Top Slicing

TOP SLICING This method, developed from methods used in the north of England iron mines, was first used in the iron mines of the United States. Later it was adapted to the mining of copper and lead-zinc ores.

Continuous Horizon.

Mining is done in horizontal slices, Io to 12 ft. thick, working downward from the top of the deposit. The first slice is taken just beneath the capping, which is sup ported by timbering. As the slice of ore is taken out lagging is laid on the floor to tightly cover the ore below. The posts sup porting the capping are then shot out and the capping caves onto the timber floor or mat. This mat is temporarily supported by the timbering of the second slice. After the ore in the second slice is taken out, a timber floor is laid and the posts are shot out. Thus the timber mat, which serves to prevent the capping from mixing with the ore receives additions as each slice is stoped. From the haulage level a raise, containing a chute and manway, is put up to the top of the ore and a main drift is driven to the boundaries of the block. Cross drifts are turned off at right angles to the main drift and are driven to the limits of the block. The ore is broken by breast stoping, and is shovelled into cars to be hauled to the chute. The back is supported by timbering. After a cross drift has been completed, the floor is covered with a layer of timber and the posts are shot out, using small charges of dynamite. The overlying capping or timber mat settles down onto the new floor. If capping in a caved drift tends to run into a working drift, the side of the latter is lagged to hold the capping.

Since the timber is not recovered, it is as poor a grade as will do for temporary support. In variations of this method a form of square-set timbering is used, and successive cuts may be two sets wide and two or three sets high.

Panel, Block and Inclined Slicing.—A large ore body is not worked in one continuous horizon, but rather is a series of blocks or panels. Inclined top-slicing was devised to save shovel ling the ore. The cuts (frequently called slices, though slice seems best applied to the entire series of cuts on one horizon) are inclined upwards from the chute at an angle of with the horizontal, so the ore will roll down to the chute. This method does not permit sorting of the ore in the stope and is more dif ficult to work than horizontal slicing. The Mitchell method of top-slicing is a modified form in which inclined slices are carried up from a chute at for from I r to 15 ft. and are then turned horizontally. It is difficult to keep the working drifts open for a long time, hence a large ore body is worked in a series of small independent units. Top-slicing is applicable to weak ore with poor walls. It is most suitable for deposits of large horizontal extent which have a capping that caves readily. It is a safe method and gives a high extraction of ore, up to 98%. However, the ore is mined by breast stoping and a large amount of timbering is required. The method is being replaced by others in which the ore is largely broken by undercutting and caving.