**6. Procedures and practical recommendations for preventing damages due to soil erosion**

The design of an earth dam or a levee is based on analytical studies of the site of construction and on personal experience of the individual designer. At a given site, it is possible to design a variety of earth dams which would be both, economical and safe. The final design depends on the quantities, types and location of the soil available for constructing the embankment, as well as the size and shape of the valley and the nature of the foundation. Sherard et al. (1967) present several typical designs of earth and earth-rock

Internal Erosion Due to Water Flow Through Earth Dams and Earth Structures 295

precaution when designing filters to protect gap-graded soils, and to avoid severe

To protect the upstream faces of earth dams, levees, and in any other situations in which erodible soils must be protected from rain currents and wave action, a layer of rock should be placed. One or more filter layers should be placed between the rock slope protection layer and the soil material that forms the earth embankment. Each transition filter layer must also satisfy the filter criteria in such a way that internal erosion or piping effect does not occur. For multi layer filters, the US Corp of Engineers (2000) and the US Bureau of Reclamation (USBR, 2000) recommend that grading curves of such filters should be more or

When filter fabrics are used, the protective filter is only the thickness of the fabric, which may be as little as 1mm. It is therefore very important that no holes, tears or gaps be allowed to form in the fabric. In this case, the openings between the filaments of a fabric should not be so large that significant loss of soil can occur. If the D85 of a soil is larger than the near maximum opening size of the fabric, little soil should be able to move through the mesh of

(c) Construction of an impervious upstream blanket Fig. 9. Measures for preventing soil erosion or piping through a pervious foundation of an

Fig. 10. Recommendations to protect levees on river banks (Auvinet et al., 2008)

(b) Impervious curtain through the pervious foundation

less parallel between each other, in order to avoid segregation or clogging.

segregation during the construction stage of any filter.

the fabric (Cedergren, 1989).

earth structure

(a) Extension of the impervious core to bedrock or impervious stratum

dams that have been constructed in the USA. Such dams vary from homogeneous earth dams (constructed on rock or impervious stratum) to dams with embankments constructed with different graduation materials and founded on either impervious or pervious soil stratums.

As it was mentioned before, internal erosion might occur through the embankment or through the dam foundation. To prevent earth erosion through the embankment, several measures might be taken. The following recommendations should be considered:


Properly designed filters should satisfy the following characteristics:


In relation to the drain design, Cedergren (1989) recommends that "designers should analyze every component of a drainage system (filters, conducting layers, collectors, outlets, and so on) to ensure that the entire system will have the necessary capacity and will function as intended". On the other hand, the criteria for the filter design was first established by Karl Terzaghi (1929) and later on modified by several authors (Sherard & Dunnigan, 1989; Wan et al., 2002) and several institutions (ICOLD, 1994; USACE, 2000; USBR, 2000; US Soil Conservation Service, 1994; among several others). Applications of these criteria to a case history and its implications are reported by Flores-Berrones et al. (2011). Sometimes it is necessary to use multi-layer filter systems, in which the characteristics for each layer should satisfy the selected design criteria for those materials surrounding the one under analysis.

For preventing soil erosion or piping through a pervious foundation of an earth structure, the following measures might be taken:


Cedergren (1989) recommends the use of high standards for all facets of design and construction, use relatively wide impervious cores and other features that hold seepage quantities and hydraulic gradients to the lowest practical levels, and provide well designed and constructed filters and drains wherever needed. It is also recommended special

dams that have been constructed in the USA. Such dams vary from homogeneous earth dams (constructed on rock or impervious stratum) to dams with embankments constructed with different graduation materials and founded on either impervious or pervious soil

As it was mentioned before, internal erosion might occur through the embankment or through the dam foundation. To prevent earth erosion through the embankment, several

d. Use properly designed filters and drains for all earth facilities exposed to the damaging

a. The filter should intercept water flowing through cracks or openings in protected soil and block the movement of eroding soil particles into it. Therefore, there must be a relationship between the size of the particles of the protected soil and the openings of

b. Filters should have enough permeability to avoid high seepage gradients or water pressures; this hydraulic condition means that the filter should act as a good drain. c. Filter grain particles should not have migration or suffusion due to the water flow action. This means that the filters should be designed to keep its internal structure

In relation to the drain design, Cedergren (1989) recommends that "designers should analyze every component of a drainage system (filters, conducting layers, collectors, outlets, and so on) to ensure that the entire system will have the necessary capacity and will function as intended". On the other hand, the criteria for the filter design was first established by Karl Terzaghi (1929) and later on modified by several authors (Sherard & Dunnigan, 1989; Wan et al., 2002) and several institutions (ICOLD, 1994; USACE, 2000; USBR, 2000; US Soil Conservation Service, 1994; among several others). Applications of these criteria to a case history and its implications are reported by Flores-Berrones et al. (2011). Sometimes it is necessary to use multi-layer filter systems, in which the characteristics for each layer should satisfy the selected design criteria for those materials

For preventing soil erosion or piping through a pervious foundation of an earth structure,

a. Continuation of the impervious zone of the embankment up to an impervious soil

b. Construction of a grout curtain or a steel sheet piling or a concrete cutoff wall, below

c. Impervious upstream blanket, in order to decrease the exit hydraulic gradient (Fig. 9c). d. Combination of recommendations 1), 2) and 3) referred above. Sherard et al. (1967) present several examples of earth and earth-rock dams that have been constructed

around the world, in which it is possible to observe alternative design solutions. Cedergren (1989) recommends the use of high standards for all facets of design and construction, use relatively wide impervious cores and other features that hold seepage quantities and hydraulic gradients to the lowest practical levels, and provide well designed and constructed filters and drains wherever needed. It is also recommended special

measures might be taken. The following recommendations should be considered:

b. Control the homogeneity of the materials during the construction process.

actions of water in their foundations or around the impervious core.

a. Obtain the best selection of the available construction materials.

Properly designed filters should satisfy the following characteristics:

c. Use transition zones between the coarse and fine materials.

stratums.

the filter.

always stable.

surrounding the one under analysis.

the following measures might be taken:

stratum or bedrock (Fig. 9a).

the impervious core (Fig. 9b).

precaution when designing filters to protect gap-graded soils, and to avoid severe segregation during the construction stage of any filter.

To protect the upstream faces of earth dams, levees, and in any other situations in which erodible soils must be protected from rain currents and wave action, a layer of rock should be placed. One or more filter layers should be placed between the rock slope protection layer and the soil material that forms the earth embankment. Each transition filter layer must also satisfy the filter criteria in such a way that internal erosion or piping effect does not occur. For multi layer filters, the US Corp of Engineers (2000) and the US Bureau of Reclamation (USBR, 2000) recommend that grading curves of such filters should be more or less parallel between each other, in order to avoid segregation or clogging.

When filter fabrics are used, the protective filter is only the thickness of the fabric, which may be as little as 1mm. It is therefore very important that no holes, tears or gaps be allowed to form in the fabric. In this case, the openings between the filaments of a fabric should not be so large that significant loss of soil can occur. If the D85 of a soil is larger than the near maximum opening size of the fabric, little soil should be able to move through the mesh of the fabric (Cedergren, 1989).

Fig. 9. Measures for preventing soil erosion or piping through a pervious foundation of an earth structure

Fig. 10. Recommendations to protect levees on river banks (Auvinet et al., 2008)

Internal Erosion Due to Water Flow Through Earth Dams and Earth Structures 297

Additionally, as it was mentioned before, soil erosion is likely to occur in certain types of soils. Among those are certain types of clay which erode by a process called dispersion or deflocculation, that occurs when the clay mass is in contact with water. If water is flowing, individual clay particles are detached and carried away through erosion channels or *pipes* that can form rapidly. As it is established by Cedergren (1989), one of the problems related with dispersive clay action, is that the deflocculation process starts as soon as there is a significant flow of water, as it can occur through poorly compacted or cracked layer in an impervious core, or along inadequate bonded contacts with rock foundations, abutments, or

The practical relevance of dispersive clays in dam engineering, started about 60 years ago after realizing that it was the main cause of piping failure of several small earth dams and levees. Most of the earth embankment failures caused by dispersive soils occur during the first filling. If there are no well designed and constructed filters upstream and downstream of the core embankment that has these clays, the probability of an internal erosion failure will be very high. This probability might increase when preexisting surface erosion caused by rainfall contributes to the formation of superficial channels that become connected to tunnels originated by internal erosion. As this type of soil is not possible to identify through the conventional index tests, it was necessary to develop certain laboratory and field

Whereas the susceptibility to erosion in cohesionless soils, such as fine sands and silts, is due to high values of water flow velocity, hydraulic gradients and seepage forces, normal clays are usually erosion resistant, except for water velocity higher than 1 m/sec. Nevertheless, for dispersive clays the erosion phenomena occur due to causes that are different to those

a. **Physic-chemical characteristics.** The erosion resistance property that normal clays have, due to the electrochemical attraction between clay particles, is reduced to a minimum in dispersive clays, due to their physic-chemical characteristics. Therefore, under a low water flow the dispersive clay particles tend to separate and taken away easily by the water current. The rate of erosion of these clay particles might be higher than the one that takes place in fine sands and silts. There are several factors that affect the dispersive action of these clays, among which are their chemistry and mineralogy, as well as the kind of salts that exist in the pore water and the circulated water. The principal difference between dispersive clays and ordinary erosion-resistant clays appear to be the nature of the cations in the pore water of the clay mass. Dispersive clays have a preponderance of sodium, whereas ordinary clays have a preponderance

of calcium, potassium, and magnesium cations in the pore water (Knodel, 1988). b. **Physical characterization.** Dispersive clays are not related to any specific geological origin, but they have been founded under alluvial environment, in lakes and in flood plain deposits. They are very important in hydraulic structures, such as earth dams, levees and channels, since many of them are constructed over such soils. In some cases marine deposits have the same pore water salts as dispersive clays, and the residual

c. **Mechanical characterization.** The external erosion or piping, caused by a water flow is very obvious and it occurs in granular or cohesionless soils. As it has been said, in this case it starts at the discharge end of a leak, at the downstream side of an earth

associated with granular soils. Such causes are due to the following characteristics:

**7.2 Identification of dispersive soils** 

procedures for its identification.

outlet conduits extending across the impervious core.

soils from such deposits are also dispersive.

Fig. 11. Recommendations to protect levees in urban areas exposed to flooding (Auvinet et al., 2008)

In addition, Figures 10 and 11 illustrate respectively some practical recommendations that should be taken into account for the protection of levees on the river banks, and levees that are built in order to protect urban areas exposed to flooding (Auvinet et al., 2008).
