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Home My WebLink AboutCT 06-27; MUROYA SUBDIVISION; GEOTECHNICAL REVIEW COMMENTS; 2010-01-11GEOCON INCORPORATED GEOTECHNICAL CONSULTANTS Project No. 07671-52-01 January 11, 2010 TaylorTaylor Woodrow Homes Incorporated 15 Cushing Irvine, California 92618 z Attention: Ms. April Tornillo 0 Subject: MUROYA PROPERTY 0 CARLSBAD, CALIFORNIA LU RESPONSE TO GEOTECHNTCAL REVIEW COMMENTS References: 1. Geotechnical Investigation, Muroya Properly, Carlsbad, California, prepared by Geocon Incorporated, dated July 14, 2009 (Project No. 07671-52-01). 2. Geotechnical Peer Review, Proposed Muroya Residential Subdivision, Carlsbad Tract 06-27, Carlsbad, California, prepared by Ninyo & Moore, dated November 11, 2009 (PrOject No. 106560003). Dear Ms. Tomillo: We prepared this letter to address geotechnical review comments by Ninyo & Moore representing the City of Carlsbad, Planning Department for the subject project. Specifically, this letter addresses items in the referenced geotechnical peer review letter. The geotechnical review comments are listed herein with the responses immediately following. Comment 1: There are minor deficiencies on the cross sections. These include mislabeling T-10 as T-4 on Cross Section A-A', not showing T-4 and T-5 on Cross Section B-B', and (also on B-B') not showing the seepage. Response: We have updated the cross sections to reflect the labeling. Figure 1 presents the updated Geologic Cross-Sections A-A' and B-B'. Comment 2: The logs for trenches T-3, T-4, and T-5 indicate that the debris fill is in "overall poor, uncompacted condition." However, Page 15 of the project geotechnical report recommends that the sui lvo ficial soils should be considered as a Type B soil asks classification for temporary excavations. Due to the poor condition of the debris fill, () consideration should be given to classify the debris fill as a Type C soil for temporary excavations. LU Response: On page 15 of the referenced geotechnical report the surficial soil where groundwater or seepage is encountered should also include the debris fill as a Type C soil and should be included on any subsequent geotechnical letters or reports. It is the 6960 Flanders Drive n San Diego, California 92121-2974 E Telephone (858) 558-6900 0 Fox (858) 558.6159 responsibility of the contractor to provide a safe excavation during the construction of the proposed project. Comment 3: Page 2 of the project geotechnical report concludes that the debris fill is "unsuitable for support of additional fill". However, cross section B-B' and Pages 7 and 12 of the report indicate that due to grading restraints for sensitive habitats, portion of the fill slope at the southeast portion of the site will be supported on debris fill. Due to the potential instability of the underlying debris fill, consideration should be given to provide additional grading recommendations in the area of the project. Response: Section 5.5, Page 7 of the. referenced geotechnical report provides recommendations for remedial grading where the limits of the debris fill may encroach upon sensitive habitat. The remedial grading for the planned fill slope should begin at the limits of grading and project down at a 1:1 inclination into formational materials as shown in the recommended grading specifications presented in Appendix C of the referenced geotechnical report. Based on these recommendations, the planned structural fill and improvements will be supported on formational materials. A small wedge of debris fill will be left in place at the toe of the planned slope within the limits of the grading due to the planned temporary excavations and the site constraints. This debris fill will not affect the performance of the planned improvements because the improvements will be supported on structural fill overlying formational materials. The planned fill over the debris fill should not be considered structural fill. Comment 4: The log for trench T-4 indicates that alluvial materials may exist at depths greater than 7112 feet. Based on the recommendations on Page 8 of the project geotechnical report, the existing alluvium should be removed and recompacted. Based on the anticipated depths of the recommended removals at the location of Trench T-8, the horizontal limits of the temporary excavations may be limited due to the adjacent Nightshade Road. Consideration should be given to provide temporary shoring recommendations for this area. Response: We assume the discussion of this comment refers to the location of trench T-8 (trenches T-4 and T-8 were mentioned). We plan on removing the alluvium in the area of Trench T-8 to formational materials; however, the grading operations will not be able to encroach the adjacent roadways and property lines. We expect the temporary excavation will descend from the roadway at an inclination of 1:1 to our removal elevations and will be limited, to a height of approximately 10 feet. Therefore, a portion of the alluvium will be left in place. We opine this wedge of alluvium will not affect the stability of the planned slope Within the area. Comment 5: The report does not provide a recommendation for the construction of a backdrain within the keyway for the 1.5:1 slope at the western portion of the project. Consideration should be given to provide such a recommendation. Response: The placement of a back drain is planned and is considered to be standard practice within the slope fill keyways. We recommend the placement of a 4-inch heel drain along the base of the backcut. We expect the drain will be located within the heel of the slope keyway and will outlet at the low point to the natural drainage. The project civil engineer should be involved during the placement of the drain to provide location of the pipe and proper outletting structures, if required. The subdrain will be as-built and shown on the final grading report. Project No. 07671-52-01 . -2- January 11, 2010 )PW Comment 6: Seepage was encountered within trench T-4. This seepage was not depicted on Cross Section B-B' or discussed as part of the slope stability analysis presented on Figure 5 of the report. Response: We encountered seepage within trench T-4; however, we did not encounter seepage at other trench locations (e.g. trench T-5) located down gradient; We expect the seepage is related to irrigation or nuisance water from the nearby developments. We consider the seepage areas to be isolated, however, drains may be required during the grading operations. The locations of the drains will be as-built and shown on the final grading report. We do not expect seepage is continuous through the represented cross-sections. Therefore, we do not believe seepage is required tb be modeled for the slope stability analyses. If you have any questions regarding this review, or if we may be of further service, please contact the undersigned at your convenience. Very truly yours, GEOCON , 77RPORATED Shawn Weedon GE 2714 SW:dmc (1) Addressee o VV C-13 Os NO. 271 12 LU tXp.05/30/11 rrI Project No. 07671-52-01 -3- January 11, 2010 N34°E A' 1r 370 A 375—,' 325 320 0 0 z Lu 270 275 225 220 MUROYA PROPERTY CARLSBAD, CALIFORNIA 0 z DISTANCE (Feet) GEOLOGIC CROSS - SECTION B-B' SCALE: V = 50' (Honz. = Vert) 350 z Q 300 ui 250 DISTANCE (Feet) GEOLOGIC CROSS - SECTION A-A' SCALE: V = 50' (Horiz. = Vert.) LEGEND Qcf ........ COMPACTED FILL (Proposed) Qudf........UNDOCUMENTED FILL Qdf........DEBRIS FILL Qal ........ ALLUVIUM Qin ........ LINDAVISTA FORMATION Tsa........SANTIAGO FORMATION T-41 ........APPROX. LOCATION OF EXPLORATORY TRENCH APPROX. REMOVAL AREA APPROX. LOCATION OF GEOLOGIC CONTACT (Queried Where Uncertain) GEOCON GO)1 INCORPORATED GEOTEO-IN1CAL CONSULTANTS 6960 FLANDERS DRIVE- SAN DIEGO, CALIFORNIA 92121- 2974 PHONE 858 558-6900 - FAX 858 558-6159 PROJECT NO. 07671 -52 -01 FIGURE 1 DATE 01-11-2010