Breaking Rock

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Except when using explosives rock is split or broken by stressing it beyond its tensile strength. Rock is stronger in compression than in tension. For example, most rock will support a heavy load upon it, but can be pulled apart relatively easily. Moreover, different rocks have different tensile strengths; that is, some are easier to break than others. When breaking rock, stress it at points of natural weakness.

Seamy rock will usually break irregularly because it has no major points of natural weakness. Moreover, holes drilled in seamy rock sometimes slip because the layers shift both horizontally and vertically. This causes the steel to jam in the hole or a feather to be bound on one side against the wedge. You can best avoid this by carefully placing holes, by keeping them as straight as possible, and by attempting to determine in advance what will happen when the rock breaks. Sometimes, however, a new hole must be drilled to free a jammed steel or wedge and feather set. Be careful to avoid extra stresses on jammed tools while working to free them. A knowledge of rock types will also help you plan the job, procure tools, place the holes, and will indicate what to expect when drilling.

Rock Types

This manual offers no 'hard and fast' rules about hole spacing and drilling depth necessary to break specific types of rock. This information is best gained from experience, depending on what is encountered and what is required at a job site. The general categories of rock that follow give broad hints about what to expect when drilling them.

Soft Rock

1. Shale—Clay, mud, and silt that is consolidated into a finely laminated structure.
2. Shist—Crystal line rock with component minerals arranged in a roughly parallel manner.

Medium Hard Rock

1. Sandstone—Sedimentary rock, usually quartz sand, cemented by silica, iron oxide, or calcium carbonate.
2. Limestone—Sedimentary rock that is formed by the accumulation of organic remains consisting mainly of calcium carbonate.
3. Marble—Metamorphic limestone that has been crystallized by a pronounced change in heat, pressure, and water content.

Hard Rock

1. Bluestone—Bluish gray metamorphic rock similar to sandstone.
2. Gneiss—Laminated metamorphic rock similar to granite.
3. Granite—Naturally igneous rock formed of crystallized quartz and orthoclase.
4. Basalt—Dense igneous rock that consists of feldspar and various minerals.

Wedges (Plugs) and Feathers

Wedges and feathers are tools designed to split rock when driven into a drilled hole or natural crack. The wedge fits in the hole between two feathers whose flat sides form a guide that prevents the wedge from jamming as it is driven into the hole. Use wedge and feathers as follows: Position the feathers in the hole so the flat sides of the wedge will be parallel to the line along which the break will occur. Drive the wedge into the slot between the feathers until the rock cracks, or until it sticks in the rock. Then tap the wedge lightly back and forth along the inside edges of the feathers until it is freed. Remove the wedge and, if necessary, begin again. Proceed slowly to allow the tools time to do their work.

Correctly placing the wedge and feathers.

To be effective, wedges and feathers must be correctly sized. The diameter of the wedge rod and feathers at the point where the rod meets the feathers must exceed the diameter of the hole. Driving the wedge between the feathers forces them against the sides of the hole and splits the rock.

Avoid unnecessary stresses on wedges and feathers by drilling holes as straight as possible. Straight holes help keep wedges and feathers from binding or jamming in the hole.