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Mirage (Plans 1-3) 2015 Geotechnical Update• __tea � y .41 f ENGINEERS + GEOLOGISTS + ENVIRONMENTAL SCIENTISTS j -May 7, 2015, - J.N. 15-216' ' CITE OF LA QlJ16VTA Mr. Brian Jacobson BT. - STANDARD PACIFIC HOMES UILDING & SAFETY DEP INLAND EMPIRE DIVISION A P P R V E D 355 East Rincon Street, Suite 300 a FOR CONS- UCTION Corona, California 92879 DATE ? Subject: Geotechnical Update Report, Palo Verde Prglect,_L. 3a h 22, 29 and 30, of. Tract 32279 and Lots 1 through 23 of Tr c 336, City of La Quinta, Riverside County, California Dear Mr. Jacobson: At your request, Petra Geosciences, Inc. (Petra) is providing herewith updated geotechnical recommendations for development of Lots 2, 9 through 12, 16 .through 22, 29 and 30 of Tract 32279 and ' Lots 1 through 23 ' of Tract 33336 within the Palo Verde project located in the City of La Quinta, California. This report is based on our,review.of available geotechnical reports by the previous consultant during original site grading Stoney -Miller Consultants, Inc. (SMC, ' 2006a, 2006b), our previous involvement within the two tracts during post -grading improvements, the requirements of the 2013 California Building Code (CBC), and our engineering judgment and professional opinion. The purpose of this report is to present geotechnical recommendations for site precise grading, and for the design and construction of the foundations for the proposed residential structures and other site improvements that are based on the current site conditions and the 2013 CBC. Site Background Site grading was performed during early- to mid -2006 under the geotechnical observation and testing by Stoney -Miller Consultants, Inc. (SMC, 2006a, 2006b). Removal or overexcavation of unsuitable surface soils was performed to competent native alluvial soils and replaced with compacted engineered fill to design grades. Engineered fills ranged in thickness between 3 and 6 feet within Tract 32279 and between 3 to 4 feet in Tract 33336. The SMC "reports concluded that, based on the observation and testing, the ' subject earthwork was performed in conformance with their, recommendations and with the City of La Quinta grading code. Offices Strategically Positioned Throughout Southern California RBOMI'V'ED ORANGE COUNTY OFFICE �J 3190 Airport Loop Drive, Suite J1,,Costa Mesa, California 92626 MAY O g ZO�S T: 714.549.8921 F: 714.668-3770 For more information visit us online at www.oetra-inc.com CITY OF LA QUINTA COMMUNITY DEVELOPMENT STANDARD PACIFIC HOMES May 7, 2015 Palo Verde/La Quinta J.N. 15-216 Page 2 Site soils are very consistent throughout the project site and consist of fine-grained silty sand (14 percent silt) that have little cohesion. Maximum density and optimum moisture content of the fill materials range from 106 to 112 pounds per cubic. foot (pcf) at optimum moisture contents between 11 and. 14 percent. Laboratory testing performed during rough grading indicated the soils are non -expansive (SMC, 2006a, 20066). SMC continued with post -grading testing (SMC, 2007) through early 2007 and .Petra took over as geotechnical consultant of record in March 2007 until site work was suspended in September of 2009. The conditions within the site are essentially the same as those completed, by 2009 with the following few exceptions. Masonry block walls have been constructed along the perimeter of the development; building I foundations and -slabs have been constructed on Lots 2, 29 and 30 'of Tract 32279; minor erosion has occurred on the vertical cuts along the front yards of many of the buildings pads; an approximately 10= foot by 10 -foot by 1 foot deep depression is located on Lot 10 of Tract 32279; and a number of boulders have been dumped on Lot 12 of Tract 33336. Recent Geotechnical Testing and Evaluation Petra has performed a recent site reconnaissance . and a representative returned to the site on April 27; . 2015 to perform in-situ density testing of near; finish pad grade soils and sampling of shallow subsurface soils for laboratory testing for moisture content. The observed fine-grained silty fine sand fill materials were generally observed to be dense in consistency and very dry, in the upper 1 to 1.5 feet below pad grades with slightly increasing moisture with increasing depth. The results of the field and laboratory testing is provided within Table III at the end of this report. Proposed Precise Grading and Construction The subject lots are presently close to their previous rough grade elevations, and -minor cuts and fills of less than 1 foot are assumed within the subject lots in order to establish proper drainage away from the proposed residences and side- yard swales. We understand that two-story, single-family homes with attached garages are proposed. The proposed single-family' residences. will be . of wood -frame, construction with first floor slabs constructed on -grade. Concrete driveways will provide access to the. adjacent streets, walkways will provide access from the, driveways to the front doors, and patios will be constructed outside of the rear doors as wells as within interior courtyards areas. The rear and side yards will be graded to collect any'surface water and deliver it to the curb and gutter of the adjacent streets. , PE'TRA SDUD AS A ROCK' GEOSCIENCES. c. STANDARD PACIFIC HOMES May 7,2015 Palo Verde/La Quinta J.N. 15-216 Page 3 CONCLUSIONS AND RECOMMENDATIONS Feasibility Based on the current design and our involvement with the project, the.*proposed residential construction within the site is feasible from a geotechnical standpoint if accomplished in accordance with the City of La Quinta requirements and our following recommendations. Site Preparation and Precise Gradin? . The surficial soils within the building pad areas have been somewhat desiccated. and eroded with the . passage of time: Therefore, remedial grading will be required to reprocess all disturbed, desiccated and . eroded surficial soils and create suitable pads for the construction of the proposed improvements. That is, following any necessary clearing operations (boulders, erosion control materials etc.), the near surface soils, should be thoroughly moisture -conditioned to near two percent above optimum to a depth of at least 12 inches below grade and then recompacted: in-place to a minimum relative compaction of 90 percent based on ASTM D1557. Scarification of :the upper building pad surfaces will not be required if the moisture conditioning is freely infiltrating into the surface. Additionally, all structural materials associated with the existing foundations on Lots 2, 29 and 30 of Tract 32279, including utility line pipes under the slab -on -grade areas, should be removed from the site. Following removal of the foundations, the building pads for these three lots should be overexcavated to a depth of 2 feet below finish grade : and the soils replaced as properly compacted fill. The limit of overexcavation should extend a minimum of 5 feet beyond the perimeter footings. Prior to placing the fill, the exposed bottom surfaces should be scarified to a depth of at least 10 inches, watered as necessary to achieve a moisture content that is slightly above optimum moisture, and then recompacted in place to a minimumrelative compaction of 90 percent. A representative . of the project geotechnical consultant should be present during all remedial grading and fill operations. In addition to the building pad areas, any eroded soils or low areas that exist between the existing adjacent street curbs or sidewalks and the front of the .lots should also be scarified and recompacted to achieve a compacted subgrade suitable for construction of driveways and any'remaming sidewalks. Street area subgrade for Cherrywood Place may also require minor preside grading and final street subgrade should be compacted to 95 percent minimum relative compaction. PETRA SOLID ASA ROCK GEOSCIENCES°c STANDARD PACIFIC HOMES May 7, 2015 Palo Verde/La Quinta J.N. 15-216, Page 4 It should be noted that the sequence of precise grading,. as recommended above, is left to grading .contractor's discretion. However, our experience indicates that for conditions where site surficial soils, exist at a moisture content well below optimum, the grading operation may be performed more efficiently if the soils, are thoroughly moisture conditioned utilizing a temporary sprinkler system for at least several days prior to recompaction. Exposed bottom surfaces areas to receive compacted fill should be observed and approved by the project geotechnical consultant prior to fill placement. No fill should be placed without prior approval from.the geotechnical consultant. The project geotechnical consultant should also be present on site during grading operations to document, proper- placement and adequate compaction of fill, as well as to observe compliance with the other recommendations presented herein. Post-Grading Considerations Precise Grading and Drainage We presume that surface drainage systems consisting of sloping concrete flatwork and graded swales will be constructed on the subject lots to collect and direct all surface. water to the curb and gutter of the adjacent streets. In addition, the :ground surface around the proposed buildings should be sloped to . provide a positive drainage gradient away from the structures. The purpose of the drainage systems is to prevent ponding of surface water within the level areas of the site and against building foundations and associated site improvements. The drainage systems should be properly maintained throughout the life of the proposed development. Section 1804.3 of the 2013 CBC requires that "The ground immediately adjacent to the foundation shall be sloped away from the building at a slope of not less than one unit vertical in 20 units horizontal (5- percent slope) for a minimum distance of 10 feet (3048 mm) measured perpendicular to the face of the wall." Further,. "Swales used for this purpose shall be sloped a minimum of 2 percent where located within 10 feet (3048 mm) of-the building foundation." These provisions fall under the purview of the Design Civil Engineer. However, exceptions to allow modifications to these criteria are provided within the same section of the code as "Where climatic or soil conditions warrant, the slope of the ground away from 'the building foundations is permitted to be PETSOLID AS A.ROCN' GEOSCIENCES°° STANDARD PACIFIC HOMES May 7, 2015 Palo Verde/La Quinta J.N. 15-216 Page 5 . reduced to not.less than one unit in 48 units horizontal- (2 percent slope).". This exemption provision appears to fall under the purview of the Geotechnical Engineer-of-Record. It is our understanding that the state-of-the-practice for projects in various cities and unincorporated areas of Riverside County, as well as throughout Southern California, has been to construct earthen slopes at 2 percent minimum gradient away from the foundations and at. 1 percent minimum for. earthen swale gradients. Structures constructed and properly. maintained under those criteria have performed satisfactorily. Therefore, considering the semi-arid climate, site soilconditions and an appropriate irrigation regime, Petra considers that the implementation of 2 percent slopes away from the structures and 1 percent swales to be acceptable for the subject lots. It should be emphasized that the homeowners,are cautioned that the slopes away from the structures and. swales to be properly maintained, not .to be obstructed, and that future improvements not to alter established gradients unless replaced with suitable alternative drainage systems.. Further,. where the flow line of the swale exists within five feet of the structure, adjacent footings shall be deepened appropriately to maintain minimum embedment requirements, measured from the flow. line of the swale: Utility Trench Backfill All utility trench backfill. should be compacted to_a minimum relative compaction of 90 percent. Trench backfill materials should be placed in. approximately 8J 12 inch-thick maximum lifts, moisture conditioned as necessary to .achieve near optimum moisture conditions, .and mechanically compacted in place with a hydra-hammer,, pneumatic, tamper or similar- equipment to achieve a minimum relative compaction of 90 percent. Native soils are generally fine-grained silty sand and. may. be used as backfill material, although the sand equivalent of these materials is anticipated to be less than 30: I A representative .of this firm should probe and .test the backfills, to document the adequate compaction has been achieved. For shallow trenches where pipe or utilities might be damaged by mechanical compaction equipment, imported sand having a .Sand Equivalent (SE) value of 30 or greater may be used for backfill. Sand backfill materials should be watered to achieve optimum (or above) moisture conditions, and then tamped with hand-operated pneumatic'tampers to ensure proper consolidation of the backfill. No specific relative compaction will be required; however,. observation, probing and, if deemed necessary, testing should be PETSOLID AS A-ROCK GEOSCIENCES. STANDARD PACIFIC HOMES _ May 7, 2015 Palo Verde/La Quinta J.N. 15-216 Page 6 .performed by a representative of this firm to verify that the backfill is adequately compacted and will not be subject to excessive settlement: Where an exterior or interior utility trench is proposed in a direction that is parallel to a building footing, the bottom of the. trench should not extend below a 1:1 plane projected downward from the bottom edge of the adjacent footing. Where this condition occurs, the adjacent footing should be deepened or the trench backfilled and compacted prior to construction of the footing. Foundation Design Considerations Seismic Design Coefficients Earthquake loads on earthen structures and buildings are a function of ground acceleration which may be determined from the site-specific. acceleration response. spectrum. To provide the design team with the . parameters necessary to construct .the site-specific acceleration response spectrum for this project, we used the computer applications that are available on . the United States -Geological Survey (USGS) website, http://geohazards.usgs.gov/. Specifically, the. Design Maps website http://earthquake.usgs.gov/designmaps/us/application.php was used to calculate the ground motion parameters. _ To run the above computer applications, site latitude, longitude, , risk category and knowledge of "Site Class" are required. The site class definition depends on the average shear wave velocity, Vs30, within the. upper 30 meters (approximately. 100 feet) of site soils. _A shear wave velocity of 600 to 1,200 feet per second for the upper 100 feet was used for the site based on engineering experience and judgment. The following table, Table 1, provides parameters required_ to construct the site-specific acceleration response spectrum based' on 2013 CBCguidelines: r" ETR SOLID AS A ROCK STANDARD PACIFIC HOMES Palo Verde/La Quinta Table 1 SEISMIC DESIGN PARAMETERS May 7, 2015 J.N. 15-216 Page 7. Ground Motion Parameters Reference Parameter Value Unit Latitude (North) - 33.6295 ° Longitude (West) - -116.2581 ° Site Class Definition Table 20.3-1, ASCE 7-10 D - Assumed Risk Category Table 1604.5, CBC 2013 II - S, - Mapped Spectral Response Acceleration Figure 1613.3.1(1), CBC 2013 1.500 g SI -Mapped Spectral Response Acceleration Figure 1613.3.1(2), CBC 2013 0.602 g Fa - Site Coefficient Table 1613.3.3(1), CBC 2013 1.0 - F,, - Site Coefficient Table 1613.3.3(2), CBC 2013 1.5 - SMs - Adjusted Maximum Considered Earthquake Spectral Response Acceleration Equation 16-37, CBC 2013 1.500 g SM, - Adjusted Maximum Considered Earthquake Spectral Response Acceleration Equation 16-38, CBC 2013 0.903 g SDs - Design Spectral Response Acceleration Equation 16-39, CBC 2013 1.000 g SDI - Design Spectral Response Acceleration Equation 16-40, CBC 2013 0.602 g To - (0.2 SDI/ SDs) Section 11.3, ASCE 7-10 0.120 s TS- (SDI/ SDs) Section 11.3, ASCE 7-10 0.602 s TL- Long Period Transition Period Figure 22-12, ASCE 7-10 8 s FPGA - Site Coefficient Figure 22-7, ASCE 7-10 1.0 - PGAM - Peak Ground Acceleration at MCE Equation 11.8-1, ASCE 7-10 0.524 g Design PGA (0.4 SDs) — Short Retaining Walls Equation 11.4-5, ASCE 7-10 0.40 g CRs - Short Period Risk Coefficient 2 Figure 22-17, ASCE 7-10 1.064 - CRI - Long Period Risk Coefficient Figure 22-18, ASCE 7-10 1.028 - Seismic Design Category' Section 1613.3.5, CBC 2013 D - PGA Calculated at the MCE return period of 2475 years (2 percent chance of exceedance in 50 years). Z PGA Calculated at the Design Level of 2/3 of MCE which is approximately equivalent to a return period of 475 years (10 percent chance of exceedance in 50 years). ' Seismic Design Category may be calculated by the structural engineer in accordance with the alternate design procedures of Section 1613.3.5.1 based on structural characteristics in addition to the ground motion parameters, this may supersede the category listed herein. References: USGS Seismic Design Web Application— http://peohazards.uses.eov/designmaps/us/application.phhp California Building Code (CBC), 2013, California Code of Regulations, Title 24, Part 2, Volume I and IL American Society of Civil Engineers (ASCE/SEI), 2010, Minimum Design Load for Buildings and Other Structures, Standards 7-10. Federal Emergency Management Agency (FEMA), 2009, NEHERP (National Earthquake Hazards Reduction Program) Recommended Seismic Provision for New Building and Other Structures (FEMA P-750). .Allowable Soil Bearink Capacities A basic allowable soil bearing capacity of 17,'5500_pounds per square:foot; including dead and live loads, , may be utilized for design of 247inch-quare-.pad-f6oting and 12 inch_wide continuous footings;founded at a minimum-depth-of-l2,inches=below the lowest adjacent final grade. This value may be increased by 20 PE■ CIf A ( SOLID ASA ROCK STANDARD PACIFIC HOMES May 7, 2015 Palo Verde/La Quinta J.N. 15-216 Page 8 percent for each additional foot of depth and by 10 percent for each additional foot of width to a maximum value of 2,500-pounds_per_square=foot:, Recommended allowable bearing values include both , dead and live loads, and may be=increased-by-on fo short duration wind and seismic forces. Footing Settlements Based on the. allowable bearing values provided above, total settlement of the footings under the anticipated loads is estimated to be less than 1 inch. Differential settlement is expected to be less than '/? inch over a horizontal span of 40 feet. The majority of settlement.is likely to take place as footing loads are applied or shortly thereafter. Lateral Resistance A passive earth pressure of 250pounds per square foot per foot of depth, to a maximum value of 2,500 pounds per square foot, may be used to determine lateral bearing resistance for footings. In addition, a coefficient of friction of 0.30 times the dead load forces may be used between concrete and the supporting . soils to determine lateral sliding resistance. The above values may be increased by one-third when designing for transient wind or seismic forces. It should be noted that, the above values are based on the , condition where footings are cast in direct contact with compacted fill or competent native soils. In cases where the footing sides are formed, all backfill placed against the footings upon removal of forms should be compacted to at least 90 percent of the applicable maximum dry density. Guidelines for Footings and Slabs on -Grade Design and Construction Very Low Expansion Potential (EI = 0 to 20) , The results of laboratory tests performed on representative samples of near -surface soils within the site at the completion of grading indicate that these material exhibit expansion potentials that are within the Very Low range (Expansion Index from 0 to 20). As such, the design of slabs -on -grade is considered to be exempt from the procedures outlined in Sections 1803.5.3 and 1808.6.2 of the 2013 CBC and may be performed using any method deemed rational and appropriate by the project structural engineer. However, the following minimum recommendations are presented herein for conditions where the_project design team may require geotechnical engineering.guidelines for design and construction of footings and . slabs on -grade the project site. GPET C SOLO ASA ROCK STANDARD PACIFIC HOMES May 7,2015 Palo Verde/La Quinta J.N. 15-216 Page 9 The. design and construction guidelines. that follow are based on the above soil conditions and may be considered for reducing the effects of variability in fabric, composition and, therefore, the detrimental behavior of the site soils such as excessive short- and long-term total and differential settlements. These guidelines have been I developed on the basis of the previous experience of this firm on projects with similar soil conditions. 'Although construction performed in accordance with these guidelines has been found to reduce post-construction. movement.and/or distress, they generally do not positively eliminate all potential effects of variability in soils characteristics and future settlement. It should also be noted that the suggestions for dimension and reinforcement provided herein are performance-based and intended only as preliminary guidelines to achieve adequate performance under the anticipated soil conditions: However, they should not be.construed as replacement for structural engineering analyses, experience and judgment. The projectstructural engineer, architect and/or civil engineer should make appropriate adjustments to slab and footing dimensions, and reinforcement type, size and spacing to account for internal concrete forces (e.g., thermal, shrinkage and expansion) as well as external forces (e.g., applied loads) . as deemed necessary. Consideration should also be given to minimum design criteria as dictated by local building code requirements. Conventional Slabs on-Grade System (Very Low Expansion) Given the very low expansion potential exhibited by onsite soils, we recommend that footings and floor slabs be designed and constructed in accordance with the following minimum criteria. Footings 1. Exterior continuous footings supporting one- and two-story structures should be founded at a minimum_depthof-1-7 inches-below the lowest adjacent final grade. Inter-ior continuous footings may be founded at a miniiiium_depth=of=l:0-inches below the tops of the adjacent finish floor slabs. 2. All continuous footings should have minimum widths of d_22and"1"5= ches for one- and two-story construction, respectively. All continuous footings should be reinforced with a -minimum of two; No..4. bars,_one top and-one-bottom: 3. Aminimum_12 inch-wide grade beam founded at the same depth as adjacent footings should be provided across garage entrances or similar openings (such as large doors or bay windows). The grade beam should be reinforced with a similar manner as provided above. PETSSOLID ASA ROCK OSC ENCES STANDARD PACIFIC HOMES May 7, 2015' Palo Verde/La Quinta J.N. 15-216 Page 10 4. Interioriso-lated,pad'footings, if required,.should be a minimumTof 24Tinches square and foundedE at a=minimum-depth=of 12 -inches -below the bottoms of the adjacent floor.slabs. Pad footings" should be reinforced with No=4--bars-spaced�a�maximum of=TS`inclieson centers, both ways,, placed near the bottoms of the footings. .5. r isolated pad footings intended'for support of roof overhangs. such as second-story-decks,I patio -covers and similar construction should be a minimum,of 24`mches.square.and founded at a'' minimu_m=deptt► ofsl8tiinches below the lowest adjacent finalgrade. The pad footings should be • reinforced with No: 4'bars spaced,a maximutii of -f8 inches on -centers, both ways, placed'near the, bottoms of the footings. Exterior isolated pad footings may need to be connected to adjacent.pad and/or continuous footings .via tie beams at the discretion of the; project structural engineer. 6.•- The minimum footing dimensions and reinforcement recommended herein may be modified (increased or decreased subject -to the constraints. of Chapter 18 of the 2013 CBC) by the structural engineer responsible for foundation design based - on his/her calculations, engineering experience and judgment. Building Floor Slabs 1. Concrete floor slabs should be a�minimum:4:inchas tl'iick and reinforced with No..3-bars,spaceda7 maxifrium:ofr24ninches-on.,centers,..6-oil ways. ''Alternatively, the structural engineer may - recommend the use of prefabricated welded wire mesh for slab reinforcement. For this condition, the welded wire mesh should be of sheet'type (not rolled) and should consist of 6x6/W2.9xW2.9 (per the Wire Reinforcement Institute, WRI, designation) or stronger. All slab reinforcement should be supported on concrete chairs or�brick to ensure the desired placement near mid -depth. ' Care should.be exercised to prevent warping of the welded wire mesh between the chairs in order to ensure its placement at the desired mid -slab position. 2. Living area concrete floor slabs and areas to receive moisture sensitive floor coveringshould be, underlain with a moisture vapor retarder consisting of a minimum 10 -mil -thick polyethylene or., polyolefin membrane that meets the minimum requirements of ASTM E96 and ASTM E1745 for vapor retarders (such as Husky Yellow Guard®, Stego® Wrap, or equivalent). All laps within., the membrane should be sealed,'.and at least 2 inches of.clean sand should be placed over the membrane to promote uniform curing of the concrete. • To reduce the potential for punctures, the membrane should be placed on a pad surface that has. been graded smooth without any sharp -protrusions. If a smooth surface cannot be achieved by grading, consideration should be given to,, lowering the pad finished grade an additional inch and then placing a 1 -inch -thick leveling course ' of sand across the pad surface prior to the placement of the membrane. At the present time, some slab designers, geotechnical professionals and concrete experts -view view the sand layer below, the slab (blotting sand) , as a place for entrapment of excess- -moisture that could adversely impact moisture -sensitive floor coverings. Asa preventive measure, the potential for moisture intrusion into the concrete slab could be reduced if the concrete is placed directly on the vapor retarder. However, if this sand layer is 'omitted, appropriate curing methods must be implemented to ensure that the concrete - slab cures uniformly. A qualified materials engineer with experience in slab design and PETRASOLID ASA ROCK ' �� GEOSCIENCES°'-. , STANDARD PACIFIC HOMES May 7, 2015 . Palo Verde/La Quinta J.N. 15-216 Page 1 I construction should provide recommendations for alternative methods of curing and supervise the construction process to ensure. uniform slab curing. Additional steps would also need to be taken to prevent puncturing of the vapor retarder during concrete placement. 3. Garage floor slabs should be a minimum 4 inches thick and reinforced in a similar manner as living area floor slabs. Garage slabs should also be poured separately from adjacent wall footings with a positive separation maintained using '/o=inch-minimum felt expansion joint material. To control the propagation of shrinkage cracks, garage floor slabs should be quartered with weakened plane joints. Consideration should be given to placement of a moisture vapor retarder below the garage slab, similar to that provided in Item 2 above; should the garage slab be overlain with moisture sensitive floor covering. 4. Pre saturation of the subgrade below floor slabs will not be required; however, prior to placing concrete, the subgrade below all dwelling and garage floor slab areas should be thoroughly moistened to achieve a moisture content that is at least equal to or slightly greater than optimum moisture content. This moisture content should penetrate to a minimum depth of 12 inches below the bottoms of the slabs. 5. The minimum dimensions and reinforcement recommended herein for building floor slabs may be modified (increased or decreased) by the structural engineer responsible for foundation design based on his/her calculations, engineering experience and judgment. Alternative Post-Tensioned Slabs on-Grade System (Very Low Expansion) In consideration of the very low expansion potential exhibited by onsite soils, any rational and appropriate , procedure may be ' chosen by the project structural engineer for the design of post-tensioned slabs on grade. Should the design engineer choose to follow the most current procedure published by the Post Tensioning Institute (PTI), the following minimum design criteria are provided. PETRA SOLID AS A ROCK Soil Information Approximate Depth of Constant Suction, feet 9 Approximate Soil Suction, pF ' 3.9 Inferred Thomthwaite Index:.. -20' Average. Edge Moisture Variation Distance, em in. feet:; Center Lift ." .. 9.0 Edge Lift ' Anticipated well, ym in. inches: Center Lift _ 0.20 Edge Lift 0.40 ' Modulus of Subgrade Reaction �- The modulus ofsub grade reaction'for design of_load bearing partitions may be_assumed to be 125 pounds r per cubic'inch. x r j Minimum Design Recommendations1.'- The soil values provided above may be utilized by the project structural engineer to design post -tensioned slabs on -ground in accordance with Section • 1808:6.2 of tNe 2013 CBC'and the P,TI publication. Thicker flooi slabs and larger footing sizes may, required for structural reasons and should govern the design if more restrictive than the minimum recommendations provided below, I • i., 3 1. Perimeter footings for both one-story and two-story structures_'should be founded at a minimum depth of 12 inches below the lowest adjacenf finished ground surface. Interior footings may, be f founded at`a minimum depth'.of 10 inches below the tops of the adjacent finish floor slabs, All ! continuous footings should be reinforced with'aminimum of two No.; 4:bars, one top and one ; bottom. Alternatively, post -tensioned tendons maybe utilized in . the perimetercontinuous footings .in lieu of the reinforcement bars.F ` 2 A minimum 12'inch-wide grade beam founded at the, same depth asradjacent footings should be provided across the.garage'entrances or similar openings (such as large doors or bay windows). The grade beam•should be reinforced in a similar.ma' nner as provided above...: 3 Exterior isolated pad footings intended for support ^of roof overhangs such as second -story decks, t . ^,•''.patio covers and similar construction..should.be a•minimum.of 24 inches square and founded at a i minimum depth of 18 inches. below, the lowest adjacent final grade. The pad footings should be reinforced with No: 4 bars spaced a maximum of 18 inches"on centers, both ways, placed near the ' \/ PET RA SOLID ASA ROCK: ` GEOSCIENCESo ' STANDARD PACIFIC HOMES May 7, 2015 Palo Verde/La Quinta J.N. 15-216 Page 13 bottoms.of the footings. Exterior isolated pad footings may need to be connected to adjacent pad and/or continuous footings via tie beams at the discretion of the project structural engineer. 4. The thickness of the floor slabs should be determined by the project structural engineer with consideration given to the expansion potential of the onsite soils; however; we recommend that a minimum slab thickness of inches be considered. 5. As an .alternative. to designing 47inch-thick post -tensioned slabs with perimeter footings as described in Items 1 and 2 above, the structural engineer may design the foundation system using a thickened slab design. The minimum thickness of this uniformly thick slab should be 8 inches. The engineer in charge of post -tensioned slab design may also opt to use any combination of slab thickness and footing embedment depth as deemed appropriate based on their engineering experience. and judgment. 6. Living area concrete floor slabs and areas to receive moisture sensitive floor covering should be underlain with a moisture vapor retarder consisting of a minimum 10 -mil -thick polyethylene or polyolefin membrane that meets the minimum requirements of ASTM E96 and ASTM E1745 for vapor retarders (such as Husky Yellow Guard®, Stego® Wrap, or equivalent). All laps within the membrane should be sealed; and at least 2 inches of .clean sand should be placed over the membrane to promote uniform curing of the concrete. To reduce the potential for punctures, the membrane should be placed on a pad surface that has been graded smooth without any sharp protrusions. If a smooth surface cannot be achieved by grading, consideration should be given to lowering the pad finished grade an additional. inch and then placing a 1 -inch -thick leveling course of sand across the pad surface prior to the -placement of the membrane. At the present time, some. slab. designers, geotechnical professionals and concrete experts view the sand layer below the slab (blotting sand) as a place for entrapment of excess moisture that could adversely impact moisture -sensitive floor coverings. As a preventive measure, the potential for moisture intrusion into the concrete slab could be reduced if the concrete is placed directly on the vapor retarder. However, if this sand layer is omitted, appropriate curing methods must be implemented to ensure that the concrete slab cures uniformly. A qualified materials engineer with experience in slab design and construction should provide recommendations for alternative methods of curing and supervise the construction process to ensure uniform slab curing. Additional steps would also need to be taken to prevent puncturing of the vapor retarder during concrete placement. 7. Garage floor slabs should be designed in a. similar manner as living area floor slabs. Consideration should be given to placement of a moisture vapor retarder below the garage slab, similar to that provided in Item 6 above, should the garage slab be overlain with moisture sensitive floor covering. 8. Presaturation of the subgrade below floor slabs will not be required; however, prior to placing . concrete, the subgrade below all dwelling and garage floor slab areas should be thoroughly moistened to achieve a moisture content that is at least equal to or slightly greater than optimum moisture content. This moisture content should penetrate to a minimum depth of 12 inches below the bottoms of the slabs. PETS SOLID AS A ROCK GEOSCIENCES STANDARD PACIFIC HOMES May 7,-2015 Palo Verde/La Quinta J.N. 15-216 Page 14 9. The minimum footing dimensions and reinforcement recommended herein. may be modified (increased or decreased subject to the constraints of Chapter 18 of the 2013 CBC) by the structural engineer responsible for foundation design based on his/her calculations, engineering. experience and judgment... Foundation Excavation Observations Foundation excavations should be.observed by a representative of this firm to document that they have been excavated into competent bearing. soils prior to the placement of forms, reinforcement or concrete. The excavations should be trimmed neat, level and square. -All loose, sloughed or moisture-softened soils and/or any construction debris should be removed prior to placing of concrete. Excavated soils derived from footing and/or utility trenches should not be placed in building slab-on-grade areas or exterior concrete flatwork areas unless the soils are compacted to at least 90 percent of maximum dry density. Retaining Walls Allowable Bearing Values and Lateral Resistance Retaining wall footings may be designed using the allowable soil bearing and lateral resistance values recommended previously :for design of residential footings founded in fill. These values are repeated below for convenience. , However, when calculating passive resistance, the upper 6 inches of the footings should be ignored in areas where the footings will not covered with concrete flatwork, or where the thickness of soil cover over the top of the footing is less than 12. inches.. Stepped retaining walls do not appear to be an element of the current design, however, if utilized, the structural design should consider the surcharge load of any superjacent walls or foundations. Allowable Soil Bearing Capacities A basic allowable soil bearing capacity of 1,500 pounds per square foot, including dead and live loads, may be utilized for design of 12-inch-wide continuous footings founded at a minimum depth of 12 inches*, below the lowest adjacent final grade. This value may be increased by 20 percent for each additional foot of depth and by.10 percent for each additional foot of width to a maximum value of 2,500 pounds per square foot. Recommended allowable bearing values include both dead and live loads, and may. be increased by one-third for short duration wind and seismic forces. mr-PETRA SOUO AS A ROCK GEOSCIENCES°0. STANDARD PACIFIC HOMES May 7, 2015 Palo Verde/La Quinta J.N. 15-216 Page 15 Lateral Resistance A passive earth pressure of 250 pounds per square foot per foot of depth, to a maximum value of 2,500 pounds per square foot, may be used to determine lateral bearing resistance for footings. In addition, a coefficient of friction of 0.30 times the dead load forces maybe used between.concrete and the supporting soils to determine lateral sliding resistance. The above values may be increased by one-third when designing for transient wind or seismic forces. It should be noted that the above values are based on the condition where footings are cast in direct contact with compacted fill or competent native soils. In cases where the footing sides are formed, all backfill placed against the footings upon removal of forms should be compacted to at least 90 percent of the applicable maximum dry density. Active and At -Rest Earth Pressures On-site soil materials have very low expansion potentials, therefore, for this condition, active earth pressures equivalent to fluids having densities of 40 and 61 pounds per cubic foot should be used for design of cantilevered walls retaining a level backfill and ascending 2:1 backfill, respectively. For walls. that are restrained at the top, at -rest earth pressures of 60 and 92 pounds per cubic foot (equivalent fluid pressures) should be used. The above values are for retaining walls that have been supplied with a proper. subdrain system (see Figure RW71). All walls should be designed to support any adjacent structural surcharge loads imposed by other nearby walls or footings in addition to the above -recommended active and at -rest earth pressures. The soil type for on-site soils used for wall backfill by the structural designer should be clearly depicted on the retaining wall plans. All structural calculations and detailed plans for the proposed retaining walls should be provided to this firm for verification purposes prior to grading and construction phases. During construction, Petra should observe the foundation excavation prior to placement of reinforcement or forms to verify the anticipated soil conditions. As with structural foundations, the foundation excavations should be free of deleterious material and loose soils prior to placing concrete. Foundation soils should be pre -moistened to near optimum moisture content prior to placement of concrete. 00 PET C�7A SOLID ASA ROCK F, STANDARD PACIFIC HOMES May 7,2015 Palo Verde/La Quinta J.N. 15-216 Page 16 Geotechnical Observation and Testing All grading and construction phases associated with retaining . wall construction, including backcut excavations, footing trenches, installation of the subdrainage systems, and placement of backfill should .be observed and tested by a representative of the project geotechnical consultant. Drainage and Waterproofing Perforated pipe and gravel .subdrains should be installed behind all retaining walls to prevent entrapment of water in the backfill (see Figure RW -1). Perforated pipe should consist of 4 -inch -minimum diameter PVC Schedule 40, or ABS SDR -35, with the perforations lain down. The pipe should be encased in a 1- foot -wide column of 3/4 -inch to 1V2 -inch open -graded gravel and the open -graded gravel should extend above the wall, footings to a minimum height equal to one-third the wall height, or to a minimum height of 1.5 feet above the footing, whichever is greater. The open -graded gravel should be completely wrapped in filter fabric consisting of.Mirafi 140N, or equivalent. Solid outlet pipes should be connected to the subdrains and then routed to a suitable area for discharge of accumulated water. The backfilled . portions of retaining walls should be coated with an approved waterproofing compound or covered with a similar material 'to inhibit migration of moisture through the walls. Wall Backfill . Recommended active and at -rest earth pressures for design of retaining walls are based on the physical and mechanical properties of the on-site soil materials. On-site soils used for backfill should be placed in' . approximately 6- to 8 -inch -thick maximum lifts, watered as necessary to achieve near optimum moisture conditions, and then mechanically compacted in place to a minimum relative compaction of 90 percent.. Flooding or -jetting of the backfill ' materials should ' be avoided. A representative of the project geotechnical consultant should observe the backfill :procedures and test the wall backfill to verify adequate compaction. Masonry Block Screen Walls Footings for masonry block walls may be designed in accordance with the bearing and lateral resistance values provided previously for building footings. However, as a minimum, the wall footings should be embedded at a minimum depth of 18 inches below the lowest adjacent final grade. The footings should PEYC SOLID AS A ROCK STANDARD PACIFIC HOMES May 7, 2015 . Palo Verde/La Quinta J.N. 15-216 Page 17 also be reinforced with a minimum of four No. 4 bars, two top and two bottom. In order. to minimize the potential for unsightly cracking related to the possible effects of differential settlement and/or expansion, positive separations (construction joints) should also be provided in. the block, walls at each corner and at horizontal intervals of approximately 20 to 25 feet. The separations should be provided in the blocks and, not extend through the footings.. The footings should be . poured monolithically with continuous reinforcement bars to serve as effective "grade beams" below the walls. Exterior Concrete Flatwork Very Low Expansion Potential (Expansion Index 0 — 20) . General The guidelines that follow should be considered,as minimums and are subject to review and revision:by the project architect, structural engineer arid/or landscape consultant as deemed appropriate. Thickness and Joint Spacing To reduce the potential of unsightly cracking, concrete walkways, patio-type slabs, large decorative slabs and concrete subslabs to be covered with decorative pavers should be at least 4 inches thick and provided with construction joints or expansion joints every 6 feet or less. Private driveways that will be designed for the use of passenger cars for access to private garages should also be at least 4 inches thick and provided with construction joints or expansion joints every 10 feet or less. Concrete pavement that will be designed based on an unlimited number of :applications of an 18-kip single-axle load in public access . areas, segments of road that will be paved with concrete (such as bus stops and cross-walks) or access roads and driveways, which serve multiple residential units or garages, that will be subject to heavy truck loadings should have a. minimum thickness of 5 inches and be provided with control joints spaced at maximum 10-foot intervals. A modulus of subgrade reaction of 125 pounds per cubic foot,may be used for design of the public and access roads. Reinforcement . All concrete_ flatwork having their, largest plan"view panel dimension exceeding 10 feet should be reinforced with a minimum of No. 3 bars spaced 24 inches on centers, both ways. Alternatively, the slab reinforcement may consist of welded wire mesh of the sheet type (not rolled) with 6x6/W1.4xW1.4, GPETRA SOLID AS A ROCK' OSC ENCES STANDARD PACIFIC HOMES May 7, 2015' Palo Verde/La Quinta . J.N. 15-216. Page 18 designation, in accordance with the Wire Reinforcement Institute (,WRI).. The reinforcement should be. properly positioned near the middle of the slabs. The reinforcement recommendations provided herein. are intended as. guidelines ,to achieve adequate performance for anticipated soil conditions. The project architect, civil and/or structural engineer should make appropriate adjustments in reinforcement type, size and spacing to account for concrete internal (e.g., shrinkage and thermal) and external (e.g., applied loads) forces as deemed necessary._ Edge Beams (Optional) Where the outer edges of concrete flatwork are to be bordered by landscaping, it is recommended that consideration .be given to the use of edge beams (thickened edges) to prevent excessive infiltration and accumulation of water under the slabs. Edge beams, if used, should be 6 to 8 inches wide, extend 8 inches below the tops ofthe finish slab surfaces:. Edge beams are not mandatory; however, their inclusion in flatwork construction adjacent to landscaped areas is intended to reduce the potential for vertical and horizontal movement and subsequent cracking of the flatwork related to uplift forces that can develop in expansive soils. Subgrade Preparation Compaction. To reduce the potential for distress to concrete flatwork, the subgrade soils below concrete flatwork areas to a minimum depth of 12 inches (or deeper; as either prescribed elsewhere in this report or determined in the field) should be moisture conditioned to at least equal to, or slightly greater than, the optimum moisture content and then compacted to a minimum relative compaction of.90 percent. Where concrete public roads, concrete segments of roads and/or concrete access driveways are proposed, the upper 6 inches of subgrade soil should be compacted to a minimum 95 percent relative compaction. . Pre-Moistening As a furthermeasure to reduce the potential for concrete flatwork cracking, subgrade soils should be thoroughly moistened prior to placing concrete. The moisture content of the soils should be at least 1.2 times the optimum moisture content and penetrate to a minimum depth of 12 inches.into the subgrade. PETSOLID ASA ROCK, GEOSCIENCES; STANDARD PACIFIC HOMES May 7, 2015 Palo Verde/La Quinta J.N. 15-216 Page 19 Flooding or ponding of the subgrade is not .considered feasible to achieve the above moisture conditions since this method would likely require construction of numerous earth berms to contain the water. Therefore, moisture conditioning should be achieved with sprinklers or a light spray applied to the subgrade over a period of few to several days just prior to pouring concrete. Pre -watering of the soils is intended to promote uniform curing of the concrete, reduce the development of shrinkage cracks and reduce the potential for differential expansion pressure on freshly poured flatwork. A representative of the project geotechnical consultant should observe and. verify the density and moisture content of the soils, and the depth of moisture penetration prior to pouring concrete: Drainage Drainage from patios and other flatwork areas should be directed to local area drains and/or graded earth swales designed to carry runoff water to the adjacent streets or other approved drainage structures. The concrete flatwork should be sloped at a minimum gradient of one percent, or as prescribed by project civil engineer or local codes, away from building foundations, retaining walls, masonry garden walls and slope areas. Tree Wells Tree wells are not recommended in concrete flatwork areas since they introduce excessive water into the subgrade soils and allow root invasion, both of which can cause heaving and cracking of the flatwork. Plan Reviews, Future Improvements and/or Grading Petra should review the precise grading plans when they become available and issue an addendum letter to this report. If additional site improvements are considered in the future, our firm should be notified so that we may provide design recommendations to mitigate movement, settlement and/or tilting of the structures. Construction of additional improvements is particularly critical on or near the tops of descending slopes. Potential problems can develop when drainage on the pads and slopes is altered in any way such as placement of fill and construction of new walkways, patios, landscape walls, swimming pools, spas and/or planters. Therefore, it is recommended that we be engaged to review the final design drawings, specifications and grading plan prior to any new construction. If we are not provided the opportunity to review these documents with respect to the geotechnical aspects of new construction and PETS SOLID AS A ROCK GEOSCIENCES" STANDARD PACIFIC HOMES ; May 7, 2015 Palo Verde/La Quinta J.N. 15-216 Page 20 grading, it. should not be assumed that the recommendations provided herein are wholly or inpart applicable to the proposed improvements. Post Grading Observations and Testing Petra should be notified at the appropriate times in order that-we may provide the following observation and testing services during the various phases of construction and precise grading. . 1. Building Construction. • Observe all footing trenches when first excavated to .verify adequate depth and competent bearing conditions. • . Re-observe all footing trenches,. if necessary, if trenches are found to be excavated to inadequate depth and/or found to contain' significant slough, saturated or compressible. materials. 2. Concrete Flatwork Construction Observe and test subgrade soils below all concrete flatwork areas to verify adequate ' compaction, moisture content and moisturepenetration prior to pouring concrete. 3. Utility Trench Backfill Observe and test placement of all utility trench backfill mains and laterals to verify adequate compaction. 4. Remedial and Precise Gradin • Following foundation demolition,'observe the overexcavation bottoms for the building pads of Lots 2,30 and 39 of Tract 32279 -prior to engineered fill placement: • Observe and test placement of any fill to be placed on the subject lots or during building pad recertification to verify, adequate compaction and moisture content. • Issue lot recertification letters, if required, during each phase of construction. PET, RA. SOLIO ASA ROCK, STANDARD PACIFIC HOMES May 7, 2015 Palo Verde/La Quinta J.N. 15-216 Page 21 5. Street Construction (Cherrywood Place) . • Observe street subgrade soils to verify adequate compaction and moisture content. • ..Observe and test all imported base and asphalt materials to verify adequate compaction - during street construction: 6: Masonry Screen and Retaining . • Observe all retaining and screen wall footing trenches when first excavated to verify adequate depth and competent bearing conditions. • Observe and document placement. of all retaining wall backdrains and waterproofing. • Observe and test placement of all retaining wall backfill soils to.verify adequate materials and compaction. REPORT LEW[TATIONS This report is based on the two proposed residential Tracts; our previous geotechnical information and our literature review of the prior consultant's grading reports. This report has been prepared consistent with that level of care being provided by other, professionals providing similar services at the same locale and in the same time period. The contents of this report are professional opinions and as such, are not to be considered a guaranty or warranty. This report should be reviewed and updated after a period of one year or if the -site ownership or project concept changes from that described herein. This report. has not been prepared for use by parties or projects other than those named or described herein. This report may not contain sufficient information for other parties or other purposes. This report may be subject to review by the controlling authorities for this project. PETRASOLID AS A ROCK' GEOSCIENCES- .. . .STANDARD PACIFIC HOMES May 7, 2015 Palo Verde/La Quinta J.N. 15-216 Page 23, REFERENCES International Building Code, 2012, 2013 California Building Code, California Code of Regulations, Title 24, Par 2, Volume 2 of 2, California Building Standards Commission, Title 24, Par 8, 2013 California Existing Building Code, Title 24, Part 10. Petra Geotechnical, Inc., 2008; Seismic Design. Parameters Updated for 2007 CBC, Palo Verde, Tract 32279 and 33336, La Quinta, California, prepared for. Standard Pacific Homes, JN 250-07, dated June 26. Stony -Miller Consultants, Inc., 2006a, Earthwork Observation and Testing Report, Rough Grading, Lots 1 through 30, Tract 32279, La Quinta; California, prepared for Standard Pacific Homes Coachella Valley Division, dated August 24. 2006b, Earthwork Observation and Testing Report, Rough Grading, Lots 1 through 23, Tract 33336, La Quinta; California, prepared for. Standard Pacific Homes Coachella Valley Division, dated August 24: . 2007, Earthwork Observation/Testing, Report, Street Improvements, Tract 32279, Rosewood Court, Cherrywood Place and Adjacent Avenue 58 La Quinta, California, prepared for Standard Pacific Homes Coachella Valley Division, dated August 24. 'Wire Reinforcement Institute (WRI), 1996, Design of Slabs on Ground. IM01- GP� CRA SOLID AS A ROCK_ . ,STANDARD PACIFIC HOMES May 7, 2015 Palo Verde/La Quinta J.N. 15-216; Page 24' TABLE III t' In -Situ Dry Density/Moisture Content NATIVE SOIL BACKFILL $toped or level ground surface , ++:? ::•{?�•.'r { ti{�7+';irs 7+/ yf,{�•.•:,+1 .� Via. .ICompacted on-sfte soil - .• '- A[�' `�iiAY+�• • t71 :••7t••r •7i •i �t .�77 �• i/,�•�4/., .. .. , v � f t :�i+;' •' •••••'„�l irl ala.; I S 'i,•„ \,<S, - r s� �. � ,. • t : .a lla aal 'IaI•!,I J\ye ��I� • , 2 :i.i�x`y �� \', •'• '?' ,•'+: :f •••*+'-�' •t ,',♦� �r�'al 'a',l'.,`' ;'t'��`,I:'�:��```', 'a ;t;: ` :,,•. -.Recommended backcut* ♦�1,: l.*?..?�ti�t„•. .•. (� l.. l �.f l., f, <J�iaio��a i�rJ` r�.. ��r•la� -g 'Y tT; ,r, ,fa /al,fa Jt: :, ,,S. a, •• J, J1. H ��41 ` ''7t /1 �4?,J, !.I l\' 4 4 J,• ,I,J,(S ll •• • '4l '' ''J ../,. f.• /•, lift 42., ,♦l,. ,`,:1Naferproofing compound F *v ` o�-�� ��; °:;.• Antall subdraln system WQk. � 1 b H.. 1<i1,.•� YI.I ',Nlinimum'12-Inch-wide column of 314” -1 1/2" ;open graded gravel wrapped in filter fabric. ?b tiQ�o �'.;;.; j; % • ai ' ;Flfter fabric (should consist of too I ' a ; \ �MW 14ON or equivalent) A Inch perforated pipe. Perforated pipe should .i: 1,:.� ,::'��E'itis:it::i:•itil•1'11i,�it;i.::;:: i•j�il{':; ijy ;.�i •J,•`r • ,'; I \ J ,aA diameter ABS SDR of 4-35 or PVC ;; Schedule 40 or approved equivalent with the ;perforations laid down. Pi eshould be laid on .a �4, ``OM1♦41J`,a,J;,;Ja,, ..; '��A��'���x Ya��• ��5 ria ,a •a- ,s la a, -f ,•, .�. r:r •.,�'la. 1,�}zryt ; 1`•Ia; r I la - F' •,,i,;;;;•; a;;;";•''�' ',,. l ����1��a�7��� �� •J, J ��,.et least 2 Inches of open -graded gravel.' 'I; ,J ,JS••, 4 ' , Y. ri �Ykt T N!'�h�'4i4 �Y; t','���}.{1. \ Ja`I',^ r. , l;' . \'•,j♦ �a � V.k Tri •.,,,,iii S 1 •, ;, .,r•..r I I; ' , LL,C♦ , alai r �W , a1/ ai`,'I�,I ;Ja;r f•♦. •'.- . �t;,lr•,,, era �,a, Y �. 1. :a\ ,e a!,.••, r.,,,i♦< ra,�. `ttL I, r,r Sri •f,1,/,f, pY,sf�i. � ,, ,\ �,Ja.,<,J,S� IS♦ .,J,<,Iyr• - ,',,,.♦'• r J �t• •: J r<,�/,'/�r�l i.i,•�/�'r :••'t♦'ate, 'r ,�.i,relc ra.� ♦.. •.,,,4 ;; J..,<,J.:,.,1 ., l\r,•..,•J,<•J,.,l,f.l,J,..LJ * Vertical height (h) and slope angle " +,,♦ ,la ♦ , � 7, `a it i,-t'J is ,.•,a,../alar,,., •=.;"-.,.,<,ta1a/\la/�/a /\.♦.41\�lJ\.•M1/ ofbackcutper soilsreport. Based "'•''''"'' on geologic conditions, configuration ,x610•♦..a�,: ,I♦/, f,J•'-•: ••ww.'\,, of backcut may require revisions (i.e. reduced vertical height, revised slope angle, etc.) PETRA RETAINING SBACKFILL U DRAIN DETAILS ' FIGURE RW -1