Internal erosion of the foundation or embankment caused by seepage is known as piping. Generally, erosion starts at the downstream toe and works back toward the reservoir, forming channels or pipes under the dam. The channels or pipes follow paths of maximum permeability and may not develop until many years after construction.
Resistance of the embankment or foundation to piping depends on:
Plastic clays with a plasticity index >15, for both well and poorly compacted are the materials which are most resistant to piping. Minimum piping resistance is found in poorly compacted, through to well-graded cohesionless soils with practically no binder. It is also found in uniform, fine, cohesionless sand, even when well compacted. Settlement cracks in resistant materials may also produce piping.
Piping can be avoided by lengthening the flowpaths of water within the dam and its foundations. This decreases the hydraulic gradient of the water flow and hence its velocity. The flowpaths can be increased by:
Seepage is the continuous movement of water from the upstream face of the dam toward its downstream face. The upper surface of this stream of percolating water is known as the phreatic surface. The phreatic surface should be kept at or below the downstream toe.
The phreatic surface within a dam can be controlled by properly designed cores or walls.
Internal drain systems
A homogeneous dam with a height of more than about 6 m to 8 m should have some type of downstream drain. The purpose of a drain is:
The effectiveness of the drain in reducing pore pressures depends on its location and extent. However, piping is controlled by ensuring that the grading of the pervious material from which the drain is constructed meets the filter requirements for the embankment material.
The design of a downstream drainage system is controlled by the height of the dam, the cost and availability of permeable material, and the permeability of the foundation.
For low dams, a simple toe drain can be used successfully. Toe drains have been installed in some of the oldest homogeneous dams in an effort to prevent softening and erosion of the downstream toe.
Horizontal drainage blanket
Horizontal drainage blankets are often used for dams of moderate height.
Drainage blankets are frequently used over the downstream one-half or one-third of the foundation area. The Bureau of Reclamation's 45 m Vega Dam is a homogeneous dam which has been constructed with a horizontal downstream drain. Where pervious material is scarce, the internal strip drains can be placed instead since these give the same general effect.
Disadvantages of horizontal drainage blankets
An earth dam embankment tends to be more pervious in the horizontal direction than in the vertical. Occasionally, horizontal layers tend to be much more impervious than the average material constructed into the embankment, so the water will flow horizontally on a relatively impervious layer and discharge on the downstream face despite the horizontal drain.,p> Where this has occurred the downstream slope is prone to slipping and piping. Repairs can be made by installing pervious blankets on the downstream slopes or constructing vertical drains to connect with the horizontal blanket. Such vertical drains are normally composed of sand and gravel.
Chimney drains are an attempt to prevent horizontal flow along relatively impervious stratified layers, and to intercept seepage water before it reaches the downstream slope. Chimney drains are often incorporated in high homogeneous dams which have been constructed with inclined or vertical chimney drains.
In some major dam projects, chimney drains have been inclined at a considerable slope, both upstream and sometimes downstream. An upstream inclined drain can act as a relatively thin core. In addition to controlling seepage through the dam and increasing the stability of the downstream slope, the chimney drain is also useful in reducing pore water pressures both during construction and following rapid reservoir drawdown.
Dimensions and permeability of drains
The dimensions and permeability of permeable drains must be adequate to carry away the anticipated flow with an ample margin of safety for unexpected leaks. If the dam and the foundations are relatively impermeable, then the expected leakage would be low. A drain should be constructed of material with a coefficient of permeability of at least 10 to 100 times greater than the average embankment material.
Thin upstream sloping core
In an earth dam with an upstream sloping core of low permeability, the foundation is assumed to be impermeable and in a steady state. Under steady state conditions the small amount of water that seeps through the core flows vertically downward in a partially saturated zone and then more or less horizontally in a thin saturated layer along the impermeable foundation. For this type of dam the downstream shell must be several hundred times more permeable than the core.
An earth dam constructed without a cutoff on permeable or semi-permeable foundations of earth or rock may lead to seepage beneath the dam creating unacceptable uplift pressures and causing instability. If an impermeable cutoff is installed to 60 % of the depth of the permeable foundation, the flow net and downstream slope gradient is only slightly modified to a lower level. A theoretical line of seepage for several depths is given here.
For an effective cutoff the positioning and depth of cutoff must be essentially 'perfect'. Since this is impossible to achieve, other methods of seepage control should be used in conjunction with cutoffs.