## 6 Intervention
### Structural Intervention: Seismic Retrofitting  
The geometry of the FE model used in the structural safety assessment of the current condition was updated to incorporate the effects of different retrofitting solutions, namely: (i) masonry injection, considering an upper and lower bound estimation of the improved masonry material properties; (ii) crack repair; and (iii) construction of buttresses at the rear wall (RW). Modelling of the Intervention  
The results of the analyses on the existing structure indicated that the foundation was shallower than what was initially assumed. Hence, the model was updated to increase the  
Figure 26: Design of geological intervention at the rock blocks at the southern end of the castle, similar to the ones carried out at the location below the tower: (**top**) location of unstable rocks, where different colours indicate different material properties, such as masonry, soil and rocks (**left**), and proposed anchors (**right**); (**bottom**) comparison of contact status under the rock blocks before and after the introduction of anchors at the southern end of the castle. Red areas within the annotated contact surfaces indicate the presence of failure areas. height of the foundation level to match the uppermost conglomerate rock level that was observed on site. In the first set of analyses, the effect of masonry consolidation and grout injections was evaluated by improving the properties of the masonry. Two scenarios were considered: (1) lower-bound scenario, with a 30% increase in the Young's Modulus of low masonry and a 20% increase for all other masonry types; and (2) upper-bound scenario, with a 50% increase in the Young's Modulus of low masonry and a 40% increase for all other masonry types. Despite greatly improving the capacity, ductility and ultimate displacement of the structure, masonry injection alone was not sufficient to improve the structural performance under code-required loads. Even with the improvement in the masonry properties, the collapse of the rear wall was still the main failure mechanism. The next set of analyses incorporated the effect of repairing and stitching of the existing cracks. This strengthening technique involves partially reconstructing the masonry surrounding the cracks and grouting. The intervention was incorporated in the model by \"closing\" the cracks that were previously modelled i.e., adopting material properties that are 80% of the improved masonry properties adjacent to the crack location. In the cases where masonry of different quality was present on each side of the crack, the properties of the crack were based on the lower-quality masonry. Table 3 shows the nonlinear material properties of the different types of masonry considered in the numerical model of the retrofitted structure. The nonlinear material properties for the masonry were computed based on the updated values of Young's Modulus, using correlations described in Section 4.2.2. For the strengthening of the rear wall, which is the most vulnerable element of the tower, the addition of buttresses was considered to be the most suitable approach to improve the performance of the wall. Two different geometries were considered (Figure 27): (A) two buttresses with base dimensions of 1 m \(\times\) 1 m; (B) two buttresses with base dimensions of 1.2 m \(\times\) 2.6 m and 1.2 m \(\times\) 2.15 m. The height of the buttress located closer to the NW2 is 5.6 m and the height of the buttress closer to the free end is 4.5 m. The material properties adopted for the buttresses were assumed to match the lower-bound estimate of the material properties of the retrofitted RW. The results of the comparative analyses concluded that the type A buttresses, which are smaller, were adequate to prevent the rear wall from collapsing. The addition of the buttresses also results in ductile failure modes for most of the loading directions and the structure was able to withstand much larger ultimate displacements.