The soil-structure interaction for underground structures is investigated in this study considering three cases of seismic excitations: major, strong and moderate earthquakes. The study of such structures buried in soil has to face various difficulties arising from both the structures (rigidity, geometry, discontinuity) and the soil (geometry, infinity, uncertainty) [1,5].

For this purpose, the boundary element method (BEM) is the most appropriate as it requires only the discretization of the frontiers, reducing therefore significantly the dimension of the problem. However, this procedure requires the determination of Green's functions, that are quite complex in the case of multilayered soils. To overcome this difficulty, one may use the thin layer method (TLM) developed by Kausel and Peek [4], in order to obtain these functions that are adequate in the case of dynamic loads in the horizontally stratified medium. The use of the BEM to calculate the displacements on the surface and in the buried structures becomes easy.

In the present paper, the Algiers subway (under construction) in multi-layered soils is analyzed under three cases of seismic excitations: major, strong and moderate earthquakes. The multilayered soil structure interaction for these underground structures with stochastic geological characteristics is thus investigated.

The height of the bottom layer is a random variable following a log-normally distribution [2,3]. A hundred samples are simulated and coupled with a deterministic formulation based on the BEM and the TLM, in order to calculate the statistics of the tunnel displacement, the ground surface displacement and transfer function to seismic acceleration [2,3,5].

The present study shows the influence, of the randomness that affects the geological properties of the layered soil, on the tunnel displacement, the surface ground motion and the transfer function with the presence of a tunnel.

The maximum value of the mean tunnel displacement corresponds to the fundamental frequencies of the excitations in whole cases and the fundamental frequencies decrease for strong and moderate earthquakes.

The fundamental frequencies of the surface ground displacement and the corresponding amplitudes depend on the earthquake characteristics. The maximum amplitude is always located at the extremity of the tunnel.

We may conclude that the tunnel influences the location of the maximum displacement, that takes place at the extremity of the tunnel, but it does not have a significant influence on the frequency content.

The amplitude of the transfer function is attenuated by the interaction between the multilayered soil and the tunnel interaction, while this interaction increases the fundamental frequency. The maximum value of the transfer function is located at the tunnel extremity.

1

M. Badaoui, "Investigation analytique et numérique du phénomène d'interaction dynamique dans les systèmes sol multicouches - structures multiples", Master Thesis, Dept. Civ. Eng., 1998.

2

Badaoui M., Nour A., Slimani A. and Berrah M.K., "Stochastic Seismic Investigation of Heterogeneous Soil Profile having Uncertain Depth", in Proceedings of The Fourth International Conference on Engineering Computational Technology, B.H.V. Topping and C.A. Mota Soares, (Editors), Civil-Comp Press, Stirling, United Kingdom, paper 114, 2004. doi:10.4203/ccp.80.114

3

M. Badaoui and M.K. Berrah, "Influence of the soil profile uncertainty on the tunnel - layered soil interaction subjected to seismic acceleration", Proceedings of the Tenth International Conference on Civil, Structural and Environmental Engineering Computing (CC2005), B. H. V. Topping (Editor), Civil-Comp Press, Stirling, UK, Rome, Italy, paper 226, 2005. doi:10.4203/ccp.81.226

4

E. Kausel and R. Peek, "Dynamic Loads in the Interior of Layered Stratum: an explicit solution", Bull. Seism. Soc. Am., 72 (5), 1459-1481, 1982.

5

A.L. Pais, "Dynamic Coupling of Multiple Structures Through Soil", PhD. Thesis, MIT, Dept. of Civ. Eng., 1988.

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