One of the most complicated problems of modern mining industry is e efficient and safe mining of highly weak unstable ore deposits. This problem acquires the particular acuteness in connection with Complex (from the hydrogeological and engineering point of view) deposits which are characterized by: great thickness of ore bodies, availability of pressure aquifers both in the very deposit and in great thickness (hundreds of meters) of loose overburden as well; Considerable depth of mining excavations (more than 500m). Modern requirements of enviromental protection predetermine acceptance of additional measures for earth surface conservation, groundwater resource and other things in entrails of earth which to the greater extent complicate mining of deposits.
Researh on strength characterisitcs of weak rock masses and load formation on supporting element of system is of the greatest importance for determination of mining system parameters under above mentioned circumstances.
It is known that rock (ore) masses Present themselves a complex system which is conditioned by observed jointing. From the point of view of rock mechanics massif structural heterogeneity reveals in relative form in its strength and strain behavior.
Particular investigation of influence of rock masses structural heterogeneity on its strength and strain behavior was made by the authors together with P.G. Labazkin with respect to one of the main principles of geomechanical characteristics of rock masses-permissible rock exposure magnitude.
As a result of these investigations a number of relationships has been set up which characterize separate factor influence on rock mass strength expressed as permissible span (L):
a)influence of bonds between slices (C1) and thickness of slice (8)
(Equation in full paper)
where A - coefficient considering the strength criterion for example on using in this case compression strength A=4 and bending strength A=2.
The performed investigations have shown that for weak unstable ore masses methods of continuous and waste- fill mining are the most efficient and often the only ones possible. It should be noted that here the term "slice" involves the broader sense of this word including both a slice of a small width (3–5m) and floors of considerable height. They have a common operation scheme of slice worked-out in specific direction by longwall mining which practically provides for that no ore pillars are left be hind it. In this case it is important to get relationships of stress-strain behavior of ore masses and fill.
The question of formation of pressure on fill mass in the process of increasing of number of worked-out and filled rooms was under investigation. It has been stated that as this number increases the loads on fill slices increase gradually and at the certain level this rise ceases. In the first instance the pressure stabilizes on the first slice, then on the second one etc until reaches the value close toγH.
The value of maximum loads (Pmax) On fill masses is proportional to the width of work-out area and stabilizes at 1–2L: