| The response of pile groups in liquefiable ground subjected to complicated dynamic loadings is a challenging and popular topic in the field of geotechnical earthquake engineering.A series of centrifuge shaking table model tests were conducted on 4×4 pile groups in liquefiable ground in this study,achieving horizontal-vertical bidirectional shaking in centrifuge tests on piles for the first time.Through the combination of centrifuge shaking table experiments and numerical simulations,three key subjects were investigated,including?1?the roles and coupling mechanism of superstructure-pile inertial interaction and soil-pile kinematic interaction,?2?the dynamic distribution of forces on each pile within the pile groups and?3?the influence of horizontal-vertical bidirectional ground motions on pile head bending moments.The main novel achievements are as follows:1.The coupling mechanism of superstructure-pile inertial effect and soil-pile kinematic effect in liquefiable ground was studied.The positive superstructure inertial force induced a positive moment on pile head,while the positive kinematic deformation did a negative moment on pile head.The soil-pile kinematic effect played a dominant role in inducing pile head moment.Also the magnitude and phase difference of inertial forces and kinematic deformations determined the pile head moment.Several important factors influencing the coupling mechanism were interpreted,including soil profiles,pile diameter,pile length,pile spacing,the stiffness of pile cap,the height of the superstructure,and properties of the ground motion.2.The dynamic distributions of bending moments on piles connected by a rigid cap within pile groups in liquefiable ground were examined.For the case of small-scale pile group subjected by horizontal ground motions,external piles experienced greater bending moments than internal piles,when the superstructure inertial force was small.And the internal piles experienced greater bending moments when the superstructure force was large.However,for the case of large-scale pile group,the distribution pattern was quite different.The external rows of piles parallel to the shaking direction experienced greater bending moments than internal rows of piles;moreover,the middle piles perpendicular to the shaking direction experienced greater bending moments than the outside piles when the superstructure inertial force was small,while the distribution pattern was opposite when the inertial force was large.The pile bending moment was determined by pile deflections and the lateral soil pressure.The negative pile deflection relative to pile top generated a negative bending moment at pile head,while the positive lateral soil pressure generated a positive bending moment at pile head.The magnitude of superstructure inertial force influenced the distribution of lateral soil pressure by affecting the relative displacement between the pile and the soil.3.The influence of the horizontal-vertical bidirectional ground motion on pile bending moment was investigated.The vertical ground motion in the bidirectional ground motion could influence the vertical total stress of the saturated sand.When the permeability coefficient of the sand was low,for example,less than 10-3m/s,the change of vertical total stress of saturated sand was mostly carried by excess pore pressure,instead of by the effective stress of the sand,which resulted in little influence on the pile bending moment.When the permeability coefficient of the sand was high,for example,more than 10-3m/s,the change of vertical total stress of saturated sand was partly carried by the vertical effective stress.So the shear stress ratio of the saturated sand changed,followed by an increase or decrease of the sand deformation and the pile head bending moment.The influence of vertical ground motions on pile bending moments depended on the magnitude of the vertical peak acceleration and the phase difference between the vertical and horizontal peak accelerations. |
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