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HomeMy WebLinkAboutCT 02-28; La Costa Condominiums; First Review of Responses to Third Party Review; 2008-04-09April 9, 2008 RECEIVED APR 112008 ENGINEERING DEPARTMENT Contract No. 148513.30 Mr. Frank Jimeno, P.E. City of Carlsbad Public Works - Engineering Division 1635 Faraday Avenue Carlsbad, CA 92008 Subject: First Review of Responses to Third-Party Review of Geotechnical Reports, Proposed La Costa Condominiums, Carlsbad, California References: 1. "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", prepared by American Geotechnical Inc., File No. 23280.01, dated February 14,2008. 3. "Revised Update Geotechnical Report, Proposed La Costa Condominiums, Lot 185, La Costa Avenue South - Unit 1, Carlsbad Tract No. 02-28, Carlsbad, California", prepared by American Geotechnical Inc., File No. 23280.01, dated February 14,2008. 2. Third Party Review of Geotechnical Reports, Proposed La Costa Condominiums, Carlsbad, California, by Bureau Veritas, Testing Engineers - San Diego, Project No. 148513.30, Dated January 11,2008 Dear Mr. Jimeno: Introduction As requested Bureau Veritas, Testing Engineers-San Diego, has reviewed the responses to our third-party review of the geotechnical documents for the subject project. The responses were presented in references 1 and 2 above. Our third party review comments were presented in reference 3. The purpose of our review was to provide an opinion on whether the geotechnical aspects of the project have been identified and appropriately addressed in the project geotechnical documents. This letter presents the results of our review of the responses. Our review is based on geotechnical data presented in the project geotechnical documents and our professional judgment. We have not performed subsurface investigation, laboratory testing or independent geotechnical analysis for the project. Testing Engineers San Diego A Bureau I 'eritas Company 7895 Convoy Court, Suite 18 San Diego. CA 92111 Main: 858.715.5800 Fax: 858.715.5810 www.us.bureauveritas.com City of Carlsbad - La Costa Condominiums Contract No. 148513.30 Results of Review Based on our review of the responses to our comments, it is our opinion that some of our review comments were adequately addressed by the project geotechnical consultant's responses. However, many of the responses were incomplete or perhaps our review comments were misunderstood by the project geotechnical consultant. Our original review comments followed by the project consultant's responses and our additional comments are enumerated below. For convenience, we have maintained the numbering system of our original review. We have also omitted the comments and responses we feel were adequately addressed. 1. Original Review Comment: 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. Consultant's Response: The geotechnical map has been revised to show geologic contacts between geologic units Our Additional Comment: While some of the geologic contacts between units are now indicated on the revised geotechnical map. Many are still not shown. Our original comment specifically identified several locations where the contacts are not indicated. The geologic contacts at those specific locations are still not shown on the geotechnical map. It is not clear why the project geotechnical consultant's map revision was incomplete. It is our opinion that our original comment as presented above, remains pertinent and should be addressed by the project geotechnical consultant. 6. Original Review Comment: On page 6 under the section entitled, "Artificial Fill (Oaf)", 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. City of Carlsbad - La Costa Condominiums Contract No. 148513.30 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. Consultant's Response: The geotechnical report has been amended to reflect that, since the Benton Engineering reports were not available for office to review, American Geotechnical has relied upon the document review performed by Leighton and Associates concerning earlier phases of grading performed under the observation and testing of Benton Engineering. Our Additional Comment: The project geotechnical consultant's response makes it clear that they have relied on the document review performed by Leighton and Associates (L&A) regarding the earlier phases of grading at the site. We are assuming that the "document review performed by Leighton and Associates" refers to information presented in the geotechnical report prepared by Leighton and Associates dated October 30, 2002. We have reviewed the L&A report. The L&A report clearly indicates that the report providing the results of observation and testing of the fill placement at the site was not available to L&A and was therefore not reviewed. Based on the above we can only conclude that neither the project geotechnical consultant nor L&A have reviewed any as-graded reports that document observations and compaction test results as well as the limits of the fill that was previously placed at the site. 9. Original Review Comment: The project geotechnical report identifies that a large portion of the site is underlain by existing 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. Consultant's 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 recompacted. City of Carlsbad - La Costa Condominiums Contract No. 148513.30 Our Additional Comment: The response does not address our original comment regarding treatment of the existing fill soils. The project geotechnical consultant's response makes it clear that they have relied on laboratory testing of the existing fill performed by Leighton and Associates (L&A). The consultant has not provided an evaluation of the suitability of the existing fill soils to support structural loads. As indicated in our previous comment No. 6, it appears that neither the project geotechnical consultant nor L&A have reviewed any as-graded reports that document observations and compaction test results as well as the limits of the compacted fill that was previously placed at the site. Although it is not stated, the consultant's response implies that the existing fill is suitable for structural support and will remain in place. If it is the consultant's opinion that the existing fill is suitable for structural support, it should be so stated. The consultant's response states that, "Furthermore, any incompetent fill material encountered during the grading process will be removed and properly recompacted." The consultant should provide a definition of the term "incompetent", and should provide a description of the method(s) and/or tests that will be used in the field during grading to determine whether any fill encountered is incompetent or not. The consultant should also address the possibility for "incompetent" fill to exist below "competent" fill and how such a condition might affect the proposed project. 10. Original Review Comment: 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 the 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 Consultant's 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 have changed since then. Hence a factor of safety in excess of 1.0 would seem appropriate for such landslides. We also believe 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 they possess. City of Carlsbad - La Costa Condominiums Contract No. 148513.30 Our Additional Comment: Our original comment was focused on one principle issue, and the consultant's response does not address that issue. The consultant's response statement, "We disagree with the reviewers comment that the existing landslides must have a factor of safety of 1.O.", makes it clear that our comment was misunderstood. Our comment did not state, "...that the landslides must have a factor of safety of 1.0." Perhaps it might be helpful to take our comment one step at a time. The consultant indicated that there were both recent and ancient landslides on the site. The report did not differentiate between the two. To help us review the consultant's landslide evaluation, our original comment asked the consultant to identify which landslides were active or recent and which were not. We do not feel that this comment is "academic". As the consultant's response to this comment will provide some of the basis for our subsequent comments regarding material strength parameters. 11. Original Review Comment: 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 7analysis 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 (or 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 units 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 City of Carlsbad - La Costa Condominiums Contract No. 148513.30 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 sub- circular. 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. It is our opinion that the project geotechnical consultant should comment on the presence of sheared bedding and fracture zones identified in the exploratory borings within the "undisturbed" formational 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. Consultant's 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 the 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. Our Additional Comment: Our original comment focused on several key issues, and the consultant's response does not address those issues. From the consultant's response it is clear that our comment was misunderstood. Perhaps it might be helpful to take our review comment one step at a time. One issue included in our original comment referred to Section D-D' and basically requested that the consultant comment on how a landslide could have occurred in a slope that had a calculated safety factor of 1.94. Another issue in our original comment related to the assumed strength parameters used in the stability analyses. The consultant has indicated in their response that it City of Carlsbad - La Costa Condominiums Contract No. 148513.30 is their opinion that the assumed shear strength parameters are reasonable. We do not agree for the following reasons: • In our original comment we noted that laboratory strength testing of the various units does not appear to have been performed by the project geotechnical consultant. • In our original comment we noted that the assumed strength parameters are significantly higher than those used by American Geotechnical, Inc. in a geotechnical report for the adjacent project to the west (La Costa De Marbella). • The strength parameters for the various units are also higher than those used in the project report by Leighton and Associates (the report that the consultant is updating). Since the consultant has stated on several occasions that they are relying on the findings and conclusions of the report by Leighton and Associates for other aspects of the project, and since the strength parameters in the Leighton report are lower than those assumed by the consultant in their analysis, it is not clear why higher values are used and how they can be justified. Our original comment requested that the consultant identify the origin or basis of the assumed strength parameters. If the basis for the parameters is not the previously referenced reports, or from actual laboratory testing then the consultant's opinion that the strength parameters are "reasonable" has no factual basis. The final issue in our original comment related to sheared bedding and sheared fracture zones identified by Leighton and associates in the "undisturbed" formational materials. That comment is still applicable, and is reiterated as follows: 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. It is our opinion that the project geotechnical consultant should comment on the presence of sheared bedding and fracture zones identified in the exploratory borings within the "undisturbed" formational 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. 17. Original Review Comment: 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 could potentially affect off-site properties. It does not appear that these failure surfaces will be removed during remedial grading and it City of Carlsbad - La Costa Condominiums Contract No. 148513.30 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. Consultant's 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. Our Additional Comment: We have attached a copy of a portion of Plate 2 from the revised update report. Although Plate 2 of the revised update report is dated the same as the original update report (May 2007), it appears that the particular portion of Cross Section that is the subject of our original comment remains unchanged from the update report. Our original comment regarding Cross Section D-D' remains applicable with the provision that if the queried surfaces indicated are not potential failure surfaces, please provide a description as to what is being depicted by them on the cross section. 19. Original Review Comment: 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. 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. Consultant's Response: It is anticipated that the backcut will be below the level of the fill. In our opinion we do not believe 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. Our Additional Comment: The consultant's response generally addresses or original comment. However, since no subsurface exploration was performed in that area, it is not clear how the consultant knows that the backcut will be below the level of fill. In our opinion, there is a potential for undermining of the existing off site fill slopes during excavation of the back cut. This condition may require temporary support for the adjacent slopes and structures. 22. Original Review Comment: Appendix C of the project geotechnical report presents the results of laboratory tests. Review of the test results indicate that 8 City of Carlsbad - La Costa Condominiums Contract No. 148513.30 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 be identified. It is our opinion that the geotechnical consultant should comment whether the laboratory tests on these three samples provide the basis for their investigation and evaluation of the engineering properties of the onsite soils. Consultant's Response: The three bulk samples were taken at representative locations throughout the site and are representative of the soil types present at the site and representative of the soil types to be used during grading. Our Additional Comment: The consultant has not addressed our original comment relating to identifying the origins of the samples, (i.e. boring or test pit number and depth). 25. Original Review Comment: 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 ratio 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. Consultant's 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. Our Additional Comment: The response to our original comment 24 indicates that the colluvium and approximate depth (five feet) were identified during field mapping of side scarps in a small surficial slump located in this area. The presence of the surficial slump would seem to indicate potential instability of the colluvial layer. Colluvium is generally known to be relatively weak and prone to failures (such as surficial slumping and mudslides). It has been our experience that clayey colluvial materials generally would not have an acceptable calculated safety factor when perched on a 1.5 to 1 (horizontal to vertical) slope. It remains 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. The consultant's response indicates that areas with potentially unstable colluvium identified during remedial grading will be removed and replaced with compacted fill. We have two comments regarding this response: City of Carlsbad - La Costa Condominiums Contract No. 148513.30 The consultant should specify the criteria that will be used to determine if colluvium encountered during grading is potentially unstable. The consultant should describe the process of removing and replacing colluvium with compacted fill on a slope (i.e. Is a stability fill created, or would a "sliver fill" be created). 26. Original Review Comment: 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. Consultant's 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' Our Additional Comment: Our original comment has not been addressed. 29. Original Review Comment: 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. Consultant's 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 failures 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 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. 10 City of Carlsbad - La Costa Condominiums Contract No. 148513.30 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 retaining structures constructed at the toe of slope could be considered. To date, we are not aware of any on-site improvements within the site design drawings for this possibility. Our Additional Comment: From the consultant's response it is not clear if the consultant is recommending construction of retaining devices to safeguard the proposed development from potential damage due to failure of an off-site slope. The consultant has identified the potential for damage due to failure of an off-site slope. The consultant should clearly state whether or not retaining devices are recommended. If such devices are recommended, they should be shown on the project plans. 33. Original Review Comment: 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. 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 recommended 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 the 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. Consultant's 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. Our Additional Comment: The response adequately addresses our original comment. However, the project grading plans provided for our review had not been revised to show the recommended subdrains. 11 City of Carlsbad - La Costa Condominiums Contract No. 148513.30 34. Original Review Comment: 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 at 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. Consultant's Response: It is our understanding that the detention basin will be lined which will limit any seepage that could impact the project. Our Additional Comment: The response generally addresses our original comment. However, our review of the project grading plans does not indicate a lined basin. We request that the project geotechnical consultant be more specific regarding the nature of the basin lining. In addition, the details of the basin lining should be included in the project plans. 39. Original Review Comment: 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 that soils with very low to low expansion potential will be imported to the site to construct the building pads, it should be stated in the project geotechnical report. Consultant's 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 necessary to provide specific design parameters for a variety of assumed conditions. Once grading is complete appropriate design parameters can be provided. 12 City of Carlsbad - La Costa Condominiums Contract No. 148513.30 Our Additional Comment: The response does not address our original comment. The consultant has identified expansive soils at the site. The consultant has stated 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 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). In the project geotechnical report, the consultant has provided recommendations that address foundation design and construction on soils that are non-expansive or have low-expansivity. Based on our review of the geotechnical documents prepared for the project site, this is a condition that clearly does not exist at the site, and probably will not exist subsequent to site grading. For the project consultant not to provide recommendations for design and construction of foundations for expansive soil conditions is, in our opinion, below the ordinary standard of practice in the geotechnical industry. 48. Original Review Comment: 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. Consultant's Response: The word aerial has been changed to areal on line 2. page 15 of the geotechnical report. Our additional comment: Our original comment remains pertinent and applicable to page 16 line 18 and page 20 line 8. 13 City of Carlsbad - La Costa Condominiums Contract No. 148513.30 The opportunity to be of service to the City of Carlsbad on this project is appreciated. If you have any questions regarding our review, please contact our office. Respectfully submitted, Testing Engineers - San Diego, Inc. A Bureau Veritas Company Jene Custenbo Senior Engineering Geol GC/VO:cs Van Olin, PE, GE Principal Geotechnical En Distribution: (5) Addressee 14 Possible limits of grading Projected Projected 40' E 48' E Projected 22'E Projected 9'WProjected 30'E / Sandstone / Silt stone roposed buttress Proposed grade Sandstone Qaystone bed *T.D.=6<Fm=40' Claystone bed T,D.=30'" T.D.=30' T.D.=40' Sandstone / Silt stone Sandy siltstone / Silty sandstone Tsa Cross-section D-D' Approximate Scale: 1" = 40'