Abstract: The analysis of changing rule of shear strength indexes of soft soil with consolidation degree under the effect of surcharge load is executed based on the theoretical derivation and indoor tests; since the conventional shear strength indexes cohesion value and internal friction angle cannot represent the mechanical character of the soft soil at any consolidation degree at the effect of the surcharge load accurately, and the existed limit equilibrium method considering the increasing of the shear strength value with the consolidation degree cannot get the data such as stress field, displacement field that are needed by the practical engineering. The comparison analysis has been done for two existed formulae, and deduced that they have equivalence property. From the viewpoint of pore water pressure, a new formula is presented; and from which we get: the internal friction angle at any consolidation degree increases with the consolidation degree monotonously; and the cohesion value decreases with the consolidation degree monotonously; when the consolidation degree is fixed and is not 100%, the cohesion value presents the linear increasing relationship with the initial consolidation pressure; but the value of the internal friction angle is not related to it. The indoor test shows that, the newly deduced formula results fit better with the test results than the existed formulae results; since the total stress parameters and the effective stress parameters have been considered comprehensively, so as to prove the validity and advantage of the newly deduced formula.

Key words: shear strength indexes, consolidation degree, pore water pressure, changing rule