144. When a wall already built is supported on beams or needles placed transversely through holes cut in the wall, as shown at b, Fig. 57, and supported at each end by posts, jack-screws, ox pumps, as shown at f, it is said to be needled; and the operation of preparing it is called needling.

Needling 59

Fig. 57.

145. Fig. 57 shows how a wall is held up by needles in order that a new wall may be built under the upper portion, or columns and girders placed to support it.

At a are shown the holes in the wall, cut at intervals to receive the needles b; these needles consist of heavy timbers to carry the upper wall c. Where the needle enters the hole in the wall, small cross-beams d are laid on the upper side, and wedged in with oak or iron wedges, as shown at e, in order to secure a larger and more even bearing on the wall. At the inner and outer ends of the needles, heavy perpendicular timbers f are placed to support the needles and carry the weight of the wall. The foot or ground bearing of these timbers is formed by three courses of heavy plank, shown at g, crossing each other at right angles to spread the weight over more surface. Wedges h, h are driven under the foot of each upright, forcing the ends of the needles up until they show a slight downward deflection, or bending at the center, thus indicating that the weight of the wall is carried on the needle. As soon as the needles carry the wall, the intermediate portions may be torn out, and the columns and girders, or other substructures, are put in place, or the excavation to the lower level begun.

In cases where the ground is soft or compressible, sheet-piling is placed around the foot of the uprights, to hold the ground in place.

146. Very frequently, especially for heavy work, when a high wall is to be underpinned, steel beams are used for the needles, jack-screws are placed under the foot of the uprights, or a crib of heavy beams is built up in place of the upright posts.