Skip to main content

Fault, parts, recognition, engineering problems

Short answer

A fault is a fracture with relative displacement. Its parts: fault plane, hanging wall, foot wall, dip, throw, heave, net slip, fault line. Recognized by offset beds, slickensides, breccia, drag folds, scarps, linear valleys, offset streams, springs, and rock changes. Problems: weak zones cause settlement/failure, leakage, tunnel collapse/inflow, slope instability, differential settlement, seismic hazard, and unreliable exploration.

Define fault and illustrate the different parts of fault. How is it recognised in the field? What engineering problems are created by the presence 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 range from a few millimetres to hundreds of kilometres.

Parts of a fault

  • Fault plane - the fracture surface along which movement occurs.
  • Hanging wall - the block lying above the fault plane.
  • Foot wall - the block lying below the fault plane.
  • Dip of fault - the angle between the fault plane and the horizontal.
  • Hade - the angle between the fault plane and the vertical (90 degrees minus the dip).
  • Throw - vertical displacement; heave - horizontal displacement.
  • Net slip - the actual relative displacement along the fault plane.
  • Fault line (trace) - the intersection of the fault plane with the ground surface.
  hanging wall
   ________    fault plane
  |        |  /
  |        | /  <- dip
  |________|/_________
            /   foot wall
   throw (vertical), heave (horizontal)

Recognition in the field

Direct evidence

  • Displacement or offset of beds, dykes, veins, streams, roads or ridges.
  • Repetition or omission (missing) of beds in a sequence.
  • Slickensides (polished, grooved surfaces) and striations on the fault plane showing the direction of movement.
  • Fault breccia, fault gouge or mylonite along the fault zone.
  • Drag folds (bending of beds near the fault plane).

Physiographic (topographic) evidence

  • Fault scarps (straight, steep cliffs) and triangular facets.
  • Linear valleys, straight stream courses and sudden changes in stream direction.
  • Offset or beheaded streams, and waterfalls and rapids along a straight line.
  • A line of springs and seepages, and a sharp line of vegetation change.
  • Alignment of sag ponds, lakes and hot springs.
  • An abrupt change of rock type across a straight line, and earthquake epicentres in a line.

Engineering 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 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.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Engineering Geology I chapter 5 questions →