Quarrying

In gneisses, sandstones and limestones, joints also occur; and while frequently convenient for the division of the beds into masses of useful size, they may be a detriment, as when they occur so close together as to fall within the limits of a block suitable or economically usable for commercial purposes. In commerce the various kinds of building stone are usually designated by the name of the locality or region in which the quarry is situated. In the case of the more important varieties this geographic name usually conveys to the architect or builder sufficient information concerning colour, texture and other properties of the material.

Methods of Quarrying.

The methods of quarrying vary with the composition and hardness of the rocks, their structure, cleav age and other physical properties; also with the position and character of the deposits or rock-masses. In the quarrying of building stone, the general purpose of the work is to separate the material from its bed in masses of form and the size best adapted to the intended use. Cutting the stone to accurate dimen sions, dressing, moulding and finishing by tooling, rubbing or pol ishing are subsequent operations not involved in quarrying. Crush ing all kinds of hard, durable stone to angular fragments for mac adam type roads and for concrete aggregate is also an operation subsequent to quarrying.

The practice of quarrying consists in uncovering a sufficient surface of the rock by removing superficial soil, sand, clay, or overborder of overlaying waste rock by some suitable process or by sinking a shaft or slope, and then with proper tools or explosive, detaching blocks of form and size adapted to the purpose in view. Frequently the exposed outer portion of the rock has been either cut up and eroded away in fissures or has been affected by the action of various atmospheric agencies, until it has become discoloured or softened by decay. This weathered material must be removed before stone can be obtained for economic use in the production of dimension stone.

The separation of blocks of building or dimension stone is ordinarily carried on by some specific variation of one of the two general present-day methods of quarrying known respectively as the "blasting" and "channeling" process. Blasting is used for the harder rocks that do not tend to shatter and channeling for most other kinds of rock. The blasting process consists of drilling holes along the outlines of the block to be removed, and then, by exploding blasting-powder in the holes, exerting sufficient force to overcome the cohesion of the rock and rend it asunder. In many quarries it is found most convenient to separate a large mass and afterwards divide it into blocks of the required size, by drilling holes for further charges of blasting powder, or by driving wedges into these to split the rock. When the rock is

stratified, or has an easily determined rift, the holes are drilled at right angles to the plane of separation.

When there is no stratification or rift, or these natural planes of separation are too far apart, or when the position of the joints is not advantageous, a row of horizontal holes must be drilled into the face or breast of the quarry, along which separation is effected either by blasting or the use of wedges. At certain American quarries, in a granite which has no rift or direction of ready cleavage, compressed air has been brought into service to effect the separation of extensive layers. A hole is drilled as deep as the desired thickness of the layer to be separated, and a small charge of dynamite is exploded at the bottom of it. This develops a cavity in which a small charge of powder is next exploded, producing a crack or crevice parallel to the surface of the rock. A pipe for conveying compressed air is now sealed into the opening, and gradually increasing pressure is introduced. This results in the gradual extension of the crevice developed by the explosion of the powder. In the absence of compressed air, water under pressure may be used and also small powder charges exploded at intervals of a few days.

Crushed-stone Quarrying.

A very large and important branch of the quarry industry has recently come into existence, since the era of improved highways and the extensive use of Portland and other hydraulic cements ; this is distinguished from "building or dimension stone" quarrying and is generally referred to as crushed-stone quarrying. The object of crushed-stone quar rying is to drill, blast with high explosives and remove or excavate the stone in the cheapest possible manner from either hillside or pit quarries, so that it may be loaded with power shovels and then be fragmented by crushing and grinding machinery of various types to commercial sizes for aggregate, or pulverized to be manu factured into cements. The quarrying of stone for flux in man ufacture of steel and glass, for burning into lime and for various other mineral industries also falls in this category. Commercial sizes for crushed stone will vary from 5 to 6 in. in greatest dimen sion (limestone fragments for lime kilns or blast furnaces) to dust, which has many commercial uses. The commonest and largest uses of crushed stone are for surfacing highways and for use as aggregate with bituminous binders or Portland cement to make concrete. Most commercial crushed-stone quarry operations differ in no essentials from open-pit mining. There are also numerous underground quarry-mining operations for the purpose of winning crushed stone for similar purposes.