Experimental study of cone penetration in silica sand using digital image correlation

The problem of cone penetration, particularly deep penetration, remains one of the most challenging in geotechnical engineering. It involves large displacements, rotations and deformation of soil elements in the path of the cone as well as complex response of the soil, including crushing and the development of large mean stresses, to the displacements imposed by the penetration process. As a result, rigorous theoretical solutions are not available for this problem, and experimental simulations of penetration provide insights that would not otherwise be available. This paper presents the results of a series of cone penetration tests performed in a half-circular chamber in sand samples prepared with three silica sands with different crushability. Cone resistance was measured, and digital images of the cone penetrating into the sand samples were acquired simultaneously during the entire penetration process. The digital image correlation (DIC) technique was then used to process these images to obtain the soil displacement field resulting from cone penetration. The results of DIC analyses and measured cone resistance suggest that the soil displacement around an advancing cone depends on the density and crushability of the sand, as well as the depth of penetration. Tests on silica sands with different degrees of crushability show that, for shallow penetration, the displacement vectors near the cone tip are essentially vertical for crushable sand, transitioning to subvertical for less crushable sands. However, for deep penetration, the displacement vectors near the cone tip are mostly vertical below the cone tip. Crushing was observed immediately below and around the cone tip for all sands tested. After passage of the cone, the crushed particles form a thin, crushed particle band of thickness equal to about 2·5D₅₀ along the shaft, with a smaller percentage of crushed particles observed within an outer band with thickness equal to 4D₅₀.