**2. Internal erosion processes**

It has been found that the soil fractions considered most susceptible to erosion are coarse silts and relatively uniform fine sands.

Cohesive soils, such as clays, are more resistant to erosion as long as chemical bonds are not destroyed [5]. It also seems that certain central materials of glacial origin are particularly sensitive to internal erosion.

According to [6], four conditions must be met for internal erosion and channeling to occur. These conditions are as follows:

There must be a seepage path and a water source.

There must be erodible material within the flow path, and this material must be transported by an infiltration flow.

There must be an unprotected outlet from which the eroded material can escape. For a channel to form, the transported material, or the material directly above it, must be capable of forming and supporting the channel's "roof."

In an earth and rockfill dam with a central core, there are mainly three processes [6] that can initiate channelization: backward erosion, concentrated leakage, and suffusion. Backward erosion is initiated at the point where the seepage exits, and erosion progresses progressively backward to form a channel. Concentrated seepage begins with a crack or soft zone emanating from the water source to an exit point (downstream). Erosion continues progressively along the walls of the erosion hole, intensifying the concentrated leak. Suffusion is the process by which fine soil particles are carried away or eroded by the voids formed by coarser particles. This phenomenon can be avoided if the soil has a well-spaced granulometric distribution and sufficiently small voids. Soils are said to be internally unstable if suffusion occurs, and internally stable if the particles are not eroded by the infiltration flow.

*Prediction of Internal Soil Erosion in Hydraulic Works DOI: http://dx.doi.org/10.5772/intechopen.113104*

**Figure 1.**

*Conceptual model for development of failure by piping from the embankment into the foundation [7].*

Piping can occur in the backfill through the foundation and from the backfill into the foundation. Conceptual piping failure development models for these three cases are shown in **Figure 1**. In addition, a failure path diagram for piping failure through backfill is shown in **Figure 1**. Similar failure diagrams for failure by piping through the foundation and backfill in the foundation can be found in [7] or [8].
