Monotonic curve can be used as envelope curve for the linking of unloading points is similar to the monotonic stress-strain curve.

Based on the plastic flow theory of soils, an elastoplastic constitutive model for describing the stress-strain behavior of the saturated sands under the action of monotonic loading is developed.

Test results show that : ( 1 ) the amount of creep deformation during monotonic primary loading depends on strain rate before creep, creep stress level, and creep time ;