Engineering Wonders of the World
Volume III

182 ENGINEERING WONDERS OF THE WORLD. INLET END OF TUNNEL AT CRAIG GOCH DAM. {Photo, by courtesy of Messrs. J. Mansergh and Sons.) The longest tunnel on the route is the Hirnant, at the lake end. This is 3,900 yards long, has a circular section with a minimum diameter of 7 feet, and falls rather more than 2 feet in the mile. The Cynynion tunnel (1,520 yards) and Llanforda tunnel (1,640 yards) are separated only by a short 183-foot syphon. The driving of the fourth and last tunnel, that carrying the pipes under the Mersey, provided the greatest of the difficulties with which the engineers had to Tunnelling- contend. Ttøg tunnel, which, under the Mersey. as Mr- 1J)eacon has pointed out, was the first ever con- structed by means of a shield and compressed air under a tidal or other river through entirely loose materials, is lined with cast-iron segments bolted together. A shaft was sunk in each bank of the river, and the tunnel driven and lined for 57 feet at the Cheshire end. The first contractors then retired. Their suc- cessors commenced a fresh tunnel at a rather higher level, and succeeded in driving it for 61 yards. Then they too were defeated by the looseness of the river bed and the frequent inroads of water. Finally, Mr. Deacon took the matter in hand, repaired the shield, and completed the tunnel in four and a half months, so placing to his credit a memorable achieve- ment. The tunnel has an inside diameter of 10 feet, and can accommodate three lines of 32-inch pipes. As the difficulties at the Mersey caused seri- ous delay in delivering the Vyrnwy water to the Liverpool reservoirs, it was decided to effect a temporary connection while the tunnel was being completed. Mr. Deacon therefore had made a Temporary Connection across the Mersey.