How do cutoffs, grouting, and drainage reduce uplift beneath gravity dams?

For gravity dams, managing uplift—the buoyant force caused by water seeping beneath the structure—is a critical design challenge to prevent instability. Engineers employ three primary methods to mitigate these pressures:1

• Cutoffs: By creating a barrier, such as a downward projection of concrete into the rock or an excavated trench backfilled with impermeable material, engineers can obstruct the path of seepage. This effectively limits the amount of water reaching the foundation area.21
• Grouting: When foundations consist of fissured rock, grout is injected to fill voids and fractures. This process reduces the permeability of the rock mass, thereby restricting the flow of water under the dam.1
• Drainage: Because achieving a perfectly impermeable cutoff is often difficult, drainage systems—such as galleries or wells—are used to intercept and relieve any water that does seep through. By providing a controlled exit for this water, the system prevents the buildup of high pore water pressures that would otherwise exert dangerous uplift forces on the base of the dam.1

Many gravity dams utilize a combination of these techniques to ensure structural integrity.1