Compressed Air Work And Diving 1909

EFFECT OF STRATA. 73 When the strata is of a soft, silty nature, no excava- tion will be required, but the shield is forcecl forward by the hydraulic rams. The area of the openings is arranged to be much less than the cross sectidnal area of the tunnel, and this helps to consoliclate the grouncl in front of the shield. The material is forced through the openings and comes out looking like clay when coming out of a machine for making wire cut bricks. As it comes through it fails behind the shield in big lumps and can then be loacled into waggons for removal. When this methocl is usecl the excavated material to be removed will be no greater than the calciilated displacement of the tunnel, and is often even less in soft grouncl. With sand or very soft material the difficulty of work with a shield is much increased, and unless great care is taken, the amount of material to be removed will much exceed the displacement. With such soft material the difficulty of regulating the air comes in. As in caisson work, if too great a volume of air is allowed to escape at the face, blows will be caused, and there will be difficulty in maintaining the pressure. If too little air is used, the water will come in at the lower le vels of the face. It is best to do with as little pressure of air as possible. Any surplus required for the Health of the workers can be allowed to escape through a valve with an automatic cut- off. The pressure that will be most suitable can be only determined by trial when passing through the strata. With close ground above and open grouncl below it will probably be found best to regulate the pressure so as to balance the water pressure at the point in the working face where the two meet. As a general rule the two pressures should balance at the level of the crown of the tunnel or a foot or two below it. When there is only a few feet of grouncl between the