In order to provide a rational basis for the design of board and pillar workings, field experiments were carried out in two South African collieries to establish the behaviour of coal measure strata when they are subjected to such mining operations. The experiments were designed to obtain the displacement pattern in the zones of expected compression and tension over pillars and boards, respectively. Mathematical analysis of the homogeneous isotropic elastic model and transversely isotropic elastic model showed that, in the cases examined, the former gave a good description of the observed displacements. From the evaluation of the experimental and theoretical results the following main conclusions were drawn:
The Young's moduli of the strata and of the coal were considerably lower, at low stresses, than laboratory tests suggested.
A large proportion of the total vertical displacement was due to the compression of the pillars.
The stress concentration in the pillars was smaller than hitherto suspected.
The intensity of the induced strain field decreased rapidly with the distance measured vertically from the seam.
Afin de trouver une base raisonnee pour projeter les exploitations en piliers et galeries, on a fait des essais dans deux houillères sud-africaines, ayant pour but de constater la conduite du terrain soumis à de telles operations. Les essais etaient conçus pour determiner le reseau des deplacements dans les zones de compression et traction prevues au-dessus des piliers et galeries. L'analyse mathematique d'un modèle elastique, isotropique, homogène et d'un autre egalement elastique mais transversalement isotropique, a prouve que le premier modèle avait fourni un bon rapport avec les deplacements reperes en realite. L'evaluation des resultats theoriques et experimentaux a rapporte les conclusions suivantes:
Les modules d'elasticite du terrain et du charbon sous contraintes faibles sont beaucoup moindres que ne l'indiquent les experiences de laboratoire.
Une grande partie du dèplacement vertical global provient de la compression des piliers.
La concentration des contraintes dans les piliers est moins elevee que l'on ne croyait auparavant.
L'Intensite du champ des deformations induites diminue rapidement lorsque la distance en sens vertical entre lui et le gisement accroît.
Zur Schaffung einer rationellen Grundlage fur die Abbauplanung im Kammerpfeilerbau wurden in zwei suedafrikanischen Zechen untertage Versuche ausgefuehrt, um das Verhalten der kohlefuehrenden Schichten unter solchen Abbaubedingungen zu ermitteln. Die Versuche wurden so angelegt, dass sich ueber die Gesetzmassigkeiten im Verschiebungsverlauf in den Zonen, in denen Druck und Zug ueber Pfeilern bezw. Abbaustrecken zu erwarten war, Aufschluss erhalten liess. Die mathematische Analyse des homogenisotropen Modells und des in einer Richtung isotropen Modells elastischer Schichten zeigte, dass das erstgenannte Modell mit den beobachteten Verschiebungen weitgehend èberreinstimmte. Die folgenden wesentlichen Schlussfolgerungen wurden aus der Auswertung der experimentellen und theoretischen Daten gezogen:
Die Elastizitatsmoduln des Nebengesteins und der Kohle waren bei niedrigen Spannungen betrachtlich niedriger als nach den Laborversuchen zu erwarten war.
Ein grosser Teil der gesamten vertikalen Verschiebung war auf den Druck zurueckzufilhren, dem die Pfeiler ausgesetzt waren.
Die Spannungskonzentration in den Pfeilern war geringer als vorher angenommen wurde.
Die Intensitàt der durch den Abbau hervorgerufenen Verformungen nahm mit wachsendem, senkrecht vom Flöz gemessenen Abstand sehr schnell ab.
In South African collieries the board and pillar method of mining is practised widely. This system is based on regularly placed, square coal pillars, which are often left intact to provide permanent support to the undermined roof. The design of such workings has been based mainly on practical experience. In order to provide a rational basis to the design procedure, information is required on the behaviour of strata and coal when they are subjected to the effects of such a method of mining. The observation of the displacement and strain patterns in the pillars and neighbouring strata provides a convenient means for obtaining an understanding of this behaviour. If a physical model is found which can account for the observed displacements, an important step will have been made towards the development of a rational design procedure. In this paper the results of two field experiments, designed to further the object outlined above, are presented together with the results of an analysis based on the theory of elasticity.
Inside a working panel a portion of the coal seam is removed and the remaining pillars, if they are remote from the solid sides, are called upon to support the total weight of the overburden. Consequently, the pillars will be subjected to an increased vertical compressive stress, while the removal of coal induces an apparent tensile stress in the boards. The resulting picture of induced stresses is one of alternating compressive and tensile zones above and below the pillars and the boards respectively. The qualitative picture of stress distribution on the roof of the seam is depicted in Figure 1.
