© ASCE. The average vertical pressure on a buried structure can be calculated using the silo theory, which assumes the translation of a vertical prism of soil above the structure that is resisted by friction on the sides of the prism. One of the key assumptions made is the value of the coefficient of lateral stress, K. In this study, an assumption regarding the rotation of principal stresses in the yielding soil has been used to calculate the average coefficient of lateral stress acting at the side of a prism of yielding soil above the horizontal buried structure. The calculated value using the proposed method agrees well with experimental observation made in literature for the value of K, and is suggested for use in the estimation of loads on buried structures, where it is expected that the structure will yield relative to a stiff body of soil.