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HomeMy WebLinkAboutCT 00-06; BRESSI RANCH; GEOTECHNICAL CONCLUSIONS AND RECOMMENDATIONS; 2002-11-07Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY To: Attention: Subject: References: Introduction November 7,2002 Lennar Homes 5780 Fleet Street, Suite 320 Carlsbad, California 92008 Ms. Kristine Zortman Project No. 971009-007 Geotechnical Conclusions and Recommendations Regarding Geology/Soils Mitigation Items GS1 through GS6 Presented in the EIR-B Document, Bressi Ranch, Carlsbad, California Geocon, Inc., .2000, Supplemental Soil and Geologic Investigation, Villages of La Costa -The Greens, Carlsbad, California, Project No. 06403-12-01, dated March 24, 2000 Leighton and Associates, 1997, Preliminary Geotechnical Investigation, Bressi Ranch, Carlsbad, California, Project No. 4971009-002, dated July 29, 1997 ----, 2001 a, Supplemental Geotechnical Investigation for Mass Grading, Bressi Ranch, Carlsbad, California, Project No. 971009-005, dated March 14,2001 ----, 2001 b, Alluvium Evaluation and Additional Geotechnical Recommendations, Poinsettia Lane and El Fuerte Street, Bressi Ranch, Carlsbad, California, Project No. 971009-006, dated September 12, 2001 Lennar Homes, 2002, Program Environmental Impact Report, EIR 98-04, Bressi Ranch Master Plan (MP 178), dated May 29, 2002 In accordance with your request, this letter presents our conclusions regarding the Geology/Soils mitigation impact, findings, and measures presented in the EIR-B report relative to the Bressi Ranch Master Plan (MPI78) project located in Carlsbad, California. The purpose of this letter is to review the mitigation measures presented in the EIR-B report and provide updated geotechnical conclusions and recommendations relative to the mitigation measures based on the subsequent geotechnical evaluations of the property since the original preliminary geotechnical investigation report, date4 July 29, 1997 was prepared (which was referenced and incorporated into the EIR report). 3934 Murphy Canyon Road, Suite 8205 • San Diego, CA 92123-4425 858.292.8030. Fax 858.292.0771 • www.leightongeo.com • • 971009-007 Conclusions The following presents a summary of our conclusions and recommendations concerning the EIR mitigation measures: GSI -Impact. There is the possibility of damage to proposed structures due to excessive settlement resultingjrom compression of the porous and/or loose topsoil, jrom swelling and shrinkage of the siltstone and claystone soils, andjrom the undocumented/trash dump located on the site. Mitigation Measure GSI. All future grading and construction of the project site shall comply with the geotechnical recommendations contained in the Preliminary Geotechnical Investigation, Bressi Ranch, Carlsbad, California (Leighton and Associates, July 1997). This report contains specific recommendations for mitigating geotechnical conditions related to soils earthwork, slope stability, and ground and surface waters. All recommendations contained in the report shall be incorporated into all final engineering and grading plans. The soil engineer and engineering geologist shall review the grading plans prior to finalization to verify the plan's compliance with the recommendations of the report. If required, third party review of the geotechnical report and final grading plans shall be conducted by the City of Carlsbad Engineering Department prior to the issuance of a grading permit. Compliance with this measure shall be verified by the City of Carlsbad. Geotechnical conclusions and recommendations concerning the mitigation of potentially compressible soils, expansive soils, and other deleterious materials were presented in our preliminary geotechnical investigation report dated July 29, 1997 (Leighton, 1997). Subsequent to our preliminary geotechnical report for the project, we have performed a number of supplemental and update evaluations/investigations of the Bressi Ranch property (referenced above). Recommendations presented in the preliminary geotechnical report (Leighton, 1997) are still considered applicable unless superseded by more recent recommendations. Leighton is currently in the process of performing an updated geotechnical investigation and grading plan review of the 40-scale mass grading plans for the Bressi Ranch project, prepared by Project Design Consultants, Drawing No. 400-8A, dated October 10, 2002. As part of our grading plan review, we will review the plans to evaluate whether or not the grading plans have incorporated the project geotechnical recommendations of the preliminary geotechnical report (Leighton, 1997) and all subsequent supplemental or update geotechnical reports. A copy of our grading plan review letter will be provided to the City of Carlsbad upon completion of the review. GS2 -Impact. Due to their potentially compressible nature, the landslide deposits within the limits of the planned grading are considered unsuitable for structural support in their present conditions. Mitigation Measures GS2. Remove (or provide some other form of stabilization) the ancient landslides which occur in areas of proposed development. Final recommendations for stabilization shall be approved prior to issuance of a grading permit. . -2- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY • • 971009-007 Preliminary and supplemental recommendations concerning the mitigation of the landslides within the limits of the proposed grading have been presented in the preliminary geotechnical report (Leighton, 1997) and are still considered applicable unless superseded by more recent recommendations. As the proposed development of the site has evolved since the preparation of the original tentative tract map, we have conducted additional/updated geotechnical investigations including our supplemental report dated March 14, 2001 (Leighton, 2001a) and our update investigation which is currently being performed. These additional investigations were performed to evaluate the affects of the mapped and/or inferred landslides on'the proposed development of the property. We anticipate that during the grading operations, the potentially compressible portions of the landslides within the limits of grading will be removed to competent material by the grading contractor under our observation. During the remedial grading operations, we will have a geologist from our firm on site to geologically map the removals and determine the depth and extent of the removals of the potentially compressible landslide deposits (as well as other compressible soils). Landslides present in the open space areas of the project, but near the limits of the proposed grading, will be evaluated, and if needed, a structural setback will be required. However, based on our geotechnical investigations of the site to date, we anticipate that all of the mapped landslides within or adjacent to the proposed limits of grading will be stabilized or completely removed to competent material during the grading operations. GS3 -Impact. On the south-central portion of the site, in an area of planned residential, a small landslide complex extends offsite. Additionally, on the east side of the site, the potential for a landslide has been observed Mitigation Measures GS3.Prior to grading and construction of the site, an off-site investigation of potential landslide areas shall be conducted to confirm the locations and extent of the potential landslides. Recommendations contained in the geotechnical investigation shall be implemented to eliminate the risk associated with the potential landslide area. Measures may include remedial grading and/or st!:uctural setbacks. Compliance with this measure shall be verified by the City of Carlsbad Our Supplemental Geotechnical Investigation Report for Mass Grading (Leighton, 2001 b) and our current geotechnical investigation of the 'planning areas along the southern property boundary of the Bressi Ranch project (currently being performed) will or have evaluated the landslides that extend off-site. While we have not performed any off-site subsurface investigations, we have utilized existing geologic data relative to the Villages of La Costa -The Greens project (Geocon, 2000). The recommendations presented in the Bressi Ranch geotechnical reports (including the recommendations we are currently preparing) to stabilize the landslides assume that no off-site grading will be performed. Since th~ Bressi Ranch property is upslope of The Greens project, development of the Bressi Ranch project will not be affected by the grading or the stabilization of the off-site landslides on The Greens project. However, the -3- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY • • 971009-007 proposed Bressi Ranch development may affect The Greens development and therefore, the geotechnical consultant for the adjacent property should review the applicable Bressi Ranch geotechnical reports. GS4 -Impact. The site is likely to be subject to at least one moderate to major earthquake during the design life of the structures. During such an earthquake, the damage from fault offse( through the site is remote, but relatively strong ground shaking is likely to occur. Mitigation Measure GS4. Allfuture development of the project site shall adhere to the Uniform Building Code and State building requirements in effect at the time specific development is proposed. Compliance with this measure shall be verified by the City of Carlsbad. Preliminary geotechnical recommendations related to seismic events (including 1997 Uniform Building Code [UBC] seismic design parameters) that may affect the Bressi Ranch development have been presented in our supplemental geotechnical report of mass grading dated March 14, 2001 (Leighton, 2001a). Additional or updated geotechnical seismic design parameters and recommendations relative to the proposed commercial and residential structures planned on the site will be provided in the applicable as-graded report(s) that will be prepared after the completion of the rough and fine grading operations of the site. GS5 -Impact. Ground water is present in the main drainages of the site. The control of groundwater in a hillside development is essential to reduce the potential for undesirable surface flow, hydrostatic pressure, and the adverse effects of ground water on slope stability. Mitigation Measure GS5. Prior to grading and construction an additional geotechnical investigation shall be conducted to identify possible future seepage areas that could occur during grading. Field recommendations for mitigation of future potential seepage, as well as for the provision of drainage in areas known to be susceptible to groundwater accumulation shall be provided. Compliance with this measure shall be verified by the City of Carlsbad Our supplemental geotechnical report of mass grading (Leighton, 200la) provided geotechnical conclusions and recommendations concerning the mitigation of ground water and potential seepage conditions. Canyon subdrains have been recommended in the majority of the drainages on the Bressi Ranch property and have been provided to the civil engineer for presentation on the grading plans. Recommendations for the placement of subdrains in the buttress and/or stability fill keys have also been provided (Leighton, 2001a). As part of our grading plan review of the project grading plans, we will review the locations of the subdrains presented on the plans and will make additional recommendations (if needed). In addition, we will provide geologic observation and mapping services during the rough and fine grading operations and will make necessary field recommendations based on the existi~g geotechnical conditions and/or if the conditions are different than anticipated. -4- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY • • 971009-007 GS6 -Impact. Geologic conditions that need to be addressed as part of grading and construction of off-site improvements include undocumented fill, topsoil, moderate to highly compressible and expansive colluvium and alluvium, and one small surficial landslide. Mitigation Measure GS6. All grading and construction associated with implementation of the off-site improvements shall comply with the geotechnical recommendations contained in the Geotechnical Feasibility Study, Off-Site Poinsettia Lane, Alicante Road and Borrow Sites within the Greens of the Villages of La Costa, Bressi Ranch Development, Carlsbad, California (Leighton and Associates, January 5, 2001). This report contains specific recommendations for mitigating geotechnical conditions related to soils earthwork, slope stability, and ground and surface waters. All recommendations contain in the report shall be incorporated into all final engineering and grading plans. The soil engineer and engineering geologist shall review the grading plans prior to finalization to verify the plans compliance with the recommendations of the report. If required, a third party review of the geotechnical report and final grading plans shall be conducted by the City of Carlsbad Engineering Department prior to the issuance of a grading permit. Compliance with this measure shall be verified by the City of Carlsbad We understand that the off-site improvement of Poinsettia Lane and Alicante Road (west of the Bressi Ranch project) will be performed by the adjacent property owner and/or his representatives. As a result, recommendations for the design, grading, and construction of the off-site improvements should be provided by the geotechnical consultant of The Greens project. If you have any questions regarding this letter, please do not hesitate to contact this office. We appreciate this opportunity to be of service. Respectfully submitted, LEIGHTON AND ASSOCIATES, INC. Distribution: (8) Addressee -5- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY · '-- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY To: Attention: Subject: References: Introduction November 7, 2002 Lennar Homes 5780 Fleet Street, Suite 320 Carlsbad, California 92008 Ms. Kristine Zortman Project No. 971009-007 Geotechnical Conclusions and Recommendations Regarding Geology/Soils Mitigation Items GSI through GS6 Presented in the EIR-B Document, Bressi Ranch, Carlsbad, California Geocon, Inc., .2000, Supplemental Soil and Geologic Investigation, Villages of La Costa -The Greens, Carlsbad, California, Project No. 06403-12-01, dated March 24, 2000 Leighton and Associates, 1997, Preliminary Geotechnical Investigation, Bressi Ranch, Carlsbad, California, Project No. 4971009-002, dated July 29, 1997 ----, 2001 a, Supplemental Geotechnical Investigation for Mass Grading, Bressi . Ranch, Carlsbad, California, Project No. 971009-005, dated March 14,2001 ----, 2001 b, Alluvium Evaluation and Additional Geotechnical Recommendations, Poinsettia Lane and El Fuerte Street, Bressi Ranch, Carlsbad, California, Project No. 971009-006, dated September 12,2001 Lennar Homes, 2002, Program Environmental Impact Report, EIR 98-04, Bressi Ranch Master Plan (MP 178), dated May 29, 2002 In accordance with your request, this letter presents our conclusions regarding the Geology/Soils mitigation impact, findings, and measures presented in the EIR-B report relative to the Hressi Ranch Master Plan (MPI78) project located in Carlsbad, California. The purpose of this letter is to review the mitigation measures presented in the EIR-B report and provide updated geotechnical conclusions and recOmmendations relative to the mitigation measures based on the subsequent geotechnical evaluations of the~ property since the original preliminary geotechnical investigation report, dated July 29, 1997 was prepared (which was referenced and incorporated into the EIR report). 3934 Murphy Canyon Road, Suite 8205 -San Diego, CA 92123-4425 858.292.8030 -Fax 858.292.0771-www.leightongeo.com \.--971009-007 Conclusions The following presents a summary of our conclusions and recommendations concerning the EIR mitigation measures: GSl-Impact. There is the possibility of damage to proposed structures due to excessive settlement resultingjrom compression of the porous and/or loose topsoil,jrom swelling and shrinkage of the siltstone and claystone soils, andfrom the undocumented/trash dump located on the site. Mitigation Measure GSl. All future grading and construction of the project site shall comply with the geotechnical recommendations contained in the Preliminary Geotechnical Investigation, Bressi Ranch, Carlsbad, California (Leighton and Associates, July 1997). This report contains specific recommendations for mitigating geotechnical conditions related to soils earthwork, slope stability, and ground and surface waters. All recommendations contained in the report shall be incorporated into all final engineering and grading plans. The soil engineer and engineering geologist shall review the grading plans prior to finalization to verify the plan's compliance with the recommendations of the report. If required, third party review of the geotechnical report and final grading plans shall be conducted by the City of Carlsbad Engineering Department prior to the issuance of a grading permit. Compliance with this measure shall be verified by the City of Carlsbad Geotechnical conclusions and recommendations concerning the mitigation of potentially compressible soils, expansive soils, and other deleterious materials were presented in our preliminary geotechnical investigation report dated July 29, 1997 (Leighton, 1997). Subsequent to our preliminary geotechnical report for the project, we have performed a number of supplemental and update evaluations/investigations of the Bressi Ranch property (referenced above). Recommendations presented in the preliminary geotechnical report (Leighton, 1997) are still considered applicable unless superseded by more recent recommendations. Leighton is currently in the process of performing an updated geotechnical investigation and grading plan review ofthe 40-scale mass grading plans for the Bressi Ranch project, prepared by Project Design Consultants, Drawing No. 400-8A, dated October 10, 2002. As part of our grading plan review, we will review the plans to evaluate whether or not the grading plans have incorporated the project geotechnical recommendations of the preliminary geotechnical report (Leighton, 1997) and all subsequent supplemental or update geotechnical reports. A copy of our grading plan review letter will be provided to the City of Carlsbad upon completion of the review. GS2 -Impact. Due to their potentially compressible nature, the landslide deposits within the limits of the planned grading are considered unsuitable for structural support in their present conditions. Mitigation Measures GS2. Remove (or provide some other form of stabilization) the ancient landslides which occur in areas of proposed development. Final recommendations for stabilization shall be approved prior to issuance of a grading permit. -2- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY • , -971009-007 Preliminary and supplemental recommendations concerning the mitigation of the landslides within the limits of the proposed grading have been presented in the preliminary geotechnical report (Leighton, 1997) and are still considered applicable unless superseded by more recent recommendations. As the proposed development of the site has evolved since the preparation of the original tentative tract map, we have conducted additional/updated geotechnical investigations including our supplemental report dated March 14, 2001 (Leighton, 2001a) and our update investigation which is currently being performed. These additional investigations were performed to evaluate the affects of the mapped and/or inferred landslides on the proposed development of the property. We anticipate that during the grading operations, the potentially compressible portions of the landslides within the limits of grading will be removed to competent material by the grading contractor under our observation. During the remedial grading operations, we will have a geologist from our firm on site to geologically map the removals and determine the depth and extent of the removals of the potentially compressible landslide deposits (as well as other compressible soils). Landslides present in the open space areas of the project, but near the limits of the proposed grading, will be evaluated, and if needed, a structural setback will be required. However, based on our geotechnical investigations of the site to date, we anticipate that all of the mapped landslides within or adjacent to the proposed limits of grading will be stabilized or completely removed to competent material during the grading operations. GS3 -Impact. On the south-central portion of the site, in an area of planned residential, a small landslide complex extends offsite. Additionally, on the east side of the site, the potential for a landslide has been observed Mitigation Measures GS3.Prior to grading and construction of the site, an off-site investigation of potential landslide areas shall be conducted to confirm the locations and extent of the potential landslides. Recommendations contained in the geotechnical investigation shall be implemented to eliminate the risk associated with the potential landslide area. Measures may include remedial grading and/or stJ:uctural setbacks. Compliance with this measure shall beyerified by the City of Carlsbad Our Supplemental Geotechnical Investigation Report for Mass Grading (Leighton, 2001 b) and our current geotechnical investigation of the planning areas along the southern property boundary of the Bressi Ranch project (currently being performed) will or have evaluated the landslides that extend off-site. While we have not performed any off-site subsurface investigations, we have utilized existing geologic data relative to the Villages of La Costa -The Greens project (Geocon, 2000). The recommendations presented in the Bressi Ranch geotechnical reports (including the recommendations we are currently preparing) to stabilize the landslides assume that no off-site grading will be performed. Since the Bres§i Ranch property is upslope of The Greens project, development of the Bressi Ranch; project will not be affected by the grading or the stabilization of the off-site landslides on The Greens project. However, the -3- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY , 971009-007 proposed Bressi Ranch development may affect The Greens development and therefore, the geotechnical consultant for the adjacent property should review the applicable Bressi Ranch geotechnical reports. GS4 -Impact. The site is likely to be subject to at least one moderate to major earthquake during the design life of the structures. During such an earthquake, the damage from fault offset through the site is remote, but relatively strong ground shaking is likely to occur. Mitigation Measure GS4. All future development of the project site shall adhere to the Uniform Building Code and State building requirements in effect at the time specific development is proposed. Compliance with this measure shall be verified by the City of Carlsbad. Preliminary geotechnical recommendations related to seismic events (including 1997 Uniform Building Code [UBC] seismic design parameters) that may affect the Bressi Ranch development have been presented in our supplemental geotechnical report of mass grading dated March 14, 2001 (Leighton, 2001a). Additional or updated geotechnical seismic design parameters and recommendations relative to the proposed commercial and residential structures planned on the site will be provided in the applicable as-graded report(s) that will be prepared after the completion of the rough and fine grading operations of the site. GS5 -Impact. Ground water is present in the main drainages of the site. The control of groundwater in a hillside development is essential to reduce the potential for undesirable surface flow, hydrostatic pressure, and the adverse effects of ground water on slope stability. Mitigation Measure GS5. Prior to grading and construction an additional geotechnical investigation shall be conducted to identify possible future seepage areas that could occur during grading. Field recommendations for mitigation of future potential seepage, as well as for the provision of drainage in areas known to be susceptible to groundwater accumulation shall be provided. Compliance with this measure shall be verified by the City of Carlsbad. Our supplemental geotechnical report of mass grading (Leighton, 2001a) provided geotechnical conclusions and recommendations concerning the mitigation of ground water and potential seepage conditions. Canyon subdrains have been recommended in the majority of the drainages on the Bressi Ranch property and have been provided to the civil engineer for presentation on the grading plans. Recommendations for the placement of subdrains in the buttress and/or stability fill keys have also been provided (Leighton, 2001a). As part of our grading plan review of the project grading plans, we will review the locations of the sub drains presented on the plans and will make additional recommendations (if needed). In addition, we will provide geologic observation and mapping services during the rough and fine grading operations and will make necessary field recommendations based on the existing geo~echnical conditions and/or if the conditions are different than anticipated. -4- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY 971009-007 GS6 -Impact. Geologic conditions that need to be addressed as part of grading and construction of off-site improvements include undocumented fill, topsoil, moderate to highly compressible and expansive colluvium and alluvium, and one small surficial landslide. Mitigation Measure GS6. All grading and construction associated with implementation of the off-site improvements shall comply with the geotechnical recommendations contained in the Geotechnical Feasibility Study, Off-Site Poinsettia Lane, Alicante Road and Borrow Sites within the Greens of the Villages of La Costa, Bressi Ranch Development, Carlsbad, California (Leighton and Associates, January 5, 2001). This report contains specific recommendations for mitigating geotechnical conditions related to soils earthwork, slope stability, and ground and surface waters. All recommendations contain in the report shall be incorporated into all final engineering and grading plans. The soil engineer and engineering geologist shall review the grading plans prior to finalization to verify the plans compliance with the recommendations of the report. If required, a third party review of the geotechnical report and final grading plans shall be conducted by the City of Carlsbad Engineering Department prior to the issuance of a grading permit. Compliance with this measure shall be verified by the City of Carlsbad We understand that the off-site improvement of Poinsettia Lane and Alicante Road (west of the Bressi Ranch project) will be perfonned by the adjacent property owner and/or his representatives. As a result, recommendations for the design, grading, and construction of the off-site improvements should be provided by the geotechnical consultant of The Greens project. If you have any questions regarding this letter, please do not hesitate to contact this office. We appreciate this opportunity to be of service. Respectfully submitted, LEIGHTON AND ASSOCIATES, INC. Distribution: (8) Addressee -5- Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY