Define fault with diagram and its importance

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 range from a few millimetres to hundreds of kilometres.

                 fault plane
                  \
   FOOT WALL       \        HANGING WALL
  ===== bed A ======\
                     \
  ----- bed B --------\ ===== bed A =====
                       \
                        \ ----- bed B -----
                         \
  The hanging wall (above the fault plane) has moved down: a normal fault.
  Throw = vertical offset of a bed; heave = horizontal offset.

Parts: fault plane, hanging wall (the block above the plane), foot wall (the block below the plane), dip, throw and heave.

Importance in civil engineering

• 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.