In the 1910s the double-track Wingatui Tunnel was driven to replace the single track tunnel known as “Chain Hills”. The original tunnel was said to be very difficult to drive because of ground conditions and the replacement encountered similar problems. In the original tunnel an accident took place in 1875 in which there were two deaths caused by a collapse of the inside of the tunnel.
This is a complete contrast from the Caversham Tunnel in which the replacement was cut through sandstone and is completely unlined in a number of areas because it is a very stable material. It is unusual to have a tunnel that is unlined on the Main Trunk.
THE NEW ZEALAND SOCIETY OF CIVIL ENGINEERS (Inc.)
Proceedings 1920 - 21
THE DUPLICATION AND RECONSTRUCTION OF THE DUNEDIN.MOSGIEL RAILWAY. (1906 - 1914).
By MR. W. R. DAVIDSON, Assoc. M. Inst. C.E.
<<Extract from the full document >>
- The approach to the Mosgiel end of the Chain Hills new tunnel required the excavation of approximately 100,000 cubic yards of clay, the maximum depth of cutting being 55ft.
- The rock through which it passes is mica schist of a particularly treacherous character, the country being waterbearing and very much faulted.
- Trouble with the large section tunnel was anticipated in view of serious difficulties met with in the old single track tunnel.
- At the start the tunnel was built in 16-ft lengths, but on very heavy ground being met with, these lengths had to be reduced to 8ft, and thereafter a standard of 12ft was adopted for all classes of ground.
- The tunnel is 19ft 9in. high with a maximum width of 25ft 71/2in. The profile is curved throughout, the walls being built to a curve of 22ft 4 1/4in. radius, and the roof arch to a curve of 13ft 6in. radius.
- The walls are 12ft 6in. in height, formation to springing, and are built of mass concrete, or hard tunnel stone set in cement mortar. For a short length where rock movement was anticipated the walls are built of brick in a piece with the arch.
- The arch throughout is built of hard burnt brick in four half-brick rings for light ground, and five and six rings for heavy ground.
- Excavation at the Abbotsford end was greatly assisted by a forward dip in the stratum, but this made bad ground doubly treacherous.
- Extremely wet and heavy ground was met with at one point where 50ft of tunnel was constructed at a cost of £2,100, or £126 per lineal yard.
- At another point the floor of the tunnel rose 10in. and it was necessary to build 72 feet of concrete invert. 2ft thick.
- Serious difficulties were met with at the Wingatui portal. Excavation was just about to begin when it was found that the rock suddenly faulted and a mass of soft mud and ancient forest detritus had to be penetrated. To give strength of attack a length of tunnel lining was built in the open. With this as a base to carry the pressure the wall of mud was eventually penetrated.
- After excavation had advanced about 10 chains a heavy rainstorm brought down a series of slips that almost buried the mouth of the tunnel.
- The carrying away of a steel flume caused the flood waters from a group of gullies above the tunnel to pour in at the portal in spite of a clay dyke built to provide against such a mishap. This entailed the pumping of about 300,000 gallons of water before work could be resumed at the face. The water draining from these gullies is now intercepted by a dam and contour flume of concrete, and led across the Wingatui cutting in a reinforced concrete flume to its natural course.
- The construction of this tunnel involved the excavation of about 62,500 cubic yards of rock and the placing of 2 1/2 millions of bricks and 5,700 cubic yards of concrete.
- The cost per lineal yard was £69
- (For comparison the cost per lineal yard for the Caversham Tunnel was £20).
