AgdaPkt 2017-01-09 Closed and Joint

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TA. - Page 66 ■ Undocumented Fills: ENGEO expects an undocumented fill layer; estimated to be up to 10 feet thick, to be present within the approximate limits of the former Redwood Creek channel, which once crossed the northeast corner of the project site, approximately where the entrance to the existing Bradford Street driveway is located, before curving to the southeast and continuing across the property line to the adjacent school district property. The creek channel was undergrounded in a culvert in the late 1950s/early 1960s, but then the culvert was filled in the late 1960s when the configuration of Redwood Creek was realigned to the east. The current Redwood Creek channel surfaces from this realigned underground culvert approximately 200 feet northeast of the project site, near Bradford Street and Main Street, where the creek is daylighted (ENGEO pp. 2 and 8). ENGEO performed a soil test (Cone Penetration Test) in the northeast corner of the project site, which revealed approximately 10 feet of sand, silt, and clay deposits, and concluded that although this material indicated a layer of undocumented fill, which could possibly extend to 10 feet thick, the planned site excavation (approximately 10 to 35 feet deep) would remove this undocumented fill layer. (p. 9) ■ Corrosivity: A corrosion consultant should provide specific design recommendations on corrosion protection for important buried metallic lines, which at a minimum should meet CBC requirements for cement used in foundation concrete. (pp. 10 and 11) ■ Grading: Grading operations should meet the standards recommended in Appendix F of the ENGEO report (Guide Contract Specifications), which are subject to review and approval by the City. In addition, grading operations should be observed and tested by a California Registered Geotechnical Engineer. (p. 11) ■ Compressible Soils: Any soft compressible soils should be removed from areas that are to receive fill or upon which structures are to be built, and should also be removed from those areas that will provide soil to be transferred for use elsewhere (i.e., to serve as borrow) (p. 11). ■ Engineered Backfill: Excavations from demolition and removal (stripping) of boulders or roots or any unsuitable materials, if below design garage elevations, should reach a suitable firm base, which should be loosened (scarified) to a depth of 10 inches, and then moisture conditioned and recompacted to provide adequate bonding with the first layer offill. (p. 11) ■ Building Foundation: The proposed residential structure maybe supported on a stiff structural mat foundation (p. 13). ■ Double -Slab Drainage System: A double -slab drainage system, which uses free -draining crushed rock (or gravel) with a slab top over the foundation mat plus perforated collector pipes leading to sumps where water is collected and removed, may be considered to reduce the risk of water ponding on the basement floor (p. 14). ■ Secondary Slab -on -Grade Construction: Slabs -on -grade should be reinforced for control of cracking, and frequent control joints should be provided to control the cracking. Such reinforcement should be designed by the Structural Engineer. Exterior slabs should be constructed with thickened edges extending at least beneath the crushed rock or gravel into compacted soil to reduce water infiltration. Slabs should slope away from the building at a slope of at least 2 percent to prevent water from flowing toward the building. (p. 14) ■ Retaining Walls: Retaining walls must be designed to resist pressure from soils and/or water (pp. 14 and I5). ■ Excavations and Shoring: ENGEO anticipates the need to use shoring techniques for below -grade excavation. Recommended techniques include driven sheet piles or soldier piles with lagging. (p. 16) ■ Dewatering Considerations: Dewatering should be performed so that water levels are maintained not less than two feet below the bottom of excavation prior to and continuously during shoring installation. The groundwater levels outside of the shoring system should not be allowed to drop significantly because lowering of groundwater levels could result in settlement of surrounding improvements. (p. 16) Item VIII (Hazards and Hazardous Materials) of this Initial Study describes County groundwater protection protocols that would be required before discharging this water into the storm or sanitary sewer system, including preparation of a site management plan that would be subject to review and approval by the San Mateo County Health System Groundwater Protection Program (GPP). ■ Stormwater Infiltration/Bioretention Areas: If bioretention areas are implemented, a subdrain or other storm drain system should be incorporated to collect and convey water to an approved outlet, considering the very low permeability of site soils. Bioretention areas should be constructed with structural side walls capable of withstanding the loads from the adjacent improvements. (p. 16) The project's drainage plan identifies: (1) an on-site, underground detention vault with media filter in Basement 1, near Bradford, which would retain and treat water from Drainage Area 92 (16,134 square feet of the roofl before the storm water enters the City's storm drain system; and (2) raised planters on the second floor outdoor terraces which would serve as a bioretention area for Drainage Area #1 (6,400 square feet of roof and the outdoor terrace itsef, before the storm water enters the City system. DMA 92 would flow directly from the roof through pipes down to the media filter and vault; DMA 91 would flow through planters first ICS