238 TUNNELING 



the lake. For an area of several hundred square feet the surface 

 of the water resembled a pot of boiling water. At times the 

 air would escape very rapidly, and again only a few bubbles 

 would show. 



It need hardly be said that the work in this gravel was veiy 

 slow. It was impossible to blast or to tear out the large boulders 

 whole, as so much surface would be exposed that an inrush of 

 water would take place despite the air pressure. The method 

 of procedure was to excavate a heading and build the biick roof 

 arch first, and then to take out the bench and build the in- 

 vert. Fig. 121 gives a number of sketches showing how the 

 work was done. A short piece of heading was taken out, the 

 top and face of the bench being meanwhile plastered with clay 

 (Sketches b and c, Fig. 121) to reduce the escape of air, and 

 then the roof arch was built and supported on side sills resting 

 on the bench. Bit by bit the roof arch was pushed forward 

 until some little distance had been completed, then the heading 

 was plastered w*ith clay and the bench taken out little by little 

 and the invert built. All the gravel except the small area 

 upon which work was actually in progress was kept thoroughly 

 plastered with clay ; and as the air escaped through the com- 

 pleted brick work very rapidly, water was allowed to cover a 

 portion of the invert (see Sketch c, Fig. 121), so as to reduce 

 the area of escape. 



When a large boulder was reached, which lay partly within 

 and partly without the tunnel section, the lining was built out 

 and around it, as shown in Sketch d, Fig. 121. The boulder 

 was then broken and taken out. All through this gravel bed 

 the cross-section of the lining is made irregular by the con- 

 struction of these pockets in the lining to get around boulders. 

 Sometimes they were on one side and sometimes on the other, 

 or on both, or at the top or bottom. In fact, there was no 

 regularity. Despite the hazard and danger of this work, con- 

 tinual progress was made, though sometimes it was only 4 ft. 

 of completed tunnel per week, working night and day ; and, if 



