Seismic analysis of horseshoe tunnels under dynamic loads due to earthquakes

Due to seismic events, such as earthquakes, the elastic waves propagate through a medium. The impact of these waves on underground structures is to provide dynamic forces and moments that may affect the stability of underground structures. The aim of this paper is to analyse the effects of seismic loads on the stability of horseshoe tunnels. As a case study, the stability state of the main access entry to C1 coal seam of Tabas collieries in Iran are analyzed using Phase2 software in static and dynamic states. It is often assumed that the effect of earthquakes on underground structures such as tunnels is negligible but the results of this study show that the stress caused by seismic loads can be harmful to the tunnel stability. It is concluded that the stress and displacement balance of forces around the tunnel are adversely affected and due to redistribution of these forces that create undue concentration in some areas, instability occurs in the tunnel. The paper also concludes that increasing the stiffness of the support system can increase the effect of the seismic loads. The analysis provided in this paper together with the conclusions obtained can serve as useful tools for the tunnel design engineers, especially in areas susceptible to seismic phenomena

The Evaluation of Empirical Coal Pillar Strength Formula Based on Uncertainty Criterion

Several empirical equations to estimate coal pillar strength have been presented in academic studies. The development processes of these equations are similar and are usually obtained by fitting the mathematical function (curve) to field data. One of the best criteria to evaluate the quality of fitting for such equations is the correlation coefficient R2, which has limited applicability. It is necessary to calculate the correlation coefficient access to the initial data for which the equation is presented; this is impossible for many coal pillar strength formulas. This paper presents a new approach based on the analysis of uncertainty amplitude to compare the coal pillar strength. This approach utilizes a combination of parameters such as Mean Squared Error (MSE), Root Mean Squared Error (RMSE), function type and degrees of freedom. The confidence level of constants is subsequently formed and the correlation coefficient becomes more comprehensive. Therefore, for an effective comparison, the efficiency and accuracy of coal pillar strength formula can be used

3D strain softening modelling of coal pillars in a deep longwall mine

In longwall coal mines, the entries on both sides of the panel play a significant role in production rate and safety of operation. With increasing production amount, the rate of conveying material through such entries increases. Therefore, it is required to design wider entries. Support of these entries, particularly in deep mines is difficult. In this paper, by using FLAC3D program code a deep longwall coal mine is modeled. The coal seam has a strain softening property, and the analysis index of stress and deformation of ribsides and coal pillars at different loading levels are determined. Strain softening parameters is studied separately for each modeled coal pillar, and based on conventional formula the pillar strength are calculated. In a modeled longwall mine, the caving material at goaf zone are fully compact. The results shows that based on Mohr-Coulomb model, the strain softening occurs at maximum cohesion and friction, and at region of decreasing the strength of pillar in stress-strain curve. Because of 3D nature of analyses, the effect of front and side abutment load on stability of pillar are studied simultaneously. Therefore, the results of this study could be suitable criteria for appraisal of pillar design method at deep longwall coal mines

Estimation of Coal Pillar Strength by Finite Difference Model

Longwall mining is now predominately used in coal mines where somewhat difficult conditions exist. As in the case of all other underground mining methods, pillars are integral parts of the mining design. The choice of shape and dimensions of the pillars has significant impact on the recovery and hence on overall productivity of the mine. The process of pillar design in longwall mining entails the selection of a safety factor, which is done by estimating the magnitude of the load applied on the pillar and the load bearing capacity of such pillars. In this paper, finite difference modeling principles have been applied to a typical coal pillar. The pillar strength is then estimated with various width/height ratios. These results have been compared with the results obtained from the conventional methods of pillar design. The effect of roof and floor quality on the strength of the typical pillar has also been evaluated in the same manner. It is concluded that although the finite difference method is not always the perfect method for such estimation, but the results clearly demonstrate that it produces more acceptable design than the conventional method, especially under undesirable conditions regarding the interface between pillars, roof and floor. An additional advantage of such method is shown to be its capability of being applied in situations where complex parameters prevail

A New Approach for Determination of Tunnel Supporting System Using Analytical Hierarchy Process (AHP)

In underground mining, the selection of support system for mine tunnel development plays a significant role in safety and economics of operations. Traditionally, such selection is on the basis of the experience of the design engineer. Nevertheless, the validity of such selection is questionable. A new approach for selecting the optimum tunnel support system based on Analytical Hierarchy Process (AHP) is proposed. In this new approach, the selection of the tunnel support system is considered as a multi criteria decision-making problem. Firstly, by using the numerical Finite Difference Method (FDM), based on technical and stability parameters of the tunnel, different support systems are specified. Then, by considering the economics and performance indices of each support system, a decision tree based on AHP model is generated and the optimum support system is selected. As a field study, the method is applied to Tabas collieries in Iran. It is concluded that the proposed support system determination is advantageous compared to other alternatives. Therefore, the proposed approach can assist the engineer in selection of optimum tunnel support system in different underground mining situations

Optimization of Chain Pillars Design in Longwall Mining Method

Chain coal pillars are parts of the structure of longwall mining system that play a significant role in the stability of the entries. With mechanization and developments in the various aspects of the method, higher efficiency in optimization of the design of chain coal pillars seems appropriate. In this paper, the three main methods of chain pillar design, namely the empirical, analytical and numerical methods are compared. Real data from the Tabas-Iran coal mines have been used in order to make the comparison process reliable. It is concluded that the most apparent advantage of the empirical method is the reliability of the results whilst the use of numerical methods enjoys the advantage of flexibility. On the other hand, the analytical methods are complex unless simplifying assumptions are made that can substantially decrease the accuracy of result which is thought to be the main advantage of the design method. A new method is therefore introduced here that combines all of the three presently used methods and by doing so, the new method has all the advantages of the three while minimizing complexities and inaccuracies associated with the use of the individual methods

Selection of Tunnel Support System by Using Multi Criteria Decision-Making Tools

Selection of the optimum support system for underground openings such as tunnels is a complex process. In this paper, a new approach, based on a combination of the Analytical Hierarchy Process (AHP), the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) and the Preference Ranking Organization METHod for Enrichment Evaluations (PROMETHEE) is introduced. For this purpose, the selection process is assumed to be a multi criteria decision-making problem. First, different support systems by using FLAC3D numerical code, based on technical, safety and stability parameters of the tunnel are specified. Then, taking economic and performance parameters as the decision criteria, by using the combination of AHP, TOPSIS, and PROMETHEE, the optimum support system is selected. As a real mine case study, this approach is used in the main access entry to C1 coal seam of Tabas collieries. Results clearly demonstrate that the proposed support system selection method is advantageous to other alternatives

Incorporating cut-off grade optimization and stockpiling into oil sands production scheduling and waste management.

In achieving maximum benefit in oil sands mining, the long-term production schedule should have the time and sequence of removing ore, dyke material and waste from the final pit limit. An optimum cut-off grade profile and stockpiling will ensure the segregation between these materials meet economic and regulatory requirements. In-pit waste management strategy for oil sands mining requires dyke construction to occur simultaneously with the advancement of mining operations. This research seeks to determine: 1) the optimum life of mine cut-off grade profile and its corresponding tonnages; 2) the time and sequence for removal of ore, dyke material and waste to maximize NPV; 3) the dyke material schedule for dyke construction to minimize construction costs; and 4) the associated impacts of stockpiling and stockpile reclamation with limited time duration. Cut-off grade optimization was used to generate an optimum grade schedule which specifies the cut-off grade, duration of mining of the grade and tonnage mined during the mine life. A heuristic framework, referred to as the Integrated Cut-Off Grade Optimization (ICOGO) model was developed in this research. It generates an optimum cut-off grade policy and a schedule for mining ore and waste, as well as overburden, interburden and tailings coarse sand dyke material for long-term production planning. Subsequently, a mathematical programming framework based on Mixed Integer Linear Goal Programming (MILGP) model was developed to generate a detailed production schedule for removal of ore, waste and dyke materials from the final pit limit. Stockpiling scenarios investigated during the study include: i) no stockpiling; ii) stockpiling and reclaiming at the end of mine life; and iii) stockpiling for one year or two years prior to reclamation. The developed models were applied to two oil sands case studies to maximize the Net Present Value (NPV) of the operations. In both case studies, the NPV generated by the ICOGO model for one year stockpiling scenario was higher than other stockpiling scenarios. For the MILGP the NPV generated for the two year stockpiling scenario was higher than the one year stockpiling scenario. In comparison, whereas the ICOGO model solved the optimization problem faster, the MILGP model results provide detailed mining-cut extraction sequencing for mining.Master of Science (MSc) in Natural Resources Engineerin

Tomografia pasywna pola prędkości i symulacje geostatystyczne w obrębie pola ścianowego

Generally, the accurate determination of the stress in surrounding rock mass of underground miningarea has an important role in stability and ground control. In this paper stress redistribution around thelongwall face has been studied using passive seismic velocity tomography based on Simultaneous IterativeReconstructive Technique (SIRT) and Sequential Gaussian Simulation (SGS). The mining-induced microseismicevents are used as a passive source. Since such sources are used, the ray coverage is insufficientand in order to resolve this deficiency, the wave velocity is estimated in a denser network and by the SGSmethod. Consequently the three-dimensional images of wave velocity are created and sliced into the coalseam. To analyze the variations of stress around the panel during the study period, these images are interpreted.Results show that the state of stress redistribution around the longwall panel can be deduced fromthese velocity images. In addition, movements of the stressed zones, including front and side abutmentsand the goaf area, along the longwall face are evident. The applied approach illustrated in this paper canbe used as a useful method to monitoring the stress changes around the longwall face continuously. Thiscan have significant safety implications and contribute to improvements in operational productivity

Productivity improvement from economic concept to an engineering tool

In broad terms, productivity is defined as the ratio of all outputs to all inputs. In this paper, productivity is distinctly used as an engineering tool and from practical viewpoint. With such engineering look, productivity is an adequate tool for evaluation of the advancement in competitive market, work assessment, profit/loss analysis, decision and even developing or changing the activity. Productivity measurement seems to be an easy task but this is a misconception. In fact the concept remains to be one of the most complex and unknown criteria. It is for this reason that attempts have been made here to accurately define productivity and hence simplify its measurement. A case study has been adopted and the productivity of Eastern Alborz Coal Mines in Iran has been calculated for years 2001-2008. The resulting values and the component models are then subject to analysis. These results are examined in terms of practicability and it is shown that the method prescribed is a pragmatic approach in all similar system situations