Relief wells are used both in the natural gas and petroleum industry and in flood control.
In the natural gas and petroleum industry, a relief well is drilled to intersect an oil or gas well that has experienced a blowout.
Specialized liquid, such as heavy (dense) drilling mud followed by cement, can then be pumped down the relief well in order to stop the flow from the reservoir in the damaged well.
Water may then flow through the soil towards the dry side of the levee, resulting in sand boil, liquefaction of the soil, and ultimately destruction of the levee.
Relief wells can prevent sand boils from occurring by relieving the water pressure as described.