Explanation

• The size of the tunnel is determined by its utility.
• For irrigation purposes, the tunnel is generally designed to run full & if the lining is of concrete, the velocity is taken as 366 cm/sec.
• In the case of road tunnels, it will depend on no. of traffic lanes & in the case of railway tunnels, it will depend on the no. of lines & type of gauge.

The shape of the tunnel is determined by the material of which the cross-section is built & the material through which the tunnel is bored.

• If the ground is made up of solid rock, then the tunnel can be given any shape.
• Tunnels in rocky terrains are generally designed with a semicircular arch with vertical sidewalls.
• In the case of soft ground such as that consisting of soft clay or sand, the pressure from the sides and the top must be resisted. A circular tunnel is generally best suited for resisting both internal and external forces regardless of the purpose for which the tunnel is used.
• To get the required shape of the tunnel section, we use Trimmers.
• Trimmers are the shot holes drilled around the periphery of a shaft or tunnel that break or trim the sides of the excavation to the shape and size required.

Commonly adopted for tunneling and the purposes these tunnels

So from the above options, Option1 is correct (Polycentric)