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Home My WebLink AboutCDP 2020-0017; SAREM RESIDENCE; Geotechnical Commentary on Soil Infiltration Characteristics, Proposed Two-Story Residence; 2020-05-28PETRA SOLID AS A ROCK GEOSCIENCE5"'0 ENGINEERS + GEOLOGISTS + ENVIRONMENTAL SCIENTISTS MR. SCOTT SAREM 5796 Armada Drive, Suite 375 Carlsbad, California 92008 May 28, 2020 J.N. 20-155 Subject: Geotechnical Commentary on Soil Infiltration Characteristics, Proposed Two-Story Residence, 4005 Skyline Road, City of Carlsbad, San Diego County, California References: Petra Geosciences, Inc., 2020, Preliminary Geotechnical Evaluation, Proposed Two-Story Residence, 4005 Skyline Road, APN 207-072-17-00, City of Carlsbad, San Diego County, California, J.N. 18-308, dated May 11. Toal Engineering, Inc., 2020, Preliminary Grading Plan for Sarem Residence, 4005 Skyline Road, Carlsbad, Drawing Sheet 1, plot-dated April 27. Dear Mr. Sarem: Per your request and to aid in design efforts by Toal Engineering (Toal), Petra Geosciences, Inc. (Petra) is providing an opinion as to the percolation characteristics of the on-site native soils at the subject site. Based on the plan prepared by Toal, a bio-retention basin is proposed in the west portion of the lot that will provide limited infiltration of runoff water with the majority being pumped to storm drain discharge. In addition, Toal indicated that pervious paving is proposed for areas of the site. This summary letter provides clarification for questions raised regarding depth to groundwater, United States Department of Agriculture- Natural Resource Conservation Service (NRCS) soil type, infiltration characteristics and use of pervious paving. Groundwater Our previous report indicated that regional groundwater is not expected to be within 50 feet of the ground surface along the upper portions of this local elevated terrace (Petra, 2020). To clarify further, groundwater is not anticipated to be within the upper 50 feet under the site at the lowest portion of the site and 80 feet at the highest portion of the site. NRCS Soil Type The NRCS soil survey website (https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx) identifies the onsite soils to consist of the Marina loamy sand ofHydrologic Soil Group "B". Offices Strategically Positioned Throughout Southern California RIVERSIDE COUNTY OFFICE 40880 County Center Drive, Suite M, Temecula, CA 92591 T: 951.600.9271 For more information visit us online al www.petra-inc.com MR. SCOTT SAREM 4005 Skyline Road. I Carlsbad Infiltration Characteristics May 28, 2020 J.N. 20-155 Page 2 We understand that the proposed bottom of infiltration is approximately 5 to 6 feet below the proposed design grades and is to be located between the proposed lower pool patio and the sport court, which is located about 20 to 30 feet from the property line. Without specific infiltration testing, it is our understanding from Toal that the City of Carlsbad provides a default value basic analysis infiltration rate of 0.2 inches per hour. The paralic or terrace deposits characterized by sandstone bedrock are expected to be exposed at the bottom of the bio-retention basin. It should be assumed that there would be variability of the infiltration rate, likely due to natural variations in soil density and gradation. The analysis infiltration value of 0.2 inches per hour is acceptable design value. Pervious Paving Pervious paving is considered to be a form of infiltration system. As with some infiltration systems, localized ponding of water and subsurface groundwater seepage should be anticipated. Geotechnical engineering practice discourages intentional ponding of water, particularly adjacent to slopes, building foundations, and settlement-or moisture-sensitive hardscape. Infiltration systems are not recommended within roughly 8 feet of building foundations or utility trenches. Problems with ground saturation in structures, such as excessive moisture vapor transmission through interior slabs, may require repairs to either block or drain excessive moisture. Should you have any questions, please do not hesitate to call. Respectfully submitted, PETRA GEOSCIENCES, INC. Jim Larwood Principal Geologist CEG 1897 JL/GRW/lv Attachment: Form 1-8 Distribution Addressee (electronic) Grayson R. Walker Principal Engineer GE 871 Mr. Caleb Rios, Toal Engineering (electronic) W:\2020-2025\2020\JD00.0-155 Scott Sarem (4005 Skyl~ lt.oed. Carllbad)\Reports\]D--JSS I JOPeroolation Letter.docx •PETRA GEOSCIENCES.,._ "'" s1i§r:i1:x2.o SOL/0 AS A ROCK Appendix I: Forms and Checklists Categorization of Infiltration Feasibility Form I-8 Condition Part t -Full Infiltration Feasibility Screening Criteria Would infiltration of the full design volume be feasible from a physical perspective without any undesirable consequences that cannot be reasonably mitigated? Criteria Screening Question Is the estimated reliable infiltration rate below proposed facility locations greater than 0.5 inches per hour? The response to this Screening Question shall be based on a comprehensive evaluation of the factors presented in Appendix C.2 and Appendix D. Yes No No Provide basis: The site will be graded such that compacted fill will cover the majority of the area as well as underlain by relatively impermeable paralic terrace deposits. The compacted fill and paralic terrace deposits possess little or no void space and therefore the infiltration rate is considered less than 0.5 inches per hour. Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative discussion of study/ data source applicability. 2 Can infiltration greater than 0.5 inches per hour be allowed without increasing risk of geotechnical hazards (slope stability, groundwater mounding, utilities, or other factors) that cannot be mitigated to an acceptable level? The response to this Screening Question shall be based on a comprehensive evaluation of the factors presented in Appendix C.2. Provide basis: Infiltration greater than 0.5 inches per hour would result in horizontal infiltration, causing seepage problems and would be detrimental to the project. No Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative discussion of study/ data source applicability. 1-3 February 2016 Appendix I: Forms and Checklists Criteri a 3 Form 1-8 Page 2 of 4 Screening Question Can infiltration greater than 0.5 inches per how: be allowed without increasing risk of groundwater contamination (shallow water table, storm water pollutants or other factors) that cannot be mitigated to an acceptable level? The response to this Screening Question shall be based on a comprehensive evaluation of the factors presented in Appendix C.3. Provide basis: Yes No Yes Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative discussion of study/ data source applicability. 4 Can infiltration greater than 0.5 inches per how: be allowed without causing potential water balance issues such as change of seasonality of ephemeral streams or increased discharge of contaminated groundwater to surface waters? The response to this Screening Question shall be based on a comprehensive evaluation of the factors presented in Appendix C.3. Provide basis: Yes Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative discussion of study/ data source applicability. Part 1 Result * If all answers to rows 1 -4 are ''Yes" a full infiltration design is potentially feasible. The feasibility screening category is Full Infiltration If any answer from row 1-4 is "No", infiltration may be possible to some extent but would not generally be feasible or desirable to achieve a "full infiltration" design. Proceed to Part 2 No "To be completed using gathered site information and best professional judgment considering the definition of MEP in the MS4 Permit. Additional testing and/ or studies may be required by the City to substantiate findings. 1-4 February 2016 Appendix I: Forms and Checklists Form 1-8 Page 3 of 4 Part 2 -Partial Infiltration vs. No Infiltration Feasibility Screening Criteria Would inf"iltration of water in any appreciable amount be physically feasible without any negative consequences that cannot be reasonably mitigated? Criteria 5 Screening Question Do soil and geologic conditions allow for infiltration in any appreciable rate or volume? The response to this Screening Question shall be based on a comprehensive evaluation of the factors presented in Appendix C.2 and Appendix D. Yes Yes Provide basis: The site will be graded such that compacted fill will cover the majority of the area as well as be underlain by relatively impermeable paralic terrace deposits. The compacted fill and paralic terrace deposits possess little or no void space. Areas at the site could be designed for infiltration rates less than 0.5 inches per hour. No Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative discussion of study/ data source applicability and why it was not feasible to mitigate low infiltration rates. 6 Can Initltration in any appreciable quantity be allowed without increasing risk of geotechnical hazards (slope stability, groundwater mounding, utilities, or other factors) that cannot be mitigated to an acceptable level? The response to this Screening Question shall be based on a comprehensive evaluation of the factors presented in Appendix C.2. Yes Provide basis: The site will be graded such that compacted fill will cover the majority of the area as well as underlain by paralic terrace deposits. The compacted fill and paralic terrace deposits possess little or no void space. Areas at the site could be designed for inftltration without increasing risk of geotechnical hazards . Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative discussion of study/ data source applicability and why it was not feasible to mitigate low infiltration rates. 1-5 February 2016 . . . Appendix I: Forms and Checklists Criteria 7 Form I-8 Page 4 of 4 Screening Question Can Infiltration in any appreciable quantity be allowed without posing significant risk for groundwater related concerns (shallow water table, storm water pollutants or other factors)? The response to this Screening Question shall be based on a comprehensive evaluation of tbe factors presented in Appendix C.3. Provide basis: Yes No Yes Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative discussion of study/data source applicability and why it was not feasible to mitigate low infiltration rates. 8 Can infiltration be allowed without violating downstream water rights? The response to this Screening Question shall be based on a comprehensive evaluation of the factors presented in Appendix C.3. Provide basis: Yes Summarize findings of studies; provide reference to studies, calculations, maps, data sources, etc. Provide narrative discussion of study/ data source applicability and why it was not feasible to mitigate low infiltration rates. Part 2 Result* If all answers from row 5-8 are yes then partial infiltration design is potentially feasible. The feasibility screening category is Partial Infiltration. If any answer from row 5-8 is no, then infiltration of any volume is considered to be infeasible within the drainage area. The feasibility screening category is No Infiltration. Partial Infiltration qo be completed using gathered site information and best professional judgment considering tbe definition of MEP in tbe MS4 Permit. Additional testing and/or studies may be required by the City to substantiate findings. 1-6 February 2016