Water Storage and Hydro Electric Dams
Dams form some of the largest engineering structures on Earth. They also often have catastrophic consequences if they fail. Dams are primarily used for water storage and hydro power generation.
Dams are usually designed for long service lives which frequently have to be extended, as the complications of removing a dam and replacing it are huge. Extending a dam’s service life probably means a complete re-appraisal of the structure, its foundations and the spillways and tunnels that carry water. This means measurement of the dam’s current state and possibly works to improve its safety. Frequently there is a need to raise the dam height to increase the capacity of the reservoir or to overcome the effects of silt build up. As the consequence of dam failure is generally totally intolerable, designs need to be conservative and works that are constructed need to be monitored carefully so that any deviation from design behaviour may be detected and its consequences evaluated.
Dams are large structures that are supported by the ground in all its forms of rock and soil. It is this interface between the natural ground and the man-made structure that provides some of the greatest engineering challenges. The ground is just as much a part of the dam as a concrete arch wall, and a part that generally has far less well defined parameters. It is what is inherited from the earth when a decision is made to build a dam at a particular location. The dam builder cannot specify the particular material properties of the ground; it must gain an understanding of what the properties are and design and build a structure that is compatible with these. Modification of the natural ground behaviour is always complex and frequently an expensive process.
Sigra’s Role in Dams
Sigra’s role in dams is threefold. In the first instance it involves the determination of the ground parameters where a dam may be constructed and secondly in the design and thirdly in the monitoring the structures and the ground once they have been built.
In the site investigation phase Sigra supplies the following services:
- Geological assessment – lithological and structural
- Rock stress measurement
- The measurement of rock properties
- The measurement of the permeability and storage behaviour of the rock mass
- The supply of expertise in drilling as part of the investigation. This includes directional drilling.
Sigra have been involved in the design of rock structures for a long period. Their expertise in the support of tunnels, drilling, drainage and grouting are useful components to dam design. Of particular importance is the company’s knowledge in dealing with rock stress and with anisotropic rock properties, both in the pre and post failure states.
Measuring and Monitoring
Sigra has the ability to measure and to install permanent monitoring systems in dams, their foundations and the surrounding slopes. Sigra’s overcore system has been used to measure the stress in concrete dams. Sigra have also installed stress change cells in rock and could do so in a dam wall to measure changes in stress that may occur therein.
More usual instrumentation for monitoring that Sigra uses includes:
This all comes with the capability to transmit this information by cable around the galleries within a dam or to send it via the cellular phone network or by radio telemetry.
Tailings dams are used to dispose of the waste from ore processing. The usually consist of finely ground material that has had the ore component extracted and which now needs to be parked for an indefinite period. This infinite time span poses problems as all structures need at least to be monitored and some need maintenance to survive. Tailings dams are no different. However, in most cases the structure of the tailings dam is less than ideal and the consequence of the dam failure can be catastrophic for those downstream.
Tailings dams have some sort of barrier which is created to prevent downstream flow. The tailings are then discharged hydraulically behind this. Particles settle out at different rates with the coarser material descending most quickly near the discharge point and the finer material travelling further over the low slope, known as the beach, to the decant pond. Layer after layer of tailings is built up by this process. When the limit of the initial barrier is reached then then it must be raised. This can be done by building a higher downstream embankment or, more usually, building this embankment upstream as this reduces the amount that it needs to be raised. Building upstream requires the material below the raised embankment being able to support the additional height.
This entire process is from a geotechnical viewpoint less than ideal. The hydraulic construction of water storage dams was halted decades ago after some spectacular failures but it is a fundamental part of tailings dam operation. The problem with this type of hydraulic construction is that consolidation only occurs through gravitational loading and that because much of the material is fine the rate of pore pressure decline is very slow. The result is frequently a soil mass that is on the edge of static or dynamic liquefaction if the material moves and as a consequence, pore pressures rise due to compaction.
To avoid liquefaction problems, it is highly desirable to have much of a tailings dam on the safe side of the compaction side of the critical state line so that pore pressures do not rise if some shear occurs. This requires some careful design, implementation and monitoring. The key to much of this is proper drainage of the tailings. This may be implemented through drainage pipes, drainage layers, or wick drains. In some cases, some positive means to ensure compaction without causing failure may be desirable. This means implementing compaction through vibrational induced shearing, but on a small enough scale not to induce general failure. For this to work there must be adequate drainage or all that is created is a localised zone of quick soil that settles back, much as it did before vibration.
The monitoring of the dam should include the monitoring of piezometric pressures and movement. The latter can advantageously include the measurement of compaction.
There are thousands of tailings dams in existence. Many of these have been built in a less than optimal manner and their stability needs to be determined. This requires an assessment of what is there. This requires visual inspections and will frequently require testing using piezo cone penetrometers or vane shear equipment. Direct testing for liquefaction potential is also useful.
Once a study has been completed it may be necessary to implement works to raise the factor of safety and reduce the probability of failure. This may require earthworks and improved drainage.
Sigra provide dam inspection, testing, monitoring and analysis services. This capability includes the installation of piezometers that record both fluid pressure and settlement and transmit this information from remote locations.