Effects of structure and strain localization in PFEM simulations of CPTu tests in a natural clay

The mechanics of cone penetration in CPTu tests in a silty clay was investigated with the PFEM, using a recently proposed finite deformation plasticity model for natural geomaterials – the FD MILAN model – which incorporates the bond strength Pt as an internal variable quantifying the effects of structure, and is equipped with a characteristic length ℓc to regularize the numerical solutions in presence of strain localization. The results of the PFEM simulations show that the deformations around the piezocone are strongly affected by the characteristic length. In particular, when ℓc/R is sufficiently small, the deformation field may be affected by clearly visible shear bands. The soil around the piezocone is subjected to a very strong destructuration process, which leads to the complete loss of bond strength in a large region around the cone tip and shaft. As a consequence, the use of literature empirical correlations in the interpretation of the CPTu test data in heavily structured soils could result in a significant underestimation of both the undrained strength and the overconsolidation ratio of the soil deposit.