Pitfalls in interpretation of cone penetration test data recovered from unsaturated geomaterials

Abstract

Laboratory-controlled and field cone penetration test (CPT) results for unsaturated geomaterials (sands, silty sands, tailings) show that suction has a pronounced influence. The penetration resistances increase significantly due to the presence of suction when compared to those for saturated or dry states, for a given relative density and net confining stress. For unsaturated sands, when suction is included in the effective stress, the same semi-empirical expressions used for saturated conditions are found to link penetration resistance to the relative density and effective confining stress. However, the same is not true for unsaturated silty sands, as saturated and unsaturated silty sands behave very differently around a penetrating cone. Suction also has implications when using classification charts. Cone penetration test results from a range of soils and tailings for both saturated and unsaturated conditions are plotted in one type of chart. Suction-induced increases in penetration resistances, and changes of soil behaviours from partially drained/undrained to drained cause incorrect classifications and assessments that geomaterials are dilative. Failure to account for suction influences, and the altered material response, may also lead to overestimation of cyclic resistance ratio.