Evaluation of the stability and suitability of artificial concrete pillars in Longhole open stoping: a case study of al Masane al Kobra mine, Saudi Arabia.
Date
2024
Authors
Mabeti, Daniel
Journal Title
Journal ISSN
Volume Title
Publisher
The University of Zambia
Abstract
This study has demonstrated that artificial concrete pillars can stabilize the active stopes, especially in broader sections of the orebodies intercepted with geotechnical challenges such as high-stress change, spalling, slabbing, sloughing, etc. FLAC3D numerical code was successfully used to simulate the stopes before and after installing artificial concrete pillars. The challenges encountered in long-hole open stoping were briefly highlighted in the statement problem of this study. The structure of the study is also briefly stated in the initial chapter of the thesis. Literature
on stope stability and pillar design theories was also reviewed. The Pillar Strength and Stress theories were also examined as the basis for artificial concrete pillars. The available constitutive models in the FLAC3D suitable for evaluating the stability of stopes and artificial concrete pillars are also discussed, and their shortcomings are noted. It was pointed out that the Hoek-Brown failure criterion was the best for analyzing rock mass properties compared to other available criteria. FLAC3D numerical code has also been described in detail, stating the steps involved in
the numerical simulation, input parameters required, boundary condition, commands for excavation, convergence involved, and interpretation of the results. Two case studies were presented, one of which involved mining a sill pillar and the other mining a crown pillar. Both case studies presented similar geotechnical challenges, such as poor rock mass, increased hydraulic radius, and collided ground support. Scaling and re-supporting were conducted before installing artificial pillars in the two stopes. Artificial concrete pillars were installed in these two stopes. The study has presented the construction of artificial concrete pillars for installation. Materials required for constructing artificial concrete pillars are aggregate
(54.5%), and (27.3%) Cement (18.2%) and reinforcement wires. The water and cement ratio must also be maintained at 0.5 of cement slurry. Cement slurry and aggregate mixing were done by using a front-end loader. The artificial concrete pillars at 28 days of age should bear above 24Mpa as a requirement to be transported underground. This challenges quality control and assurance if the design mixing ratio is not achieved, as the concrete blocks will crumble during transportation or at the packing stage. Loading and unloading of the packs was done using telehandler.
After the concrete packs are installed in the stope, welded wire mesh is also installed around, and afterwards, 75-100 mm thick shotcrete is sprayed. Geotechnical data was collected through underground mapping, laboratory testing, and field measurements. Underground mapping aided in defining the materials to be used in the FLAC3D numerical code. At the construction stage, samples were extracted from the mixed concrete and taken for laboratory testing. Uniaxial compressive strength, density, Young's modulus, internal friction angle, and cohesion were tested on these samples. The material properties were used as input parameters in FLAC3D numerical code to simulate the stability of the stope before and after artificial concrete pillars were installed. The results for zones of displacement magnitude, local force ratio, state by average failure and convergence were
considered in the analysis comparing before and after installing artificial concrete pillars. The results for zones of displacement with artificial concrete pillars installed presented a reduction in both cases by 97% and 99 % in cases 1 and 2, respectively. Zones' local force ratio presented an 85% and 71% reduction in cases 1 and 2, respectively, when installing artificial concrete pillars. Also, the zone convergence results showed a further reduction of 80% in case 1 and 63% in case 2 when artificial concrete pillars were installed. Zone state failures by average also showed a
significant improvement in tensile and shearing in the roof and sidewalls of the two stopes. Field measurements were conducted on the installed convergence pins using a tape extensometer. Both scenarios indicate a substantial 70% reduction in displacements before and after for the crown pillar's roof and a 60% reduction for the sill pillar. Notably, the sidewalls in both cases experienced a significant 10% reduction for the crown pillar and a 5% reduction for the sill pillar. These results underscore the effectiveness of the author's approach, reinforcing the success of the research and enhancing structural stability. The above analysis has revealed that the designed artificial concrete pillars can be effectively used as a support system to prevent roof collapse in active stopes. FLAC3D numerical simulation and field measurements showed the stope's stable condition after installing artificial concrete pillars. Therefore, the designed artificial concrete pillars can be used to meet the needs of safety of mining operations; however, real-time stability monitoring of the concrete pillars must be emphasized at some key positions. The investigation has also revealed that artificial concrete pillars can replace original ore pillars in many mines worldwide; the problem encountered with such is the self-failure mechanism of artificial concrete pillars, which has been puzzling the safety of operations in many mines.
Description
Thesis of Doctor of Philosophy In Mining Engineering