A NUMERICAL APPROACH TO DYNAMIC UNILATERAL CONTACT PROBLEM OF SOIL-PILE INTERACTION UNDER SECOND-ORDER EFFECTS Professor A. A. LIOLIOS Democritus University of Thrace, Dept. Civil Engineering, GR-67100 Xanthi, Greece. (e-mail: LIOLIOS@demo.cc.duth.gr) ABSTRACT The paper deals with a numerical approach for the dynamic soil-pile interaction, considered as an inequality problem of structural engineering. So, the unilateral contact conditions due to tensionless and elastoplastic softening/fracturing behaviour of the soil as well as due to gapping caused by earthquake excitations are taken into account. Moreover, second-order geometric effects for the pile behaviour due to preexisting compressive loads are taken also into account. The numerical approach is based on a double discretization and on mathematical programming. First, in space the finite element method (FEM) is used for the simulation of the pipeline and the unilateral contact interface, in combination with the boundary element method (BEM) for the soil simulation. Next, with the aid of Laplace transform, the problem conditions are transformed to convolutional ones involving as unknowns the unilateral quantities only. So the number of unknowns is significantly reduced. Then a marching-time approach is applied and finally a nonconvex linear complementarity problem is solved in each time-step. References [1] Panagiotopoulos, P.D., "Hemivariational Inequalities and Applications", Springer Verlag, Berlin (1993). [2] Maier, G., Incremental Elastoplastic Analysis in the Presence of Large Displacements and Physical Instabilizing Effects, Int. Jnl Solids and Structures, Vol. 7, 345-372, (1971). [3] Wolf, J.P.,"Soil-Structure-Interaction Analysis in Time Domain", Prentice-Hall, Englewood Cliffs, N.J. 1988. [4] Liolios, A.A., "A Boundary Element Convolutional Approach to Unilateral Structural Dynamics", in: Antes, H. (Ed.), IABEM 93-Extended Abstracts, Inst. Appl. Mech., Techn. Univ. Braunschweig, 1993. 