Section 4-4
The characteristics of the Section 4-4 FEM  are shown in Figure 11. The soil is characterized by 4-node plane strain  elements and the maximum size of computation mesh elements is determined by the  following relation, in order to perform correctly the next dynamic analysis:
\(h\le h_{\max}\ =\ \frac{V_s}{\left(5\ -\ 8\right)f_{\max}}=6.83\ m\)
\(\)where fmax  is the maximum frequency of the seismic signals adopted in this study, equal to 15 Hz. The dimensions of the model have been fixed in order to avoid the  boundary effects and in this case have  been taken equal to three times the size of the structure (in horizontal and  vertical direction). The structure is represented, in the first approach, by linear elastic beams while  for the soil an elastic perfectly plastic model with Mohr-Coulomb strength  rule, characterized by the mechanical properties shown in Table E, was adopted.  The contact between soil and walls was modeled  by using elastic-perfectly plastic interface elements, with a friction angle  equal to 20° [10]. The values of the stiffness of the interface elements are  evaluated according to [6]:
\(K_n=\frac{E_{0ed,i}}{t_v}\)
\(K_t=\frac{G_i}{t_v}\)
where:
\(G_i=R\cdot G_{soil}\)
\(E_{0ed,i}=2G_i\frac{\left(1-\nu_i\right)}{\left(1-2\nu_i\right)}\)
and                    
Known the geometry and the values of the  static loads, the results are obtained in terms of bending moments acting on  the retaining walls (Figure 12). The maximum values of the bending moment  acting on the retaining walls obtained in static conditions is equal to 600  kNm.