Seite - 1540 - in Book of Full Papers - Symposium Hydro Engineering

5. TUNNELS Tunnels need to be designed and constructed in an efficient manner for the best performance. The Bureau of Indian Standards code IS: 4880-1976 “Code of practice for design of tunnels conveying water” (Parts 1 to 4)[1] provides guidelines for design of a tunnel under various situations. The following are the salient points. 5.1. TUNNEL LAYOUT The first aspect that needs to be decided for a tunnel is the alignment, that is, the route layout of the tunnel in plan. Figure 5 shows the possible alignment for the tunnel water conveyance system for a diversion of water through tunnel. The inlet structure geometry must be hydrodynamic, both in plan and in longitudinal profile to avoid flow separation. The flow separation may occur in the transition river section with irregular section for the tunnel section, usually circular or horseshoe sections The layout is usually governed by the geological features of the surrounding hills. Complicated geological conditions and extraordinary geological occurrences such as intra-thrust zones, very wide shear zones, geothermal zones of high temperature, cold/hot water springs, water charged rock masses, intrusions, fault planes, etc. should preferably be avoided. Sound, homogeneous isotropic and solid rock formations are the most suitable for tunnelling work. However, in the Himalayan region, such conditions are rather rare compared to the hills of peninsular India. This is because the Himalayan geological formations are mostly sedimentary in nature whereas the peninsular upland of the country is of igneous nature. Hence, geological investigations have to be carried out in detail before a tunnel alignment is finalized. 5.2. TUNNEL BODY The flow in a river diversion tunnel should occur in supercritical scheme to provide quick acceleration required by deploying of upstream structure. In 1540