Discrete element modeling (dem) of cone penetration testing on soil with varying relative soil density

Authors: SYED, ZAMIR - Iowa State University; TEKESTE, MEHARI - Iowa State University; Way, Thomas - Tom

Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: 3/10/2017
Publication Date: 7/16/2017
Citation: Syed, Z., Tekeste, M., Way, T.R. 2017. Discrete element modeling (dem) of cone penetration testing on soil with varying relative soil density. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE). 1701608:11.

Interpretive Summary: Soil cone penetration resistance is measured using a steel cone which is pushed vertically down into the soil. During this process, the vertical force pushing downward on the cone and the depth of the cone beneath the soil surface, are measured. The cone penetration resistance is useful in determining soil firmness and the extent to which the soil has been compacted. Discrete Element Modeling (DEM) is a computer simulation method which simulates a large number of particles and their interactions with solid surfaces which contain them or pass between them. DEM was used in this project to simulate a steel cone, having a cone apex angle of 30 degrees and a base diameter of 12.5 mm, penetrating into a sandy loam soil. These computer model results were compared with laboratory measurements made with an actual cone penetrating into actual soil. When the initial density of the soil was relatively low, meaning the soil was relatively loose, the cone behavior predicted by the DEM simulation was in close agreement with the laboratory measurements. When the initial soil density was relatively high, there was less agreement between the DEM simulation and the laboratory measurements. These results are expected to be helpful in improving the application of DEM to soil-machine interactions, and in promoting the accuracy and usefulness of simulation for equipment development.

Technical Abstract: Modeling soil-tool interaction is essential for equipment design and performance evaluation on soil behavior responses under loading. Computational tools based on particle-based mechanics such as Discrete Element Modeling (DEM) and Smoothed Particle Hydrodynamics (SPH) have potential in modeling large strain soil dynamic behaviors from soil-tool interaction. The objective of this study is to validate the accuracy and robustness of DEM calibration methodology as it relates to soil deformation during cone penetration on varying initial soil relative density. The influence of factors such as DEM material properties and cone to particle size ratio, on DEM cone penetration simulation are investigated. The paper presents a comparison of DEM-predicted cone penetration resistance and laboratory-measured penetration data for a Norfolk sandy loam soil. Soil mechanical behavior was modeled with a Hertz-Mindlin (HM) contact stiffness model and a new coupled frictional law for static and rolling resistance coefficients. The DEM material properties were calibrated using residual strength from a direct shear test. DEM simulations were performed using LIGGGHTS (LAMMPS improved for general granular and granular heat transfer simulations) open source DEM code. Cone penetrometer experiments using an ASABE standard cone with 12.5 mm base diameter and a 30-degree cone tip were used to validate the calibrated DEM model. DEM prediction of the cone penetration resistance trend and steady-state values are in close agreement with the laboratory-measured data for a relative density range of 5 to 30%. At higher density states (relative density of 90%), the DEM calibration requires further improvement.