Effects of Cyclic Shear Direction on Earthquake-Induced Pore Water Pressure and Settlement of Clay Layer

Abstract eng:
In this paper, normally consolidated specimens of Kaolinite clay, Tokyo bay clay and Kitakyushu clay were tested under undrained uni-directional and multi-directional cyclic simple shear. The effect of cyclic shear direction on the pore water pressure accumulation during undrained cyclic shear and on the settlement after cyclic shear were investigated by focusing on the normalized pore water pressure ratio (DU) and post-cyclic settlement in strain (Dε), which are defined as the ratio of those induced by multi-directional cyclic shear ((U dyn /σ’ v0 ) M and ε M ) to those induced by uni-directional one ((U dyn /σ’ v0 ) U and ε U ), and also focusing on the difference of pore water pressure ratio (∆U) and post-cyclic settlement (∆ε), which are defined as the difference between (U dyn /σ’ v0 ) M , ε M and (U dyn /σ’ v0 ) U , ε U , respectively. It is shown from the experimental results that for all specimens, DU and Dε firstly reach their peaks (DU max and Dε max ) at the shear strain amplitude of about γ DUmax = 0.1% - 0.4% for U dyn /σ’ v0 and γ Dεmax = 0.1% - 0.2% for ε, then decreasing to smaller values. For clays used in this study, plasticity index is in the range from I p = 25.5 to 63.8, then the maximum normalized pore water pressure ratio increases from DU max = 2.19 to 3.12, which is smaller than the maximum normalized post-cyclic settlement being from Dε max = 2.62 to 3.62. As to the difference of pore water pressure ratio and post-cyclic settlement, ∆U and ∆ε firstly increase with the shear strain amplitude up to their maximum value (∆U max and ∆ε max ) and decrease. For the range of I p from 25.5 to 63.8, ∆U max decreases from 0.34 to 0.20, being smaller than ∆ε max which also falls from 2.56% to 1.02%, respectively. Meanwhile the shear strain amplitude for these peak values increases from γ ∆Umax = 0.3% to 1.2% and from γ ∆εmax = 1.0% to 1.2%. Therefore, it is suggested that the higher the Atterberg’s limit (plasticity index) of clay, the smaller the effect of cyclic shear direction on the pore water pressure and the settlement becomes.

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