**9. Geotechnical measurements**

Studies carried out during the design phase can never give a definitive result. Because the research studies for tunnel design are limited and the acceptances to be made require that tunnel projects always be controlled with the measurements made during the tunnel excavation. This is because not all the works carried out during the tunnel design phase

**Figure 21.** *Tunnel excavation section (a) longitudinal section (b) front view.*

**Figure 22.** *ADECO-RS full-face excavation method.*

#### **Figure 23.** *Hard rock TBM section for water tunnels.*


#### **Table 8.**

*Most common TBM with or without face support [34].*

reflect the tunnel behaviour exactly. Every tunnel project requires revision during tunnel excavation. For this purpose, geotechnical measurements made during the tunnel excavation are of great importance and it is an indispensable condition of tunnelling.

A correct programming of measurements during the tunnel excavation will both prevent uncertainties in the tunnel project and contribute to the formation of a safer and more economical outcome. The geotechnical measurements to be made in the tunnel are:


#### **9.1 Deformation measurements**

During the tunnel excavation, deformation measurements should be carried out every 10 or 20 m for solid rocks and every 2 m for weak rocks. Deformation measurements are made by electronically reading targets placed along the tunnel section with a theodolite. These measurements provide extremely important information regarding tunnel behaviour. According to the results of the measurements, the deformations in the tunnel can be determined and the support systems and the excavation sequence can be revised. The measurement frequency can be adjusted on-site according to the deformation rate occurring in the tunnel. If the measurements made during the first week after the excavation remain constant, they are continued weekly, while in cases where the deformation increases daily, two readings can be made daily (**Figure 24**).

#### **9.2 Convergence measurements**

During tunnel excavation, convergence measurements should be made with tape extensometers (**Figure 25**) to see the closures in the tunnel. In addition to deformation measurements, convergence measurements are also made to determine on which side and how much the closure is.

#### **9.3 Extensometer measurements**

Extensometer measurements are made in the tunnel in order to see the plastic zone thickness around the tunnel (**Figure 26**). They play an important role in determining the length of the bolts by detecting the loosened zone thickness around the tunnel. In

**Figure 24.** *Deformation target points in circular tunnel.*

*Tunnels DOI: http://dx.doi.org/10.5772/intechopen.109608*

**Figure 25.** *Tape the extensometer section in the tunnel.*

addition, they give information about the magnitude of the loads that will come to the outer lining depending on the loosened zone thickness. Extensometer lengths are determined according to the tunnel diameter. Extensometers can be applied individually as well as in single, double or triple types. Measurements are started immediately after the placement of extensometers and are made daily or weekly depending on the frequency of movement.
