33. When the soil is very bad a shield ([Pl. XII], Fig. 20) is used to prevent the earth in front and above from caving into the gallery. In starting out from the shaft the following method is adopted: As soon as the top sheeting is sufficiently advanced and the shaft sheeting is forced down about 1 foot, the top plank of the side sheeting is inserted and driven forward about 2 feet, and the earth at the top of the gallery is excavated for from 6 inches to 1 foot in advance. A piece of plank a foot wide and in length equal to the width of the gallery is then placed directly under the top sheeting and against the face of the excavation, and is held in place by braces at its ends secured to the shaft lining. The shaft sheeting is then lowered another foot, the next plank of the side sheeting inserted, the earth excavated, and a second plank of the shield placed in the same way as before. This is continued until the entire face is covered. The top and side sheeting are then driven forward, and the top plank of the shield is removed and replaced in advance; after which each plank is removed and replaced in succession, as above described.
34. Inclined Galleries.—Method of fixing the slope.—If the gallery instead of being horizontal is ascending ([Pl. XII], Fig. 21) or descending (Fig. 22), the proper slope is obtained by using a slope-block whose edge is equal to the rise or fall of the gallery in one interval. This is placed upon the lower of two consecutive ground-sills, and the proper height of the other is determined by a mason’s or spirit level (Fig. 21). If a field-level or a mason’s level properly marked for the slope is used, the slope-block may be dispensed with (Fig. 22).
35. Position of Frames.—In driving descending galleries better progress will be made and less material used if the frames are set at right angles to the axis of the gallery ([Pl. XII], Fig. 22); and this is the usual custom. In driving ascending galleries this is impracticable, and the frames are set vertically (Fig. 21). In all other respects inclined galleries are driven in the same manner as horizontal ones.
36. Partly-lined Galleries.—In very firm soil side sheeting may be omitted entirely, and in that less firm the side planks need not be in contact. When the side sheeting is omitted the width of excavation may be reduced to the clear width of the gallery, and the stanchions be let into the side wall flush with its surface. In this case the ground-sills are frequently omitted, the stanchions resting upon wooden blocks, stones,or directly upon the earth.
To save material, the planks of the top sheeting are sometimes more or less separated also. This can only be recommended when rapid and temporary work is required with limited materials; and in these cases the earth between the planks should be supported by a packing of sticks, brush, etc., etc.
37. Change of Direction in Galleries.—In changing the direction of a gallery, the new direction is laid off by using a carefully made angle-templet ([Pl. XI], Fig. 7) or slope-block, field-level, etc., and it is prolonged in the new direction by the methods already described. When the soil is firm enough to stand safely while excavating and lining one gallery interval, even if somewhat short, no difficulty exists in changing the direction of a gallery in either a vertical or horizontal plane, since the excavation in the new direction may be made so large that the miner working in it can place the new frames and introduce the sheeting and wedges. The gallery can then be carried on without diminution in size.
When the soil is bad, however, special arrangements must be made for introducing the sheeting.
38. Change of Slope.—To pass from a horizontal to an ascending gallery ([Pl. XII], Fig. 21) it is only necessary to give the top sheeting the proper angle by holding down its back end with a piece of scantling placed across the gallery for that purpose; and, to give the side sheeting the proper inclination, cutting trenches in the bottom of the gallery for the lower pieces, if necessary.
In passing from a horizontal to a descending gallery ([Pl. XII], Fig. 22) the roof may be carried forward horizontally, and the floor given the desired pitch by increasing the height of the consecutive frames, until enough head-room is obtained to allow the top sheeting for the descending gallery to be inserted at the proper height and angle. The frame at this point is made with a cap-sill (upon which the sheeting rests directly), and a second cross-piece below it, serving as a cap-sill for the descending gallery. From this point forward the frames may be set perpendicular to the axis of the gallery, as previously stated.
If the descending gallery is very steep and the horizontal pressure of the soil great, it may be necessary to strengthen the stanchions of the last two or three vertical frames by cross-pieces near their upper ends.