In other tunnels the complete section has been excavated and timbered, and the work of building commenced from the foundation of the side walls. A strong continuous invert from side wall to side wall is necessary where passing through soft swelling clay or loose strata intersected with small streams of water. Where the material is very solid and dry, it is not necessary to introduce inverts, but the foundations of the side walls should be laid at such a depth below rail-level as not to be affected by drain-water running through the tunnel. The side walls and arching may be either of masonry or brickwork, but should be of the best description, especially for the facework. For brick arching only the best hard-burnt bricks should be used, and the inner or exposed ring should consist of selected hard fire-bricks to withstand the heat and gases escaping from the funnels of the locomotives. The thickness of the side walls and arching will depend upon the description of material to be supported. In some places a comparative thin lining may be sufficient, while in others extra thickness must be given to resist the great pressure exerted by expanding clay and loose wet strata. Weeping-holes, or small drain-pipes, placed low down must be left in the side walls every three or four yards, or closer in very wet places, to allow the water collected at the back of the walls to escape into the side drains of tunnel. In building the arch portion every effort should be made to have close solid work without any open joints or spaces through which the water may run, and the crown of the arch and a few feet down on each side should be coated with cement or asphalte to lead all water away from the top to the sides. Water dripping from the under side of the arch on to the line is a great destructor of the permanent way materials, especially the fastenings; and bolts, nuts, fish-plates, and spikes placed in a wet dripping tunnel will not last half the time they would out in the open line, where they would have the sun and wind to dry them. [Illustration: Fig. 212, 213, 214, 215, 216, 217, 218, 219] Small arched recesses or niches should be formed in the side walls at 176 convenient distances to serve as refuges for platelayers or others working in the tunnels. It is most essential that the space between the masonry and brickwork lining and the facework of the excavation should be carefully filled in and hard packed, so as to prevent the possibility of pieces of rock or other material falling on to the top of the arch. The neglect of this precaution may lead to a casualty years after the tunnel has been completed.