**1. Introduction**

This chapter describes the earth erosion caused by the water flow and seepage that occurs through earth dams, earth embankments and some other structures constructed with earth, such as canal systems, dikes, reservoirs and levees. The erosion in levees on river banks and in levees constructed to protect urban areas exposed to flooding is also discussed. It first describes the mechanism of the soil erosion and the importance of such phenomenon, particularly the damages and consequences that such erosion might produce when it becomes out of control. For instance, one of the main causes of earth dam failures all over the world is the so called piping event, which occurs due to the constant migration of soil particles towards free exits or into coarse openings; this event might occur through the earth embankment or its foundation soil. Another cause of constant earth structure failures is due to uncontrolled saturation and seepage forces. In this context, phenomena known as rapid filling and rapid drawdown, which occur in earth structures subjected to sudden changes of water level (increments or decrements) that modify flow conditions inside a soil mass are assessed. Examples of both failures are given in this chapter.

Each one of the main factors that affect the occurrence of the earth erosion phenomenon is described with detail. Among these factors are: a) the erodibility of the soil; b) the water velocity inside the soil mass; c) geometry of the earth structure. Other important factors discussed here are the homogeneity or anisotropy of the earth structure and its foundation soil, the soil graduation and degree of compaction of the materials used during the construction process; the hydraulic conductivity of such materials, the upstream water energy head, as well as the hydraulic gradient. The importance and the way that each of these factors affect the earth erosion are presented. The calculation of the seepage forces and their effects in slope stability are also described.

The main graphical and numerical methods used for the analysis of the erosion problem considering steady-state and transient flow conditions are discussed. The advantages and shortcomings of each one are emphasized.

Description of the existing procedures for preventing damages due to soil erosion is given in this chapter. Some remediation methods for solving hydraulic problems related to piping or internal erosion, such as impermeable flexible walls, impermeable blankets, grouting

2000).

1968)

Material

in that zone.

Lane criteria (Casagrande, 1968)

resistance are the uniform fine cohesionless sands.

Internal Erosion Due to Water Flow Through Earth Dams and Earth Structures 285

The first engineers that analyzed this problem were Blight (1910) and Lane (1935), as cited in Casagrande (1968), who defined the susceptibility to soil erosion through a percolation factor *C*, in terms of the horizontal and vertical paths of the water flow, the type of soil and the water head between the upstream and downstream water levels of a hydraulic structure. Figure 1 illustrates the definition of the percolation factor and Table 1 gives the minimum values of *C* recommended by these engineers to avoid soil erosion. Unfortunately, this criterion did not work well for all cases and its use is not recommended (Flores-Berrones,

Fig. 1. Dam example given by Blight (1910) to define the percolation factor *CB* (Casagrande,

Table 1. Minimum values of percolation factors to avoid piping, according to Blight and

In 1967 Sherard et al. published a table which gives a rough empirical relationship between piping resistance in earth dam embankments and soil types. Such table indicates that soils with the greatest piping resistance are the well compacted high plasticity clays, the intermediate are the well graded coarse sand and sand gravel mixtures, and the least piping

The soil erosion in earth structures, particularly in earth dams and levees, might occur through the embankment, the foundation or from the embankment to foundation (Figs. 2a, 2b and 2c). This kind of erosion has the following phases: a) initiation and continuation of erosion, b) progression to form a pipe, and c) formation of a breach (Fell et al., 2003). The initiation of the soil erosion usually starts at the exit point of the seepage, and retrogressive erosion results in the formation of a "*pipe*". In fact, this is the reason why this erosion phenomenon is also called *piping* (see Fig. 2c). The removal of a small portion of the earth embankment or foundation by erosive action at any point, particularly at the exit part of the downstream slope, accentuates the subsequent concentration of seepage and erosive forces

1 3

*t b <sup>C</sup> h* 

(Lane criteria)

*L*

*B b t <sup>C</sup> h* 

Fine sand and silt 18 8.5 Coarse sand 12 6.0 Gravel and sand 9 3.0 Boulders, gravel and sand 4 2.5

(Blight criteria)

procedures and drainage blankets are also presented. A short section with some recommendations to protect river banks from the erosive attack of water (such as rockfill, *bolsacreto* or *colchacreto* system –concrete bags–, breakwaters, sheet pile walls, etc.) is also included. The construction of graduation filters to prevent piping and movement of erodible soils is also presented. Special emphasis is given in the actual filter design criterion that is recommended by the US Army Corps of Engineers (2000), U.S. Bureau of Reclamation (2000) and the U.S. Soil Conservation Service (1994). Together with these recommendations, those given for earth dams design by A. Casagrande (1968) for avoiding piping and internal erosion in earth dams are given.

Several devices that have been developed to assess how resistant earth materials are to water flow are presented. Additionally, the main recommended laboratory and field tests to analyze soil dispersion or erosion are discussed. A description of laboratory tests to verify the best suitable material to use as a filter and protect a dam core against piping or internal erosion is also given.

Two practical examples related to drainage failures caused by piping and by uncontrolled saturation and seepage forces are presented to illustrate the content of this chapter. In particular, analyses to assess how transient flow caused by rapid filling and drawdown affects soil erosion in typical levees constructed to protect urban areas exposed to flooding are performed by numerical modeling based on finite element method (FEM).

Finally, several recommendations for preventing or solving problems related to soil erosion are presented, together with the main conclusions of this chapter.
