Improvement of piezocone test interpretation for partial drainage conditions and for transitional soils

The present study shows the results of experimental analyses of field cone penetration tests as well as calibration chamber mini-piezocone tests on soils of intermediate permeability (silts, clayey and sandy silts). The penetration rate varied across over three orders of magnitude to provide information on partially drained and undrained tip resistance, excess pore water pressure and friction sleeve. Whilst previous experimental researches essentially focused on tip resistance and pore water pressure measurements, it is worthwhile underlying that the present study is one of the first experimental studies that explored the effect of penetration rate on sleeve friction measurements. As the penetration rate is reduced, moving from the undrained conditions to the fully drained conditions, friction sleeve systematically decreases, together with the expected results in terms of increasing tip resistance and decreasing excess porewater pressure. The obtained experimental database of penetration measurements on intermediate soils can be added to the previous worldwide collected data to develop a new general interpretation procedure for cone tests in transitional soils. Besides, numerical analyses have been carried out by using the Finite Element Method. The Updated Lagrangian technique has been adopted to simulate the large strain penetration process. Both the Modified Cam Clay constitutive model and the Mohr Coulomb model have been used to compare numerical simulation results with, respectively, the experimental results on kaolin clay (Randolph and Hope, 2004; Schneider et al., 2007) and those obtained with the present study. The problem of piezocone miss-interpretation in case of transitional soils, such as loose silt mixture has been dealt with an empirical methodology, based on the calibration of the Soil Behaviour Type index using soil characteristics inferred from reference boreholes. Moreover, a new approach has been proposed to overcome miss-interpretation of piezocone test results for soil layers belonging to vadose zones in which the effective stress state is controlled by suction. This procedure allows for the correction of the Soil Behaviour Type (SBT) index, in order to allocate correctly the investigated soils inside SBT classification charts (Robertson, 1990). In addition to that, the applied method has suggested a procedure, based on piezocone measurements, to estimate the effective stress state in the case of a homogeneous soil layer in which a vadose zone above the water table is present.