Liquefaction Consequences of Stratified Deposits of Silty Soils

在2010-2011年坎特伯雷地震序列中，土壤液化对新西兰基督城的建筑物和生命线造成了重大破坏。新西兰的设计和施工方法与美国相似，因此，从这些地震中吸取的教训可以直接应用到美国。许多由地震引起的破坏案例都被当前的设计程序很好地捕捉到了。 然而，许多其他情况并非如此。 在城市的几个含有粉土沉积物的地方，土壤液化的发生和影响被高估了。 大多数液化案例和实验室实验都集中在干净砂土的响应上。 粉质土在几个特性上不同于纯砂。 粉土中含有不同粒度和形状的颗粒，具有不同的矿物学特征。不同的沉积过程形成淤泥沉积物。因此，粉质土壤沉积物的反应与干净的沙子沉积物不同，这并不奇怪。 然而，工程师们目前主要依赖于基于干净沙子的抗震性能的设计程序。这种做法可能会导致在粉质土场地的预测和实际性能之间的显着差异。 因此，迫切需要研究在2010-11年坎特伯雷地震期间受到强烈震动并预计会发生地震但没有表现出液化迹象的场地的粉质土分层沉积物的地震响应。当前程序对液化触发的过度预测似乎是由于无法捕获分层粉质土沉积物的循环响应。 通过调查分层粉质土沉积物场地的液化触发，将获得深入了解，常规程序表明应已液化，但现场观察表明未液化。 目前要评估的假设是，需要对分层土壤沉积物的土壤-水系统响应进行评估，以捕获所观察到的无液化表现的情况。 将评估将静力锥贯入试验（CPT）结果与土壤细粒含量和塑性相关的经验相关性，并将测试细粒含量在细砂/粗粉砂沉积物中不是有意义参数的假设。 通过天然和制备的土样的循环测试程序，将开发关于粉质土的循环响应的基本见解。 目前对粉质土的循环响应还缺乏令人满意的认识。 通过有效的数值模拟，捕获的非线性，有效的应力响应的分层粉质土沉积物，有关的关键机制和可能的原因，缺乏液化的表现在现场，简化程序表明应该液化的见解将被开发。这项工作将导致建议如何评估循环响应的分层土壤网站，其中包括一个替代的液化评价方法。 该补助金支持与新西兰的国际研究合作，这将促进美国和新西兰的知识。 美国博士生，谁是针对女性，将大大受益于参与这项国际研究。 该奖项的共同资助由NSF国际科学与工程办公室提供。

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.