Akihiro Hamanaka

Behaviour of grouting material used for floor reinforcement in underground mines

In underground coal mines, floor heave is a major problem that is caused by groundwater, high strata stress and weak floors. Controlling the stability of the roadways is the key to maintaining the safety and efficiency of underground mining. Appropriate cement grouting is an effective and attractive floor reinforcement technique that can control excessive floor heave and closure. Cement grouting also reduces the permeability of the rock mass and consequently controls the movement of groundwater into the roadway. Injecting cement grout into roadway floors provides a more uniform reinforcement throughout the floor. It may even be possible to reinforce an extremely fractured floor. In this respect, this technique is superior to floor bolting/dowelling. This technique may be performed either as a pre- or post- development reinforcement. It is very difficult, however, to reinforce a floor that is not heavily fractured by the injection technique. Therefore, before floor injection, floor blasting is performed in order to create an adequate number of fractures in the floor. Fracturing by blasting also relieves and improves stress conditions in the floor. From this point of view, the behaviour of grouting injection as floor reinforcement material is discussed. That is to say that the results of an injection analysis, an injection experiment and a viscosity measurement to understand the injection process of cement grout into soil voids were studied. The injection analysis was performed by combining two analyses, the analysis of the clogging process, and the analysis of the transportation process. As to the analysis of the transportation process, since a cement-based grouting material can be described as a Bingham fluid, the equation of plug flow related to a Bingham fluid inside a circular pipe, instead of Darcy’s law, was expanded and analysed. The injection experiment was performed under low-pressure on a decomposed granite sample. The injection s experiment was performed using three mixtures with cement/water ratios (C/W) of 1/10, 1/5, and 1/3.5, respectively, to examine the influence of variable cement/water ratios. The viscosity model is proposed from the result measured with the aid of Brookfield viscometer. To calculate yield stress, viscosity data obtained with the Brookfield viscometer was used. However, for the determination of plastic viscosity, because some cement particles will be in the state of aggregation and so on, the viscosity measurements obtained by the use of the Brookfield viscometer are overestimated in comparison with a theoretical equation that assumed that cement particles are dispersed in the solution completely. However, it is thought that cement particles cannot form aggregates easily in the soil voids or cracks. Therefore, the plastic viscosity model of grouting material is expected to follow the proposed theoretical equation. Finally, the assumption that the grouting material was injected in the state of laminar flow was verified by data obtained from the Lugeon test. The field data which are satisfied with this condition is simulated using the plastic viscosity and the yield stress model of grouting material.

Effects of Rock Mass Conditions and Blasting Standard on Fragmentation Size at Limestone Quarries

The size distribution of fragmented rocks depends on not only the blasting standard but also the mechanical properties, joint system and crack density of rock mass. As, especially, the cracks in the rock mass are heavily developed at the limestone quarries in Japan, the joints and/or cracks in the rock mass have big impacts on the blasting effects such as the size of fragmented rocks. Therefore, if the joint system and/or crack density in the rock mass can be known and evaluated in quantity, the blasting operation can be done more effectively, efficiency and safety. However, the guideline for designing the appropriate blasting standard based on the rock mass condition such as mechanical properties, joint system and/or distribution of cracks, discontinuities, from the scientific point of view, has not been developed yet. Therefore, a series of blasting tests had been conducted in different mines and faces, geological conditions and blasting standards in order to know the impacts of each factor on the blasting effects. This paper summarizes the results of blasting tests and describes the impacts of rock mass conditions and blasting standard on the size of fragmented rocks.

Preliminary Study on Design of Longwall Mining from Final Highwall at Mae Moh Lignite Mine in Thailand

The EGAT Mae Moh lignite mine is the largest open-pit coal mine in Thailand. The total geological and economical reserves of Mae Moh coal field are approximately 1,140 Mt and 825 Mt, respectively. The annual production is 12 Mton, which represents 90 % of the total coal production in Thailand, and tends to decrease gradually. In this mine, the development of underground mine from final highwall is considered after the open-pit mining operation is finished. Longwall mining is one of the common methods for extracting coal in various thickness of coal seams and is much more effective, productive and safer than any other mining methods. The productivity of longwall mining is potentially high because it is basically a continuous operation requiring fewer workers and allowing a high production rate to be sustained. Due to unfavorable geological and geotechnical conditions such as weak strata and huge final pit slopes in this mine, however, various studies on the introduction of underground mining systems have been conducted so far. Optimizing boundary and/or safety pillars and panel design for both safety and economics is a key part of planning longwall mines. This paper discribes the applicability of the longwall mining system from final highwall and its suitable design taking the slope stability into consideration by means of three-dimensional explicit finite-difference program FLAC 3D producted by ITASCA.

Study on Backfilling of Soil for Revegetation at the Rehabilitation Area in Indonesian Coal Mine

Mining operation of open cut mines gives serious impacts on surrounding environment such as disturbance of the tropical rainforest, the pollution of surface and/or ground water, subsidence and erosion. Rehabilitation is one of the important and considerable topics for an environment protection under such situations. Adequate rehabilitation program will solve these environmental issues and recover the ecosystem. The basic concept of rehabilitation in open cut mines is the creation of a stable and self-sustaining land surface in post mine surface involved the creation of conditions suitable for the establishment and permanent development of a dense cover of vegetation. In usual, a lot of open cut mine keep the topsoil individually in the natural forest during soil stripping process and spread the topsoil to rehabilitation area for success of revegetation. However, there are some considerable points for dealing with topsoil in situ such as the soil properties and severe compaction by heavy equipment. Therefore, the proper topsoil management has to be considered for establishment of vegetation in post mine surface.

Study on the Propagation of Blast-Induced Ground Vibration and Its Control Measure in Open Pit Mine

In modern mining industry, the blasting technique is one of the most adopted techniques for rock excavation and fragmentation due to its economical and efficient aspects. However, compared with other methods, the use of explosives in blasting is limited by law as it may have a severe impact on the surrounding environment such as vibration, noise and dust. Especially, a ground vibration induced by blasting has to be paid much attention in the mining operation as it may give an obvious impact on the surrounding facilities and buildings and In many established safe level blasting criteria, the ground vibration level is characterized by using three parameters; duration, amplitude (peak particle velocity :PPV) and frequency. Current studies on the blast-induced ground vibration focus on two parameters amplitude and frequency that suggested by USBM when the concerned subject is structure, i.e.. Hence, the control of PPV and dominant frequency are very important in order to design an appropriate blasting standard and minimize its environmental impacts.

Research on exploiting residual coal around final end-walls by highwall mining system in China

To improve the resources’ recovery ratio and the economic benefits of surface coal mines, we present a residual coal extraction technology from final end-walls with highwall mining system in China. Based on Mark–Bieniawski formula and tributary area theory, the evaluation methodology of highwall mining parameters under the thick cover was given. Furthermore, the pillar design chart in the highwall mining system under the thick cover was also concluded. In order to examine the rationality of the pillar design with the presented methodology, numerical simulation was also carried out. The outcome shows that it will require a larger pillar width with the thickening of the cover; hence, the recovery ratio will decrease correspondingly. To improve coal recovery ratio, the backfilling technology should be considered under the thick cover. According to two-dimension numerical simulation results, the calculated methodology for pillar stability of highwall mining system under the thick cover presented in this article is acceptable. It also found that keeping the pillar width-to-height ratio above 1 is a key measure to maintain the pillar stable and integral under the thick cover. Meanwhile, it can be concluded that the bigger safety factor is demanded for roof of openings under much thicker cover and higher stress ratio. In other words, the highwall mining system is feasible to extract the residual coal around end-walls in Chinese surface coal mines. Thus, not only coal recovery ratio can be improved but also can increase the production life of surface coal mines in China.

Optimization of Dump Bench Configuration to Improve Waste Dump Capacity of Narynsukhait Open Pit Coal Mine

In next two years, the current waste dump of Narynsukhait coal mine is predicted insufficient to accommodate the overburden as limited of the waste dump capacity. Thus, redesigning waste dump is paramount to increase capacity of the dump in future. This paper describes current condition of waste dump of Narynsukhait coal mine and then discusses the optimization of waste dump geometry by analyzing the effect of different waste dump’s bench configuration on slope performance. Optimization of the geometry is carried out by investigating and comparing the performance of geometrical combinations of bench height, bench angles and number of safety berm by means of numerical modeling. The model shows that increasing height of bench is able to induce shear stress in the bench and may initiate bench instable. However, the shear stress can be limited by having safety berm and/or reducing bench angle to satisfy the stability criteria.

Ex Situ UCG model experiments with oxygen enriched air in an artificial coal seam

Underground Coal Gasification (UCG) demands precise evaluation of the combustion area in the coal seam. Especially, the monitoring of fracture activity in the coal seam and around rock is important not only for efficient gas production but also for estimation of subsidence and gas leakage to the surface. For this objective, laboratory experiments were conducted using the simulated UCG models. This paper also investigated gas energy for coal consumption, the production gas quantity and heat value, the application of oxygen element balance in the gasification reaction process, and the gas composition obtained in this study. During burning of the coal, temperatures inside the coal, contents of product gases and acoustic emission (AE) activities were monitored successively under the control of feeding gas (air/oxygen and steam) flow rate. Comparison of the temperature variation and accumulated AE event curves revealed a close correlation between them. The local change of temperature inside the coal induced fractures with AE. The AE activity was related closely to the local changes of temperature inside the model. The evaluation of gas energy recovery calculated from the obtained product gas provided a fair evaluation for the coal consumed, and the quantity of gas product and calorific value obtained from the UCG process.

Numerical Study on Effect of Longwall Mining on Stability of Main Roadway under Weak Ground Conditions in Indonesia

The purpose of this research is to study the effect of longwall mining on the stability of main roadway in the underground coal mine. The PT GDM (Gerbang Daya Mandiri) underground coal mine in Indonesia, where the rocks are weak, was selected as a representative study site. To accomplish the objective of the research, the finite difference code software FLAC3D was used as a tool for the numerical simulations. The longwall mining of several panel and barrier pillar widths at various depths was simulated and discussed. Based on the simulation results, it indicates that the effect of coal panel extraction on the main roadway stability depends on the width of panel and barrier pillar. The greatest effect occurs when the large panel width and the small barrier pillar width are applied, whereas the smallest effect happens when the narrow panel width and the large barrier pillar width are adopted. In this paper, therefore, to maintain the stability of the main roadway with the aim of maximizing the coal recovery, the appropriate size of panel and barrier pillar width is proposed for each mining depth for this underground coal mine.

Feasibility of Chain Conveyor Cutter for Ocean Disposal Site’s Construction

The concept of ocean disposal site is proven technology. One of the most concerns in ocean disposal site is the leakage of pollutants into the ocean from waste materials. In order to prevent its happening, technically, a vertical impermeable wall is constructed around the disposal site. However, the conventional method fails to guarantee zero leakage due to method’s limitation. This paper presents the use of CCC (chain conveyor cutter) method to construct vertical impermeable wall. The feasibility of CCC for ocean disposal site is evaluated by means of pollutant dispersion model, and moreover, the result is compared with conventional method. The result indicates that the CCC method is technically feasible for ocean disposal site’s construction. Additionally, this method shows better as for technique and economic point of view than that of the conventional method. One of advantage of CCC method is the ability to construct homogeneous wall in regard to strength and permeability, which cannot be found in the conventional ocean disposal technology.