**3.2 First case study: the ancient walls of the city of Cagliari (Sardinia, Italy)**

 The study area lies in the northwestern sector of the ancient walls of Cagliari. Apart from the Roman works, of which few traces remain, the original body of the walls, built by the Pisans (Republic of Pisa), dates back to the thirteenth century; in literature, these walls are also referred to as "the Pisan walls" or "the medieval walls." Three centuries later—that is, in in the sixteenth century, after the advent of firearms, in particular the artillery—the Spanish, who at that time dominated Cagliari, decided to modify the defense line incorporating the medieval walls in an embankment coated with limestone blocks; the latter, apart from few modifications and consolidation works, is the current structure of the walls. **Figure 5** shows in detail (the circle) the area in which two up-hole tomographies have been carried out

**Figure 5.**  *The northwestern sector of the Spanish walls. The circle indicates the survey area.* 

**Figure 6.**  *Position of the two acquisition arrays.* 

 to provide information for the geotechnical modeling of the walls, in view of the construction of an underground car parking, right at the foot of the walls itself.

As for data acquisition geometry, the geophones were placed on the outer surface of the coating at intervals of 1.6 m, and the shots were performed in their respective holes at 1–2 m spacing, starting from the bottom.

 Data processing has been performed by means of a software based on a nonlinear optimization technique (ASA, adaptive simulated annealing) [11] that works in terms of modeling, starting from the set of the first arrival times and the spread geometry. **Figure 6** shows the position of the two acquisition arrays, and **Figure 7**  shows their schematic view. The results after processing and interpretation are in

**Figure 7.**  *Schematic view of the up-hole seismic tomographies across the ancient walls.* 

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

**Figure 8.**  *Up-hole tomography UH1 interpreted.* 

**Figure 9.**  *Up-hole tomography UH2 interpreted.* 

**Figures 8** and **9**. The two tomographies show very similar characteristics, and the ancient medieval wall, the limestone basement, and the filling on both sides of the medieval wall are clearly depicted. On the contrary, the tomographies show no trace of the external coating of the current walls, though its presence is certain. This is a clear sign that thickness of the coating is relatively small and therefore has not been covered satisfactorily by the seismic rays, as deductible from **Figure 7**.

From the two up-hole tomographies and several drillings, the base model for the geotechnical assessment has been compiled as shown in **Figure 10**.

## **3.3 Second case study: a road embankment along a hillside**

 **Figure 11** shows a satellite view of a road that runs along the side of a hill in Sardinia, Italy. A few years from the construction of the road, after an exceptionally rainy season, progressively pronounced fracture lines, indicated in the figure, appeared on the asphalt. It was the beginning of a landslide that affected the background and the road itself. **Figure 12** shows a detail of a fracture. After 1 month of monitoring, since fracturing—and therefore the landslide—progressed, it was decided to verify the conditions of the subsoil through geophysical techniques. Then, two seismic refraction tomographies perpendicular to the axis of the road, as shown in **Figure 13**, were executed. Data acquisition was carried out with a 48-geophone spread, detector interval of 2.7 m, and same interval for shots (hand hammer with vertical stacking); the processing was performed with the same software employed in the previous case study. The position of the two seismic lines is in **Figure 13**, and the two P-wave velocity sections are in **Figures 14** and **15**.

 Tomography SRT1 (**Figure 14**) exhibits a surface zone 5–14 m thick that extends along the whole section. It is composed of the natural unconsolidated overburden and the artificial body of the road embankment, with P-wave velocity in the range 400–800 m/s. The underlying marls are initially much altered for a thickness in the range 2–10 m. Worth of notice is that just under the road, there is a 15–20 m wide zone that deepens for about 20 m, in which, after the demolition of the embankment, water was found. At the base, at depths of 15–20 m from the surface, except

#### **Figure 10.**

*Geotechnical model deduced from the two up-hole seismic tomographies and four geognostic boreholes indicated in the figure.* 

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

#### **Figure 11.**

*The stretch of road affected by the landslide phenomenon and the elevation profile.* 

#### **Figure 12.**

*Detail of a fracture in the asphalt at the beginning of the landslide phenomena.* 

**Figure 13.**  *Position of the two seismic refraction tomographies.* 

**Figure 14.**  *Seismic refraction tomography SRT 1 interpreted.* 

**Figure 15.** 

*Seismic refraction tomography SRT 2 interpreted. The SRT 2 is parallel to SRT 1, and the scale indicates the alignment.* 

in the abovementioned area below the road, there are integer marls. At least three faults can be identified.

The seismic tomography SRT 2 (**Figure 15**) exhibits features similar to those of SRT 1. In this case at least two faults can be identified. The two tomographies will constitute the basis for the road slope stability study.
