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Home My WebLink AboutCT 00-06; BRESSI RANCH; REVISED GRADING AND PLACEMENT OF A SEGMENTAL RETAINING WALL; 2003-08-11v Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY To: Attention: Subject: Introduction Lennar C01l1111unities August 11,2003 -00 Gr cY Project No. 971009-009 5780 Fleet Street, Suite 320 Carlsbad, California 92008-4700 Ms. Kristine Zortman Geotechnical Recommendations Concerning Revised Grading and Placement of a Segmental Retaining Wall along the Existing 24-Inch Forced Sewer Main, Poinsettia Lane Station No. 84+00 to 85+50, Bressi Ranch, Carlsbad, California In accordance with your request, this letter presents our geotechnical recommendations concerning the revised grading and placement of a segmental retaining wall along the existing 24- inch forced sewer main south of Poinsettia Lane (between Station No. 84+00 and 85+50) in Carlsbad, California. Due to the location of the existing forced sewer main, d~pth of the potentially compressible alluvium, and location of the proposed toe-of-slope for Poinsettia Lane, potentially compressible material will be left-in-place beneath the forced sewer main and proposed embankment fill. In order to minimize the impact of differential settlement of the forced sewer main due to loading of the proposed embankment fill, we recommend that either the toe-of-slope be moved away from the sewer main or alternative remedial grading operations be performed to minimize differential settlement of the alluvial soils below and adjacent to the sewermam. Findings On July 22, 2003, five excavations were made within the limits of the proposed grading along the anticipated alignment of Poinsettia Lane between Station No. 80+00 and 92+00 (north of the existing forced sewer main). Based on our geotechnical observations, the depth of the potentially compressible alluvial soils ranges from less than 5 to 19+ feet in depth below the existing ground surface. Geotechnical analysis indicated that with the exception of the proposed slope along Poinsettia Lane between Station No. 84+00 and 85+50, the proposed grading (including the anticipated remedial grading operations) should not impact the existing sewer main. 3934 Murphy Canyon Road, Suite 8205. San Diego, CA92123-4425 858.292.8030. Fax 858.292.0771. www.leightongeo.com 971009-009 Conclusions and Recommendations Between Station No. 84+00 and 85+50 along the south side of Poinsettia Lane, we recotnll).end that the proposed toe of slope be moved a minimum of 15 feet away from the northern side of the existing 24-inch forced sewer main. In order to move the toe-of-slope, we understand that the proposed slope will need to be constructed with a slope inclination greater than 2 to 1 (horizontal to vertical) or a mechanically stabilized earth or segmental retaining wall will need to be constructed. If a retaining wall is planned, the wall should be placed in the upper half of the slope. We also recommend that the removal of the potentially compressible alluvium be made by starting 5 feet away from the existing sewer main and excavating a 1 to 1 (horizontal to vertical) slope down and away from the sewer main until competent formational material is encountered. The I to 1 slope should be heavily benched as fill is placed to remove additional compressible . material. Figure 1 presents the existing geotechnical conditions, proposed grading, and the approximate location of the retaining wall in a plan view. Figure 2 presents a cross-section of the existing geotechnical conditions and proposed grading. Figure 3 presents our recommended remedial grading. Figure 4 presents a cross-section showing a typical segmental retaining wall. Segmental Retaining Wall Preliminary Design Recommendations Based on the geotechnical conditions of the site, the recommended soil parameters presented on Table 1 should be utilized in the design of the proposed segmental retaining wall. These parameters are based on our previous investigations of Bressi Ranch and should be confirmed by this office at the completion of grading (but prior to construction of the wall). The wall should be designed in accordance with the standard design procedure for southern California, including at a minimum; the consideration of internal stability, external stability, global stability, seismic stability, bearing, drainage, erosion and appropriate safety factors for 1?oth design materials as well as design. Temporary sloping should be performed in accordance with current OSHA requirements. Table 1 Retaining Wall Soil Parameters Soil Property Reinforced Zone Retained Zone Foundation Zone Internal Friction Angle (degrees) 28 28 28 Cohesion (pst) 0 0 100 Total Unit Weight (pet) 130 130 130 -2-Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-009 A maximum moment magnitude earthquake of moment magnitude 7.0 on the Rose Canyon/Newport-Inglewood Fault Zone could produce an estimated peak horizontal ground acceleration 0.31g at the site. The ground acceleration was modeled using the 1995b/1997 attenuation equation of Abrahamson & Silva for a rock site. Water should be prevented from infiltrating into the reinforced 'soil zone. The retaining wall should be provided with a 4-inch diameter, SDR 35 perforated drainage pipe surrounded by 2 cubic foot (per lineal foot) of %-inch crushed rock wrapped in filter fabric (Mariti 140N or equivalent). Additional backdrains (or panel drains) may be necessary as determined by field observations at the time of construction. A concrete swale (if required by the City of Carlsbad or the retaining wall designer) should be provided where an ascending slope is located above the retaining wall. All drains and swales should outlet to suitable locations as determined by the project civil engineer. We also recommend a minimum horizontal setback distance from the face of slopes for the retaining wall footing as indicated on Table 2. This distance is measured from the outside bottom edge of the footing (horizontally to the slope face) and is based on the slope height and type of soil. However, the foundation setback distance may be revised by the geotechnical consultant on a case- by-case basis if the geotechnical conditions are different than anticipated. Table 2 Minimum Footing Setback from Slope. Faces Slope Height Minimum Recommended Foundation Setback Less than 5 feet 5 feet 5 to 15 feet 7 feet Greater than 15 feet 10 feet Appropriate surcharge pressures should also be applied for walls that are influenced by improvements or vehicular traffic within the retained or reinforced zones. The wall design engineer should also select grid design strengths based on deflections tolerable to the proposed improvements and global stability. This office should review final plans prior to commencement of work. - 3 -Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-009 If you have any questions regarding this letter, please contact this office. We appreciate this opportunity to be of service. Respectfully submitted, LEIGHTON AND ASSOCIATES, INC. William D. Olson, RCE 45283 Senior Project Engineer ~KW~ Randall K. Wagner, CEG 1612 Director of Geology Attachments: Figure 1 -Geotechnical Map and Proposed Segmental Retaining Wall Location Figure 2 -Detail of Existing Geotechnical conditions and Proposed Grade Figure 3 -Recommended Remedial Grading Figure 4 -Alternative 1: Segmental Wall in upper Portion of Slope with 2: 1 Fill Slope Distribution: (4) Addressee (2) Lennar Communities Attention: Ms. Lisa Galloway (1) Lennar Communities Attention: Mr. Jim Urbina (1) Proj ect Design Consultants Attention: Ms. Karen Mossberg (1) Kimley-Horn Attention: Mr. Mike Knapton -4-Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY LEGEND Qal ..... 't_/ Quaternary Alluvium Geologie contact (queried where uncertain) GT·6 ~ Trench location 1----11 Geologic cross-section 80 (fj .. -.~" ~~sB~~~~W~ NORTH ."-;;;;;;'_ ---;-",,"~;--~ __ .,_ -tr: \_, _ ! i ~ \ .. ~)(. 'f ~ i \\ .,." '1<0 -" ,f-" /'. "I/jl\.. 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'I. ~// ---'·,l .. \)l" ~" " \\ 1,,1 \ "', \ \~, ~f\ .,. , ..... , ~ " ~d" <tI. "">" '-. ... \\\ \ ... . (l~'o; ;, .' , .. ' ~ (Qf" . .... ...... '. ", •... :. 'j. '. \ 'v,o-; '; " ...., \ \ .. -.. -. "'-t." .: ~ ~ .. "+. : '. ';;" .• ' I: ~r.:: ~.:~:. I:': '\ ./ I /j __ p"';;~!: .... ,.-.i,-. ~ I II P~T';-;;-" -, '''-.JjII :;':;;::'-;;-; ............... ~ , IW , < \. \ '--"'90 , . ~ __ _ . '----'--.... ..,--' " --~~~~~----------~.~ .. ,--~-----........... ' •• ". > •• ¥~ ., Figure ,No. 1 e e LEGEND Af Proposed fill Afo Older existing fill Qa I/Qcol Quaternary Alluvium I Colluvium Tsa Tertiary Santiago Formation GT·6 Exploratory trench location ~ -II) II) .... c: ~ z 0 i= « > w oJ w 200 180 160 12" FM 24" steel 24" FM Afo pipe (to be abandoned) ,.,. ...... ' ~/~aI/QCOI- Estimated p'revious removal limits Trench GT·6 Existing profile 2 ...... ...... 1~"'" ......... Tsa Proposed grade Poinsettia Lane I ... -...... ------_____ L _______ _ Qal/Qcol ---- 200 -Q) ~ c: 180 z o 160 ~ > w oJ w 140 140 DETAI'L OF EXISTING GEOTECHNICAL CONDITION'S AND PROPOSED GRADE Poinsettia Lane Station No. 85+00 N100W Project No. 971009-009 Scale 1"=20' ----~~~~~----------Engr.lGeol. WDO/RKW Drafted By ____ .:..K::.A.:M.:.!.-______________ _ D~e ~41~3 ----~~~-~----Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY c Figure No.2 e e LEGEND Af Proposed fill Afo Older existing fill Qal/Qcol Quaternary Alluvium I Colluvium Tsa Tertiary Santiago Formation GT·6 Exploratory trench location ....... -Q) Q) .... 2- 12" FM 2001 24" FM z 180 I ~fo I 5' 0 i= « > w .J w 160 Estimated j:1revious removal limits Tsa 24" steel pipe (to be abandoned) GT·6 2 1V~ Existing profile ~~ ~~~ Proposed grade Poinsettia Lane ,,-------L " -------------- ~~~ , ... ,,,.,.,,,. ~~ Recommended frontcut Qal/Qcol Remove compressible soil to competent formational material and replace with compacted fill -- Tsa 200 -Q) Q) .... c ~ 180 z 0 ~ > W .J w 160 140 140 RECOMME·NDED REMEDIAL GRADING Poinsettia Lane Station No~ 85+00 N10 0 W - Project No. 971009-009 Scale 1"=20' --~~~--~------" Engr.lGeol. WOO/RKVV " Drafted By __ .:.KA;::..::M.:.:· _" .'--________ _ Date _=8--=1-=.1-,-0:...:3~ ___ _ . Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY cf Figure No.3 e e LEGEND Af Proposed fill Afo Older eXisting fill Qal/Qcol Quatemary Alluvium I Colluvium Tsa Tertiary Santiago Formation GT -6 Exploratory trench location -or- Existing grade Proposed toe-ot-slope Recommended segmental wall Proposed grade Poinsettia Lane -- --Q) .f! c: '" 200 12" FM 24" steel pipe (to be removed) 200- ~ -Q) .f! c ~ 180 p,.-exls"" ~ ~om. 9 < 2 > 1 w ...J W •••••••••••••• -----_ ... --------_ .. __ ... _--_ .. 160-1 0 ~ ~ M M ~ w --" ~ 1-160 _s-J-____ r-.I Tsa Tsa 140 140 N100W -... AL TERN'ATIVE 1: SEGMENTAL WALL INUP'PER PORTION OF SLOPE WITH 2:1 FILL SLOPE' Poinsettia Lane Station No. 85+00 Project No, 971009·009 Scale _...!.1_ .. =...!2~0~·...,_---- Engr.lGeol. WDO/RKW Drafted By _.!-"KA=M~ ____ _ Date _..:8..:..;~1:.....:1"-·0::.;3=__ ___ _ Leighton and Associates, Inc. A LEIGHTON. GROUP COMPANY c , Figure No.4 ·, • e -