Undrained shear strength estimation through instrumented free-fall penetration tests using spherical and conical penetrometers

Abstract

This paper presents (i) results from a series of instrumented free-fall penetration (FFP) tests performed, under laboratory-controlled conditions, in a large-scale testing facility and (ii) a simple methodology to predict undrained shear strength su from FFP test data without resorting to assumptions on the values of different parameters. Comparison of soil resistance derived from acceleration data recorded during FFP tests with soil resistance recorded during constant rate penetration (CRP) tests using a ball penetrometer reveals unique relationships between dynamic and constant-rate penetration resistances for different shapes of FFP tools. Based on regression analysis, expressions are proposed for calculation of resistance ratio ξr for three different shapes of FFP tool. The proposed expressions for ξr further facilitate the prediction of undrained shear strength su employing acceleration data recorded during FFP tests. FFP-based predictions of su show excellent agreement with CRP-based su prediction in three different laboratory testbeds. Furthermore, the proposed methodology successfully predicts su profile at a field test site using FFP test data reported in the literature.