Abstract
Backward erosion by piping is one of the processes that threaten the stability of river embankments in the Netherlands. During high river stages, groundwater flow velocities underneath the embankment increase as a result of the steepened hydraulic gradient. If a single outflow point exists or forms, the concentrated flow can entrain soil particles, leading to the formation of a subsurface pipe. The processes controlling this phenomenon are still relatively unknown due to their limited occurrence and because piping is a subsurface phenomenon. To study the initiation of piping, we performed laboratory experiments in which we induced water flow through a porous medium with a vertically orientated outflow point. In these experiments, we explicitly considered grain size variations, thus adding to the existing database of experiments. Our experiments showed that the vertical velocity needed for the initiation of particle transport can be described well by Stokes’ law using the median grain size. We combine this with a novel method to relate bulk hydraulic conductivity to the grain size distribution. This shows that knowledge of the grain size distribution and the location of the outflow point are sufficient to estimate the hydraulic gradient needed to initiate pipe formation in the experiment box.
Highlights
Using Stokes’ law, the flow velocity measured in the vertical channel at the initiation gradient for backward erosion was used to determine the grain size for which the upward and downward forces balance out
The D50 performed slightly better than the D70 for predicting the flow velocity required to initiate backward erosion
Unless the grain size distribution is exceptionally broad, both D50 and D70 can be used as an indicator for predicting the velocity required to initiate backward erosion, especially for smaller grain sizes
Summary
One of several key mechanisms threatening the stability of dikes is piping [1,2,3,4,5,6]. For this process, the occurrence of pervious layers overlain by impervious cover layers is of concern [7]. The impervious cover layer prevents pressure and water from being released over wider areas (due to seepage), causing a concentration of flow to a single exit point which either occurs naturally or through engineering works that compromise the integrity of the cover layer (see Figure 1).The hydraulic gradient between the river and the seepage point can be of sufficient magnitude to entrain sand particles in the underlying pervious layer, during high river stages. When allowed to continue progressing upstream, this could eventually lead to a slump type failure in an overlying embankment [5,8,9]
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