A new interpretation method for matric suction correction to cone penetration test results in unsaturated sand

Understanding the effects of matric suction on the tip resistance (qc) profiles from cone penetration tests (CPTs) is of great importance for correct estimation of unsaturated soil properties. However, CPTs in unsaturated soil profiles have rarely been investigated, especially for cases with varying suction (or moisture content) with depths, making the interpretation of the qc data challenging. In this study, a new and practical approach was developed to interpret the qc profiles in unsaturated samples by viewing the unsaturated soil profile as a two-layer system distinguished by the varying moisture content and separated at the depth where air-entry suction was found. A series of centrifuge CPTs were conducted in uniform unsaturated sand profiles prepared at different relative densities (DR), all with a controlled water table that generated a hydrostatic suction distribution. For a given DR, the presence of suction increases the values of qc substantially, especially in the transition zone of the sand’s water retention curve. The suction effects are more prominent for looser sand. A resistance ratio (η) is newly defined, and the excellent match between the measured and calculated data shows that η can be used to correct the unsaturated qc profiles to the saturated counterpart, hence allowing the use of existing CPT correlations to estimate the soil property values.