The Polish mining industrie has at the time being at its disposal powered supports for 140 longwalls. These are supports of different types manufactured in Poland, USSR, GFR and United Kingdom. The attained considerable experience enables the generalised specification of conditions of correct selection of powered supports to the determined geological and mining conditions. It has been found by experiments that for safe maintenance of workings in powered supports following items are of vital importance:
properties of roof rocks,
gabarites of workings /height and span/,
load bearing capacity of supports,
type of support and magnitude of roof surface connected with it exposed for flitting the support set.
In this connection the criteria are specified for quantitative determination of roof class by establishment of its index number L. According to it the empirical formulae are given for determining the permissive width of path, the period of starting the longwalls and criteria of provoking caving of suspending roofs. The necessary load bearing capacity of supports has been established according to the roof class and the height of workings as well as criterion of permissive surface of roof exposure for flitting the support sets which in turn determines the type of support usable in the given geological and mining conditions.