Effect of irregularities of earthquake loading on seismic compression of sand
Dynamic simple shear tests on different types of sands under multidirectional earthquake loading were simulated with finite element method. Reduced order bounding surface model was employed to study the effect of characteristics of earthquake loading on the seismic compression of sands. The parameters of model were determined based on results obtained from laboratory dynamic simple shear tests. The study results show that the characteristics of earthquake loading such as loading sequence, asymmetry, loading direction, loading type and input loading dimension have great influence on seismic compression of sand. The vertical strain of sand increases dramatically when sand specimen is subjected to high stress pulse; also its increment is greatly influenced by time and sequence of stress pulse; the vertical strain of sands increases as the asymmetry of earthquake loading grows under identical conditions; negative loading results in increase of vertical strain of sand due to its compaction effect, which is opposite to positive loading effect; 20 cycles were used simply as the number of equivalent cycle in some current codes for computation of seismic compression, whose equivalences for vibration-type and impact-type of seismic loading is greatly different; two times of seismic compression of sand induced by one component loading was taken as the sum of seismic compression of sand induced by actual multidirectional earthquake loading in Tokimatsu and Seed (1987) procedure, which is in doubt.