b. Inner lining

The outer lining used in the tunnel is made to ensure the stability of the tunnel after the tunnel excavation.

Shotcrete: Shotcrete is used in tunnels as a basic carrier element. Shotcrete applied immediately after the excavation not only prevents the ground from loosening by preventing atmospheric effects but also has the feature of carrying the loads coming from the ground. Shotcrete application is divided into dry and wet in tunnels.

Steel rib: With the increase in the thickness of shotcrete in tunnels, it acts as reinforcement in the concrete and can resist the first deformations that will occur after excavation in the tunnel. Steel rib can be divided into I type, H type, lattice girder and TH type (sliding rib).

Rock bolt: The rock bolt is an integral part of the external support as a support system. It is a support system that is necessary both to prevent block slipping due to discontinuities in rocks and to connect the plastic zone to the elastic zone in weak rocks. Rock bolts are used passively in tunnels. It is divided into SN, PG, swellex and

**Figure 13.** *SN-type rock bolt.*

self-drilling bolt (IBO). Rock bolt diameters are generally used between 28 mm and 51 mm depending on the soil type.

In SN-type bolt applications, firstly the hole is drilled, then the borehole is filled with injection and finally, the bolt is placed into the hole (**Figure 13**).

In PG-type bolts, after the hole is drilled, the PG bolt is driven into the hole. In the last stage, the grout is pumped into the hole using the grout pipe (**Figure 14**).

Swellex-type bolts are used in tunnels opened in rocks with water ingress. After drilling the hole, the swellex bolt is placed. Afterwards, high pressure is applied from the bolt mouth, allowing the bolt to swell and hold onto the rock (**Figure 15**).

IBO (injection boring outside) is known as self-drilling bolts. In the application, the hole is drilled with the help of the bit attached to the bolt and the bolt remains in the borehole. In long bolts, bolts connected with coupling are added every 3 or 4 m and placed in the hole together with the drill (**Figure 16**). The injection process is completed by filling the entire drilling through the IBO bolts.

Forepoling/umbrella: Forepolings are not basically considered as a carrier element. The main purpose of the forepoling is to provide the stability of the ceiling in order to prevent slips that may occur during the excavation (**Figure 17**). Forepoling diameters and types vary depending on the ground.

Inner Lining: The inner lining concrete is made after the completion of the outer supports of the tunnel in order to give the tunnel its final form and to provide an

*Swellexs type rock bolt.*

architectural view. In general, inner-lining concrete is not considered as a load-bearing element. However, in weak or very weak rock conditions, it has recently been designed as a carrier element. The inner lining concrete is a necessary structure for the placement of drainage pipes required for tunnel drainage, as well as for the placement of ventilation fans and electro-mechanical devices to be used in the tunnel (**Figure 18**).
