Hydro technical tunnels can be divided into two groups, non- pressure tunnel (spillway tunnel, diversion tunnel, tailrace tunnel, etc) in which the flow takes place with the free surface exposed to the atmosphere and pressure tunnel (headrace tunnel) in which flow takes place with the pressure. Generally, the pressure tunnel is designed for the circular section and non-circular and non-pressure tunnel of other shapes too (Flat arch roof tunnel with vertical wall, semi circular type roof tunnel with vertical wall, egg shape tunnel, horse shoe type tunnel). The minimum diameter of the tunnel is fixed with the consideration of the transportation, excavation, and hauling during the tunneling and should be greater than 2m in case of circular section and in case of other shapes should be greater than 1.9m in width and 2.1 meter in height.
The design of the tunnel depends upon the physical characteristics of the rocks through which are to be aligned. Among them the main characteristic is the rock strength- which is expressed as in hardness coefficient (f) for the different types of rock. Out of the total internal pressure, some is transferred to the surrounding rock and rest resisted by the lining itself. The thickness of lining depends upon the load to be shared by the lining. For the high strength rock and without its defective laying (faults, cracks) it may not be necessary to do lining but in order to decrease the friction usually concrete lining of 15-30 cm is preferred.
Generally, in case of the pressure tunnel, it is necessary to do the concrete lining such that it takes few part of the load to it. Principally the lining is done in two layers. Lining is constructed of the concrete of 25- 45 cam minimum thickness *depending upon the internal pressure and radius). External concrete layer takes the rock pressure and works in the compression condition. Thin layer of RCC is constructed, which takes the pressure of the flow and works in the tension condition. Internal layer could be also laid on the reinforcement mesh above the concrete lining with the shotcrete (mixture of cement and sand laid in high pressure). In case of very high pressure, the internal lining could also be done with the steel plate.
Beneath the tunnel generally drainage is to be provided for the release of the underground water pressure on the concrete lining as well as minimize the seepage flow through it.
During the concrete lining, it is practically not possible to place the concrete firmly on the upper portion of the tunnel. So, after the concreting, cement grouting is done by injecting the cement with very high pressure through the drill hole provided at the concrete lining. The cement lining will also help in filling the cracks if it exists on the surrounding rocks due to the different causes (blasting, natural fault etc).
Necessity of tunnel lining is when,
Internal water pressures are high i.e. above 100 m water head. In particularly sound rock formations, tunnels can remain unlined even when the pressures are as high as 200m of water. But if the pressure are greater than this lining becomes essential.
In the strata of low strength and where the rock is anisotropic.
Hydraulic resistance is designed to be low. Diversion tunnels for instance, are usually low pressure tunnels but they are invariably lined as the drainage capacities have to be high and therefore, the hydraulic resistance has to be low.