Conventional plasticity assumes that a yield surface exists and the direction of plastic strain increment (DPSI) is uniquely dependent on the current stress state. Triaxial stress probing tests of yield and plastic flow of sand have been conducted using discrete-element modelling with polyhedral particles resembling the shapes of Toyoura sand. It is found that a yield surface does not exist, but a memory surface (MS) separating two types of distinct sand behaviour can be established. Within the MS, the DPSI is primarily controlled by the stress increment, and the magnitude of plastic strain increment is insensitive to the stress increment direction. When the stress state is on or outside the MS, a much larger plastic strain increment is observed if the stress increment points outside the MS, and the DPSI is dependent on both the current stress state and stress increment. The shape and size of the MS, which can be modelled by the SANISAND yield function, are dependent on the soil density and evolve with plastic strain.
