Geology in tunnel site selection

A tunnel passes through the ground, so the rock mass is both the load and the structure. Site and alignment are selected on the basis of the following.

Geological factors

1. Rock type and strength: hard, massive, homogeneous rocks (granite, gneiss, quartzite, limestone) are best; avoid soft, weak or swelling rocks (shale, phyllite, clay).
2. Geological structure: the tunnel axis should cross the strike of beds at a large angle; avoid alignment parallel to the strike, folds and fault planes. Avoid faults, shear zones and thrusts (MBT, MCT).
3. Rock mass quality: RQD, RMR, Q values; joint spacing and orientation.
4. Groundwater: inflow, high pressure and water-bearing fractures or karst increase risk; select alignment above the water table when possible.
5. Overburden (rock cover): enough cover for stability but not too deep (squeezing, rock burst); low cover under rivers and valleys is a weak zone.
6. Portal sites: stable slopes with good rock, free from landslide, rock fall and flood.
7. Weathering and alteration at the portals and along the route.
8. Special hazards: gas, hot water, high temperature, rock burst, squeezing, karst cavities.
9. Seismicity and active faults.
10. Construction facilities: access, muck disposal and water supply.

Selection is based on mapping, geophysics (seismic refraction, resistivity), exploratory drill holes and geological sections along the alignment.