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HomeMy WebLinkAboutGPA 2017-0003; OAKMONT OF CARLSBAD; TRANSMITTAL OF GEOTECHNICAL INFORMATION; 2017-09-25GEOCON INCORPORATED GEOTECHNICAl ■ Project No. 06442-32-29 September 25, 2017 Oakmont Senior Living ENVIRONMENTAL ■ 9240 Old Redwood Highway, Suite 200 Windsor, California 95492 Attention: Ms. Hannah Daugherty Subject: TRANSMITTAL OF GEOTECHNICAL INFORMATION CARLSBAD OAKS NORTH LOT I CARLSBAD, CA LIFO RN IA MATER I ALSO References: 1. Final Report of Testing and Ohservation Services During Site Grading, Carlsbad Oaks North Business Park, Phase 1, Lots 1 through 9, Car/shad, California, prepared by Geocon Incorporated, dated August 30, 2006. 2. Addendum to Final Report of Testing and Ohservation Sen'ice:-, During Site Grading, Carlsbad Oaks North Bw;iness Park -Phase I, Lot 1, Carhbad, California, prepared by Geocon Incorporated, dated October 30, 2008. 3. Update Geotechnical Correspondence, Carlsbad Oaks North Lot I, Carlsbad, Cahfornia, prepared by Geocon Incorporated, dated June 28, 2017 (Project No. 06442-32-29). 4. Preliminaty Grading and Drainage Plan, Oakmont of Carlsbad, Lot I of Tract No. 14926, prepared by Alliance Land Planning & Engineering, Inc., dated June 30, 2017. Dear Ms. Daugherty: In accordance with your request, Gcocon Incorporated has provided geotechnical engineering services on the subject project. Specifically, we have performed two in-situ permeability tests to aid in evaluating the on-site storm water BMP design. The following information is provided to support storm water BMP design in accordance with the 2016 City of Car/shad Storm Water Standards. STORM WATER MANAGEMENT INVESTIGATION We understand storm water management devices arc being proposed in accordance with the 2016 City of' Car/shad Storm Water Standards. If not properly constructed, there is a potential for distress to improvements and properties located hydrologically down gradient or adjacent to these devices. 6960 Flanders Drive ■ San Diego, California 92121-2974 ■ Telephone 858.558.6900 ■ Fax 858.558.6 \59 Factors such as the amount of water to be detained, its residence time, and soil permeability have an important effect on seepage transmission and the potential adverse impacts that may occur if the storm water management features arc not properly designed and constructed. We have not performed a hydrogeological study at the site. If infiltration of storm water runoff occurs, downstream properties may be subjected to seeps, springs, slope instability, raised groundwater, movement of foundations and slabs, or other undesirable impacts as a result of water infiltration. Hydrologic Soil Group The United States Department of Agriculture (USDA), Natural Resources Conservation Services, possesses general information regarding the existing soil conditions for areas within the United States. The USDA website also provides the Hydrologic Soil Group. Table I presents the descriptions of the hydrologic soil groups. If a soil is assigned to a dual hydrologic group (AID, BID, or CID), the first letter is for drained areas and the second is for undrained areas. TABLE 1 HYDROLOGIC SOIL GROUP DEFINITIONS Soil Grnup Soil Group Definition Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These A consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of B moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission. Soils having a slow infiltration rate when thoroughly wet These consist chiefly of soils C having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission. Soils having a very slow infiltration rate (high runoff p9tential) when thoroughly wet. These D consist chiefly of clays that have a high shrink-swell potential. soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission. The subject sheet-graded pad is underlain by compacted fill placed above the Point Loma formation. After completion of the proposed grading operations, the property would consist of compacted fill over Pont Loma Formation. The compacted fill and formational materials should be classified as Soil Group D. In addition, the USDA website also provides an estimated saturated hydraulic conductivity for the existing soil. Table 2 presents the information from the USDA website. The Hydrologic Soil Group Map presents output from the USDA website showing the limits of the soil units. The USDA information is presented in Appendix B. Project No. 06442-32-29 - 2 -September 25, 2017 TABLE2 USDA WEB SOIL SURVEY -HYDROLOGIC SOIL GROUP Map Unit Approximate Hydrologic ksn of Most Map Unit Name Symbol Percentage Soil Group Limiting Layer of Property (Inches/ Hour) Cieneba coarse sandy loam Cili2 44 D 1.98 -5.95 Huerhuero loam H,D 56 D 0.00-0.06 In-Situ Testing We performed two Soil Moisture, Inc. Aardvark Permeameter tests at the locations shown on the attached Site Plan, Figure I. Test P-1 was located in the bottom of an existing basin. Some standing water was observed in a portion of this basin. Test P-2 was hand angered until practical refusal was encountered on the Point Loma Formation contact. The test borings were 4 inches in diameter. The results of the tests provide parameters regarding the saturated hydraulic conductivity and infiltration characteristics of on-site soil and geologic units. Table 3 presents the results of the field saturated hydraulic conductivity/infiltration rates obtained from the Aardvark Penneameter tests. The data sheets are presented in Appendix A. We applied a feasibility factor of safety of 2 to the test results. Soil infiltration rates from in-situ tests can vary significantly from one location to another due to the non-homogeneous characteristics inherent to most soil. TABLE 3 FIELD PERMEAMETER INFILTRATION TEST RESULTS Geologic Test Depth Field-Saturated Field Test No. Hydraulic Conductivity, Infiltration Rate Unit (feet, below grade) k,ar (inch/hour) (inch/hour) P-1 Qcf 2.4 0.0002 0.0001 P-2 Kp 3.75 0.002 0.001 STORM WATER MANAGEMENT CONCLUSIONS The Site Plan, Figure 1, presents the existing property and the locations of the in-situ infiltration test locations. Soil Types Compacted Fill -Compacted fill exists across the property. The proposed storm water BMP's will be founded in compacted fill placed above very dense formational materials. The compacted fill is comprised of sandy/clayey silt. The fill has been or will be compacted to a dry density of at least 90 percent of the laboratory maximum dry density. In our experience, compacted fill does not possess Project No. 06442-32-29 -3 -Sqitt:mbt:r 25, 2017 infiltration rates appropriate for infiltration BMP's, as demonstrated by the in-situ testing. Hazards that occur as a result of fill soil saturation include a potential for hydro-consolidation of the granular fill soils and/or swelling of the expansive soils, long-term fill settlement, differential fill settlement, and lateral movement associated with saturated fill relaxation. The potential for lateral water migration to adversely impact existing or proposed structures, foundations, utilities, and roadways, is high. Therefore, full and partial infiltration should be considered infeasible. Section D.4.2 of the 2016 Srorm Water Standards (SWS) provides a discussion regarding fill materials used for infiltration. The SWS states: • For engineered fills, infiltration rates may still be quite uncertain due to layering and heterogeneities introduced as part of construction that cannot be precisely controlled. Due to these uncertainties, full and partial infiltration should be considered geotechnically infeasible and liners and subdrains should be used in areas where infiltration BMP's are founded in compacted fill. • Where possihle, infiltration BMPs on fill material should be designed such that their infiltrating sw:face extends into native soils. The underlying formation below the compacted fill is expected between 5 to 10 feet below proposed finish grades after remedial grading is performed. Full and partial infiltration should be considered geotechnically infeasible within the compacted fill and liners and subdrains should be used. If the infiltration BMP's extended below the compacted fill, partial infiltration may be feasible. • Because (?f the uncertainty ofjill parameters as well as potential compaction of the native soils, an infiltration BMP may not he.feasible. Therefore, full and partial infiltration should be considered geotechnically infeasible and liners and subdrains should be used in the fill areas. • If the source (?f fill material is defined and this material is known to he of a granular nature and that the native soils he/ow are permeable and will not he highly compacted, infiltration through compacted fill materials may still be feasible. In this case, a project phasing approach could be used including thefO!lowing general steps,(!) collect samples.from areas expected to be used fbr fill, (2) remold samples to approximately the proposed degree of compaction and measure the saturated hydraulic conductivity of remolded samples using laboratory methods, (3) {f infiltration rates appear adequate fOr infiltration, then apply an appropriate factor of sa.fCty and use the initial rates fhr preliminary design, (4) fOllo;ving placemen/ r~ffi/1, conduct in-situ testing to refine design infiltration rates and adjust the design as needed. However, based on the discussion above, it is our opinion that infiltrating into compacted fill should be considered geotechnically infeasible and liners and subdrains should be used. Infiltration Rates The results of the unfactored infiltration rates (i.e. field saturated hydraulic conductivity) for Tests P-1 and P-2 were 0.0002 inches per hour (iph) and 0.002 iph, respectively. After applying a feasibility factor of safety of 2.0, the infiltration rates obtained for P-1 and P-2 are 0.0001 and 0.001 iph, respectively, The infiltration test results show the on-site soil permeability is variable across the site. A Project No. 06442-32-29 September 25, 2017 single design rate for an area could not be accurate based on the variability. Therefore, based on the results of the field infiltration tests, anticipated grading, and our experience, full and partial infiltration should be considered infeasible. The results of the permeability testing are presented in Appendix A. Groundwater Elevations Groundwater is expected to be encountered at depths greater than 100 feet below the site, therefore groundwater is not expected to be a factor. Groundwater mounding is caused when infiltration is allowed and the lateral hydraulic conductivity is relatively low causing an increase in the groundwater table. Groundwater mounding is not likely. Soil or Groundwater Contamination Based on review of the Geotracker website, no active cleanup sites exist on or adjacent to the subject site. In addition, we are not aware of any contaminated soils or shallow groundwater on the site that would preclude storm water infiltration. An environmental assessment was not part of our scope of work. Slopes Existing slopes exist on the perimeter of the property. lnfiltration of storm water adjacent to cut or fill slopes should be avoided. Fill slopes will exhibit instability if water is allowed to saturate the compacted fill. Cut slopes may exhibit daylight seepage. Storm Water Management Devices Based on the discussion above, both infiltration tests did not meet the minimum feasibility criteria for full or partial infiltration. To limit the adverse impacts of storm water infiltration, i.e. lateral water migration, daylight water seepage, etc., the design should include liners and subdrains. The impermeable liners should consist of a high-density polyethylene, HDPE, with a thickness of about 30 mil or equivalent Polyvinyl Chloride, PVC. The liner should surround the bottom and sides of the infiltrating surface and should extend slightly above the high water elevation. The subdrain should be perforated, installed near the base of the excavation, be at least 4-inches in diameter and consist of Schedule 40 PVC pipe. The final segment of the subdrain outside the limits of the storm water BMP should consist of solid pipe and connected to a proper outlet. Any penetration of the liner should be properly waterproofed. The devices should also be installed in accordance with the manufacturer's recommendations. Project No. 06442-32-29 . 5 . September 25. 2017 Storm Water Standard Worksheets The Storm Water Standard manual stipulates the geotechnical engineer complete the Categorization of" Infiltration Feasihili(v Condition (Worksheet C.4-1 or Form l-8) worksheet information to help evaluate the potential for infiltration on the property. A completed Form I-8 is presented in Appendix B. The regional storm water standards also have a worksheet (Worksheet D.5-1 or Form 1-9) that helps the project civil engineer estimate the factor of safety based on several factors. Table 4 describes the suitability assessment input parameters related to the geotechnical engineering aspects for the factor of safety determination. TABLE 4 SUITABILITY ASSESSMENT RELATED CONSIDERATIONS FOR INFILTRATION FACILITY SAFETY FACTORS Consideration High Medium Low Concern - 3 Points Concern - 2 Points Concern - 1 Point Use of soil survey maps or Use of well permeameter simple texture analysis to or borehole methods with Direct measurement with estimate short-term accompanymg localized (i.e. small- infiltration rates. Use of continuous boring log. scale) infiltration testing Direct measurement of Assessment Methods well penneameter or infiltration area with methods at relatively high borehole methods without localized infiltration resolution or use of accompanying continuous measurement methods extensive test pit boring log. Relatively (e.g., infiltrometcr). infiltration measurement sparse testing with direct Moderate spatial methods. infiltration methods resolution Predominant Soil Silty and clayey soils Loamy soils Granular to slightly Texture with significant fines loamy soils Highly variable soils Soil boring/test pits Soil bonng/test pits Sde Soil Variability indicated from site indicate moderately indicate relatively assessment or unknown variability homogenous soils homogenous soils Depth to Groundwater/ <5 feet below 5-15 feet below > 15 feet below Impervious Layer facility bottom facility bottom facility bottom Based on our geotechnical investigation and the previous table, Table 5 presents the estimated factor values for the evaluation of the factor of safety. This table only presents the suitability assessment safety factor (Part A) of the worksheet. The project civil engineer should evaluate the safety factor for design (Part B) and use the combined safety factor for the design infiltration rate. Project No. 06442-32-29 - 6 -Scptcmhcr25, 2017 APPENDIX A AARDVARK TEST RESULTS FOR CARLSBAD OAKS NORTH -LOT 1 CARLSBAD, CALIFORNIA PROJECT NO. 06442-32-29 APPENDIX B FORM 1-8 FOR CARLSBAD OAKS NORTH -LOT 1 CARLSBAD, CALIFORNIA PROJECT NO. 06442-32-29 Map Unit Description: Cieneba coarse sandy loam, 30 to 65 percent slopes, em ded--San Diego County Area, California Carlsbad Oaks North -Lot 1 San Diego County Area, California CIG2-Cieneba coarse sandy loam, 30 to 65 percent slopes, eroded Natural Resources Conservation Service Map Unit Setting National map unit symbol: hb9s Elevation: 500 to 4,000 feet Mean annual precipitation: 12 to 35 inches Mean annual air temperature: 57 to 64 degrees F Frost-free period: 200 to 300 days Farmland classification: Not prime farmland Map Unit Composition Cieneba and similar soils: 85 percent Minor components: 15 percent Estimates are based on observations, descriptions, and transects of the mapunit. Description of Cieneba Setting Landform: Hills Landform position (two-dimensional): Backslope Landform position (three-dimensional): Side slope Down-slope shape: Convex Across-slope shape: Convex Parent material: Residuum weathered from granite and granodiorite Typical profile H1 -Oto 10 inches: coarse sandy loam H2 -10 to 14 inches: weathered bedrock Properties and qualities Slope: 30 to 65 percent Depth to restrictive feature: 4 to 20 inches to paralithic bedrock Natural drainage class: Somewhat excessively drained Runoff class: Medium Capacffy of the most limiting layer to transmit water (Ksat): High (1.98 to 5.95 in/hr) Depth to water table: More than 80 inches Frequency of flooding: None Frequency of ponding: None Available water storage in profile: Very low (about 1.0 inches) Interpretive groups Land capability classification (irrigated): 7e Land capabifity classification (nonirrigated): 7e Hydrologic Soil Group: D Ecological sffe: SHALLOW LOAMY (1975) (R019XD060CA) Hydric soil rating: No Web Soil Survey National Cooperative Soil Survey 912012017 Page 1 of 2 Map Unit Description: Cieneba coarse sandy loam, 30 to 65 percent slopes, ero ded--San Diego County Area, California Minor Components Vista Percent of map unit: 1 O percent Hydric sail rating: No Las posas Percent of map unit: 5 percent Hydric sail rating: No Data Source Information Soil Survey Area: San Diego County Area, California Survey Area Data: Version 10, Sep 12, 2016 USDA. Natural Resources oiliiii Conservation Service Web Soil Survey National Cooperative Soil Survey Car1sbad Oaks North -Lot 1 9/20/2017 Page 2 of 2 Map Unit Description: Huerhuero loam, 9 to 15 percent slopes--San Diego County Area, California San Diego County Area, California HrD-Huerhuero loam, 9 to 15 percent slopes Natural Resourees Conservation Service Map Unit Setting National map unit symbol: hbcp Elevation: 1,100 feet Mean annual precipffation: 12 to 20 inches Mean annual air temperature: 57 degrees F Frost-free period: 260 days Farmland classification: Not prime farmland Map Unit Composition Huerhuero and similar soils: 85 percent Minor components: 15 percent Estimates are based on observations, descriptions, and transects of the mapunit. Description of Huerhuero Setting Landform: Marine terraces Down-slope shape: Concave Across-slope shape: Concave Parent material: Calcareous alluvium derived from sedimentary rock Typical profile H1 -Oto 12 inches: loam H2-12 to 55 inches: clay loam, clay H2 -12 to 55 inches: stratified sand to sandy loam H3 -55 to 72 inches: Properties and qualities Slope: 9 to 15 percent Depth to restrictive feature: More than 80 inches Natural drainage class: Moderately well drained Runoff class: Very high Capacity of the most limiting fayer to transmit water (Ksat): Very low to moderately low (0.00 to 0.06 in/hr) Depth to water table: More than 80 inches Frequency of flooding: None Frequency of ponding: None Salinity, maximum in profile: Nonsaline to very slightly saline (0.0 to 2.0 mmhos/cm) Sodium adsorption ratio, maximum in profile: 25.0 Available water storage in profile: Moderate (about 6.6 inches) Interpretive groups Land capability classification (irrigated): 4e Land capability classification (nonirrigated): 4e Hydrologic Soil Group: D Web Soil Survey National CooperaUve Soil Survey Carisbad Oaks North -Lot 1 9120/2017 Page 1 of 2 Map Unit Description: Huerhuero loam, 9 to 15 percent slopes---San Diego County Area, California Ecological site: CLAYPAN (1975) (R019XD061CA) Hydric soil rating: No Minor Components Las flores Percent of map unit: 1 0 percent Hydric soil rating: No Oliventain Percent of map unit: 3 percent Hydric soil rating: No Unnamed Percent of map unit: 2 percent Hydric soil rating: No Data Source Information Soil Survey Area: San Diego County Area, California Survey Area Data: Version 10, Sep 12, 2016 L'S[),I. Natural Resources iiliiia Conservation Service Web Soil Survey National Cooperative Soil Survey Carlsbad Oaks North -Lot 1 9/20/2017 Page2of2