Structural control on geo-hazards

Structural control means that the orientation and nature of geological structures (bedding, foliation, joints, faults, folds) decide whether a slope will fail, and in what manner.

1. Dip of bedding/foliation versus slope:
   • Dip slope (dip direction same as the slope, dip less than the slope angle): bedding planes daylight, giving planar slides.
   • Dip into the slope (reverse): stable against sliding, but toppling is possible.
   • Dip across the slope: wedge or lateral movement.
2. Joint sets: two intersecting joint sets form a wedge whose line of intersection daylights in the slope face (wedge failure).
3. Faults, shear zones and thrusts: crushed, clay-rich weak zones act as slip surfaces and conduits for water; many large landslides lie along the MBT and MCT zones.
4. Folds: limbs dipping out of the slope are unstable; fold hinges have fractured rock.
5. Joint spacing and persistence: closely spaced, continuous joints lead to rock fall and toppling.
6. Weak interbeds (clay, mica schist, shale layers) form planes of sliding.
7. Discontinuity condition: smooth, clay-filled, wet joints have low friction.

Conditions for planar failure: the strike of the plane is within about 20 degrees of the slope face, and the dip of the plane is less than the slope angle but more than the friction angle. These are checked by stereographic projection (kinematic analysis).