Browsing by Author "Modika, Moses"

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    A holistic approach to the design of two tonnes temporary support systems (nets, mechanical props, hooks and roof bolt face plates)
    (University of the Witwatersrand, Johannesburg, 2024) Modika, Moses
    In 2019, Thembelani Mine experienced a high number of injuries caused by Falls of Ground (FOGs) in the face area. To address this issue, safety nets with a capacity of two tonnes were urgently introduced. The two tonnes safety nets were intended to hold FOGs with a maximum mass of 1.78 tonnes, thereby providing a factor of safety of 1.1. However, in 2020 a FOG accident occurred during a drilling shift when a rock with a mass of 1.65 tonnes fell, leading to the death of an employee. The square headboard mechanical props with headboard dimensions of 130 mm x 130 mm, which were supposed to hold the safety net, were pulled out by the FOG, thereby allowing the safety net to collapse with the FOG. The slipping out of the square headboard mechanical props was due to a smaller headboard surface area of 130 mm x 130 mm in contact with hangingwall and a pretensioned load of 0.86 tonnes to 1.43 tonnes which could not sustain a falling rock with a mass of 1.65 tonnes. As a result, the square headboard props were immediately replaced with T-shaped Grease box mechanical props with average pretension load of 2.23 tonnes, larger and longer headboards of 370 mm x 100 mm to increase resistance to slipping out. The T-shaped Grease box mechanical props come with 16 mm flexible G-hooks attached to the headboards. The T-shaped Grease box mechanical props were tested at a drop test facility together with two tonnes Rock- stop safety nets and found to be able to hold two tonnes mass when installed at a standard spacing of 1.5 m x 1.5 m. This indicated that components of the temporary support systems other than the two tonnes Rock-stop safety net were not considered during the roll out. These components include 12 mm diameter flexible G-hooks, roof bolt face plates with a thickness of 4.5 mm, and square headboard mechanical props with welded 12 mm U-hooks and headboard dimensions of 130 mm x 130 mm. Therefore, it was decided to conduct a study to test the temporary support systems holistically using installation scenarios as practiced at Thembelani Mine as opposed to testing each of the constituents of the systems individually. The aim of the tests was to assess if all components fulfil the requirements of the two tonnes temporary support systems from system test point of view. iv While conducting the study, it was found that the safety nets were introduced without considering all aspects of the temporary support systems, such as safety net deflection parameters, hooks, mechanical props, and roof bolt face plates. Moreover, it is necessary to maintain a safe working height within a specific stoping width, which is the vertical distance between the footwall and the safety net in the hangingwall after holding a falling rock. This is to ensure that the employee underneath the net does not get struck by the FOG which deflected with the safety net. The study aimed to identify the items that did not meet the two tonnes systems' requirements and recommend improvements to ensure they function as expected. The study was conducted at available drop test facilities such as NCM and Videx, simulating the installation scenarios as practiced at Thembelani Mine. The study revealed that the two tonnes Rock-stop safety nets, along with 6 mm face plates, 14 mm Carbon steel S-hooks, or 14.5 mm Spring steel G-hooks, fulfilled the requirements for the two tonnes systems. The two tonnes Rock-stop safety nets and T-shaped Grease box mechanical props with flexible 16 mm G- hooks also fulfilled the two tonnes’ requirements. The study determined that the systems that fulfilled the two tonnes’ requirements were effective for all stopes at Thembelani Mine with heights of 1.2 m and 1.4 m and flat excavation ends with a height of 3.2 m, with deflection limits of 0.35 m, 0.55 m and 1.3 m respectively. In 2022, an alternative to the safety nets, the MINAX 80/3 Blast-on Mesh (BOM) with 8 mm blast-on cable, was introduced. The BOM underwent a similar study to the Rock-stop safety net, and the drop tests showed that the deflections obtained with the BOM in various scenarios as installed at Thembelani Mine were all above 0.35 m but less than 0.55 m. Hence, BOM can be used in stopes and flat excavations with heights of 1.4 m and 3.2 m, respectively.