Shallow Penetrometer Penetration Resistance

Abstract

Shallow penetrometers—such as the hemiball and toroid—were conceived as potential in situ testing devices with the ability to measure: (1) soil strength parameters during vertical penetration, (2) soil consolidation characteristics during dissipation tests postpenetration, and (3) interface friction during torsional loading. Knowledge of the response of soil to such tests is critical to the design of subsea pipelines and the ability to measure the response of soil to all three types of test using a single device in situ from a mobile testing platform, such as a remotely operated vehicle (ROV), would be highly advantageous. Potential benefits of the employment of such devices could include significant time and cost savings and improved spatial measurement density, since more tests could be conducted along the route of a pipeline if an ROV is used as a mobile in situ testing platform. This paper presents an assessment of the ability of the hemiball and toroid to measure soil strength parameters directly from their response to vertical penetration. A large deformation finite-element approach was employed to model the penetration process and initial simulations were validated against small-strain analyses published in the literature. A comprehensive parametric study was then conducted investigating the impact on normalized penetration resistance of soil unit weight, shear strength gradient and penetrometer-soil interface friction. A forward model was derived from the parametric analyses and its inverse performance (i.e., the ability to infer soil parameters from force-displacement response) was assessed using additional large deformation analyses with randomly assigned material parameters within realistic bounds. Both variants of shallow penetrometer investigated are found to be well suited to inferring soil strength parameters directly from their response to vertical penetration.