Mechanism of sample disturbance caused by tube penetration: Model tests on Toyoura sand

Abstract

The mechanism by which samples are disturbed as the sampling device penetrates the soil during sample collection was examined based on pore water pressure behavior, void ratio (e), and the movement of soil particles in model tests on Toyoura sand. Samplers with tubes with inner diameters of 35, 45, and 75mm and cutting edge angles of 6° and 90° were used for the tests. The penetration speed (Sp) was in the range of 0.6–5.8cm/s, while the relative density (Dr) was in the range of 6%–83%.All the soil particles inside the tubes, with the exception of those close to the tube wall, shifted simultaneously due to local disturbance, which caused a lubricating effect. The distance from the tube wall (Dw) to which the soil was affected by wall friction during tube penetration was about 0.35–1.35mm, similar to that of clayey material, which is nominally less than 2mm. Moreover, the results were consistent with those for the tube sampling of Niigata sand deposits, for which Dw was unrelated to the tube diameter and Dr. This is contrary to the common belief that the movement of sand particles and changes in e are small for a 90° cutting edge angle in a tube 45mm in diameter. However, this phenomenon can be explained by the existence of an area of disturbance. For Toyoura sand, this area of disturbance was formed more easily by a 90° cutting edge angle than by a 6° cutting edge angle.