High Resolution 1:10,000 scale Mapping Strategy of Multi ... - NDMA

Water Table
Potential Failure
Plane S
σ
Figure App 5.1: Force diagram for thin to thick translational slides
The resisting force of earth materials, whether consolidated bedrock or
unconsolidated sediments, is the shear strength (S) of the materials (Figure App 5.1
and Figure App 5.2). Shear strength is a combination of forces, including the slope
normal component of gravity (Figure App 5.3 ) or normal stress (σ), pore pressure
(μ) within the material, which counteracts the normal stress, cohesion of the material
(C), and the angle of internal friction (φ). Shear strength is given by the Coulomb
Equation:
S = C + (σ - μ )tanφ (Eq 1)
Normal stress is the vertical component of gravity, resisting downslope movement
(Eq 2).
σ = γzcosβcosβ (Eq 2)
G
The role of water is especially critical in slope stability, but it is incorrect to think of its
role as that of lubrication. Water plays a dual role. In increasing the unit weight of
material, it increases both the resisting (normal stress) and driving (shear stress)
forces. It also creates pore pressure, which opposes the normal stress and therefore
reduces the resisting force or shear strength of the material (it is subtracted from
µ
τ
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