An explicit method for in-situ determination of undrained elastic shear modulus using ROBOCONE torsional and vertical modules

An innovative site investigation tool, termed ROBOCONE, is being developed by integrating robotic modules capable of lateral, vertical and torsional displacement, into the shaft of conventional cone penetrometer. This study presents an explicit method for determining undrained elastic shear modulus by coupling the response of the vertical and torsional modules, so-called t–z and τ–θ modules respectively. A comprehensive finite element modelling framework was developed to investigate soil-ROBOCONE interaction and quantify the influence of module’s aspect ratio on measured stiffness. For each module, deformation components at the interface and in the surrounding soil were decomposed in order to evaluate the end effects on soil stiffness. Coupling the τ–θ and t–z module responses effectively eliminate the influence of interface stiffness – a parameter difficult to determine prior to testing. These findings enhance the efficiency of site investigation by enabling early-stage characterisation of soil properties in offshore projects without the need for offshore sampling and subsequent laboratory testing, while also providing practical guidance for optimizing ROBOCONE design.