Discrete element modelling of cone penetration testing in granular materials

Falagush, Omar (2014) Discrete element modelling of cone penetration testing in granular materials. PhD thesis, University of Nottingham.

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Cone penetration testing (CPT) is one of the most versatile devices for in situ soil testing. With minimal disturbance to the ground, it provides information about soil classification and geotechnical parameters. Several researchers have used different numerical techniques such as strain path methods and finite element methods to study CPT problems. The Discrete Element Method (DEM) is a useful alternative tool for studying cone penetration problems because of its ability to provide micro mechanical insight into the behaviour of granular materials and cone penetration resistance.

This study uses three-dimensional DEM to simulate the cone penetration testing of granular materials in a calibration chamber. Due to the geometric symmetry of this study a 90 degree segment of the calibration chamber and the cone penetrometer was initially considered followed by a 30 degree segment to allow for the simulation of smaller particle sizes and to reduce computational time. This research proposes a new particle refinement method, similar to the mesh refinement of finite-element modelling, in the sense that a large number of small particles were brought into contact with the cone tip, while the large particles were distanced further away from the cone, to reduce computational time effectively. Using a radius expansion method for sample preparation and assigning a constant mass to each particle in the sample was found to reduce computational time significantly with little influence on tip resistance.

The effects of initial sample conditions and particle friction coefficient were found to have an important influence on the tip resistance. In addition, prohibiting particle rotation was found to increase tip resistance significantly compared to when the particles were permitted to rotate freely. Particle shape in this study was simulated by replacing the spheres with simple two-ball clumps and was found to have an important effect on the tip resistance.

DEM simulations of biaxial tests were conducted to investigate the effect of initial sample conditions, particle shape and particle friction coefficient on the stress-strain behaviour of granular materials. All the above mentioned parameters were found to have a significant effect on the stress-strain behaviour of granular materials. Biaxial test simulations were also conducted to obtain basic granular material properties to derive analytical CPT solutions from continuum mechanics principles. Some of the DEM simulation results were found to be in good agreement with the analytical solutions that used a combined cylindrical-spherical cavity expansion method.

Particle crushing was simulated during the cone penetration tests by replacing a broken particle with two new equi-sized smaller particles with mass conserved. The results showed considerable reduction in the tip resistance for the crushing model compared to the non-crushing model and this reduction increased as the confining stress increased.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: McDowell, G.R.
Yu, H-S.
Keywords: Discrete element method, penetrometers, soil penetration test, granular materials
Subjects: T Technology > TA Engineering (General). Civil engineering (General) > TA 703 Engineering geology. Rock and soil mechanics
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Civil Engineering
Item ID: 14134
Depositing User: EP, Services
Date Deposited: 31 Oct 2014 11:05
Last Modified: 14 Sep 2016 17:09
URI: http://eprints.nottingham.ac.uk/id/eprint/14134

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