**8. Tunnel excavation systems**

Circular tunnel cross-section is always the most accurate method for tunnel stability and stress distribution (**Figure 19**). However, in most cases, this is not possible. Horseshoe section is often applied in mine galleries and water tunnels (**Figure 20**).

As a tunnel excavation method, it is preferred that it is completed as a full section, and the stress distributions are created once, in terms of tunnel stability. However, in large-diameter tunnels, the excavation process is carried out gradually. While tunnel excavation is divided into top heading and bench in solid rock, invert excavation is also carried out in weak rocks (**Figure 21**).

In solid rocks, the distance between the top heading and the bench can be longer due to the low deformations. This distance can be up to 70–100 m in solid rocks. On weak grounds, the top heading, bench and invert distance should be minimum to complete the ring. The distance between the top heading and the bench is between 15 and 25 m in weak ground conditions. Temporary invert is performed to prevent

#### **Figure 18.**

*Tunnel inner lining section (a) tunnel section (b) application in site.*

deformations that may occur in the top heading in very weak soils. Thus, deformations are limited by closing the ring in the top heading.

In addition, the ADCEO-RS (The Analysis of Controlled Deformation in Rocks and Soils) method also suggests a near-full cross-section excavation in both rocks and weak soils [32, 33]. In this method, the tunnel face and ceiling section are reinforced with fibre bolts, and the tunnel excavation can be done as a full section (**Figure 22**). Thus, the tunnel is excavated close to the circular, and the ring is closed immediately.

Tunnel excavation in long tunnels (L > 5 km) can be conducted using a Tunnel Boring Machine (TBM). It is generally preferred in metro tunnels and water tunnels (**Figure 23**). The TBM machine carries out the excavation with mechanical excavation and segments or supports are placed right behind it.

The type and selection of TBM completely depend on the type of soil or rock. While open TBM is preferred for solid rocks, earth pressure balanced (EPB) type TBMs are preferred for very weak rocks. TBM types are summarised in **Table 8** [34].

**Figure 20.** *Horseshoe tunnel section.*

**Figure 19.**
