Abstract: According to the theory based on sem-i infinite homogeneous isotropic elastic body, the coefficient of elastic uniform compression changes in inverse proportion to the square root of base area. However, the theoritical conclussion disagrees with a number of results of tests. The paper demonstrates the essential reason causing the disagreement. The paper considers that assuming soils in range of sem-i infinite to be a homogeneous isotropic elastic body is not in accord with practice. In fact, the elastic modulus of soils changes with depth. After considering the factor, the variation rate of coefficient Cz with base area F will be much lower than that with F. This conclusion is demonstrated by a number of computational results. Meanwhile, the paper indicates that the determination of value of Cz for layered soil structure characterized by different mechanical properties, adopted by/Code for Design of Dynamic Machine Foundation0 (GB50040 -96) is unreasonable.