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HomeMy WebLinkAboutGEOTECHNICAL RECIEW OF PGP - TR 25479 LOTS 222-224, 234-237 AND 251-254 Riverside County - 38655 Shy Canyon Drive,Suite A Murriew,California 92563 951-600 9271 -� past i-}present-I future it's in oaev science Engineers,Geologists Environmental Scientists April 19,2010 J.N. 121-10 Mr. Edgar Gomez RI('fLM0NJ)AMERICAN HOMES 5171 California Avenue, Suite 120 Irvine, California Subject: GeotectinicalReview of Precise Grading Plans,Tract 25479,Dots 222-224,234- 237 and 251-254, Rosetta Canyon Project, City of Lake Elsinore, Riverside County,California. References: See Attached List. Dear Mr: Gomez: We have reviewed the precise grading plans for both the model lots (Lots 222-223) and the production homesites (Lots 224; 234-237 and 251-254) within the Rosetta Canyon project, Tract 25479 located in the City of Lake Elsinore;California,prepared by Husker and Associates,Inc,(plot- dated April 15,2010). In addition; we have reviewed the referenced reports and have performed a recent site reconnaissance of the subject lots. The purpose of this report is to present geotechnical recommendations for the design and construction of the foundations for the proposed residential structures and other site improvements that are based on the current grading plans and the 2007 California Building Code. These recommendations are based on our recent geotechnical activities(Petra,2010)and our review of the previous geotechnical reports for Tract 25479 by Albus-Keefe&Associates; Inc. (Albus, 2003, 2004a, 2004b). i I i RICHMOND AMERICAN HOMES April 19,2010 Rosetta Canyon Project J.N. 121-10 Page 2 PREVIOUS GIIADING Rough grading of Tract 25479 reportedly tools place approximately 6 years ago between March and September 2004 with grading observations and testing performed by Albus-Keefe&Associates,Inc. (Albus, 2004b). The Albus-Keefe rough grade report indicates that following removals of unsuitable soils,the subject building pads were replaced with compacted fill to the design finish pad grades. The compacted fill placed within the subject tract consists generally of silty sand with gravel, cobbles and boulders up to 3 feet in diameter. The subject building pads are underlain by approximately 11 to 34 feet of engineered fill. In addition subdrains are located at depth within portions of Lots 234-236 and 253-254. SITE RECONNAISSANCE Based on a recent site reconnaissance, conditions within the site are essentially the same as those described in the grading report by Albus-Keefe with the exception of 5- to 6- foot high masonry screen walls that have been constructed at the rear of Lots 224 and 234-237 and along the side of Lots 222, 224, 251 and 254. In addition, wood fencing is located at the rear of Lots 222-223 and along the side yards of Lots 234,236 and 237 as well as wrought iron fencing located at the rear of Lots 251-254. The pad surfaces and front yard fill slopes of the subject lots exhibit generally minor erosion; in addition, due to the amount of time that has elapsed since the completion of grading, the ground surfaces of the subject lots have become somewhat desiccated and covered with a minor to locally moderate growth of low weeds,grasses,shrubs-and minor amounts of scattered construction debris. The areas between the font yard fill slopes and adjacent streets are also approximately 1 foot below finish grade. i i RICHMOND AMERICAN HOMES April 19, 2010 Rosetta Canyon Project J.N. 121-10 Page 3 PRECISE GRADING PLAN REVIEW Proposed Constriction Review of the precise grading plans for the subject lots prepared by Hunsaker and Associates,Inc. indicates that one-and two-story single-family homes with attached garages are proposed within the subject lots. The proposed single-family residences will be of wood-frame construction with first floor slabs constructed on-grade.Lots 234-237 and 25 1-254 are to have deepened footings adjacent to the side yard swales. 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. The rear and side yards will be graded with earthen swales to collect and convey surface water to the curbs and gutters of the adjacent street. A temporary asphalt parking lot will be constructed within Lot 224. The parking lot pavement will consist of 3 inches of asphalt placed directly on compacted subgrade soils. Proposed Precise Grading The subject lots are presently at their previous rough grade elevations and minor cuts and fills of less than 1 foot are proposed within the subject lots in order to establish proper drainage away from the proposed residences and to the front of the lots. CONCLUSIONS AND RECOMMENDATIONS Feasibility Based on our review of the precise grading plans and our previous involvement with the proj ect,the precise grading is feasible and practical from a geotechnical standpoint if accomplished in accordance with the City of Lake Elsinore requirements and our recommendations. Site Preparation and Grading The surface of the subject pads are covered with a light to locally moderate growth of vegetation and site pad grade soils are slightly desiccated and slightly eroded due to the passage of time. Therefore, i RICHMOND AMERICAN HOMES April 19, 2010 Rosetta Canyon Project J.N. 121-10 Page 4 in addition to clearing the lots of vegetation and debris, and bringing the subject lots to proposed finish grade elevations and establishing proper drainage, 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 clearing/grubbing operations, the near surface soils,where desiccated, should be scarified to a depth of at least 8 inches,thoroughly moisture conditioned and recompacted in place in accordance with recommendations provided herein. In addition, should any eroded or low areas exist between the existing curb and gutter and the front of the lots, these areas will also require scarification and recompaction to achieve a level and compacted subgrade suitable for construction. Ultimately, all temporary improvements (asphalt parking lot and landscaped areas) on Lot 224 should be demolished and hauled offsite. Following demolition,the upper surface of the building pad may require additional/deeper grading and should be properly reconditioned,as recommended above, and tested. 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-Gradine Considerations Precise Grading and Drainage Facilities Surface drainage systems consisting of sloping concrete flatwork and graded earth swales are proposed on the subject lots to collect and direct all surface waters to the adjacent street.In addition, the ground surface around the proposed buildings will be sloped at a positive gradient away from the structures.The purpose of the precise grading is to prevent ponding of surface water within the level RICI- MOND AME+'RICAN HOMI E+,S April 19, 2010 Rosetta Canyon Project J.N. 121-10 Page 5 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 1803.3 ofthe 2007 California Building Code 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 ofthe code as"Where climatic or soil conditions warrant,the slope of the ground away from the building foundations is permitted to be 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 Riverside County, as well as throughout Southern California,has been to provide a 2 percent gradient away from the foundations a I percent minimum for earthen swale gradients. Structures constructed and properly maintained under those criteria have performed satisfactorily. Therefore,considering the semi-arid climate,site soil conditions and an appropriate irrigation regime, Petra Geotechnical, Inc. considers that the implementation of 2 percent slopes away from the structures and I percent swales to be suitable 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 IUCHMOND Ali ERICAN HOMES April 19, 2010 Rosetta Canyon Project J.N. 121-10 Page 6 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 8- to 12-inch-thick maximum lifts, moisture conditioned as necessary to achieve near optimum moisture conditions,and mechanically compacted in place with a hydra-harmner, pneumatic tamper or similar equipment to achieve a minimum relative compaction of 90 percent. A representative of this firm should probe and test the backfills to verify adequate compaction. 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 performed by a representative of this firm to verify that the backfill is adequately compacted and will not be subject to 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. RICHM®ND AMERCCAN HOMES April 19, 2010 Rosetta Canyon Project J.N. 121-10 Page 7 Foundation Design Considerations 2007 CBC Seismic Design Coefficients Structures within the site should be designed and constructed to resist the effects of seismic ground motions as provided in Section 1613 of the 2007 California Building Code(CBC). The method of design is dependent on the seismic zoning, site characteristics, occupancy category, building configuration, type of structural system and on the building height. For structural design in accordance with the 2007 CBC, a computer program, Earthquake Ground Motion Parameters Version 5.09a, developed by the United States Geological Survey (USGS, 2007) was utilized to provide ground motion parameters for the subject site. The program includes hazard curves,uniform hazard response spectra and design parameters for sites in the 50 United States,Puerto Rico and the United States Virgin Islands. Based on the latitude,longitude and site classification,seismic design parameters and spectral response for both short periods and 1-second periods are calculated including Mapped Spectral Response Acceleration Parameter, Site Coefficient,Adjusted Maximum Considered Earthquake Spectral Response Acceleration Parameter and Design Spectral Response Acceleration Parameter. The program is based on USGS research and publications in cooperation with the California Geological Survey for evaluation of California faulting and seismicity(USGS, 1996a, 1996b;2002;2007). The following 2007 CBC seismic design coefficients should be used for the proposed structures. These criteria are based on the site class as determined by existing subsurface geologic conditions, on the proximity of the site to the nearest fault and on the maximum moment magnitude and slip rate of the nearest fault. RICHMOND AMERICAN DOMES April 19,2010 Rosetta Canyon Project J.N. 121-10 Page 8 2007 CBC Section 1613 Seismic Design Coefficients Site Latitude 33.7048 Site Longitude 117,3231 Mapped Spectral Response Acceleration Parameter, S, (Figure 1613.5(3) for 0.2 1.500 second) Mapped Spectral Response Acceleration Parameter, S1 (Figure 1613.5(4) for L0 0.600 second Site Class Definition (Table 1613.5,2) D Site Coefficient, Fa(Table 1613.5.3(1) short period) 1.0 Site Coefficient,F, Table 1613.5.3 2) 1-secondperiod) 1.5 Adjusted Maximum Considered Earthquake Spectral Response Acceleration Parameter, L500 Sms (Eq. 16-37 Adjusted Maximum Considered Earthquake Spectral Response Acceleration Parameter, 0.900 Smt . 16-38 Desi n Spectral Res onse Acceleration Parameter, SDs (Eq. 16-39 1.000 Design Spectral Response Acceleration Parameter, SDI (Eq. 16-40 0.600 Allowable Soil Bearin.-Capacities An 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 width and/or depth, 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. Footing Settlements Based on the allowable bearing values,total settlement of the footings is anticipated to be less than 1 inch and differential settlement is expected to be less than%of an inch over a span of 30 feet. The majority of these settlements are likely to take place immediately after placement of the footing loads. RICHMOND AMERICAN DOMES April 19,2010 Rosetta Canyon Project J.N. 12140 Page 9 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 0.40 tunes 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 short duration wind or seismic forces. The above values are based on footings placed directly against compacted fill. In cases where the footing sides are formed,all backfill placed against the footings should be compacted to at least 90 percent of maximum dry density. Minimum Footing and Floor Slah Recommendations Results of previous laboratory tests by Albus-Keefe, and subsequent laboratory testing by Petra, indicated that onsite soils for the subject lots exhibit a very low and low expansion potential as determined in accordance with ASTM D 4829. For Lots 222-224 and 251-254 a very low expansion potential (expansion index between 0 and 20) is appropriate. For Lots 234-237 a low expansion potential (expansion index between 21 and 50)is deemed appropriate. Based on these results,two separate foundation recommendation categories are presented herein. Lots 222-224 and 251-254 (Very Low Expansion Potential) The design of slab-on-ground foundations will be exempt from the procedures outlined in CBC Sections 1.802.3.2 and 1805.8.2. However,based on the existing soil conditions within the site,.it is recommended that footings and floor slabs constructed on soils with a very low expansion potential be constructed and reinforced in accordance with the following minimum criteria. The design and construction recommendations that follow are based on the above anticipated soil conditions and may be considered for minimizing the effects of long term total and differential settlement. These recommendations have been developed on the basis of previous experience of this RICHM®ND AMIERICAN HOMES April 19,2010 Rosetta Canyon Project J.N. 121-10 Page 10 firm on proj ects with similar soil conditions.Although construction performed in accordance with these recommendations has been found to minimize post-construction movement and/or cracking, they generally do not positively mitigate all potential effects of future settlement. The structural engineer should also design the foundations in consideration of the anticipated amounts of total and differential settlement. Based on this design, thicker floor slabs, larger footing sizes and/or additional reinforcement may be required and should govern the design if more restrictive than the minimum recommendations provided below. 1. Footings a. Exterior continuous footings should be founded at a minimum depth of 12 inches below the lowest adjacent final grade for one-story construction and at a minimum depth of 18 inches below the lowest adjacent final grade for two-story construction. Interior continuous footings may be founded at a minimum depth of 12 inches below the bottoms of the adjacent floor slabs. In addition, all continuous footings should have a minimum width, of 12 and 15 inches, for one-story and two-story construction, respectively. All continuous footings should be reinforced with a minimum of two No. 4 bars, one top and one bottom. b. Interior isolated pad footings should be a minimum of 24 inches square and founded at a minimum depth of 12 inches below the bottoms of the adjacent floor slabs. Pad footings should be reinforced withNo.4 bars spaced a maximum of 18 inches on centers,both ways, near the bottoms of the,footings. c. Exterior isolated pad footings intended for support of roof overhangs such as second-story decks,patio covers and similar construction should be a minimum of 24 inches square,and founded at a 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, near the bottoms of the footings. 2. Building Floor Slabs a. Unless a more stringent design is recommended by the architect or structural engineer,we recommend a minimum slab thickness of 4 inches for both living area and garage floor slabs, and reinforcement consisting of No. 3 bars spaced a maximum of 24 inches on centers, both 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, in 1 RICHMOND AMERICAN HOMES April 19,2010 ? Rosetta Canyon Project J.N. 121-10 Page 11 accordance with Wire Reinforcement Institute (WRI) designation, or stronger. All slab reinforcement should be properly supported to ensure placement near mid-depth. b. Living area concrete floor slabs should be underlain with a moisture vapor retarder consisting of a minimum 10-mil thick polyethylene or polyolefin membrane that meets the E minimum requirements of ASTM E 96 and ASTM E1745 for vapor retarders (such as Husky Orange Guard, Stego, 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 removing an additional inch fiom the pad and then placing a 1-inch-thick leveling course of sand across the pad surface prior to the placement of the membrane. i c. Garage floor slabs should be a minimum 4 inches thick and reinforced in a similar manner as living area floor slabs. Garage floor slabs may also be poured separately from adjacent wall footings with a positive separation maintained with 3/4-inch-minimum felt expansion joint materials, and quartered with weakened plane joints. A 12-inch wide grade beam founded at the same depth as adj acent footings should be provided across garage entrances. The grade beam should be reinforced with two No. 4 bars, one top and one bottom. d. Presaturation of the subgrade soils below floor slabs will not be required;however,prior to placing concrete, the subgrade soils below living area and garage floor slabs should be prewatered to promote uniform curing of the concrete and reduce the potential for the development of shrinkage cracks. Lots 234-237 (Low Expansion Potential) For lots that have a low expansion potential (EI between 21 and 50),the 2007 CBC specifies that slab-on-ground foundations constructed on expansive soils require special design considerations in I accordance with the Wire Reinforcement Institute (WRI)publication"Design of Slab-on Ground Foundation", which was last updated in 1996. The design procedures outlined in the WRI publication are based on the weighted plasticity index of the different soil layers existing within the upper 15 feet of the building site. Based on our limited laboratory testing of site soils, the clay portion of the site soils, a weighted plasticity index of 12 can be assumed for the subject lots. The WRI publication also states that the weighted plasticity index of each building site should be RICHMOND AMERICAN HOMES April 19,2010 Rosetta Canyon Project J.N. 121-10 Page 12 modified(multiplied)by correction factors that compensate for the effects of sloping ground and the unconfined compressive strength of the soil andbedrock materials. Since all of the buildings will be constructed on level building pads and based on the estimated unconfined compressive strength of the on-site soils,it is recommended that the weighted plasticity index(12)be multiplied by a factor of 1.2 in order to determine the value of the effective plasticity index (per Figure 9 of the WRI publication). In surmnaiy, an effective plasticity index of 14 should be used for the subject lots. The design and construction recommendations that:follow are based on the above soil conditions and may be considered for minimizing the effects of slightly expansive soils and long term differential settlement. These recommendations have been developed on the basis ofprevious experience of this firm on projects with similar soil conditions.Although construction performed in accordance with these recommendations has been found to minimize post-construction movement and/or cracking, they generally do not positively mitigate all potential effects of expansive soils and future settlement. The effective plasticity index provided above should be utilized by the proj ect structural engineer to design slab-on-ground foundations with an interior grade beam grid system in accordance with the WRI publication. Based on this design, thicker floor slabs, larger footing sizes and/or additional reinforcement may be required and should govern the design if more restrictive than the minimum recommendations provided below. 1. Footings a. Exterior continuous footings should be founded at a minimum depth of 18 inches below the lowest adjacent final grade. Interior continuous footings may be founded at a minimum depth of 12 inches below the bottoms of the adjacent floor slabs. In addition,all continuous footings should have a minimum width of 12 and 15 inches, for one-story and two-story construction,respectively. All continuous footings should be reinforced with a minimum of two No. 4 bars, one top and one bottom. b. Interior isolated pad footings,if any,should be a minimum of 24 inches square and founded at a minimum depth of 12 inches below the bottoms ofthe adjacent floor slabs.Pad footings should be reinforced with No.4 bars spaced a maximum of 18 inches on centers,both ways, near the bottoms of the footings. RICHMOND AMERICAN HOMES April 19,2010 Rosetta Canyon Project J.N. 121-10 Page 13 d. Exterior isolated pad footings untended for support of roof overhangs such as second-story decks,patio covers and similar construction should be a minimum of 24 inches square,and founded at a 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,near the bottoms of the footings. e. The spacing and layout of the interior concrete grade beam system required below floor slabs should be determined by the project architect or structural engineer in accordance with the WRI publication using the effective plasticity index provided previously. 2. Building Floor Slabs a. The project architect or structural engineer should evaluate minimum floor slab thickness and reinforcement in accordance with the WRI publication based on the effective plasticity index provided previously. Unless a more stringent design is recommended by the architect or structural engineer,we recommend aminimum slab thickness of 4 inches for both living area and garage floor slabs,and reinforcement consisting of No. 3 bars spaced a maximum of 18 inches on centers,both 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,in accordance with Wire Reinforcement Institute(WRI) designation, or stronger. All slab reinforcement should be properly supported to ensure placement near mid-depth. b. Living area concrete floor slabs 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 E 96 and ASTM E1745 for vapor retarders ( such as Husky Orange Guard, Stego, 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 removing an additional inch from the pad and then placing a 1-inch-thick leveling course of sand across the pad surface prior to the placement of the membrane. c. Garage floor slabs should be a.minimum 4 inches thick and reinforced in a similar manner as living area floor slabs. Garage floor slabs should also be poured separately from adjacent wall footings with a positive separation maintained with 3/4-inch-minimum felt expansion joint materials, and quartered with weakened plane joints. A 12-inch wide grade beam RICHMO1®D AMERICAN HOMES April 19,2010 Rosetta Canyon Project J.N. 121-10 Page 14 founded at the same depth as adjacent footings should be provided across garage entrances. The grade beam should be reinforced with two No. 4 bars, one top and one bottom. d. Presaturation of the subgrade soils below floor slabs will be required. That is, prior to placing concrete, the subgrade soils below living area and garage floor slabs should be prewatered to achieve a moisture content that is at least 1.2 times the optimum moisture content. Tlus moisture should penetrate to a depth of approximately 12 inches into the subgrade to promote uniform curing of the concrete and reduce the potential for the development of shrinkage cracks. Foundation Excavation Observations Foundation excavation should be observed by a representative of this firm to verify 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 diy density. Soluble Sulfate and Chloride Analvsis Results of both the previous laboratory tests performed by Albus-Keefe during grading, and our laboratory testing during the due diligence period,indicate that site soils were found to contain water soluble sulfate contents of less than 0.10 percent. Based on Section 1904.3 of the 2007 CBC, concrete that will be exposed to sulfate-containing soils shall comply with the provisions of ACI 318-05, Section 4.3. According to Table 4.3.1 of the ACI 318-05,a Negligible exposure to sulfate can be expected for concrete placed in contact with the onsite soil materials; therefore,no special cement will be required. RICHMOND AMERICAN How April 19, 2010 Rosetta Canyon Project J.N. 121-10 Page 15 Soil Corrosivity Albus-Keefe did not conduct any corrosion tests with regards to ferrous materials during the grading operation to develop the subj ect lots. Based on limited sampling and laboratory testing by Petra for soil pH,minimum resistivity and chloride indicate that the soils within the subject tract are neutral with respect to pH (pH of 7.3). Soil resistivity was found be 1,400 ohm-cm, and had a chloride concentration of 155 ppm. The results of these tests indicate that on-site soils are corrosive to ferrous metals and copper. Petra does not practice corrosion engineering; therefore, we recommend that onsite soils be tested and analyzed near or at the cornpletion of precise grading by a qualified corrosion engineer to evaluate the general corrosion potential of the onsite soils and any impact on the proposed construction. Exterior Concrete Tlatwork Thickness and Joint Snaring To reduce the potential of unsightly cracking, concrete walkways, patio-type 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. Concrete driveway slabs should beat least 4 inches thick and provided with construction joints or expansion joints every 10 feet or less. Reinforcement All concrete patio-type slabs, driveways and walkways greater than 10 feet in width should be reinforced with No. 3 bars spaced 24 inches on centers, both ways. Alternatively, the slab reinforcement may consist of welded wire mesh of sheet type (not rolled) with 6x6/W2.9xW2.9 designation in accordance with Wire Reinforcement Institute(WRI). The reinforcement should be properly positioned near the middle of the slabs. - . RICHMOND AMERICAN HOMES April 19,2010 Rosetta Canyon Project J.N. 121-10 Page 16 Subgrade Preparation As a further measure to reduce cracking of concrete flatwork, the subgrade soils below concrete flatwork areas should first be compacted to a minimum relative compaction of 90 percent and then thoroughly wetted to achieve a moisture content that is at least equal to, or slightly greater than, optimum moisture content. This moisture should penetrate to a depth of 12 inches into the subgrade and should be maintained in the soils during placement of concrete. Pre-watering of the soils will promote uniform curing of the concrete and minimize the development of shrinkage cracks. 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. FUTURE IMPROVEMENTS AND/OR GRADING 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 grading, we take no responsibility for misinterpretation of our recommendations presented herein. 1 RICHMOND AMERICAN HOMES April 19,2010 Rosetta Canyon Project J.N. 121-10 Page 17 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. L Building, Construction • Observe all footing trenches when fist 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. • Observe pre-saturation of subgrade soils below all living and garage floor areas to verify moisture content and penetration. 2. Concrete Flatwork Construction i Observe and test subgrade soils below all concrete flatwork areas to verify adequate compaction,moisture content and moisture penetration prior to pouring concrete. 3. Utility Trench Backfili • Observe and test placement of all utility trench backfill to verify adequate compaction. 4. Precise Grading i • Observe and test placement of any fill to be placed on the subject lots to verify adequate compaction. PETRA i IUCHM®ND AMERICAN HOMES April 19, 2010 Rosetta Canyon Project J.N. 121-10 Page 18 CLOSURE r This report is subject to review by the controlling authorities for this project. We sincerely appreciate this opportunity to be of service. Please do not hesitate to call the undersigned ifyou have any questions regarding this letter, Respectfully sub-mitted, PETRA GE®TECHNICAL, INC. Gr yson R. Walker, GE Principal Engineer EXF.'�y-�> " �1 pr GE 871 7C FOFGP�.� Distribution: (3)Edgar Goniez (1 .pdf)Esmeralda Cardona (1 .pdf) Michelle Meyer (1 .pdf)David Nelson (1 .pdf) Ben Etemadi RICE MOND AMERICAN HOMES April 19, 2010 Rosetta Canyon Project J.N. 121-10 Page 19 REFERENCES Albus-Keefe&Associates,Inc.2003,Rough Grading Plan Review,Tracts 25478,25479 and Of-site Roadway Corridor,Ramsgate Project,Lake Elsinore,Riverside County,California,prepared for Centex Homes, dated September 30. 2004a, Supplemental Foundation Design Recornmendations, Tracts 25478 and 25479, Rosetta Canyon Development, Lots 1 Through 254, Tract 25479, City of Lake Elsinore, County of Riverside, California, ,prepared for Centex Homes, dated August 13. 2004b,Rough Grading Report,Rosetta Canyon Development,Lots 1 Through 254,Tract 25479,City of Lake Elsinore,County of Riverside,California,,prepared for Centex Homes, dated September 27. Petra,2010,Due-diligence Geotechnical Assessment,Rosetta Canyon Projects,Tract 25476(Lots 1 through 14, 18 through 25, 189 through 201 and 214 through 221), Tract 25477 (Lots 1 through 8, 74 through 81 and 116 through 123), Tract 25478 (Lots 44, 45 and 75), Tract 25479(Lots 222 through 224,234 through 237 and 251 through 254)and Tract 32503 (Lots 19 through 54),City ofLake Elsinore,Riverside County,California;prepared for Richmond American Homes, dated March 9. International Conference of Building Officials, 2007, 2007 California Building Code, California Code of Regulations, Title 24, Par 2, Volume 2 of 2, California Building Standards Conunission, Based on 2006 International Building Code, 2007 California Historical Building Code, Title 24,Par 8, 2007 California Existing Building Code,Title 24,Part 10. Ift . ,