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BLACK AVENUE RESIDENTIAL DEVELOPMENT 648-26-1 Page 5 4.3.1 Background During strong seismic shaking, cyclically induced stresses can cause increased pore pressures within the soil matrix that can result in liquefaction triggering, soil softening due to shear stress loss, potentially significant ground deformation due to settlement within sandy liquefiable layers as pore pressures dissipate, and/or flow failures in sloping ground or where open faces are present (lateral spreading) (NCEER 1998). Limited field and laboratory data is available regarding ground deformation due to settlement; however, in clean sand layers settlement on the order of 2 to 4 percent of the liquefied layer thickness can occur. Soils most susceptible to liquefaction are loose, non-cohesive soils that are saturated and are bedded with poor drainage, such as sand and silt layers bedded with a cohesive cap. 4.3.2 Analysis As discussed in the “Subsurface” section above, several sand layers were encountered below the design groundwater depth of 30 feet. Following the liquefaction analysis framework in the 2008 monograph, Soil Liquefaction During Earthquakes (Idriss and Boulanger, 2008), incorporating updates in CPT and SPT Based Liquefaction Triggering Procedures (Boulanger and Idriss, 2014), and in accordance with CDMG Special Publication 117A guidelines (CDMG, 2008) for quantitative analysis, these layers were analyzed for liquefaction triggering and potential post-liquefaction settlement. These methods compare the ratio of the estimated cyclic shaking (Cyclic Stress Ratio - CSR) to the soil’s estimated resistance to cyclic shaking (Cyclic Resistance Ratio - CRR), providing a factor of safety against liquefaction triggering. Factors of safety less than or equal to 1.3 are considered to be potentially liquefiable and capable of post- liquefaction re-consolidation (i.e., settlement). The CSR for each layer quantifies the stresses anticipated to be generated due to a design- level seismic event, is based on the peak horizontal acceleration generated at the ground surface discussed in the “Estimated Ground Shaking” section above and is corrected for overburden and stress reduction factors as discussed in the procedure developed by Seed and Idriss (1971) and updated in the 2008 Idriss and Boulanger monograph. The soil’s CRR is estimated from the in-situ measurements from CPTs and laboratory testing on samples retrieved from our borings. The tip pressures are corrected for effective overburden stresses, taking into consideration both the groundwater level at the time of exploration and the design groundwater level, and stress reduction versus depth factors. The CPT method utilizes the soil behavior type index (IC) to estimate the plasticity of the layers. 4.3.3 Summary Our analyses indicate that minor soil layers below a depth of 30 feet could potentially experience liquefaction triggering that could result in post-liquefaction total settlement at the ground surface up to ¼ inch based on the Yoshimine (2006) method. As discussed in SP 117A, differential movement for level ground sites over deep soil sites will be up to about two-thirds of the total settlement between independent foundation elements. In our opinion, differential settlements are anticipated to be negligible across a typical residential foundation. As discussed in the “Subsurface” section above, several sand layers were encountered below As discussed in the “Subsurface” section above, several sand layers were encountered below feet. Following the liquefaction analysis framework in thefeet. Following the liquefaction analysis framework in the Soil Liquefaction During EarthquakesSoil Liquefaction During Earthquakes (Idriss and Boulanger, 2008), (Idriss and Boulanger, 2008), CPT and SPT Based Liquefaction Triggering ProceduresCPT and SPT Based Liquefaction Triggering Procedures and Idriss, 2014), and in accordance with CDMG Special Publication 117A guidelines and Idriss, 2014), and in accordance with CDMG Special Publication 117A guidelines 2008) for quantitative analysis, these layers were analyzed for liquefaction triggering and 2008) for quantitative analysis, these layers were analyzed for liquefaction triggering and liquefaction settlement. These methods compare the ratio of the estimated cyclic liquefaction settlement. These methods compare the ratio of the estimated cyclic CSR) to the soil’s estimated CSR) to the soil’s estimated resistance to cyclic shaking (Cyclic resistance to cyclic shaking (Cyclic CRR), providing a factor of safety against liquefaction triggering. Factors of CRR), providing a factor of safety against liquefaction triggering. Factors of safety less than or equal to 1.3 are considered to be potentially liquefiable and capable of postsafety less than or equal to 1.3 are considered to be potentially liquefiable and capable of post settlement).settlement). The CSR for each layer quantifies the stresses anticipated to be generated due to a designThe CSR for each layer quantifies the stresses anticipated to be generated due to a design level seismic event, is based on the peak horizontal acceleration generated at the ground level seismic event, is based on the peak horizontal acceleration generated at the ground surface discussed in the “Estimated Ground Shaking” section abovesurface discussed in the “Estimated Ground Shaking” section above overburden and stress reduction factors as discussed in the procedure developed by Seed and overburden and stress reduction factors as discussed in the procedure developed by Seed and Idriss (1971) and updated in the 2008 Idriss and Boulanger monograph.Idriss (1971) and updated in the 2008 Idriss and Boulanger monograph. The soil’s CRR is estimated from the inThe soil’s CRR is estimated from the in samples retrieved from our borings. The tip pressures are corrected for effective overburden samples retrieved from our borings. The tip pressures are corrected for effective overburden