Date: August 4, 2015 Project No.: 608-4-2 Prepared For: Mr. Sajeev Madhaven, AIA SAN FRANCISCO UNIFIED SCHOOL DISTRICT 135 Van Ness Avenue - Room 207a San Francisco, California 94102 Re: Geotechnical Consultation for Storm Infiltration Robert Louis Stevenson ES Modernization 3051 34th Avenue San Francisco, California Dear Mr. Madhaven: This supplemental letter provides a summary of our findings and geotechnical recommendations for storm water infiltration for the above-referenced project. As you know, we have previously performed a geotechnical investigation for the site and presented the findings and recommendations in our report dated July 9, 2015. Recommendations for storm water infiltration provided herein are based on our current project understanding and information provided to date. The recommendations provided herein supplement the recommendations presented in our previous report. Please refer to our report for recommendations not discussed herein. We reviewed the following documents for the preparation of this letter: A drainage plan titled, Robert Louis Stevenson Elementary School, Stormwater Management Plan, Drainage Management Plan, San Francisco, County of San Francisco, California, prepared by BKF, dated July 16, 2015. A cross section detail titled, Section at Rain Garden, prepared by BKF, dated July 27, 2015. THE PROJECT As part of the campus modernization project, we understand a new infiltration system will be installed to the west of the proposed classroom building located in the northwestern corner of the campus, near the corner of 35 th Avenue and Pacheco Street. Based on our correspondence with the project Civil Engineer, the system will have a footprint of approximately 800 square feet and will collect watershed from the roof of the proposed classroom building and the proposed concrete flatwork to the north of the building. See Figure 1, Site Plan, for the approximate layout of the rain garden in relationship to the proposed building. We understand that this project is being designed for a 2-year storm event with a 24-hour runoff volume of 2,320 cubic feet and according to San Francisco Public Utilities Commission (SFPUC) requirements.
Overflow pipe inlets will be included in the proposed system. The edge of the infiltration system will be located anywhere from about 7 to 10 feet away from the western edge of the proposed classroom building. SOIL PERMEABILITY AND STORM WATER INFILTRATION FIELD INFILTRATION TESTS Infiltration tests were performed to estimate the rate of infiltration through the native soils onsite. Two infiltration tests, Perc-1 and Perc-2, were performed near the northern and southern limits of the proposed infiltration system using a Guelph permeameter device, and another infiltration test, I-1, was performed adjacent to Perc-1 using a Double-Ring Infiltrometer on July 17, 2015. The approximate locations of the infiltration tests are shown on Figure 1. All three of our infiltration tests were performed in sands with a very low fines content, which was generally consistent with the subsurface conditions encountered in our exploratory borings for the campus modernization investigation. Please refer to our geotechnical report for more details regarding subsurface information from our investigation. Guelph Permeameter Tests To estimate the infiltration rate and hydraulic conductivity of the soils, we performed two in-situ field permeability tests using a Guelph permeameter by SoilMoisture Equipment Corp., Model #2800, in accordance with ASTM D5126 near the locations shown on the attached Site Plan. We coordinated the test locations and depths with BKF Engineers. Generally, the Guelph permeameter is a constant head device, which uses two water-filled chambers to measure infiltration rate in a shallow borehole. A constant head level is established in the borehole and the rate of water outflow into the surrounding soil is noted. The rate of flow when it reaches a steady state, or constant rate, is used to determine the soil characteristics such as the saturated conductivity and permeability. Our field permeability tests were performed at the terminal boring depth of approximately 4 feet. These depths correspond to a depth of approximately 12 inches below the adjacent City sidewalk, which we understand will be the bottom of the infiltration system. Test results are summarized in Table 1 below. Table 1: In-Situ Field Guelph Permeameter Test Results Location / Depth (ft.) Perc-1 (4ft) Perc-2 (4ft) Infiltration Rate 3.6 in/hr 19.8 in/hr Double-Ring Infiltration Test We also performed one infiltration test using a double-ring infiltrometer in accordance with ASTM D3385 test methods (constant head) at a depth of approximately 12 inches below existing site grades. Similar soil conditions were encountered in the shallow double-ring infiltration test compared to the Guelph Permeameter tests that were performed within small diameter borings. The rings were embedded at least 6 inches below the exposed soil level, filled with approximately 8 to 10 inches of water and allowed to presoak for at least 30 minutes before starting the test readings. The double-ring infiltration test was conducted for Project No. 608-4-2 Page 2 August 4, 2015
approximately 2 hours, until a constant infiltration rate of approximately 16 inches per hour was established. Reliability of Field and Laboratory Test Data Test results may not be truly indicative of the long-term, in-situ permeability. Other factors including stratifications, heterogenous deposits, overburden stress, and other factors can influence permeability results. In addition, for stratified soils such as those encountered at the site, the average horizontal permeability is typically greater than the average vertical permeability. General Comments and Design Considerations As discussed, tests were performed at discrete locations, depths, and samples. In addition, some disturbance in preparing the tests can occur. Therefore, the above results can vary significantly and are not representative over the entire site. Localized areas/depths containing higher or lower permeable materials or variable ground water conditions can increase or decrease the actual infiltration rates, respectively. Therefore, we recommend the potential for variations be considered when evaluating the soil infiltration capacity or performance. In addition, we recommend the project civil engineer give consideration for handling/discharging of water when the infiltration rate is not sufficient or during a large storm event. We also recommend that subsurface water infiltration techniques and/or devices be in accordance with local agencies guidelines and requirements. We recommend you contact the San Francisco Public Utilities Commission for additional information and approval, as required. CONCLUSIONS AND RECOMMENDATIONS FINDINGS AND RECOMMENDATIONS Based on our experience and engineering judgment, soils typically have fair to good vertical permeability with medium to high infiltration rates ranging from about 3½ inches per hour to 19½ inches per hour, respectively. In relation to the above mentioned values, all three of the in-situ field tests and laboratory tests indicate fair to good permeability and infiltration rates at the depth, location, and samples tested. Based on our field test data and our engineering judgement, we would recommend a design infiltration rate of 4 inches per hour to account for any variations in the subsurface materials. Based on the results of our field infiltration tests, the undisturbed native soils may be classified as Hydrologic Soil Group B, according to the United States Department of Agriculture National Engineering Handbook (Chapter 7 Hydrologic Soil Groups). In addition, historic high ground water maps prepared by the California Geologic Survey indicate ground water on the order of 50 feet below the existing ground surface (CGS, 2000) across the site. We recommend any underground percolation systems take into account the estimated depth to ground water referenced above. In addition, the project civil engineer should review the above information and provide additional recommendations including the dry well construction, as deemed necessary. Based on our review of the previous report and the results of our infiltration tests, we would recommend that the upper 2 feet of the soils below the bottom of the retaining wall and all building foundations (see Figure 2) be over-excavated and recompacted to at least 90 percent Project No. 608-4-2 Page 3 August 4, 2015
relative compaction, as determined by ASTM 1557. The bottom of the excavation should be prepared according to the subgrade preparation recommendations provided in our geotechnical report. We also recommend that a heavy duty vibratory smooth or pads foot drum roller be used for compaction efforts. The purpose of this recommendation is to minimize the potential for settlement due to water infiltration under the foundations in this area. CLOSURE We hope this provides the information you need at this time. The findings and recommendations provided in this report have been prepared for the sole use of the San Francisco Unified School District, specifically for the campus modernization project at Robert Louis Stevenson Elementary School in San Francisco, California. Our professional services were performed, our findings obtained, and our recommendations prepared in accordance with generally accepted geotechnical engineering principles and practices at this time and location. No warranties are expressed or implied. If you have any questions or need any additional information from us, please call and we will be glad to discuss them with you. Sincerely, Cornerstone Earth Group, Inc. Matthew A. Anderson, P.E. Project Engineer Scott E. Fitinghoff, P.E., G.E. Senior Principal Engineer MAA:SEF:maa Copies: Attachments: Addressee (1 by email) Figure 1 Site Plan Figure 2 Conceptual Cross Section at Rain Garden Project No. 608-4-2 Page 4 August 4, 2015