The basal stability of soil nailed excavation systems has been investigated using Terzaghi’s ultimate bearing capacity theory; and a simplified procedure is proposed for estimating basal heave stability. It is assumed that the basal heave failure is caused by bearing capacity failure; but only part of the failure surface presented in Terzaghi’s bearing capacity is mobilized to evoke basal heave failure. The important factors affecting basal stability have been identified; and correlations between them and factor of safety are discussed from the results of parametric studies. Comparison with other methods has shown that the method proposed is more reliable to estimate basal heave stability.