Tuesday, August 4, 2009

Earth Dams and Tailings Dams

Earth dams for reservoirs and ring dyke tailings dams are similar in many ways; they both provide a boundary for the retained liquid and are both of similar shape. Yet what sets them apart is the way in which they retain the fluid and the construction internally which acts in different ways.

Earth Dams

Earth dams have a very specific use; to retain water inside the boundary and provide support so that it is safe and will not fail due to the weight of the retained water. The construction of an earth dam has two distinct parts; one of which to provide a water tight layer so that water cannot pass through and the other to provide restraint. Figure 1 shows a common cross section of an earth dam.

As seen from figure 1, the earth dam contains a clay core that provides the dam with an almost impermeable layer at which water cannot pass through and a soil or rock mass to each side providing restraint. Other parts of this dam include an upstream blanket to reduce water seeping under the wall, and drainage blankets on the downstream wall for water to drain freely away.

Earth dams although can be found to be constructed in many different ways due to the size, amount of water being held back, location and availability of materials and safety factors.

The Core

Comprised of a low permeable (k=10-8, 1 0-9), low plasticity (CL & SC or SC-CL) usually high clay content. Compaction of the core is a vital part and usually compacted to a level of 95% to 98% RD. The core can be placed in many different ways throughout the dam cross section yet never downstream from centre. Different positioning of the core provides the dam with different properties and different flow paths, and is usually optimized depending on the situation.


Can vary in composition and usually dependent on local materials but always of higher grain size than that of the core. The shoulders provide restraint for the core from near horizontal forces applied by the water. On the outer edges of the shoulder is usually a form of rip-rap or large boulders to prevent wave action from erosion of the shoulder. Although the shoulders have a large mass and strategically placed, movement of the wall is generally expected and can be as much as 20mm per year.


Upstream BlanketUsually located on the upstream side of the dam and usually made of a material similar to that of the core, the blanket provides another almost impermeable layer so that there is less seepage through the dam foundation. A blanket also has a good ability of increasing flow paths beneath the dam as seen in the diagrams below.


Filters are installed between materials of high grain size and materials of low grain size to prevent boundaries from mixing. Materials of these filters are of a grain size between these two materials and most commonly sand.

Tailings Dams

Tailings dams are very similar to normal earth dams for reservoirs yet precautions for failure of these structures are monitored much closer. Tailings dams are built to allow the remains from processing ores to settle out from the water that is used in the processing.
The construction of tailings dams can vary significantly and is mainly dependent on the type of tailings being stored and the topography of the storage facility. Vick, SG states that there are four main types of tailings storage, Water Retention Dam, Upstream Dam, Downstream Dam and Centreline Dam; these are displayed below.

From the types of tailings dam it can be seen that most start in the same way with the conical shape seen in a normal earth wall, yet as the amount of tailings increases the wall is also heightened in addition and in different ways depending on the design. The four types all have advantages and disadvantages depending on their construction; the following table shows some of these properties.

Monitoring of tailings dams is much more strict than that of an earth fill dam, this is due to the some time hazardous materials being held. For instance a tailings dam for a Uranium processing plant can be extremely harmful containing over a dozen radioactive nuclides including thorium, radium and radon. If not controlled properly, dust can be blown from these ponds entering food chains and bodies of water.

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