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7C <br /> Page 57 <br /> D. Encounterina Groundwater in Bav Mud Durinq Excavation <br /> Poten#ial Impact 11-4: Excavation for the praposed projecYs below-grade <br /> installa#ions, such as utifities, might be difficult due to the high groundwater and relatively <br /> shalfow depth of the weak Bay mud. This condition represents a potentially sign�cant impact. <br /> Mitigation 91-4: Require that the detailed, design-level geotechnical <br /> investigations recommended under Mitigafions 11-1 and 11-2 include analyses of groundwater <br /> and Bay mud within the limits af all foundation and utility canstructian. The analyses shall make <br /> recommendations regarding dewatering techniques, slope and shoring requirements for <br /> excavatians, buffer zones for construction equipment near siope edges, the potential for <br /> requiring light grading equipment, subgrade remediation techniques, and the use af excavated <br /> fill. Project gra�ing a�d construction plans shai! incorparate the results anu re��r�r�en�ations <br /> of these analyses. In generaE, these recommendati�ns are expected to include, but not be <br /> limited to, the following provisions: <br /> 1. Soil excavated from below the water leve{ would be saturated and would <br /> require drying before it can be used as fill. Continuous dewatering of <br /> excavatians might be required. <br /> 2. The sides of the excavations should be sloped or shored to minimize ground <br /> movements; steep cuts are not possible in Bay mud unless shoring is used. <br /> Typically, when excavation extends into Bay mud, 4:1 or flatter slopes are <br /> required to reduce movements. <br /> 3. Keep cons#ruction equipment away from the edges of slopes in arder to <br /> reduce vibration degradation. Consider light grading equipment to avoid <br /> creating ruts in the subgrade, a situation that migh# require pumping. <br /> Remediate rutted subgrade areas by over-excavating the soft area to a depth <br /> of 18 to 24 inches, placing a geotextife af the bottom af the over-excavation, <br /> and backfilling with suitable granular material. <br /> 4. Do not use excavated Bay mud as fill beneath structures ar pavement. When <br /> placing substantial filf directly on top of Bay mud, such as in the proposed <br /> reduced and reconfigured marinas, th�fill might need to be placed in thin, <br /> uniformly thick lifts to prevent"mud waves"from forming. Mud waves can <br /> damage structural elements already installed, particularly piles and <br /> underground utilities. <br /> 5. Ttie projec#civil engineer shali certify that afl re{evant provisions of ihe <br /> detailed, design-level geotec#�nical investigations have been incorporated into <br /> the grading, dredging, and cons#ruction plans. <br /> 6. Alf earthwork and site preparation shall be performed under the direct <br /> supervision of a certified engineering geologist or geotechnical engineer. <br /> Findings: Imp{ementation of these measures is feasible and would reduce this <br /> impact to a fess-than-signrficant level. <br /> Atty/ResoJReso.1772 36 <br /> 121307 <br />