I9I0.] DAVIES— TUNNEL CONSTRUCTION. 179 



made without immediate support ; but as soon as it is enlarged, the 

 soil will cave in, unless given support to hold the material in situ. 

 This is the purpose of the shield, and it is also the theory upon which 

 tunnels are constructed by the ordinary mining methods. With the 

 latter method air pressure has always been used to keep the water 

 back. Narrow advance headings are driven corresponding to each 

 side wall, and about at the springing line, in which timber mud sills 

 are laid, acting as foundation for the temporary steel sets which 

 support the roof. Then as the soil in the upper portion of the 

 tunnel between side bearings is removed, length by length, these 

 steel centers are put in place to carry the wooden planks known as 

 " laggings," which are placed tight together to give continuous solid 

 support to the external soil. Then the lower portion of the excava- 

 tion is removed and posts with lagging put in below the sills, to 

 hold the roof and sides until the permanent lining of reinforced 

 concrete can be put in place. 



In these subaqueous tunnel operations, as the influx of water 

 is the " bugaboo " of the " sand hog," for the restraint of which air 

 pressure is employed, so the greatest danger arising from the use 

 of air pressure is the " blow out." Safety in such operations is 

 dependent on the maintenance of a nice balance between the air 

 pressure within and the water pressure without the tunnel. In 

 the case of a diving bell or caisson, the exact amount of air pres- 

 sure necessary to balance the column of water extending vertically 

 from the rim of the diving bell or cutting edge of the caisson to 

 the surface of the water, can be automatically determined. If too 

 much pressure is used, the excess will escape under the cutting 

 edge, and if not enough, the water will rise in the working chamber 

 above the cutting edge. In the case of a tunnel, however, no such 

 exact determination can be had, as the bottom of the tunnel, being 

 deeper, requires a greater pressure to exclude the water than the top, 

 and if this pressure is used, there will not be enough water pressure 

 at the top to prevent the air from escaping, and if only enough is 

 maintained to balance the water at the top, the bottom will be 

 flooded. Fortunately the overlying material, to a greater or less 

 extent, dependent on its character, permits of the maintenance of 

 a greater pressure than that due to the hydrostatic head, so that the 



