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Home My WebLink AboutHDP 02-10; Dunn Residence; Hillside Development Permit (HDP) (31)1 n - TAYLOR-HUNTER ASSOCIATES, INC. pMcchrrrrl Iqinnring. ~lo~y k snvimnncnW consultants Geotechnical Engineering Geology Environmental Consulting Civil Engineering December 5,2002 THA Project No. G02-00 199 Mr. Jim Dunn 45 16 Cassanna Way, #SO5 Oceanside, California 92057 818 Civic Centet Drive Oceanside,CA 92054 Td: 760.721.9990 Fax: 760.721.9991 w.taylor-hun!er.m SUBJECT ASSESSMENT OF SLOPE REGRESSION 99052 HIGHLAND DRIVE, CARLSBAD, CALIFORNIA Dear Mr. Dunn: 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 approximately 2.5 acres in the City of Carlsbad, California. The parcel is identified as APN 156-05 1-24 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 portior, of the lot is limited to a roughly M acre area in the southeast corner of the parcel. This is an area where undocumented/uncompacted fill was placed over native terrace deposits more than S6 years 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: b Aerial photograph dated Dec. 9, 1966 b Aerial photograph dated Jan. 14, 1988 (stereo pair) b Aerial photograph dated June 1, 1934 b Aerial photograph dated Aug. 12, 1998 (stereo pair) - Mr. Jim Dunn December 5,2002 Page 2 b 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 17, 1989. b Preliminary Site Plan Exhibit prepared by Aquaterra Engineering (undated) b Current CAD drawing prepared by Aquaterra Engineering Copies of the aerial photographs are included as Attachment 1 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 copies included in Attachment 1. 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. DISCUSSION 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 e). It appears that rhe fill materiai 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 slope gradient shortly after the fill was placed was nominally between about 1.2: 1 and 1.5: 1 (horizontal: vertical). Figure s also shows the location of the top of slope 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:l. The slope angle has flattened over the past 35 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-HUNTER ASSOCIATES, INC. I w Mr. Jim Dunn December 5,2002 Page 3 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 fill. 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 manhole. - The 1989 geotechnical report included the conclusion that the fill slopes should be considered unstable and recommended that all of the uncompacted fill soils 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 overlying 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 2:1, 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) 72 1-9990. Sincerely, TAYLOR-HUNTER ASSOCIATES, I TAYLOR-HUNTER ASSOCIATES, INC. I ".- - x DATE December 4.2002 SLOPE LOCATION - 1966 TO 1998 ' ~FZOR-HUNTEI; ASSOCIATES INC. D~wNBY I L. Taylor Dunn Property, Highland Drive ~*ngkrremg.OcdoplMd-lmnjtonb ..-I, r a ____ n__,_ 1. - ma Carlsbad. CA Source: Continental Aerial Photo, Inc., C123-3-14. 12 Aug.1998 PROJECT NAME PROJECT NUMBER --*?- DATE TAYLOR-HUNTER ASSOCIATES INC. DRAWNBY 0.okchnkal.~np,g.obpyod.mhomntolcomGkonb -EPA, E Dunn PrOPertY G02-00199 1998 AERIAL PHOTOGRAPH December 4,2002 L Taylor Carlsbad, CA Dunn Property, Highland Drive A.......~ @-..I... I- - Inn' PROJECT NAME Dunn PWJW PROJECT NUMBER GO240199 DATE December 4,2002 TAYLOR-HUNTER ASSOCIATES INC. g&ddeal- wand mimnmntal &ltonh. I L. Taylor I Appx. Scale: I" = IW I I 1988 AERIAL PHOTOGRAPH Dunn Property, Highland Drive Carlsbad,-CA I Source: USGS Terraserver, Image date 01 June 1994 I ---IrAC.I*.. 1994 AERIAL PHOTOGRAPH Dunn Property, Highland Drive Carlsbsd, CA _I TAYLOR-HUNTER !wechnkd Mahaa WbY PROJECT NAME Dunn Property PROJECT NUMBER G02-00199 DRAWN BY December 4,2002 L. Taylor -- " DATE TAY LO R - H U N T E R ASS 0 C I AT E S INC gcoEcdnkol sngh.aing. g-b 9- coi&Hl~ RrA, F A-mr Cnla. I" = IM' 1966 AERIAL PHOTOGRAPH Dunn Property, Highland Drive Carlsbad, CA