## 3 The Geomechanical Monitoring System and the Numerous Available Geotechnical Measures  
For many years, the Cava Madre has been equipped with a detailed monitoring system. The variety of instruments and the complexity of the system are due to the singularity of the excavation (large dimensions), as well as the historical-architectural importance of the extracted material and the site. The instrumentation for geomechanical monitoring was installed and integrated on several occasions, both to follow the development of the excavations and to take into account the installed supports; the purpose of the monitoring measures is that of static control of the excavations and supports in relation to the work progress and of comparison with the assessments deriving from the continuous updating of the numerical modeling. The monitoring system is aimed at geomechanical control of the behavior of the excavation contour walls in terms of displacements, of the support systems like concrete beams in terms of stresses and strains, and of the rebars and tendons in terms of reaction forces. The scope of the monitoring system is also the periodic update of the geomechanical model of the cavern zone and its correlation with the advancement of the excavation works. To date, most of the instrumentation is managed with periodic reading through the use of data acquirers, these being electric vibrating wires or strain gauge transducers (Figure 5). Some measurements are performed using topographical methods. Finally, weather conditions are continuously recorded, more specifically the air temperature. The instrumentation currently in operation consists of numerous types of transducers, briefly summarized below:  
* topographic targets
* strain gauge bases on old concrete strut-beams  
Figure 4: (**a**) Image of an instrumented rock specimen tested in the laboratory: the specimen appears as white marble, pink and gray finely veined; (**b**) a digital core obtained by the analysis of a borehole wall, with traces of crossing joints (on the right). * strain gauge bases on metal struts located in the inner part of the cavern
* jacks for measuring stresses in old concrete strut-beams
* strain gauge bases in the new concrete strut-beams
* pressure cells in the new concrete strut-beams
* annular load cells placed below the distribution head of the stranded tensors
* straight wire pendulums  
To estimate the stress state in the wall of the cavern and at its foot, a lot of measurements were available (17 flat jacks were used, 11 of which were left in place to evaluate the stress variations over time). The results of the investigations highlighted a predominantly compressive state of stress everywhere in the rock mass around the cavern, despite the shape of the cavern elongating in the vertical direction and an ascertained high level of horizontal in situ stresses. Furthermore, two tests were carried out on the concrete strut beams present at the time in the entrance area, which were subsequently demolished. The stress state in the deeper zones of the rock was evaluated with borehole slotter stressmeter tests. A tendency was found for the k\({}_{0}\) parameter (the ratio of the in situ horizontal stress over the vertical one, i.e., the coefficient of lateral thrust at rest) to be greater than unity; it means that the horizontal lithostatic stress is greater than the vertical one. Of particular interest are the measurements of displacement of the lateral walls and, in particular, of convergence, i.e., the distance between two topographic targets placed on opposite walls of the cave. The convergence is able to control the deformation of the cavity in relation to the progress of the excavation operations and the entry into the operation of the support works. Convergence is able to show a possible evolutionary trend of displacements once excavations are completed or suspended, and the extent of deformations connected to very specific excavation activities.