**4.1 Identification of the geotechnical problem**

The geotechnical problem consists of the need of predicting the effects of the excavation in the area adjacent to the bastion and to assess the risk of compromising the stability of the latter. The geognostic and geophysical investigations made it possible to define the internal composition of the bastion and the physical properties of the materials, but they do not allow identifying the actual dimensions of the external coating, that is, the containment wall, and leave uncertain the mechanical properties of

*Application of Seismic Tomography and Geotechnical Modeling for the Solution of Two Complex… DOI: http://dx.doi.org/10.5772/intechopen.81876* 

the materials, such as the cohesion and the internal friction angle of the Mohr Coulomb model. However, the geognostic and geophysical surveys have made it possible to reconstruct a geomechanical model, whose completion requires a sensitivity analysis of

#### **Figure 16.**

*Case 1: flowchart of iterative analysis modeling—inverse solution.* 

the results as the geometric and geomechanical characteristics of the external containment wall of the bastion vary. **Figure 16** shows the flowchart of the whole procedure.

## **4.2 Modeling and results**

Calculations were set based on reliable values of geotechnical properties (**Table 1**) called "hypothesis B" and four values (2.5, 2.0, 1.5, and 1.0 m) of the wall thickness. The modeling process allows the calculation of the safety factor (SF) in the three


#### **Table 1.**

*Geotechnical properties of the materials for the hypothesis B.* 

**Figure 17.**  *Left to right: pre-excavation, first excavation, and second excavation.* 

**Figure 18.** 

*Results for the hypothesis B: SF versus excavation steps and wall thickness (m).* 

possible excavation steps, that is, (1) without excavation, (2) excavation of the only loose material, and (3) excavation extended to a portion of the underlying rock (**Figure 17**). The results are shown in histograms of **Figure 18**.

The hypothesis B has been modified in A and C that differ for the cohesion and the tensile strength. The results obtained are illustrated in the diagrams of **Figure 19**.
