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HomeMy WebLinkAboutPD 2021-0003; BRIGDEN RESIDENCE; LIMITED SOIL INVESTIGATION AND BEARING VALUE EVALUATION; 2021-01-13 13 January 2021 Premier Pools and Spas Job No. 20-12991 5431 Avenida Encinas, Suite B Carlsbad, CA 92008 Attn: Mr. Aaron Temme Subject: Limited Soil Investigation and Bearing Value Evaluation Brigden Property Retaining Wall 1953 Cobalt Drive Carlsbad, California Dear Mr. Temme: In accordance with your request, a representative of Geotechnical Exploration, Inc. has visited the subject site and performed an evaluation of the soil conditions in the area of the proposed new retaining wall on the southwest side of the existing residence. It is our understanding that the proposed retaining wall will be approximately 6 feet high and utilize a continuous footing founded in firm compacted fill soil. As part of our investigation, we observed and evaluated the soil conditions in the slope area where the new retaining wall will be built. The original field work, conducted on November 2, 2020, consisted of observing the slope area where the proposed retaining wall will be built. Our observations revealed that this area of the site remains the same and is underlain by stiff, sandy clay fill soil. The ascending slope above the 6-foot vertical cut for the retaining wall has a gradient of approximately 2.0:1.0. The vertical cut is supported by temporary shoring consisting of 4x8 plywood sheets and wood posts. The on-site fill soil is considered to have a low to medium expansion potential with an Expansion Index of less than 90. The backfill soils should consist of low expansive soils or gravel. Brigden Property Retaining Wall Job No. 20-12991 Carlsbad, California Page 2 Based upon our observation, probing of the exposed fill soil, it is our opinion that the new foundation for the retaining wall can be founded directly into the existing compacted fill soil. 1. It is our opinion that the stiff compacted fill soils will provide adequate bearing strength for the proposed retaining wall foundations. New footings placed in stiff compacted fill soils can be designed for an allowable soil bearing capacity of 1,500 pounds per square foot (psf). The static allowable bearing capacity may be increased 500 psf for each additional foot in width over 1 foot, up to a maximum of 4,000 psf. We do recommend that the proposed footings and slabs contain at least a nominal amount of reinforcing steel to reduce the separation of cracks should they occur. The allowable static soil bearing capacity may be increased one-third for structural design including seismic or wind loads. The final reinforcing schedule and wall structural details should be provided by the design engineer. 2. The proposed footings should have a minimum depth of 12 inches and a width of at least 4 feet, founded in the stiff compacted fills. The dimensions and steel reinforcing for the retaining wall should be provided by the structural designer. 3. Site-specific seismic design criteria to calculate the base shear needed for the design of the residential addition are presented in the following table. The design criteria has been obtained from the California Building Code (CBC) 2019 edition, and is based on the distance to the closest active fault and soil profile classification. Brigden Property Retaining Wall Job No. 20-12991 Carlsbad, California Page 3 4. The proposed retaining wall should be designed in accordance with Section 1613 of the 2019 CBC, which incorporates by reference the ASCE 7-16 for seismic design and the following parameters should be utilized. We have determined the mapped spectral acceleration values for the site based on a latitude of 33.1109 degrees and longitude of 117.2668 degrees, utilizing a program titled “ATC Seismic Hazards,” provided by ATC, which provides a solution for ASCE 7-16 (Section 1613 of the 2019 CBC) utilizing digitized files for the Spectral Acceleration maps. In addition, we have assigned a Site Classification of D-Stiff Soil. The response parameters for design are presented in the following table. The design spectrum acceleration vs. Period T is attached. TABLE I Mapped Spectral Acceleration Values and Design Parameters Ss S1 Fa Fv Sms Sm1 Sds Sd1 0.981g 0.357g 1.108 1.94 1.087g 0.693g 0.724g 0.461g 5. The liquefaction of saturated sands during earthquakes can be a major cause of damage to buildings or structures. Liquefaction is the process by which soils are transformed into a viscous fluid that will flow as a liquid when unconfined. It occurs primarily in loose, saturated sands and silts when they are sufficiently shaken by an earthquake. On this site, the risk of liquefaction of foundation materials due to seismic shaking is considered to be remote due to the relatively shallow, dense nature of the natural-ground material and the lack of a shallow static groundwater surface under the site. No soil liquefaction or soil strength loss is anticipated to occur due to a seismic event. Brigden Property Retaining Wall Job No. 20-12991 Carlsbad, California Page 4 6. The active earth pressure (to be utilized in the design of cantilever, non- restrained walls) for the proposed retaining wall should be based on an Equivalent Fluid Weight of 38 pounds per cubic foot (for level backfill only) if low expansive import soils are used. For 2.0:1.0 sloping backfill, we recommend an equivalent fluid weight of 52 pcf. Additional loads applied within the potential failure block should be added to the active soil earth pressure by multiplying the vertical surcharge load by a 0.31 lateral earth pressure coefficient. For restrained wall conditions, we recommend an equivalent fluid weight of 56 pcf for level backfill and 71 pcf for 2 to 1 sloping backfill. Surcharge loads applied within a horizontal distance to the retaining wall height may be converted to lateral pressures by multiplying by a factor of 0.47. If seismic soil increment is required or if the exposed height exceeds 6 feet, the unrestrained walls with level backfill should be designed for a triangular pressure soil seismic increment of 13 pcf, in addition to the regular static loading, with zero pressure at the top and the maximum pressure at the bottom of the wall. 7. The allowable passive earth pressure of the stiff fill soils (to be used for design of shallow foundations and footings to resist the lateral forces) may be based on an Equivalent Fluid Weight of 260 pcf. This allowable passive earth pressure should only be considered valid for design if the ground adjacent to the foundation structure is essentially level for a distance of at least three times the total depth of the foundation and is properly compacted or dense natural soil. An allowable Coefficient of Friction of 0. 35 times the dead load may be used between the bearing soils and concrete foundations, walls or floor slabs. Brigden Property Retaining Wall Job No. 20-12991 Carlsbad, California Page 5 8. Adequate measures should be taken to properly finish-grade the site after the new retaining wall and other improvements are in place. Drainage waters from this site and adjacent properties are to be directed away from perimeter foundations, floor slabs, and footings, onto the natural drainage direction for this area or into properly designed and approved drainage facilities. Proper subsurface and surface drainage will help minimize the potential for waters to seek the level of the bearing soils under the foundations, footings, and floor slabs. Failure to observe this recommendation could result in undermining, differential settlement of the building foundation or other improvements on the site, or moisture-related problems. It is not within the scope of our services to provide quality control oversight for surface or subsurface drainage construction or retaining wall sealing and base of wall drain construction. It is the responsibility of the contractor to provide proper surface and subsurface drainage. 9. Due to the possible build-up of groundwater (derived primarily from rainfall and irrigation), excess moisture is a common problem in below-grade structures or behind retaining walls. These problems are generally in the form of water seepage through walls, mineral staining, mildew growth and high humidity. In order to minimize the potential for moisture-related problems to develop, the backfill side of all structure retaining walls must be adequately waterproofed and drained. Proper subdrains and free-draining backwall material (such as gravel or geocomposite drains such as MiraDrain 6000 or equivalent and Total Drain collector at the bottom) should be installed behind all retaining walls on the subject project in addition to wall waterproofing. Geotechnical Exploration, Inc. will assume no liability for damage to structures that is attributable to Brigden Property Retaining Wall Job No. 20-12991 Carlsbad, California Page 6 poor drainage. Proper surface drainage should also be implemented behind and in front of the retaining wall. Geotechnical Exploration, Inc. recommends that we be asked to verify the actual soil conditions revealed in footing excavations prior to form and steel reinforcement placement and test the backfill compaction. We should also review the retaining wall plans before construction. The contractor should follow OSHA safety guideline recommendations where applicable. Should you have any questions regarding this matter, please contact our office. Reference to our Job No. 20-12991 will help to expedite a response to your inquiries. Respectfully submitted, GEOTECHNICAL EXPLORATION, INC. _______________________________ ______________________________ Jay K. Heiser Jaime A. Cerros, P.E. Senior Project Geologist R.C.E. 34422/G.E. 2007 Senior Geotechnical Engineer Hazards by Location Search Information Coordinates:33.1109, -117.2668 Elevation:235 ft Timestamp:2021-01-13T15:01:12.050Z Hazard Type:Seismic Reference Document: ASCE7-16 Risk Category:II Site Class:D Basic Parameters Name Value Description SS 0.981 MCER ground motion (period=0.2s) S1 0.357 MCER ground motion (period=1.0s) SMS 1.087 Site-modified spectral acceleration value SM1 * null Site-modified spectral acceleration value SDS 0.724 Numeric seismic design value at 0.2s SA SD1 * null Numeric seismic design value at 1.0s SA * See Section 11.4.8 Additional Information Name Value Description SDC * null Seismic design category Fa 1.108 Site amplification factor at 0.2s Fv * null Site amplification factor at 1.0s CRS 0.899 Coefficient of risk (0.2s) CR1 0.91 Coefficient of risk (1.0s) PGA 0.429 MCEG peak ground acceleration FPGA 1.171 Site amplification factor at PGA PGAM 0.502 Site modified peak ground acceleration TL 8 Long-period transition period (s) 235 ft Report a map errorMap data ©2021 G.E.I Response Letter Response to City of Carlsbad Third-Party Geotechnical Review March 5, 2021     05 March 2021 Premier Pools and Spas Job No. 20-12991 5431 Avenida Encinas Carlsbad, CA 92008 Attn: Mr. Aaron Temme Subject: Response to City of Carlsbad Third-Party Geotechnical Review Brigden Residence 1953 Cobalt Drive Carlsbad, California References: Hetherington Engineering, Inc., Third-Party Geotechnical Review (first), Proposed Pool/Spa Retaining Walls, 1953 Cobalt Drive Carlsbad, California. Project No 9288.1 Log No. 21295 dated February 2, 2021. Geotechnical Exploration Inc., Limited Soil Investigation and Bearing Value Evaluation, Brigden Property Retaining Wall, 1953 Cobalt Drive Carlsbad, California. Project No. 20-12991, dated January 13, 2021. bHA, Inc., Grading Plans for: 1953 Cobalt Drive, GR2021-0003, undated Dear Mr. Temme: In accordance with your request, Geotechnical Exploration, Inc. herein responds to the City of Carlsbad Third-Party Reviewer comments in a memo with completion date February 2, 2021 (Referenced above), with respect to the planned residential project at the subject property. For clarity purposes we include the comments and responses to them. Comment No. 1: The Consultant should provide a location map for the site. Response: Refer to Figure No. 1 for Vicinity Map. Comment No. 2: The Consultant should provide a detailed site description. Brigden Residence Job No. 20-12991 Carlsbad, California Page 2   Response: The lot is known as Assessor’s Parcel No. 213-290-09, in the City of Carlsbad, County of San Diego, State of California. For the purposes of this report, the proposed residence is assumed to face northward to Cobalt Drive but, in fact, faces northwest. Refer to Figure No. I, the Vicinity Map, for the site location. The generally pentagonal shaped lot, consisting of 10,466 square feet, is bordered on the west by single-family residential developments at a higher elevation; on the north by Cobalt Drive; and on the south by single-family residential properties and on the east by Estrella De Mar Road. Refer to Figure No. II, the Plot Plan. Our site visit revealed that the property is currently developed with a two story, single-family residence with an attached garage on a relatively level building pad. Vegetation on the site consists primarily of ornamental landscaping including shrubbery and several mature trees. Approximate elevations across the property range from approximately +241 feet above MSL at the southwest property corner of the site to approximately +223 feet above MSL at the northeast property corner, respectively. Survey information concerning elevations across the site was obtained from the topographic survey template of the preliminary grading plan by bHA, Inc., undated (see Plot Plan, Figure No. II). Comment No. 3: The Consultant should provide a detailed description of proposed site grading, structures/improvements, foundation type, etc. Response: It is our understanding, based on our communication with the client, that the project will consist of the addition of a swimming pool and spa in the rear yard as well as associated improvements, which will make use of retaining walls, continuous footings and slabs on grade. In addition, it is also our understanding that the proposed retaining wall along the southwest side yard is to be constructed of masonry block and continuous concrete foundation. Comment No. 4: The Consultant should review the project grading, foundation and pool plans, provide any additional geotechnical recommendations considered necessary, and confirm that the plans have been prepared in accordance with the geotechnical recommendations provided in the referenced report. Response: Geotechnical Exploration Inc. has reviewed the project grading, foundation and pool plans and found them to be in accordance with the recommendations provided in the referenced report. The geotechnical recommenda- tions presented in this addendum report reflect any necessary updated geotechnical recommendations. Brigden Residence Job No. 20-12991 Carlsbad, California Page 3   Comment No. 5: The Consultant should provide a geotechnical map/plot plan utilizing the latest grading plan for the project to clearly show (at a minimum) a) existing site topography, b) proposed structures/improvements, c) proposed finished grades, d) locations of subsurface exploration, e) geologic units/contacts, and f) remedial grading limits, etc. Response: This addendum report includes a geotechnical map/plot plan utilizing the latest grading plan provided to this office by the client for the project and including a) existing site topography, b) proposed structures/improvements, c) proposed finished grades, d) locations of subsurface exploration, e) geologic units/contacts, and f) remedial grading limits. Comment No. 6: The Consultant should address the feasibility of the project. Response: It is our opinion that the project is feasible from a geotechnical standpoint. We found the site to be well suited for the proposed swimming pool and retaining wall project. Comment No. 7: The Consultant should provide a statement regarding the impact of the proposed grading and construction on adjacent properties and improvements. Response: Based on the available information at this stage, it is our opinion that the proposed site development would not destabilize neighboring properties or induce the settlement of adjacent structures or improvements if designed and constructed in accordance with our recommendations. Comment No. 8: The Consultant should provide grading recommendations (remedial grading, temporary excavations, compaction criteria, etc.). Response: All utility trenches and retaining walls should be backfilled with properly compacted fill. Backfill material should be placed in lift thicknesses appropriate to the type of compaction equipment utilized and compacted to a minimum degree of compaction of 90 percent based upon ASTM D1557-12e1 by mechanical means. Any portion of the trench backfill in public street areas within pavement sections should conform to the material and compaction requirements of the adjacent pavement section. Backfill soils placed behind retaining walls should be installed as early as the retaining walls are capable of supporting lateral loads. Backfill soils behind retaining walls should be low expansive (Expansion Index less than 50 per ASTM D4829-19). Our experience has shown that even shallow, narrow trenches (such as for irrigation and electrical lines) that are not properly compacted can result in problems, particularly with respect to shallow groundwater accumulation and migration. Brigden Residence Job No. 20-12991 Carlsbad, California Page 4   As stated in CBC 2019, Section 1705.6 Soils: “Special inspections and tests of existing site soil conditions, fill placement and load-bearing requirements shall be performed in accordance with this section and Table 1705.6 (see below). The approved geotechnical report and the construction documents prepared by the registered design professionals shall be used to determine compliance. During fill placement, the special inspector shall verify that proper materials and procedures are used in accordance with the provisions of the approved geotechnical report.” A summary of Table 1705.6 “REQUIRED SPECIAL INSPECTIONS AND TESTS OF SOILS” is presented below: a) Verify materials below shallow foundations are adequate to achieve the design bearing capacity; b) Verify excavations are extended to proper depth and have reached proper material; c) Perform classification and testing of compacted fill materials; d) Verify use of proper materials, densities and ft thicknesses during placement and compaction of compacted fill prior to placement of compacted fill, inspect subgrade and verify that site has been prepared properly Section 1705.6 “Soils” statement and Table 1705.6 indicates that it is mandatory that a representative of this firm (responsible engineering firm), perform observations and fill compaction testing during excavation operations to verify that the remedial operations are consistent with the recommendations presented in this report. All grading excavations resulting from the removal of soils should be observed and evaluated by a representative of our firm before they are backfilled. The pool excavation is anticipated to be in firm soils and relatively safe temporary vertical wall excavation may remain for two months. After that time, additional inspections should be made to ensure their adequacy and safety. The contractor should follow Cal-OSHA recommendations at the job site as applicable to the trade. At the time of our representative’s site visits, any area of excavation that may be unstable and/or unsafe can be evaluated. The contractor’s OSHA knowledgeable person at the site can call our office if any unstable soil conditions are observed. Quality control grading observation and field density testing for the purpose of documenting adequate compaction has been achieved and acceptable soils have been utilized to properly support a project applies not only to fill soils supporting primary structures (unless supported by deep foundations or caissons) but all site improvements such as stairways, patios, pools and pool decking, sidewalks, driveways and retaining walls, etc. Observation and testing of utility line trench backfill also reduces the potential for localized settlement of all of the above including all improvements outside of the footprint of primary structures. Brigden Residence Job No. 20-12991 Carlsbad, California Page 5   Comment No. 9: The Consultant should provide hardscape minimum thickness, reinforcement, and joint spacing from a geotechnical standpoint. Response: As a minimum for protection of on-site improvements, we recommend that all exterior pedestrian concrete slabs be 4 inches thick and be founded on properly compacted and tested fill, with No. 3 bars at 15-inch centers, both ways, at the center of the slab, and contain adequate isolation and control joints. The performance of on-site improvements can be greatly affected by soil base preparation and the quality of construction. It is therefore important that all improvements are properly designed and constructed for the existing soil conditions. The improvements should not be built on loose soils or fills placed without our observation and testing. For exterior slabs with the minimum shrinkage reinforcement, control joints should be placed at spaces no farther than 15 feet apart or the width of the slab, whichever is less, and also at re-entrant corners. Control joints in exterior slabs should be sealed with elastomeric joint sealant. The sealant should be inspected by the owner every 6 months and be properly maintained. Comment No. 10: The Consultant should provide swimming pool recommendations. Response: The recommended allowable soil bearing capacity of the on-site, dense formational or properly compacted fill material is 1,500 psf. The friction coefficient is 0.40. A static active equivalent fluid weight of at least 45 pcf should be used for pool walls with level backfill using low expansive soils and 62 pcf for any sloping backfill. Soil seismic increment values presented previously in this letter should be included in the pool shell design. Surcharge loads applied within a horizontal distance equal to the pool depth should be considered by converting the vertical load to horizontal load using a conversion factor of 0.47 Comment No. 11: The Consultant should specify the sulfate exposure category (ACI 318) based on soluble sulfate testing and provide recommendations for sulfate resistant concrete, if necessary, or default to a severe exposure category, if testing is not available. Response: Geotechnical Exploration Inc. recommends that a soluble sulfate test be performed of the near surface soils to be in contact with foundations and concrete elements after grading is completed, to determine the sulfate exposure category and recommend the appropriate sulfate resistant concrete prior to pouring concrete. Alternatively, if these chemical tests are not performed, we recommend using concrete with a compressive strength of 4,500 psi, cement type V, with a water cement ratio no higher than 0.40. Brigden Residence Job No. 20-12991 Carlsbad, California Page 6   Comment No. 12: The Consultant should provide a list of recommended observation and testing during site grading and construction. Response: This addendum report provides a discussion and a list of recommended observation and testing during site grading and construction. Geotechnical Exploration, Inc. recommends that we be retained to verify the actual soil conditions revealed during site grading work and footing excavation to be as anticipated in the "Limited Soil Investigation and Bearing Value Evaluation" and this addendum report for the project. In addition, the placement and compaction of any fill or backfill soils during site grading work must be observed and tested by the soil engineer. The contractor should contact our firm when fill or backfill soils are to be placed. It is the responsibility of the grading contractor and general contractor to comply with the requirements on the grading plans as well as the local grading ordinance. All retaining wall and trench backfill should be properly compacted. Geotechnical Exploration, Inc. will assume no liability for damage occurring due to improperly or uncompacted backfill placed without our observations and testing. This opportunity to be of service is sincerely appreciated. Should you have any questions concerning the following report, please do not hesitate to contact us. Reference to our Job No. 20-12991 will expedite a response to your inquiries. Respectfully submitted, GEOTECHNICAL EXPLORATION, INC. _______________________________ ______________________________ Jaime A. Cerros, P.E. Hector G. Estrella, C.E.G. Senior Geotechnical Engineer Engineering Geologist R.C.E. 34422/G.E. 2007 P.G. 9019/C.E.G. 2656 Exp. 5/31/22 VICINITY MAP Brigden Residence 1953 Cobalt Drive Carlsbad, CA.Figure No. I Job No. 20-12991 SITESITESITE Thomas Bros Guide San Diego County pg 1127-E5 Approximate Limits ofRemedial GradingBrigden Residence1953 Cobalt DriveCarlsbad, CA.Figure No. IIJob No. 20-12991REFERENCE: This PLOT PLAN was prepared from an existing undated GRADING PLAN by bha, INC. and from on-site field reconnaissance performed by GEI.Scale: 1” =20’(approximate)LEGENDNOTE: This Plot Plan is not to be used for legal purposes. Locations and dimensions are approximate. Actual property dimensions and locations of utilities may be obtained from the Approved Building Plans or the “As-Built” Grading Plans.20-12991-p.aiPLOT PLAN ANDSITE SPECIFICGEOLOGIC MAPMarch 2021Observations made in existingretaining wall foundation excavation.( no exploratory test excavations )Artificial FillQafGEOLOGIC LEGENDTsaSantiago Formation (concealed)QafTsaQafTsa