In the gold mines of South Africa, conventional shaft pillars do not provide adequate protection to the shaft and associated service excavations at depths exceeding approximately 3 km. Alternative methods of protection being considered generally involve extraction of the reef around the shaft at the start of mining operations, which solves the problem of high stress build-up, but permits large deformations of the rock mass to occur. These deformations, particulary the reef-level displacement discontinuity, could cause severe problems to the shaft steelwork and lining. Numerical modelling has shown that deformation can be controlled to a degree by the use of backfill, and suitable mining layouts. Delaying shaft steelwork and lining installation until residual deformations are tolerable also alleviates the problem of excessive deformations.
Dan les mines d'or d'Afrique du Sud, des piliers des puits conventionnels ne fourniasent pas une protection suffisante au puits et aux puits de service auxilliaires à l'exces de 3 km environ. Dea techniques de protection en consideration engagent, en general, l'extraction du filon autour du puits au commencement des operations minières ce qui rèsolvent Ie problème du generateur de contrainte mais qui laissent la deformation non restreinte à la masse rocheuse. Ces deformations, particulièrement le deplacement discontinu au niveau du filon provoqueraient de graves problèmes au structure en acier et au soutènement du puits. Modelage numerique a indique que la deformation peut être controllee à un certain degre par l'usage de terre de remblayage et la planification appropriee des minea. Le retardement de l'instsllation du structure en acier et du soutenement du puits juaqu' aux deformations residuelles sont tolerables allège Ie problème des deformations excessives.
Die konventionellen Schachtpfeiler in den Suedafrikanischen Goldminen bieten keinen ausreichenden Schutz fuer den Schacht und die verbundenen Hilfsausschachtungen in Tiefen von mehr als ca. 3 km. Alternative Schutzmethoden, die erwogen werden, befassen sich durchweg mit dem Abbau des Erzganges rings um den Schacht bei Beginn des Abbaus; dies löst zwar das Problem von hohem Spannungsaufbau, lasst aber eine vollkommen freie Verformung der Gesteinsmasse zu. Diese Verformungen, vor allem die Verschiebungsdiskontinuitat auf der Ebene des Erzganges, könnte ernste Probleme fuer die Stahlkonstruktion des Schachta und seine Auskleidung verursachan. Numerisches Modellieren hat gezeigt, dass Verformungen zu einem bestimmten Grade unter Kontrolle gebracht werden können durch Hinterfuellung und durch geeignete Abbauplanung. Das Problem uebermasaiger Verformungen kann auch dadurch verringert werden, indem man die Stahlkonstruktion im Schacht und das Anbrigen der Auskleidung verzögert, bis verbleibende Verformungen annehmbar sind.
The conventional method of protecting vertical shafts and their associated service excavations from the effects of mining of tabular orebodies, is to leave a sufficiently large block of unmined reef(shaft pillar)such that mining induced plus virgin stresses are kept below damaging levels. In the gold mines of South Africa, depths of shaft-reef intersections are beginning to exceed 3 km. At these great depths, virgin stresses are so high that shaft pillar sizes required to reduce induced stresses to acceptable levels become impractically large(Wagner and Salamon 1973).Also, depending on the strength of the particular rock mass, virgin stresses alone may be above damaging levels. Shaft pillars therefore no longer provide an adequate method of protection at great depth.
Since any unmined reef in the vicinity of the shaft will become highly stressed at great depth, the principle of protection for all alternatives being Considered is to -either carefully engineer the location of any unmined reef (in the form of crush pillars or satellite pillars), or to completely mine the reef around the shaft, the ‘shaft-reef’, prior to Commencement of regular stoping activities.
Complete removal of the shaft-reef reduces stress levels but permits unrestrained deformation of the rock mass to occur prior to total closure. These deformations may be acceptable from a rock mechanics Point of view, but they assume particular importance when considering the stability and design of shaft steelwork and linings.
The purpose of this paper is to review the possible alternatives to use of deep shaft pillars, and to qusntify certain practical methods that are available to reduce or virtually eliminate the undersirable effects of strata deformation.
Various alternatives to shaft pillars have been discussed in the past(More O'Ferrall 1983)but little experience has been accumulated by the Industry in their planned application. A review of some of the more important methods of shaft system protection being considered will highlight some of their common features, and will help illustrate their differences when compared to the use of conventional shaft pillars.
A logical progression from a solid shaft pillar experiencing, high stress levels would be to overstope service excavations and thereby leave a segmented pillar, or a series of satellites.
