FLOOD06514 - Laserfiche WebLink

Home Browse Search

Mechenillms of SIr.embenk Feilur. e like draw down or seepage. protection against hydraulic erosion may not be the best treatment On the other hand. geotechnical failure may represent a delayed response to continuin l scour at the bank toe. in which case toe protection against hydraulic erosion is essentilJ. Other contributory causes of bank failure include boat-generated waves and turbulenc,:. ice and debris jams. and traffic of animals and vehicles. When geotechnical factors alone are involved. this usually results in mass failure of the embankment material. Several dfferent types of mass failure can occur in banks. These include sliding along a deep failm: surface. shallow slips. and lock failures (Thome. 1982). Different factors affectingnass failures are discussed in the following paragraphs. Soil Typt, - The type and scale of failures vary with soil type. Not all the factors identified above are relevant to a particular bank because of local differences in soil types. soL water regime. and the variable nature of many bank soils. Bank Slope Geometry. Two separate processes which may affect bank slope geometry are surface erosion and toe scour. Surface Water and Groundwater Flow Regime - Two processes of importance here are seepage and infillration. High pon' water pressure in the bank material. particularly after a rapid lowering of the water levd in the channel. will reduce the effective stresses in the bank material and this can triggl,r a deep-seated rotational failure. Such excess pressure or those from steady seepage can encourage surface erosion or toe scour. Banks composed of non-cohesive silty sands or sandy silts are most prone to piping due to steady seepage. Similar failures rarely occur in banks composed of gravel. or coarse- medium sand. because the lift forces will rarely exceed the submerged unit weight of the material. Cohesive soil can generally withstand bigher hydraulic gradients than noncohes lve soils and seepage- induced failures are less likely to occur than in silty material. Note that where fine-grained particles are removed by suffusion. the resulting coarse- grained material with its larger voids may become more susceptible to surface erosion. Rainwater or surface water drainage into the bank, particularly through cracks or fissures. (an cause an increase in the unit weight of the material. It also leads to higher pore pressure. bank strength may decrease and, combined with the increased weight, can lead to failure. SUIthargt: Loading - Any excess loading on top of the bank will increase its susceptib Jity to mass failure. Tension Cracking - Tension or desiccation cracks fonning in cohesive soils reduce the stability IIf the bank. particularly if they subsequently fill with water and/or undergo freeze-thaw cycles. The root nat provided by vegetation can modify the geotechnical properties of the soil. a and in ~rticular can improve its shear and tensile strength. Vegetation can therefore ,., 18 Colorado Erosion Control Manual