184 DAVIES— TUNNEL CONSTRUCTION. [February i8, 



placed side by side at the same elevation, as is usual, dangerous 

 grade crossings would have been established at three points of junc- 

 ture, which would have impaired safe and rapid operation. The 

 idea was therefore conceived of placing the tracks for trains moving 

 in opposite directions close together, but one above the other, so 

 that at points of intersection they would be on dififerent levels. At 

 the three junction points where each single tunnel diverged, it was 

 necessary to build a switch chamber or section of tunnel, having 

 the width of a single tunnel at one end and gradually widening 

 until sufficient width was obtained for the construction of the two 

 diverging tubes ; and owing to the separation of grades in the tubes, 

 these chambers needed to be double decked. As the shield is only 

 adapted to the construction of a fixed size and form of tunnel, it 

 could not be used, and some other method had to be devised. 



To construct these chambers by underground mining methods 

 involved very serious hazards, due to their great size and the un- 

 stable character of the material at the roof level. On the other 

 hand, to dig an open pit from the surface was not feasible, owing 

 to the great depth and danger of infiltration of water from the river 

 nearby into the excavation. The method finally adopted was to 

 build these sections of tunnels on the surface as monoliths of 

 concrete reinforced with steel, and then to sink them as pneumatic 

 caissons to their final position. They were built complete with the 

 exception of the bottom of the lower chamber, which was left open 

 so as to serve as a working chamber for the excavators. The ma- 

 terial excavated was passed out through shafts extending through 

 the upper chamber, and thence through an air lock to the surface, 

 whence it was removed or dumped on the descending caisson in 

 order to give additional weight to cause the caisson to sink by over- 

 coming the friction of the surrounding material on the sides. 

 When each caisson had been sunk to the proper level, they were 

 completed by an inverted concrete arch closing the bottom and 

 sealing the connecting tunnels joined to them. In some cases the 

 shields were driven to the caissons, then rolled through and con- 

 tinued on the opposite end. Each of these caissons took about 

 three months to sink, and the largest weighed twelve thousand tons. 

 During the construction and sinking, by arrangement with the rail- 



