The triaxial tests results, performed with use of undisturbed cohesive soils samples, were described in this paper. Shear strength parameters – internal friction angle (ϕ’), cohesion (c') and undrained shear strength were evaluated for soils of various values of plasticity index. Based on tests results analysis the relationship between internal friction angle and plasticity index was presented in respect to used standards and literature. The attention was drawn to the reasons of cohesion overestimating. The recommendations of Eurocode 7 for interpretation of strength tests were evoked too.

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