**9.4 Inclinometer measurements**

Inclinometer measurements are made from the surface to see possible movements in the portal sections or on the tunnel route (**Figure 27**). The inclinometer is selected according to the thickness of the land movement or possible loose zone in the drilled boreholes and generally goes down to the level of the tunnel. By means of inclinometers, possible movements in the portal section are determined and the necessary information is provided for the portal design, and it can be determined whether possible movements on the tunnel route affect the tunnel structure.

#### **9.5 Stress cell**

Stress cells are placed on the ground or concrete after excavation to see the stresses acting on the supports. In addition, stress cells are placed in the inner lining of concrete and the stresses that occur in the lining are determined. With the determination of the incoming loads, necessary revisions can be made in the shotcrete or inner lining concrete design/implementation. Because, when the assumptions made in tunnel lining calculations often differ with the site conditions, it is extremely important to determine the incoming loads on site. The lining thicknesses can be determined by making back analyses by means of the loads determined in the field. Stresses can be measured by placing the stress cell in both the radial and tangential directions (**Figure 28**).

#### **9.6 Strain gauge measurements**

Strain gauges are placed in shotcrete or interior lining concrete to see the strains in the lining and to detect the incoming stresses (**Figure 28**).

#### **9.7 Piezometric measurements**

Piezometric measurements can be carried out to measure the water pressure or the water level around the tunnel. These measurements are of great importance for inner lining design, especially in the long term.

#### **Figure 26.**

*Extensometer measurements in tunnel (a) installation section in tunnel, (b) in tunnel application and (c) triple type extensometer.*

### **9.8 Geotechnical face maps**

After the tunnel excavation, the geological units should be defined and perimetric maps should be made. The changes in geological conditions with these maps are extremely important in terms of evaluating the changes in geological conditions between those envisaged in the project and the geological conditions actually encountered. If there is a difference between the encountered geological conditions and the predicted geological conditions, the tunnel design may need to be revised according to the new conditions.

**Figure 27.** *Inclinometer typical section (a), measurements on site (b).*

**Figure 28.** *Stress cell and strain gauge application on site.*
