Rock-fill dams are a kind of dam that combines porous yet firmly packed gravel, sand, or silt in one section with an area of concrete, metal, clay, or some other substance that completely blocks water. The section of compacted granular soil helps slow the flow of water while the impervious zone provides strength and stability. Rock-fill dams are one of two types of embankment dams, the other being earthen dams.
These structures are effective because the force of the river or reservoir hits the impervious zone and is transferred to the firmly packed transition zone where water can slowly begin to filter through the dam. When clay is used for the impervious zone, this is known as a composite dam.
Rock-fill dams are made of two main parts, the main rock-fill or transition zone and the impervious zone. Synthetic filters made of specifically graded soil are also sometimes used to reduce internal erosion (that is, to stop fine grains from moving around within the dam).
If you're ever assigned to build such a dam or you decide that you want to build a rock-fill dam on your property, it's important that you pay attention to the following points:
- Stability is critical. Both the foundation and the abutment must be stable throughout the life cycle of the dam.
- Seepage must be controlled to ensure safe operation. Seepage control measures should be in place before the dam's initial use.
- The spillway must be strong enough to prevent overtopping of the embankment.
- Reservoir slopes should be carefully designed.
Advantages of Rock-fill Dams
One of the most important aspects of a rock-fill dam is that it can be very cheap to build since materials can be sourced from near the dam's location. This type of dam can also be suitable in areas that don't present the best conditions for deep foundations. Another key benefit is that the dam can be planned in areas that are susceptible to cold conditions, and in hot and humid climates, as well. Moreover, they can be built even in rough weather, and they take far less time to build than many other types of dams.
How to Create a Rock-fill Dam
The best way to build a dam is to start by placing the material in controlled lifts. Steel-drum roller compactors are used so the material can be locked into place to reduce settlement during subsequent years. These rollers can also be used to avoid material segregation due to high lift rock fall on the slopes.
The dam height can be naturally increased by adding more rocks behind the diaphragm without encroaching on the reservoir area. The main rock-fill provides support for its own weight and stability, while the membrane holding the water creates an impervious zone. This membrane can be made of wood, concrete, steel, asphalt, or stone. The dam is then made water-tight by continuing the impervious face upward.
Always consult your local building codes and water regulations before proceeding.