Effects of thickness of roof layers on optimum design of truss bolt system using finite element modeling techniques

In underground excavations, optimum design of reinforcement systems is largely based on geological features of the surrounding rock such as in-situ stress distribution, rock strength properties, thickness of the layers, etc. In current design of truss bolt systems these parameters are yet to be considered. In this study, effects of changing thickness of roof layers on optimum design of truss bolt have been investigated using three stability indicators, namely reduction in the loosened area above the roof, number of plastic points and horizontal movement on the first bedding plane. Total of 7 different bedding configurations have been generated and 100 different truss bolt designs have been tested on each bedding configuration. Results showed that by changing the thickness of the roof layers, the optimum design of truss bolt changes drastically. In highly laminated formations, it has been demonstrated that a gently inclined bolt angle is more effective, while by increasing the thickness of roof layers, truss bolt systems with a higher bolt angle and longer bolts, i.e. similar to systematic rock bolt systems, responds better

Strata Movement and Fracture Propagation Characteristics due to Sequential Extraction of Multiseam Longwall Panels

Multiseam longwall mining-induced strata deformation and fracture propagation patterns are different from those of single-seam mining. This difference is due to interaction of the caved zones as a result of longwall mining activity at different coal seams, which severely impacts formation of subsidence and permeability of the strata after multiseam mining. Understanding this phenomenon is of great importance in order to predict the multiseam subsidence reliably, evaluate the risk of water inrush and take suitable preventive measures, and determine suitable locations for placing gas drainage boreholes. In this study, scaled physical modelling techniques are utilised to investigate strata deformation, fracture propagation characteristics, and vertical subsidence above multiseam longwall panels. The results show that magnitude of the incremental multiseam subsidence increases significantly after multiseam extraction in comparison with single-seam mining. This increase occurs to different extent depending on the multiseam mining configuration. In addition, interstrata fractures above the abutment areas of the overlapping panels propagate further towards the ground surface in multiseam extractions compared with single-seam extractions. These fractures increase the risk of water inrush in presence of underground/surface water and create highly permeable areas suitable for placing gas drainage boreholes

A study on truss bolt mechanism in controlling stability of underground excavation and cutter roof failure

The truss bolt reinforcement system has been used in controlling the stability of underground excavations in severe ground conditions and cutter roof failure in layered rocks especially in coal mines. In spite of good application reports, working mechanism of this system is largely unknown and truss bolts are predominantly designed based on past experience and engineering judgement. In this study, the reinforcing effect of the truss bolt system on an underground excavation in layered rock is studied using non-linear finite element analysis. Different indicators are defined to evaluate the reinforcing effects of the truss bolt system. Using these indicators one can evaluate the effects of a reinforcing system on the deformation, loosened area, failure prevention, horizontal movement of the immediate layer, shear crack propagation and cutter roof failure of underground excavations. Effects of truss bolt on these indicators reveal the working mechanism of the truss bolt system. To illustrate the application of these indicators, a comparative study is conducted between three different truss bolt designs. It is shown that the design parameters of truss bolt systems, including tie-rod span, length, and angle of the bolts can have significant effects on the reinforcing capability of the system