Errori sulla valutazione dei parametri di resistenza conseguenti all’interpretazione dei risultati di prove di taglio diretto eseguite a velocità eccessivamente elevate

In routine geotechnical design, strength properties of clayey soils obtained from shear box tests carried out with a too high shear strain rate are often provided. Relatively fast shear tests can not ensure that drained conditions apply during all the duration of the test; therefore, soil strength parameters, obtained assuming that the effective stress normal to the shear plane coincides with the applied total stress, are clearly wrong. In this paper, the experimental results obtained in shear box tests carried out with shear strain rates greater than the maximum values suggested by international standards and recommendations are reported and compared with the correct ones. The study clearly shows that values of soil strength parameters obtained interpreting the results of shear box tests carried out too fast may be, depending on overconsolidation ratio of the soil, smaller or higher than those representative of real soil behaviour, with relevant effects on the geotechnical design of structures.