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HomeMy WebLinkAboutCT 97-02; RANCHO CARILLO VILLAGES A-D; INTERIM REPORT OF TESTING AND OBSERVATION SERVICES DURING SITE GRADING; 1998-07-20c2r fl-6i INTERIM REPORT OF TESTING AND OBSERVATION SERVICES• DURING SITE GRADING LOT NOS. I THROUGH 4 (MODEL LOTS) VILLAGE C RANCHO CARRILLO CARLSBAD, CALIFORNIA PREPARED FOR CONTINENTAL RANCH INC. SAN DIEGO, CALIFORNIA T JULY 1998 GEOCON 0 INCORPORATED GEOTECHNICAL CONSULTANTS Project No. 05845-12-16 July 20, 1998 Continental Ranch Inc. 12230 ElCamino Real, Suite 300 San Diego, California 92130 Attention: Mr. Dave Lother Subject: LOT NOS. 1 THROUGH 4 (MODEL LOTS) VILLAGE C RANCHO CARRILLO CARLSBAD, CALIFORNIA INTERIM REPORT OF TESTING AND OBSERVATION SERVICES DURING SITE GRADING Gentlemen: In accordance with your request and, our proposals dated December 6, 1996 and April 1, 1998 , we have provided compaction testing and observation services during the mass grading and fine grading of the subject site. Our services were performed during the period of January 2, 1997 through June 16, 1998. The scope of our services included the following: Observing the grading operation, including the removal and/or processing of loose topsoils, colluvium, alluvium. Performing in-place density tests in fill placed and compacted at the site. Performing laboratory tests to aid in evaluating the compaction, expansion, and shear strength characteristics of various soil conditions encountered and/or used for fill. Providing on-site geologic consultation services to verify that grading was performed in substantial conformance with the recommendations of preliminary project geotechnical report. Preparing an "As-Graded" Geologic Map. Preparing this interim report of grading. The purpose of this report is to document the grading operations within the first phase of site development, particularly the model lots. A final report of testing and observations for all of Village C will be prepared upon completion of grading operations. 6960 Flanders Drive 0 San Diego, California 92121-2974 U Telephone (619) 558-6900 U Fax (619) 558-6159 GENERAL The grading contractor for the mass grading phase of the project was Sukut Construction, Incorporated. The fine grading operations were performed by Signs and Pinnick. The project mass grading plans were prepared by Rick Engineering Company and are entitled Grading Plans for Rancho. Carrillo, Sheet 24 with the City of Carlsbad approval dated November 14, 1996. The fine grading plans were also prepared by Rick Engineering Company and are entitled Grading and Erosion Plans for Rancho Carrillo, Villages A, B, C, and D, C. T. 97-02, Sheet 7, with the City of Carlsbad approval dated May 6, 1998. The project soils report is entitled: Preliminary Geotechnical Investigation, Villages A, B, C, and D, Rancho Carrillo, Carlsbad, California, prepared by Geocon. Incorporated, dated March 3, 1993, updated on July 14, 1997. References to elevations and locations herein were based on surveyor's or grade checker's stakes in the field and/or interpolation from the referenced. Grading Plans. Geocon Incorporated did not provide surveying services and, therefore, has no opinion regarding the accuracy of the as-graded elevations or surface geometry with respect to the approved grading plans or proper surface drainage. I . GRADING Mass grading of Village C occurred during the period of January 2, 1997 through May 11, 1998. Fine grading for the lots reported herein was performed during the period of May 22 through June 16, 1998. Grading began with the removal of brush and vegetation from the area to be graded. The subject lots are cut lots and no remedial grading and fill placement have occurred within this area. Laboratory testing was performed on a representative sample of finish grade soil to determine expansion characteristics. The results of the laboratory test is summarized in Table I. Slopes No significant cut or fill slopes exist within the subject area. S u bdrains No canyon subdrains are located within this portion of the project. Project No. 05845-12-16 -2- July 20, 1998 Finish Grade Soil Conditions Based on laboratory test results, the prevailing soil conditions within approximately the upper 3 feet of rough pad grade on each lot have an Expansion Index of 14 (Table I), and are classified as having a "very low" expansion potential as defined by the Uniform Building Code (UBC) Table 18-I-B. SOIL AND GEOLOGIC CONDITIONS The soil and geologic conditions encountered during grading were found to be similar to those described in the project geotechnical report. In general, the subject lots are underlain by formational soils of the Delmar Formation. The enclosed As-Graded Geologic Map (Figure 1) depicts the general geologic conditions observed. No soil or geologic conditions were observed during grading which would preclude the continued development of the property as planned. CONCLUSIONS AND RECOMMENDATIONS 1.0. General 1.1. Based on observations and test results, it is the opinion of Geocon Incorporated that the grading, which is the subject of this report, has been performed in substantial conformance with the recommendations of the previously referenced project soil report. Soil and geologic conditions encountered during grading which differ from those anticipated by the project soil report are not uncommon. Where such conditions required a significant modification to the recommendations of the project soil report, they have been described herein. 2.0. Seismic Soil Stability 2.1. The site is underlain by dense formational soils of the Delmar Formation. Near surface permanent groundwater is not present. Therefore, it is our opinion that the soils on the site are stable and the potential for liquefaction is remote. 3.0. Future Grading 3.1. Any additional grading performed at the site should be accomplished in conjunction with our observation and compaction testing services. All trench backfill in excess of one-foot ProjeetNo. 05845-12-16 -3- July 20, 1998 thick should be compacted to at least 90 percent relative compaction. This office should be notified at least 48 hours prior to commencing additional grading or backfill operations. 4.0. Foundations 4.1. The foundation recommendations that follow , are for one- or two-story residential structures and are separated into categories dependent on the thickness and geometry of the underlying fill soils as well as the Expansion Index of the prevailing subgrade soils of a particular building pad (or lot). The recommended minimum foundation and interior concrete slab design criteria for each Category is presented in Table 4.1. TABLE 4.1. FOUNDATION 'RECOMMENDATIONS BY CATEGORY Foundation Minimum Continuous Footing Interior Slab Category Footing Depth Reinforcement Reinforcement (inches) I 12 One No. 4 bar top and bottom 6 x 6-10/10 welded wire mesh at slab mid-point II 18 Two No. 4 bars top and bottom No. 3 bars at 24 inches on • center, both directions III 24 Two No. 5 bars top and bottom No. 3 bars at 18 inches on • S center, both directions CATEGORY CRITERIA Category I: Maximum fill thickness is less than 20 feet and Expansion Index is less than or equal to 50. Category II: Maximum fill thickness is less than 50 feet and Expansion Index is less than or equal to 90, or variation in fill thickness is between 10 feet and 20 feet. Category III: Fill thickness exceeds 50 feet, or variation in fill thickness exceeds 20 feet, or Expansion Index exceeds 90, but is less than 130. Notes: All footings should have a minimum width of 12 inches. Footing depth is measured from lowest adjacent subgrade. All interior living area concrete slabs should be at least 4 inches thick for Categories I and II and 5 inches thick for Category III: All interior concrete slabs should be underlain by at least 4 inches (3 inches for Category Ill) of clean sand or crushed rock. All slabs expected to receive moisture-sensitive floor coverings or used to store moisture-sensitive materials should be underlain by a vapor barrier covered with at least 2 inches of the clean sand recommended in No. 4 above. S Project No. 05845-12-16 -4- July 20, 1998 4.2. Foundations for either Category I, II, or III. may be designed for an allowable soil bearing pressure of 2,000 pounds per square foot (psf) (dead plus live load). This bearing pressure may be increased by one-third for transient loads such as wind or seismic forces. 4.3. No special subgrade presaturation is deemed necessary prior to placing concrete, however, the exposed foundation and slab subgrade soils should be sprinkled, as necessary, to maintain a moist condition as would be expected in any such concrete placement. 4.4. Recommended foundation category for each lot is presented in Table 4.2. TABLE 4.2. SUMMARY OF RECOMMENDATION FOUNDATION CATEGORY Lot Numbers Foundation Category 1,2,3, and 4 I 4.5. Where buildings or other improvements are planned near the top of a slope steeper than 3:1 (horizontal:vertical), special foundations and/or design considerations are recommended due to the tendency for lateral soil movement to occur. For fill slopes less than 20 feet high, building footings should be deepened such that the bottom outside edge of the footing is at least 7 feet horizontally from the face of the slope. Where the height of the fill slope exceeds 20 feet, the minimum horizontal distance should be increased to W3 (where H equals the vertical distance from the top of the slope to the toe) but need not exceed 40 feet. For composite (fill over cut) slopes, H equals the vertical distance from the top of the slope to the bottom of the fill portion of the slope. An acceptable alternative to deepening the footings would be the use of a post-tensioned slab and foundation system or increased footing and slab reinforcement. Specific design parameters or recommendations for either of these alternatives can be provided once the building location and fill slope geometry have been determined. For cut slopes in dense formational materials, or fill slopes inclined at 3:1 (hori- zontal:vertical) or flatter, the bottom outside edge of building footings should be at least 7 feet horizontally from the face of the slope, regardless of slope height. Swimming pools located within 7 feet of the top of cut or fill slopes are not recommended. Where such a condition cannot be avoided, it is recommended that the portion of the swimming pool wall within 7 feet of the slope face be designed assuming that the adjacent soil provides no lateral support. This recommendation applies to fill slopes up to 30 feet in height, and cut slopes regardless of height. Project No. 05845-12-16 - 5 - July 20, 1998 For swimming pools located near the top of fill slopes greater than 30 feet in height, additional recommendations may be required and Geocon Incorporated should be contacted for a review of specific site conditions. 4.6. Although other improvements which are relatively rigid or brittle, such as concrete flatwork or masonry walls may experience some distress if located near the top of a slope, it is generally not economical to mitigate this potential. It may be possible, however, to incorporate design measures which would permit some lateral soil movement without causing extensive distress. Geocon Incorporated should be consulted for specific - recommendations. 4.7. As. an alternative to the foundation recommendations for each category, consideration should be given to the use of post-tensioned concrete slab and foundation systems for the support of the proposed struótiires. The post-tensioned systems should be designed by a structural engineer experienced in-post-tensioned slab design and design criteria of the Post-Tensioning Institute (UBC Section 1816). Although this procedure was developed for expansive soils, it is understood that it can also be used to reduce the potential for foundation distress due to differential fill settlement. The post-tensioned design should incorporate the geotechnical parameters presented on the following table entitled Post- Tensioned Foundation System Design Parameters for the particular Foundation Category designated. TABLE 4.3. POST-TENSIONED FOUNDATION SYSTEM DESIGN PARAMETERS Post-Tensioning Institute (PT!) Design Parameters Foundation Category I II III Thomthwaite Index -20 -20 -20 Clay Type - Montmorillonite Yes Yes Yes Clay Portion (Maximum) 30% 50% 70% Depth to Constant Soil Suction 7.0 ft. 7.0 ft. 7.0 ft. Soil Suction 3.6 ft. 3.6 ft. 3.6 ft. Moisture Velocity 0.7 in/mo. 0.7 in/mo. 0.7 in./mo. Edge Lift Moisture Variation Distance 2.6 ft. 2.6 ft. 2.6 ft. Edge Lift 0.41 in. 0.78 in. 1.15 in. Center Lift Moisture Variation Distance 5.3 ft. 5.3 ft. 5.3 ft. Center Lift 2.12 in. 3.21 in. 4.74 Project No. 05845-12-16 -6- July 20, 1998 4.8. UBC Section 1816 uses interior stiffener beams in its structural design procedures. If the structural engineer proposes a post-tensioned foundation design method other than UBC Section 1816, it is recommended that interior stiffener beams be used for Foundation Categories II and III. The depth of the perimeter foundation should be at least 12 inches for Foundation Category I. Where the Expansion Index for a particular building pad exceeds 50 but is less than 91, the perimeter footing depth should be at least 18 inches; and where it exceeds 90 but is less than 130, the perimeter footing depth should be at least 24 inches. Geocon Incorporated should be consulted to provide additional design parameters as required by the structural engineer. 4.9. The recommendations of this report are intended to reduce the potential for cracking of slabs due to expansive soils, differential settlement of deep fills or fills of varying thicknesses. However, even with the incorporation of the recommendations presented herein, foundations, stucco walls, and slabs-on-grade placed on such conditions may still exhibit some cracking due to soil movement and/or shrinkage. The occurrence of concrete shrinkage cracks is independent of the supporting soil characteristics. Their occurrence may be reduced and/or controlled by limiting the slump of the concrete, proper concrete placement and curing, and by the placement of crack control joints at periodic intervals, in particular, where re-entry slab corners occur. 5.0. Retaining Walls And Lateral Loads 5.1. Retaining walls not restrained at the top and having a level backfill surface should be designed for an active soil pressure equivalent to the pressure exerted by a fluid density of 30 pounds per cubic foot (pcf). Where the backfill will be inclined at no steeper than 2.0 to 1.0, an active soil pressure of 40 pcf is recommended. These soil pressures assume that the backfill materials within an area bounded by the wall and a 1:1 plane extending upward from the base of the wall possess an Expansion Index of less than 50. For those lots with finish grade soils having an Expansion Index greater than 50 and/or where backfill materials do not conform to the above criteria, Geocon Incorporated should be consulted for additional recommendations. 5.2. Unrestrained walls are those that are allowed to rotate more than 0.001H at the top of the Wall. Where walls are restrained from movement at the top, an additional uniform pressure of 7H psf (where H equals the height of the retaining wall portion of the wall in feet) should be added to the above active soil pressure Project No. 05845-12-16 -7- July 20, 1998 5.3. All retaining walls should be provided with a drainage system adequate to prevent the buildup of hydrostatic forces and should be waterproofed as required by the project architect. The use of drainage openings through the base of the wall (weep holes, etc.) is not recommended where the seepage could be a nuisance or otherwise adversely impact the property adjacent to the base of the wall. The above recommendations assume a properly compacted granular (Expansion Index less than 50) backfill material with no hydrostatic forces or imposed surcharge load. If conditions different than those described are anticipated, or if specific drainage details are desired, Geocon Incorporated should be contacted for additional recommendations. 5.4. In general, wall foundations having a minimum depth and width of one foot may be designed for an allowable soil bearing pressure of 2,000 psf, provided the soil within 3 feet below the base of the wall has an Expansion Index of less than 90. The proximity of the foundation to the top of a slope steeper than 3:1 could impact the allowable soil bearing pressure. Therefore, Geocon Incorporated should be consulted where such a condition is anticipated. 5.5. For resistance to lateral loads, an allowable passive earth pressure equivalent to a fluid density of 300 pcf is recommended for footings or shear keys poured neat against properly compacted granular fill soils or undisturbed natural soils. The allowable passive pressure assumes a horizontal surface extending at least 5 feet or three times the surface generating the passive pressure, whichever is greater. The upper 12 inches of material not protected by floor slabs or pavement should not be included in the design for lateral resistance. An allowable friction coefficient of 0.4 may be used for resistance to sliding between soil and concrete. This friction coefficient may be combined with the allowable passive earth pressure when determining resistance to lateral loads. 5.6. The recommendations presented above are generally applicable to the design of rigid concrete or masonry retaining walls having a maximum height of 8 feet. In the event that walls higher than 8 feet or other types of walls are planned, such as crib-type walls, Geocon Incorporated should be consulted for additional recommendations. 6.0. Slope Maintenance 6.1. Slopes that are steeper than 3:1 (horizontal: vertical) may, under conditions which are both difficult to prevent and predict, be susceptible to near surface (surficial) slope instability. The instability is typically limited to the outer three feet of a portion of the slope and usually does not directly impact the improvements on the pad areas above or below the Project No. 05845-12-16 - 8 - July 20, 1998 slope. The occurrence of surficial instability is more prevalent on fill slopes and is generally preceded by a period of heavy rainfall, excessive irrigation, or the migration of subsurface seepage. The disturbance and/or loosening of the surficial soils, as might result from root growth, soil expansion, or excavation for irrigation lines and slope planting, may also be a significant contributing factor to surficial instability. It is, therefore, recommended that, to the maximum extent practical: (a) disturbed/loosened surficial soils be either removed or properly recompacted, (b) irrigation systems be periodically inspected and maintained to eliminate leaks and excessive irrigation, and (c) surface drains on and adjacent to slopes be periodically maintained to preclude ponding or erosion. Although the incorporation of the above recommendations should reduce the potential for surficial slope instability, it will not eliminate the possibility, and, therefore, it maybe necessary to rebuild or repair a portion of the project's slopes in the future. 7.0. Drainage 7.1. Adequate drainage provisions are imperative. Under no circumstances should water be allowed to pond adjacent to footings. The building pads should be properly finish graded after the buildings and other improvements are in place so that drainage water is directed away from foundations, pavements, concrete slabs, and slope tops to controlled drainage devices. LIMITATIONS The conclusions and recommendations ëontained herein apply only to our work with respect to grading, and represent conditions at the date of our final observation June 16, 1998. Any subsequent grading should be done in conjunction with our observation and testing services. As used herein, the term "observation" implies only that we observed the progress of the work with which we agreed to be involved. Our services did not include the evaluation or identification of the potential presence of hazardous or corrosive materials. Our conclusions and opinions as to whether the work essentially complies with the job specifications are based on our observations, experience, and test results. Subsurface conditions, and the accuracy of tests used to measure such conditions, can vary greatly at any time. We make no warranty, expressed or implied, except that our services were performed in accordance with engineering principles generally accepted at this time and location. We will accept no responsibility for any subsequent changes made to the site by others, by the uncontrolled action of water, or by the failure of others to properly repair damages caused by the uncontrolled action of water. The findings and recommendations of this report may be invalidated Project No. 05845-12-16 - 9 - July 20, 1998 \. wholly or partially by changes outside our control. Therefore, this report is subject to review and should not be relied upon after a period of three years. If you have any questions regarding this report, or if we may be of further service, please contact the undersigned, at your convenience. Very truly yours, GEOCON INCORPORAt ..Cn David F. LeakéU'l RCE 22527 AS:DFL:dmc (2) Addressee (4/del) Continental Ranch (Job Site) Attention: Mr. Ray MacDóugle 2 ,&-•. AU /X' IPI SADR I ,I •!778 u' I cERTIRED • —i fr t'Ø t(fl't ENGINEER1N I Ali Sadr GEOLOGIST CEG 1778 c.&.#' Project No. 05845-12-16 -10- - July 20, 1998 TABLE I SUMMARY OF LABORATORY EXPANSION INDEX TEST RESULTS Sample No. Moisture Content Dry Density (pci) Expansion Index I Before Test (%) After Test (%) C-E 13.1 23.0 102.3 14 Project No. 05845-12-16 : . July 20, 1998.