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HomeMy WebLinkAboutCT 02-28; La Costa Condominiums; Recommended Remedial Grading Procedures; 2007-05-31A COSTA 1ONDOMINIUMS y 3 1,2007 EN. 23280. ^•^ _ \— — ' acme Center Blvd. 5600 Spring Mtn. 22725 Old Canal Rd. Suite 112 Suite 201 Yorba Linda, CA 92887 San Diego, CA 92121 Las Vegas, NV 89 712 Fifth Street, Suite B Davis, CA 95616 AmericanGeotechnical Inc.WWW. SOIL, KXJNDAtlON & GtOLOGIC STUDIES American Geotechnical, Inc. SOIL, FOUNDATION AND GEOLOGIC STUDIES May 31,2007 File No. 23280.01 Mr. Kwan So CALSO, LLC 2683 Costebelle Drive La Jolla California 92037 Subject: RECOMMENDED REMEDIAL GRADING PROCEDURES LA COSTA CONDOMINIUMS Lot 185 of La Costa South Unit 1, Carlsbad Tract No. 02-28 Carlsbad, California Project No. 980161-004 References: "Grading Plans for Banich, Powers, Calso Landslide Stabilization, 2416 Sacada Circle, Carlsbad, California," prepared by American Geotechnical Inc., dated March 27, 2007. "Grading and Erosion Control Plans for La Costa Condominiums, Carlsbad Tract No. 02-28, Drawing No. 422-4A, 9 sheets," prepared by O'Day Consultants, dated, 2005. "Update Geotechnical Report, Proposed La Costa Condominiums, Lot 185, La Costa Avenue South - Unit 1, Carlsbad Tract No. 02-28, Carlsbad, California," prepared by American Geotechnical Inc., dated May, 2007. Dear Mr. So, American Geotechnical is pleased to present this report containing our recommended remedial grading procedures for the grading and construction of the southern side of the La Costa Condominiums project (lot 185 of La Costa South Unit 1, Carlsbad Tract No. 02-28) located in Carlsbad, California. These procedures were prepared in order to reduce the risk to existing homes along the ridgeline during remedial grading for the proposed La Costa Condominium project. The geotechnical recommendations for grading and construction of the project have been provided in the referenced report (American Geotechnical, May 2007). The recommendations are still considered valid and should be followed unless superceded by recommendations in this report or recommendations made during or after grading of the site. 22725 Old Canal Road, Yorba Linda, CA 92887 • (714) 685-3900 • FAX (714) 685-3909 5600 Spring Mountain Road, Suite 201, Las Vegas, NV 89146 • (702) 562-5046 • FAX (702) 562-2457 5764 Pacific Center Blvd., Suite 112, San Diego, CA 92121 • (858) 450-4040 • FAX (858) 457-0814 712 Fifth Street, Suite #B, Davis, CA 95616 • (530) 758-2088 • FAX (530) 758-3288 File No. 23280.01 May 31, 2007 Page 2 American Geotechnicaljnc SOIL, FOUNDATION AND GEOLOGIC STUDIES We appreciate the opportunity to be of service. If you should have any questions, please do not hesitate to contact us. Sincerely, AMERICAN GEOTECHNICAL Craig Chase Chief Engineering Geologist C.E.G. 2225 Via Fax & Mail Edred T, Marsh Principal Engineer G.E. 2387 ETM/CC; kb Distribution: Mr, Kwan So - (3) 22725 Old Canal Road, Yorba Linda, CA 92887 • (714) 685-3900 • FAX (714) 685-3909 5600 Spring Mountain Road, Suite 201, Las Vegas, NV 89146 • (702) 562-5046 • FAX (702) 562-2457 5764 Pacific Center Blvd., Suite 112, San Diego, CA 92121 • (858) 450-4040 • FAX (858) 457-0814 712 Fifth Street, Suite #B, Davis, CA 95616 • (530) 758-2088 • FAX (530) 758-3288 nie NO 232800, MAmerican Geotechnical, Inc. May 31,2007 PageS 1.0 BACKGROUND Based on the results of previous investigations at the site, the north-facing hillside along the southern side of the site possesses potential slope instability conditions that warrant special design and construction considerations. These conditions include: 1) a slope failure that occurred on and below lot 47 (2416 Sacada Circle) during the winter of 2004/2005; 2) adverse geologic conditions such as existing ancient landslides, clayseams, zones of fractured claystone and out-of-slope bedding; 3) manufactured fill and cut slopes constructed at slope inclinations of 1.5:1 (horizontal to vertical) that are steeper than current standards allow without some type of additional reinforcement for stabilization; 4) seepage zones and surface drainage from the adjacent residential lots on the ridgeline that are directed down the hillside (instead of toward the street); and 5) the close proximity of the existing residential structures on the ridgeline to the anticipated backcut needed to construct the buttresses and stability fills. Because of these conditions a full- time monitoring program is recommended during remedial grading, 2.0 REMEDIAL GRADING PROCEDURES The following procedures are recommended during the grading of the site. 2.1 Survey staking of the Buttress and Stability Fill Backcut and Property Boundary In order to facilitate the proper placement of the recommended slope inclinometers behind the top of the proposed buttress and stability fill backcuts and to install the slope survey monuments along the property boundary, the backcut/limits of grading and property boundaries should be located and staked by the project civil engineer/surveyor. 2.2 Slope Inclinometer and Survey Monument Installation Prior to any of the remedial grading operations, approximately 6 slope inclinometers and several slope survey monuments should be installed. The approximate locations of the slope inclinometers are presented on the attached Remedial Grading Map (Plate 1). The survey monuments will be spaced somewhat evenly across the hillside. FileNo. 23280.01 IIAmerican Geotechnical, Inc. May 31,2007 Page 4 Slope Inclinometers In order to install the slope inclinometers, access roads and drill pads will need to be constructed. Based on the staking of the top of the buttress and property lines, the optimal location of the slope inclinometers can be determined prior to construction of the access road(s) and drill pads. The slope inclinometers are anticipated to be on the order of 80-100 feet in depth. The slope inclinometer casings will be backfilled with a lean sand/cement slurry. For planning purposes, installation of the slope inclinometers, placement of the slurry backfill should take approximately 10-12 working days to complete. At least two sets of initial readings must be performed prior to any slope grading. Once grading is started, the frequency of the readings should be increased to full-time (at least twice a day). The daily readings should be continued until the buttress or stability fill soils are placed to the top of the backcut. We anticipate that our field staff will perform the slope inclinometer monitoring and that the findings will be generated and analyzed daily in the field to minimize any delays. Slope Survey Monuments We also recommend the placement of surface survey monuments above the proposed buttress and stability fill key backcut along the property boundary (or above) as another method of monitoring any potential slope instability conditions. The survey monuments can consist of metal pins or short pieces of rebar driven into the ground. The monuments should be surveyed for horizontal and vertical movement on a weekly basis with more frequent readings (about 2 times a week) performed when grading is ongoing near the monuments. The results of the survey readings should be given to the geotechnical consultant immediately after the readings are taken. In addition, a fairly high level of precision is desirable for monitoring purposes to help identify even minor levels of slope movement. The precision of the horizontal and vertical components should be on the order of plus or minus 0.01 feet (approximately 1/8 inch.) File NO 2328001 HAmerican Geotechnical, Inc. May 31,2007 Page 5 2.3 Excavation of the Buttress and Stability Fill We recommend that the buttress and stability fill key backcut be done in at least four sections in order to minimize the potential impacts to the residences along the ridgeline. The approximate locations of the sections are presented on the attached Remedial Grading Map (Plate 1). Section C contains the recent slope failure which occurred on and below Lot 47 (2416 Sacada Circle) and should ideally be completed first. Although not considered a requirement, the remaining sections can then be excavated from west to east (i.e. Section A first and Section D last). It is imperative that all excavations take place during the dry season to minimize potential problems associated with water. During construction, temporary cuts between the sections should be excavated at inclinations of approximately 1 to 1 (horizontal to vertical). We recommend that the grading contractor consider utilizing steep side cuts with flatter ramps out the front of the section for equipment access. An access/haul road for the construction equipment may be graded on the downhill side of the top of the backcut; however the access/haul road excavation on the uphill side should be limited to a maximum height of approximately 5 feet. Roads/ramps into and out of the section through the front may be excavated at inclinations on the order of 3:1 to 4:1 (horizontal to vertical) or steeper. Once the excavations for the buttress and stability fill keys have started, the excavation and construction should be performed as quickly as possible to minimize the time any backcut is left- exposed. In general, exposed backcut heights greater than half of the total height of the backcut should not be left open over any weekend. Work should be planned so that excavations within approximately 20 feet of the basal rupture surface of the ancient landslides or clayseams are not left overnight. Work should also be planned so that all fills are at least 20 feet above the basal rupture surface or clayseams prior to finishing for the day. Geologic observations or the results of the slope inclinometer or slope survey monitoring programs during the grading operations may dictate additional or revised recommendations. Fi,eNO 232800! ilAmerican Geotechnical, Inc. May 31,2007 Page 6 Any necessary supplies (such as gravel, filter fabric and subdrain pipe) should be on-site prior to starting the backcut excavations in order to minimize time delays. The grading contractor should also have the necessary resources, equipment, and manpower to quickly and efficiently perform the needed work. We also recommend that periodically during the day, a representative of the grading contractor and the geotechnical consultant visually observe the backcut and the area above it for indications of slope movement (i.e. cracks or fissures). Any indications of movement should be immediately communicated to the project team so that steps to remediate the conditions can be made. 3.0 POTENTIAL SLOPE INSTABILITY ISSUES DURING CONSTRUCTION If during the excavation of the buttresses and stability fill keys, indications of slope movement are observed (i.e. based on visual observations or the results of the slope inclinometers or survey monuments), steps should be taken immediately to evaluate the condition. The first step should be to stop the grading and fill in the excavation with soil until the movement is stopped and alternative recommendations can be provided. The actual mitigation should be based on the existing geologic conditions and the status of the grading operations. Additional geologic mapping or a subsurface investigation may be needed to determine the extent of the instability conditions. Additional slope stability analysis may also be needed to evaluate the conditions and provide corrective measures. Measures such as excavating the lower portion of the buttress and stability fill keys in even smaller sections (i.e. on the order of 25 to 50 foot wide sections); enlarging the buttress and stability fill keys; or providing alternative slope stabilization may be warranted. Alternative slope stabilization measures would likely consist of providing 2-3 rows of tiebacks near the top of the backcut prior to proceeding with any additional excavation.