Environmental Engineering Reference
In-Depth Information
Figure 13.5
Discontinuities at all geological scales.
Plate 13.2
Erosion of escarpments, river valleys and gulleys
at all scales, controlled by regional fracture patterns in
Wyoming, USA. Major structures, several kilometres long,
are aligned left-right and diagonally top left-bottom right.
Photo: Ken Addison.
imminent rock mass failure, and its criteria are summarized in a
Mohr-Coulomb
equation:
where τ = shear stress,
c
= cohesion, σ = normal stress and = the angle of internal
friction. This is an extremely useful way of summarizing and quantifying forces
mobilized against the rock mass and three components of
shear strength
which resist it.
Cohesion
is provided by electrostatic and magnetic bonds between minerals,
intergranular cement and water. The first two are acquired during the various processes of
rock formation. Pore water in moderate quantities exerts a 'suction' force, through
surface tension, and its presence depends on rock porosity.
Normal stress
is the
anchoring weight of rock overlying a point in the mass and depends on rock density and
