Loading...
HomeMy WebLinkAboutCDP 2023-0042; MCGERVEY ADU; UPDATE LIMITED GEOTECHNICAL EVALUTION LETTER; 2023-08-16Geotechnical • Geologic • Coastal • Environmental 5741 Palmer Way • Carlsbad, California 92010 • (760) 438-3155 • FAX (760) 931-0915 • www.geosoilsinc.com Scott and Lori McGervey 830 Citrus Place Carlsbad, California 92008 August 16, 2023 W.O. 7272-A 1-SC Subject: Update Limited Geotechnical Evaluation, Design Parameters, Proposed Additional Construction at 830 Citrus Place, Carlsbad, San Diego County, California, Assessor's Parcel Number (APN) 206-040-31-00 Dear Mr. and Mrs. McGervey: In accordance with your request and authorization, GeoSoils, Inc. (GSI) is providing the following summary of our update limited geotechi:,ical evaluation for the proposed additional construction at the subject site. The purpose of this study was to update the preliminary geotechnical conclusions and recommendations provided in GSI (2017 [see the Appendix]), relative to the currently proposed additional site development shown on the architectural plans prepared by BGI Architecture ([BGI], 2023). In addition, this update evaluation modernizes the geotechnical aspects of the I proposed development for consistency with the 2022 California Building Code (California Building Standards Commission [CBSC], 2022). The scope of our services included: a site reconnaissance; limited engineering analyses; and preparation of this summary report. Unless specifically superseded herein, the conclusions and recommendations contained in GSI (2017) are still considered valid and applicable, and should be appropriately implemented during the balance of project design and construction. PROPOSED DEVELOPMENT Based on our review of BGI (2023), GSI understand,s that the currently proposed site development includes the construction of a single-story addition along the southeastern corner of the existing residential structure. BGI (2023) also shows that the existing detached garage will be razed and a new two-story garage and accessory' dwelling unit (ADU) will be constructed near the southern portion of the property. The building addition and new garage/ADU are anticipated to consist of wood framing with typical footings and concrete slab-on-grade floors. Building loads are currently unavailable but assumed to be typical of relatively light residential construction. BACKGROUND GEOTECHNICAL STUDY In Spring of 2017, GSI performed a limited geotechnical evaluation of the subject property. The study included ·reviews of readily available published geologic maps and literature and aerial photographs; subsurface exploration and the geologic logging of two (2) shovel-excavated test pits' and one (1) hand-auger boring; laboratory testing of collected soii samples; engineering analyses; and the preparation of a summary report (GSI, 2017). In the explored areas of the property, we found 1 foot to 3¾ feet of undocumented artificial . fill at the surface. Approximately 1 foot of colluvium was found underlying the undocumented fill in a test pit excavated near the southwestern corner of the existing residential structure. Quaternary-age old paralic deposits were encountered beneath the undocumented fill and colluvium at depths ranging between approximately 2 and 3¾ feet below the existing grades. However, the upper approximately 1 foot of the old paralic deposits in the test pi' excavated near the southwestern corner of the existing residential structure was weathered. Groundwater was not encountered in the test pits and boring to the explored depths. The undocumented fill, colluvium, and weathered old paralic deposits were considered potentially compressible in their existing state and, therefore, unsuitable for supporting the proposed settlement-sensitive 'improvements. We concluded that the unweathered old paralic deposits, occurring at depths on the order of 3 to 3¾ feet below the existing grades, were suitable bearing materials at the subject property. Expansion index testing performed on a representative sample of the onsite soils yielded an expansion index less than 5, which correlated to very low expansion potential (i.e., expansion index range of Oto 20). Sieve analysis testing conducted on a representative soil sample indicated that the sample was composed of silty sand (Unified Soil Classification System [USCS] designation -SM). A representative sample of the onsite soils was also subjected to laboratory testing to evaluate corrosion potential. The test results indicated that the sample was neutral with respect to soil acidity/alkalinity; moderately corrosive to exposed, buried metals when saturated; presented negligible sulfate exposure to concrete (i.e., Exposure Class SO per Table 19.3.1.1 of American Concrete Institute [ACI] 318-14 [ACI, 2014); and contained a relatively low concentration of soluble chlorides. In order to mitigate the potential for the propo~ed building foundations and concrete slab-on-grade floors to succumb to damaging settlements, GSI recommended that the new building foundations extend into suitable, unweathered old paralic deposits. We also,, recommended remedial grading beneath the new concrete slab-on-grade floor areas .. Moreover, we recommended that existing building footings subjected to additional loads • be underpinned into the unweathered old paralic deposits. 1As an alternative to remedial grading beneath the ·new concrete slab-on-grade floors, we recommended the use of structural slabs, designed to span between the deepened footings without relying on the McGervey 830 Citrus Place, Carlsbad File:e:\wp21 \7200\7272a1 .ulg GeoSoils, Inc:. W.O. 7272-A1-SC August16,2023 Page 2 underlying, unmitigated soiJs for support. Lastly, we recommended that the proposed development not surcharge the existing Caltrans retaining wall to the northeast of the subject property by extending new building foundations below a 1: 1 (horizontal:vertical [h:v]) plane projected up and toward the subject property from the heel of the retaining wall footing. A copy of GSI (2017) is included in the Appendix, following the referenced documents. RECENT SITE RECONNAISSANCE On July 28, 2023, a GSI representative performed a visual reconnaissance of the subject property to evaluate the c.urrent surficial conditions thereon. Based on our observations, the surficial conditions within the proposed development areas were generally similar to those present at the time of our field exploration performed in preparation of GSI (2017). ONSITE SOILS AND STORM WATER INFILTRATION FEASIBILITY According to the United States Department of Agriculture / Natural Resources Conservation Service's (USDA/NRCS's) "Web Soil Survey" website (http://websoilsurvey.sc.egov. usda.gov), the onsite soils consist of the Marina loamy coarse sand, 2 to 9 percent slopes. The attributes of these soils are summarized in the following table: CAPACITY OF THE SOIL TYPE LANDFORM NATURAL RUNOFF MOST LIMITING HVDROLOGIC (MAP UNIT) SETTING DRAINAGE CLASS LAVER TO SOIL GROUP CLASS TRANSMIT WATER (Ksat) Marina loamy coarse sand, 2 to 9 percent Somewhat Moderately high to slopes Ridges excessively Medium high (0.57 to B (USDA/ NRCS drained 1.98 inches per hour) map symbol -MIC) While the USDA/NAGS soil data suggest that infiltration of storm water into the onsite soils to meet any required permanent post-construction storm water best management practice (BMP) objectives may be feasible, it is our opinion that infiltration of storm water into the onsite earth materials, in any volume, has a high potential to result in perched groundwater (mounding) accumulating upon the unweathered old paralic deposits. The perched groundwater would migrate laterally and induce settlement of existing fill soils and the underlying low-density colluvium and weathered old paralic deposits within unmitigated McGervey 830 Citrus Place, Carlsbad File:e:\wp21\7200\7272a1 .ulg GeoSoils, Inc. • I W.O. 7272-A1-SC August 16, 2023 Page3 portions of the subject prope,rty, as well as similar soils on the adjacent properties. This has the potential to adversely affect the existing and proposed onsite improvements and the existing development on the adjacent properties, causing distress. Thus, the infiltration of storm water into the onsite soils is not considered sound engineering practice and is not recommended, from a geotechnical perspective. UPDATED SEISMIC SHAKING PARAMETERS The following table summarizes the updated site-specific design criteria obtained from the 2022 California Building Code (CBC), Chapter 16 Structural Design, Section 1613, Earthquake Loads (California Building Standards Commission, 2022) and American Society of Civil Engineers (ASCE) 7-16 (ASCE, 2017). The short spectral response uses a period of 0.2 seconds. I 2022 CBC SEISMIC DESIGN PARAMETERS I SITE SPECIFIC PARAMETER VALUE PER I ASCE 7-16 Risk Category!1l I, II, or Ill Site Class D (Default) Spectral Response -(0.2 sec), s. 0.873 g Spectral Response -(1 sec), S1 0.680 g Site Coefficient, Fa 1.2(2) Site Coefficient, F v 2_5(3) (Section 21.3) Maximum Considered Earthquake Spectral 1.~29 g!4) Response Acceleration (0.2 sec), SMs (Section 21.4) Maximum Considered Earthquake Spectral 1.110 g(S) Response Acceleration (1 sec),SM1 (Section 21.4) 5% Damped Design Spectral Response 0.886 g(G) Acceleration (0.2 sec), S08 5% Damped Desigri Spectral Response 0.737 g!7l Acceleration (1 sec), s01 (Section 21.4) McGervey 830 Citrus Place, Carlsbad File:e:\wp21\7200\7272a1 .ulg GeoSoils, Inc. 2022 CBC or ASCE 7-16 REFERENCE Table 1604.5 Section 1613.2.2/Chap. 20 ASCE 7-16 (p. 203-204) ✓ Section 1613.2.1 Figure 1613.2.1 (1) Section 1613.2.1 • Figure 1613.2.1 (2) Table 1613.2.3(1) Table 1613.2.3(2) Section 1613.2.3 (Eqn 16-36) Section 1613.2.3 (Eqn 16-37) Section 1613.2.4 (Eqn 16-38) Section 1613.2.4 (Eqn 16-39) W.O. 7272-A1-SC August 16, 2023 Page4 I 2022 CBC SEISMIC DESIGN PARAMETERS I SITE SPECIFIC 2022 CBC or ASCE 7-16 PARAMETER VALUE PER REFERENCE ASCE 7-16 PGAM -Probabilistic Vertical Ground Acceleration may be assumed as about 50% 0.559 g ASCE7-16 (Eqn 11.8.1) of these values. Seismic Design Category D(B) Section 1613.2.5/ ASCE 7-16 (Section t 1.6) (p. 85: Table 11.6-1 or 11.6-2) 1. Risk Category to be confirmed by the project architect or structural engineer. 2. PerTable11.4-1 ofASCE7-16. • 3. Per Section 21.3 of ASCE 7-16, if S1 ~ 0.2 then F v is taken as 2.5. 4. Per Section 21.4 of ASCE 7-16, SMs = (1.5)(805) = (1.5)(0.886 g) = 1.329 g. 5. PerSection21.4ofASCE7-16,SM1 = (1.5)(801) = (1.5)(0.737g) = 1.110g. 6. Per Section 21.4 of ASCE 7-16, S05 shall be taken as 90 percent of the maximum spectral acceleration (S.) obtaine,d from the site- specific spectrµm at any period within the range from 0.2 to 5 seconds, inclusive. 7. Per Section 21.4 of ASCE 7-16, S01 shall be taken as the maximum value of the product TS. obtained from the site-specific spectrum from the period within the range of 1 to 5 seconds, inclusive. 8. Per Tables 11.6-1 and 11.6-2 of ASCE 7-16, S08 (0.886 g) ~ 0.50 and S01 (0.737 g) ~ 0.2: Thus, the seismic design category is 11D11• UPDATED PRELIMINARY RECOMMENDATIONS- All earthwork should be performed in· accordance with Appendix Chapter J of the 2022 California Building Code (CBSC, 2022i, the requirements of the City of Carlsbad, and the preliminary geotechnical recommendations contained in GSI (2017). Foundation and concrete slab-on-grade floor design and construction should conform to the requirements of the 2022 California Building Code and the preliminary geotechnical recommendations • provided in GSI (2017). Upon request, alternative recommendations for remedial grading ' can be provided to avoid deepened building foundations. However, the implementation / of the alternafo,e recommendations may require tt'le acquisition of a grading permit and the preparation of a gradi_ng plan. Based on our review of the footprints for the proposed building addition and garage/ADU shown on BGI (2023), there appears to be sufficient separation from the existing Caltrans retaining wall. Thus, the potential for the proposed building addition and garage/ADU to surcharge the retaining wall is considered low, and the need to' deepen the building foundations to avoid surcharging the offsite wall is not considered warranted at this time. Any proposed ancillary site improvement (driveway, walkways, patios) underlain by potentially compressible undocumented fill, colluvium, and weathered old paralic deposits may experience distress resulting from adverse settlements. Thus, these imp~ovements may require increase maintenance and repairs, and have a reduced service life if remedial earthwork is not performed. This should be disclosed to all interested/affected parties. McGervey 830 Citrus Place, Carlsbad File:e:\wp21\7200\7272a1 .ulg GeoSoils, Inc. W.O. 7272-A1-SC August 16, 2023 Page 5 It is possible that some differential movement may occur between the existing residential structure and the proposed building addition, especially during a seismic event. Thus, the project structural engineer should consider designing the interface between the existing and new construction for differential movement on the order of at least ¼-inch. PLAN REVIEW Final project plans (grading, precise grading, foundation, landscaping, etc.), should be , reviewed by this office prior to construction, so that construction is in accordance with the conclusions and recommendations of this report and GSI (2017). Based on our review, supplemental recommendations or further geotechnical studies may be warranted. LIMITATIONS The materials encountered on the project site and used for our analysis are believed representative of the area; however, soil and bedrock materials vary in character between excavations and natural outcrops or conditions exposed during earthwork construction. Site conditions may vary due to seasonal changes· or other factors. Inasmuch as our study is based upon our review, engineering analyses, and laboratory data, the conclusions and recommendations presented herein are professional opinions. These opinions have been derived in accordance with current standards of practice, and no warranty is express or implied. Standards of practice are subject to change with time. This report has been prepared for the purpose of providing soil design parameters derived from testing of a soil sample received at our laboratory, and does not represent an evaluation of the overall stability, suitability, or performance of the property for the proposed development. GSI assumes no responsibility or liability for work or testing performed by others, or their inaction; or work performed wh~n GSI is n_ot requested to be onsite, to evaluate if our recommendations have been properly implemented. 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. McGervey 830 Citrus Place, Carlsbad File:e:\wp21\7200\7272a1 .ulg GeoSoils, Inc. W.O. 7272-A1-SC August16,2023 Page 6 The opportunity to be of service is greatly appreciated., If you have any questions' concerning this report, or if we may be of further assistance, please do not hesitate to contact any of the undersigned. Respectfully submitted, GeoSoils, li:,c. \arJ~ ~e,_.,,-stepW, Coover ~~7neering Geologist, Geotechnical Engineer, GE 2057 ,1-P!_ Ryan B. Boehmer Staff Geologist HBB/SJC/JPF/sh Attachment: Appendix -References and GSI (2017) Distribution: (1) Addressee (PDF via email) (1) BGI Architecture, Attention: Mr. John Beery (PDF via email) McGervey 830 Citrus Place, Carlsbad File:e:\wp21\7200\7272a1 .ulg GeoSoils, Inc. W.O. 7272-A1-SC August16,2023 Page 7 . . APPENDIX REFERENCES American Concrete Institute, 2015, Guide to concrete floor and slab construction (ACI 318-15): reported by ACI Committee 302, dated June. American Concrete Institute, 2014, Building code requirements for structural concrete (ACI 318-14), and commentary (ACI 318R-14): reported by ACI Committee 318, dated September. American Society of Civil Engineers, 2017, Minimum design loads and associated criteria • for buildings and other structures, provisions, ASCE Standard ASCE/SEI 7-16. BGI Architecture, 2023, Scott & Lori McGervey kitchen addition & detached ADU/Garage,. 830 Citrus Place, Carlsbad, CA 92008, 13 sheets, various scales, job no: 2302, dated July 7. California Building Standards Commission, 2022, California Building Code, California Code of Regulations, Title 24, Part 2, Volumes 1 and 2, based on the 2021 International Building Code, effective January 1, 2023. GeoSoils, Inc., 2017, Limited geotechnical evaluation relative to allowable bearing value, lateral bearing pressure, lateral sliding resistance, and seismic design parameters, proposed additional construction at 830 Citrus Place, Carlsbad, San Diego County, California, Assessor's Parcel Number (APN) 206-040-31-00, W.O. 7272-A-SC, dated May 12. GeoSoils, Inc.