Engineering significance of fault

A fault is a fracture in the rock mass along which the two blocks have moved relative to each other, parallel to the fracture surface. The displacement may be a few millimetres to hundreds of kilometres.

Engineering significance (effects and problems)

• Weak zone: crushed, sheared rock (breccia, gouge) has low strength, high compressibility and high permeability, so foundations may settle or fail.
• Leakage: a fault zone acts as a channel for water, causing seepage under dams and reservoir leakage.
• Tunnelling: sudden inflow of water, roof collapse, squeezing and heavy support requirements when a tunnel crosses a fault.
• Slope instability: a fault plane dipping out of the slope gives a ready sliding surface for landslides and rock slides.
• Differential settlement: a structure built across a fault, with different rock properties on the two sides, can settle unequally.
• Seismic hazard: active faults can move again and cause earthquakes and surface rupture, so dams, bridges and buildings must be kept away from them.
• Uncertainty in exploration: displaced or missing beds make the correlation of boreholes and the estimation of rock depth unreliable.

Precautions

• Avoid building dams, big bridges and tunnels across active faults, or provide a special design for the expected displacement.
• Treat the fault zone with excavation, concrete plugs, grouting and drainage.
• Map faults carefully by surface mapping, trenching, drilling and geophysical survey before design.