6.2.4 Interface elements

The finite element method is based on continuum mechanics and is incapable of effectively evaluating the loading and displacement conditions induced by relative displacement between materials.Retaining walls used in excavation are stiff while the adjacent material，soil，is relatively soft.When the retaining wall deforms，relative displacement may be generated between the soil and the wall.To simulate the relative displacement between soil and structures during excavation，interface elements are sometimes used in analysis.

As shown in Figure 6.9，an interface element is an element connecting structures and soil，with or without thickness，which has a quite large normal stiffness but relatively small shear stiffness so that it can simulate the relative displacement between soil and structures.Concerning the studies of interface elements，readers can refer to relevant literature（Goodman et al.，1968；Pande and Sharma，1979；Sachdeva and Ramakrishnan，1981；Desai and Nagaraj，1988；Sharman and Desai，1992）.

Figure 6.9 Interface element

Though interface elements can rationally simulate the relative displacement between soil and structures，since extra parameters，which are not easily obtained from conventional soil tests，will be introduced，and numerical instability during analysis often occurs，they have to be used especially carefully.If interface elements are not to be adopted，the soil in the vicinity of the structure can be considered to divide into fine elements.When the retaining wall is deformed，these fine soil elements can easily attain the plastic state，which will then produce larger deformation.

This section only succinctly elucidates the basic principles of the finite element method.As for other inferring processes and important related theories，please refer to related publications.