Shafts are also sunk in soft ground by constructing a concrete or metal shell resting on a cutting shoe on the surface. The material inside is dug out and the shell sinks of its own or added weight. The first section of the shell may be from 5 to 10 feet long. As this section sinks other sections are added. This is called the caisson method. It is advantageous in wet ground and when the shafts are to be left as a permanent manhole. If a permanent shaft is to be left in an excavation being braced with wood, the permanent lining should follow within 20 to 30 feet of the shaft excavation. This is done to avoid the difficulty of maintaining a great length of temporary wood shaft with the danger of collapse, or of blocks or other objects falling on the workers below. The distance between shafts is controlled by the depth and size of the tunnel, surface conditions, and the character of the material being tunneled. Except where surface conditions are crowded the shallower the cover to the tunnel the more frequent the shafts. The advantage of frequent shafts lies in the possibility of removing excavated material from the tunnel promptly, and in making ventilation of the tunnel easier. The saving made by the construction of numerous shafts must be balanced against the extra cost of the shafts. For the shallowest tunnels the shafts are seldom placed closer than every 500 feet. =165. Timbering.=—After the shaft has been excavated to the proper grade the tunnel is struck out either by cutting through the wooden sheeting or by removing portions of the caisson lining. Practically all tunnels except those in solid rock must be framed to some extent. Some of the types of frames used in tunnel construction are shown in Fig. 116. Different combinations of these may be used in different classes of materials. In solid rock which remains firm on exposure no timbering is necessary. Where the roof only need be supported and the sides are strong enough to be used for support, a timber “hitch” or frame supported on the sides of the tunnel may be used. This is suitable for loose rock roofs with solid rock sides. Timbering such as is shown in the lower left hand corner of Fig. 116 becomes necessary in extremely soft, wet, or swelling material, where the bottom and sides as well as the roof tend to push in. The remaining frame in Fig. 116 shows a form frequently used and lying between the two extremes indicated. In wet tunnels a channel may be cut in the bottom below the sill for drainage purposes as shown in this form. The needle beam method of timbering is also shown in Fig. 116. This method of timbering is used mainly near the heading because of the speed and ease with which it can be installed, but it is undesirable because of the space occupied.