Research On The Rock Mass Quality Assessment Of Underground Excavation And Reinforcement Scheme Optimization Techniques

The greatest threat to the personal safety and the normal operation of the construction is posed by the instability of the rock mass surrounding the underground excavation. In order to assess and improve the stability of the rock mass, many effective methods and measures are necessary to be taken to analyse and control the deformation and instability of the rock mass surface, in the mean while, it is important to strengthen the structure of surrounding rock of underground excavation engineering research, to predict and control the stability of the surrounding environment. Therefore, many researchers and engineers have been introduced and developed. All of these methods have limitations, but they are valuable tools if applied appropriately and with care. In this paper, many assessment systems are introduced, particularly the Tunnelling Quality Index (Q) system and the Key Block theory, which are applied based on the same rock mass geotechnical parameters of the Sandton South Shaft on the Gautrain Tunnel, with the aim of evaluating their application conditions and limiting factors, achieving an optimal reinforcement scheme at last.Based on an understanding of the structural geology and stress regime in the field of the underground construction, the Sandton South Shaft was constructed entirely within the Archaean Granite Suite at a shallow level, and the North and East walls of the vertical shaft were constructed near a diabase-granite contact. In addition, some effective drainage mitigation measures are implemented. Therefore, in this situation, the underground rock mass stress and the groundwater have only minor effects on the stability of the shaft.The conclusions of the paper including:(1) Sufficient knowledge and understanding of the tectonic, hydrogeology and stress field is necessary before evaluating the underground excavation rock mass stability, as well as analysis the elements which could impact the stability of engineering;(2) As the most widely used and the most simple evaluation method for engineering rock mass stability assessment, Q system could generally divide the shaft into different sections in accordance with the engineering rock mass quality for stability evaluation and reinforcement scheme design. But because of the existing limitations, the safety and support efficiency of the reinforcement scheme in certain part by using the Q system alone is not reasonable;(3) The Key Block theory just supplies the gap of the Q system. It could optimize the existing reinforcement scheme by analyzing the instability in certain part. By applying the Dips and Unwedge programs, the ultimate reinforcement scheme can be achieved, which could guarantee both the efficiency of the scheme and the safety of the underground excavation;(4) During the process of optimization, the Key Block theory is not suitable for non-blocky or plastic rock mass, and it is unuseful for the instability problem in the corner or the edge;(5) The Unwedge program is difficult to search the accurate position of Key Blocks. Thus, the Key Blocks determined by the Unwedge program should be examined by in situ reconnaissance;(6) This thesis combined two different methods together for rock mass stability assessments, which belong to different classifications. It not only guarantees the stability of underground rock mass, but also improves the support efficiency of materials. The optimized the ultimate reinforcement scheme satisfied both the overall and local stability.In the end, the paper discussed the possible trends in future.