Design of pillars using field trials and back analyses

Examination of collapsed pillar cases outside of the empirical limits of Salamon and Munro’s in situ database has highlighted the need for additional parameters to be considered in the design of coal pillars. These include the influence of discontinuities, surrounding strata characteristics and the effects of deterioration and time.
 
An underground pillar monitoring exercise was successful, until the dynamic collapse failure of the pillar, which was not recorded due to the manual nature of the recording. A continuous logging system is vital for further such experiments. Increased certainty is required with respect to whether the pillar was confined by a goaf, before more definite conclusions can be made. This requires a different support strategy, such as long hole cable anchors.
Joint frequency, joint condition and joint orientation have an important effect on the strength of coal pillars. A new methodology has been developed to take these effects into account. An evaluation of the field data showed that pillars without any joint structures are likely to be about 10 per cent stronger than predicted by the empirical Salamon and Munro equation.
 
A simple and flexible technique to incorporate the effect of scaling, or time effects, into pillar design, has been produced. Pillars may be designed by specifying a probability of survival for a given number of years. Alternatively, it might be required that the probability of survival for an indefinite period should not be less than a specific value. Real rates of scaling for different geotechnical areas are required for the technique to become practical.
An analysis of geotechnical data according to the dimensions of discontinuity and roof rating from a substantial database allowed the identification of eight distinct geotechnical areas in South African collieries.
The size effect at the in situ scale, if present at all, is negligible. A representative value of in situ critical rock mass strength, Θc, is 6.3 MPa. It has been shown that significant (in the statistical sense) differences in coal material strength detected in the laboratory can lead to a modification of Θc.
 
The effect of variations of the pillar / roof or floor contact conditions on the width to height ratio (w/h) effect on pillar strength have been catered for by means of a design chart. The 99 per cent confidence interval for the in situ contact friction angle is 21.0° to 24.8°.
 
The results of these investigations have been put together to form the basis of a new methodology for pillar design. A pillar design flowchart has been produced. The modular approach to pillar design, with explicit quantification of the influence of Θc, jointing, w/h and roof / floor contact conditions, allows scope for site or geotechnical area specific pillar design. However, this also requires site specific measurements of geotechnical parameters.
 
There are a number of issues not yet resolved. Among them are:
1) the proposed new design methodology requires underground verification
2) the effect of coaltopping has not been determined
3) the factors that form part of the pillar design methodology have been assumed to act in series; this is not necessarily the case.
 
A trial period is suggested in which rock engineers make use of the new methodology in different areas and compare results so that confidence is built up in their applicability in different geological settings.
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