Grant-funded Project Nr. 330/2004/B-GEO/PøF
Final Report

Project title:2D-numerical modelling of soil and rock laboratory tests using distinct element method (DEM)
Research leader:Mgr. Karel Sosna, 2003
Co-researcher: Ing. Jan Boháè, CSc.; Mgr. Branislav Kuthan
Period of project:2004-2004
Overall grant:125 000 CZK

Project Results

Three principal models of laboratory tests commonly used for soil and rock were modelled in 2D space using the software PFC2D.  (Particle Flow Code). Variuos features of granular materials were analysed. Calibration of individual numerical parameters to the mechanical behaviuor of real soil or rock material will be carried out later. Numerical tests did not involve water or any other influence by fluid matter.

Three main models test were simulated in accordance with real soil sample geometry:
1) Biaxial test model with axial load and constant confining stress with following results:
- peak strength  (reached after 2-4 % of axial strain) is a function of the magnitude of confinig stress
- Young´s modulus is directly linked to the values of numerical particle stiffnesses
- peak stength is influenced by bond strength and friction coefficient (only higher friction produces peaks in axial stress)
- volume changes are controlled by the initial sample porosity, dilatancy occured when porosity was smaller
- evoluton of shear bands can be uniquely identified by displacement field and strain inhomogeneities
- loss of strength within bonded particles is a function of continuous debonding (bond breakage)
- frictional sliding is mostly relevant within unbonded particles

2) Oedometer test model with axial load and rigid confinement (stationary walls)
- modulus is controlled by the friction coefficient when over 1 % of strain is reached
- initial sample porosity limits the final strain but does not influence stiffnes
- loading curve is linear when unbonded particles are used, unloading results in the increase of modulus
- stress-strain curve for bonded particles is nonlinear as a result of bond breakage, bond strength is the limiting parameter

3) Direct shear test model with axial load and shear straining applied on the left and right wall
- the sample exhibits peak and residual strength
- lower initial porosity induces higher values of peak friction angle
- shear bands can be distinguished by displacement field, accumulated rotation of particles is higher
- streess-strain curve evolution is related the shear rate (lower shear rate generates lower straining)  

The following paper is being prepared:
Kuthan, B. (2005). Numerical model of a laboratory test using PFC2D, Geotechnika – in preparation.