is mapped 4 mil_es (6 Km) SW from the site. Although considered inacLive, a number of geologic faults are mapped nearby in the peninsula. such are the pilarcitos and San Mateo Faurts, etc.
There is al-so a number of active faults on the East Bay. The Hayrrard and Cal-averas Faults are l-ocated 20 miles (lZ Km) NE and 29 miles ENE of the site, respectively. The Concord Fault is ]ocated 33 mil-es NNE of the site (Figure 2D) .
Al-l- of these faults are currently exhibiting creep movements and micro-seismic activity, and are capable of generat.ing major earthquakes with t.he capacity for widespread d.amage to both man-made and natural structures. Major Bay Area earthquakes last occurred on the Hayward, San Andreas and Calaveras FauLts in the year 1858, 1989 and 1861, respectively. Other small faults are mapped in the immediate area, although none are associated with any seismic activitrz or considered active.
Ar-Lrrougrt n'1 fL^,,-1^ ..its -.:^ r-E. rs not yet possible to accurately predict when and where an earthquake will occur on the basis of currenL technology, it is reasonabl-e to assume that the proposed structure will be subjected during their useful tife to at least one moderate to severe earth- quake. During such an earthguake, the danger from faul-t offset thru the site is remote, but strong shaking of the site wi]I IikeIy occur.
Soil liquefaction is not consid.ered a hazard at the site, but soil slumping or surface sloughing could occur on sloping ground due to the strong shaking of a major seismic event, particularly if soils are saLuraLed. However, foundations built on competent strata, dlthough may suffer some damage, shou1d perform satisfactorily during a strong event. In addition, wood-framed buildings are generally flex- ible enough to sustain some seismic deformations with minor or moderate structural damage. An effective surface drainage will cont.ribute to maintaining higher shear strength, and hence stable slopes.
Additional 2010 Cafifornia Buildinq Code (CBC) Seismic Paramecers.
The CBC requires for seismic design the determination of the mapped spectral accel-eration values for short period (Ss) and for 1 second. (sr) (from Figure 1613.5(3), Ref. 13). rn our presenr case, the exrsting soil profile may be assimilated to a C site cl-ass of soil L).pe, i.e. 'Very Dense Soil and Soft Rock' (Table ]-613,5.2, Ref . 13). other CBC seismic coefficients are calculated as follows :
Site coordinates 37 .504735 deg N, L22.48953G deg W (NAD27) and hence, Ss = a.847, Fa
\ t
-
lt v <9 kat
-
v.JJrl
1.0, Sms = t.847, Sds = L.23L
1.3, Sm1 = 1.22L, Sd1 = 0 .814
The proposed building will have a II occupancy category. the building will have a D seismic design category.
Therefore,
CONCLUSIONS
Based on our field and office studies, it is our opinion that from a geotechnical engineering standpoint, the site is suitabl-e for the proposed residence and retaining walls, provided t,hat Ehe recommendations presented in this report are incorporated into the design and construction of the proposed addition structure.
Because of soil- characteristics, we recommend that the foundation for rL^ *-^^^-^.1 l^, urrE r/lvyvDsu rrouso consist of a drilled pier and grade beam sysLem embedded in Hard, weathered, clay conglomerate. The new retaining waI1 should be supported on a p:-er/fooLing combination.
The pier foundation must be designed and built to provide adequate vertical and horizontal restraint. The verticaf restraint will be prowided by a combination of skin-friction and end-bearing action of pi"t" penelrating the Hard conglomerate. The horizontal restraint wiII also be provided by passive resistance of Hard layers.
Ground shaking will be the major cause of earthquake damage. The subject dwelling wilI consist of a wood-framed, two-storey building supported by a relatively rigid system resting or embedded in competent soil strata.
The controlling seismic event will be produced by the San Andreas Fault, whose maximum credible Peninsula Branch Event would have a M7.1 on the Richter Sca1e. The site may be wulnerable to seismically triggered landslides. Further, the building period for a 2- storey sLructure may faI1 within range of maximum spectral values, and therefore, Lhe structure may be subjecu to high base skrear and high shear stresses. Seismic CBC (2010) spectraf values for design are shown in the previous SEISMICITY Section.
RECOMMENDATIONS
The following are recommendations for the successfuf completion and maintenance of the project. Because the recommendations are partly general and partly specific t.o certain items of concern identified above, recommendation implementation should be discussed wit.h an experienced Soil-s Engineer. A Soils Engineer must be retained to:
- Review the foundation, grading, and drainage plans
prior to construction.
Update this report if necessary because of observed
changies or delays.
Inspect the excavation operations, particularly the
drilling of pier boreholes; and the instaflation of
surface drains and sub-drains.
- Prepare a Final Soils Engineer's Report that indicates whether construetion was done according to expected
soil-s characteristics, or new features were encountered which reguired special engineering considerations.
We request to be allowed to provide these additional engineering services so that there will be no question about interpreting our recommendations which are based on our findings and conclusions.
A. Site Preparation and Gradinq
The area of t.he proposed addition should be cleared and stripped so sufficient dept.h to remove any obstructions, debris, and all surface vegetation. These material-s should be removed from the site. If any ^L-!-,,^F..i ^-^ I LrrJbL!LrL.ururrD rsuch as tree root systems) are removed below the planned finished grades, the resulting holes must be backfilled with approved materiafs that are compacted to the reguirements given bel-ow.
Mature, native brush should be considered grenerally as contributors to slope stability and erosion protection. Not more than necessary native brush should be removed for construction. Any proposed landscape should also include fire-resistant, native trees and bushes. n,.^ r^ uuE uv ^^.i'l Dvrr ^]y-asLicity, on siLe materials are not recommended for use as fill or backfill materials. Any imported fill used at the site should be a non-expansive soil wit.h a plasticity index of 12 or less. A11 f ilt and backf il-l material-s placed at the site must not contain rocks or lumps greater than 5 inckres in their greatest d.imension, with not more than l-5 % larger than 2 .5 inches.
A11 sub-grade surfaces that will receive fill-, should be scarified to a depth of 5 inches, moisture-conditioned wet of optimum, and compacted to Lhe requiremenLs given below. A11 structural fill and backfill material-s placed at the site shoul-d be compacted to aL least 902 rel-ative compaction by mechanical means only, as determined by ASTM Test Designation D1551-10.
In addition, structural fil1s (including waI1 backfills) equal or greater than 4 feet in thickness should be compacted to at least 95eo relative comoaction over their futl depth. AI1 new fills should be keyed into compact materials and compacLed in lift.s not exceeding B inches i-n un-comoacted t.hickness.
We recommend that any new cut slopes, and any slopes disturbed by the construction operations be heavily planted Lo minimize sloughing and erosion (Ref. 15) . A1I finished grades should slope at least 2% in such a manner thaL surface water will not run over exposed slopes or col-l-ect against obstructions. For other grading details, the reader is referred to the CBC (Ref. 15).
B. Drainaqe
Particul-ar attention must be given to both surface and subsurface drainage at this site. Even low-plasticity soils may be affected by drying:-and-wett.ing cycles, and must be protected by a careful-l-y planned drainage system to avoid water percolation near the foundations.
In addition, there is always the risk of a slide occurring in sloping terrain, and a good drainage plan wiII decrease such risk.
No concentrated water should be allowed to run over any prepared s1ope. Rough and finish grading should be planned to prevent any ponding of waLer on the surface, except that a safe sediment pond may be necessary if grading is done during the winter.
Either 3- or 4-inch, rigiid, perforated pipe with solid clean-ouLs shoul-d be placed behind retaining wa1ls. Perforated pipes should be surrounded with at leasL one cubic foot per linear foot of freedraining material consisting preferably of cl-ean, uniform, 1- Lo 2- inch gravel or drain rock.
Slope runoff should be intercepted behind retaining wa1l-s by installing peripheral swales that empty into small- catch basins for proper disposal. Drainage must not be allowed to colfect and pond anywhere on the siEe. Crawl spaces beneaEh floors can coflect seepage if t.he rough interior grade is lower than the finisLr grade around th.e outside. Finished ground surfaces must be sloped at l-east 2% away from foundation wa1ls for positive drainage. Do not introduce surface runoff into perforated sub-drains (See Figures 4 and 5).
Roof runoff shoul-d also be col-l-ected and directed away from foundations. Do not allow down-spouts to deposit runoff where it. can saturate foundation soiI. A1I down-spouls should be connected by solid pipe and al-Iow to discharge, ideally, onto paved surfaces or into city storm drains. If this is not possible/ storm runoff should be dissipated on site using a dissipation area shown in Figure 4A.
Notice that 5-inch Lo l-2-inch boulders or rock fragments may be used for rip-rap. Notice also that the 4-inch dissipation 'T' pipe must be built perfectly l-evel for gentle water dissipation downslope.
Pressure treated wood, railroad ties, etc. may be used to maintain a perfectly horizontal 'T' pipe.
Installation and operation of automatic sprinkler systems must be carried out careful-ly so as not produce excessive amounLs of water.
Further, irrigation either manual or automatic should be kept to a minimum. Landscaping should consist of drought and fire resistant enaai oe nf |- raaa anrl Jrrrahaa
-- -=D AIIU UUDIIED.
Proper house maintenance should include annual flushing with a garden hose of all sub-drains, catch basins and down-spout piping. If any pipes become clogged, t.hey should be cleared so that hydrostatic water pressures do not lessen the shear strengLh of the soi1s. Likewise, ground surfaces should be maintained to promote good drainage and to prevent erosion and foundation soil saturation.
C. Drilled Pier Foundations
Dri11ed, friction piers must be designed according to the parameters given below. Minimum pier diameter shoul-d be L6 inches. Piers should penetraLe the Hard, weathered conglomerate with a minimum pier length (measured from the original ground elevation) of 10 feet for the main structure and 8 feet for secondary struetures (driveway retaining walIs etc.). Final- pier depths to be decided in the field by Soils Engrineer. The following summary table may be used for pier design :
Table 1 - SoiI Parameters for Foundation Design
T-)ani- h 1€{- )
svverr \!e/
Soil
Charact.
Skin Frct Pull-out
(Psf ) Res. (psf )
Pass Res Bear]-ngi
/nnf\ /*\ Droc /lrcf) r!veg
Fi1l and TonsoilTli craaard
(Creep Depth)
Sti-ff to
Hard Weathrd
Conglomerate 400 200
4+
Hard,
Weathered
Conglomerate
means applied to
600 300
where (*) 1.5 pier diameters.
fUU
However, when embedded in Hard, gravelly conglomerate, and provided t.hat pier boreholes will be clean from all debris, piers may be designed to account for the end-bearing effect. In these cases, the a1lowable end-bearing pressure will be 4 kips per square foot, plus 10% per foot of penetration starting from 5-foot depths, to a maximum of 12 ksf for dead plus live Ioad, with a one-third increase for al-lloads including wind or seismic. The allowable bearing pressure, ed, inside the Hard gravelly conglomeraLe is given by :
and
qa = 4 ( 1 + 0.1 ( L - 6 ) ) ksf
L = pier length (ft) below original ground
If the end-bearing effects are added, the total pile capacity will be cal-culated as follows :
Pier capacity = Shaft resistance + Tip resistance / 3
The following pier capacities have been calculated for foundation design. For given pier spacings, and hence pier foads, pier lengths may be obtained from Table 2 in order to minimize foundation costs :
Table 2 - vertical- Pier Capacities in kips (1 kip = 1,000 lbs) D
( in)
L(fr)
810]-2141518
L4 r_9 24 30 3s 40
15 22 28 34 40 46
where :
D = Pier Diameter in inches
L = Pier length (ft) below original ground
Creep Pressures on Pi-ers
Pier creep earth pressures are assumed to act thru the soil depth with creep potential (Creep Depth), which in this case is esLimated to be at an average depLh of 2 feet from the original ground.
Creep pressures will be accounted for by designing piers to resist a v^^F-s^.r'l
--
'1 lcuLarryurq! - -oteraf pressure diagram of stress equal to the creep depth t.imes 200 pcf -efw, i.e., creep stress = 2 x 300= 500 psf in this case, appli-ed 1 foot below ground.
I2
r6
Creep pressures will be applied to one pier diameter. Passive soil resj-stance, existing only below the Creep Dept.h of 2 feet on average, will be applied to 1.5 diameters and is given on Tabl-e 1 above and in Section H be1ow.
Piers in Pl-astic Cl-ay
Because piers are to be drilled through native, plastic c1ay, we recommend that the grade beams be designed to resist a 1,500 psf uplift pressure.
Pier Construction
Piers might not be installed deeply or firmly enough for fixity at their bottoms in order to act as vertica1 cantilever beams. Depths to fixity wou1d depend on the lengt.h of embedment into passively acting soil-. However, if a minimum embedment of 8 feet is provided, fixedend effects aL pier bottoms may be assumed for structural- design.
AlI piers must be tied together with tie beams or grade beams that extend up and down the slope between the piers as wel-f as across the slope between the piers. The spacing of the piers should be determined by the Structural Engineer, but shou]d not be closer than five pier diameters. Pier foundations become more efficient when usinq fewer, widelw spaced, larqer piers.
AII pier holes shoul-d be dry and reasonably free of l-oose cuttings and fall-ing debris prior to instal-ling reinforcing steel and placing concrete. Some of the pier holes may encounter different soil conditions that assumed thru their design depthsi such piers will be evaluated on an individual basis at the time of construction.
In addition, care must be taken during the pier hole drilling operation to werify that pier holes reach the underlying dense materials, and that boufders or locally unconfined rock ouEcrops are avoided.
Pier borehofes should not be left un-poured over 24 hours and by no means over 48 hours. If water is encountered in any of the pier excavations, pumping may be required to remove Lhe mud from the hofes. If open boreLroles are caving-in, a drill--and-pour teckrnique may have to be implemented.
D. Sha]l-ow Foundations
Footings should only be used as a supplement t.o drilled piers for retaininq wal1s. Retaining walls may be designed using piers for wertical and horizontal (passive) foads supplemented by short footings of, say, 2 Lo 4 feet to provide a rigid bending joint at the base of the wal-l-. The footing trench bottoms should be thoroughly 10 compacted. The footings may be designed for maximum arlowable bearing pressures as follows :
Footinq Depth (ft) Alfowabfe Pressure (psf)
3.0 4.0
> 5.0
1, 500
2,000
2 ,500
3,000
Retaining walls should be waterproofed, sub-drained with perforat.ed pipes, and densely backfilled prior to framing. Footings may be designed for the al1owable bearing pressures shown above, for dead plus li-ve loads, with a one-third increase for al-l- loads including wind and seismic. The allowab1e bearing pressures are net values; therefore, ttre weight of the footings can be neglected for design purposes. A11 foundation bearings must be reasonably dry and free of cuLLings or fall-ing debris prior t.o installing steel bars and pouring UUIIU! ^^6 ^e^F C LC ^ .
E. Concrete Slabs on Grade
Framed wood flooring is preferable for the living areas of buildings.
However, where slabs are to be used, the following is recommended :
A minimum of 4 inches of crushed rock should be used between compacted sub-grade and concrete sl-abs. Any over-excavated sub-grade should be filled with densely compacted crushed rock or other nonexpansive fill material. A plastic membrane under the slab may be used as l-ocal vapor barrier. The membrane should be placed over 2 inches of sand over the crushed rock. Post-tensioned slabs may not need vapor barriers if the concrete is permanently in compression.
Prior to pJ-acing sIabs, sub-grades may be pre-swelled by flooding for a few days prior to placement of vapor barriers or concrete, partir:ularlrz 'if fhe slab is structurallv connected to the foundation.
Alternatively, the top 12 inches of soil may be replaced by a compacted, non-plastic, sub-base, imporL fill materlal.
Concrete slabs should be at least 4 inches thick and reinforced, preferabl rz wi th har reinforcement. Wire mesh is acceptabl-e if is carefu11y Lentered in the slabs. Exterior slabs, garage, carport s1abs, and driveways should be free-floating and separate from foundations.
Weakened-p1ane expansion joints should be made in exposed, nonstructural slabs at about. 10-foot intervals. Reinforcinq should be continuous through expansion joints.
tL
Concrete walks should be reinforced concrete over sand or gravel. If truck traffic passes over concrete wa1ks, they should be 5-inch reinforced sl-abs over 5 inches of rock. Similar concrete pads should be placed wherever a debris box or a trailer storage is ant.icipat.ed.
F. Pavement
The customary driveway section consists of a 4-inch, reinforcedconcrete slab as described above, or a 2-inch cover of plant-mixed asphalt. (a thicker "engineered" R-Value design can be prepared upon requesL). Either pavement should be placed over at least 4 inches of CalTrans Class II Aggregate Base rock.
Instal-} pawement according to the CalTrans Standard Specifications, Sections t6, L9,26, and 39. Compact the sub-grade to 95? relative compaction (ASTM D1557) aL a moisLure content of 2% over optimum moisLure, and tkren rock tack, and pave immediately to keep the soil from drying and subjecL to swell heave the following winter. Base rock should be Cal-Trans Class II aggregate, asphalt should be plantmixed Type B. Base rock should also be compacted to 95% and Lacked.
Asphalt shoul-d be sealed after paving.
G . Lateral- Pres sures
Partiall-y buried foundations and retaining walIs must be designed to resist active lateral soil pressures. Active pressures behind backdrained reLaining walIs and partially buried foundations may be taken as follows:
Wall Type
Unrestrained, Slope < 4:L
Unrestrained, Slope < 3:1
TTnroql- rainod Q'l nne
- e!s+rrvv/ ).1
Restrained
Active Pressure (pcf-efw) *
35 40
qo
Add an additional uniform fateral
pressure of 8 x H (psf), where H =
height of backfill above retaining
waI1 foundation in feet.
(*) Pounds per cubic foot-equivalent fluid weight
A 50% pressure increase should be used for shoring design.
For unrestrained wal-Is, add an addit.iona1 uniform pressure equal to one-third the maximum surcharge foad applied to the wall backfill.
For restrained walls this additional uniform design pressure should be one-half the maximum surcharqe.
L2
- Seismic pressures will be accounted by adding a uniform pressure = 8 x H psf, where H = height in feet of retaining wall. (Soil parameters may be increase by l/3 for rapid dynamic loading).
The above pressures assume that sufficient drainage will be provided behind the wall-s to prevent the build-up of hydrostatic pressures from surface and subsurface water infiltration. Adequate drainage may be provided by a sub-drain system consisting of either 3- or 4- inch diameter perforated pipes bedded in free-draining material. The free-draining material should consist of clean, uniform, 1- Lo 2-inch qravel- or crushed rock.
Retaining walIs behind non-Iiving spaces shoul-d have a walf drainage sysLem incfuding conventional drainage bfanket material (suckr as Miradrain) , and free-draining material. The free-draining material shoul-d be wrapped with a slmthetic filter fabric forming a "drainage burrito'r, and placed behind the wafl with aE least 1 foot of t.hickness and should extend to within 2 feet of finished grade.
The perforated sub-drain pipes should be connected to a system of closed pipes that leads to the city storm drainage system if possible, or that. dischargies in the paved street gutter. Compaction of backfill shoul-d be conducted in such a way that will- not damage the wall (See SecLion A for backfill placement and compaction) . The upper 1 foot of backfill shoufd consist of compacted on-site or imported clayey materials (Figure 5).
Construction equipment surcharges should also be anticipated in de- ^..: --^ ^--] Dr'.,rr, q*s tstr-^ urrs construction specifications should provide for adeguate concrete curing time before allowing the backfilt to be compacted.
Sliding friction and passive resistance should be taken as discussed in Section H below. On hillsides we recommend spudding wal1 footings with reinforced concrete piers designed to take bending stresses.
Besides being provided with proper sub-drains, we recommend that al-lretaining walls behind living spaces be thoroughly water-proofed, preferably by hot-mopping, or better yet, with sealing synthetic membranes such as HLM5000, PermaSeal, Bituthene, Miradrj- or equivalent.
Lined surface swales shoutd be provided behind any retaining walls having an exposed sloping surface draining Lowards them. The swales/ that wil-I collect runoff water from the slopes, shoul-d be sloped to drain to suitable discharge facilities. The top of the walls shoufd extend at least 5 inches above the swale in order to retain minor erosion or sloughing materials.
13 The walfs must. be supported on pier/footing foundations, that are designed in accordance with the recommendations presented previously under SecLions C, "Drilled Piersrr and D, "Sha1]ow Foundations".
LaLeral load resistance can be developed in accordance with the recommendations presented below.
H. Lateral Load Resistance
Lateral loads on footings resting on Hard conglomerate may be resisted by friction with a friction coefficienE of 0.4.
Lateral Ioads on piers may be resisted by passive pressures acting against the sides of the piers or keyways. Eguivafent passive pressures as shown on Table I :
Between 0 and 2 feeL, use 0 Pcf-efw,'
Between 2 and 5 feet, use 350 Pcf-efw;
below 6 feet, use 500 Pcf-efw Lo a
max. vafue of 8,000 psf.
Passive resistance of drilled piers will be calculated by applying passive stresses on 1.5 pier diameters.
I,IMITATIONS
The recommend.ations presented herein are based on the soil condiLions revealed by our Lest borings and laboratory procedures according to generally accepted geotechnical engineering principles and practices.
This warranty is in lieu of all other warranties eiLher expressed or impl ied.
IL musL be understood that for this report to be vafid, the owner should ensure that necessary steps are taken to carry out the recommendations of the report in the field. Any added risk incurred by the choice of al-ternative construction meEhods which depart from our recommendations wiII be borne by the owner. Further, this report must not be construed as any guarantee or insurance agiainst any L)pe of soil- f ailure.
The recommend.ations in this report are general in nature and are subject to adoption or revision as the construction circumstances warrant. We shoul-d, be notified for supplemental- recommendations should unusual situations be encountered during construction. We may be consulLed for supplemental ad.vice, or Lo provi-de assistance in interpreting our findings and recommendations, or to inspect various aspects of const,ruction.
t4
Our recommendations are walid as of the present time. However, future conditions may change conditions due to legislation, improvement of engineering knowledge, natural process, or manrs works. Therefore, this report is subject to review and its validity may decrease with the passage of time.
Finally, careful desigrn and construction cannot guarantee that damage will not occur if a disaster strikes. Disaster may strike, for instance, in the form of either a large, destructive landslide, or a ^.1 Dayrrr!ruq'L, -*.i€.i^-.^F rr€d.fb1z earthquake, The owner afone undertakes such risk, and therefore, the owner should obtain home insurance if available against earthquake and landslide damage.
REFERENCES
1. USGS (Brabb and Pampeyan) , L972, Preliminary Geologic Map of San Mateo Countv. Miscell-aneous Fie1d Studies Ma'o MF-328. 2a. USGS (Brabb and Pampeyan) , 1972, Preliminary Map of Landslide Deposits San Mateo County, California. Miscellaneous Field Studies Map MF-344.
2b. USGS (Wieczorek, et aI), 1985, Map Showing Slope Stability During Eart.hquakes in San Mateo County, Cal-ifornia.
3. Of f icial Map County of San Mateo, California (1,9'76) ,
Geotechnical- Hazard Slmthesis Map, Leighton & Associates Geotechnical Engineers and the San Mateo CounLy Planning Department.
4. State of California, L982, Revised Official Map, Special Studies Zone.
5. USGS (Stover, Reagor, Baldwin, Brewer) , 1990, Prefiminary fsoseismal map for the Santa Cruz (Loma Prieta) California Earthquake of 10-l-8-89, Open FiIe Report 90-l-8.
6. USGS (Youd and Hoose) , L978, Location Map of Ground Failures in t.he San Francisco Bay Counties Region, PP 993.
7. Caf ifornia Division of Mines & Geology (Hart), t975, Faul-E Hazard Zones in California, Special Publication 42.
8. USGS (Borcherdt) , 1975, Studies for Seismic Zonat.ion of the San Francisco Bay Region, Professional- Paper 94I-A.
rK
9. USGS (Herd) , L979, Seismic Zonation of the San Francisco Bay Region, Circ. 807, p. 10.
10. Seed & Idriss , L982, Ground Motions and SoiI Liquefaction During Earthquakes, Earthquake Engineering Research
Institute.
11. U.S. SoiI Conservation Service, 1981, SoiI Survey of San Mateo Countv.
12. USGS (Rantz) , t97t, Mean Annual Precipitation Data for the San Francisco Bay Region, BDC 32.
13. Amerj-can Society for Testing Materials, Annual Standards.
L4. Architectural Plans and Topographic Survey by JC Engineering, January, 201L.
15. California Building Code, 20:.0 Edition.
16. ABAG, L995, Manual of Standards for Erosion and Sediment, Control Measures.
L6
APPENDTX
S UIIIIIT EIIIGITTEEIi INTG
..,/ i/ //
)t9 y'o\* t
WASTE WATER I
TRaTMENT
Lil.,ltA MAI-I q"
? SANCHA fuOBE
! H/SrcF/C SIE
PEDftC
,(s ro,trt
I
\/
Refr AAA San Mateo co. Map
q4,\4t€
FIGUREl-SITELOCATTON
RocKAwAy I
BEACH I
- r,.*9Q---.
p lt7 .o$1
SUIIITIT EIVGINEERING
#$
ril
h
"ir"'
t
r,,-
T'rt
)?*r.
q,,
:
"d-
'
q,
$x
t
Ref. : USGS
L
Scale r I =
(Brabb and pampeyan), L7TZ
1 mite
FTGURE 2A - REGIONAL GEOI,OGY
$il j
S UMIT IT EI[G[ IlIEEII T IWC
MORI POINT
Ref. San Mateo County Official Map
of Geologic Hazards (1973)
tr I
ScalerlE2'OOO
ll
FTGURE 2C - REGIONAL LAND STABTLTTY
+J
.4
rdiiQ_/
\
:I
t'3y,
?,t
q
'11;
SUPTFTIT ENGI I1IEE RI FIG
l
I
I
3Al
I
I
I
I
I
nCn
I
CA llt tt,', t--j* I
l/ i'l I f:. +"6W
$TAT[ I
Iraci
I
I
I
1 Mr:ri Poi
l
I
f
I
)
\
)
PA{TK
i..
i
l
I
l
I
I
78 SHAT?P
Point $an P
Devils
I
I
I
ro
n
)
i€'
44 198 5
2 , OOO'
?
I
i
I
I
s,
FTGURE ZB - REGIONAL LAND STABTLITY
SETSMIC SLOPE STABTLTTY
SUIIIIIT ENGIIUEEII.ING
f
6.5 "'.1"
'. lP
E
71 6.4*
.,
NOVATO
AN RAFAEL
tO
1..$'1conu
..o
\.c- ..?
irr
o
)
o
o
6. Or
6 v
\
\
a
SAN MA
ROJECT SITE
$oo-t-
?W
HAYwARD \.,
l\.. n* \i
a
I
I
a
6.7 ( 1e76 )
FIGLJRE 2D - AcTIvE FAULT zoNE6
Ref, r Calif. Div. of Mines and Geology (*) t Hose Probable Magnltude or Maxl-mum Credlble Earthquake
SUffI}TTT NNGINTEErITNG
-,t\\fr
Ql-gruu
s$-s-"sii- --d
I
LEGEND
#ror.r Boring ts- I I Lo"ation
FIGURE 3 . SITE PLAN AND SOIL BORING
LOCATIONS
S UDTITIT TqNGIISEERINIG
ROO!' DttAlN
tfLD(i. lv^t,t, ------+
II|IN -__rr
coMt,AcTuD t' MIN
CL{YUY SOIL S}T^L
CONNECT ALL DOWNSPOUTS
AND CATCH tsASINS TO
SOUD PIPE
BUILDING DRATN (NTS)
FLUSH ANNTIALLY
-4" #fiiDff6 ntcto rtnn
/ ht/ 2z MIN sLoPE
l4;
t.4
CLEANOUT DETATL (rurS)
FIGURE 4 SUBDRAIN DETAILS
SUMMIT ENGINEERING
C.APPED 'CLEANOUT
f_> 6'
^+r
-.,1-
I
I
t ei'
I
I
rl
tr
I
lfl* a(-t _Al --\> -.....- Storm Dra_i ns -- '1)-
P LAN VTEW
Ferfeet-1y 4" g pvc
Level Perf. Pipe
<lllfimrn errvrvA-A
FJGURE 4A * DISSIPATION AREA
SI]II}IIT ENGINEERING
Bou].ders
Concrete
| .1 l,)'l ?1i'r
or Eguivalent
\ 1 I
--4 MTN
4 q
4 a
4,, PERFORATED RIGID PIPE
HOLES DOWNT2% MIN SLOPE
L coMPAcrED lr.\c^r tLL --\
A I
"l
I
al
l
t
I
I
to' rtqa4'Mfu+t',FE:s 4
" | 3/4o vQ 2" ctaAfl.uvif.
" -r--------.t-W-
/ oR R?UND #4VE L
:'_J
12" MIN
FIGURE 5 RETAINING WALL DETAITS
S{IMMIT ENGINEERING
PROJECT No. 1loL0s DATE: 3/7/zoLL
PROJECT MME: New Residence
LOCATION: Bay View Road, Pacifica CA
DRILUNG METHOD: Minuteman tr/ 3-in Augerrl4o-1b Harnrner HOLE DIAM:3. 5 in DRILUNG CONTRACTOR Acdess Drilling Co. DRILLER: Jose
l.,lATERlAL
, DESCRIPTION
Era b, t?
LI -v
"o-
-2 -
-4 -
-6 -
.8 -
-10-
.-L2-
J4-
-L5 -
*t:
2 4
N.E. . N<rt Encountorrd
[ 3-in Eample
N sPr sempta il
| 6rgb ssmplt
2.5-in
sample
NOTES:
ijlack, silty clayr'lopsoil and
fi11, moist. Soft CL
a:T
Ll \-
color chanqe to oranc{e-brorrin clai;
inoist. Stif f to l'. Si:if f
harder dr j 1ling tn g.ravellY clay'
rock fraqr:lts' sharpr harci.V. Stif
harcler drilling w,/ depth in claY;
c:rave1 conqlomerate. ilard
5'j1ow grind in hard ciravel (v. wea
thereci qreensLone-i ). llard
SUMMIT ENGINEERING
6045 ShirleY Drive
Oakland, CA 9461 I
PROJECTNo 110105 DATE: 3/7/?aLI
PROJECT lllAME: Ne',s Resicj.ence
LOCATION: Bar.. Vier,u Roarl-, Paci-fica GWL DEPTH: N. E. i-'A DRILUNGMETHOD: Minuteman w/ 3-in Augerrl4O-1U Hamrner HOI..E DIAM:3. 5 in DRILUNG CONTRACTOR Acdess DriLting co. DRILLER: Jose
I,IATERIAL
Ee , DESCRIPTION
.oI
..e.2
-16
I
20 2
4 N.E. - Not Encolntcrad
[ 3-ln eampte
N sPr samptc n
t Grab samplr
z. f,-an
sample
r-. T
l'1ed. brorvn, sittir clay, noist,
soine gravel i'I'=4€;,"1ot PI=2O?/"
iied, stif f to gtif f
narcler ctrillitq{, crancre*i:rown cla',''
w,/ abundant gravelr sirarp hard frcl
inent* r l-ic;ir't -.'elloisish*lt:rownr 1.^r€3,*
i:her congio::terater clrillrng harcier
r.il depth. Slow crinc!, iiardSUMMIT ENGINEERING
6045 Shirley Drive
Oakland, CA 94611
PROJECTNo. 1101o5 BORING No. il-:l DATE: 3/1/2orr
PROJECT MI,IE: New Residence
LOCATION: Bay View Road, Pacifica fA GWL DEPTH: N.E.
DRILUNG METHOD: Minuteman w/ 3-in -Auger, f4g-1b Haruner HOLE DIAM:3. 5 in DRILUNG CONIRASTOR Acdess Drillinq Co. DRILLER: Jose
Ee
o
2 4
6 I
o
2 4
6 8
o
2 4
N.E. . Not Encqntorod
[ 3-ln eample
N sPT semptc n
I Grab samptc
2.5-in
sampJ-e
NOTES;
tiedium brown to orange-bro'.Elr c1a',r
ror/ gravel, :rroist. Stiff to V.Stiff
harder drillinq r,;l depth in clay
cor-:c1J-omerate, weathered, abundt
a-re1. Slow srind. Fiard
{_-L
BOfi = 4 feeL
SUMMIT ENGINEERING
6045 Shirley Drive
Oakland, CA 9401 I