TUNNELS, RAIL AND ROAD

New Zealand's ubiquitous hills and mountains have made tunnels a normal part of railway construction. Thus the Auckland-Wellington Main Trunk line of 426 miles includes 7¾ miles of tunnels. Few of the 25 tunnels are in really stable country. Full support has had to be given to the ground by timbering as excavation proceeded, and a substantial, permanent, concrete lining placed to “hold” the country.

*Curved

One of the first tunnels built in New Zealand connected Christchurch with its port of Lyttelton by single-track railway. It was begun in 1861 and cost £200,000 for a total length of 1 ½ miles. A two-lane road tunnel has now been built only a few chains away and almost parallel to the rail tunnel. It cost £3 million for 1 ? miles (with ancillary works) and was completed in February 1964. The construction entailed precise surveying and levelling, drilling and blasting of rock, earthmoving, draining, concrete lining, lighting and ventilation – this last a most important consideration in road tunnels. This tunnel is fully ventilated using the “cross-ventilation” system with inlet and outlet exhaust fans, each capable of moving 320,000 cu. ft. of air per minute. Electric monitoring and measuring devices will quickly detect noxious gases (especially carbon monoxide) and, by remote control of ventilating machinery, will keep them below danger level. The tunnel is lined throughout with ceramic tiles and has special lighting installed. There are 2 ½ miles of special approach roading, with bridges, toll plaza, traffic stations, and roundabout. The tunnel itself and its buildings contain interesting (and for New Zealand entirely novel) uses of precast, pre-stressed concrete. The breakthrough took 12 months; an average of 23 ft of completed tunnel was driven each day (800 cu. yd. of rock excavated) by a three-shift construction team.

The construction of the Otira Tunnel through the Southern Alps has had possibly a greater influence than any other single factor on the development of the West Coast of the South Island. This singletrack railway tunnel is 5 ? miles long. It was begun in 1908 and, after many difficulties, finished in 1921. It is 15 ft 6 in. high and 15 ft wide. There are no lights in the tunnel. Natural ventilation has proved sufficient, as the railway is electric.

The Tawa Tunnels, giving better railway access from the west coast of the North Island to Wellington, were excavated only with great difficulty. The two tunnels aggregate 3 ½ miles in length and are on a grade of about 1 in 120. The finished dimensions of the tunnel inside the 2 ft-thick concrete lining are width, 26 ft and height, 20 ft. The dangerously unstable and variable nature of the ground made it necessary to “timber” the whole tunnel immediately after excavation. Each of the 8-ft long arch segments making up a “set” was cut from 10 in. × 10 in. timber; the spacing of these sets was generally at 5 ft 3 in. centres, but at times it was necessary to place them at 1 ft 4 in. centres–almost touching. These tunnels were completed in 1937.

The 5·46-mile railway tunnel through the Rimutaka Ranges 30 miles north of Wellington was driven through better country. The contractor was therefore able to use “full face” mechanised excavation techniques throughout, the maximum progress being 425 ft of excavation in a week. The tunnel was begun in October 1953 and completed in November 1955. It is 15 ft 4 in. wide and 17 ft high.

The Homer Tunnel, which links Milford Sound and the Hollyford Valley, had many unusual construction difficulties. The Hollyford portal, at an elevation of 3,000 ft, is at the foot of vertical rock faces 2,000 ft high, at the head of a glaciated valley. Destructive snow avalanches, accompanied by tremendous air blasts, caused several fatal accidents and often damaged outside installations, including the destruction of the substantial reinforced concrete shelter which extended some 10 ch out from the portal. The air blasts have been known to shear off 12-in. hardwood poles 4 ft above ground level, blow over a crawler tractor weighing 4 tons, and shear off 20-in-diameter beech trees without uprooting them. The tunnel, 20 ft by 12 ft in cross section, had to be driven down hill on a grade of 1 in 10 through some 3,500 ft of extremely hard granite-type rock. When the tunnel was completed, the rock continued to “pop” in places, and so regular scaling of loose rock had to be carried out. Rock bolting was also used.

Kaimai Railway Tunnel

The Kaimai Railway Tunnel, which is to be driven through the heavily forested Kaimai Range as part of a new 15 ½-mile railway deviation linking the eastern Waikato district with the Bay of Plenty, will have a completed length of 29,000 ft. It will be the longest tunnel in New Zealand, exceeding the length of the 28,864-ft Rimutaka Tunnel by 136 ft. Work on the earthworks as part of the construction of the tunnel approaches had already begun by October 1965. The tunnelling contract is to be let at the end of 1966, and it is hoped that the whole deviation will be ready for use about July 1970.

This major construction project, including the tunnel, station facilities, bridges, track, signalling, communications, and other equipment, is estimated to cost some £5,700,000. It is expected that more than 1,000,000 tons of railway freight will pass through the tunnel during the first year after it has been completed. Ministry of Works engineers are making extensive use of modern technology in their preparatory work, including helicopters to check surveys and to place trigometrical stations at various points in the ranges. To measure distances, they are using a tellurometer, which is an electronic measuring device worked by radar impulses.

by R.W.S.