Takashi Sasaoka

Evolution and effect of the stress concentration and rock failure in the deep multi-seam coal mining

Supports crushing accident occasionally occurs in the protected seam exploitation of deep multi-seam coal mining structure and results in adverse effect to the production. To prevent its recurrence in a newly developed working field, a 3D numerical extraction model was built based on the geologic and mining conditions of Jining coal mine to evolve the changes, state and characteristics of the reconstructed vertical and lateral stress in rock interlayer after protective seam exploitation. Stress release and increase zones of this mining structure were separated. Mining-induced localized stress concentration and the interlayer rock failure behavior were explored. The action of concentrated stress on the hydraulic supports in protected seam was discussed upon the major stress redistribution. Using the infinitesimal strain method, a mechanical model was created to further explore, from the vertical and lateral directions, the cause and mechanism of localized stress concentration and rock failure behavior in rock interlayer. The field investigation was finally performed to verify the numerical and mechanical results, and the essential control measures were proposed to prevent this accident. Key findings of this study bring some new insights into the deep multi-seam coal extraction and help to promote a more reliable underground mining.

Geochemistry of waste rock at dumping area

Geochemical characteristics and the increase in the total reactive surface area of the sulphide-bearing rock due to weathering processes at dumping area, are expected to enhance the oxidation of sulphides, leading to the generation of acid mine drainage (AMD). The study of the changes of geochemical characteristics of sulphide-bearing rocks showed that the oxidation uniformly occurred at the surface layer of 2- and 10-year-old waste rock dumps. The changes of mineralogical composition were also investigated, particularly for the cases where significant formation of clay minerals occurred. The study showed that the formation of clay minerals after the weathering processes took place at the dumping area, suggested that the generation of AMD would be minimised because the interior of the waste rock dump provided a barrier to oxygen and water.

Application of the retained gob-side gateroad in a deep underground coalmine

Abstract Zhuji coalmine, a typical deep underground coalmine located in Huainan city, Anhui province of China, is characterised by rich gas, high in situ stress and low permeability. These geotechnical conditions cause serious problems including outburst, excessive gas emissions in the upper corner of the coal face, and low resource recovery resulting from large remaining coal pillars when using conventional mining methods. Retained gob-side gateroad (RGSG) is proposed to resolve these problems by eliminating the large coal pillars and extracting gas simultaneously. To achieve a scientific and reasonable application of the RGSG in the Zhuji coalmine, stress evolution in the surrounding rock of this roadway was studied using FLAC3D numerical simulation method. Then, a staged supporting strategy and corresponding parameters were proposed in terms of the characteristics of stress evolution. Finally, the effectiveness of the proposed method was discussed based on the field measurements and roadway maintenance was carried out for reuse during the adjacent panel excavation.

Development of a new covering strategy in Indonesian coal mines to control acid mine drainage generation: a laboratory-scale result

The waste dump of sulphide-containing rocks is one of the potential acid mine drainage sources, since it contains a huge amount of readily oxidised sulphide mineral, due to its exposure to air and water. The application of the dry cover system is regarded as one of the best practices since it prevents acid mine drainage of the waste rock dump at the surface coal mine. However, the implementation of the dry cover system in field practice has faced several obstacles due to the limited number of cover materials. The nature of geological condition is considered to be a controlled issue, whilst the problem is the mining method and equipment size. This article describes the acid generation mechanism and its control, application of cover system and the problems that are faced in Indonesian coal mines, whilst discussing the preliminary laboratory results of multi-layer cover systems. It finally proposes a new covering strategy in an attempt to overcome the problem.

Geotechnical issues in the application of rock bolting technology for the development of underground coal mines in Indonesia

The application of a rock bolting system for Indonesian underground coal mines has been attempted for several years without any success. Recently, two new underground mining trials were conducted in East Kalimantan, Indonesia, in order to investigate the possibility of using rock bolting to address the technical challenges of underground coal mines under weak geological conditions. Unfortunately, one of the trials failed and was given up due to a fatal roof fall accident. However, in another trial, even though roof falls have occurred within the working area, the bolted roadways showed reasonable stability over time. Moreover, a comprehensive monitoring system has been installed in order to monitor the ground behaviour and its characteristics. This application of a comprehensive monitoring system has enhanced the ability to increase the level of support and to react to changing/deteriorating conditions. This paper describes the conditions and geotechnical issues for underground mines in Indonesia, and discusses the applicability of the rock bolting system to address problems associated with weak strata conditions in Indonesia.

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.

Application of Coal Ash to Postmine Land for Prevention of Soil Erosion in Coal Mine in Indonesia: Utilization of Fly Ash and Bottom Ash

The increase in the number of coal-fired power plants with the increase in coal production and its consumption has caused the problem of the treatment of a large amount of coal ash in Indonesia. In the past studies, coal ash was applied to postmine land with the aim of improving soil conditions for plant growth; however, heavy rain in the tropical climate may cause soil erosion with the change in soil conditions. This study presents the effects of application of coal ash to postmine land on soil erosion by performing the artificial rainfall test as well as physical testing. The results indicate that the risk of soil erosion can be reduced significantly by applying the coal ash which consists of more than 85% of sand to topsoil in the postmine land at the mixing ratio of over 30%. Additionally, they reveal that not only fine fractions but also microporous structures in coal ash enhance water retention capacity by retaining water in the structure, leading to the prevention of soil erosion. Thus, the risk of soil erosion can be reduced by applying coal ash to topsoil in consideration of soil composition and microporous structure of coal ash.

A review study of predictive model blast vibration attenuation equation by using neural network as an evaluator

Over the years, a number of empirical attenuation equations (AEs) have been proposed. However, many established AEs are not accurate enough and sometimes they are confusing to use, particularly when the parameter associated with blasting and geological condition changes. Nowadays an accurate AE is an important requirement for a coal mine such as Kaltim Prima Coal (KPC) whose mining area in the East Kutai regency is located close to a residential area, the Sangatta town. In this study, several important and widely used predictors were used to predict peak particle velocity, while a back propagation artificial neural network is used as a comparator to evaluate the established AEs. Through this study, it is proposed that susceptibility assessment of conventional AEs be employed as tool to evaluate established AEs in a more adaptable way.

Interaction between physical and chemical weathering of argillaceous rocks and the effects on the occurrence of acid mine drainage (AMD)

The disintegration of rocks by weathering plays an important role in the occurrence of Acid Mine Drainage (AMD), which is the environmental problem caused by the exposure of sulfide minerals to water and oxygen. The weathering of rocks is, generally, classified into physical or chemical weathering. However, there are few studies that focus on the complex interaction between physical and chemical weathering of rocks and on the effects of the interaction on the occurrence of AMD. This paper elucidates the complex interrelation between physical and chemical weathering of rocks as well as the progress of AMD through leaching test and weathering test with argillaceous rocks taken in open-cast coal mine in Indonesia in addition to sample analysis before and after the wetting and drying cycle: the rock samples were exposed to oxygen and water during the cycle. The results indicated that the argillaceous rocks which consist of sulfide and/or sulfate caused chemical weathering with micro-cracks on the surface of rocks through the dissolution of soluble iron and sulfur during the occurrence of AMD. Additionally, physical weathering of rocks due to clay minerals was accelerated by chemical weathering with the development of cracks with the occurrence of AMD in the argillaceous rocks containing kaolinite and pyrite. Although weathering of rocks also accelerated AMD, it was concluded that the sulfur content, the form of sulfur and iron in rocks, and the supply of oxygen significantly contributed to the occurrence of AMD.

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.