Numerical Study on Roadway Stability under Weak Geological Condition of PT Gerbang Daya Mandiri Underground Coal Mine in Indonesia

This paper aims to assess the roadway stability of the PT Gerbang Daya Mandiri (GDM) underground coal mine. A numerical analysis method using 3D finite difference code (FLAC 3D) was used to investigate the failure zone behavior of the roadway at various overburden depths (50 m, 100 m, 200 m, and 300 m). The outcome of this research was the most appropriate support system of the roadway. The results of numerical analyses indicated that the excavation depth affected the thickness of failure zone, and the capacity of the support system was significantly associated with an increase of the overburden depth. Steel set, cablebolt, and rockbolt supports were assessed in this paper. The steel set is selected as the main support system in GDM coal mine, and it is effective to stabilize the roof and sidewalls of the roadway until 200 m depth. As the failure zone becomes larger at the deeper sites, the cablebolt support is introduced to control the floor stability, and the use of rockbolt in combination with steel set is suggested to support the roof and sidewalls

EVALUATION OF ROCK MASS QUALITY BASED ON ROCK MASS RATING AND GEOLOGICAL STRENGTH INDEX METHOD FOR TUNNEL CONSTRUCTION IN PIYUNGAN-PATUK AREA, YOGYAKARTA SPECIAL PROVINCE, INDONESIA

The tunnel excavation plan is proposed to solve the transportation problem in Piyungan-Patuk area, which is located in the eastern part of Yogyakarta special province, Indonesia. This tunnel will be built in the area that contains various types of rock, i.e. andesitic lava, andesitic breccias, pumiceous breccias and tuffaceous sandstone. Before this underground structure will be constructed, the main problems that may occur during the excavation need to be considered which relating to rock mass instabilities. Thus, to minimize and avoid those problems, it is necessary to do the detail site investigation related to geological conditions in order to estimate the rock mass quality in this area. This research is mainly aimed to evaluate the rock mass quality by applying two empirical methods such as Rock Mass Rating (RMR) and Geological Strength Index (GSI). The purpose of RMR system is to classify the quality of rock mass by using surface and subsurface (borehole) data, in order to guide the excavation method and also to give the recommendation of tunnel supports as well as the unsupported span and stand-up time. Furthermore, GSI system is not only aimed to estimate the rock mass quality, but also the modulus of deformation (Ed), compressive strength (�c) and tensile strength (�t) of the rock mass. Moreover, according to RMR and GSI method, the relationship between RMRbasic and GSI rating is able to conduct in form of linear equation in order to compare the rock mass quality condition between both methods. Besides, depended on the rock mass conditions in the study area, this research has also tried to figure out the engineering potential risks that may occur during the tunnel construction and attempted to suggest the appropriate method to control and prevent such those potential risks. Based on RMR determination by using surface data, the qualities of rock masses in the study area are classified to be good and fair rock, the good rocks are found on andesitic lava and andesitic breccias (RMR=62-77, unsupported span is 8.3 m, stand-up time is 333.33 days

A Stope Mining Design with Consideration of Hanging Wall When Transitioning from Open Pit Mining to Underground Mining for Sepon Gold Mine Deposit, Laos

This study investigates the transition from surface to underground mining at the Sepon Gold mine. The stability of surface slopes is assessed prior to commencing underground operations. Stope mining is adopted based on ore body characteristics and geological features. Finite element numerical analysis, employing the Generalized Hoek–Brown criterion, evaluates slope stability for surface slopes and opening stopes. The pit design comprises a 70° slope angle, 20 m height, and 10–15 m safety berm, meeting stability requirements with a factor of safety of 2.46. Underground mining design includes a 62° ore body dip, a 50 m thick crown pillar to prevent surface subsidence, and 5 m wide, 5 m high stopes. Sill pillars are left for support after each level of excavation. Rock bolts are employed in specific stope areas where necessary, with continuous monitoring for surface deformation. This study analyzes the influence of stope sizes on the pit wall and pit bottom stability, identifying slope failures near the hanging wall close to the pit bottom during underground mining. A slight increase in the displacement zone is observed on the upper crest of the top footwall. Overall, the findings demonstrate successful stability in the transition from surface to underground mining at the Sepon Gold mine

PREDICTION OF MULTI-SEAM MINING-INDUCED SURFACE SUBSIDENCE IN UNDERGROUND COAL MINE IN INDONESIA

This paper attempts to predict the surface subsidence induced by multi-seam longwall mining in the PT Gerbang Daya Mandiri (GDM) underground coal mine in Indonesia. Several numerical models of multi-seam longwall mining under various depths were built in the finite difference code software “FLAC3D” which was used as a tool for numerical simulations. Effect of mining sequence and influence of lower seam mining were firstly investigated. The angle of draw (AoD) and maximum surface subsidence (S_) were used to describe characteristics of the surface subsidence. Based on simulated results, it is indicated that the undermining provides a better mining sequence in multi-seam longwall mining compared to the overmining. Mining the coal seam in an undermining order will not cause any difficult mining conditions in a lower seam, whereas some ground control problems in an upper seam are expected when the coal seam is mined in an overmining order. Under all mining depths in the undermining, extracting the lower seam panels significantly influences the magnitude of surface subsidence. The AoD and S_ increase significantly after all panels in the lower seam is mined. This indicates that very large surface subsidence is expected when multi-seam mining is applied at GDM underground coal mine. An application of some countermeasures such as adopting a large pillar width and a small panel width is suggested in this underground coal mine in order to minimize the surface subsidence caused by multi-seam longwall mining. Minimizing the surface subsidence by adopting a large pillar width and a small panel width is therefore numerically investigated in this paper. Based on simulated results, it is found that the AoD and S_ decrease significantly when larger pillar width and narrower panel width are adopted. The use of larger pillar width and narrower panel width result in smaller AoD and S_

Study of Characteristics of Surface Subsidence in Longwall Coal Mine under Poor Ground Conditions in Indonesia

This paper aims to study the characteristics of surface subsidence induced by longwall mining under poor ground conditions in Indonesia by means of numerical simulation techniques using finite difference code “FLAC3D”. The effect of mining depth in cases of single panel and multi-panel longwall mining, the influence of panel and pillar widths, and the impact of backfilling material, were incorporated into the FLAC3D software. The simulated results indicated that the angle of draw and maximum surface subsidence were significantly associated with the depth of mining, the number of extracted panels, the width of panel and pillar, and the type of backfill. In single panel mining, the largest maximum surface subsidence is observed in case of the shallowest mining depth, and it gradually decreases as the depth increases. In contrast, the angle of draw increases with increasing the mining depth. In multi-panel mining, the angle of draw and maximum surface subsidence increase as the mining depth increases. Moreover, the angle of draw and maximum surface subsidence decrease when the narrow panel and large pillar widths are adopted, and the backfilling materials are applied

STUDY ON STABILITY CONTROL OF COAL MINE TUNNEL EXCAVATED IN WEAK ROCK MASS IN INDONESIA

This paper focuses on the stability analysis and support design of the coal mine tunnel excavated in weak rock mass in an Indonesian underground coal mine through numerical simulations using the FLAC3D software. The PT Gerbang Daya Mandiri (GDM) coal mine situated in Indonesia was selected as a mine site in this study. According to the results of a series of numerical simulations, the stability of the mine tunnel decreases by increasing the depth and stress ratio. Ground control problems, for example falling roof, sidewall collapse, and floor heave are expected unless an appropriate support system is anticipated. Three support systems, including friction rockbolt, steel arch, and shotcrete are discussed as methods to stabilize the roof and sidewalls of the mine tunnel. From the simulated results, the steel arch is considered to be the most effective support method when compared with other support systems. The steel arch which is installed with closer space and larger cross-section delivers a better stability control to the roof and sidewalls of the mine tunnel. Although the stability of the roof and sidewalls of the mine tunnel can be maintained effectively by the steel arch support, the occurrence of floor heave is expected when the mining depth is increased. To control the floor stability of the mine tunnel, three techniques by applying cablebolt, invert-arch floor, and grooving method are therefore investigated and discussed. Based on simulated results, the heaving of the floor is well controlled after the cablebolt, invert-arch floor, and grooving methods are applied. Nevertheless, it is found that controlling the floor heave by cablebolt support could be the most suitable method comparing with other support systems in terms of the installation process, providing flat and safe working conditions of the floor, and economy. Additionally, the cablebolt with closer row space and longer length works more effectively to control the heaving problem of the floor

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

This paper aims to assess the roadway stability of the PT Gerbang Daya Mandiri (GDM) underground coal mine. A numerical analysis method using 3D finite difference code (FLAC 3D) was used to investigate the failure zone behavior of the roadway at various overburden depths (50 m, 100 m, 200 m, and 300 m). The outcome of this research was the most appropriate support system of the roadway. The results of numerical analyses indicated that the excavation depth affected the thickness of failure zone, and the capacity of the support system was significantly associated with an increase of the overburden depth. Steel set, cablebolt, and rockbolt supports were assessed in this paper. The steel set is selected as the main support system in GDM coal mine, and it is effective to stabilize the roof and sidewalls of the roadway until 200 m depth. As the failure zone becomes larger at the deeper sites, the cablebolt support is introduced to control the floor stability, and the use of rockbolt in combination with steel set is suggested to support the roof and sidewalls

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