Associated flow model can describe the elastic-plastic constitutive relationship of metal perfectly,but for soils it dos not work that well. This is because the direction of plastic strain increment of soil is notunique, which has been proved by experiments. Traditional elastic-plastic theory is based on the assumptionthat the direction of plastic strain increment is unique, so neither associated flow nor non-associated flow cansolve the constitutive problem satisfactorily. On the basis of generalized potential theory, the traditionalunrecoverable deformation contains two parts: the quasi-elastic-plastic, which obeys elastic rule, andpure-plastic, which obeys associated flow rule. The present paper presents a elastic-plastic model consideringquasi-elastic-plastic deformation for soil, and studies the decomposing criterion of plastic strain incrementand the determination of quasi-elastic parameters. The new model can solve the problem of non-uniquenessof the direction of plastic strain increment, it is illustrated by two examples.
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