Geological problems in tunnel construction

Tunnel construction in the Himalaya meets many geological problems, which depend on the rock mass, structure, water and stress.

1. Rock fall and roof collapse (overbreak): in highly jointed, blocky or weak rock, wedges fall from the crown; worst when joints dip steeply and the tunnel axis is parallel to strike.
2. Squeezing ground: weak rocks (phyllite, schist, shale, clay-rich shear zones) under high overburden deform slowly into the opening, closing it; needs yielding support.
3. Swelling ground: clay minerals (montmorillonite) and anhydrite swell with water and damage lining.
4. Rock burst and spalling: in brittle, hard rock (granite, gneiss, quartzite) under high stress, the rock breaks violently; common under high overburden in the Higher Himalaya.
5. Water inflow and flooding: fault zones, karst cavities and fractured rock give heavy inflow and high pressure; wash out infill, causing flow of sand and collapse.
6. Faults, shear zones and thrusts: crushed rock with low strength; collapses and big water flow (MBT, MCT).
7. Running and flowing ground: loose sand or saturated silt flows into the face.
8. Karst cavities in limestone and dolomite.
9. Gas and high temperature: methane, hydrogen sulphide, hot water.
10. Portal instability: landslides and rock fall at the entry.
11. Abrasive rock (quartzite) wears cutters in TBM.

Remedies: pre-investigation, probe drilling, forepoling, pipe-roof, grouting, systematic rock bolts and shotcrete, steel ribs, drainage, yielding support and a controlled advance rate.