They are employed to support load caused by changing ground movement or faulted and fractured rock. They are designed so that when the ground load exceeds the design load of the arch as installed, yielding takes places in the joint of the arch, permitting the overburden to settle into a natural arch of its own and thus tending to bring all force into equilibrium. The arc is stronger after this yielding than before because of increased joint overlap. Lewis, 1964.