Keywords: buried structure, soil-structure-interaction, dynamic analysis, finite element method.

Buried structures such as tunnels, shelters, culverts and other underground infrastructure are important civil engineering structures that provide communities with necessary facilities. The buried structures are often built near or under engineering structures such as buildings and roadways. In this case the buried structure may experience excessive deformation due to the transfer of part of the surface load to the buried structure. For a proper design of buried structures to be conducted, the interaction between buried structures and surface loading needs to be investigated.

Surface loading could be static or dynamic. The influence of static loads on buried structures is well investigated. However, the effects of dynamic surface loading such as impact load have been the subject of relatively few studies. For example, Kiger et al. [1] investigated experimentally the behaviour of buried structures under dynamic loading. Their results demonstrated the importance of the interaction effects of shallow burial depths on the structure response. In addition, soil arching was found to affect considerably the pressure on the structure roof. Chen et al. [2] demonstrated numerically that soil arching is a significant feature of the behaviour of dynamically loaded buried structures although their study assumed full bond at the soil-structure interface. However, in fact the structure response to dynamic loading is significantly affected by the characteristics of the soil-structure interaction. The friction along the interface affects how the loads are transferred from the surrounding soil to the structure. A strong interface results in beneficial shear transfer effects (from structure to soil) and hence reduced structure deformations as shown by Ramirez [3]. Lysmer et al. [4] conducted a non-linear analysis using soil a cap model to study the effects of an aircraft impact load on underground concrete tunnels. The results showed that the depth of embedment has a considerable effect on the vertical response of the structure, with the structure response decreasing by increasing the burial depth of the structure.

In this paper the finite element method is used for the analysis of the interaction between an impact loaded reinforced concrete (RC) circular footing and a buried cylindrical RC structure. A two-dimensional axisymmetric elasto-plastic finite element modelling is developed, using a numerical code ABAQUS/Explicit. The soil is modelled as an elasto-plastic material using the Drucker-Prager Cap model. The influence of interface frictional effects at the soil-footing and soil-structure contact zones is investigated. The structure response during and immediately after the impact is investigated in terms of the pressure on, and the velocity of, the structure roof respectively. In addition the effect of the burial depth of the structure are studied

The results for the roof vertical displacement at the midspan show that whilst the maximum displacement obtained with the coefficient of friction of 0.0 and 0.5 is about 9.0 mm, the equivalent value given by the fully bonded mesh is only 2.34 mm. Thus the tangential slip at the interfaces is an important aspect of the overall problem, although a wide range of reasonable coefficients of friction gives effectively the same result. The results indicate that as the coefficient of friction decreases the structure displacements increase. To investigate the possible effects of significant soil plasticity on the impact response of the specified system, the results for the vertical displacement at the midspan obtained using the elastic properties and the cap model are compared. It is found that inclusion of soil plasticity has no significant difference on the overall behaviour of the problem. This is because of the specified geometry and boundary conditions, the soil deforms primarily due to volumetric compression with shear stresses generally low.

The results of the contact pressure on top of the structure roof indicates that the contact pressure is considerably higher at the supported edge than at the centre of the roof. This clearly demonstrates relief to the structure caused by the soil arching over the roof as it deflects. Investigation into the influence of burial depth shows that the contact pressure is significantly attenuated at deeper depths, which in reality may amplify the protective power of buried shelter structures.

1

S.A. Kiger, J.V. Getchell, T.R. Slawson, and D.W. Hyde "Vulnerability of Shallow-Buried Flat Roof Structures", Tech. Report SL-807, US Army Engineering Waterways Experiment Station, Vicksburg Miss. 1984.

2

H.L. Chen, S.P. Shah and L.M. Keer "Dynamic response of shallow-buried cylindrical structures", Journal of engineering mechanics, Vol.116, No.1 1990.doi:10.1061/(ASCE)0733-9399(1990)116:1(152)

3

J.A. Ramirez "Performance Based Seismic Evaluation of Underground Structures", Final Report No. 0000136, Purdue Research Foundation. 2004.

4

J. Lysmer, P. Arnold, M. Jakub and N.J. Krutzik "Dynamic Behaviour of Tunnels under Impact Loads", Journal of Nuclear Engineering and Design. Vol.85, Part1. PP55-69.1985. doi:10.1016/0029-5493(85)90272-9

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