The URL can be used to link to this page

Home My WebLink AboutCT 73-18; RANCHO DEL PONDEROSA UNIT 1; GEOLOGICAL AND FAULT STUDY; 1973-11-23I· 'I :..1, . :'·1' 1 : '1 I I· .',1 '.1 ·1 I I, " :1 I I ,I ·1 I TABLE OF CONTENTS TRANSMI,TTAl lETTER SCOPE FilE AND lITERATURE RESEARCH . FI ElD EXPLORATION lABORATORY TESTS GEOLOGY FACTORS DETERr~INING ACTIVITY AND APPLICATION TO THE SUBJECT FAULT CONCLUSIONS AND RECOMMENDATIONS FIGURE 1 -GEOLOGIC MAP AND SITE PLAN FIGURE 2 -GENERALIZED CROSS SECTIONS T -1,. T -2, T-3 FI GURE 3 -GENERAL! ZED CROSS SECTIONS T -4 & T-5 FIGURE 4 ~ EPICENTER AND FAULT MAP PAGE. 1 1 2 2 . . '2 ~. 6 .. " WOODWARD· GIZ·IENSKI & ASSOCIAJES CONSULTING SOIL AND fOUNDATION ENGINEERS AND \lEOLOGIITI . ., < •• , ' . . '. I ·1 '.1 I I '.1 I I I I I I I I ,I I I I I WOODWARD· ·GIZIENSKI & ASSOCIATES CONSULTING SOIL AND fOUNDATION ENGINEERS AND GEOLOGISTS . An affiliate of Woodward -C;lyde C9118u1!ants 3467 Kurtz 'Street San Diego California 9211Q (714) 224.29.11 Ponderosa Homes November 23, 1973 Project No. 72-253 2082 Business Center Drive Suite 100 Irvine, California 92664 Attention: Mr. Donald Gayle ENGINEERING GEOLOGICAL AND FAULT STUDY' PONDEROSA HOMES DEVELOPMENT OLIVENHAIN ROAD SAN DIEGO COUNTY, CALIFORNIA In accordance wit.h your request and our proposa" authorized September 14, - 1973, we have made an engineering geological and fault study of the subject site located north of, and adjacent to, Olivenhain Road near its intersection with Rancho Santa Fe Road in the 01ivenhain'area of San Diego County. Ca'l;-fornia. A preliminary soil investigation of the site has been .made' and reported i.n two reports dated November 17, 1972 and September 13. 1973. The attached report presents the results of our investigation and our conclusions pertaining to the site. The project geologi.st and engineer . 'assigned to this project are Braven R. Smillie and Richard P. While, 'respectively. WOODWARD-GIZIENSKI & ASSOCIATES SFG/BRS/RPW/mf (4) Ponderosa Homes (1) Rick Engineering Company Attachments -; . , ; -I -- I: I ·1 ~. . I. I I '1, '1- -I 1- '1 :1 I .1 1 ·1 I, I ·Project No. 72-253 SCOPE The purpose of this study is to explore and evaluate a fault zone 'which was foundin the southwestern portion of the site during our.prelim1nary soil investigation. In par.ticular·our main 'objectives are to determine the general seismic setting of the areas in which the subject site ;-s loca'ted, the extent of the faulting on site, the general width of the zone .affected,· and to evaluate the 'genera1 degree of activity of the fault zone. FILE AND LITERATURE RESEARCH In order 'to express mea'ningful opinions in regard to the geologic and seismic setting and condftions at the subject project, ·the available ~eologic literature, aerial photographs and prior reports pertinent to . ..:. the general surrounding area were reviewed. References of special va'lue· in this study included: (1) "Seismic Risk in San Diego" McEuen and Pinckney, San Diego- 'Society of Natural History, Volume' 17, No.4, July 1972. (2) -"Crustal Strain and Fault Movement Investigation" Departme'nt . of Water Resources Bulletin No. 116-2. (3) "Preliminary report on Recency -of Faulting in the Greater San Di ego Area, Ca 1 i forni a" U. S. G. S. " Zi.ony and Buchanan, ,1972. (4) "Offshore Extension of the Rose Canyon Pault, San Djego,.·" Californian·U.S.G.S. -George W. Moore, 1972. . -. (5)' ,"Geology and Mineral Resources of San Diego County, California" County Report 3, Harold Weber, Jr. 1963 .. (6) Stereographic aerial photos from Sheet 2, 3 and 7 (scal~ 1:20,000) U.S. Department of Agriculture, flown in 1964 by' Cartwright Aerial Surveys. Inc. WOODWARD· GIZIENSKI & ASSOCJATES CONSULTING SOIL AND fOUNDATION ENGINEElIS AND GEOLOGISTS .-,::,.' , .. " -.'.-- . -~ ':.-'i-~ ,-, .'. .. ':: : ... ..,. ' .. ·1 I .,,1 ,I " . ·1 I "·1 "I· , , ~ I· ·1 I . 1 I I I' I· ,'1 . :"'1 I Project No. 72-253 . 'Page 2 FIELD EXPLORATION The field work for thisstudywqs done during -the p~rioc;l 'oi September 18 and 19, 1973 under the supervision of a project engineering geologist from our firm. Fol1owing a field geologic reconnai.ssance of the site, and surroundi ng area, four backhoe trenches up to~ ~Q9IJt~fLilLdgRib, were . - excavated along the projected trend of the fault zone to augment information __ ~_ ~_ ~ ____ '''--~ ___ ... _ _. _ -~ -~ _._ ~_~ ' •• ~ ______ <_ ~ • ___ ~ _ ~ ____ • w"-_ __ __ obtai'ned from a test ttench and borings m~ge .. _earl1er. The trenches averaged _____ '~_.J.-------------.--~-u~ ~ '. ----------approximately 7 ft in depth and ranged in length from 120 to 150 ft. The C -'. f."' -,- total length of all trenches was approximately 645 ft. The appro?<imate locatio.ns of the te'sftrenches are shown on the, Geologic Map and Site Plan, Fig. 1. Figs. 2 and 3 are generalized cross-· sections of the test trenches and are ba'sed on fi:eld logs of the trenches prepared by the geologist. LABORATORY TESTS. Samples ,were obtained of soils overlying individual fault breaks ; n the zone, but not offset by these breaks. Age determi nati,ons were made - on these samples by Teledyne IS~~0pes.,.Co.rporatibn of Westwood, Ne~ Jersey . , GEOLOGY Regional Geologic Setting The San Diego Region lies within the Peninsular Range Pr~vlnce of California. This province is a well defined geologic and physiographic unit extending southeastward from the Los Angeles Basin to the, tip of Baja,'. California. The subject site 'lies on a broad coastal band of relatively horizontally stratified Tertiary sediments, about 2 miles west of the boundary . . with the IIHighland" Jura-Cretaceous Age metavolcanic rocks. -------~ ,-... WOODWARD· GIZIENSKI & ASSOCIATES CONSULTING SOIL AND fOUNDATION :[NGIN[ERS AND GEOLOGISTS I 'I· '? I I, I, I I ,I I ,I I I I I I I I' I ,I Project No .. 7~-253 . Page 3 Regional Faults A review o.f the existing geologi'cal literature.' including the Department of Water Resources Bunetin No. 116~2 entitled "Crustal St~airi and Fault Movement Investigationll and th~ California Division of Mines .' and Geology "Provisional Fault Map of California" and uPreliminary Earthquake' . ' Epicenter Map of California. 1934-1971 11 indicate's that the closest active fault, along~which epicenters of magnitude 4 or greater have been recorded,. is along the Elsinore Fault zone located some ~5 miles northeast of the -_. ----------~ ---'. subject sit~. In this regard, the. paper prepared by McEuen and Pinckney , , ' concludes that, based on current available data "a Richter Magnitude 7.3 't--------_~ .:. earthquake on the Elsin'ore Fault, having a statistical repea~ __ inter~a.1 of 60 years, appears to 'be the maximum probable ear.thquak'e for San oiego.'" . --, --,'-' -,,,--. ---._---_.--.-- Fig. 4; the uFault.and Epicenter Map", was prepared from data given in the reports referenced above. It indicates the approximate 10cati·on of the subject site with respect to the Els1noreFault zone and to a postUlgted . ..-- offshore extension 'of the Rose Canyon Fault, some 7 miles to the southwest~ Historically 'the ,Rose Canyon ,fault has been noted for·its general quiescense and f~r its general lack of topographic features commonly'associated with . Holocene faulting such as sag depressions, offset draihages or ~harp1y defined fault scarps.' For these reasons the Rose Canyon fault has not been considered "active" for this study . .. In addition to the larger faults des,cribe.d .above, there are a' few relatively smali short faults located in the general area of the SUbject site. These faults, which average only a few miles in length, are shown on ~he "Epicenter'and Fault Map". The nearest of these are two short zones of faulting within approximately 4 miles of the site, neither of which WOODWARD· GiZIENSKI & ASSOCIATES CONSULTING SOIL AND fOUNDATION ENGINEERS AND GEOLOGISTS I I' '. 'I I' I I" I I I I :1 I, I I I I I I :1 ·Proj~ct· No . .72-Z53 . page 4 '. 'is located in an area of epicenter clustering. Due to this fact, the,lack .. of topographic expressfon, and the relati~ely short length of the faults, they have been classified as inactive. Site Geology, . The' subject si te l occupi es an. area of approximately' 121 acres and -~ , ' is situated on the northerly slope of Encinitas Creek which drains westerly , I , ~ i~to Green Valley. Elevations across the site range from a'pproximate1y 95 ft at the southwest corner to over 275 ft (MSLD) on a hilltop near the northeast corner of the property. ' As noted in our earl i e.r reports on thi s si te, the natural subs,urface soils and geologic formations on the site' can generally be divided into \ the following groups: . . /'(l) 'An overburden"consisting of ,a mantle,of resi-dual soils (topsoil ~--------~~ -------.. -. -------...........,.-----~-----"---- , . (2) Alluvial-colluvial soils in the mair subtributary draws. , These are usually reworked soil mantle m~terials deposited by the action of water and wind i'n the case of alluvial soils, and gravity in the case of coJ1uvial soils. - ---~ '(3) Underlying the overburden and alluvial-colluvial soils ar~ ., foundmassive,-p-oorl~ to moderately indurated sandstones, which appear ~ -to ~par~,_o~~~_~~e_.!o~_~eL£?_~mation of Eocene Age, and _~~_a~inated siltstone- claystone unit, which appears to be similar to the Del Mar Form~tion, also of Eocene Age. The major Eocene Age formational sediments are generally horizonatally stratified, although some expos'ures of the sandstone unit ind;'cate a relativ1y strong tendency towards crossbedding and 'moderate dips(5 to 10 degrees) (})i WOODWARD· GIZIENSKI & ASSOCIATES CONSULTING SOIL AND fOUNDATION ENGINUIIS AND GEOLOGISTS I I .1 I I ~I I 'I ,:·1 I '1 I ·1 '1 I I- I I I. Project·No. 72-253 .Page 5 toward the west-southwest ~lo.ng Olivenhain Road. Inspection of a road cut along Dlivenhain Ro~d, and data from test borings made during the soil investigation, revealed the existence . . of a fault which traverses the property·in a northeast-southwest direction; Further investigation, including this study; has defined a zone of faulting along that trend approximately 100 ft wide. The fault ~lanes in this zone .have a trenq of approximately N 4p,oE with a dip" of . -, . . " ) 85°E from the horizontal. The vertical compone,nt of movement 'along the fault plane has been up on the nort,hwest side and down on the southeast, placing older Tertiary mudstones in juxtapositioti~with' younger sands.~.., at the south end of the site. The vertical offset on the fault is not known· but test boring results i'ndicate that it is at least 45 ft. A "ClOSl! examination of each break witbin the zone revealed no wide gouge or crusheq zones where:·the faul t crosses' the sandstone uni t, 'placing' sands' on' either side of the break, the break appears ceme~ted and IIhe~ledll., Inspections of surface exposures and our test trenches reveal numerous trends' of joints and fractures withi.n the 'sandstone unit; most of' them are cemented or IIhealed" as a"rei..tne 'fau'lt ,breaKs in.. sandstone.' ";Jhe grouping and attitude of· these joint patterns indicate's that they probably resulted from the same compressional forces ~hich caused the fault'descriPed- above. FACTORS DETERMINING ACTIVITY AND APPLICATION TO THE SUB.JECT FAULT In order to express a meaningful opinion as to the relat'ive lIactivityll of the fault zone which trav.erses the subject property, and its effect upon the' proposed development, the following items were considered: (1) Seismic History of the area. As discussed in this report WOODWARD· GIZIENSKI & ASSOCl'ArES CONSULTING SOIL AND FOUNDATION ENGINEEIIS AND GEOLOGISTS :'::-.1' I :".1 ;1 ,I ,.1 ·1 I ;1-.- .,1 '1· ·1 :·1 I I ':1 '1' I I Project No. 72-253 ·Page 6 ,under' "Regional Faults" available historic records and seismic instrument data revealed no significant seismic activity·near the sybject,site. and none apparently reTated to the nearest mapped faults, approximately 4 ·miles to·the east {see· Fig 4 "E~icenter and Fault Map"} .' (2) Topographic Express;,on. Our field reconnaissance and study of aeri a 1 photographs of the area fa i1 ed to revea 1 any s i,gni fi cant topographi c . eXQression of the subj~ct fault zone such as sag ponds, parallel valleys 4C ~ £ ,~ .' _ • _ • or offset ridges on or near the property. (3) Age dating of materials undisturbed by the fault. Two samples of alluvial soils deposited across~ the fault ·zone. butundisturbe'd by th~ individual' fault breaks,. were obtained' for age dating. One of these samples which had a high percentage of organic materia" was tested by the IIRadioca'rbon" method. The other whi ch contained c~rbonate cementati o'n was tested by . ...- . . the "Carbonate/carbon II method. The locations where the samples were obtained < • '. , ~ are shown on the generalized cross sections of trencn.T4. Fig 3., Radiocarbon' age determination o~ ,sample Al. the younger of '~he two soils was 670 years {plus or minus;··i40 years}. The age determination of the carbonate-cemented sample. A2, 'lIas 8,950' ,Years (plus or minus 150 years)., The signifi'canc'e ' of the age determinations of these undisturbed sedime'ntary deposits is ' that there' probably. has been no vertical movement along the sub'jec( fault~ zOne within ~t least the la~t ~,800 years. CONCLUSIONS AND RECOMMENDATIONS , ' (l)-Based on our studi·es t~ date, it i·s our opi.ni on that the closest acth:e fault on which repeated' seismi.c events have occurred is . the Elsinore Fault zone located some 25 miles northeast of tbesubject s.ite~ WOODWARD· GIZIENSKI & ASSOCIATES CONSULTING SOIL AND FOUNDATION ENGINEUS AND QEOLOGISTS @. --c· I I :1 '·1 '·1: I I '1 .1:' "1 I :1 I ·1 .. ·1 '·1 I. I I Proj~ct'No. 72-253 'Page 7. Based on current avai 1 able data, an earthguake of approximate Ri chter . Magnitude 7.300· the Elsinore Fault, having a statistical repeat interval of 60 years appears to be the maximum probable earthquake' for the subject l • • • site a~d for the San·Diego area in general. For ~ost construttion, the maximum probable earthquake is recommended for c\esign. (2) The fault which extends through. the subject site has a northeast -. southwest trend, is approximately 100 ft wide, and incorporates an offset of at least 45 ft. The approximate location of the fault is shown on the attached Site Plan. The zone consists of numerous breaks, each incorporating a very narrow band of gouge and minor zone of crushing. In most"cases the fractures in the sandstone material have been cemented and "healed". (3) Based on the general sei smi c hi s.tory of the a,rea, the general . . lack of topographic expression, and the age ·dating of materials undisturbed by the fault, it is our· opinion that the subject fault, for the proposed '. . ~ development, maybe considered inactive in nature. It is further our opinion that the subject fault is similar to sho~t nearby faults located some 4 e miles from the site which have been previously classed as inactive. (4) It is our opinion that the fault will have little affect on the proposed subdiVision with respect to the actual physical nature· of the zone. It is recommended however, that a geologic inspection. of .=' . -aJl cut siopes and fill keys be made by Woodward-Gizienski &'Associ.ates duri n9 gradi ng to i denti fy and map the actual occ.urenc.e of the faul t breaks, and to determine if the zone, or fractures associated with it, will create adverse stability problems in cut slopes"or 'fill foundations. Additional recommendations for slope correctionS can be presented at that time if this shoul·d be required. .. @y WOODWARD· GIZIENSKI & ASSOCIATES CONSULTING SOIL AND fOUNDATION ENGINEER. AND GEOLOGISTI