The use of large quantities of simple deformation measuring instrumentation for establishing mine design parameters for a deep potash mine is described. Interpretation of results provided information on long term roadway and pillar stability, including widths of yield zones around pillars and the time-dependent variation of these zones, of particular significance due to the rheological behaviour of potash material. The resultant parameters relating roadway and pillar geometry to geological and mining conditions enabled more realistic planning of mine layouts.
Un grand nombre d'instruments simples a ete utilise pour mesurer la deformation, afin d'etablir les paramètres applicables à l'elaboration d'un plan de mine de potasse profonde. L'interpretation des resultats obtenus a fourni des renseignements sur la stabilite à long terme des voies et des piliers, notamment sur la largeur des zones autour des piliers sujettes à flechir et sur la variation de ces zones en fonction du facteur temps; ce dernier point est particulièrement important à cause du comportement rheologique de la potasse. Les paramètres ainsi degages - qui mettent en rapport d'une part la geometrie des voies et des piliers, et d'autre part la geologie et les conditions creees par. l' exploitation - ont permis d'elaborer des plans plus realisties de percees souterraines.
Die Definierung der Kriterien fuer die Abbauplannung eines Kaliflozes in grosser Teufe durch die Verwendung von grossen Mengen einfacher Verformungsmessungenist erleutert. Auswertung der Daten gab Information ueber die langzeitige Stabilitat von Strecken und Restpfeilern, einschliesslich des Durchmessers der plastischen zone um die Restpfeiler und die zeitabhangige Veranderung dieser Zonen, welche von besonderer Bedeutung ist wegen des rheologischen Verhaltens des Kaligesteins. Die resultierenden Kriterien die Strecken-und Restpfeilergeometrie mit den geologischen und bergmannischen Bedingungen verbinden, er laubten·eine realistischere Plannung des Abbauzuschnittes.
Potash mining at Boulby, in North Yorkshire, England, was commenced by Cleveland Potash Limited in 1973 on a lease area shown in Figure 1. The potash seam mined at Boulby lies at depths in excess of 1,100m below the surface making this mine ore of the deepest potash mines in the world. As a result of this great depth, the stability of mining layouts requires careful consideration, particularly in view of the high stress levels anticipated and the marked rheological behaviour of potash material. Apart from problems resulting from the depth, mining methods and layouts have been further restricted as a result of the existence of a National Park on the surface, directly above a large area of the mine lease. Planning permission for the mine was only granted on the condition that no damaging subsidence was caused on the surface. However, the major restriction on the mining layouts was the presence of massive, water-bearing Bunter sandstone beds approximately 120m above the seam. It was imperative that the mining layouts and sequence of development be calculated in such a way that the minimum necessary tensile strains be developed at the base of the sandstone beds to prevent the possibility of a water inrush into the mine workings. In the light of the above foreseeable problems, the mine management initiated a rock mechanics research project with the Department of Mining Engineering, University of Newcastle upon Tyne, during the exploration stage of the mine development. The initial work consisted of fundamental research into the mechanical and rheological properties of core obtained from exploration boreholes(2, 6). On the basis of these properties, interpretations were made, related to design constraints for the underground excavations including shaft and mine excavations. These were followed by laboratory and mathematical modelling and 'in situ' measurements. Monitoring of ground conditions has continued during the course of the mine development, providing more realistic estimates for the parameters associated with the mine design. The work described in this paper primarily concerns the 'in- situ' investigations conducted at the mine. An extensive instrumentation programme, based on relatively cheap and easily installed equipment, was established in an attempt. to develop an empirical model for the behaviour of the potash and adjacent evaporite strata. Subsequent sites involved areas of experimental pillar dimensions and roadway configurations. An overcoring, stress measurement operation was conducted to measure the 'in situ' virgin stress field. All of these results have been used in conjunction with results from numerical modelling and laboratory work in order to define suitable panel geometries for mining layouts on the criteria of:-
room and pillar stability,
minimum subsidence induced effects at the base of the Bunter sandstone and on the surface,
an efficient and economic extraction ratio.
The potash seam which is mined has an average thickness of 3.4m throughout most of the lease explored to date. The seam forms part of the Middle Evaporite series of the Zechstein deposits which extend across the North Sea and into Europe. Figure 2 shows a simplified geological section in the vicinity of the seam.
