Abstract: As the result of the direct shear test and triaxial test, strength index of over-consolidated soil is not in conformity to that of normally consolidated soil. Shear strength index under the pressure which is greater or less than the previous consolidation pressure is obviously different; and the test data should be respectively processed. Prior to shearing, the sample pretreatment should be in the condition of dead weight stress or preconsolidation pressure. The comparison of the shear strength index under the vertical stress or confining pressure which is smaller than and greater than previous consolidation pressure shows: the former cohesion obtained by unconsolidated undrained shear test is smaller than the latter; on the contrary, the internal friction angle is larger; it is just the opposite in consolidated undrained shear test; the stress history simulated through experiment for remolded soil is difficult to reflect the actual strength characteristics, so test should be carried out by using undisturbed soil. Laboratory test should determine the previous consolidation pressure and K0; and then, according to the actual engineering stress state, the shear strength index can be obtained.

Key words: stress history, shear strength, previous consolidation pressure, over-consolidation