A comprehensive study is made of an approximate model for the computation of the dynamic pressures and associated forces induced by ground shaking on rigid embedded vaults (2, 10). For rigidly supported vaults, it is shown that, due to the lack of shearing resistance of the medium surrounding the vault, the model is unable to predict the forces induced under static conditions of loading, and that it generally leads to much higher results than those obtained by available exact solutions (20). The soil surrounding the vault is hence modelled by a new set of impedances (20). It is shown then that the effect of base flexibility is to reduce the forces induced on massless vaults. Evaluation is also made of the effect of the vault mass. It is finally shown that the difference in the responses computed by the original and modified models decreases in importance with increasing flexibility.