Summary

Results of tests carried out in mines and concerning the influence of the dimension of the solid elements on its stregth during the tests by the method of compressing the solid from a cut are given for six different types of coal. Laboratory tests were carried out on the strength of model pillars with the dimension ratio of D/h from 1.0 to 20. The results of tests are determined by empirical formulae. On the basis of the theory of plasticity the strength of pillars with a high value of D/h was explained. Two experimentally found criteria of exceeding the strength of thin specimens (model pillars) are given. In conclusion empirical formulae useful to calculation of the maximum and minimum strength of pillars are given; they take into account the strength of coal specimens, the influence of scale effect and the influence of the ratio of D/h.

Introduction

The methods of a proper utilization of testing results seem to be the basic problem of the actual development of the knowledge in the field of rock mechanics. A wide development of apparatus and of underground measurement methods and also of laboratory tests lead to a state in which there are possibilities of a sufficiently precise testing both of rock properties and of state of stresses or strains prevailing within. However, the precise methods of utilization of results of those tests for planning mining technologies and for prediction of hazards which can occur, fall far behind. Coefficients and assumptions taken for calculations are not precise sometimes; consequently, relatively precise results of direct tests are not duly utilized for planning.

The determination of an effective strength of coal in a pillar is one of more simple cases of such a situation. The observations made so far seem to indicate that three substantial factors influence on the strength of a pillar, namely: coal strength, scale effect and dimension ratio. All these factors have to exmined jointly. The works carried out thus far by a number of investigators (Greenwald — Howarth — Hartmann, Stear, Skinner and others) were mainly focussed on the coal strength examined on specimens or in the solid without taking into account the strengthening resulting from a low slenderness of pillars. Under conditions of the Lower and Upper Silesian Coalfields, where thin seams are mainly worked, the problem of the influence of pillar slenderness on its strength seems to play the basic role. It seems to be confirmed by the results of recent tests carried out in South Africa [1] where by stock-taking of existing mine pillars and by their classification onto stable and instable ones the values α and β occurring in the equation for pillar strength were determined. In those tests being of a retrospective nature, the determination of the cube strength of coals from particular pillars was, however, not possible. In the Polish coalfield the coal strength determined in the laboratory ranges from about 30 to 250 kp/cm2 (exceptionally up to 550 kp/cm2). With such considerable variations a precise analysis of the inflence of the strength of coal matter on the strength of pillars is indispensable.

Résumé

On a présenté les résultats des essais effectués dans les mines, concernant l’influence des dimensions de l’élément du massif de charbon comprimé sur sa résistance lors des essais conduits par la méthode de compression du massif à partir de la saignée pour six différents types de charbon. On a effectué les essais de laboratoire sur la résistance des stots de modèle à rapport des dimensions de D/h de 1,0 — 20. Les résultats des essais ont été déterminés par formules empiriques. Sur la base de la théorie de plasticité on a expliqée la résistance des stots ayant une haute valeur de D/h. On a donné deux critères prouvés expérimentalement de dépasser la résistance des échantillons minces (stots de modèle). En conclusion on a donné les formules empiriques qui servent au calcul de la résistance maximule et minimale des stots, compte tenu de la résistance des échantillons de charbon, de l’influence de l’effet d’échelle et de l’influence du rapport de D/h.

Zusammenfassung

Es werden die Untertage-Untersuchungsergebnisse hinsichtlich des Einflusses der Druckgrösse eines Kohlenfestebestandteiles auf dessen Festigkeit bei Untersuchungen mittels der Methode der Kompression der Kohlenmassive vom Schram aus für 6 verschiedene Kohlengattungen angegeben. Es wurden Laboratoriumuntersuchungen der Festigkeit von Modellpfeilern mit einem D/h-Dimensionsverhältnis von 1 bis 20 durchgeführt. Die Untersuchungsergebnisse werden mittels empirischer Formeln dargestellt. Anhand der Plastizitätstheorie wird die Festigkeit von Pfeilern mit grossen D/h-Werten erörtert. Es werden zwei im Laboratorium festgestellte Kriterien der Festigkeitsüberschreitung dünner Proben (von Modellpfeilern) angegeben. Abschliessend werden empirische Formeln für die Berechnung maximaler und minimaler Pfeilerfestigkeiten angeführt, die die Festigkeit der Kohlenproben, die Beeinflussung des Skala-Koeffizienten sowie den Einfluss des D/h-Verhältnisses berücksichtigen.

This content is only available via PDF.
You can access this article if you purchase or spend a download.