What is joint; engineering significance of joint and fault

A joint is a fracture or crack in a rock along which there has been no (or negligible) displacement of the two sides.

Significance of joints

• Joints reduce the strength and bearing capacity of the rock mass; blocks bounded by joints can slide or fall.
• They control slope stability (plane, wedge and toppling failures) in cuts and dam abutments.
• They give paths for seepage, so reservoirs leak and tunnels get water inflow.
• Weathering is faster along joints.
• In tunnels they cause overbreak and roof falls; the tunnel axis should be at a high angle to the main joint set.
• They help in quarrying blocks, but closely spaced joints give only small blocks.
• Grouting, rock bolts and shotcrete are used for treatment.

Significance of faults

• 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 may settle unevenly, because the rock properties differ on the two sides.
• 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.