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C.F. Wan, R. Fell, M.A. Foster
This paper presents the findings of experimental investigation of the rate of piping erosion of soils conducted at the University of New South Wales.
Two tests, namely the Slot Erosion Test and the Hole Erosion Test, have been developed to study the erosion characteristics of a soil. The erosion characteristics are described by the Erosion Rate Index, which indicates the rate of erosion due to fluid traction, and the Critical Shear Stress, which represents the minimum shear stress when erosion starts. Results of the two laboratory erosion tests are strongly correlated. Values of the Erosion Rate Index span from 0 to 6, indicating that two soils can differ in their rates of erosion by up to 106 times. Coarse-grained soils, in general, are less erosion-resistant than fine-grained soils. The Erosion Rate Indices of coarse-grained cohesionless soils show good correlation with the fines and clay contents, and the degree of saturation of the soils, whereas the Erosion Rate Indices of fine-grained cohesive soils show moderately good correlation with the degree of saturation. The absence of smectites and vermiculites, and apparently the presence of cementing materials, such as iron oxides, improves the erosion resistance of a fine-grained soil.
The Hole Erosion Test is proposed as a simple index test for quantifying the rate of piping erosion in a soil, and for finding the approximate Critical Shear Stress corresponding to initiation of piping erosion. Knowledge of these erosion characteristics of the core soil of an embankment dam aids assessment of the likelihood of dam failure due to piping erosion in a risk assessment process.
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2002 Papers
2002 – Experimental Investigation of the Rate of Piping Erosion of Soils in Embankment Dams
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C.F. Wan, R. Fell, M.A. Foster
This paper presents the findings of experimental investigation of the rate of piping erosion of soils conducted at the University of New South Wales.
Two tests, namely the Slot Erosion Test and the Hole Erosion Test, have been developed to study the erosion characteristics of a soil. The erosion characteristics are described by the Erosion Rate Index, which indicates the rate of erosion due to fluid traction, and the Critical Shear Stress, which represents the minimum shear stress when erosion starts. Results of the two laboratory erosion tests are strongly correlated. Values of the Erosion Rate Index span from 0 to 6, indicating that two soils can differ in their rates of erosion by up to 106 times. Coarse-grained soils, in general, are less erosion-resistant than fine-grained soils. The Erosion Rate Indices of coarse-grained cohesionless soils show good correlation with the fines and clay contents, and the degree of saturation of the soils, whereas the Erosion Rate Indices of fine-grained cohesive soils show moderately good correlation with the degree of saturation. The absence of smectites and vermiculites, and apparently the presence of cementing materials, such as iron oxides, improves the erosion resistance of a fine-grained soil.
The Hole Erosion Test is proposed as a simple index test for quantifying the rate of piping erosion in a soil, and for finding the approximate Critical Shear Stress corresponding to initiation of piping erosion. Knowledge of these erosion characteristics of the core soil of an embankment dam aids assessment of the likelihood of dam failure due to piping erosion in a risk assessment process.
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2006 Papers
2006 – The Ability of Monitoring to Detect Internal Erosion and Slope Instability in Embankment Dams
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Robin Fell
Internal erosion and piping within embankment dams may initiate in cracks caused by differential settlement or desiccation, in cracks caused by hydraulic fracture and in very poorly compacted layers of soil. It generally cannot occur unless one of these defects is present because backwards erosion, the other mechanism for internal erosion, will not occur in embankments under normal gradients and will not occur in cohesive soils unless gradients are exceptionally high.
As a result it is very unlikely that it will be possible to detect initiation of erosion with piezometers, and the most likely successful method is seepage observation and monitoring. However the time from the first detection of increased seepage to breach of the dam may be very short-a matter of hours in some situations.
Thoughtfully positioned and read piezometers are more likely to be successful in identifying the critical gradients which may lead to the onset of backwards erosion in cohesionless soils in the foundation of dams.
Piezometers are more useful in establishing the pore pressures for use in analysis of stability, but in most cases where stability is marginal undrained strength analysis is required and the pore pressures and effective strengths alone are not sufficient to assess stability. In a number of cases differential settlements, and acceleration of settlements have proven valuable in detecting the on-set of instability and the conditions in which internal erosion and piping to initiate. Once these conditions are recognised more detailed survey monitoring and borehole inclinometers can be valuable in better defining the geometry of instability.
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