The information of actual size of the chambers and of the pillars applied in some ore deposits of the USSR is concerned, and the results of experimental studying the pillar deformation as well as those of evaluating the stable spans of the chambers under different geological conditions are analysed.
Auf der Grundlage der umfanreichen Untersuchungen in einigen Erzlagerstatten der UdSSR werden tatsachlige Abmessungen von Pfeilern und Kammern angegeben. Die Ergebnisse der experimentallen Untersuchungen zum Verformungsverhalten von Pfeilern und zur Einschatzung Von standfesten Kammerspannweiten in verschiedenen geologischen Bedingungen sind angefuhrt.
Les donnees reeles des dimensions des chambres et des piliers, utilises aux certains gisements minerals de l''URSS et les resultats des etudes experimentales de l''aptitude à la deformation des piliers et d''estimation des travees stables des chambres dans les Conditions geologiques differents sont exposees.
The mining practice has accumulated a rich experience of studying the stability of large underground openings. Especially great diversity of geological conditions is observed in the course of exploiting the ore deposits. Ore bodies usually have a bedlike form distorted by folds or disjunctives, which results in considerable variability of their inclination and thickness. During the Working of deposits the room-and-pillar systems are often applied that cause the formation of rooms (chambers) whose section is large and length is considerable. The bearing members are ore pillars formed like separate columns or strips arranged uniformly If seen on a contour plan (see Fig. 1).
Some data on the geological conditions, chambers and pillars at different ore mines in the USSR are given in Table 1 (in accordance to ).
The ore pillars left are either lost or extracted in the course of the further mining of the deposit. In the first case it is desirable to leave the least size pillars at an optimum ratio between the pillar cross-section and the span (the width) of the chambers. The problem consists in the evaluation of the strength of the rocks in the pillars and the load on them which depends upon the areas of the extracted ore.
To study the behaviour of pillars under the increasing load, experiments were carried out at some mines. These experiments consisted in subsequent extracting a certain number of the pillars and in observing the neighbouring pillars. The stress and the strain (longitudinal and lateral) were measured in the pillars, and their characteristics were also evaluated (ultrasonic tests and electrometry).
(table in full paper)
(figure in full paper)
Experience accumulated during many years of observation shows that there are at least four stages in the process of pillar deformation for which peculiar dependences between the stress and the strain are typical. Fig. 2 illustrates typical diagram " 6 - ξ " as well as the corresponding rheological model.
Stage I of pillar loading is pure elastic behaviour with Young modulus E. Only occasional fractures appear at this stage. When the stress exceeds some level 6e -limit of elasticity - stage II of visco-elastic creep begins. The edge zones of the pillar begin getting demolished, and separate cracks appear in the central part of it. Stage II is imitated on the rheological model by block consisting of elements E1, η1, 6e. Stages I and II are stable. If the stress exceeds the long-term strength of the rock 6ℓ stage III of viscous creep begins.