It is known that the use of compressible material (e.g tire chips) on the backfill can significantly reduce the load against retaining wall. This reduction of earth pressure is achieved through a mechanism of simulating the quasi-active or the intermediate active state. When the soils move to active state, the active earth pressure is reduced. If there exists a highly compressible material between soil and retaining wall, the backfill soil approaches the active state. In this paper, this mechanism has been numerically evaluated. In the numerical simulation, the modified Cam clay model with super-subloading yield surface and rotational hardening concept (SYS Cam clay model) was used. The SYS Cam clay model, which was introduced by Asaoka et al. (2002), is the elasto-plastic model that takes into the account the soil structure, overconsolidation and anisotropy. Using this model, the typical responses of sand with various densities can be described. The earth pressures at rest for both backfill conditions (only soil and tire chips and soil) were calculated and the reduction mechanism made clear.