Abstract:The nonlinear theory of the active earth pressure produced by the interaction of two media,which are composed of strain-hardening medium and strain-softening medium for the soil masses behind the retaining wall,is studied by using catastrophe theory when the wall back is vertical and smooth. The necessary and sufficient conditions leading to the active earth pressure are presented. It is found that the formation of the active earth pressure relies mainly on both the stiffness ratio k of the media and the homogeneity index. It is also illuminated that the classical Coulomb¢s and Rankine¢s theory on the earth pressure is a special example of k = 0 when the backfill surface is horizontal and the backfill is composed of a strain-softening medium. It is shown that the relation between the earth pressure strength sa and the depth z is nonlinear,i. e. the sa -z relation is linear for k = 0. The sa-z relation is approximately linear for the relatively small shear modulus Gh of the strain-hardening medium,and the sa-z relation is obviously nonlinear for the relatively large value of Gh. By making contrast between the theoretical curves and experimental or in-situ monitored data,the nonlinear earth pressure theory is proven to be effective.