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HomeMy WebLinkAboutMS 2018-0002; OAK AVENUE PARCEL MAP; RESPONSE TO THIRD PARTY GEOTECHNICAL REVIEW; 2020-04-17Geotechnical. Geologic. Coastal • Environmental 5741 Palmer Way • Carlsbad, California 92010 • (760) 438-3155 • FAX (760) 931-0915 • www.geosoilsinc.com April 17, 2020 P1ECEIIVTF1Th W. O1'493-A-SC Mr. Temujin Matsubara APR 24 2020 1391 Oak Avenue Carlsbad, California 92008 LAND DEVELOPMENT ENGINEERING Subject: Response to Third-Party Geotechnical Review, Proposed Construction at 1391 Oak Avenue, Carlsbad, San Diego County, California 92008 References: 1. "Review of geotechnical Report: Oak Avenue Parcel Map, 1391 Oak Avenue, Carlsbad, California 92008," Project No. 226816-00101.67, dated of January 10, 2020, by NV5 West, Inc. "Geotechnical Evaluation for Proposed Construction at 1391 Oak Avenue, Carlsbad, San Diego County, California 92008," W.O. 7493-A-SC, dated October 12, 2018, by Geosoils, Inc. "Grading Plans For: Oak Avenue Parcel Map (1391 Oak Avenue, Carlsbad, California 92008), Drawing No. 522-2A, dated December 3, 2019, by Pacific Overland Engineering & Surveying. Dear Mr. Matsubara: In accordance with the request of Mr. Temujin Matsubara, this summary report presents the responses to the third-party reviewer's comments, prepared by NV5 West, Inc. The scope of our services has included a review of the geotechnical report and grading plans (see References 1 & 2), a review of the January 10, 2020 third-party geotechnical review by the City's geotechnical engineering reviewer (see Reference 3), analysis of data, and preparation of this review response. For convenience, portions of the review comments are reproduced below, in italics, followed by our response. REVIEW RESPONSE Review Comment No. 1 The project geotechnical consultant should clarify whether the garage is attached or detached. The importance of this clarification is discussed in more detail in the following comment. Response to Comment No. 1 From the details shown on sheet 3 of the project grading plans (see Reference 3), the proposed construction appears to consist of a main residence supported by a pier and beam foundation, with an attached garage supported by concrete slab-on-grade floor. Review Comment No. 2 It is not clear if the above recommendation is to mitigate a potential transition condition. The geotechnical consultant should review the project plans and provide recommendations, if necessary, regarding the potential for a transition condition underlying the proposed structure. If as stated, the intent of the above recommendation is to provide at least 3 feet of compacted fill below the proposed finished grade throughout the entire building area, then the quantity for remedial grading stated on the project grading plan under earthwork quantities (130 cubic yards) appears to significantly underestimate the quantity of remedial grading and should be recalculated by the Civil Engineer. Response to Comment No. 2 Recommendations to mitigate a potential cut-fill transition condition and updated remedial grading limits are provided below: Treatment of Existing Ground Removals should consist of all surficial deposits of fill, colluvium, and weathered paralic deposits. Based on our site work, removals depths on the order of 2 to 21/2 feet should be anticipated. These soils may be re-used as fill, provided that the soil is cleaned of any deleterious material and moisture conditioned, and compacted to a minimum 95 percent relative compaction per ASTM D 1557. Removals should be completed throughout all areas expected to bear any slab-on-grade foundations. 2. In addition to removals throughout areas expected to bear slab-on-grade foundations, overexcavation/u ndercutting of the underlying formational soil should be performed in order to provide for at least 3 feet of compacted fill below finish grade. Undercutting should be completed for a minimum lateral distance of at least 5 feet beyond the slab-on-grade foundation envelope. Once removals and overexcavation are completed, the fill should be cleaned of deleterious materials, moisture conditioned, and recompacted to at least 95 percent relative compaction per ASTM D 1557. Review Comment No. 3 The project geotechnical report recommends allowable bearing values for footings bearing entirely in properly compacted engineered fill. The referenced project plan indicates that the pier foundations might bear in natural soil. The geotechnical consultant should review the project plans and if foundations are to bear in natural ground, revised recommendations should be provided. Mr. Temujin Matsubara W.O. 7493-A-SC 1391 Oak Avenue, Carlsbad April 17, 2020 File: e:\wp10\7400\7493a.rtt GeoSods, Inc. Page 2 Response No. 3 Recommendations for pier and beam foundations are provided below: DRILLED PIER AND GRADE BEAM FOUNDATION RECOMMENDATIONS The proposed structures underlain by left-in-place undocumented artificial fill and colluvium, may be supported by a drilled, cast-in-place, concrete pier and grade beam system (drilled piers) with structural concrete floors. Any slab on grade would need to span between the pier and grade beam system, in accordance with the recommendations of the structural engineer. This may necessitate less settlement-sensitive improvements around the property that are not pier supported ([avers. etc.). Actual pier design should be finalized by the project's structural engineer and consider the structural capacity of the pier(s) used. The integrity of the piers should be checked by the structural engineer or civil engineer specializing in structural analysis. Pier holes should be drilled straight and plumb. Locations (both plan and elevation) and plumbness should be the contractors responsibility. The grade beams spanning should be a minimum of 24 inches by 24 inches in cross section and supported by drilled piers, a minimum of 24 inches in diameter which are placed at a maximum spacing of 8 feet on center and a minimum of 3 pier diameters apart and supporting all structural columns. The design of the grade beam and piers should be in accordance with the recommendations of the project structural engineer, and utilize the following geotechnical parameters. Pier and grade beam design and construction shall minimally conform to the applicable sections contained in the 2019 CBC. Foundations Design Criteria - Drilled Piers The drilled pier foundation for the building should gain vertical support from friction and end bearing in the dense unweathered paralic deposits underlying the site. Drilled piers for residential foundations are intended to resist vertical, and lateral loads due to imposed structural loads and not provide lateral stability/stabilization of slopes. The drilled piers should be at least 12 inches in diameter and should extend at least 5 feet into unweathered paralic deposits. The effects of pier groups should be evaluated when the preliminary foundation drawings are made available. Soil parameters to be used in pier and grade beam design are provided below. All the parameters provided are computed based on soil strength only, structural strength of the piers should be checked by the structural engineer or civil engineer specializing in structural analysis. Creep Zone: 5-foot vertical zone below a given point on the slope face, and projected upward, parallel to the slope face. Creep Load: The creep load projected on the area of the grade beam should be taken as an equivalent fluid approach, Mr. Temujin Matsubara W.O. 7493-A-SC 1391 Oak Avenue, Carlsbad April 17, 2020 File: e:\wp10\7400\7493a.rtt GeoSoils, Inc. Page 3 having a density of 60 pcf. For the caisson, it should be taken as a uniform 900 pounds per linear foot of drilled pier's depth, located above the creep zone. Point of Fixity: Located a distance of 1.5 times the caisson's diameter, below grade, or the creep zone, whichever controls. Passive Resistance: Passive earth pressure of 300 psf per foot of depth, to a maximum value of 3,000 psf may be used to determine drilled pier depth and spacing, provided that they meet or exceed the minimum requirements stated above. To determine the total lateral resistance, the contribution of the creep prone zone above the point of fixity, to passive resistance, should be disregarded. No contribution from soil/concrete friction on the bottom of slabs (if any) should be included in passive calculations. The upper 12 inches of passive resistance forthe drilled piers should be neglected unless confined by slabs or pavement. Additional lateral resistance may be obtained from lateral pile deflection. For a 1/4 inch lateral pile deflection, a lateral load of 10 percent of vertical capacity can be utilized. A more refined lateral load capacity may be provided when the pier head conditions (fixed, free), layout and elevations are provided by the structural consultant and/or architect on this project. Allowable Axial Capacity: Shaft capacity: 350 psf applied over the surface area of the shaft within formation only. Tip capacity: 3,500 psf. Assumes clean dense tip condition. Evaluated in the field by the geotechnical consultant. Pile/Fill Settlement 1/2 inch between heavy and lighter loaded piles, and should be less than 1/4 inch for post construction static and dynamic settlement. ClDH (Drilled Pier) Construction The excavation and installation of the drilled piers should be observed and documented by the project geotechnical engineer to verify the recommended depth. Mr. Te.mujin Matsubara W.O. 7493-A-SC 1391 Oak Avenue, Carlsbad April 17, 2020 File: e:\wp10\7400\7493a.rtt GeoSoals, Inc. Page 4 The drilled holes should be cased, specifically below the water table to prevent caving. The bottom of the casing should be at least 4 feet below the top of the concrete as the concrete is poured and the casing is withdrawn. Dewatering may be required for concrete placement if significant seepage or groundwater is encountered during construction. This should be considered during project planning. The bottom of the drilled pier should be cleared of any loose or soft soils before concrete placement. The exact depths of piers should be determined during the final precise grading plan review. Proper low slump concrete should be used and should be delivered through tremie pipe. We recommend that concrete be placed through the tremie pipe immediately subsequent to approved excavation and steel placement. Care should be taken to prevent striking the walls of the excavations with the tremie pipe during concrete placement. Vibration of concrete to reduce the potential for segregation should be performed. Review Comment No. 4 In the report provided to NV5 for review, Appendix 0 (or any of the report appendices for that matter) did not contain laboratory test results for relatively undisturbed and bulk soil samples. In the report that was provided for NV5s review, Appendix D was titled, "General Earthwork and Grading Guidelines". The consultant should clarify the reported location of the laboratory program for the relatively undisturbed and bulk soil samples. Response No. 4 It appears that this issue was a typographical error, since the laboratory testing data was intended to be removed from the appendices. All relevant laboratory data is provided within the report's "Laboratory Testing" section (pages 8-10 of Reference 2). Review Comment No. 5 On page 7, the referenced geotechnical report provides recommended shaking parameters. The report text indicates that the parameters are based on 2016 CBC. However the title of the parameters table indicates that theyare 2013 CBC seismic design parameters. Itis likely that this is a typographical error, but it should be corrected by the geotechnical consultant. Response No. 5 After performing an additional search for 2016 CBC seismic design parameters on the subject site, it appears that the table titled "2013 CBC seismic design parameters" was a typographical error. For convenience, and to avoid confusion, the table of 2016 seismic design parameters is repeated below: Mr. Temujin Matsubara W.O. 7493-A-SC 1391 Oak Avenue, Carlsbad April 17, 2020 File: e:\wp10\7400\7493a.rtt GeoSoils, Inc. Page 5 2016 CBC SEISMIC DESIGN PARAMETERS PARAMETER i VALUE 2016 CBC REFERENCE Risk Category II Table 1604.5 Site Class D Section 1613.3.2/ASCE 7-10 (p. 203-205) Spectral Response - (0.2 sec), S. 1.133 g Section 1613.3.1 Figure 1613.3.1 (1) Spectral Response-(1 sec), S, 0.435 g Section 1613.3.1 Figure 1613.3.1 (2) Site Coefficient, Fa 1.047 Table 1613.3.3(1) Site Coefficient, F, 1.565 Table 1613.3.3(2) Maximum Considered Earthquake Spectral 1.186 g Section 1613.3.3 Response Acceleration (0.2 sec), SMS (Eqn 16-37) Maximum Considered Earthquake Spectral 0.681 g Section 1613.3.3 Response Acceleration (1 sec), SM1 (Eqn 16-38) 5% Damped Design Spectral Response 0.791 g Section 1613.3.4 Acceleration (0.2 sec), S,, (Eqn 16-39) 5% Damped Design Spectral Response 0.454 g Section 1613.3.4 Acceleration (1 sec), S, (Eqn 16-40) PGA, - Probabilistic Vertical Ground Acceleration may be 0.47g ASCE7-1O(Eqn 11.8. 1) assumed as about 50% of these values. Seismic Design Category D Section 1613.3.5/ASCE 7-10 (Table 11.6-1 or 11.6-2) Review Comment No. 6 On page 7, the referenced geotechnical report suggests that perimeter retaining wall footings may need to be deepened into unweathered very old paralic deposits or unweathered Santiago Formation. The Santiago formation was not mentioned anywhere else in the referenced report or in the exploratory excavation logs. It is not clear whether the Santiago formation was encountered. It is likely that this is a typographical error, but it should be corrected by the geotechnical consultant. Response No. 6 The Santiago Formation underlies the very old paralic deposits at depth. The statement of interest by the reviewer was a generic statement, since the depth of the deep foundations and the Santiago Formation were not know. It is likely that deep foundations will extend into unweathered very old paralic deposits, but if they extend to the Santiago Formation, that is also satisfactory. Mr. Temujin Matsubara W.O. 7493-A-SC 1391 Oak Avenue, Carlsbad April 17, 2020 File: e:\wp10\7400\7493a.rtt GeoSoils, Inc. Page 6 Civil Engineer, RCE The opportunity to be of service is sincerely appreciated. If you should have any questions, please do not hesitate to contact our office. Respectfully submitted, GeoSoils, Inc. NAL e CL 0. 1340 t Certified i \ Engineering / John P. Franiin v t~h Geologist )jEngineering Geologist, MJS/JPF/DWS/mn Distribution: (1) Addressee (email) (2) Pacific Overland Engineering & Surveying, Mr. John P. Strohminger (2 wet signed & email) GeoSoils, Inc.