The study sought to establish the applicability of rock mass classification as a primary input to wall control blasting at a hard-rock mine in Botswana. Conventional rules of thumb are used to develop blast designs based on parametric ratios with insufficient consideration of the rock mass factors that influence the achievability of final wall designs. Control of the western highwall of the mine’s pit had proved to be challenging in that the designed catchment berms and wall competence were perpetually unachievable from the pit crest to the current mining levels. This exposed the mining operation to safety hazards such as local wallrock failure from damaged crests, frozen toes, and rolling rockfalls from higher mining levels. There was also an effect of increased standoff distances from the western highwall, which reduce the available manoeuvring area on the pit floor and subsequently the factor of extraction that is safely achievable. The study investigated the application of rock mass classification and the blastability index (BI) as a means to improve wall control. This was achieved by establishing zones according to rock type forming the western highwall rock mass, in which the distinguishing rock mass classification factors were categorized and used to establish the suitable wall control designs through a design input tool (DIT). The DIT consolidated rock mass classification methodologies such as the Geological Strength Index (GSI) and the Rock Mass Rating (RMR) and related them to the BI and discontinuities of the rock mass to produce a tool that can be used to develop objective wall control designs. The designs developed by the tool inherently take into account the rock mass characteristic factors at the centre of the rock mass classification methods above and significantly reduce the dependence on rules of thumb. It was found that this approach yields designs with explosive energy measures that are consistent with the rock-breaking effort and the behaviour of discontinuities, all the while remaining biased towards preservation of perimeter wall rock. The work presented in this paper is part of an MSc research study at the University of the Witwatersrand.
Highwall mining involves driving a series of parallel entries with web pillars in between. These entries are driven by using a continuous miner with attached conveying system to extract locked up coal behind the highwall slope when the open cut coal mine reaches its ultimate limit. These entries are driven unmanned, unventilated and unsupported. Therefore, a detailed knowledge of structural features ahead of operations is essential in assessing the stability of these entries. Highwall mining operations can greatly benefit from accurate structural mapping of rock mass defects. The stability of these entries can be suitably assessed for any major roof failure by adopting discrete fracture network based structural modelling to characterize and delineate the regions of possible major roof failures to prevent damage to the conveyor and burial of the expensive continuous miner. In this paper, a generalized framework is described based on photogrammetric survey, digital mapping and discrete fracture network based structural modelling to characterise such failures for an Indian highwall mining operation. A sensitivity analysis is undertaken to demonstrate the significance of structure persistence in the geotechnical assessment. Such analysis would provide more insight into designing highwall mining layouts and in predicting possible impending highwall failures, and indirectly facilitates reducing machine downtime for better management of highwall mining operations.
Highwall mining operations offer the opportunity to improve coal extraction rates significantly provided geotechnical risks are managed properly. These risks include slope failure as well as roof collapse within the highwall entries induced by local failure, potentially resulting in loss of the continuous miner and conveying system. The Guidelines for Open pit Design  presents detailed recommendations for the management of uncertainty and risk in open cut slope design and the work described in this paper takes place within this framework. In this paper, we describe a framework that uses photogrammetric survey, digital mapping and analysis, stochastic structural modelling for the stability analysis of structurally controlled failures at an end user site in India. This study uses data previously acquired at an Indian coal mine site .
2. Site details
Sasaoka , T. (Kyushu University) | Shimada, H. (Kyushu University) | Hamanaka, A. (Kyushu University) | Sulistianto, B. (Institute Technology Bandung) | Ichinose, M. (Center of Urban Infrastructure, Environment and Resources) | Matsui, K. (Kyushu University)
Indonesia is the second largest coal exporter to Japan. Almost all the coal is produced from open cut mines. However, a great deal of coal is left in the highwalls of the mined-out pits. The use of highwall mining systems has increasingly come into play in the US and Australia, when overburden depth exceeds economical recovery in open cut mine. However, the stability of the highwalls is always a major safety concern in such mining. This paper describes the characteristics of the highwall mining system and discusses the appropriate highwall mining system for use in Indonesia.
Indonesia produces over 400 Mt of clean coal and is the second largest coal exporter to Japan, accounting for about 32 Mt of coal annually. Over 99% of the coal produced in Indonesia comes from surface mines. More open cut mines will be developed and more coal will be mined in order to fill the great demand both in Indonesia and the rest of the world.
In open cut mines in Indonesia, there are many sites where mining operations have developed long highwalls which have been abandoned due to the current economics. Mining operations have been transferred to lower stripping ratio blocks of coal or overlying seams. In some cases, coal lies buried beneath spoil heaps or is covered with mud and water. It is estimated that there is a great deal of coal beneath abandoned and working highwalls. Considering these situations, it seems to be worth introducing highwall mining systems into Indonesian open cut mines.
Final highwalls of open cut mines can form the starting point for other mining methods, such as highwall or underground mining. In its basic application, highwall mining is a technique utilized after an open cut portion of a reserve has been mined, sometimes prior to the introduction of underground mining (Seib, 1993). In this system, the coal seam is mined by remotely operated equipment, such as an auger machine or a modified continuous miner incorporated into a highwall mining system. Major issues of highwall mining systems are less coal recovery due to leaving coal as the pillars and the instability of openings and highwalls due to the pillar and roof failures.This paper describes the characteristics of the highwall mining system and discusses the appropriate highwall mining system for Indonesia.
Karekal, Shivakumar (CSIRO Earth Science and Resource Engineering) | Subramanian, Srikrishnan Siva (Central Institute of Mining and Fuel Research) | Porathur, John Loui (Central Institute of Mining and Fuel Research)
Highwall mining operation involves driving a series of parallel unsupported, unmanned and unventilated excavations into a coal seam exposed at the open pit Highwall using a remotely operated continuous miner with attached conveying system. These parallel excavations are separated by web pillars of pre-designed width which are critical to the Highwall mining operations. The Highwall slope must remain stable during Highwall mining operation to ensure safety of workers and machinery. In this paper, Highwall slope stability is investigated with respect to different Highwall mining parameters using FLAC3D numerical modeling software. The parameters included in the study are: (i) single seam and multiple seams Highwall mining excavations with different width to height ratios; (ii) different Slope angles; (iii) different excavation heights; and (iv) different cover depths. A narrow strip of rock mass is considered by taking a plane of symmetry. The modeling results reveal that stability of open pit slopes have profound influence on the Highwall mining parameters, and the web pillar design can affect the stability of Highwall slopes. In designing Highwall slopes for an open pit, the design must include Highwall mining excavations, otherwise, near critical failure slopes could become critical and fail with Highwall excavations. In authors’ knowledge, this work is the first attempt at exploring the effect of Highwall mining parameters on overall slope stability.
Porathur, J.L. (Central Institute of Mining and Fuel Research) | Karekal, S. (Central Institute of Mining and Fuel Research and CSIRO) | Roy, P.P. (Central Institute of Mining and Fuel Research) | Verma, C.P. (Central Institute of Mining and Fuel Research)
Abstract: Highwall mining is a mechanized and productive method compared to conventional underground mining methods. It leads to added recovery of coal which is otherwise lost in the pit below the highwall. The major challenge envisaged is however the designing of web pillars in complex Indian geomining conditions, invariably comprising of multiple seams, some in close proximity, and multiple-pass extractions in thick seams. The method has been recently introduced in India in December 2010 with the operation of ADDCAR Continuous Highwall Mining Machine at Ramagundem Opencast Project-II (OCP-II) of Singareni Collieries Company Ltd. (SCCL). After successful extraction of two seams at OCP-II, the machine is now extracting at neighbouring Medapalli Opencast Project (MOCP) of SCCL.
The assessment of the stability performance of open pit highwalls has traditionally been based on consideration of surface movement rates. Although generally accepted in industry, this approach presents challenges for establishing appropriate threshold values to allow for safe mining operations and does not address the issue of total strain. Surface movement monitoring data from a limited number of open pit mining operations in North and South America has been evaluated in terms of highwall strain, rock mass quality and inferred mechanism of instability. Threshold strain levels for key stages of highwall stability proposed from previous work has been reviewed and are endorsed based on evaluation of additional data. An empirical highwall strain criteria for the assessment of highwall stability performance is postulated based on relatively consistent trends in highwall strain data.
L'evaluation du niveau de stabilite des hauts murs de carrières a traditionnellement ete basee sur les taux de mouvements de surfaces. Bien que cette approche soit generalement acceptee à travers l'industrie, elle presente des defis pour la definition de seuils appropries concernant la securite des operations minières et n'adresse pas la notion de deformations totales. Des donnees de mouvements de surface provenant d'un nombre limite d'operations minières à ciel ouvert d'Amerique du Nord et du Sud ont ete evaluees en terme de deformation des hauts murs, de qualite de la masse rocheuse et des mecanismes inferes d'instabilite. Des seuils de deformation pour differentes phases clefs de stabilite de hauts murs ont ete revus et sont endosses, base sur l'evaluation de donnes additionnelles. Un critère de deformation empirique pour l'evaluation de la stabilite des hauts murs est postule, base sur des tendances relativement consistantes de donnees de deformations de hauts murs.
Die Beurteilung der Stabilitat von hohen Felsböschungen basiert traditionellerweise von Oberflachenbewegungen. Obwohl dieses Verfahren im Allgemeinen akzeptiert wird, stellt es jedoch gewisse Probleme fuer die Festlegung von Grenzwerten fuer die Sicherheit dar. Die totalen Dehnungen werden bei dieser Betrachtungsweise vernachlassigt. Eine beschrankte Anzahl von Oberflachenbewegungs Daten von Bergbauwerke in Nord- und Suedamerika sind in Bezug auf die Dehnungen von hohen Felsböschungen, Felsqualitat und davon abgeleiteten Bruchmechanismen untersucht worden. Grenzwerte fuer die Belastung von hohen Felsböschungen wahrend den massgebenden Bauetappen aus bisherigen Projekten sind begutachtet und durch zusatzliche Untersuchungen bestatigt worden. Gestuetzt auf relativ konsitenten Trends, ist hier ein empirisches Verfahren der Dehnungen in hohen Felsböschungen zur Beurteilung der Stabilitat vorgestellt.
Open pit mining results in the extraction of large volumes of rock and accompanying stress relaxation of the rock mass. The amount of stress relaxation that occurs will be governed by the overall geometry of the open pit in terms of the highwall angle, overall depth, and the rock mass quality. The stability performance of a large open pit highwall is affected by stress relaxation as the rock mass dilates and rock mass strength deteriorates. The amount of stress relaxation that occurs along open pit highwalls is commonly measured using survey-monitoring prisms.
Surface movement monitoring data can be evaluated to quantify the amount of movement as highwall strain. where the authors have been involved, and data provided by others, has been evaluated in terms of highwall strain, rock mass quality and inferred mechanism of instability. This paper discusses stress relaxation in open pit mining, defines highwall strain, outlines threshold strain levels for key stages of highwall stability and presents an empirical highwall strain criteria for open pit practitioners to consider as an additional approach for the assessment of highwall stability.
Stress Relaxation with Open Pit Mining
Open pit highwalls undergo unloading during the mining process.
ABSTRACT: The Montana Tunnels Mine was mined out to a depth of 900 feet at the end of 1999. The first signs of instability were recognized in 1996 along the upper slope area of the northwest sector when the pit was mined out to a depth of approximately 500 feet. To extend the life of mining operations, it has been proposed to extend and steepen this highwall to a depth of 1000 feet. Slope stability analyses were carried out using the UDEC software program to assess the stability and deformation of the proposed highwall design. Back analysis of the 1996 instability along the upper slope section and highwall deformation to date has indicated reasonable agreement with the observed mode of failure and displacement monitoring results, The results of detailed analyses of the proposed highwall design have indicated very limited additional deformation, further development of tension cracks is unlikely and large-scale instability is not expected to occur.