Predicting the block size of the rocks of ore deposits

Abstract

THEWhen ledge rocks are worked by blasting, the grain-size composition of the spoil depends largely on the jointing. Information on the state of the rock, obtained by detailed surveying, helps to predict the natural block size of ledge rocks. In this connection, the principal difficulty is that core descriptions give only qualitative features, and the core yield remains the only source of information on the block size of these rocks.To enable data on the core yield to be used for assessing the natural block size of ledge rock, in a number of open-cut ore mines we performed comprehensive investigations, 'including the following: a) the degree of jointing in the direction of borehole drivage to the surfaces of the slopes of the benching banks; b) the fragment size of the core in boreholes intersecting the exposure in which the jointing was surveyed; c) the dependence of the core yield on the drilling procedure and condition and on the strength and jointing of the rocks in these boreholes. These conditions revealed the following:Under identical drilling conditions (hard-alloy bits 93-112 mm in diameter, length of operating cycle 1.0-2.0 m), the ratio of the distances between the natural uncemented joints separating the rocks along the bbrehole axis to the actual size of the core fragments is 7-30;When the boreholes are drilled in identical rocks the ratio of the mean sizes of the core fragments is directly proportional to the ratio of the inner diameters of the drill bits;When the boreholes are drilled with hard-alloy bits in rocks with Protod'yakonov hardness more than 6, the core yield is virtually independent of the hsrvluess of the rock; Additional crushing of the core during drilling by fracture and torsion has no effect on the core yield if the size of the natural blocks is 2-3 times greater than the inner diameter of the drill bit;The influence of abrasion of the core as it is twisted can be disregarded if the ratio of the operating cycle to the core diameter is not more than 10-15;The following relation exists between the core yield (k, %) and the mean size of the blocks along the borehole axis (l, m):which holds true with a correlation coefficient of 0.76 for the following conditions: drilling with hard-alloy bits of diameter 93 mm or more with mud circulation for an operating cycle of length not more than 2 m and a core yield of not less than 60%. Equation (1) enables us to use the data of detailed surveying for predicting the block size.A rational procedure for processing the data is described below.On the composite plan of the mine workings and on the geological cross sections we draw all the faults and folds and the stratigraphic and lithologie contacts, and establish the boundaries of the structural-tectonlc blocks within which the block size of the rocks can be regarded as constant.On this graph we plot all the data on the block size of the rocks, obtained from large-scale measurements of the jointing or by processing photographs and stereopairs, and the information on the yield of oversized material ...