## 4 Monitoring of the Technical Condition of Existing Buildings  
Monitoring of the technical condition of existing buildings is the most important element when controlling construction works and ensuring the safety of these objects [33; 34]. The scope of observation of the structures of existing buildings depends on their location and the distance from a new investment. It usually includes the geodetic measurements of horizontal and vertical displacements for walls in the direct vicinity of the executed excavation, building observation, and possibly the measuring of the width of existing cracks with feeler gauges. An exemplary instrument for measuring the width of cracks is shown below in Figure 5. Other less frequently used monitoring methods are strain gauge, which measures deformation using foil strain gauges, and piezometric which is installed to control groundwater levels. The frequency of measurements should be adapted to the course of the construction works and the current technical condition of the building. Conducting traditional geodetic measurements allows the results of displacements to be obtained in quite significant time intervals, which is often insufficient in relation to the significant dynamics and the speed of the phenomena occurring in the excavation. Therefore, systems that allow for the continuous measurement of displacements of existing buildings and the obtaining of measurement results on-line are becoming more and more popular. One of them is the innovative hydrostatic leveling cells system, which measures the vertical displacements of structures in real time with high accuracy using a network of connected sensors [35]. The system is characterized by high accuracy measurement and was designed to measure the vertical movement of structural elements. Its measuring range applies to  
Figure 5: An exemplary instrument used to measure the width of cracks (photograph by authors). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type of Building Structure** & **Limit** & **Limit** \\
**Type of Building Structure** & **Displacement (SLS)** & **Displacement (ULS)** \\  & **[s\({}_{\text{k}}\)]\({}_{\text{u}}\), mm** & **[s\({}_{\text{k}}\)]\({}_{\text{n}}\), mm** \\ \hline Masonry buildings with timber floor slabs or Klein’s floor slabs & 5–7 & 15–18 \\ Masonry buildings with rib and slab floors or RC floors, prefabricated building & 7–9 & 20–25 \\ Monolithic reinforced concrete buildings & 9–11 & 23–35 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Approximate values of the maximum limit displacements for buildings according to [5]. displacements of up to \(\pm\)500 mm. The mounted sensors create a map of the 2D measurement network, which can then monitor the vertical displacements of the structure over the entire surface. Contour line plots can be generated over time to directly detect which area should be analyzed. All the sensors of the system are connected to each other with the same liquid pipe and then connected to the liquid expansion tank, which is located above the entire measuring system. This method of system construction allows the appropriate pressure to be forced inside. The vertical displacements of individual sensors mounted on the structure cause pressure changes inside the system, which are measured in the place of the installed sensors. When the sensor moves vertically, the corresponding liquid height changes accordingly, independent of all the other sensors. As liquid pressure can be measured very accurately at each sensor, vertical movement can be calculated from the density of the used liquid. Each sensor is also connected to each other sensor by an air pipe in order to normalize the reference pressure between the sensors. When the structure settles down, the measuring cells settle with the structure, in turn increasing the pressure in the system. Therefore, a settlement measurement can be performed. Lifting of the structure is similarly detected, i.e., when the liquid pressure in the system is reduced.