Keywords: pressure, dynamic, discrete element method, earthquake, soil-structure interaction, soil.

A model based on a discrete element method (DEM), was developed to evaluate dynamic earth pressures acting on buried structures. This is particularly relevant in the context of building basement design, where it is proposed that the present recommendations overestimate the dynamic soil pressures induced by earthquakes.

Seismic soil pressures acting on buried structures have been calculated for many years using the Mononobe-Okabe approach. As the assumptions involved in this analysis do not apply to the case of buried structures, because wall displacements or deformations are not large enough to produce a fully plastic stress condition in the soil, an interest in modelling more accurately the phenomena has moved researchers and committees to develop other analysis schemes. Despite the effort of Veletsos & Younan [1], Ortigosa & Musante [2] and, Ostadan & White [4], the dynamic behaviour of the soil-wall system in buried structures is still not well understood. The method consists of a bidimensional formulation based on the plane strain assumption. The constitutive laws of the modelled materials match the dynamic behaviour; the results shown in this paper were calculated using equivalent linear elastic dynamic properties for the soil, assuming that for the motions considered, the strains were not large enough to induce considerable plastic behaviour.

The proposed model considers the buried structure embedded within the soil, so that it is not fixed at any point. Therefore, when an earthquake excites the soil, the structure moves with the soil mass, this difference from all other investigations related is significant, because the expected relative displacement between the wall and the soil, which produces the soil pressure, are expected to be closer to the real behaviour of the soil-wall system. The input motions were applied at the rock level, by assuming them to be the same as the ones recorded at stations on the rock. The response at the surface of the model is then recorded and compared with the motions measured on the same soil type, according to Chilean seismic standard "NCh 433", finding them to be equivalent if one compares peak ground accelerations, velocities, displacements or elastic spectra.

The results confirm the hypothesis that dynamic soil pressures produced by earthquakes are overestimated by commonly used design methodologies, as Wood [3] or in the Chilean case seismic standard "NCh 433". The results show that the dynamic earth pressures are not directly dependant on the depth of the buried structure; in fact, they appear to be reasonably constant, particularly for deep buried structures. The results also show that the soil unit weight has very little importance on the calculated pressures, and that the dynamic elasticity modulus is a much more relevant parameter. The effect of the superstructure or the degradation of soil properties have not been taken into account.

1

Veletsos A.S. and Younan A.H. (1994) Dynamic Soil pressures on Rigid Vertical Walls. Earthquake Engineering and Structural Dynamics, Vol. 23, 275-301. doi:10.1002/eqe.4290230606

2

Ortigosa P. and Musante H. (1991) Seismic Earth Pressures Against Structures with Restrained Displacements. Proceedings of the 2nd International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, 621-628.

3

Wood J.H. (1973) Earthquake Induced Soil Pressures on Structures. Ph.D Thesis, California Institute of Technology, Pasadena, California.

4

Ostadan, F. and White, W. (1998) Lateral Seismic Soil Pressure an Updated Approach. US-Japan SSI Workshop. September 22-23, San Francisco, California, EE.UU.

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