Investigation of the causes of stope failure :

Abstract

The most prominent underground extraction techniques in Ugandan metalliferous mines are self and semi-artificially supported open stope mining. Taking a closer look at this, the integral part of the global stability of underground metal mines with numerous working levels and old workings within the same reef puts the crown pillars that stands in the vertical plane between the two open stopes on analysis, because they are significantly affected by the mechanical and physical properties of the rock mass, structural weakness, initial state of the horizontal stress and geometry of the pillar in fresh working zones, however in areas with old developments, new activities are affected by conditions of abandoned excavations. Considering stope development at Green Stone Resources Limited in Busia Eastern Uganda no integrated design and failure assessment criterion was available in its domain up to the date of this project but, generally development and failure mitigation criterion relied on the past experiences and rule of thumb. That led to short termed life of bored tunnels and continuous risky failure at stopes in the new shaft's ore extraction level 54m by the year 2014/2015. In this report practical identification of the causes of stope failure at GRL has been discussed and method(s) which will aid engineers in designing stopes with optimum life at any level in GRL considering all the local conditions has been developed. Country rock failure criterion, blast fragmentation extent, geometry of influential pre-existing excavations, and in-situ rock mass classification are principled parameters which has been assessed in detail. Chapter 2 of this work covers the basic failure theories, modified empirical and theoretical methods of stable stope design and failure assessment. These methods were used in determination of theoretical values for stope span and thickness of crown pillars and wall rocks required on ground and also computation of stress levels in pillars for the average 2.2m span of the new shaft drives and 6m crown pillar thickness between the old and new drives. Chapter 3 has the four phases that sighted techniques used for gathering and reducing field and laboratory data essential for stope failure analysis. The scaneline surveying used in the field to determine the structural formation of the rock mass and laboratory tests which were conducted to determine the mechanical properties of the rock mass. Chapter 4 entails results and discussion for surface excavation, old and new shafts mappings, natural rock structure joint survey and stereography, precipitation and dewatering, point load test and TCS, rock drilling, explosive strength and fragmentation analysis. That identified a fault zone striking the mine at 165 0 at a bearing of 015 0 with width >5m as the actual cause of stope failure. Chapter 5 is the derived conclusions and recommendations basing on the computed results from the analysis.