Engineering importance of thrusts

A thrust is a low-angle reverse fault in which an older rock is pushed over a younger rock. The importance of the HFT (MFT), MBT and MCT in civil engineering is:

1. Crushed, sheared and weak rock gives low bearing capacity, tunnel collapse, large overbreak and squeezing, so extra support is needed.
2. Slope instability: landslides and debris flows are concentrated in the thrust zones.
3. Seismic risk: the thrusts are active, so earthquake loading, surface rupture and ground shaking affect dams, bridges and buildings. Structures must not be placed across them.
4. Groundwater: thrust zones are conduits for water; heavy seepage, springs and inrush in tunnels.
5. Rock contrast: rocks of different strength and permeability meet at the thrust, giving differential settlement and leakage under dams.
6. Site investigation must locate the thrust accurately, with trenching and geophysical methods, before alignments of roads, tunnels or dams are fixed.