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HomeMy WebLinkAboutCT 81-29; ALICANTE HILLS; GEOLOGIC AND SOILS INVESTIGATION; 1981-06-22JI ;,1 j'l ~'I ... ·1 'i """' Fl' ;:1' . JI ';1 "'1 '. . 'Soii M~chanics ~ Geology • 'Foundation Engi'neering , , ,1446'East Chestnut Avenlle • Santa Ana, California 92701 • (7141 '547-6703. • • ._. _ ~ M _ .... _ '" _ ~ __ -~ _ .-.-.... -'. JUne 22, 1981 w.o. 837-0C Daon Corporatiori 4041 MacArthur Boulevard Building A -3rd Floor Newport Beach, California 92660 Attention: ·Subject: Gentlemen: Mr. Arne Hamala Preliminary Geologic and Soils Engineering Investigation, Alicante Hills -Single Family - Dwellings, La Costa Area, City of Carlsbad, California. Th,e following presents a geotechnical 'summary-.report and .gra·ding . plan review for the s.ubject pr()perty. 'The purpose of this study :was to determine the feasibility of de~eiopi'l1g the site for ·sing.le family.detached.reside~tial housing. Op.r inVestigations of this project bega~ in, January '1981 as a part ,of an over~ll review. of.' La costa Properties. Moie detailed geological reco~naissance mapping a'nd -inspections were performed in June, 198.1. Our investigations also included review of :previous grading plans ~:r;:om Rick Engineering, various aerial photographs with ster~ogra,phic' coverage and avallable' geotechnical reports., GENERAL SITE b~SCRIPTION Location The subject property is located approximately 'in' the central' portion of La Costa, north of the golf· course occupying ,san Marcos , " .. ,-' '. ,-' : 1 Los Angeles Office • 5650 VanNuys Boulevard, Van Nuys, California 91401 • (21~J 785-2158 '" , ~" r:1 L f..:;.'., ~·::I ' <~ •• >, : . .:,: ~::;I .. ' ~2 ,_ ~I" lEI ~~I' 0 b ' fl ~ , [J, f?II L. [:. ~. rl ~, EI EI f:I' .- LI [I I~~I t"..;: I ILl' Daori Corpora~ion . ~;'. Creek. ~he elongate site consists of a previously graded hillside, , ' developed more thaI). six years ago as a part of ' the Alicante Hills project"Carlsbad Tract 75-6. Whereas Alica.nte Hills consists of 87+ acres, the sUbject'singfe fami1,y detached site represents approximately 15 acres along the northern boundary. Previous Gr'ading Original grading on the site cons;Lsted mostly of cut e,xcavation, w~th ~inorfilling, creating a series of flat building pads~ '; The rear 'of the parcel is marked by a south facing, l~:l (horizon- tai to vertical) cut slope varying up to 35 ,feet high. The fronts, of the graded pads are bordered by low cut/f,ill slopes which face Alfil Way~ the, primary access route. '" Existing Condition The graded pad surfac~s are vacant. M;inor' trash debris and dump fill deposits are scattered throughout the site. Local ~rosion, from pad and off;;! te r:unoff, have affected the s,lopes to 'a minor extent. PROPOSED DEVELOPMENT The current development, plan includes' placing filIon the property, to raise elevations and to provide rear access' for 38 ,single .family , • > '. • detached' units. The existing slope -facing Alfil Way vlould be modified in configuration, flattened to 2:land'iaised,:to a max:imum . " . -' . height of 30 f~et. The ~~~stingrearcut slope' would ,not be altered. Access to units would continue'to be f,rom Alf{lWay, via four entry drives. GeoSoils, Inc. '. ' II ;--j °1 ~:.~, .... :'""1 . , c..,,: :-::;1 .. , I ~ ::11 ;.~~ ""I .,'; , " :.' -"- 1;;;1 ,,'. ,:." ~ -";1 --" Daon Corporation Page 3 June 22, 1981 w.O.' 837-0C The source of fill for the project is not yet known, although expected to be from the La Costa area. GEOTECHNICAL CONDITIONS . General The subject property lies with a north-south trending ridge sequence terminating at San Marcos Creek. This vicinity marks t~e transition from hard volcanic rQck of the higher eastern areas to younger sedirrientary rocks which characterizes the western portions of La Costa. The distribution of geologic materials and fill encountered on the property are approximately delineated on the enclosed Site Plan ~Plate I). Volcanic Bedrock Most of the graded'-.areas on the site expose a variable series of volcanic-rocks mapped as the Santiago Peak Volcanics (map symbol Jsp) • These consist chiefly of harQ., dark colored basalt and andesite in a fractured and blocky condition. Weathering has typically produced reddish hues along fracture and/oI-exposed surfaces. Abundant exposures of the volcanic rocks occur on the '. -- rear cut slope and in isolated areas of-the graded pads and front -slope. The volcanics a.r~ relatively hard, although conventional grading techniques appear to have been utilized for previous grading. Local areas of hard rock exposed at pad grade suggest some difficulty with finish g.rading_. Trenching in. the volcanics is likely to experience difficulty. GeoSoil-s, I ne. " ".' [,1: fll' l~ El fl '~I r~1 ~ :,'" I', :~ (I ~ EI tI f.1 L , [I [J a"I' I ' , ;.~ ... Daon Corporation Sedimentary Be¢lrock Page 4 June 22, 1981 w.o. 837-0C Lin'ii ted exposure of sedimentary rocks also occur on the property. These consist of a sandy breccia-conglomerate which overlies the volcan;i.cs, 'grading to a white sandstone to the south. The breccia-conglomerate contains angular volcanic fragments mixed with 'rounded cobbles in a sandy-matrix. These units are considered a part of the Del Mar Formation. An 'irregular soil zone between the volcanics and sedimentary rock appea:t:'s to represent an ancient topsoil horizon (paleosol). 'This zone consists of a sandy clay which is distinctively red.' Thi,s clayey zone is regarded as highly expansive. The sediments appear to have been easily excavated. No evidence for bedding or other significant 'planar features were noted., Fi.ll Fill has bee,n placed over some portions of the site during original grading. They occur as a narrow swale filling and various deposits placed to create flat pads. The fill materials were derived from the local bedrock and'are relatively minor in thickness and extent. These deposits appear dry to damp, ano. reasonably dense, wher~ . ' exposed, although further analysis and/or testing. may be ;r:-eq'uired at the beginning of grading. Minor dump fill and surficial fill are scattered about the property. These soils ar~ loose and dry. GeoSoils, Inc. .. ,' . ',.' : ;':1 • .! ~ ..;:..Li: •• ""'" '.; ~ ;Y'I :'1' " ?-I 1:"" ":.~~ i-~I '., .. ; "".1 .... ~~ JI' "11 ' i . , ..... "'1 :: : .... :~"'I '" , .. -' 31, "TIl ., ~~~i .,' ~I "I baon Corporation Structural Features, Landslides, Groundwater Page 5 " June 2;2, 1981 W.O~ 837-0C No major structural features were identified during our site investigations. The ,volcanic rocks' expose steeply inclined fracture surfaces in random directions. No evidence of lapd- sliding was observed nor were groundwater seepages or springs encountered. CONCLUSIONS AND ~ECOMMENDATIO~S The following conclusions and recommendations are based lip on, our geologic reconnaissance ,of the site, our knowledge of the site materials and experience in geotechnical applications. In general, the site is suited for the planned development from a soils engineering and geologic standpoint. No evidence exists of impending geologic instability or adverse soil conditions. General Conditions 1. Natural rock units underlying the property are suitably dense and should adequately support residential structures and proposed fills. 2. Most of the on site materials are regarded as having a' ,low swell potential, except for the ancient topsoil zone which would be highly expansive. 3. Proposed excavations alo~g Alfil Way may encounter hard rock which may m~ke it difficult to cut the proposed slope or to form a fill key. Heavy rippi~glitilizi~g a D~8 or equivalent machinery would be required. The need for blast- ing is not anticipated, ,although no shallow seismic soundings GeoSoils, Inc. -. ' , [I" m" f::1 [I ~~ , [) WI~ It.< " ,', ~ > t:!il '';' {~ ~ tl tI ~ EI [I II [I [I [J fl ~ t. LI .J Daon Corporation Page 6 June 22, 1981 W.O. 837-0C were performed to define this. Trenching in the volcanics would be expected to be difficult. 4. Existing fill deposits vary from loose (chiefly in pad surface areas) to reasonably compact _ (deeper fills). 5-. Excavation wi thin the volcanic rock is likeiy to produce oversize rock which would requi're special ,handling during grading. 6., There is an existing storm dr~"in line in the westerly third of site and perhaps other utilities. Care should be taken during grading to avoid damage to these facilities. Import Soils 1. Import soil locations should be inspected and, if necessary, tested by the soils engineer for its suitability and design parameteJ;'s. 2. It is expected that import soils would come from a local source, probably from the Delmar Formation. These materials vary from sands -to clays and are generally moderately to critically expansive. Clay soils should be mixed with other ,soils to improve strength and soil characteristics. GeoSoils, Ine. JI i;1 :"1 ~' ~I ;)1 t1_ Daon Corporation Page 7 June 22, 1981 w.o. 837-0C 3.' Foundation parameters and other soil criteria needed for construction should be determined when the borrow-source is known anq/or at completion of grading. Treatment of Existing Ground .. ;:1 1. All heavy concentration of vegetation and trash should be ~~I -., ~.J '"~I '.' ,,,,, ...;.' ~I ;'e' .', ~ '"I ';:1 " , ""-I :~ .! '-'1 ~. J: j J. :~I . , '''1 -,-<-, :1 wasted from the site. 2. All loose fill deposits which occur in areas of the pad surface should be 'removed to bedrock ,or suitable fill. 3. Surficial removals should be performed across the entire site to expose bed~ock or compacted fill. Tpese removals are expected to vary from six to twelve inches deep, a~though local removal of fill up to three feet may occur. 4. Subsequent to removals and after approval of the soils engineer, the underlying ground should be sdarified to a depth of six inches, watered to achieve near optimum moisture content and compacted mechanically. General Grading Criteria 1. Onsi te soils are considered sui table for reuse as com~acted fill. 2. The cut portion of the slope along Alfil Way should be graded initially apd a key e~cavated prior to placing fill. This is necessary in order to geologically inspect t~e condition of the finished slope to determine its suitability. GeoSoils, Inc. , , .\', Daon Corporation Pag~ 8 June 22, 1981 w.o. 837 .... 0C 3. The fill key should be at least 15 feet wide and tilted slightly inward so that the heel of the key is a~ least one foot deeper than the toe. 4. Subdrains are not considered necessary at this site, although this determination shoul~ be made in the field during grading. 5. All fill soil approved by the soils' engineer s'hould be spread in thin lifts, watered to achieve hear optimum moisture content and compacted. 6. During filiing, benching should be accomplished into the rear of the key until the upper pad is reached. 7. Large rocks or rock fragments encountered du;r:-i!lg grading should be placed as windrows or individually buried in the deep fills. Rock should be maintained at least five feet below structures or beneath any proposed excavations for ,utili ties', foundations or pools. Windrows should. be placed under the direction of the soils engineer. 8. Any remaining cut areas or shallow fills near structures should be overexcavated to accomplish, a minimum fill depth of three feet';to provide uniform foundation support. Thi,s' recommendation may be increased slightly depending ~pon the soil type and/or foundation depths. 9. All fill should be compacted to a minimum of 90 percent reiative compaction as determined by ASTM D-1557-70. GeoSoils, Inc. " I ""1' '"\ "- w_ ' .. ' :"1 ~=I' .,., I:JI 31 ·~1J j -:":'11 .; ~~ .L '~I -'I Daon Corporation Page 9 June 22, 1981 w.o. 837-0C 10. Contin~ous inspection and periodic testing of the till should be maintained during the earthwOrk to aid the contractor in performing his work. Where testing irt~i­ cates less than 90 percent relative compaction and/or inadequate moisture content; the contractor should be informed and additional effort applied until subsequent testing indicates the minimum compaction standard. Earthwork Factors 1. Grading of existing onsite materials is generally minor in extent. The volcanics would be expected to bulk on the order of 1 to 3 percent. The sedimentary rocks should' be similar from cut to fill. 2. Balance of offsite materials would depend upon their location and materials. 3~ No subsidence of the natural ground is anticipated. Fill Slopes 1. Fill slopes at a gradient of 2:1 a~e cohsidered satisfacto~y with respect to gross stability. 2. The outer surface of the fill slopes should be back-rolled every three to four feet as the slopes are constructed. The backrolling should be adequate to bring compaction to wi thin two feet of the surface. When the slopes rea'ch their design height, they should be shaped, sheepsfooted and then grid rolled. GeoSoils, Inc. ~I'· It Daon Corporation Pag.e 10 June 22, 1981· W.O. 837-0C 3. Alternately, slopes may be overfilled and then cut back [I to their compacted core. ~ ~ LI [J ~. EI . 8.J tI III 4. Slopes built with cohesive soils tend to be moisture sensitive and can be involved in surficial failures or "carpet" failures during periods of prolonged rainfall of irrigation malfunction. Experience has shown, however, that slope compaction as described above combined with deeply rooted vegetation and nominal irrigation can mitigate this potential. Drainage away from the top of slopes is mandatory. At all t,imes, avoid saturation or des~ccation of the slope materials, since these conditions tend to deteriorate the slope. Irrigation facilities shou~d be turned of£ during the rainy season. Maintenance of slopes should include good drainage, elimination of burrow- ing rodents, correction of defective irrigation facilities and controlled slope vegetat~on growth. Existing Cut Slope 1. The existing cut slope along the nO:r;'th property line is considered adequate as graded. 2. Minor erosion has occurred along the top of sl~pe due to runoff from above. This runoff should be controlled to avoid continued erosion. A brow ditch and/or modification . of the .grades above the slope may accomplish this • GeoSoils, I ne. . I ~ 1 ' II ;ii' . :~I ,~I, ~'I • .i ""'ll J ~'I ". "'I i~' ,-. .. . 91 Daon Corporation INVESTIGATION LIMITATIONS Page 11 June 22, 1981 w.o. 837-0C The materials encountered on the project site and utilized in our laboratory investigation are believed representative of the total area; however, soil and bedrock materials vary in charac- teristics between excavations and natural outcrops. Since our investigation is based upon the site materialsobserved, selective laboratory testing and engineering analyses, the con~ clusionsand recommendations are professio'nal opinions. These opinions have been derived in accordance with current standards of practice and no warranty is expressed 'or implied. This opportunity to be of service is sincerely appreciated and £f you have any questions pertaining to this report, please call. Respectfully submitted, GeoSoils, Inc. By: By: JAS/ARK/se Encl: Site Plan (in pocket) . Dist: (6) Addressee GeoSoils, Inc.