Bruce Hebblewhite

Numerical Study of the Mechanical Behavior of Nonpersistent Jointed Rock Masses

AbstractEstimating the mechanical properties of nonpersistent jointed rock masses is one of the most challenging problems in practical rock engineering due to the complex interaction of rock joints and intact-rock bridges. In this paper, the effect of joint geometrical parameters of nonpersistent rock mass on uniaxial compressive strength (UCS) and the deformation modulus was studied by using the discrete-element particle flow code PFC3D. In this numerical approach, the intact material is represented by an assembly of spherical particles bonded together at their contact points, and the joint interface is explicitly simulated by slip surfaces that are applied at contacts between the particles lying on the opposite sides of the joint interface. The failure process is simulated by the breakage of bonds between particles. A previous study of the authors has shown that this approach is capable of reproducing the mechanical behavior of nonpersistent jointed rock masses by a comparative study against physical ex...

Analytical solutions for mining induced horizontal stress in floors of coal mining panels

Abstract From the computational mechanics point of view, analytical solutions for any practical engineering problem are very valuable and important because (1) any numerical method, although powerful for dealing with practical problems, has to be validated before it is put into applications; (2) analytical solutions, in many circumstances, are often the sole resource to be used for validating a numerical method. Besides, analytical solutions can also provide a better understanding of basic physics behind practical engineering problems. For the above reasons, analytical solutions for the mining induced horizontal stress in the floors of coal mining panels have been rigorously derived in this paper. In particular, the following main factors are considered in the process of deriving the analytical solutions: (1) effect of the mining depth of a coal panel which reflects the virgin stress in a panel floor; (2) effect of the primary driving force on the top surface of the panel floor; (3) effect of the direct interaction of the panel floor with its underlying strata; (4) effect of the interaction of the panel floor with its left and right side media; (5) effect of the size and the material properties of the panel floor. Since the present analytical solution is derived from the mechanical model, which can be easily and exactly modelled by any numerical method such as the finite element method, it provides a very useful tool for validating such a numerical method for solving this kind of problem in engineering practice. Finally, the present solutions have been used to carry out a parametric study so as to draw many interesting conclusions for the problem considered.

Failure of rockbolts in underground mines in Australia

Over the last 15 years there has been an increasing incidence of failure in rockbolts used in underground mines in Australia. Failures have also been observed in the United Kingdom where Australian Technology rockbolting is also used. Most of the failures in the United Kingdom were found to be initiated by corrosion pits, but in Australia, the fractures were considered likely to be due to stress corrosion cracking (SCC). This paper reports a metallurgical study of 44 failed rockbolts from four different underground mines in Australia. The study confirmed that failure was generally due to SCC and showed that this was usually initiated by bending of the bolts that occurred due to lateral movement of the rock strata. It also showed that many of the failed bolts had very low toughness with Charpy impact values of 4–7 Joules.

Experimental Studies on the Mechanical Behaviour of Rock Joints with Various Openings

The mechanical behaviour of rough joints is markedly affected by the degree of joint opening. A systematic experimental study was conducted to investigate the effect of the initial opening on both normal and shear deformations of rock joints. Two types of joints with triangular asperities were produced in the laboratory and subjected to compression tests and direct shear tests with different initial opening values. The results showed that opened rock joints allow much greater normal closure and result in much lower normal stiffness. A semi-logarithmic law incorporating the degree of interlocking is proposed to describe the normal deformation of opened rock joints. The proposed equation agrees well with the experimental results. Additionally, the results of direct shear tests demonstrated that shear strength and dilation are reduced because of reduced involvement of and increased damage to asperities in the process of shearing. The results indicate that constitutive models of rock joints that consider the true asperity contact area can be used to predict shear resistance along opened rock joints. Because rock masses are loosened and rock joints become open after excavation, the model suggested in this study can be incorporated into numerical procedures such as finite-element or discrete-element methods. Use of the model could then increase the accuracy and reliability of stability predictions for rock masses under excavation.

Analysis of Mining-induced Valley Closure Movements

Valley closure movements have been observed for decades in Australia and overseas when underground mining occurred beneath or in close proximity to valleys and other forms of irregular topographies. Valley closure is defined as the inward movements of the valley sides towards the valley centreline. Due to the complexity of the local geology and the interplay between several geological, topographical and mining factors, the underlying mechanisms that actually cause this behaviour are not completely understood. A comprehensive programme of numerical modelling investigations has been carried out to further evaluate and quantify the influence of a number of these mining and geological factors and their inter-relationships. The factors investigated in this paper include longwall positional factors, horizontal stress, panel width, depth of cover and geological structures around the valley. It is found that mining in a series passing beneath the valley dramatically increases valley closure, and mining parallel to valley induces much more closure than other mining orientations. The redistribution of horizontal stress and influence of mining activity have also been recognised as important factors promoting valley closure, and the effect of geological structure around the valley is found to be relatively small. This paper provides further insight into both the valley closure mechanisms and how these mechanisms should be considered in valley closure prediction models.

Evaluation of valley closure subsidence effects under irregular topographic conditions

Abstract When mining occurs beneath or in the vicinity of valleys and other forms of irregular surface topography, the observed vertical subsidence at the base of the valley is less than that would be expected in flat terrain, while the observed horizontal movement of valley sides is greater than that in flat terrain. The reduction in subsidence at the bottom of the valley is referred to as upsidence and the convergence of two sides of the valley is termed valley closure. Researchers have identified the phenomenon of valley closure and upsidence as being very significant parts of non-conventional subsidence effects over the past 15 years in Australia. This paper aims to review and evaluate the recent developments in studying mining induced valley closure subsidence effects. The main issues addressed in this paper are: empirical predictions of valley closure and upsidence; numerical modelling approaches; and mechanisms behind this behaviour based on field measurements.

Parameter Study on Prediction Methods for TBM Penetration Rate

Penetration rate prediction of Tunnel Boring Machine (TBM) is the first step to advance prediction process of mechanized tunnelling. In this research, influence of effective parameters on TBM penetration rate is investigated by sensitivity analysis of three main TBM performance prediction methods; Norwegian University of Science and Technology (NTNU), rock mass index (RMi) and QTBM. Based on these analyses, it is shown that applied thrust per disc and joint spacing in NTNU and RMi models have more influence on penetration rate. In QTBM model, Q value, applied thrust per disc and induced biaxial stress are more effective.

Development of predictive methods for strain at the surface due to longwall coal mining

Abstract Longwall coal mining can result in impacts on natural and built features at the surface. The potential for impacts can be assessed using predicted mine subsidence parameters including strain. Improved predictive methods for strain have been developed as part of recent research using a large database of ground monitoring data from the Australian coalfields. Strain is predicted using a two-step process. Firstly, the net horizontal movements are predicted across each of the curvature zones above the active longwall. The distributions of strain are then predicted within each of these zones based on these net horizontal movements. The methods provide the site-specific predictions of strain anywhere above the active longwall based on the local surface topography, survey bay length and selected confidence level. The advantage of these methods is that they consider the potential for irregular anomalous movements, which are often not included in other existing predictive methods. The new predictive methods therefore can then be used to more reliably assess the potential for impacts on surface features located above longwall coal mining.

Numerical analysis on mining-induced fracture development around river valleys

ABSTRACT This paper presents a detailed study of the mechanisms contributing to fracture development around river valleys associated with mining operations. Due to the geology and geomorphology of the Southern Coalfield of New South Wales, Australia, non-conventional subsidence effects usually occur. The influences associated with valley closure and upsidence are principally tensile and shear fracturing/cracking of the river and underlying strata, which act as underground flow paths for surface water. This paper explicitly simulates the mining-induced fracture development around the valley structure, utilising a distinct element method modelling technique with Voronoi tessellation. The fracture propagations within intact rock as well as along existing discontinuities are simulated in this study. It is demonstrated that the presence of natural geological discontinuities and the mining-induced stress field play an important role in determining the extent and pattern of fracture propagation. Analysis of the mining-induced explicit fracturing system in the vicinity of valley provides an improved understanding of the near-surface hydrological cycle and enables effective remediation of the mining-induced adverse impacts on river valleys.

Applicability of a joint constitutive model: correlation with field observations

Abstract This paper presents the results of numerical studies that illustrate the performance of a new joint constitutive model developed for the stability analysis of large-scale jointed rock masses. The joint model features the reduction of stiffness and strength due to joint initial opening and scale effect, which has been demonstrated by performing a series of direct shear tests. The study thus focuses on the model’s applicability to field-scale rock joints. The movement of a real rock slope is first investigated numerically where the mechanical behaviour of joints is represented by the Mohr–Coulomb model and the proposed joint model. The slope is appraised to be much less stable by the simulation using the developed joint model, which agrees more with the qualitative observation on the site. The response of jointed rock masses to excavation is also examined by simulating two underground rock structures where the mechanical behaviour of joints obeys the developed model in the numerical modelling. Predictions from numerical simulations are in good agreement with field monitoring data surrounding the caverns. Thus, the new joint constitutive law can be utilised to evaluate the stability of large-scale rock structures with sufficient capability.