Abstract

This paper explores the merit of including cyclic episodes in cone penetrometer tests to measure changes in sleeve resistance during cycling. Tests were carried out in a geotechnical centrifuge in a kaolin clay and a dense silica sand. The data from the tests in clay indicate that the cone sleeve mobilises the remoulded undrained shear strength during the initial penetration, but that with continued cycling, the mobilised interface shear strength reduces to around one-third of the remoulded undrained shear strength before increasing. The initial reduction is considered to be due to local consolidation around the cone sleeve (leading to total stress reduction), whereas the subsequent increase is attributed to consolidation- induced strength hardening. Tests in sand also indicate a reduction in sleeve friction during cycling, consistent with the shear band contraction mechanism associated with friction fatigue of piles in sand, but with no consolidation-induced hardening behaviour as these tests were drained. The findings suggest that cyclic cone penetrometer tests may be a convenient means of gathering data for geotechnical design for problems where the cyclic response at the clay/structure interface is of interest. Considerations on the implications for offshore site investigations are provided.

Abstract

Classical Cone Penetration Test (CPT) or CPTu (when water pore pressure is also measured) can provide so far only strength parameters of soils, specifically the tip resistance and the lateral friction. This article presents the numerical simulation in a (virtual) calibration chamber, using the Discrete Element Method (DEM), of a CPT-based test proposed in the quest for possibilities to determine soil deformability parameters as well. It is a non-standard test characterized by force-controlled cycles applied to the penetrometer tip that is movable independently of the penetrometer body. Very small irreversible displacements of the tip are observed over the first cycles whose amplitudes span a region of low fractions of the tip resistance, that is subsequently assimilated to a pseudo-elastic domain within which, deformation moduli can be derived from the slopes of the force-displacement curve properly interpreted. Results also reveal a loading level beyond which, these slopes and the corresponding deformation moduli, significantly decrease while the irreversible displacements of the tip increase substantially.