ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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distribution system workers to a substantial risk of loss, injury, or death. The City's engineering design process for the project would ensure the development of structures that meet or exceed current state engineering and design standards. The CBC allows facilities to be designed using an alternative design basis from an approved national standard. For concrete tanks and similar structures, standard practice is to use ACI 350,"Code Requirements for Environmental Structures,"in conjunction with ACI 350.3 "Seismic Design of Liquid- Containing Structures."Although not required by the code,the City designs wastewater treatment facilities as "essential facilities"and uses corresponding importance factors.An essential facility is defined by the CBC as a structure that is necessary for emergency operations subsequent to a natural disaster. This results in 25-50% higher structural design strengths. This same approach is used for the design of all tank structures under the latest version of ACI 350 and ACI 350.3,thereby reducing the inelastic damage that is likely to be seen in a major earthquake event. Because the City designs wastewater treatment facilities as"essential facilities"with 25-50% higher structural design strengths, construction of the proposed project would not expose people or structures to seismic-related structural and nonstructural damage and associated hazards. This would be a less-than-significant impact. IMPACT Geology, Soils, and Seismicity—Risks to People and Structures Caused by Seismic-Related Ground 4.7-2 Failure. Based on the underlying soil conditions in the project area and the shallowness of the groundwater table, construction of the proposed project has the potential to expose people or structures to seismic-related ground failure, including liquefaction and differential settlement. Therefore, this impact is considered potentially significant. Because the project area is located in the CBC's Seismic Zone 3,the level of anticipated seismic ground shaking is lower than for many other areas in California. However, strong ground shaking(corresponding to zone VII to VIII on the Modified Mercalli Scale)may still occur as a result of large,distant earthquakes, causing general alarm and moderate damage. In addition, Seismic Zone 3 implies a minimum horizontal acceleration of 0.3 g for use in earthquake resistant design(where damage to a single-family dwelling typically begins at 0.2 g). Liquefaction occurs when saturated soil loses shear strength and deforms as a result of increased pore water pressure induced by strong ground shaking during an earthquake. As the excess pore pressure dissipates,volume changes are produced within the liquefied soil layer,which can manifest at the ground surface as settlement of structures, floating of buried structures, and failure of retaining walls. Soil types most susceptible to liquefaction are saturated, loose, and sandy soils. According to Kleinfelder reports (1997 and 2004), subsurface soils at the WQCF site include interbedded medium-dense silty and"clean" sand within the upper 40 feet of subgrade soils. In addition, groundwater levels were found to be high. Therefore, interbedded sand layers at the WQCF and pumping station sites,and along wastewater collection system, effluent outfall pipeline, and recycled water distribution system pipeline alignments may be susceptible to liquefaction in the event of a large magnitude earthquake along the nearest segments of the Great Valley fault. This potential for liquefaction would not be expected to be widespread in the project area given the discontinuous and localized nature of these sand layers at the WQCF site. Soil permeability at the WQCF site is rapid,but surface runoff is very slow. High groundwater levels during the winter and spring months in combination with slow surface runoff could create saturated surface soil conditions and high soil moisture content. During strong ground shaking, saturated soils at the WQCF site could undergo liquefaction. Given these conditions, if project structures and facilities are not designed or constructed appropriately, a large seismic event could expose WQCF,pumping station,wastewater collection system, and recycled water distribution system workers to a substantial risk of loss, injury, or death. Because strong ground shaking may occur at the project area as a result of distant earthquakes, subsurface soils at the WQCF site include interbedded medium-dense silty and"clean"sand, and project area groundwater levels are high, construction of the proposed project has the potential to expose people or structures to seismic-related ground failure,including liquefaction and differential settlement. This would be a potentially significant impact. EDAW Manteca WQCF and Collection System Master Plans EIR Geology, Soils,and Seismicity 4.7-18 City of Manteca