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12/1712007 D. Encounterina Groundwater in Bav Mud Durina Excavation Potential Impact 11-4: Excavation for the proposed project's below-grade installations, such as utilities, might be difficult due to the high groundwater and relatively shallow depth of the weak Bay mud. This condition represents a potentially significant impact. Mitigation 11-4: Require that the detailed, design-level geotechnical investigations recommended under Mitigations 11-1 and 11-2 include analyses of groundwater and Bay mud within the limits of all foundation and utility construction. The analyses shall make recommendations regarding dewatering techniques, slope and shoring requirements for excavations, buffer zones for construction equipment near slope edges, the potential for requiring light grading equipment, subgrade remediation techniques, and the use of excavated fill. Project grading and construction plans shall incorporate the results and recommendations of these analyses. In general, these recommendations are expected to include, but not be limited to, the following provisions: 1. Soil excavated from below the water level would be saturated and would require drying before it can be used as fill. Continuous dewatering of excavations might be required. 2. The sides of the excavations should be sloped or shored to minimize ground movements; steep cuts are not possible in Bay mud unless shoring is used. Typically, when excavation extends into Bay mud, 4: 1 or flatter slopes are required to reduce movements. 3. Keep construction equipment away from the edges of slopes in order to reduce vibration degradation. Consider light grading equipment to avoid creating ruts in the subgrade, a situation that might require pumping. Remediate rutted subgrade areas by over-excavating the soft area to a depth of 18 to 24 inches, placing a geotextile at the bottom of the over-excavation, and backfilling with suitable granular material. 4. Do not use excavated Bay mud as fill beneath structures or pavement. When placing substantial fill directly on top of Bay mud, such as in the proposed reduced and reconfigured marinas, the fill might need to be placed in thin, uniformly thick lifts to prevent "mud waves" from forming. Mud waves can damage structural elements already installed, particularly piles and underground utilities. 5. The project civil engineer shall certify that all relevant provisions of the detailed, design-level geotechnical investigations have been incorporated into the grading, dredging, and construction plans. 6. All earthwork and site preparation shall be performed under the direct supervision of a certified engineering geologist or geotechnical engineer. Findings: Implementation of these measures is feasible and would reduce this impact to a less-than-significant level. Atty/Reso/Reso.1772 121707 36 14829 Muff# 613