3.8 Tunnel Final Lining

•The final lining of the tunnel is made of M35A20 grade Reinforced Concrete with addition of polypropylene fibre (C3H6)n.
•Experiments revealed that Polypropylene fibre reinforced concrete maintains its compressive strength at high temperatures also. The PP fibre reinforced concrete thus will not fail during fire outbreak inside the tunnel.
•The quantity of PP fibre is maintained 3kg per m3 of concrete.

Polypropylene Fibre

3.7.1 Swellex Mn24

Swellex Mn24

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