NUMERICAL MODELLING OF GEMSTONE EXPLOITATION USING EXAMINE 2D: A CASE STUDY OF KOMU, GEMSTONE DEPOSIT, OYO STATE, NIGERIA

Abstract

The Mohr- Coulomb and Hoek Brown models are the most widely used constitutive laws to describe the elasto plastic behaviour of rock materials under load. They are available in many numerical modelling codes including Phase2 and Examine2D, which were selected to perform the numerical modelling analysis in this research. The boundary element method was selected together with Hoek- Brown model to develop models of gemstone exploitation for the study. The design procedure for underground excavation in Komu mine field is developed through simulation using Examine2D, a mine design software which employs boundary element method of numerical analysis. Parameters defining in-situ field stress (σ1, σs and σz), rock mass elastic properties (modulus of elasticity and poisson’s ratio), and rock mass strength parameters. The uniaxial compressive strength of the pegmatite in the area is done with the formula UCS= Pmax APmax D 2)2 = 4PmaxπD2 . Models of differentdimensions (3 m x 4 m, 4 m x 5 m, 4 m x 6 m, 5 m x 6 m, circle of radius 5 m and modified cylindrical 4.716 m x 4.206 m) for the underground excavations were designed on Examine 2D using boundary element method of numerical analysis. Horseshoe models of 3 m x 4 m, 4 m x 5 m, 4 m x 6 m and 5 m x 6 m dimensions; circular model of of radius 5 m and modified cylindrical model of 4.716 m x 4.206 m dimensions are simulated for underground excavation. Results of the excavation models show that all the models indicate strength factor higher than 1 along the proposed exavation boundaries, which is the threshold value required for safe underground excavation. The circular model shows highest relative stability compared to the rest models; this is due to stress differential along excavation boundary with failure being propagated from the angular corners of the excavation, this renders the horseshoe models less favourable as the side walls forms a sharp angle with the invert and the crown of the excavation in the horseshoe shape design