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HomeMy WebLinkAboutCT 97-02; Rancho Carrillo Villages A, B, C & D-Continental; Rancho Carrillo Villages A, B, C & D; 1997-07-14UPDATE REPORT RANCHO CARRILLO VILLAGES A, B, C AND D CARLSBAD, CALIFORNIA PREPARED FOR CONTINENTAL RANCH INC. SAN DIEGO, CALIFORNIA JULY 1997 GEOCON INCORPORATED GEOTECHNICAL CONSULTANTS Project No. 05845-12-04 July 14, 1997 Continental Ranch Inc. 12636 High Bluff Drive, Suite 300 San Diego, California 92130 Attention: Mr. Dave Lother Subject: Reference: RANCHO CARRILLO VILLAGES A, B, C, AND D CARLSBAD, CALIFORNIA UPDATE REPORT 1. Preliminary Geotechnical Investigation for Rancho Carrillo Villages A, B, C, and D, Carlsbad, California, prepared by Geocon Incorporated, dated March 3, 1993 (Project No. 04787-12-05) 2. Supplemental Geotechnical Investigation [for] Rancho Carrillo, Carlsbad, California, prepared by Geocon Incorporated, dated June 20, 1996 (Project No. 04787-12-09) 3. Tentative Map C. T. 97-02/P. U. D. 97-02, Rancho Carrillo Villages A, B, C, and D, prepared by Rick Engineering Company, stamp dated July 1, 1997 Gentlemen: In accordance with your request, we are providing this update to the above-referenced geotechnical report. This update is required to meet the requirements of the City of Carlsbad in that a period of three years has elapsed since the referenced preliminary geotechnical report was issued. As a part of this update, we have reviewed the referenced tentative map indicating the planned finish grade conditions. For reference purposes, the geologic mapping from the geotechnical reports has been overlain on the attached 80-scale version of the tentative map. It is our opinion that the conclusions and recommendations, as presented in the referenced geotechnical reports are valid and remain applicable to the proposed development. Furthermore, based on our review of the 40-scale tentative map, it is our opinion that the proposed grading is in general conformance with the recommendations of the referenced geotechnical reports. 6960 Flanders Drive • San Diego, California 92121-2974 • Telephone (619) 558-6900 • Fax (619) 558-6159 For clarity purposes, the attached letter prepared by Geocon Incorporated, dated May 14, 1997, is included in this update to aid in understanding the intent and implementation of placing "granular" soil within fill slopes as discussed in the geotechnical report. If you have any questions regarding this correspondence, or if we may be of further service, please contact the undersigned at your convenience. Very truly yours, GEOCON INCORPO David F. Leak RCE 22527 DFL:dmc (6) Addressee (2) Rick Engineering Company Attention: Mr. Craig Kahlen Project No. 05845-12-04 -2-July 14, 1997 GEOCON INCORPORATED GEOTECHNICAL CONSULTANTSProject No. 05845-12-01 May 14, 1997 VIA FACSIMILE AND U. S. MAIL Mr. Dave Lother Continental Ranch Inc. 12636 High Bluff Drive, Suite 300 San Diego, California 92130 Subject: RANCHO CARRJJLLO CARLSBAD, CALIFORNIA CONSULTATION - GRANULAR SOIL Gentlemen: In accordance with your request, this correspondence has been prepared to discuss the intent and implementation of placing "granular" soil in the outer 15 feet of fill slopes on the subject project. The intent of our recommended "granular" slope zone was to provide a completed slope with shear strength characteristics that under normal conditions would not be susceptible to surficial sloughing or shallow slope failure. Therefore, soils with low compacted shear strength or soils which may lose strength due to subsequent excessive expansion after placement are not desirable in the slope zone. Additionally, even though a thin layer of such soil may not be sufficient to significantly reduce the strength of the overall slope zone, such a condition could cause subsurface water to perch on the layer and create a nuisance slope seepage condition or destabilize a localized portion of the slope. Since silty clayey soils are most susceptible to a loss of strength due to soil expansion and often possess low strength characteristics even in a properly compacted condition, their placement within the slope zone should be kept to a minimum to reduce the risk for slope distress. Essentially all material on the project site can be used in the slope zone provided it is adequately mixed or blended with other materials to yield a soil condition which contains at least 30 percent sand size particles and/or does not possess an Expansion Index in excess of 90. It has been our experience that when soil meets such conditions, its shear strength characteristics will be sufficient to provide acceptable surficial slope stability. Where the soil does not meet the above criteria, lengthy laboratory testing would be required to determine that the long term shear strength of the soil actually used is acceptable. The term "granular" was used in the project soil report to preclude the use of very clayey, highly expansive soil in the slope zone unless such materials were altered (by mixing or blending with sandy soils) to yield a granular texture. It has been common practice on most grading projects in San Diego County to limit the placement of low strength, highly expansive clayey soils to fill areas beyond the slope zone. The use of "false" slopes to meet the "granular" slope zone criteria is typically not necessary and the implementation of the recommendation is not normally a separate pay item. 6960 Flanders Drive • San Diego, California 92121-2974 • Telephone (619) 558-6900 • Fax (619) 558-6159 From a practical viewpoint, the stiff, clayey soils that comprise a majority of the on-site topsoils, colluvium and alluvium, will be very difficult to blend and/or mix with sandy soils due to their plasticity characteristics. Alternatively, the mixing of fbrmational claystones and siltstones with sandy soils should be less difficult and is frequently accomplished, particularly when the mixing occurs in the cut area prior to placement If you have any questions regarding this letter, or if we may be of further service, please contact the undersigned at your convenience. Very truly yours, GEOCON INCORPORATED Thomas V. RCE 20427 TVL:sc (1) Addressee (1) Continental Ranch Inc. (via Facsimile) Attention: Mr. Mike Lombardi Project No. 5845-12-01 - 2 - May 14,1997