Weight of the Soil in the Failure Block

The weight of the soil in the failure block is an instrumental parameter in both the driving and resisting forces. The gravitational force on the mass of the bank "inside" of the failure plane is the primary driver of bank failure. However, the component of this weight normal to the failure plane also increases the frictional resistance to failure.

W_i sinβ = The component of the weight down the failure plane, driving the soil into the water.
W_i cosβ tanΦ^'_i = The frictional resistance of the soil along the failure plane, where:
W_i cosβ = component of the weight normal to the failure plain
Φ^'_i = friction angle (which can be measured in the laboratory with triaxial testing or in situ with borehole shear equipment).

Cohesion

Cohesion is the inter-particle attraction in a soil matrix. For very fine soils (generally less than 0.0625 mm), particularly those composed of clay minerals, the electrochemical forces between particles can be stronger than the frictional forces. These electrochemical binding forces resist failure in cohesive soils such that:

c^'_iL_i =The effective cohesion per unit length (c^'_i) acting along the length of the failure plane in a soil layer L_i. (Note: cohesion is actually a shear strength that acts over an area, but L_i becomes an area when it is projected along the streamwise or longitudinal direction).