The location of the impermeable zone in a rockfill dam involves the same factors as it does in the case of an earth dam.
The upstream deck has a number of advantages:
- It is more stable under the water load, because the downward force of the water produces frictional resistance to sliding
- The permeable rock embankment develops no uplift, since the embankment permits no movement of water upward from the foundation.
- The impermeable deck can easily be inspected and repaired if necessary.
- During construction the height of the dam can be increased by dumping only on the downstream side and extending the membrane upward on the sloping surface.
The disadvantages of an upstream deck are:
The central core location has a number of advantages:
- The deck is vulnerable to weather and wave attack.
- If constructed of earth, sudden drawdown greatly reduces its stability and may cause it to slide.
- Settlement of the rock embankment tends to produce tensile cracks in the membrane.
- The core is equally supported and is more stable during a sudden drawdown (if constructed from earth).
- Settlement of the rockfill induces compressive stresses in the core, tending to make it more compact.
- There is less core volume and less cross sectional area for leakage for a given height of dam and thickness of core.
The choice for dams with impermeable zones depends largely on the stability of the core material. If it is strong, a near upstream location is often the most economical. However, if the core material is weak a central location is better.