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Home My WebLink AboutCT 14-09; UPTOWN BRESSI RANCH; ADDENDUM TO UPDATE REPORT; 2017-08-02GEOCON INCORPORATED r I I I I{ECORD COPY -~/1..(~3/ In:tiai GEOTECHNICAL • ENVIRONMENTAL• Project No. 02108-32-01 August 2, 2017 Shea Homes 9990 Mesa Rim Road San Diego, California 92121 Attention: Mr. Greg Ponce Subject: ADDENDUM TO UPDATE REPORT UPTOWN BRESSI RANCH CARLSBAD, CALIFORNIA Reference: Update Report and Change ofGeotechnical Engineer of Record, Bressi Ranch, Lots 29 Through 32, Carlsbad, California, prepared by Geocon Incorporated, dated April 24, 2017. Dear Mr. Ponce: In accordance with your request, we have prepared this update correspondence providing revised geotechnical recommendations for the subject site in Carlsbad, California. Based on recent discussions with you, recommendations considering chemical soil stabilization using either lime or cement additives are desired as an alternative to the recommendations presented in the referenced report. The recommendations presented in the referenced report remain applicable unless superseded herein. It is our understanding that chemical stabilization is desired for the retail portion of the project only. MODIFICATIONS AND RECOMMENDATIONS The primarily geotechnical consideration for site development is the presence of "high" to "very high" expansive soils identified in the recent and previous studies. In order to reduce the effects of soil expansion on the proposed improvements, special design and remedial grading recommendations will be necessary. It should be noted that incorporation of the recommendations herein will not eliminate the potential for adverse impacts due to the highly expansive soil, especially for lightweight improvements such as slabs-on grade, curb and gutter structures, etc. In accordance with your request, we have provided supplemental geotechnical services to evaluate the feasibility of chemical soil stabilization using either lime or cement to reduce the adverse impacts of the highly expansive soils that are present. Laboratory soluble sulfate content testing was performed in accordance with California Test Method 417 and the test results are presented in Table I. These tests were performed to determine the feasibility of using lime or cement as an additive to stabilize the existing clayey subgrade soils. 6960 Flanders Drive • San Diego, California 92121-2974 • Telephone 858.558.6900 • Fa)( 858.558.6159 \ 6 - TABLE 1 SUMMARY OF LABORATORY WATER-SOLUBLE SULFATE CONTENT TEST RESULTS CALIFORNIA TEST METHOD 417 Sample No. Water-Soluble Sulfate Content(%) 2016 CBC Classification S-1 0.063 Not Applicable (SO) S-2 0.100 Moderate (SI) The laboratory tests indicate that the subgrade soils have a water-soluble sulfate content of less than 5,000 parts per million (ppm). Therefore, using lime or cement as an additive to the clayey soils is feasible. The amount of lime or cement content needed to stabilize the clayey subgrade soil was determined by the Texas Method, which consists of performing a series of plasticity index (PI) tests at different lime and cement contents until the Pl generally stabilizes at its lower bound. The test results are presented in Tables 2 and 3. TABLE 2 SUMMARY OF LABORATORY PLASTICITY INDEX VS. QUICK LIME CONTENT TEST RESULTS ASTM D4318 Plasticity Index Sample 0°/o 2o/o 4o/o 6°/o 8°/o S-1 36 37 22 9 NP S-2 33 26 12 12 NP NP -Non-Plastic TABLE 3 SUMMARY OF LABORATORY PLASTICITY INDEX VS. CEMENT CONTENT TEST RESULTS ASTM D 4318 Plasticity Index Sample 0°/o 2°/o 4o/o 6°/o 8°/o lOo/o S-1 ---34 26 26 18 NP S-2 32 30 23 24 27 NP NP -Non-Plastic Based on the results of our testing, we recommend a lime or cement content of approximately 7 percent by weight or greater. We understand that cement treatment is the preferred option from a cost standpoint. As presented in the referenced report, remedial grading of the upper 3 feet of existing sheet-graded material across the site, including finish grade in cut areas, is recommended (see Section 1.0 and Figure 2). This operation can be performed concurrently with the chemical stabilization in areas of proposed cut. In building pad areas, the upper 3 feet of soil should be treated with at least 7 percent Project No. 02108-32-01 - 2 -August 2, 2017 cement by weight, thoroughly mixed with the on-site soils, and compacted to at least 90 percent relative compaction. Based on discussions with Pavement Recycling Systems, the stabilization contractor, the upper approximately 2 feet of material would be stockpiled and treated, and the lower !-foot would be treated in-place. Following the treatment, mellowing period, and compaction of the lower !-foot, the stockpiled and treated soil would be placed in accordance with our recommendations. The lateral limits should extend at least 5 feet outside the building footprint. In all other areas, including flatwork and pavement areas, the upper I foot of subgrade soil should be treated with 7 percent cement or greater and placed at 90 percent relative compaction in sidewalk areas and at least 95 percent in roadway areas. A cost benefit evaluation is recommended for pavement areas to compare a hypothetical cement-treated pavement section (i.e. 4-inches of AC over 4-inches of aggregate base) to an in-situ pavement section (i.e. 4-inches of AC over 9-inches of aggregate base). In addition, soils proposed for retaining wall backfill will be cement-treated to lower the expansion index to acceptable levels (i.e. expansion index of 90 or less). Based on the laboratory test results, we expect the cement-treated soil to exhibit an expansion index between 30 and 80. The treated backfill should be placed within a I: I projection measured from base of wall. The in-situ and treated-soil laboratory expansion index test results are presented in Table 4. TABLE4 SUMMARY OF LABORATORY EXPANSION INDEX TEST RESULTS ASTM D4829 Moisture Content Sample No. Dry Density (pct) Expansion Index Before Test(%) After Test(%) S-1 (in-situ) 17.2 37.4 89.3 112 S-1 (7% Lime) 18.5 34.2 83.9 29 S-1 (7% Cement) 18.6 39.6 82.7 78 S-2 (in-situ) 15.3 36.7 91.9 96 S-2 (7% Lime) 16.5 34.4 86.9 64 S-2 (7% Cement) 19.7 36.1 82.4 37 The cement-treated subgrade soils should be placed and compacted in accordance with the recommendations contained in Section 27 of the Ca/trans Manual and Section 301 of the Standard Specifications for Public Works Construction. The subgrade stabilization should be observed and tested by Geocon Incorporated to verify that the project specifications discussed above have been met and the typical stabilization procedures provided in Section 27 of the Ca/trans Manual and Section 301 of the Standard Specifications for Public Works Construction were followed. The foundation design parameters presented in the referenced report remain applicable. The actual foundation design category for each structure or lot will be provided after the site is fine graded and the laboratory expansion index testing completed. Pavement recommendations considering treated subgrade soils are presented in our letter titled Lime/Cement-Treated Pavement Section Recommendations, Uptown Bressi Ranch Retail, Carlsbad, California, dated July 21, 2017. Project No. G2108-32-0I - 3 - August 2. 2017 Should you have any questions regarding this correspondence or desire additional information, please contact the undersigned. Very truly yours, GEOCONINCORPORATED I . I ' /, I I ,_: ~ • I Trevor E. Myers RCE 63773 TEM:DBE:dmc ( e-mail) Addressee Project No. G2 I 08-32-0 I ,/z. ! . ~; _::-......_____.i..,/ -'- David B. Evans CEG 1860 --- - 4 -August 2, 2017