Nonlinear interpretation of fall-cone tests in K0-consolidated clay

The fall-cone test is widely used for estimating the undrained shear strength of soft clays, yet its conventional interpretation is largely based on an empirical proportional relationship between soil strength and cone penetration. The applicability of this relationship to clays formed under one-dimensional stress histories remains insufficiently understood. This study investigates the fall-cone response of normally consolidated Malaysian kaolin prepared under K 0 conditions. Fall-cone tests were performed using two cone configurations, while undrained shear strength was independently evaluated through unconsolidated-undrained triaxial compression, laboratory vane shear, unconfined compression and miniature cone penetration tests. The results show that the conventional proportional relationship does not fully capture the variation of undrained strength with the penetration response. Instead, a power-law relationship provides a more consistent description of the observed nonlinear behaviour. This relationship is reproduced across the different independent strength measures, indicating that the nonlinear trend is not dependent on a particular reference test. The results also show that the strength–penetration relationship varies with cone configuration. Specifically, the fitted power-law exponents were d = 1.4326 and 2.0413 for the 45° and 60° cones, respectively. Cross-validation and comparison with published fall-cone data provide further support for the proposed interpretation, while also indicating that soil state and stress history may influence the penetration response. These findings suggest that fall-cone measurements in K 0 -consolidated clay are better interpreted through a cone-dependent nonlinear strength–penetration relationship than through a single proportional calibration, providing a more appropriate framework for undrained strength assessment under one-dimensional consolidation conditions.