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<br />Mechanism. of SIr..mbank F.llura <br /> <br />FIGURE 3. 1 <br /> <br /> <br />Bet.", <br /> <br />~ <br />~~ <br /> <br />..... <br /> <br />a) Shallow failure <br /> <br />.Sballow bank angle <br />.Failure nearly parallel to &lope at~' <br />.UlU&lly in con-cohesive banks <br />.Vesctation will help to stabilize bank <br /> <br />Befo", <br /> <br /> <br />~ <br /> <br /> <br />~ <br /> <br />c) P\anar/llab failure <br /> <br />.Sleep or near vertical banks <br />OOoop _ion omcb <br />-uufe affected by groundwater <br /> <br />e <br /> <br />Types of mass failure of banks (Reprinted from Hemphill and Bramley 89) <br /> <br />1'~ ~ ~ ~~BT:.. <br />~ "--- " ~ ----~ <br /> <br />~ <br /> <br />Bet.", <br /> <br />~ <br /> <br />~ <br /> <br />Bet.", <br /> <br />Before <br /> <br />..... <br /> <br />b) Planar failure <br /> <br />.Steep or vcrtica11w angle <br />"'Frequently in DO[l-cohcsive banIa <br />-Water nonnally k ,wer that bank height <br /> <br />~~ <br />Wow - <br />Table -. <br />Bd... <br /> <br />~J <br /> <br />~ <br /> <br /> <br />c) Rotational failure with weak zone <br /> <br />o Muaive rotational r.i1ure <br /> <br />"'Very larse volume of slipped material <br />"'Erosion of bank threalenl stability <br />ofyallcys:ide <br />*Tension cnc:b up vaDey side <br /> <br />..... <br /> <br />"'Failure sudace dictated by weak zone <br />"'See also comments for type (d) <br /> <br />Cohesive Sand ........ Desiccation <br /> <br /> <br /> <br />~ <br /> <br />Before Before <br /> <br />d) Rotational faill" in hcmogencou. ..\ Failure of Composite bulk <br />ma&erial &I' <br /> <br />AIle, <br /> <br />,- <br /> <br />AIle, <br /> <br /> <br />.umany in cohClive material <br />"'Failun:: may extend beyond toe <br />"'Affected by gnnllldwater <br />"'On moderately t igh or steep banks <br /> <br />-OCCUR only where upper cohesive <br />t.yeroverlies erodible sand <br />-Failure by lenIior'l part of ovcthanging <br />b1ocl< <br /> <br />h) Failure of compoeite bank. <br /> <br />"Occurs as type (g) <br />-Failure can be by shear <br />-Failure with upper lOil in tension, <br />fonowed by rotation <br /> <br />bank malt,rial is relatively homogeneous), the failure surface may follow a circular arc <br />(Figure 3.1-<<1). In other cases. the shape of the surface may also include logarithmic <br />spimls or even planar sections (Figure 3-I-e). The Ialler is common where layers of <br />weak material dictate the actual shape of the failure surface. The volume of materials <br />involved in such failures is relatively large. AIthoogh the failure surface often passes <br />through VIe toe of the bank, this is nol always the case, and the possibility of failure <br />surfaces extending beyond the toe must not be overlooked. <br /> <br />Tension cracks parallel to the river bank are often observed on the surface prior to such <br />a failure. The eventual failure surface will nonnally follow such cracks, and their <br />position will indicate the approximate extent of any potential failure. If the tension <br /> <br />e <br /> <br />20 <br /> <br />COlorado Erosion COntrol Manual <br />