**2. Geotechnical modeling**

The combined use of geophysical surveys and geomechanical modeling can make it possible to solve complex geotechnical problems for which geognostic surveys alone prove insufficient [1]. As illustrated graphically in **Figure 1**, the proposed procedure is composed of different phases. These are (1) analysis, (2) measurements, (3) processing, (4) preliminary model, (5) in situ surveys, (6) model revision, and (7) back and sensitivity analysis [2]. The above phases are connected to each other in an iterative way as indicated in the graph, until a satisfactory level of coherence between processing results and direct observations is reached. Actually, there may be cases in which it is difficult to know with precision the complete geometry of the problem, the lithology, and the mechanical properties of natural or artificial materials, as they are not directly investigated. In the present cases, it is possible to carry out iterative evaluations in which the formulation of an initial geomechanical model suggests typology and modality of in situ investigations. In other terms, the initial model integrates and improves the interpretative capacity of the geomechanical model whose processing, in turn, must be compared with direct observations (back analysis or inverse problem). The latter observations could be the observed stability or the instability witnessed, for instance, by easily detectable break surfaces. The last phase of the procedure consists of the sensitivity analysis [3].

Thus, we arrive at the calculation of a safety factor in instability or dimensioning of a stabilization intervention or dimensioning of an additional work that modifies the initial situation without altering the original equilibrium or designing a future monitor system. The two real cases are discussed below.

The first one deals with an excavation in the historic city center for the construction of an underground car parking, in an area flanking a sixteenth century bastion of unknown construction features; the second case is a highway slope affected by rotational instability [4].

In both cases, it is necessary to have a definitive geomechanical model, based on which to design interventions whose effects must be foreseen, starting from initial uncertainties on geometrical and geomechanical aspects. In both cases, the procedure develops starting from direct observations and special in situ investigations. The latter allows to apply a back analysis procedure (or inverse solution) able to highlight critical aspects of the problem and/or to refine the forecast geomechanical model.

**Figure 1.** 

*Flowchart of an iterative analysis modeling—inverse solution.* 

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

In the case of the excavation for the construction of the underground car parking, the uncertainties relate to the geometry, composition, and geomechanical properties of the adjacent bastion, obviously apart from the direct observation of the current equilibrium conditions.

 In the case of the landslide road, the uncertainties regard the composition of the slope, the stratigraphy and geomechanical properties, and the interaction with a temporary groundwater activated by extraordinary meteoric contributions [5], apart from the direct observation of the landslide in progress.

In both cases, the in situ investigations cannot be exhaustive, due to the discontinuity of the acquired information and to uncertainties implicit in the methodology followed.

Although the nature of the two problems is not the same—feared instability in one case and already occurred instability in the other—they lend themselves to exemplifying the possibility of identifying solutions through the combined action between the geomechanical model and in situ surveys. Among the latter, geophysical-geognostic surveys are those that offer the possibility to acquire the most information possible, by extension and quality.
