**2. Soil erosion mechanism in earth structures**

Erosion in earth structures due to water flow occurs when the erosion resistant forces are less than the seepage forces that tend to produce it, in such a way that the soil particles are removed and carried with the water flow. The resistant forces depend on the cohesion, the interlocking effect, the weight of the soil particles and the kind of protection they have downstream, if any. Since the seepage through an earth structure is not uniform, the erosion phenomenon increases where there exists a concentration of seepage and water velocity; in places where this concentration emerges at the downstream side, the erosive forces on the soil particles might become very significant. This accentuates the subsequent concentration of seepage and erosive forces there.

This erosion process might occur at any crack that exists in the earth structure, due to differential settlements, seismic movements, tension stresses, or holes caused by dry roots or gnawing animals (rabbits, rats, etcetera). The existence of cracks is also due to shrinkage drying or swelling due to saturation. Favorable internal erosion conditions also exist in contacts between soils and rigid walls, concrete structures, interface with bedrock foundation, etcetera. Areas where ark effect is present are also very susceptible to internal erosion. In all the previous cases, if the vertical effective stresses are reduced by the effect of the water flow, then the existing crack might propagate in such a way that it will create the hydraulic fracture phenomenon.

The erosion starts at any point where the seepage water discharges and works toward the reservoir, gradually enlarging the seepage channel. Depending of the stage of this process, the occurred damage might be classified as a simple "incident", an accident, or a complete failure.

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

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

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

Finally, several recommendations for preventing or solving problems related to soil erosion

Erosion in earth structures due to water flow occurs when the erosion resistant forces are less than the seepage forces that tend to produce it, in such a way that the soil particles are removed and carried with the water flow. The resistant forces depend on the cohesion, the interlocking effect, the weight of the soil particles and the kind of protection they have downstream, if any. Since the seepage through an earth structure is not uniform, the erosion phenomenon increases where there exists a concentration of seepage and water velocity; in places where this concentration emerges at the downstream side, the erosive forces on the soil particles might become very significant. This accentuates the subsequent concentration

This erosion process might occur at any crack that exists in the earth structure, due to differential settlements, seismic movements, tension stresses, or holes caused by dry roots or gnawing animals (rabbits, rats, etcetera). The existence of cracks is also due to shrinkage drying or swelling due to saturation. Favorable internal erosion conditions also exist in contacts between soils and rigid walls, concrete structures, interface with bedrock foundation, etcetera. Areas where ark effect is present are also very susceptible to internal erosion. In all the previous cases, if the vertical effective stresses are reduced by the effect of the water flow, then the existing crack might propagate in such a way that it will create the

The erosion starts at any point where the seepage water discharges and works toward the reservoir, gradually enlarging the seepage channel. Depending of the stage of this process, the occurred damage might be classified as a simple "incident", an accident, or a complete

are performed by numerical modeling based on finite element method (FEM).

are presented, together with the main conclusions of this chapter.

**2. Soil erosion mechanism in earth structures** 

of seepage and erosive forces there.

hydraulic fracture phenomenon.

failure.

erosion in earth dams are given.

erosion is also given.

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, 2000).

Fig. 1. Dam example given by Blight (1910) to define the percolation factor *CB* (Casagrande, 1968)


Table 1. Minimum values of percolation factors to avoid piping, according to Blight and Lane criteria (Casagrande, 1968)

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 resistance are the uniform fine cohesionless sands.

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 in that zone.

Internal Erosion Due to Water Flow Through Earth Dams and Earth Structures 287

to rapid filling and drawdown conditions due to these oscillations of river water level and also to the seepage forces generated by rain infiltration at the crown of the levees protecting the margins of rivers are observed. Instability problems in river banks commonly begin with erosion, which in some parts (depending on the type of soil) causes *piping* and might result

Landslide

(1)

in landslides as shown in Figure 3 (Auvinet & Lopez-Acosta, 2010).

Piping

**3. Factors affecting the earth erosion phenomenon** 

Fig. 3. Evidences of instability in river banks caused by erosion (Auvinet & Lopez-Acosta,

Main factors affecting the erosion phenomenon are: a) the erodibility of the soil; b) the water velocity inside the soil mass or the water velocity on a river; c) geometry of the earth

*Erodibility* can be defined as the relationship between the velocity of the water flowing over the soil and the corresponding erosion rate experienced by the soil. This definition of erodibility presents some problems because water velocity is a vector quantity which varies everywhere in the flow and is theoretically zero at the soil-water interface. It is preferable to quantify the action of the water on soil by using the shear stress applied by the water on the soil at the water-soil interface. Thus, erodibility of a soil can be defined by the relationship between the erosion rate *Z* and the shear stress *τ* at the soil-water interface (Briaud, 2008):

> . *Z f*

Fig. 4. Proposed erosion categories for soils and rocks based on shear stress (Briaud, 2008)

Beginning of erosion

structure through its size and shape.

2010)

This effect, due to concentrated water leaks, varies somehow from what is called *suffusion* or internal instability, which implies the internal movement of soil particles due to the adjustment of internally unstable soils; this is the case of gap graded or very broadly graded soils, such as coarse sands and gravels with small quantities of fine soils.

Fig. 2b. Piping in the foundation initiated by backward erosion (After Fell et al., 2003)

Fig. 2c. Piping from embankment to foundation initiated by backward erosion (After Fell et al., 2003)

The soil erosion problems also might occur in river banks. In tropical regions the intense rainfalls originate large and quick variations of the water surface of rivers. Problems related

This effect, due to concentrated water leaks, varies somehow from what is called *suffusion* or internal instability, which implies the internal movement of soil particles due to the adjustment of internally unstable soils; this is the case of gap graded or very broadly graded

INITIATION CONTINUATION PROGRESSION BREACH/FAILURE

concentrated leak

Backward erosion in progresses to form a pipe

Backward erosion in progresses to form a pipe

Breach mechanism forms

Breach mechanism forms

Breach mechanism forms

Continuation of erosion Enlargement of

Fig. 2a. Piping in the embankment initiated by concentrated leak (After Fell et al., 2003)

INITIATION CONTINUATION PROGRESSION BREACH/FAILURE

INITIATION CONTINUATION PROGRESSION BREACH/FAILURE

Fig. 2c. Piping from embankment to foundation initiated by backward erosion (After Fell et

The soil erosion problems also might occur in river banks. In tropical regions the intense rainfalls originate large and quick variations of the water surface of rivers. Problems related

Fig. 2b. Piping in the foundation initiated by backward erosion (After Fell et al., 2003)

Continuation of erosion

Continuation of erosion

soils, such as coarse sands and gravels with small quantities of fine soils.

Concentrated leak forms and erosion initiates along walls of crack

Leakage exits from the foundation and backward erosion initiations

Leakage exits from the foundation and backward erosion initiations

al., 2003)

to rapid filling and drawdown conditions due to these oscillations of river water level and also to the seepage forces generated by rain infiltration at the crown of the levees protecting the margins of rivers are observed. Instability problems in river banks commonly begin with erosion, which in some parts (depending on the type of soil) causes *piping* and might result in landslides as shown in Figure 3 (Auvinet & Lopez-Acosta, 2010).

Fig. 3. Evidences of instability in river banks caused by erosion (Auvinet & Lopez-Acosta, 2010)
