## 6 Sustainable Strengthening and Seismic Improvement Techniques
### Structural Damage Restoration Techniques
#### 6.4.1 Non-Invasive Technique on the Example of the City Fortress Walls and Tower Motrilja  
When planning the rehabilitation techniques, the following conditions should be taken in consideration: that the concept of the building should not be changed during rehabilitation interventions, that the architectural appearance of the building remains authentic, and that the investment costs are as low as possible [38]. All these conditions are met, and it is possible to apply them in the rehabilitation of the walls of the fortress, dungeon, and Tower Motrilja because the levels of damage are almost identical and the construction material is the same (as well as the recommended rehabilitation techniques). The basic principle of this technique is that dilapidated structures, whenever possible, should be repaired rather than replaced with new ones. The most common and important structural problems that need to be solved in the rehabilitation of the city fortress and the buildings within its walls are due to ground subsidence or damage caused by collapse due to deterioration over time, the effects of natural disasters (earthquakes), or ancient war destruction. The actual condition of the walls of the city fortress was determined on the site. Although the walls look quite solid from the outside (Figure 18), the situation is quite different from the inside of the walls of the fortress (Figure 19): clearly expressed vertical cracks are visible, which should be filled and strengthened by the injection methods already mentioned (Section 6.1, Figures 16 and 17). The wall was repaired only from one side, which is unacceptable. This issue should be approached studiously and repair and strengthening of all walls, on both sides should be performed, as well as foundation consolidation. Before proceeding with the rehabilitation of cracked walls, it is necessary to assess the extent of mechanical damage in cracked structures, detect the presence of cavities and defects, assess moisture content and capillary growth, detect surface deterioration, and assess the mortar and the mechanical and physical properties of the stone. In addition, consolidation of the foundations should be carried out, which includes their rehabilitation by soil reinforcement, jet grouting, repair with expanding compounds, or insertion of micro-piles. This way, the tension in the soil will be reduced and the building will be weighted at the foundation level. The goals of the injection technique are to fill cracks, thereby increasing the continuity and strength of the wall, and to fill the gap between two or more layers of wall when they are poorly connected. Towers, as prominent parts of buildings, should be strengthened due to the danger of overturning or excessive tilting of walls and columns, with the consequent collapse of the roofs and floors they support. In the example of Tower Motrilja, methods for rehabilitation are presented, which include the technical measures provided for in the text above. The rehabilitation and renovation of Tower Motrilja included the measures proposed under level 2, Section 6.3. Using the regulations for eliminating the consequences of the earthquake, the practice and suggestions of the Institute for the Protection of Cultural Monuments, the proposed and implemented actions for its recovery are shown in the following text. In order to preserve the original features of the building and solve the construction of the tower, it was necessary to keep the shape of the mezzanines and replace worn-out material due to the introduction of a modern function into the building (Figure 20). Tower Motrilja has five floors (Basement + Ground +2 Floors +Attic), built in a massive structural system, with facade walls 1.30-1.50 m thick, made of stone, with lime plaster as filling. The foundation of Tower Motrilja is polygonal (octagonal), with a total width of about 8.30 m (Figure 21).