Thesis (Ph. D.) - Asian Institute of Technology, 1992

A dissertation s ubmitted in pa rtia l f ulfillment of the requirements for the Degree of Doctor of Engineering, School of Engineering and Technology

Undisturbed composite samples was obtained from the interface of the cement piles supporting a test embankment in the AIT campus. Constant volume and constant normal stress direct shear test were performed on the interface material. Based on these data a softening parameter x is defined to depend only on the plastic shear displacement uP. A yield function, F, is then formulated to coincide with the Mohr-Coulomb failure criterion of the interface material. The Mohr-Coulomb strength parameters, 6 , is dependent on the plastic shear displacement uP during strain softening. The variation of the softening parameter 'K ·, generates a family of yield loci. The plastic potential function G is formulated by comparing the numerical result of plastic displacement increments def ( = A.(oG/ oa 1 ) ) with the field of experimentally determined plastic displacement increments d~ ( Id~ I= J (du P) 2 + (dvP) 2 ) which intersects the function at every point at right angle. The dvP is plastic normal displacement increment of interface. Within the yield loci the pre-peak shear behavior of the interface is described by a piece-wise linear elastic relationship. The elastic shear stiffness Kn for shear displacement is obtained from the experimental observations and is expressed as a function of the initial normal stress and the current interface stress level. During the phase when yielding takes place the post-peak strain softening behavior is characterized by the softening modulus M . and the non-associated stress-strain relationship is derived in terms of yield function F, the plastic potential G, the softening parameter x and the contraction angle tp ( tan tp = d vP I duP ). The constitutive law so developed for the elastic behavior and the yielding are then used to simulate the direct shear behavior of the interface materials. Comparisons of the experimental observations with predictions from the developed elastoplastic model reveal that the model makes successful predictions for stresses at pre-peak and post-peal< states. Moreover, the model combined with non-associated flow rule gives more approximate predictions for the volume change of interface.