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|Title:||CENTRIFUGE AND NUMERICAL MODELLING OF SOFT CLAY-PILE-RAFT FOUNDATIONS SUBJECTED TO SEISMIC SHAKING||Authors:||SUBHADEEP BANERJEE||Keywords:||earthquake, pile, clay,strain softening, resonance period,dimensionless moment||Issue Date:||27-Oct-2009||Citation:||SUBHADEEP BANERJEE (2009-10-27). CENTRIFUGE AND NUMERICAL MODELLING OF SOFT CLAY-PILE-RAFT FOUNDATIONS SUBJECTED TO SEISMIC SHAKING. ScholarBank@NUS Repository.||Abstract:||The behavior of pile foundations under earthquake loading is an important factor affecting the performance of structures. Observations from past earthquakes have shown that piles in firm soils generally perform well, while those installed in soft or liquefiable soils are more susceptible to problems arising from ground amplification or excessive soil movements.
The current thesis presents the details and results of a study on the seismic response of pile-raft systems in normally consolidated kaolin clay due to far-field earthquake motions. The research comprises four major components: (1) element testing using the cyclic triaxial and resonant column apparatus to characterize the dynamic properties of kaolin clay, the results of which were subsequently incorporated into a hyperbolic-hysteretic constitutive relationship; (2) dynamic centrifuge tests on pure kaolin clay beds (without structure) followed by 3-D finite element back-analyses; (3) dynamic centrifuge tests on clay-pile-raft systems and the corresponding 3-D finite element back-analyses and (4) parametric studies leading to the derivation of a semi-analytical closed-form solution for the maximum bending moment in a pile under seismic excitation.
The element test results showed that strain-dependent modulus reduction and cyclic stiffness degradation feature strongly in the dynamic behaviour of the clay specimens. In the centrifuge tests involving uniform clay beds without piles, the effects of modulus reduction and stiffness degradation were manifested as an increase in the resonance periods of the clay layers with the level of shaking and with successive earthquakes. For the pile systems tested, the effect of the surrounding soft clay was primarily to impose an inertial loading onto the piles, thereby increasing the natural frequency of the pile over and above that of the pile foundation alone. There was also some evidence that the relative motion between piles and soil leads to aggravated softening of the soil around the pile, thereby lengthening its resonance period further.
In terms of the bending moment response, the maximum bending moment was recorded near the fixed head connection between the pile and the raft. The bending moment was found to increase almost linearly with the scaled earthquake ground motion. It was also observed that the bending moment increases with the flexural rigidity of the pile material and with increasing added masses on the pile raft.
The centrifuge model tests were back-analysed using the finite element code ABAQUS. The analyses, which were carried out using a user-defined total-stress hyperbolic-hysteretic constitutive relationship (HyperMas), gave reasonably good agreement with the experimental observations. The ability of the numerical model to reasonably replicate the centrifuge tests suggests that the former may be used to analyze conditions not considered in the centrifuge experiments, as well as to carry out sensitivity studies. To facilitate the parametric studies, the method of non-dimensional analysis, using Buckingham-o 0b s theorem, was carried out to derive the dimensionless terms associated with the maximum bending moment in a seismically loaded pile. The resulting semi-analytical solution for the maximum bending moment was calibrated through parametric studies involving the pile length, moment inertia, pile and soil modulus, mass of the raft and peak ground motion.
|Appears in Collections:||Ph.D Theses (Open)|
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