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Home My WebLink AboutCDP 02-42; DUNN RESIDENCE; ASSESSMENT OF FILL SLOPE REGRESSION; 2002-12-05·' • December 5, 2002 THA Proj~ct No. Go2-ooi99 Mr.Jim Dunn 4316 Cassanna Way, #S05 Oceanside, California 92057 .::2:,.~~~~ a...,., .I TAYLOR-HUNTER ASSOCIATES;l,NC. _.:. -~ c.otecbnlcal eiicineering, ceology &: environmental consultants SUBJECT: AsSESSMENT OF FILL SLOPE }U;GRESSION 2202 HIGHLAND DRIVE, CARLSBAD, CALIFO}J.NIA Dear Mr. Dunn: Geoteclmical Engi11eering Geology Envir011mental Consulting Civil Engi1teeri11g 818 Civic Center Drive Oceanside, CA 92054 Tel: 760.721.9990 Fax: 760.721.9991 www.taylor-hunter.com At your request, Taylor-Hunter Associates, Inc. (THA) has performed a limited review and evaluation of slope regression issues affecting your property located. at 2202 Highland Drive in Carlsbad, California. The project involves a parcel of approxjmately 2.5 acres in the City of Carlsbad, California. The parcel is identified as APN 156-051-24 and is located at.the northern terminus of Highland Drive {Figure I). The lot is currently vacant and unimproved (Figure 2). We understand that due to restrictions imposed by the hillside development regulation in City of Carlsbad zoning code and the local coastal program, the buildable portion of the lot is limited to a roughly ¼ acre area in the southeast corner of the parcel. This is an area where undocumented/uncompacted fill was pl~ced over native terrace deposits more than s6'1jekrs ago. We understand that defining the historical limits and of the fill in this area is one of the issues affecting determination of the limits of the buildable area. The purpose of our evaluation has been to assess whether the geometry and configuration of the fill slopes in the southeast portion · of the parcel have changed over time and, if so, to evaluate the changes that have occurred. THA has reviewed the following documents in connection with our evaluation: ..,. Aerial photograph dated Dec. 9, 1966 ..,. Aerial photograph dated Jan. 14, 1988 (stereo pair) ..,. Aerial photograph dated June I, 1994 .,.. Aerial photograph dated Aug. 12, 1998 (stereo pair) Mr.Jim Dunn December 5, 2002 Page2 • ~ A report entitled "Geotechnical Investigation, Proposed Lot Split, Three-Acre Site, 2202 Highland Drive, Carlsbad, California!' prepared by Ron Gutier and Erik Nelson dated February I 7, 1989. ~ Preliminary Site Plan Exhibit prepared by Aquaterra Engineering (undated) ~ Current CAD drawing prepared by Aquaterra Engineering Copies of the aerial photographs are included as Attachment I to this letter. It is noted that the quality of the original aerial photographs that THA reviewed is significantly better than the quality of the cop~es included in Attachment I. In addition to reviewing the. above-listed documents, THA visited the site to observe current conditions and verify interpreted information from the review of aerial photos. D.(SCUSSION Available geological information contained in the literature and the 1989 geotechnical report indicate that near surface deposits at the site consist of Eocene Santiago Formation, Pliocene aged Lindavista Formation (older terrace deposits) and manmade fills. The fill soils were placed primarily in the southeast portion of the parcel. The northwest facing slope in the southeastern portion of the property is composed of undocumented/uncompacted fill soil (see Figure 2). It appears that the fill material comprising the slope was placed in the early-to mid- 1960's. THA evaluated changes at the site based on detailed examination of the aerial photographs. Available stereo pairs were examined using a Sokkia MS27 stereoscope. Because of the angle of the sunlight in the 1966 aerial photograph, the top of slope can be discerned clearly. Figure 3 presents an enlarged copy of the 1966 photo with the top-of-slope indicated. The 1966 photo indicates that the fill was initially placed with a relatively steep slope face. Based on inspection of the aerial photo, we estimate that the sl'ope gradient shortly after the fill was placed was nominally between about 1.2:1 and 1.5:1 (horizontal: vertical). Figure 3 also shows the location of the top of slqpe in 1998 based on inspection and interpretation of the 1998 stereo aerial photographs. As illustrated by Figure 3, the most severe slope regression has occurred in the vicinity of the storm drain manhole, where the top of slope has receded by as much as 20 feet or more as a result of erosion apparently caused by leakage of water from the manhole. More typically, the· top of slope has regressed 5 to 9 feet · along the width of the fill slope. The inclination of the fill slope currently ranges from about 1.5;1 to 2:1. The slope angle has flattened over the past 3·5 to 40 years as a result of many processes including erosion, sloughing, slope creep, and settlement. Impacts from foot traffic and dumping of debris fill on the slope have probably also contributed to flattening of the fill slope. TAYlOR-tlUNTER ASSOCIATES, l~C. Mr.Jim Dunn December 5, 2002 Pages • • The overall result of these processes has been regression of the top-of-slope towards the south and some amount of down-slope movement of the fiU. As noted above, the amount of slope regression is generally 5 to 9 feet and is more than 20 feet in the immediate vicinity of the storm drain manhble. - The 1989 geotechnical report included the conclusion that the fill slopes should be considered unstable and recommended that all of the uncompacted fill soil~ should be removed and replaced as compacted fill. We concur with the conclusions and recommendation as it relates to the fill slopes. Earthwork associated with removal and recompaction of the fill will affect essentially all of the fill soils that comprise the slope above about elevation 125 to 130 feet. The contact between the native deposits and ove:rlying undocumented fill soil is nominally 40 to 60 feet horizontally from the current top of slope. The engineered fill will need to be properly keyed and benched into firm native deposits. If the new fill slope is to be steeper than 2: I, the slope will need to be reinforced and/ or one or more retaining walls will need to be constructed. We also understand that the development will involve relocating approximately 250 linear feet of an existing storm drain that traverses the property. The conceptual plans indicate that the storm drain pipe is located within an easement that traverses the central portion of the property. Relocation of the storm drain will involve excavating a significant portion of the existing fill slope. If you have any questions, please do not hesitate to call me at (760) 721-9990. TAYLOR-HUNTER ASSOCIATES, INC. PROJECT NAME J:lufin-Property , PROJECTNUMBER -~02-00199 FIGURE 1. SITE LOCATION AND VICINITY MAP Durin Property, Highland Drive Carlsbad. CA ·,,~t:-:,' WEST View towards northwest from southeast.area of parcel. Top of fill .slope in foreground. • EAST View towards southeast from south-central area of parcel. WEST ~,_--.......... _.. PROJECT NUMBER G02-00199 • FIGURE 2. PHOTOGRAPHIC MOSAICS --· .. -------""'"" nATr= December 4, 2002 SHOWING CURRENT CONDITIONS TAYLOR· H UN TE R ASSOCIATES, INC. DRAWN BY L. Taylor Dunn Property, Highland Drive geolechnlcal englnNrlng, geology and envlronmtntal consultcnis R~AI F NTR Carlsbad, CA Dunn Propertv - PRO.lECTNUMBER Go2-001_99 FIGURE 3 •. COMPARISON OF TOP OF ~=-..,..,.-,, ... '--'D'--'-A~TE-=-=-'-~=~D_ec_e_m~be'""'r-4,-2-00_2 ____ __. SLOPE LOCATION -1966 TO 1998 TAYLOR-H UN T E-R ASSOC I A TES, INC. DRAWN BY L Taylor Dunn Property, Highland Drive geotechnlcal engineering, geology and envkonmental comultants ..,,,_., ~ _ • • • --· r. .. rlch,,,,I r A PROJECT NAME. Dunn Property • ATTACHMENT 1 COPIES OF ... t\ERIAL PHOTOGRAPHS DUNN PROPERTY -2202 HIGIIl,AND DRIVE, CARLSBAD <' Source: Continental Aerial Photo! Inc., C123-3-14, 12 Aug.1998 ~ r"rriolr:'ttu·t-i rE"R Assoc IATE s,1Hc. geotechnlcol engineering, geology and environmental collSll~ants PROJECT NAME . PROJECT NUMBER DATE DRAWN BY Durin Property G.02-00199 December 4, 2002 L. Taylor - 1998 AERIAL PHOTOGRAPH Dunn Property, Highland Drive Carlsbad, CA Spurce: Continental Aerial Photo, Inc., SD-2-8, 14 Jan. 1988 PROJECT NAME Dunn Property .~~ ~· ..-DATE December 4, 2002 PROJECT NUMBER G02°00199 r :::t ~«. ·~ TAY LO R-HUNTE R ASSOCIATES ,INC. 1-'b~RA=W~N~B~Y'----'----+~L'-. T"'a~yl.c.cor _____ -'"-_-1 geotechnlcol engineering, goology and environmental consultants SCALE Approx. Scale: 1" = 100' 1988 AERIAL PHOTOGRAPH Dunn Property, Highland Drive Carlsbad, CA Source: USGS Terraseiver, Image date 01 June 1994 PROJECT NAME Durill P-roperty PROJECT NUMBER G02.,0Q199 ~"!!';~ .. ---·.:. TA YL O R . H U N TE R .AS S O C I A TE S , INC. DRAWN BY L. Taylor, geotechnical engineming, geology and environmental consultants i-;_ "'c,"',-'"'11.,c:..:i::C!.!..!"-'---~-1-c:::,.;_nn.:..:~;;:.-v,:=.C,'-~-~·-~--.-.-_-,,-.-n,-----i D T December 4, 2002 1994 AERIAL PHOTOGRAPH Dunn Property, Highland Drive Carlsbad, CA