DEM study on mechanism of standard penetration test of sands

Compared to laboratory tests, in-situ tests such as the Standard Penetration Tests (SPT), which avoid sampling disturbance effects, have received widespread attention and application. However, the interpretation of in-situ test results often relies heavily on empirical approaches. Studying the mechanism of in-situ tests provides a theoretical foundation for interpreting the results, thereby offering an effective approach to addressing this issue. To elucidate the underlying test mechanisms, Discrete Element Method (DEM) simulations of SPT were conducted at five different test depths. Key macroscopic phenomena were systematically analyzed. The findings align with the established academic and industrial observations, demonstrating the validity of the simulations. By leveraging the advantage of DEM in providing detailed micro-scale information, this study examined the microscopic behavior of the soil. The particle displacements led to changes in the spatial distribution of inter-particle contacts, generating new force transmission channels. To further elucidate the test mechanism, a novel DEM stress calculation method is proposed to analyze the stress distribution in the soil. These newly formed channels facilitated stress redistribution within the soil, resulting in increased stress near the sampler tip. This strengthened the constraints on the soil, thereby increasing the resistance the soil could provide during subsequent deformations. Since the energy delivered to the soil per hammer blow remained nearly constant, higher resistance reduced the penetration depth per blow, thereby increasing penetration difficulty.