Liquefaction Consequences of Stratified Deposits of Silty Soils

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Soil liquefaction caused significant damage to buildings and lifelines in Christchurch, New Zealand during the 2010-2011 Canterbury Earthquake Sequence. New Zealand design and construction methods are similar to those in the U.S., so lessons learned from these earthquakes are directly transferrable to the U.S. Many of the cases of liquefaction-induced damaged were captured well by current design procedures. However, many other cases were not. The occurrence and effects of soil liquefaction were over-predicted in several parts of the city that contained silty soil deposits. Most liquefaction case histories and laboratory experiments have focused on the response of clean sands. Silty soils differ from clean sands in several characteristics. Silts contain different sized and shaped particles with different mineralogy. Different depositional processes create silt deposits. Therefore, it should not be surprising that silty soil deposits responded differently than clean sand deposits. Yet, engineers currently rely on design procedures largely based on the seismic performance of clean sands. This practice can result in significant discrepancies between predictions and actual performances at silty soil sites. Thus, there is a pressing need to investigate the seismic response of stratified deposits of silty soils at sites that were strongly shaken and predicted to liquefy during the 2010-11 Canterbury earthquakes but did not exhibit evidence of liquefaction.The over-prediction of liquefaction triggering by current procedures appears to be due to their inability to capture the cyclic response of stratified silty soil deposits. Insight will be gained by investigating the liquefaction triggering of sites with stratified silty soil deposits that conventional procedures indicate should have liquefied, but field observations indicate did not liquefy. The current hypothesis to evaluate is that an assessment of the soil-water system response of stratified soil deposits is required to capture the observed cases of no liquefaction manifestations. Empirical correlations that relate cone penetration test (CPT) results to the fines content and plasticity of the soil will be evaluated, and the hypothesis that fines content is not a meaningful parameter in deposits of fine sand/coarse silt will be tested. Through a program of cyclic testing of natural and prepared soil specimens, fundamental insights regarding the cyclic response of silty soils will be developed. A satisfactory understanding of the cyclic response of silty soils is currently lacking. Through validated numerical simulations that capture the nonlinear, effective stress response of stratified silty soil deposits, insights regarding key mechanisms and probable reasons for the lack of manifestations of liquefaction at sites that simplified procedures indicate should have liquefied will be developed. The work will lead to recommendations on how to assess the cyclic response of stratified soil sites which include an alternative liquefaction evaluation method. This grant supports an international research collaboration with New Zealand that will advance knowledge in the U.S. and in New Zealand. The U.S. PhD student, who is targeted to be female, will benefit greatly from participating in this international study. Co-funding of this award has been provided by the NSF Office of International Science and Engineering.