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Home My WebLink AboutCT 11-01; ROBERTSON RANCH WEST VILLAGE; RESPONSE TO 2ND REVIEW; 2012-01-03(oiy!-7 12 Geotechnical. Geologic Coastal • Environmental 5741 Palmer Way • Carlsbad, California 92010 (760) 438-3155 • FAX (760) 931-0915 • www.geosolsEnc.corn January 3, 2012 Jp6145-A7/E1 -Sc lIVED FEB 212013 LAND DVEL °PMENT ENGINEERING Attention: Ms. Teresa Sousa, Mr. John Buller, and Mr. Erik Pfahler Subject: Response to 2nd Review for CDP 11-10/HDP 11-02/SUP 11 -03/HMP 11-04 - ECR Southbound Widening view Comments (Engineering), Rancho Costera Project (Robertson Ranch West Village), Carlsbad, San Diego County, California Oct 17 6145-A5 Dear Ms. Sousa, Mr. Buller, and Mr. Pfahler: In accordance with your request and authorization, GeoSoils, Inc. (GSI) has reviewed Comments No. 2 and 8 (and one page of redline notes), included in the latest City review comments (City of Carlsbad, 2011 a) regarding the application for a Land Use Permit in the City of Carlsbad, with respect to the widening of a portion of southbound El Camino Real. The purpose of this letter is to address the geotechnical aspects of the review document. The scope of services has included a review of the referenced documents and plans (see the Appendix), engineering/geologic analysis, analysis of data (including revised details; "Detail - Bioretention Area At El Camino Real Median," "Detail - Bioretention Area At El Camino Real Parkways," and "Detail - Fully Lined Flow-Through Planter," all dated December 27, 2011 or later), and preparation of this review response. The bioretention details reviewed are in general accordance with the GSI recommendations (i.e., 6-inch deepening of curb below the pavement section, adjoining the bioretention areas in the median, and metal grate for clean-out, filter fabric impermeable liners, etc. Based on the latest details provided, permeable payers are porposed on grade adjoining the bioretention areas, with 12-inch deepening of the curbs below the pavement section along the median where the curb abuts the payers.. REVIEW RESPONSE For ease of review, the pertinent City review comments are repeated below in italics, followed by GSl's response Shapell Homes 8383 Wilshire Boulevard, Suite 700 Beverly Hills, California 90211 City Engineering Review Comment No. 2 "Revise the preliminary Storm Water Management Plan (SWMP) to move the post-construction bmp 's for 'Z' street to the preliminary storm water management plans for Robertson Ranch West Village (CT 11-01). Provide soil engineer recommendations for the bio filtration basins proposed as part of this project, especially those located in the medians and located at the toe of slopes. Add these recommendations to the preliminary SWMP. Verily if the native soils can accommodate the infiltration rates or if impermeable liners will be required. If impermeable liners are required, revise the sizing of basins to match raised planters per the bmp calculator and verify the increased area can be met for the project. Address the redlines, revise the document and return the redlines along with 2 new preliminary S WMP's with the next submittal." GSI Response to Review Comment No. 2 See discussion following City Engineering Review Comment No. 8 City Engineering Review Comment No. 8 "Provide soil engineer recommendations on the proposed bioretention areas for this project. Clarify whether onsite soils have adequate soil percolation rates for the proposed bioretention areas. Clarify if a liner (or other measure) is required for bioretention areas, especially for those located in the median or at the toe of slopes." GSI Response to Review Comments No. 2 and 8 In preparation of our supplemental report for the development of Rancho Costera (GSI, 2011d), the infiltration rate of water into existing fill, and sedimentary bedrock was evaluated using a double ring infiltrometer. As indicated in GSI (2011), an infiltration rate of 0.18 inch/hour for existing artificial fill, and between 0.16 and 0.06 inch/hour for the bedrock materials was obtained at a depth of approximately ±3 feet, and may be utilized for design of the proposed detention systems. According to the County of San Diego Guidelines for Determining Significance Surface Water Quality (2007), these infiltration rates marginally fall into Hydrologic Soil Group: Soil Type C (Soils having slow infiltration rates); however, are on the border of Soil Type D (High runoff potential). Accordingly, GSI has assumed the soil at the site is Soil Type D, in light of potential soil variability (clay content, cementation, etc.). In consideration of overall slope stability, bioretention structures located at the tops or toes of slopes should be provided with an impermeable liner. Due to the presence of expansive soils, and/or potential for subgrade failure in pavement areas due to elevated soils moisture contents/yielding, the saturation of soils adjacent to bioretention areas located within the median or shoulder of ECR will result in a greater potential for distress to adjacent pavements, flatwork, etc. As such, impermeable liners and/or thickened Shappell Homes W.O. 6145-A7/E1-SC Coastal Dev. Permit ECR, Carlsbad GeoSoils, Inc. January 3, 2012 File: e:\wpl2\6100\6145a7e1.rt2r Page 2 edges/cut off walls for adjoining curbs should be provided. Mirafi 140N or equivalent, should be provided in bioretention areas between soil backfill of contrasting permeabilities (i.e., sand/compost mix and underlying Class II gravel). The details entitled "Detail - Bioretention Area At El Camino Real Median," and "Detail - Fully Lined Flow-Through Planter," dated December 27, 2011 generally appear to meet these conditions. General Design/Construction Recommendations Based on site work performed to date, site soils are generally considered to be relatively fine grained, and are considered to posses a relatively low permeability (i.e., low percolation/infiltration rate). Thus, an appropriate factor of safety, per the controlling authorities requirements, should also be incorporated into the design calculations. The following comments and/or recommendations should also be considered during design (structural and civil), and implementation of the proposed detention/infiltration and/or other BMP systems onsite: As with any BMP detention/infiltration or bioretention device, localized ponding and groundwater seepage should be considered. Thus, a designed system with an impermeable liner at the bottom, and the sides, is recommended. Similarly, as with any BMP detention/infiltration or bioretention device, proper maintenance and care will need to provided. Best management maintenance practices should be followed at all times, especially during inclement weather. Should regular inspection and/or required maintenance not be performed, the potential for malfunctioning of the detention/infiltration systems will increase. Areas adjacent to, or within, the detention/infiltration or bioretention system that are subject to inundation should be properly protected against scouring, undermining, erosion, and soil expansion, in accordance with the recommendations of the design civil engineer. All inlets, outlets and piping from these temporary drainage features should be properly backfilled and installed per City standards. Provisions for the maintenance of bioretention/detention areas from any siltation, debris, and/or overgrown vegetation (i.e., root systems) should be provided. An appropriate regular and periodic inspection and maintenance schedule will need to adopted and provided to all interested/affected parties. Any proposed utility backfill materials located within the proposed area of the BMP may become saturated. This is due to the potential for piping, water migration, and/or seepage along the utility trench line backfill. If utility trenches cross and/or are proposed near the detention/infiltration systems, cut-off walls or other water barriers will likely need to be installed to mitigate the potential for piping and excess water entering the utility backfill materials and being transported into and saturating Shappell Homes W.O. 6145-A7/E1-SC Coastal 0ev. Permit ECR Carlsbad GeoSoils, Inc. January 3, 2012 File:e:\wpl2\6100\6145a761 .rt2r Page 3 subgrade areas nearby. Any nearby planned utilities, either adjacent or underlying, should utilize a geotextile filter fabric if open graded gravels are utilized, in order to reduce the potential for piping and associated distress. Any proposed footings and/or foundations should maintain a minimum of 1:1 horizontal to vertical (h:v) distance down and away from the base of the footing and/or foundation to any adjacent detention/infiltration or bioretention system. If a 1:1 (h:v) down and away distance cannot be maintained, a deepened footing and/or foundation will be required. The landscape architect should be notified of the location of the proposed detention/infiltration or bioretention system(s). If landscaping is proposed over the detention/infiltration or bioretention system, consideration should be given to the type of vegetation chosen and their potential effect upon subsurface improvements (i.e., some trees/shrubs will have an effect on subsurface improvements with their extensive root systems). The potential for surface flooding, in the case of detention/infiltration or bioretention system blockage, should be evaluated by the design engineer. Without periodic maintenance, these temporary detention basins may overtop, flood and/or saturate nearby soils, and/or cause erosion/undermining of slopes and structures. Furthermore, slopes in areas susceptible to flooding/inundation should be protected, as indicated previously. As the infiltration testing conducted for this project is specific to the anticipated nature of the artificial fill and bedrock materials encountered onsite, any changes to the design of the BMP's and/or estimated size or depth of the system, should be reviewed by this office. Depending upon the nature of any changes, proposed depth of the systems, and the requirements of the reviewing entity, additional infiltration testing may be warranted. Alternatively, bioretention areas may utilize Soil Type D, with lined bottoms and sides. Final grading and improvement plans, as well as structural foundation plans, should be submitted to this office for review and comment, as they become available, to reduce any misunderstandings between the preliminary recommendations presented herein. If project designs are found to differ substantially from those stated herein, appropriate recommendations would be offered at that time. Shappel I Homes W.O. 6145-A7/E1-SC Coastal Dev. Permit ECR Carlsbad GeoSoils, Inc. January 3, 2012 File: e:\wpl2\6100\6145a7e1 .rt2r Page 4 PLAN REVIEW Final project plans should be reviewed by this office prior to construction, so that construction is in accordance with the conclusions and recommendations of the reports for the project. Based on our review, supplemental recommendations and/or further geotechnical studies may be warranted. LIMITATIONS The materials encountered on the project site and utilized for our analysis are believed representative of the area; however, soil and bedrock materials vary in character between excavations and natural outcrops. Site conditions may vary due to seasonal changes or other factors. Inasmuch as our study is based upon our review and engineering analyses and laboratory data, the conclusions and recommendations are professional opinions. These opinions have been derived in accordance with current standards of practice, and no warranty, either express or implied, is given. Standards of practice are subject to change with time. GSI assumes no responsibility or liability for work or testing performed by others, or their inaction; or work performed when GSI is not requested to be onsite, to evaluate if our recommendations have been properly implemented. Use of this report constitutes an agreement and consent by the user to all the limitations outlined above, notwithstanding any other agreements that may be in place. In addition, this report may be subject to review by the controlling authorities. Thus, this report brings to completion our scope of services for this portion of the project. Shappell Homes W. 0. 6145-A7/E1-SC Coastal Dev. Permit ECR Carlsbad GeoSoils, Inc. January 3, 2012 File:e:\wpl2\6100\6145a7e1.rt2r Page 5 The opportunity to be of service is sincerely appreciated. If you should have any questions, please do not hesitate to contact our office. Respectfully submitt dQAL GeoSoils, Inc. cYf c oi n ohn an Ii YEongineering ç'-) Geologist,CEG--t340 RGC/J PF/ATG/jh N. 3E23 0 - Andrew T. Gu tell Geotechnical Engineer, GE 2320 Distribution: (1) Addressee (1 via US Mail and 1 via email) (3) O'Day Consultants Inc. - Mr. George O'Day and Mr. Keith Hansen (3 wet signed and 1 via email) Shappell Homes Coastal Dev. Permit ECA, Carlsbad GeoSoils, Inc. Fi1e:e:\wp12\6100\6145a7e1 .rt2r W.O. 6145-A7/E1-SC January 3, 2012 Page 6 APPENDIX REFERENCES Aquatext, Scouring Velocity, The Free On-line Aquaculture Dictionary, http://www.aguatext.comltables/scouring.htm. Amimoto, Perry Y., 1981, Erosion and Sediment Control Handbook: Division of Mines and Geology, Department of Conservation, May. Carlsbad, City of, 2011 a, 2 nd Review for CDP 11 -10/HDP 11-02/SUP 11 -03/HMP 11 -04 - ECR Southbound Widening, dated November 2. 2011b, 2" review for MlP 02-03(C)CT 11-01/HDP 11-01/SUP 11 -02/HMP 11-03 - Robertson Ranch West Village, dated October 7. County of San Diego, Department of Public Works, Department of Planning and Land Use, 2007, Guidelines for determining significance surface water quality, dated July 30. GeoSoils, Inc., 2011 a, Geotechnical review and response to 2nd City review comments (engineering), Rancho Costera Project (Robertson Ranch West Village), Carlsbad, San Diego County, California, W.O. 6145-A5, dated October 17. 2011 b, Geotechnical Review of coastal development permit plans for El Camino Real Rancho Costera Project, Carlsbad, San Diego County, California, W.O. 6145-A4, dated September 12. 2011c, Geotechnical response to 18t review for MP 02-03 (C)/CT 11-01/HDP 11-0101/SUP 11-02/HMP 11-03-Robertson Ranch West Village, dated June 23, 2011, by Planning Division, City of Carlsbad," W.O. 6145-A4-SC, dated August 17. 2011 d, Supplement to the updated geotechnical investigation for Rancho Costera (formerly Robertson Ranch West Village), Carlsbad, San Diego County, California, W.O. 6145-Al-SC, dated June 6. 2011 e, Geotechnical investigation for the planned improvement of El Camino Real, between Cannon Road and Tamarack Avenue, Rancho Costera (formerly Robertson Ranch West Village), Carlsbad, San Diego County, California, W.O. 6145-E-SC, dated May 11. 2010, Updated geotechnical investigation for Robertson Ranch West Village, Carlsbad, San Diego County, California, W.O. 6145-A-SC, dated October 11. Morgan, R.P.C., 2005, Soil Erosion and Conservation, Third Edition, Blackwell Publishing GeoSoils, Inc. Toy, T.J., Foster, G.R., and Renard, K.G., 2002, Soil Erosion, processes, prediction, measurement, and control, John Wiley & Sons, pub. O'Day Consultants, Inc., 2011a, Coastal development permit for El Camino Real Rancho Costera, Job No. 10-1307, Sheets 1 through 11, 40-scale plans, dated January, printed November 9 and November 11. 2011 b, Vesting master tentative map for Rancho Costera, Job No. 101307, Sheets 1 through 20, 40-scale plans, revised October 24, printed October 25-28. United States Geological Survey (USGS), 1984, Physical basis and potential estimation techniques for soil erosion parameters in the precipitation-runoff modeling system (PRMS), Water-Resources Investigations Report 84-4218. Shappell Homes Appendix FiIe:e:\wpl2\6100\6145a7e1 .rt2roDRAFT GeoSoils, Inc. Page 2