3.7 Insertion of Rock bolts

•A Rock Bolt is a long Anchor bolt for stabilising rock excavations. A rock bolt transfers the load from unstable exterior to confined interior of rock mass.
•Two types of Rock Bolts are used in Rohtang Tunnel:

    1) Swellex Mn24

     As the name suggests, Swellex swells when water pressure is applied at its opening. It anchors large broken Rocks.

    2) Injection Bolt

    Injection bolt is a hollow steel tube of 35mm dia provided with holes

    of diameter 5mm throughout its length. When cement is grouted through

   Injection bolt, the grout is released from these holes are broken pieces of

   are bound together.

3.6 Secondary Shotcrete

•After installation of lattice girders, M40A10 grade concrete Reinforced with steel fibre is sprayed over the lattice girders to a thickness at which lattice girders are completely covered.

•Steel Fibre provide strength and flexibility to the shotcrete.

•The quantity of steel fibre is kept 35kg per m3 of concrete.

•Length of fibre = 35mm

•Diameter of fibre = 0.54mm

•Fibre count : 15500 fibre per kg.

Steel Fibres

 

3.5.1 Lattice Girder Installation

Pre-fabricated Lattice Girders

Installation of Lattice Girders

3.5 Lattice Girder Installation

• Lattice girders are the supporting elements in tunnels and they normally consists of steel bars laced together in a triangular pattern.
• They are made to suit the shape of the tunnel.
• The main purpose of lattice girders is to provide temporary support to the rock which may fall readily.
• The secondary purpose of lattice girders is to provide an indication if sufficient thickness of shotcrete is applied.
• They also help in resisting tunnel deformations during rock bursts.
• Lattice girders are not used in case of Rock Class 1, 2 and 3.

3.4 Installation of square wire mesh

     After primary shotcreting, Square wire mesh with 15 cm "mesh" is installed throughout the top face of the tunnel. The wire mesh acts as primary reinforcement to the tunnel lining. The diameter of steel bars in the mesh is 6mm.

Installation of Wire mesh

3.3.1 Shotcreting Video

3.3 Primary Shotcrete protection

•The primary stabilisation of the tunnel excavation is achieved by shotcrete.

•The shotcrete stabilises the excavated rock surface by filling the cracks produced in the rocks

•Primary shotcrete consists of M40A10 grade concrete. The shotcrete is sprayed throughout the excavation with the help of a computer operated machine called MYCO POTENZA.

•Along with shotcrete, an accelerator admixture is pumped so as to harden the shotcrete concrete immediately.

•If accelerator is not used, the shotcrete may not stick to the excavated surface.

•The accelerator commonly used is Sodium Silicate.

•The quantity of accelerator is kept 7% of the volume of shotcrete.

MYCO potenza for Shotcreting

3.2 De-fuming, Mucking and Scaling

• It takes about 30 minutes to expel the fumes out of the tunnel. All the workers and engineers are supposed to move out of the tunnel during this process.
• After De-fuming, mucking operation starts. Mucking is the operation of removing the blasted rock from the tunnel face.
• In Rohtang tunnel, a conveyor belt is used to convey the Muck out of the tunnel. The conveyor mechanism has been installed by ROWA Tunneling Logistics.
• The facilities are designed in such a manner, so that muck volume of 600 tons per hour can be effected.
• The removal of loose rock which has not been removed during blast is called as Scaling. Usually this operation is done using Hydraulic hammer.
  
   
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3.1 Drilling and blasting

•The face of the tunnel is made smooth with the help of hydraulic hammers.

•Holes of diameter 48mm to 51mm are drilled on the face of top heading or benching with the help of a machine called Boomer.

•The maximum length of the hole is kept 3m.

•The holes are filled with explosives and connected to a non electric detonator. The detonating mechanism is placed at least 500m distance from blast site.

•The detonator used for explosion is a non electric detonator and is commonly called as Shock Tube Initiator.

Boomer for hole drilling

Cortex tube for detonation purpose

Explosive

3. Tunneling Approach

The excavation of Rohtang Tunnel is done on the basis of NATM (New Austrian Tunnelling method). This method can be described as "Design as you monitor" approach, by providing support to the tunnel on the basis of Rock class. The NATM has following sequence:


1. Drilling and blasting.
2. De-fuming (it takes 30 minutes)
3. Mucking and scaling.
4.Primary shotcreting with M40 grade concrete.
5. Installation of square wire mesh throughout the tunnel.
6. Lattice girder installation.
7. Secondary shotcreting with M40 grade concrete reinforced with steel fibres.
8. Drilling holes for Rock bolts.
9. Inserting Rock bolts.
10. Cement grouting of Rock bolts.
11. Tunnel final lining with reinforced M35 grade concrete.


I'll explain each process in detail.