Page:Walker (1888) The Severn Tunnel.djvu/137

78 would be, on what line and at what level it should be driven to pass through the most favourable ground, and encounter the least risk? To determine these questions, it was necessary to put down borings and ascertain the nature of the strata. The geology of the district was carefully studied by Mr. Richardson, borings were taken at many points, and the strata were supposed to lie horizontally. But in the execution of the work they were found to be much contorted, as will be seen by the longitudinal section of the tunnel.

The tunnel was adopted by the Great Western Railway Company in preference to a bridge, and the length of 4½ miles, which was originally fixed for tunnelling, was the length between the points at which the gradients rose, to 60 feet from the surface.

Tunnelling may be in rock, or in very soft or loose strata.

Tunnels may be required to be executed with great speed; or if they are short, speed may be no object. The consideration of speed will determine, to a large extent, how the tunnel should be carried out.

Forty years ago there were continual discussions as to whether the best method of constructing a tunnel of any length was by driving a top heading or a bottom heading. If the tunnel is short, and in solid rock, no heading may be necessary.

In the long tunnels through the Alps, which generally required no lining, a top heading was driven by one gang of men, widened out by other