The use of X-ray computer tomography to investigate particulate interactions within opencast coal mine backfills

Restoration of opencast coal sites frequently involves the controlled compaction of heterogeneous arisings, and accurate prediction of the settlements associated with such backfill is difficult. Attention has previously focussed on improving the specifications used to control backfilling as a way of both reducing the magnitude of the settlements and improving their predictability. However, there still exists a large degree of uncertainty about the fundamental particulate interactions that occur within a fill. The range of mechanisms previously considered to be influential on creep behaviour is described. A current research programme is investigating the use of X-ray computer tomography (CT) as a means of nondestructively observing particle interactions during settlement, in conjunction with both long-term creep tests and short-term compressibility tests. The principles of this radiographic technique, which is relatively new to geotechnical engineering, are described, and findings are presented that illustrate the potential of the method. Interim findings indicate that large particles are distributed on an apparently random basis within a fill, and the nonuniform distribution of voids is clearly demonstrated. Moreover, the results presented conflict to some degree with the general consensus of opinion that particle crushing is a major mechanism in the settlement process. Rather, local collapse into small voids left by compaction, and relative sliding and rotation of particles, seem to be the dominant factors for a range of compaction efforts. Particle splitting is discernible on some sections, but this mechanism appears to be less significant than others. Research is continuing into the time dependency of the observed mechanisms, the effects of moisture content changes and the effects of heterogeneous initial particle strengths. (C) 2003 Elsevier B.V All rights reserved.