10.4 Internal erosion
(1)P Filter criteria shall be used to limit the danger of material transport by internal erosion.
(2)P Where an ultimate limit state due to internal erosion can occur, measures such as filter protection shall be applied at the free surface of the ground.
(3) Filter protection should generally be provided by use of natural non-cohesive soil that fulfils adequate design criteria for filter materials. In some cases, more than one filter layer may be necessary to ensure that the particle size distribution changes in a stepwise fashion to obtain sufficient protection both for the soil and the filter layers.
(4) Alternatively, artificial filter sheets such as geotextiles may be used provided it can be established that they sufficiently prevent transport of fines.
(5)P If the filter criteria are not satisfied, it shall be verified that the critical hydraulic gradient is well below the design value of the gradient at which soil particles begin to move.
(6)P The critical hydraulic gradient for internal erosion shall be established taking into consideration at least the following aspects:
- direction of flow;
- grain size distribution and shape of grains;
- stratification of the soil.
10.5 Failure by piping
(1)P Where prevailing hydraulic and soil conditions can lead to the occurrence of piping (see figure 10.3), and where piping endangers the stability or serviceability of the hydraulic structure, prescriptive measures shall be taken to prevent the onset of the piping process, either by the application of filters or by taking structural measures to control or to block the ground-water flow.
- 1 free water table
- 2 piezometric level in the permeable subsoil
- 3 low permeability soil
- 4 permeable subsoil
- 5 possible well; starting point for pipe
- 6 possible pipe
NOTE Suitable structural measures are:
- application of berms on the land side of a retaining embankment, thus displacing the possible starting point of piping farther away from the structure and decreasing the hydraulic gradient at this point;
- application of impermeable screens below the base of the hydraulic structure by which the ground-water flow is either blocked or the seepage path is increased, thereby decreasing the hydraulic gradient to a safe value.
(2)P During periods of extremely unfavourable hydraulic conditions such as floods, areas susceptible to piping shall be inspected regularly so that necessary mitigating measures can be taken without delay. Materials for such measures shall be stored in the vicinity.
(3)P Failure by piping shall be prevented by providing sufficient resistance against internal soil erosion in the areas where water outflow may occur.
(4) Such failure can be prevented by providing:
- sufficient safety against failure by heave where the ground surface is horizontal;
- sufficient stability of the surface layers in sloping ground (local slope stability).
(5)P When determining the outflow hydraulic conditions for the verification of failure by heave or of local slope stability, account shall be taken of the fact that joints or interfaces between the structure and the ground can become preferred seepage paths.