Formulas are obtained for the speeds of propagation of gravity-induced rarefaction and condensation density discontinuities in vertical sand columns. A rarefaction wave is induced in a column of sand at rest by removing the support at the bottom of the column. A condensation shock is induced by reintroducing the support, which stops the sand from falling. The theoretical prediction of the speed of propagation of the leading edges of the rarefaction wave corresponds well with the speed measurements obtained in a preliminary experiment.

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