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350, Construct an addition for new , Studies & ReportsAmerican Geotechnical A CALIFORNIA CORPORATION October 18, 1988 File No. 1645 County of Los Angeles Department of Public Works, Geology Section 900 So. Fremont Avenue, Third Floor Alhambra, CA 91803 SUBJECT: RESPONSE TO GEOLOGIC AND GEOTECHNICAL REVIEW SHEETS dated August 1 and August 10, 1988, respectively, Lot 6, Tract 29383 20 Buggywhip Drive RollingHills, California Gentlemen: In response to the County of Los Angeles' Geotechnical Engineering Review Sheet dated August 10, 1988, several. items are requested for additional information. For your convenience we have reiterated the items in question followed by American Geotechnical's.response. Item 1: Item 2:, Response: "Provide the design parameters and analysis used to determine the diameter, depth, and spacing of the caissons. This analysis must show that the site will be stabilized by the chosen caisson schedule." "The geologist (and) the soils engineer must make a finding in accordance with Section 309., Los Angeles County Building Code." Design criteria for underpinning foundation systems was provided in the American Geotechnical May 31, 1986 report, Figures 11 and 12. Therein conventional design criteria are provided for friction piles. Criteria was based on the results of laboratory testing and engineering analyses as are presented in the May 1986 report. The actual structural load- ings, caisson and grade beam dimensioning have been the responsibility of the project struc- tural engineering, South Bay Engineering. 25202 Crenshaw Boulevard -Suite 101-Torrance, CA 90505 (213) 539-9983 5755 Oberlin Drive -Suite 104-San Diego, CA 92121 (619) 457-2711 1240 Van Buren -Suite 210 -Anaheim, CA 92807 (714) 666-2241 File No. 1645 October 18, 1988 Page 2 American Geotechnical A CALIFORNIA CORPORATION Damage to the structure was identified in the May 1986 report as primarily slope influences. Although settlement of the fill may have been a minor contributing factor, the slope influences were considered more significant. The structure at the site is quite close to the descending fill slope which near the top of slope is moderate to steep. Since the distress to the structure was not resulting from gross instability, criteria for remedial treatment was developed to mitigate against those processes contributing to distress. It was, recommended that the structure be underpinned and that underpinning be extended through the fill and through existing landslide debris underlying the site. Although there was not an intent to increase gross stability in excess of the County criteria, undoubtedly there'will'be considerable benefit by the caissons penetrat- ing the landslide surface. Presently,,we estimate there are 59 deep caissons planned. which penetrate the landslide surface by 18 to 25 feet each. Although a rough estimate of factor -of -safety improvement could be made based upon available information and assump- tions as to conditions beyond those reviewed, analyzing the factor -of -safety improvement is beyond the scope of the current consulting. It must be kept in mind that the underpinning is considered "remedial treatment" and site conditions will undoubtedly be improved substantially as a result of the foundation system as proposed. If the County finds it, essential that additional stability calcula- tions be conducted, the client should consider drilling additional borings to further delin- eate subsurface geometry including the land- slide and shear key originally constructed. Item 3: "Clarify that the orientation of Section A -A' analyzed is the most critical.". Response: Cross Section A -A' presented in the original report was considered representative of the underlying geotechnical conditions.' The location of A -A' was selected because it coincided with the best geologic information available from subsurface exploration. Consid- eration of a possibly more critical cross File No. .1645 October 18, 1988 Page 3 American Geotechnical A CALIFORNIA CORPORATION section, a second Cross Section X-X'.has been prepared and has been included herein. It must be kept in mind that X-X' utilizes projected geologic information whereas borings were on line with Cross Section A -A'. In Cross Section X-X' the landslide head scarp was modified slightly from Cross Section A -A' based on additional view of stereoscopic aerial photographs.. 'It is hoped that the_information contained herein is sufficient to allow permitting of the remedial construction. If additional information is required, please do not hesitate to. call. Respectfully„„pubmitted, AMERIC • �EQtTECHNICAL r fr Gregory s)N Fxtrei��o��Q R f�`o f f Principalrf��,wt•},eee Prs cs a3 Geologist R.C.E. 26098 . C.E.G. 1183 G.E. 103 Distribution: 2 - City of Rolling Hills 2 Portuguese Bend Road •Rolling Hills, CA 90274 l South Bay Engineering 304-Tejon Place Palos Verdes Estates, CA 90274 Attn: Rick Morales 1 Russ Barto #3 Malaga Cove Plaza, #202 Palos Verdes Estates, CA 90274 Attn: Max Wuthrich 2 - Allstate Insurance Co. P.O. Box 17100 17100•Pioneer Boulevard, #200 Artesia,. CA 90701 Attn: Rosie Barnes ELEVATION (FT) X 990- 980 970- 960- 950- 940- 930- 920- 910 900- 890- 88b 870- 860- 850- ERODED SCARP IDENTIFIED FROM 1972 PHOTOS Tm Qls o OBI 4q;( 16 TRUE DIP � \ ��G �G1 5 O sci Tm EMBEDMENT. PER CASSION SCHEDUAL EMBEDMENT PER CASSION SCHEDUAL EXISTING GRADED SLOPE AND PAD GRADE Qaf -990 -980 -970 -960 - 950 - 940 PREEXISTING NATURAL SLOPE -930 -920 9 - 910 ?� ? -900 Tm -890 -880 - 870 -860 -850 SCALE: 1"=16' (AA) NOIIVA 13 GEOLOGIC CROSS SECTIONLX-X� American Geotechnical F.N. 1645 OCT. 1988 PLATE 1 ELEVATION (FT) X 990- 980- 970- 960- 950- 940- 930- 920- 910- 900- 890- 886- 870- 860- 850- ERODED SCARP IDENTIFIED FROM 1972 PHOTOS Tm Qls �O 0� O(<' 7 \„I 16° TRUE DIP 9 TRUE 13 ° TRUE DIP \ G�O 0� co(''�0 _,�0 �05 bO �p TRUE DIP 19 ° \ \ Tm EMBEDMENT PER CASSION SCHEDUAL EMBEDMENT PER CASSION SCHEDUAL EXISTING GRADED SLOPE AND PAD GRADE Qaf PREEXISTING NATURAL SLOPE Tm --990 -980 -970 -960 -950 -940 - 930 -920 - 910 - 900 -890 -880 - 870 -860 -850 SCALE: 1"=16' (Id) NOI1VA313 GEOLOGIC CROSS SECTION ,jX-X' American Geotechnical F.N. 1645 OCT. 1988 PLATE 1