D. Stead

Quantifying progressive pre-peak brittle fracture damage in rock during uniaxial compression

This paper presents the findings of an extensive laboratory investigation into the identification and quantification of stressinduced brittle fracture damage in rock. By integrating the use of strain gauge measurements and acoustic emission monitoring, a rigorous methodology has been developed to aid in the identification and characterization of brittle fracture processes induced through uniaxial compressive loading. Results derived from monocyclic loading tests demonstrate that damage and the subsequent deformation characteristics of the damaged rock can be easily quantified by normalizing the stresses and strains observed in progression from one stage of crack development to another. Results of this analysis show that the crack initiation, sci, and crack damage, scd, thresholds for pink Lac du Bonnet granite occur at 0.39sUCS and 0.75sUCS, respectively. Acoustic emissions from these tests were found to provide a direct measure of the rapid release of energy associated with damage-related mechanisms. Simplified models describing the loss of cohesion and the subsequent development of microfractures leading up to unstable crack propagation were derived using normalized acoustic emission rates. Damage-controlled cyclic loading tests were subsequently used to examine the eAects of accumulating fracture damage and its influence on altering the deformation characteristics of the rock. These tests revealed that two distinct failure processes involving the progressive development of the microfracture network, may occur depending on whether the applied cyclic loads exceed or are restrained by the crack damage stress threshold. # 1999 Elsevier Science Ltd. All rights reserved.

Changes in acoustic event properties with progressive fracture damage

Laboratory results from uniaxial compression tests performed on pink Lac du Bonnet granite samples indicate that a number of stages in the progressive failure process of rock can be identified through the combined analysis of strain gauge and acoustic emission (AE) data. Testing focused on identifying the crack initiation (σ ci ) and crack damage (σ cd ) stress thresholds, two key components in the brittle fracture process. Results from this testing also showed that the properties of the acoustic events are markedly different throughout loading most notably before and after crack initiation. This proved valuable in substantiating observations made using strain gauge data. This paper explores the use of acoustic emissions and the interpretation of the acoustic event properties in identifying the different stages of crack development.

Developments in the analysis of footwall slopes in surface coal mining

Abstract Surface mining of coal, particularly in areas of mountainous topography, often involves the formation of extensive footwall slopes parallel to the strata dip. Due to structural deformation steep dips on the limbs of folds may be encountered in association with thrust faults, jointing and residual shear strength conditions. Such an environment necessitates a rigorous assessment of footwall stability in order to ensure safe and economic exploitation of the coal. This paper provides a detailed review of the factors influencing footwall slope instability in surface coal mining and the major instability mechanisms which have been recognized within the published literature. The analysis of footwalls in the design stage and the back analysis of footwall slope failures has in general been undertaken using predominantly two-dimensional limit equilibrium techniques often incorporating a simplistic elastic column analysis. The application of numerical modelling techniques to surface coal mine footwalls has received little attention. In this paper the authors attempt to illustrate the potential for investigating footwall failure mechanisms and stability using principally the distinct element method.

The effect of neighbouring cracks on elliptical crack initiation and propagation in uniaxial and triaxial stress fields

Abstract The study of brittle fracture and its relationship to strength is a fundamental part of rock mechanics and a number of other engineering disciplines. The initiation, propagation and coalescence of these fractures results in the degradation of material strength and eventually leads to failure. Results show that the interaction of stresses between neighbouring tips of elliptical cracks aligned parallel to the direction of loading can have a significant influence on one another in terms of crack initiationand propagation. Depending on the distance separating each crack and the loading conditions imposed on the medium surrounding them, the addition of neighbouring cracks can act to either suppress or promote crack growth.

Design of tabular excavations in foliated rock: an integrated numerical modelling approach

SummaryThis paper presents a rock mechanics design methodology applicable to steeply dipping orebodies typical of many underground hardrock mines. The first stage in the design process is the characterization of the rock mass using bothin situ and laboratory data. The effects of anisotropy on rock mass behaviour are discussed with reference to laboratory and field observations. The second stage involves the use of a number of selected numerical modelling techniques to investigate ground response in the near-field rock mass surrounding the mining excavations. This study shows that the use of several numerical methods in conjunction, allowing for the advantages of each method to be maximized, provides a more comprehensive analysis of the different facets of stope design. This approach differs from those in the literature which seek to compare the different numerical methods in order to select just one method best suited for a problem. The design methodology employed emphasizes the importance of developing an understanding of ground deformation mechanisms as opposed to predicting absolute behaviour.

ENGINEERING GEOLOGY IN PAPUA NEW GUINEA: A REVIEW

Abstract Papua New Guinea is a country where geology has a major impact on civil and mining engineering projects. It is situated in a geologically active region and hence is subject to earthquake- and volcanicity-related hazards. Combined with a high annual rainfall, rugged topography and intense weathering, this makes Papua New Guinea an extremeley challenging and interesting environment for the engineering geologist. Several large-scale open pit mining operations such as Panguna and Ok Tedi have involved a considerable input from geological engineers both in mine design and in the development of the necessary infrastructure. Environmental concerns have become increasingly important requiring the design of tailings dams to limit the effect of mineral extraction. The construction and maintenance of roads is an important factor in the growth of Papua New Guinea and both major and minor roads have been adversely influenced by slope instability problems. Numerous slope failures have occurred along the countrys main route, the Highlands highway, on several occasions resulting in road closure. The development of hydroelectric power has also involved a significant input from geological engineers in the design and construction of dams and ancillary structures. This paper examines the influence of geology on the development of Papua New Guinea.

Benchmark testing of numerical approaches for modelling the influence of undercut depth on caving, fracture initiation and subsidence angles associated with block cave mining

Abstract This paper reports the findings from a benchmark study testing several numerical methods, with a focus on the influence of undercut depth on block caving-induced surface deformation. A comparison is drawn between continuum v. discontinuum treatments of the modelled geology. Results were evaluated with respect to different simulated levels of ground disturbance, from complete collapse to small-strain subsidence. The results show that for a given extraction volume, the extent of ground collapse at surface decreases as undercut depth increases. The presence of sub-vertical faults was seen to limit the extent of the modelled caving zones. In contrast, the extent of small-strain surface subsidence was seen to increase with increasing undercut depth. The faults in this case did not have the same limiting effect. Overall, the findings emphasise the importance of balancing model simplification against the need to incorporate more complex and computationally demanding representations of the rock mass structure.

Ground deformation associated with mining a remnant footwall pillar in a surface coal mine

ABSTRACT Data is presented from a ground deformation monitoring program conducted at a surface coal mine over a two year period. Monitoring focused on the mining of a coal measures sequence on a footwall slope. Mining plans had aimed to isolate a remnant pillar block, 12500 m2 in plan, carrying a 132kV pylon tower and thus to maintain the integrity of a cross-pit power line. Pillar deformation which arose from shear on inclined strata is reviewed and related to the excavation sequence by strike cut and dip cut mining, involving remedial backfill buttressing. The monitoring program was based principally on an array of borehole inclinometers, supported by ground tiltmeter and conventional survey methods. The paper discusses the performance of the monitoring practice and strategy adopted in this mine production environment.