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Benk Lon from Comblnecl Cau... . e cracks fill with water then the lateral water pressure which is created will further increase the likelihood of failure. In cases where the potential tension crack depth is large in proportion to the total depth of the slope (>30%), the failure may be more appropriately analyzed as a planar-type failure. Where the outside of an eroding meander bend lies at the edge of the river valley, the possibility exists thai further erosion could bigger a massive landslide stretching up the valley slope (Figure 3-1-f). This type of failure is relatively rare but the effects are severe enough that if such a failure is thought likely, advice should be sought immediately from an experienced geotechnical engineer. Composite banks consist of a layer of fine silty sand or sandy silt overlaying alluvial deposits of coarse non-cohesive soil. Failures of composite bank occur primarily because the lower bank portion is more frequently exposed to flowing water and, since this comprises more erodible material, the results is that the upper bank will be undercut A cantilever overhang fonns. and three separate modes of subsequent failure have been noted, namely: I) Tension failure (Figure 3-I-g) 2) Shear failure 3) Beam-type failure (Figure 3-I-h) 3.3.1 Geotechnical Parameters . In the majority of the failure modes discussed above. it is the resistance of the bank material to shear which determines whether or not failure actuaIly occurs. For composite river banks, tensile effects within the soil may determine the mode of failure. Pore water pressure plays an important role in detennining the strength of a soil. Any increase in pore water pressure within the voids reduces the grain to grain contact stresses. the effective stress, and hence the ability of tbe material to resist defonnation. Conversely, negative pore water pressures arising from a partial drying out of die soil increases the contact stress, and hence the shear strength. In some circumstances in fully saturated soils, the pore water pressure may increase under conditions of steady or transient seepage so that the effective stress is reduced to zero with a complete loss of shearing resistance. Where this occurs under transient conditions of shock loadings such as by the wave level motion induced by boats. or an earth tremor, it is tenned liquefaction. Complete bank failure usually results. 3.4 Bank Loss from Combined Causes - Loss of bank material is often caused by a combination of hydraulic and geotechnical causes. Classic examples include erosion of composite banks and degradation. Literally hundreds of scenarios develop which involve these forces worldng in conjunction to fail bank lines. In that section a few of the more common types are presented. Many of the references cited present more de1ailed discussion of this type of failure. e Colorado Erosion COntrol Manual 21