=166. Shields.=—Shields are used in tunneling in soft wet material and are particularly suitable for work under air pressure. They are used in rock tunnels where water is anticipated or air pressure is used. The shields often save the expense and difficulty of timbering as the masonry of the sewer follows closely behind the shield. Fig. 118 shows the arrangement for a shield for tunneling in soft material in the construction of the Milwaukee sewers. The shield has an exterior diameter of 9 feet 4 inches and an overall length of 9 feet 8⅛ inches. The cutting edge section is 20 inches long. The shell is made of one inch plate to the back of the jack chambers and one-half inch plate in the tail. The shield is driven by ten 60–ton hydraulic jacks. The jacks are shown in position in the figure. These jacks rest against the finished tunnel lining and serve to consolidate it at the same time that they push the shield into the material to be excavated. The face of the tunnel is cut with a pick and shovel while the jacks are removed one at a time and a new ring of lining is put in place. The lining may be temporary timbering to receive the thrust of the jacks, but it is usually desirable that the permanent lining follow immediately behind the shield. Since the shield is larger than the outside of the lining the space left by its passage should be grouted immediately after it has passed. =167. Tunnel Machines.=—Tunnel machines have been used successfully on sewer tunnels in soft materials, but not in rock.[95] The machines are of different types, but in general consist of a revolving cutting head, equipped with knives, and driven by an electric motor. The bearing on which the shaft for the cutting head rests is supported against the sides of the tunnel. The muck is carried away by means of a conveyor and dumped into muck cars without rehandling. Rapid progress can be made with these machines in suitable conditions. [Illustration: FIG. 119.—Method of Drilling and Loading Rock Tunnel Face. Courtesy, Aetna Power Co. ]