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PGA West - Signature TR 36537 BCPR2020-0016 - Geotechnical Report78-075 Main Street #G-203 ■ La Quinta, California 92253 ■ Telephone 760.579.9926 ■ Fax 951.304.2392 Project No. T2572-22-06 September 24, 2020 WOODBRIDGE PACIFIC GROUP, LLC 27271 Las Ramblas, Suite 100 Mission Viejo, California 92691 Attention: Mr. Robert Richards Subject: UPDATED GEOTECHNICAL RECOMMENDATIONS 2019 CBC SEISMIC DESIGN PARAMETERS SIGNATURE AT PGA WEST – HACIENDAS & ESTATES PGA BOULEVARD AT SIGNATURE WAY LA QUINTA, CALIFORNIA Reference: 1) Due Diligence Geotechnical Investigation, PGA West, Tract 36537, LA Quinta, California, prepared by Geocon West Incorporated dated October 31, 2013 (Project No. T2572-22-01). 2) Report of Observation and Testing Services During Grading, The Signature at PGA West Tract 36537-1 All and Tract 36537-2 Lots 8-31 and Multifamily, LA Quinta, California, Grading Permit No. BGR-14017, prepared by Geocon West Incorporated dated December 3, 2014 (Project No. T2572-22-03). Dear Mr. Richards: In accordance with your request, and with respect to the Proposal No. IE-2594, dated June 4, 2020, Geocon West, Inc. (Geocon) has provided the updated geotechnical recommendations for finish grading, reconditioning of previously graded lots, seismic design parameters in accordance with the 2019 California Building Code (CBC) and ASCE 7-16, and post tensioned slab and conventional foundation recommendations. The seismic design parameters presented herein are based on the referred sections, figures and formula (CBC 2019 and ASCE 7-16) and does not include parameters based on site-specific ground motion hazard analysis. The information, conclusions, and recommendations provided in the reference geotechnical reports remain applicable unless revised information or recommendations are provided herein. BCPR2020-0016 SFDx8 02/23/2021 Geocon Project No. T2572-22-06 - 2 - September 24, 2020 FINISH GRADING OF LOTS 1-7 AND 32-38 When development on the site stopped in 2014, Lots 1-7 and 32-38 was left below finished grade and soils were stockpiled on the lots. These lots were classified as Category II foundations based on the underlying fill geometry. This classification could change if the soils used to complete the lots is expansive. The Foundation Categories for the remaining lots are provided in the referenced Report of Observation and Testing Services during Grading dated December 3, 2014. The completion of grading for these lots should entail the removal of the stockpiled soils to the elevation of the last recorded density test. Followed by scarification, moisture conditioning, and compaction of the previously tested fill soil. Then fill placement in accordance with the referenced reports can proceed until finish grade elevations are reached. Geocon should be on site full time during fill placement to perform observation and testing during grading. Once grading is complete soil samples should be collected and submitted to the laboratory for expansion potential and sulfate content testing. Final Foundation Categories will be provided in a geotechnical report of observation and testing for these lots. RECONDITIONING AND RECERTIFICATION OF PREVIOUSLY GRADED LOTS The remaining lots within the tract should be reconditioned prior to foundation construction. Reconditioning can be accomplished on a per phase basis and should entail removal and/or scarification of the upper two feet of soil across the lot, moisture conditioning and recompaction in accordance with the referenced reports. Geocon should be on site to perform geotechnical observation and testing to verify that the soils contain adequate moisture and density at the -2-foot elevation, and to observe and test as soils are moisture conditioned and compacted. Upon completion, Geocon will prepare a Lot Recertification letter for each phase. Geocon Project No. T2572-22-06 - 3 - September 24, 2020 SEISMIC DESIGN CRITERIA – 2019 CALIFORNIA BUILDING CODE Table 1 summarizes site-specific design criteria obtained from the 2019 California Building Code and ASCE 7-16. TABLE 1 2019 CBC SEISMIC DESIGN PARAMETERS Parameter Value 2019 CBC Reference Site Class D Section 1613.2.2 MCER Ground Motion Spectral Response Acceleration – Class B (short), SS 1.500g Figure 1613.2.1(1) MCER Ground Motion Spectral Response Acceleration – Class B (1 sec), S1 0.600g Figure 1613.2.1(2) Site Coefficient, FA 1.000 Table 1613.2.3(1) Site Coefficient, FV 1.700 Table 1613.2.3(2) Site Class Modified MCER Spectral Response Acceleration (short), SMS 1.500g Section 1613.2.3 (Eqn 16-36) Site Class Modified MCER Spectral Response Acceleration – (1 sec), SM1 1.020g Section 1613.2.3 (Eqn 16-37) 5% Damped Design Spectral Response Acceleration (short), SDS 1.000g Section 1613.2.4 (Eqn 16-38) 5% Damped Design Spectral Response Acceleration (1 sec), SD1 0.680g Section 1613.2.4 (Eqn 16-39) Lat.= 33.6417 Long.= -116.2627 Table 2 presents the mapped maximum considered geometric mean (MCEG) seismic design parameters for projects located in Seismic Design Categories of D through F in accordance with ASCE 7-16. TABLE 2 ASCE 7-16 PEAK GROUND ACCELERATION Parameter Value Site Class D Mapped MCEG Peak Ground Acceleration, PGA 0.589g Site Coefficient, FPGA 1.1 Site Class Modified MCEG Peak Ground Acceleration, PGAM 0.648g Geocon Project No. T2572-22-06 - 4 - September 24, 2020 Conformance to the criteria in Tables 1 and 2 for seismic design does not constitute any kind of guarantee or assurance that significant structural damage or ground failure will not occur if a large earthquake occurs. The primary goal of seismic design is to protect life, not to avoid all damage, since such design may be economically prohibitive. Considering an earthquake with the magnitude of 7.34 and peak ground acceleration of 0.65, we recommend a total seismic settlement on the order of about one inch with a differential seismic settlement on the order of about ½ inch over a horizontal distance of 40 feet. The project structural engineer and architect should evaluate the appropriate Risk Category and Seismic Design Category for the planned structures. The values presented herein assume a Risk Category of II. Table 3 presents a summary of the risk categories. TABLE 3 CBC 2019 RISK CATEGORIES Risk Category Building Use Examples I Low Risk to Human Life at Failure Barn, Storage Shelter II Nominal Risk to Human Life at Failure (Buildings Not Designated as I, III or IV) Residential, Commercial and Industrial Buildings III Substantial Risk to Human Life at Failure Theaters, Lecture Halls, Dining Halls, Schools, Prisons, Small Healthcare Facilities, Infrastructure Plants, Storage for Explosives/Toxins IV Essential Facilities Hazardous Material Facilities, Hospitals, Fire and Rescue, Emergency Shelters, Police Stations, Power Stations, Aviation Control Facilities, National Defense, Water Storage Geocon Project No. T2572-22-06 - 5 - September 24, 2020 FOUNDATION AND CONCRETE SLABS-ON-GRADE RECOMMENDATIONS The foundation recommendations presented herein are for proposed residential structures. We separated the foundation recommendations into two categories based on either the maximum and differential fill thickness or Expansion Index. We anticipate the majority of structures will be Category II due to the geometry of the underlying fill and native materials. However, the category may be increased to Category III where expansive soils are present within three feet of finish grades in the building pads. The foundation category criteria for the anticipated conditions are presented in Table 4. Foundation Categories are provided in the December 3, 2014 report of rough grading. Foundation Categories for Lots 1-7 and 32-38 will be determined once grading for those lots is complete and expansion potential of the soils has been tested. TABLE 4 FOUNDATION CATEGORY CRITERIA Foundation Category Maximum Fill Thickness, T (Feet) Differential Fill Thickness, D (Feet) Expansion Index (EI) II T<50 D<20 EI<90 III T>50 D>20 90<EI<130 Post-Tensioned Foundation Design We understand that post-tensioned concrete slab and foundation systems was used for the previously constructed homesat the site. The post-tensioned systems should be designed by a structural engineer experienced in post-tensioned slab design and design criteria of the Post-Tensioning Institute (PTI), as required by the 2019 California Building Code (CBC Section 1808). Although this procedure has been developed for expansive soil conditions, we understand it can also be used to reduce the potential for foundation distress due to differential fill settlement. The post-tensioned design should incorporate the geotechnical parameters presented on Tables 4 and 5 for the particular Foundation Category designated. The parameters presented in Table 5 are based on the guidelines presented in the PTI, design manual. The foundations for the post-tensioned slabs should be embedded in accordance with the recommendations of the structural engineer. TABLE 5 POST-TENSIONED FOUNDATION SYSTEM DESIGN PARAMETERS Post-Tensioning Institute (PTI) Third Edition Design Parameters Foundation Category II III Thornthwaite Index -20 -20 Equilibrium Suction 3.9 3.9 Edge Lift Moisture Variation Distance, eM (feet) 5.1 4.9 Edge Lift, yM (inches) 1.10 1.58 Center Lift Moisture Variation Distance, eM (feet) 9.0 9.0 Center Lift, yM (inches) 0.47 0.66 Geocon Project No. T2572-22-06 - 6 - September 24, 2020 Slabs that may receive moisture-sensitive floor coverings or may be used to store moisture-sensitive materials should be underlain by a vapor retarder. The vapor retarder design should be consistent with the guidelines presented in the American Concrete Institute’s (ACI) Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials. In addition, the membrane should be installed in accordance with manufacturer’s recommendations and ASTM requirements and installed in a manner that prevents puncture. The vapor retarder used should be specified by the project architect or developer based on the type of floor covering that will be installed and if the structure will possess a humidity-controlled environment. The bedding sand thickness should be determined by the project foundation engineer, architect, and/or developer. However, we should be contacted to provide recommendations if the bedding sand is thicker than 6 inches. Placement of 3 inches and 4 inches of sand is common practice in Southern California for 5-inch and 4-inch thick slabs, respectively. The foundation engineer should provide appropriate concrete mix design criteria and curing measures that may be utilized to assure proper curing of the slab to reduce the potential for rapid moisture loss and subsequent cracking and/or slab curl . We suggest that the foundation engineer present concrete mix design and proper curing methods on the foundation plans. It is critical that the foundation contractor understands and follows the recommendations presented on the foundation plans. The foundations for the post-tensioned slabs should be embedded in accordance with the recommendations of the structural engineer. If a post-tensioned slab foundation system is planned, the slab should possess a thickened edge with a minimum width of 12 inches and extend below the clean sand or crushed rock layer. If the structural engineer proposes a post-tensioned foundation design method other than the 2019 CBC: • The criteria presented in Tables 4 and 5 are still applicable. • Interior stiffener beams should be used for Foundation Categories II and III. • The width of the perimeter foundations should be at least 12 inches. • The perimeter footing embedment depths should be at least 12 inches, 18 inches and 24 inches for foundation categories I, II, and III, respectively. The embedment depths should be measured from the lowest adjacent pad grade. Geocon Project No. T2572-22-06 - 7 - September 24, 2020 Our experience indicates post-tensioned slabs are susceptible to excessive edge lift, regardless of the underlying soil conditions. Placing reinforcing steel at the bottom of the perimeter footings and the interior stiffener beams may mitigate this potential. Because of the placement of the reinforcing tendons in the top of the slab, the resulting eccentricity after tensioning reduces the ability of the system to mitigate edge lift. The structural engineer should design the foundation system to reduce the potential of edge lift occurring for the proposed structures. During the construction of the post-tension foundation system, the concrete should be placed monolithically. Under no circumstances should cold joints form between the footings/grade beams and the slab during the construction of the post-tension foundation system. Category II, or III foundations may be designed for an allowable soil bearing pressure of 2,500 pounds per square foot (psf) (dead plus live load). This bearing pressure may be increased by one-third for transient loads due to wind or seismic forces. We estimate the total static settlement under the imposed allowable loads to be about one inch with differential static settlement on the order of ½ inch over a horizontal distance of 40 feet. Also, we estimate the total seismic settlement to be about one inch with differential seismic settlement on the order of about ½ inch over a horizontal distance of 40 feet. Isolated footings, if present, should have the minimum embedment depth and width recommended for conventional foundations for a particular foundation category. The use of isolated footings, which are located beyond the perimeter of the building and support structural elements connected to the building, are not recommended for Category III. Where this condition cannot be avoided, the isolated footings should be connected to the building foundation system with grade beams. For Foundation Category III, consideration should be given to using interior stiffening beams and connecting isolated footings and/or increasing the slab thickness. In addition, consideration should be given to connecting patio slabs, which exceed 5 feet in width, to the building foundation to reduce the potential for future separation to occur. Special subgrade presaturation is not deemed necessary prior to placing concrete; however, the exposed foundation and slab subgrade soil should be moisture conditioned, as necessary, to maintain a moist condition as would be expected in such concrete placement. Geocon Project No. T2572-22-06 - 8 - September 24, 2020 Where buildings or other improvements are planned near the top of a slope steeper than 3:1 (horizontal to vertical), special foundations and/or design considerations are recommended due to the tendency for lateral soil movement to occur. • Building footings should be deepened such that the bottom outside edge of the footing is at least 7 feet horizontally from the face of the slope. • Geocon should be contacted to review the pool plans and the specific site conditions to provide additional recommendations, if necessary. • Swimming pools located within 7 feet of the top of cut or fill slopes are not recommended. Where such a condition cannot be avoided, the portion of the swimming pool wall within 7 feet of the slope face be designed assuming that the adjacent soil provides no lateral support • Although other improvements, which are relatively rigid or brittle, such as concrete flatwork or masonry walls, may experience some distress if located near the top of a slope, it is generally not economical to mitigate this potential. It may be possible, however, to incorporate design measures that would permit some lateral soil movement without causing extensive distress. Geocon Incorporated should be consulted for specific recommendations. • Any recommendation or design method shall provide a satisfactory safety factor not less than the safety factors based on the recommendations provided in 2019 CBC Section 1808.7. The recommendations of this report are intended to reduce the potential for cracking of slabs due to expansive soil (if present), differential settlement of existing soil or soil with varying thicknesses. However, even with the incorporation of the recommendations presented herein, foundations, stucco walls, and slabs-on-grade placed on such conditions may still exhibit some cracking due to soil movement and/or shrinkage. The occurrence of concrete shrinkage cracks is independent of the supporting soil characteristics. Their occurrence may be reduced and/or controlled by limiting the slump of the concrete, proper concrete placement and curing, and by the placement of crack control joints at periodic intervals, in particular, where re-entrant slab corners occur. Geocon should be consulted to provide additional design parameters as required by the structural engineer. Geocon Project No. T2572-22-06 - 9 - September 24, 2020 For slabs-on-grade underlain by compacted fill we recommend that a modulus of subgrade reaction of 125 pounds per cubic inch (pci) be utilized for the design of the slab foundation bearing in newly placed engineered fill. This value is a unit value for use with a one-foot square footing. The modulus should be reduced in accordance with the following equation when used with larger foundations: where: KR = reduced subgrade modulus K = unit subgrade modulus B = foundation width (in feet) For preliminary analysis, a vertical modulus of subgrade reaction on the order of 30 pounds per cubic inch (pci) may be used. The modulus should be varied between 20 and 40 pci to evaluate the effects that variable seismic settlement may have on the slab. The slab should be designed with enough stiffness to significantly reduce the potential distortion that could occur due to soil differential settlement during earthquake event. Conventional Foundation Design Category II or III foundations may be designed for an allowable soil bearing pressure of 2,500 pounds per square foot (psf) (dead plus live load). Foundation categories are defined in Tables 4 and 5 and will be evaluated once site grading has been completed. This bearing pressure may be increased by one-third for transient loads due to wind or seismic forces. We estimate the total static settlement under the imposed allowable loads to be about one inch with differential static settlement on the order of ½ inch over a horizontal distance of 40 feet. Also, we estimate the total seismic settlement to be about one inch with differential seismic settlement on the order of about ½ inch over a horizontal distance of 40 feet. Table 6 presents minimum foundation and interior concrete slab design criteria for conventional foundation systems. TABLE 6 CONVENTIONAL FOUNDATION RECOMMENDATIONS BY CATEGORY Foundation Category Minimum Footing Embedment Depth (inches) Continuous Footing Reinforcement Interior Slab Reinforcement II 18 Four No. 4 bars, two top and two bottom No. 3 bars at 24 inches on center, both directions at slab mid-point III 24 Four No. 5 bars, two top and two bottom No. 3 bars at 18 inches on center, both directions at slab mid-point Geocon Project No. T2572-22-06 - 10 - September 24, 2020 The embedment depths presented in Table 6 should be measured from the lowest adjacent pad grade for both interior and exterior footings. The conventional foundations should have a minimum width of 18 inches and 24 inches for continuous and isolated footings, respectively. Footings may be designed to resist lateral loads using an allowable coefficient of friction between soil and concrete of 0.35. The footings may also be designed using a passive resistance exerted by an equivalent fluid weight of 250 pounds per cubic foot (pcf) with a maximum of 2200 psf where the footings are poured neat against compacted fill materials. The allowable passive pressure assumes a horizontal surface extending at least 5 feet, or three times the surface generating the passive pressure, whichever is greater. The upper 12 inches of material in areas not protected by floor slabs or pavement should not be included in the design for passive resistance. The passive resistance may be increased by one-third when considering loads of short duration, such as wind or seismic forces. Isolated footings, if present, should have the minimum embedment depth and width recommended for conventional foundations for a particular foundation category. The use of isolated footings, which are located beyond the perimeter of the building and support structural elements connected to the building, are not recommended for Category III. Where this condition cannot be avoided, the isolated footings should be connected to the building foundation system with grade beams. The requirements for concrete and rebar presented in this report are preliminary recommendations. The Project Design/Civil/Structural Engineer should provide the final recommendations for structural design of concrete and rebar in accordance with the latest version of the applicable codes and standards. Should you have any questions regarding this report, or if we may be of further service, please contact the undersigned at your convenience. Very truly yours, GEOCON WEST, INC. Lisa A. Battiato CEG 2316 Mehrab Jesmani PhD, PE 81452 LAB:MJ:JJV:hd Attachments: LIMITATIONS AND UNIFORMITY OF CONDITIONS Distribution: (e-mail) Addressee Geocon Project No. T2572-22-06 September 24, 2020 LIMITATIONS AND UNIFORMITY OF CONDITIONS 1. The recommendations of this report pertain only to the site investigated and are based upon the assumption that the soil conditions do not deviate from those disclosed in the investigation. If any variations or undesirable conditions are encountered during construction, or if the proposed construction will differ from that expected herein, Geocon should be notified so that supplemental recommendations can be given. The evaluation or identification of the potential presence of hazardous or corrosive materials was not part of the scope of services provided by Geocon. 2. This report is issued with the understanding that it is the responsibility of the owner, or of his representative, to ensure that the information and recommendations contained herein are brought to the attention of the architect and engineer for the project and incorporated into the plans, and that the necessary steps are taken to see that the contractor and subcontractors carry out such recommendations in the field. 3. The findings of this report are valid as of the date of this report. However, changes in the conditions of a site can occur with the passage of time, whether they are due to natural processes or the works of man on this or adjacent properties. In addition, changes in applicable or appropriate standards may occur, whether they result from legislation or the broadening of knowledge. Accordingly, the findings of this report may be invalidated wholly or partially by changes outside our control. Therefore, this report is subject to review and should not be relied upon after a period of three years. 4. The firm that performed the geotechnical investigation for the project should be retained to provide testing and observation services during construction to provide continuity of geotechnical interpretation and to check that the recommendations presented for geotechnical aspects of site development are incorporated during site grading, construction of improvements, and excavation of foundations. If another geotechnical firm is selected to perform the testing and observation services during construction operations, that firm should prepare a letter indicating their intent to assume the responsibilities of project geotechnical engineer of record. A copy of the letter should be provided to the regulatory agency for their records. In addition, that firm should provide revised recommendations concerning the geotechnical aspects of the proposed development, or a written acknowledgement of their concurrence with the recommendations presented in our report. They should also perform additional analyses deemed necessary to assume the role of Geotechnical Engineer of Record.