Soil Failure Mechanisms and End-Bearing Factors of Dynamically Conical Penetrometers in Clay: LDFE-Based Insights

Dynamically conical penetrometers, such as free-fall penetrometers (FFPs) and dynamically installed anchors (DIAs), are promising solutions for offshore site investigation and anchoring in clay. However, the influence of soil failure mechanisms on the end-bearing factor remains insufficiently understood, which limits the reliability of dynamically installed anchor (DIA) design and the interpretation of free-fall penetrometer (FFP) test data. This study aims to investigate the soil failure mechanisms around dynamically conical penetrometers and their influence on the evolution of the end-bearing factor (Nc) over a wide embedment range up to 50 times the penetrometer diameter, using 3D large deformation finite-element (LDFE) analysis. Three distinct failure mechanisms around the dynamic penetrometers are identified for the first time: shallow embedment failure, trapped cavity mechanism, and deep failure mechanism. The transition depths between these mechanisms increase with the normalized undrained shear strength and the aspect ratio of the penetrometer. According to soil failure mechanisms, the evolution of Nc is divided into four stages: (1) initial increase during shallow embedment failure; (2) stabilization at a shallow depth; (3) linear growth governed by the trapped cavity mechanism; and (4) stabilization under the deep failure mechanism. Additional analyses show that increasing soil rigidity raises Nc and extends the critical stabilization depth linearly in shallow failure. Based on these findings, a new procedure is proposed to estimate Nc considering evolving soil failure mechanisms, aspect ratio, and soil rigidity. The findings contribute to a more accurate interpretation of soil strength in FFPs and to a more precise calculation of end resistance during DIA penetration.