Vane Shear Strength in Terms of Effective Stresses

Experience with the shear strength determined with the field vane has shown that the measured values of the strength, normalized with the effective overburden stress, suv/σ'vo, are often lower than 0.1 for normally consolidated quick clays of low plasticity. This undrained shear strength ratio is less than one third of the corresponding undrained strength measured in a triaxial active test in the laboratory. For more plastic clays and overconsolidated clays, however, the difference between field vane and triaxial active shear strength is found to be less. The ratio of the two strengths might even be close to unity in some cases. The explanation for this phenomenon is not that the insertion of the field vane causes disturbance and remoulding in soft, sensitive clays. With a revised failure criterion for soft, contractant clays, it is possible to express the vane strength in terms of effective stresses, and thus to explain the differences in the shear strengths measured by the vane and in the laboratory. It is concluded that the directly measured vane shear strength values do not constitute, in most cases, a representative strength for stability analysis, for instance for embankments and excavations. Amethod for correcting the vane shear strength for stability analysis in practice is proposed