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PI/1J~lCO DYIVe Trrrnont mld Pipeline Project E/IVironr«ent«[ Ckecklist a-iii) Less than Significant. Liquefaction is a soil behavior in which a soil undergoes a rapid loss in effective strength due to the development of high excess pore water pressure generated by earthquake ground shaking. Soils that are most susceptible to liquefaction are clean, loose, uniformly graded, saturated, fine-grained sands or saturated soft to firm low plasticity silts that lie near to the ground surface, depth usually considered to be less than 50 feet. The geotechnical study commissioned for the project (DCM Engineering 2005) concluded that, while the regional mapping of liquefaction hazard levels by others shows that the liquefaction hazard to be moderate to high along the pipeline alignment, based upon exploratory borings taken along the pipeline alignments that did not encounter any layers of saturated silts or sands of low to medium density, the risk of liquefaction is considered to be low. Therefore, this would be less than significant. a-iv) No Impact. The site is generally flat and no landslides are present within the project site and pipeline alignment (DCM Engineering 2005). Therefore, this would be no impact. b, c, and d) Less than Significant. There will be limited grading and excavation activities associated with construction of the fluoridation building and the two facility vaults over a relatively small area of approximately 2,450 square feet or less than 0.06 acres. The pipeline will be installed with open trenching and the spoil material trucked off site. "I'hcre will be no large scale mass grading activities that will disturb the ground surface over an extensive area. Implementation of the erosion control Best Management Practices (BMP's), as discussed in the Project Description and Section VIII, Hydrology and Water Quality will reduce erosion and loss of soil in the limited areas of disturbance during construction. Construction will be in conformance with all storm water management and discharge control provisions contained in Chapter 9.14 of the City of Pleasanton's Municipal Code and the National Pollutant Discharge Elimination System Permit (NPDF,S) for construction activities. As described in the Project Description, the City of Pleasanton standard specifications include development of a Storm Water Pollution Prevention Plan (SWPPP) for construction. The SWPPP will identify Best Management Practices necessary to control erosion and sedimentation during construction. The SWPPP will require that the measures applied to prevent and control erosion are inspected and kept in working order throughout project construction. Grading will be prohibited from October 15 to April 1 to reduce city-wide impacts. These measures will mitigate off-site erosion during construction and will decrease hazards of strong ground shaking and reduce impacts to less than significant. The project area and adjacent areas are located in flat terrain and would not be subject to landslides. Excavations of the pipeline trench will be in soils classed according to Cal/OSHA as "type C' in the upper 3.5 feet, overlain with clayey and sandy fill soils and with depth medium to stiff to stiff saturated native lean clay and fat clay "type B soils. Groundwater is anticipated to occur in some portions of the pipeline trench. Based upon the geotechnical investigation recommendations, shoring will be required so that the trench walls do not collapse affecting personnel who enter the trench, adjacent existing utilities, pavements and structures and resist effects from adjacent vehicular traffic, construction City of Pleasanton 29 Winzler & Kelly Draft Initial Study/Proposed Negative Declaration August 2006 04263501