Shafts vary in size—from the smallest in which a man can work (about 2' × 4'), to any size that may be required, seldom exceeding 10' × 10'.
Great galleries are used for descent into a ditch, and when it is wished to pass cannon through them.[13]
Common galleries are used for descent into a ditch, and for communications. Troops can pass through them “by twos.”
Half galleries answer for general purposes of attack. They allow the miner to work freely in different positions without being cramped, but are small enough to admit of rapid driving. Branches and small branches are driven out from the galleries to the mine-chambers, etc. They can be driven rapidly for short distances (10 to 20 feet); but when of greater length the earth is removed from them with difficulty, they are not easily ventilated, and are too small for use as communications.
20. Shaft and Gallery Linings.—In very firm soil it is sometimes practicable to drive small shafts and galleries short distances without lining them; but if these are to stand for any length of time, there is always danger of their falling in, particularly if shaken by the explosion of mines in their vicinity. When it is considered safe to use them, however, the shafts should be elliptical in plan, and the roofs of the galleries should be pointed arches. As a rule, however, both shafts and galleries should be lined. Those which are permanent in their character—as the main galleries of the countermines of a permanent work—are lined with masonry. Masonry linings may be of brick, stone, or concrete walls and arches. The smaller galleries constructed during a siege, and all the shafts and galleries of the attack, are lined with wood. Wooden linings are of two general types, known as cases, and frames and sheeting.
21. Cases ([Pl. XI], Figs. 11 and 12) are made of plank, from 6" to 12" wide, each case consisting of a cap-sill, a ground-sill, and two stanchions. The cap and ground-sills are cut to a length equal to the clear width of the shaft or gallery plus twice the thickness of the stanchions; a rectangular notch is cut in each end to receive a corresponding tenon cut on the stanchion. The length of the stanchions between shoulders is equal to the clear length of the shaft or height of the gallery. The length of the tenons is generally equal to the thickness of the cap and ground sill (usually 2´´), and their width about three inches. Notches are cut in the sides of all the pieces of the case, as shown in the figure, for convenience in handling them.
In grand galleries the tenons at the top of the stanchions are usually shorter than the thickness of the cap-sill, and those at the bottom, as well as the mortises in the ground-sill, are omitted. The stanchions are kept from collapsing by blocks nailed to the ground-sills. These blocks are 2" thick, and wide enough (about 9´´) to so guide the wheels of a gun-carriage as to prevent the axle striking the stanchions.
In cases for smaller galleries also the tenons are sometimes omitted at the bottom of the stanchions, the mortises in the ground-sills cut an inch or two deeper, and the stanchions kept from collapsing by keys driven in the mortises ([Pl. XI], Fig. 13).
22. Shaft and Gallery Frames ([Pl. XI], Figs. 14, 15, and 16) are made of scantlings, halved together at the ends, as shown in the figures. Sheeting is made of boards or planks of the necessary thickness, sawn to proper lengths, and bevelled at the ends. When sawn lumber is not available, the frames may be made of saplings, and in some cases poles may be used for sheeting.
The middle of each cap and ground sill, both in frames and cases, is distinctly marked by a shallow saw cut or otherwise.