Engineering significance of joint and fault

Engineering significance of joints

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

• Joints reduce rock mass strength and bearing capacity, and the rock fails along them.
• They control slope stability: planar, wedge and toppling failures in cuttings and dam abutments.
• Joints give paths for seepage, so water leaks from reservoirs and flows into tunnels and excavations.
• Weathering is faster along joints.
• In tunnelling they produce overbreak, roof falls and wedge failures; the tunnel axis is best oriented across the joint strike.
• They make quarrying easier, but close jointing gives only small blocks.
• Grouting, rock bolts and shotcrete are needed in highly jointed rock.

Engineering 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, 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.