Tunnel Contour Quality Index in a drill and blast tunnel: Definition, analysis and effects
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The drill and blast (D&B) method is applied in the excavation of a wide range of underground spaces, such as tunnels, rock caverns and mines. It has been progressing rapidly to reduce construction time and cost with the introduction of more effective and accurate hydraulic and computerized drilling jumbos, higher capacity of loading and transport equipment, and more advanced explosives and detonating systems. Especially, the development of technology in the drilling jumbo and measuring equipment made it possible for tunneling engineers to perform more sophisticated and comfortable operations by the feedback from recorded data. The result from blasting in a drill and blast tunnel is generally evaluated through pull percentage (the ratio of actual pull length to drilled length per round), the level of induced vibration and noise, and quality of the excavated contour. Among these, the quality of the contour, which means the condition of the tunnel perimeter, may be characterized by overbreak/underbreak and contour roughness. Many researchers have been performing various experiments and studies to optimize contour quality since it is directly related to construction time and cost as well as management costs in unlined water tunnels. In addition to studies on the mechanisms which determine contour quality, another important subject with regards to contour quality is to establish ways to be able to quantify and characterize it for more detailed analyses. However, there has not been any concept developed for this purpose, although some indices to evaluate the result of a tunnel blasting or ways to reduce overbreak have been proposed. Hence, this thesis presents an index, TCI (Tunnel Contour Quality Index) to evaluate the contour quality of traffic tunnels or underground spaces. TCI is defined as a fraction form consisting of one constant for range adjustment, three elements and three weights for the elements. Each element consists of one correction factor and one parameter: either total overbreak, the ratio of contour length (RCL) or longitudinal overbreak variation for considering increased cross sectional area, increased contour length and the variation of the increased area in the longitudinal direction respectively. RCL can be estimated by total overbreak and deviation difference (average difference of deviation between two neighboring measured points in a tunnel section). TCI is classified into TCIT for the evaluation of an entire tunnel or more than five blasting rounds, and TCIR for the evaluation of only one or each blasting round. In addition, the index for head loss used for a water tunnel, where the increase in cross sectional area results in the decrease of head loss, is also suggested. The effects of drilling and rock mass on contour quality were analyzed, with the help of drilling information produced automatically from computerized drilling jumbos and software, on the basis of field investigations. The result shows that when total overbreak is classified, according to the causes, into the overbreak by starting position of the contour hole, the overbreak by look-out and the overbreak by other factors, the three causes account for approximately 40% : 50% : 10% of total overbreak respectively, for the Q-value range between 1 and 100. Overbreak by other factors, deviation difference and longitudinal overbreak variation generally decrease with higher Q-value or RBS (Relative Block Size, RQD / Jn, Jn : rating for the number of joint sets), although the correlation between them is quite low. And the smallest overbreak by other factors and deviation difference were found when the joint orientation was between 30° and 0°, and between 60° and 90° respectively, to the left of the tunnel excavation direction. Estimation models for total overbreak and deviation difference are suggested on the basis of rock mass quality (Q-value or RBS) and drilling conditions that are given from drilling logs. In the end, TCI can be estimated with the models prior to blasting. The estimation models for shotcrete and muckpile volume are suggested using the models as well. To check the effect of TCI on construction time, the existing simulation tool based on the NTNU (Norwegian University of Science and Technology) construction time estimation models has been upgraded so that it is able to estimate the construction time including rock support, in an Excel spreadsheet. Construction time and advance rate were analyzed with the variation of Q-value for seven sizes of a road tunnel, using the software. It is found that the decrease of TCIR by 10 units results in an increase of 5.2% in cycle time. The study may be divided into three parts in general, for which different approaches have been applied; The first part describes the definition of TCI, and has been performed on the basis of a literature study, schematic analysis, site investigations and assumptions. The second part focuses on the analysis of major factors affecting TCI, as well as the estimation models for TCI, shotcrete volume and muckpile volume. The analysis was made based on site investigations and a statistical approach. The third part describes the effects of TCI on construction time, based upon the definition of TCI as well as upgraded construction time estimation models. Lastly, to make TCI into a more effective index for the evaluation of contour quality, analyses on TCI distribution according to various types of tunnels are necessary. Based on the distribution, a guideline or specification as well as the weights of TCI having scientific grounds need to be provided for the better management of tunnel contour quality.