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HomeMy WebLinkAboutCT 02-28; La Costa Condominiums; Responses to the Third Party Review; 2008-02-14American Geotechnical, Inc. SOIL, FOUNDATION AND GEOLOGIC STUDIES February 14, 2008 File No. 23280.01 Mr. Frank Jimeno, P.E. CITY OF CARLSBAD PUBLIC WORKS - ENGINEERING DIVISION 1635 Faraday Avenue Carlsbad, California 92008 Subject: RESPONSES TO THE THIRD PARTY REVIEW PROPOSED LA COSTA CONDOMINIUMS Lot 185, La Costa Avenue South - Unit 1 Carlsbad Tract No. 02-28 Carlsbad, California Reference: "Third Party Review of Geotechnical Reports" prepared by Testing Engineers - San Diego, Inc. dated January 11, 2008 Dear Mr. Jimeno: American Geotechnical is pleased to present the responses to the questions from the referenced report. We appreciate the opportunity to be of service. If you have any questions, please do not hesitate to contact us. Sincerely, AMERICAN GEOTECHNICAL Edred T. Marsh Principal Engineer G.E.2387 ETM:ks Distribution:Mr. Frank Jimeno, P.E. - (2)Via Mail Only 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 MAmerican Geotechnical, Inc. 1. The Geotechnical Site Map (plate 1) does not indicate the contact between many of the geologic units (for example the contact between Tsa and Af located between the southern portions of Cross Sections A-A' and D-D', or the contact between Tsa and Qcol near Cross Section L-L' and the contact between Tsa and Af in the northeast corner of the site. It is our opinion that the geotechnical map should be revised to show geologic contacts, and the legend should be revised to indicate the map symbol used to depict geologic contacts. In addition, the symbols used on the map to depict geologic contacts should be in general accordance with standard geologic mapping conventions. Response: The geotechnical map has been revised to show geologic contacts between geologic units 2. The first line in the geotechnical map legend includes a symbol Af/Tsa which depicts existing fill, buried Santiago Formation. That symbol appears to be used only once in the southwest corner of the map. In the other areas of the map, the symbol af/Tsa is circled. That circled symbol is not indicated on the map legend. By geologic mapping convention, then a symbol is circled it means that the unit is buried. However, the map legend is not clear on this particular symbol. In the case of this project we would expect that the "Af" in the circled Af/Tsa would not be buried. In any event, the map symbols and map legend do not match, and the map symbols are not consistent for this unit. It is our opinion that the geotechnical map and legend should be revised to correct any inconsistencies. In addition, the symbols used on the map to depict the various units, should be in general accordance with standard geologic mapping conventions. Response: The geotechnical map has been revised to depict buried geologic units using the conventional map symbol 3. The manner in which the geotechnical site map and legend depict exploratory borings performed by various geotechnical consultants is generally clear. The symbols for the various numbered borings are preceded by an acronym of abbreviation of the company name followed by the boring number. However, there are a number of borings in the northern portion of the site whose symbol is not indicated on the geotechnical map legend. The origin of these borings is not indicated on the map or described in the project geotechnical report. It is our opinion that the geotechnical map legend should be revised to include a symbol and description of these borings. Response: The geotechnical map & cross section have been revised to clearly differentiate exploratory borings performed by different geotechnical consultants. 4. There are two exploratory borings shown on the geotechnical map that are annotated with the symbol LB-1 (one near the northern end of Cross Section K-K' and another near the northern end of Cross Section l-l'). It is our opinion that the geotechnical map should be revised to indicate the correct location and identification of the borings shown. LB-1 is shown on the Cross Section l-l" and again on Cross Section K-K'. This appears to be related to the above mentioned error since the cross sections are not located near each other. It is our opinion that the consultant should check the boring locations and numbers and revise the cross sections to reflect the correct boring numbers. Response: The geotechnical map & cross sections have been revised to show the correct identifications and locations of the Leighton large diameter boring, LLB-1 and the Leighton small diameter boring, LSB-1 File NO 2328001 IIAmerican Geotechnical, Inc. February 14, 2008 PageS 5. The boring logs in the project geotechnical report identify the borings performed by American Geotechnical, Inc. as LGB-1 and LGB-2. The Geotechnical Site Map (Plate 1) identifies these borings as AGLB-1 and AGLB-2. It is our opinion that the map and report should be consistent in their identification of these borings. The report does not indicate the type of rig used to drill LGB-1 and LGB-2, and it is also not indicated on the boring logs. It is our opinion that the consultant should provide a description of the drilling and sampling methods used. Response: The boring logs have been revised to correctly identify the American Geotechnical large diameter borings as being AGLB-1 & AGLB-2, and the general description of the type of boring rig/drilling company and sampling methods have been added to the boring logs and the geotechnical report. 6. On page 6 under the section entitled, "Artificial Fill (Qaf)", the project geotechnical report indicates that previous site grading was observed and tested by Benton Engineering, Inc. in about 1970. The report then states that, "According to the referenced report (Benton, 1979), the fill was observed and tested during grading and the results were presented in a report issued in 1970. This report was not available for review." There are several comments relating to the above statement. a. The statement, "...the referenced report (Benton, 1979)...", would seem to indicate that the 1979 report was reviewed and included in the references presented in Appendix A. Our review of the references presented in Appendix A indicates that a 1979 report by Benton Engineering is not listed. It is our opinion that the geotechnical consultant should clarify whether the referenced report by Benton Engineering, Inc. was actually reviewed, and if so, it should be included along with the other references in Appendix A. b. The statement, "...the fill was observed and tested during grading and the results were presented in a report issued in 1970. This report was not available for review.", makes it clear that although the 1970 report was not available for review, the project geotechnical consultant has some knowledge of the observation and testing of the previous fill that was placed on the site, and that it was documented in a report in 1970. If the report was not available for review, it is not clear how the consultant knows that the fill was observed and tested during grading, or even how the consultant even knows of the existence of such a report. It is our opinion that the project geotechnical consultant should provide clarification and reference for the statements regarding the previous grading and existing fill soils at the site. Response: The geotechnical report has been amended to reflect that, since the Benton Engineering reports were not available for our office to review, American Geotechnical has relied upon the document review performed by Leighton & Associates concerning earlier phases of grading performed under the observation and testing of Benton Engineering. 7. The legend for the geotechnical map includes a symbol AGTP-2 and indicates that is shows the approximate location of test pit of American Geotechnical. There are no symbols on the geotechnicai map annotated as AGTP. There are however, symbols on the map annotated TP-1 and TP-2. If these symbols that are meant to indicate the test pits by American Geotechnical then it is our opinion that either the geotechnical map or the map legend should be revised so that the annotation is consistent. File NO 2328001 IlAmerican Geotechnical, Inc.February 14, 2008 ' Page 4 Response: The geotechnical map has been amended to identify the American Geotechnical test pits labeled, AGTP-1, AGTP-2, AGTP-3 8. There are two test pits identified as TP-2 shown on the geotechnical map. It is our opinion that the geotechnical map should be revised to reflect the correct annotation for each test pit. Response: The geotechnical map has been amended to identify the American Geotechnical test pits labeled, AGTP-1, AGTP-2, AGTP-3 9. The project geotechnical report identifies that a large portion of the site is underlain by exiting fill soils. Although there are no test results or evaluation of the existing fill presented, on page 6, the report indicates that the existing fill is moderately well compacted. On page 13 under section 6.2.1 entitled, "Site Preparation", the report indicates that, "Prior to grading, the site should be cleared of all surface and subsurface obstructions including things such as existing structures, fill..." From that section and statement, it appears that the consultant's opinion is that the fill is unsuitable to remain in place and should be removed. Recommendations for treatment of the existing fill is not mentioned in any of the other sections of the report. It is our opinion that the consultant should provide clarification regarding the treatment of the existing fill soils. Response: The geotechnical report has been amended to reflect that American Geotechnical has reviewed the laboratory testing of the existing fill material performed by Leighton Engineering. The comment on Page 13 regarding fill refers to any "loose, dump fill" that may exist prior to grading. Furthermore, any incompetent fill material encountered during the grading process will be removed and properly compacted. 10. The project geotechnical report indicates the presence of ancient landslides and recent landslide movement along the southern portion of the site. The report does not indicate which landslides are active and which are not. Recent landslide movement indicates that calculated safety factors for the existing conditions should be less than 1.0. Appendix D (Slope Stability Calculations) of the project geotechnical report indicates that the existing conditions have calculated safety factors well in excess of 1.0. This would seem to indicate that assumptions or factors used in analyses may not be representative of the actual site conditions. As an example, the stability analysis of the existing landslide condition along Cross Section D-D' was calculated with a circular failure surface and a block failure surface. The calculated safety factors for the two failure surfaces were reported as 1.34 and 2.36, respectively. The calculated safety factors appear to be higher than would be expected for an unbuttressed landslide condition. It is our opinion that the consultant should identify which landslides are active and comment on why the existing calculated safety factors of those landslides are greater than 1.0. Response: This question relates to the landslides mapped at the site. We disagree with the reviewers comment that the existing landslides must have a factor of safety of 1.0. Clearly a recent landslide, such as the one below the Banich property, would be expected to have a factor of safety slightly in excess of 1.0. However, the other landslides moved quite some time ago and the groundwater and surface topography has changed since then. Hence a factor of safety in excess of 1.0 would seem appropriate for such landslides. We also believe that this question by the reviewers is rather academic, since we have proposed to remove all the ancient landslides during grading, no matter what the value of the factor of safety that they possess. File NO. 23280.01 IlAmerican Geotechnical, Inc. February 14, 2008 Page 5 11. As a follow up to the previous comment number 10, we will use Cross Section D-D' from the project geotechnical report as one example to illustrate why it is our opinion that the assumptions or factors used in the stability analyses may not be representative of the site conditions. If an analysis of the pre-landslide or no-landslide condition was performed with the soil strength parameters used in the analysis of Cross Section D-D' the calculated safety factor would be similar to, but likely greater than, the calculated safety factor of 1.94 for the no-landslide condition reported for the proposed back cut along D- D'. If the pre-landslide safety factor of the slope was 1.94 (ir greater), it would not seem plausible for a landslide to have occurred in that slope, and as represented in the project geotechnical report, there do not appear to be any geotechnical factors or conditions present that would cause the occurrence of a landslide in the subject slope. However, the landslide is present, which would indicate that the assumptions or factors used in the analysis are not representative of the existing site conditions. Some of the factors which may not be representative are discussed in the following sections. a. The assumed soil strength parameters used in the stability analyses may be too high. The project geotechnical report appears to use assumed shear strength and unit weight parameters for the various units in the stability calculations presented in Appendix D of the report. Laboratory testing of these unit does not appear to have been performed by the project geotechnical consultant. We note that the assumed soil strength parameters are significantly higher than the parameters used in the referenced geotechnical report by American Geotechnical, Inc. dated May 18, 2007 (reference 4) for an adjacent project to the west (la Costa De Marbella). It is our opinion that the project geotechnical report should identify the origin or basis of the assumed soil parameters used in the analysis. b. The project geotechnical report indicates that the onsite landslides occurred along shear surfaces developed within the claystone beds and laminations of the Santiago Formation. We generally agree that bedding plane shears form the basal slip surfaces of the landslides. It is also our experience that the back of the landslides are also sheared zones that can generally develop along high-angle joint or fracture systems. The failure surface for such landslides would not be a circular failure surface as analyzed in the slope stability calculations (Appendix D of the project geotechnical report). The failure surface would have a relatively flat basal failure surface (parallel to bedding), and have a relatively steep back scarp that was planar to subOcircular. The stability analysis of such a failure surface would include modeling of the relatively weak sheared basal rupture and sheared back scarp. The soil strength parameters along the sheared bedding and fracture surfaces would typically be lower than those assumed in the project geotechnical report. Our review of the boring logs that were downhole-logged by Leighton and Associates, Inc., and presented in the project geotechnical report, indicates that factures and sheared fracture zones are common in the "undisturbed" formational materials beyond the limits of the landslides. The presence of relatively weak zones developed along joint and/or fracture systems could help explain the presence of landslides in slopes that are generally comprised of relatively high strength geologic materials. The comments should address why the presence of these relatively weak zones is not reflected in the slope stability analyses performed for the subject project. File NO. 23280.01 HAimerican Geotechnical, Inc.February 14, 2008 Page 6 Response: As a follow-up to item No. 10, the reviewers make reference to cross-section D-D' and the back cut factor of safety of 1.94. While this factor of safety does seem rather high, it is because all of the landslide debris (weaker strength material) has been removed from the cross section, furthermore, it should be noted that many of the other backcuts have much lower factor of safety values. For example, the backcut factor of safety for Section A-A' is 1.21, the factor of safety for the backcut at Section B-B' is 1.29 and the factor of safety for the backcut at Section k- k'is 1.39. In our opinion, the assumed shear strengths that we have used in our slope stability analyses are reasonable. We would also like to point out that we will shortly be excavating the backcut for the Banich repair and we will be able to fully evaluate conditions at that time. If the Banich backcut is substantially different than as expected, then we could make the appropriate changes to our slope stability analyses at that time. 12. The report does not specify the seismic factor used to calculate seismic stability. It is our opinion that the consultant should indicate the seismic factor used in the calculations. Response: The seismic coefficient used in the stability analysis was 0.15 13. The northern and eastern limits of the landslide in the southwest portion of the sire (near Cross Section D-D') and the northern limits of landslides in the eastern portion of the site (near Cross Sections B-B' and K-K') are not clearly delineated on the geotechnical map. It is our opinion that the geotechnical map should be revised to show the limits of the landslides (similar to the limits of the landslide near Cross Section C-C'. Using standard geologic mapping symbolization, if the limits are buried, they should be dotted, if the limits are uncertain and buried, they should be dotted and queried. Response: The geotechnical map has been revised to show the limits of the landslide boundaries using conventional map symbols. 14. As shown on the Geotechnical Site Map, in the northeast portion of the site, an irregular area roughly 20 feet by 40 feet is indicated by light green shading (the same shading used for the landslide shown on the map). This feature spans the geologic contact between Qcol and Tsa. The feature is not annotated with a map or in the map legend. It is our opinion that the consultant should identify this feature and revise the geotechnical map and legend so that it is appropriately depicted. Response: The geotechnical map has been revised to identify the shallow surficial failure located in the northeast portion of the site. 15. On Plate 3, Cross Section I-!' identifies a "proposed buttress". This buttress is not indicated on the geotechnical map and the buttress is also not indicated in the stability calculations presented in Appendix D. It is our opinion that if the consultant is recommending a buttress in this area, the location and recommended limits of the buttress should be presented appropriately on the geotechnical map. Stability calculations for the buttress should also be included. Response: The cross section l-l' on Plate 3 has been revised to correct the typographic error that misidentified the proposed grade as a proposed buttress. 16. The cross sections presented on Plates 2 and 3 generally include depictions of existing and proposed greades. Some of the cross sections also depict proposed remedial landslide grading. File NO. 23280.01 HAmerican Geotechnical, Inc. February 14, 2008 Page? We assume that the benching depicted at the back and toe of the landslide on the cross sections indicates that the landslides will be completely removed during remedial grading. Cross Section D-D' depicts proposed remedial landslide grading similar to the other cross sections. However, since Cross Section D-D' lacks the depiction of benching at the back and toe of the landslide, it is not readily apparent if the landslide will be completely removed. Our review of the stability calculations presented in Appendix C would seem to indicate that the landslide may be removed during remedial grading. It our opinion that the consultant should clarify the recommended remedial grading, and if the landslide depicted on Cross Section D-D' is to be removed during grading, the cross section should be revised to show the benching at the back and toe so that the cross section is consistent with the other cross sections presented. Response: Cross Section D-D' on Plate 2 has been revised to show the proposed stability buttress and fill benching behind the existing landslide to be removed during remedial grading. 17. As a follow up to our previous comment 16, Cross Section D-D' apparently depicts queried failure surfaces that extend southerly to the property boundary and beyond the approximate limit of buttressing (as shown on the Geotechnical Site Map). It would appear that these failure surfaces will be unsupported during excavation of the recommended buttress backcut, and that failure of these surfaces will be removed during remedial grading and it does not appear that these failure surfaces were modeled in the slope stability calculations presented in the project geotechnical report. It is our opinion that the project geotechnical consultant should address potential failure of the recommended backcut along these surfaces, and if necessary provide appropriate recommendations to mitigate the potential to damage to adjacent properties if a failure were to occur. Response: No failure surfaces were identified above the grading limits shown. What we believe as the actual conditions were modeled in the slope stability analysis. Although not anticipated, any potential instability above the grading limits will be closely monitored during remedial grading work. 18. Cross Section B-B' depicts proposed remedial landslide grading. We assume that the benching depicted at the back and toe of the landslide indicates that the landslide will be completely removed both on site and on the adjacent property to the south. Cross section B-B' does not depict the proposed finished grade of the slope along the section. It is our opinion that Cross Section B-B' should be revised to depict the proposed grades. Response: Cross Section B-B' on Plate 2 has been revised to depict the proposed 2:1 (H:V) final slope grade after remedial grading for landslide removal has been performed. 19. As a follow-up to the previous comment number 18, Cross Section B-B' depicts a relatively steep backcut proposed to remove the existing landslide. According to the cross section, this backcut could undermine support at the toe of an existing fill slope and residential structure on the adjacent property to the south. The existing subsurface conditions indicated the cross section for this existing fill slope do not match the conditions modeled in the computer slope stability analysis for this section presented in Appendix D. It does not appear that the stability analysis for Cross section B-B' included analysis for potential failure of the existing fill slope either as it exists or in its undermined condition. It is our opinion that the project geotechnical consultant should review the cross section and stability analysis for this area and comment on the slope stability. File NO 23280 01 HAmerican Geotechnical, Inc. February 14, 2008 Page 8 The consultant's review and comment should also address temporary stability of the existing fill during backcut excavation and provide recommendations for temporary support of the slope and existing structure, if needed. Response: It is anticipated that the backcut will be below the level of the fill. In our opinion we do not believe that the stability of the fill slope will be undermined by the backcut. However, during remedial grading the slope above the excavation will be closely monitored. 20. The cross sections presented on Plates 2 and 3 do not present any groundwater data. Typically groundwater and seepage conditions with depths encountered are annotated adjacent to the boring symbols on cross sections. Considering the importance of groundwater to the stability of the project area especially during grading when relatively steep temporary slopes will be excavated into the hillside, it is our opinion that the cross sections should be modified to depict the groundwater and seepage conditions as encountered in the borings. Plotting the groundwater conditions on the cross sections will also help the project grading contractor identify potential troublesome areas and assist in planning his work. Response: The cross sections on Plate 2 & 3 have been revised to show perched groundwater and seep zones encountered in the exploratory borings. 21. Surface water seepage was indicated on the Site Plan (Figure 1) contained in the reference Landslide Stabilization Recommendations report by American Geotechnical, Inc., dated March 27, 2007 (reference 5). Since the Landslide Stabilization Recommendations report is technically not a project document, and for reasons presented in the previous review comment number 19, it is our opinion that the seepage condition should be reported in the project geotechnical report and identified on the geotechnical map. Response: The geotechnical map has been revised to show active surface seep zones identified on the site. The geotechnical report has been revised to include a description of the identified surface seep zones. 22. Appendix C of the project geotechnical report presents the results of laboratory tests. Review of the test results indicate that laboratory tests were performed on three samples. The report does not identify the origin of the samples (i.e. boring or test pit number and depth). It is our opinion that the origin of the samples should comment whether the laboratory tests on these three samples provide the bases for their investigation and evaluation of the engineering properties of the onsite soils Response: The three bulk samples were taken at representative locations throughout the site and are representative of the soil types present at the subject site and representative of the soil types to be used during remedial grading. 23. The project geotechnical report appears to use assumed shear strength and unit weight parameters for the various units in the stability calculations presented in Appendix D of the geotechnical report. Laboratory testing of these units does not appear to have been performed by the project geotechnical consultant. It is our opinion that the report should identify the origin or basis of the assumed soil factors used in the analysis. File NO. 2328o.oi LHAmerican Geotechnical, Inc. February 14, 2008 Page 9 Response: Data used to performed the slope stability analysis that are presented in Appendix D of the Geotechnical report come from two different sources: 1 - Update preliminary Geotechnical report by Leighton and Associates dated October 30, 2002 2- Landslide Stability Recommendations by American Geotechnical in March 2007. Those data were obtained by back calculation performed on the cross section of the active landslide 24. There appear to be no borings in the hillside above the flat pad area in the easternmost portion of the site in the vicinity of Cross Sections l-l' and Cross Section L-L'. The map depicts an area mapped as colluvium. Cross Section L-L' depicts a slope with a ration of approximately 1.5 to 1 (horizontal to vertical) in a unit mapped as colluvium. The colluvium is perched on the existing slope. Cross section l-l' also depicts colluvium perched on the existing slope. The depth of the colluvium is queried on Cross Section l-l', but is not queried on Cross Section L-L'. The basis for the geology depicted on the Geotechnical Site Map (Plate 1), and the cross sections is not clear. The geotechnical conditions differ from those presented in previous geotechnical reports. Since these cross sections are used to analyze stability in this area, it is our opinion that the project geotechnical consultant should provide clarification for the basis of the geotechnical conditions depicted for this area. Response: The area of Colluvium shown on the geotechnical map was identified by our project geologist during field mapping of the site. The approximate depths of colluvium were estimated based upon observation of the side scarps of the small surficial slump located halfway up the slope in this area (shown on the geotechnical map). Cross section L-L' has been revised to indicate the approximate depths of the colluvium with query marks. 25. As a follow-up to the previous comment number 24, the stability calculations for Cross Sections l-l' and L-L do not appear to include analysis of the stability of the colluvium perched on the relatively steep hillside above the project site. Colluvium is generally known to be relatively weak and prone to failures in sloping terrain. Cross Section L-L' depicts a slope with a ration of approximately 1.5 to 1 (horizontal to vertical) within the unit mapped as colluvium. It is our opinion that the stability analysis for this area should include analysis of the stability of the colluvium. If the colluvium is found to have a less than acceptable factor of safety against failure, the project geotechnical consultant should provide appropriate recommendations for mitigation of potential instability. Response: We do not believe that the thin veneer of colluvium will adversely affect on-site or off- site improvements. Although not anticipated, areas with potentially unstable colluvium identified during remedial grading will be removed and replaced with compacted fill. 26. The Geotechnical Site Map (Plate 1) of the project geotechnical report depicts an approximate limit of buttressing and possible off-site grading along Cross Section L-L'. Review of Cross Section L-L' (Plate 3) indicates that the limit of buttressing is not reflected on the cross section. It is our opinion that the project geotechnical consultant should revise the cross section so that the recommended buttress in this area is correctly depicted. Response: Cross Section L-L shows the limits of the proposed buttress - it extends from the private site road to approximate 30' behind the retaining wall. The limits of the proposed buttress have been revised on the geotechnical map and the proposed buttress backcut benches have been revised on cross section L-L' ™«N°-2328001 HAiTierican Geotechnical, Inc.February 14, 2008 ' Page 10 27. On the Geotechnical Site Map (Plate 1) of the project geotechnical report, a line (shown as long dashes with dots) identifies, "possible limits of grading". Based on the map and cross sections, the nature of the grading within this limit is relatively clear in the western approximately two-thirds of the site. However, the nature of this grading is not clear in the eastern approximately one-third of the site. The possible grading is not shown on the map or cross sections, and it is not discussed in the project geotechnical report. It is our opinion that the project geotechnical consultant should discuss the nature of possible grading in this area. In addition, the "possible limits of grading" line is not closed at the northern property line (indicating that the possible limit extends north beyond La Costa Avenue. If this line is, as it is annotated, the "possible limits of grading", it should be closed along the northern property line along La Costa Avenue. Response: The geotechnical map and cross sections have been revised to clearly indicate the proposed "possible limits of grading" - including the northern boundary. The eastern boundary of the possible limits of grading is outlined on the geotechnical map. The geotechnical report has been revised to include a discussion on the proposed limits of possible grading (p.14 Section 6.2.3) 28. The proposed remedial landslide grading shown by the project geotechnical consultant on Cross Section K-K' indicates a finished slope on the order of 90 feet high with a slope ratio of 1.5 to 1 (horizontal to vertical). The report does not address the stability of this slope nor does it provide recommendations that should be incorporated into design and construction so that this slope will have an acceptable factor of safety against deep- seated and surficial instability. It is our opinion that the project geotechnical consultant should address the stability of this relatively steep and high slope. Response: The proposed finish slope gradient along section K-K' is actually approximately 2:1 (H:V) not 1.5:1. The cross section has been revised to show the correct slope gradient symbol. 29. On page 19, in the section entitled, "Off-Site Fill Slopes", the geotechnical report states that slope stability analysis indicates that the 1.5 to 1 (horizontal to vertical) off-site fill slopes above the southern property boundary are subject to surficial failure. The report also indicates that if one of these slopes were to experience a failure, there could be some impact on the proposed condominium development. The report mentions some methods that could be considered to mitigate the potential for damage to the project due to a failure of one of the off-site slopes. It is our opinion that the project geotechnical consultant should provide an analysis/description of the types of failure and impacts that might be experienced by the proposed development due to a failure of one of the off-site slopes. The project geotechnical consultant should also review the project design and comment whether the design has incorporated appropriate features to mitigate the potential for damage to the proposed project due to failure of the off-site slopes. Response: On page 19 of our May 31, 2007 geotechnical report, we specifically address off-site slopes. As we stated in this section, it is our opinion that these off-site slopes could be subjected to surficial failure if seepage develops parallel to the slope face. The reviewers requested that we provide analysis/descriptions of the types of failures that might be experienced by the proposed development due to the failure of one of these off-site slopes. Since surficial slope failures tend to be rather shallow, it is our opinion that the proposed work on our property will not have any affect on the surficial stability of these off-site slopes. If the off-site slopes should fail, we would anticipate that the failure mass could move downslope as a shallow slump or could transform itself into a mudflow. File NO. 2328o.oi IlAmerican Geotechnical, Inc. February 14, 2008 Page 11 The reviewers also requested that we review the project design and comment on whether the design has incorporated appropriate features to mitigate the potential for damage to the proposed project due to failure of the off-site slopes. As we stated on page 19 of our May 31, 2007 geotechnical report, we indicated that the construction of additional retaining structures mid-slope or increasing the strength and height of retaining structures constructed at the toe of slope could be considered. To date, we are not aware of any on-site improvements included within the site design drawings for this possibility. 30. As a follow-up to the previous comment number 29, the project geotechnical report does not include an analysis or statement regarding the gross or mass stability of the off-site 1.5 to 1 (horizontal to vertical) slopes and the potential impact of these slopes on the proposed project. It is our opinion that the project geotechnical consultant should comment on the gross stability of existing slopes and impacts to the project site due to potential slope instability. The project geotechnical consultant should provided appropriate recommendations to mitigate potential adverse impacts, if necessary. Response: In this question, the reviewers request further information on the off-site slopes. In particular, the reviewers request an analysis or statement regarding the gross stability of the off- site 1.5 to 1 (horizontahvertical) slopes. In Appendix D, our gross slope stability analyses are for the entire slope, including the on-site and off-site portion of the slope. We have performed analysis for gross slope stability, which includes both the on-site and off-site portions of the slope. 31. On Page 12, the project geotechnical report recommends that off-site drainage that is currently being directed over the southern slope be collected and directed to a suitable disposable area. It is our opinion that the project geotechnical consultant should review the project plans and comment whether the design has incorporated appropriate features to control surface drainage currently directed to the site from off-site sources. Response: We have reviewed the plans and it is our opinion that our recommendations concerning off-site surface drainage have been incorporated into the design. 32. The project geotechnical report indicates that nearly the entire slope along La Costa Avenue (northern project boundary) will be stabilized with a buttress. However the buttress is not continues easterly along La Costa Avenue beyond the proposed main entrance to the project. The majority of the existing slope along La Costa Avenue both east and west of the proposed main entrance is comprised of existing fill. If a buttress is recommended to stabilize the existing fill slope and proposed improvements along La Costa Avenue west of the main entrance, it is not clear why it would not be continued to stabilize the existing fill slope and proposed improvements east of the main entrance. It is our opinion that the geotechnical consultant should comment on the reason that the recommended buttress is not continued east of the proposed main entrance. Response: The existing North facing slope located on the east side of the main entrance along La Costa Avenue will be cut to a flatter angle during grading, therefore any loose surficial fill soils located on the slope will be removed during grading. 33. Review of the project geotechnical report indicates that nearly all of the onsite slopes will require a stabilization buttress. The standard detail for buttress subdrains provided in the project geotechnical report indicates that the recommended vertical spacing of buttress subdrains (backdrains) is about 12.5 feet, and the horizontal spacing of buttress subdrain outlets should be limited to about 100 feet. It is our opinion that the locations of the recommended subdrains and the subdrain detail should be shown on the project plans. File NO. 23280.01 HAiTierican Geotechnical, Inc. February 14, 2008 Page 12 Based on the recommendations for subdrains presented in the project geotechnical report, it appears that on the order of 50 to 100 separate subdrain outlets may result from the remedial grading (not counting the outlets for the recommended retaining wall subdrains). The subdrain detail shows outlets discharging at the slope face. On page 26 under section 6.8 entitled "Site Drainage", the project geotechnical report states that, "No drains should be allowed to empty adjacent foundations or over slopes." It is our opinion that the project geotechnical consultant should provide recommendations regarding appropriate discharge for the recommend subdrains that will be constructed as a part of this project. The recommendations should consider protection of the outlets, periodic clearing of vegetation or other blockage, and designation of, or recommendations for who should be the responsible party for maintaining the outlets. In view of the number of discharge outlets that may result from proposed grading, consideration should be given to discharging all subdrain outlets into the project storm drain system. In addition, the locations of the drains and outlets should be surveyed and the locations shown on the as- graded map for future reference. Response: The subdrains will be connected via a tight line and directed to the proposed concrete "V" ditches. The grading plans will be revised to depict approximate locations of proposed drains. 34. Sheet 3 of the referenced project grading plans prepared by O'day Consultants (reference 3) shows a proposed detention basin in the western portion of the site a the toe of the slope that descends from the southern site boundary. It appears that this is an unlined basin and that it is located up-gradient from proposed residential structures and improvements. The proposed detention basin is not addressed in the project geotechnical report it should be noted that the referenced geotechnical report by Leighton and Associates, Inc. dated October 30, 2002 (reference 4), indicates that ponding of water should be avoided. It is our opinion that the project geotechnical consultant should review the proposed detention basin and comment whether water infiltration from the unlined basin could adversely affect the proposed improvements, and if necessary, provide appropriate recommendations to mitigate any adverse impacts. Response: It is our understanding that the detention basin will be lined which will limit any seepage that could impact the project. 35. The geotechnical map (Plate 1) presented in the reference report by Leighton and Associates, Inc. dated October 30, 2002 (reference 4) indicates the presence of a colluvium-filled drainage underlying a portion of the site adjacent to La Costa Avenue westerly of the proposed main entrance to the project. The Leighton map indicates that the colluvium is now buried by existing fill soils. It is not clear why the project geotechnical consultant has chosen not to include that feature on the geotechnical map provided in the project geotechnicai report. It is our opinion that the project geotechnical map should depict the known geotechnical features underlying the site, and that the project geotechnical map should be revised to indicate this feature. Response: As part of our investigation we. reviewed the Leighton boring log, LSB -2 that identified buried colluvium beneath fill material. Both the description and the dry densities given for the colluvium generally matched that of the fill material. Also the Leighton lab test results for the colluvium yielded a low hydrocollapse potential. Consequently we concluded that the Leighton investigation may have misidentified the colluvium. However, we have revised the geotechnical map to show the area of buried colluvium identified by Leighton & Associates. File NO 23280.01 HAmerican Geotechnical, Inc. February 14, 2008 Page 13 36. We understand that repair for a slope failure of the north-facing slope along the southern site boundary is currently being performed. Recommendations for the slope repair were presented in the referenced American Geotechnical, Inc. report dated March 27, 2007 (reference 5). Repair of the slope includes reconstruction with geogrid reinforcement, subsurface and surface drainage features with a surface drainage energy dissipater. It is not clear how the repaired slope and its features will interface with stabilization measures that are recommended for the adjacent slopes on the subject La Costa Condominiums project. The project geotechnical documents do not address the interfacing of the slope stabilization that is ongoing and the proposed stabilization that is planned for the subject project. It is our opinion that the project geotechnical consultant should comment on the proposed remedial slope grading at the site and its impacts on the slope stabilization reconstruction that is currently being done on the adjacent property to the south. The comments should include discussion on the interfacing of the current slope stabilization and the proposed remedial grading stabilization that is recommended for the subject project (i.e. grading interface, geogrid interface, surface and subsurface drainage features interface). Response: The currently on-going slope repair will blend with the planned finished slope elevations during remedial grading for the condominium development. The subsurface drainage will be directed to the east side of the buttress and will daylight to the surface so that during subsequent grading operations the drains will be connected and continuous. The grading plans (see No.33) will reflect proposed drain locations. 37. As a follow-up to the previous comment number 36, it is our opinion that the project geotechnical consultant should comment on whether the conditions exposed or encountered during the current slope stabilization will require modifications to the recommendations presented in the project geotechnical documents for the subject La Costa Condominiums project. Response: We do not anticipate modifications. 38. Our review of the project geotechnical report indicates that the project geotechnical consultant has drilled only one exploratory boring within the boundary of the subject project. The geotechnical report includes logs of exploratory borings drilled during previous investigations by other consultants. The consultant's interpretation of subsurface conditions (identification and characterization of subsurface geologic units and existing fill, geometries of landslides, characterizations of groundwater underlying the site, etc.) appears to be based on data developed during previous investigations by other consultants. If the consultant is relying principally on the geotechnical data developed by others, it should be so stated. The project geotechnical consultant should comment on the applicability and suitability of the work performed by other geotechnical consultants as it relates to the proposed development. Unless the project geotechnical consultant provides alternate analysis, the consultant should stat that he agrees with the interpretations and recommendations presented in the reports of the other consultants. If the project geotechnical consultant does not agree with the findings presented in the previous geotechnical reports, he should so state and provide the reason(s) for disagreement along with appropriate geotechnical data/analysis supporting his position. Previous reports that are relied on for characterization of the site (not just the logs of borings) should become permanent additions to the project geotechnical documents. The previous geotechnical reports should also be submitted for third-party review. File NO. 23280.01 HAmerican Geotechnical, Inc. February 14, 2008 Page 14 Response: American Geotechnical has reviewed the previous geotechnical investigation reports prepared by Leighton & Associates and the large diameter boring logs prepared by Benton Engineering during their earlier investigation of the site. We generally concur with their geologic descriptions, findings and site characterizations because they substantially conform with our field mapping and subsurface explorations results. American Geotechnical has reviewed, analyzed and revised all of the geologic cross sections contained within our report. Based on our analysis of the results of previous investigation performed by Leighton & Associates and Benton Engineering and our own investigation of the recent slope failures affecting the southern slope, we have developed extensive remedial grading recommendations that are appropriate and comprehensive. 39. The project geotechnical report indicates the presence of expansive soil within the planned development, and on Page 20 states that, "Because of the large quantities of moderately to highly expansive soils present within the building pad areas, it may not be possible to selectively grade the building pad areas without importing material such that the overall characterization of the soils within five (5) feet of grade are either non- expansive or have low-expansivity." The expansion index testing (2 tests) presented in the project geotechnical report resulted in medium and high expansion potential. In consideration of the expansive soil conditions expected at the site, it is not clear why the foundation recommendations provided in the project geotechnical report address foundation design and construction on soils that are non-expansive or have low- expansivity. Note that the referenced geotechnical report by Leighton and Associates, Inc. dated October 30, 2002 (reference 4), presents foundation recommendations for soils with expansion potential ranging up to very high (post-tensioned foundation and slab system). Unless the project geotechnical consultant is certain that soils with very low to low expansion potential will be imported to the site to construct the building pads, it is our opinion that the recommendations for foundation design and construction provided in the project geotechnical report should reflect the expansive soil conditions that are expected at the site. If the project geotechnical consultant is certain the soils with very low to low expansion potential will be imported to the site to construct the building pads, it should be state in the project geotechnical report. Response: Because we do not know what the expansivity of near surface soils will be until the site is graded we do not think it is necessary to provide specific design parameters for a variety of assumed conditions. Once the grading is complete appropriate design parameters can be provided. 40. The project geotechnical documents recommend that a representative of the project geotechnical consultant be on site during construction to provide observation and testing services. It is our opinion that an engineering geologist should also be present on site during construction to observe and map the geologic conditions exposed/encountered. The mapped geologic conditions should be presented on an as-graded geologic map and included with the as-graded geotechnical report for the project. Response: We agree that an engineering geologist should be on-site during grading. We recommend that an engineering geologist map the geologic conditions encountered and provide additional recommendations as needed during earthwork operations. 41. The project geotechnical documents recommend installation of slope inclinometers and surface survey monuments above the proposed buttress backcut along the southern property boundary. The project documents indicate that the inclinometers and survey monuments will be surveyed regularly to monitor potential slope instability. File NO. 23280.01 IlAmerJcan Geotechnical, Inc. February 14, 2008 Page 15 It is our opinion that the results of the monitoring should be provided by the City on a weekly or at least bi-weekly basis, and if significant movement is detected the results should be provided on a daily basis. Response: This consultant believes that if significant movement is detected that work will stop immediately whether or not the readings are provided to the city. However, if the city requests results of the monitoring we would certainly provide the data. 42. When borings are not located along a cross section, but are projected to the cross section, the distance and direction of projection is typically annotated on the cross section. Our review indicates that many of the borings depicted on the cross sections were "projected" to the cross section lines. Of all the borings that were projected only one boring was annotated to indicate that it was projected (B-4 on Cross Section J-J'). The distance of projection was not provided. It is not clear why the consultant chose to indicate that one of the borings was projected, but failed to do so for all the other borings that were projected. Response: The Cross-Sections on Plate 1 & 2 have been annotated to show the distance and direction of projected borings. 43. The total depth of the borings was not shown on the cross sections. Typically when borings are depicted on cross sections, the total depth of the boring is annotated, usually above or below the boring symbol. Response: The cross-sections on Plate 1 & 2 have been annotated to show the total depth of each boring. 44. LB-5 is located approximately on the Cross Section C-C', but is not shown on the cross section. The consultant has shown AGLB-2 which is located further from the cross section than LB-5. It is not clear why the LB-5 was not included on the section. Response: We chose to show AGLB-2 on Cross Section C-C' instead of LLB-5 because it is more recent and therefore more representative of the current geologic conditions (i.e. AGLB-2 was drilled and downhole-logged after the recent landslide depicted on the geotechnical map occurred) 45. The geologic cross sections (Plates 2 and 3), depict buildings which, although not identified as such, appear to be existing structures outside of the project boundary. Cross Section J-J' (Plate 3) is the only section that depicts a building within the property boundary. Although this building is also not identified, we assume that this building does not exist since it is not on the project plans or addressed in the site description. To eliminate confusion, if buildings are shown on cross sections, they are typically identified as either existing or proposed as appropriate. Response: The cross-sections on Plates 2 & 3 have been annotated to identify existing versus proposed buildings. 46. On page 5 paragraph 2, the first two lines of the project geotechnical report make reference to a report by Leighton and Associates in 1999, and indicate that the report is referenced in Appendix A. the 1999 Leighton report is not listed in Appendix A, as stated. File NO. 2328001 HAmerican Geotechnical, Inc. February 14, 2008 Page 16 Response: The appendix has been updated to include the Leighton and Associates report dated 1999 47. On page 15 line 1, it appears that some unit of measure was apparently omitted after the statement that, "A minimum of five (5)..." Response: The report has been amended to state "a minimum of five (5) feet" 48. On page 15 line 2, page 16 line 18 and page 20 line 8, it seems that the consultant has misused the term "aerial" as it relates to the extent of the proposed improvements. Typically, the term "aerial" refers to airborne or in flight, or something similar. Perhaps the more correct term would be areal. Response: The word aerial has been changed to areal on line 2, page 15 of the geotechnical report.