Otira Tunnel, Midland Railway

Category
Engineering Work (eg road, bridge, sawmill, dam)

Description
This 8,529m tunnel was the longest in the British Empire at the time of its construction.

Driven through wet shale and rotten rock, this 8529 m tunnel was opened in 1923 and was then the longest in the British Empire. It completed the Christchurch to Greymouth rail link. The Westport Stockton Coal Co., used electric locomotives from the Stockton Coal Mine to Millerton brakehead above Ngakawau. Installed about 1905 and superseded about 1950 by the aerial ropeway as the major means of bringing coal down from the coal plateau to the railway at Ngakawau.

The Midland Company had intended to cross Arthurs Pass by using the Abt rack rail system on a grade of 1 in 15 rising some 600 feet (180 m) from Bealey Flat station (the present Arthurs Pass township) to Arthurs Pass station at the summit of the pass itself – then descending 1500 vertical feet again mostly at 1 in 15 above the Otira Gorge on the slopes of Mount Rolleston to regain the Rolleston riverbed (a tributary of the Otira) a short distance away from the present western Otira tunnel portal – some 6.5 miles of rack railway involving two engine reversals. A number of short tunnels were required in the Otira Gorge to avoid broken and unstable country.

This would not be considered as a practical solution for a trunk railway intended to carry heavy traffic. Each rack rail engine could haul about as much as its own weight on such a grade at a speed of little more than walking pace whether traveling up or down grade. Thus a very limited tonnage of freight could have been taken over the line each day. During the nineteenth century the alternatiave to a railway was for freight - the bullock team and for passengers - the Cobb & Co coach. It was not until the early twentieth century that a feasible alternative of a tunnel 5 miles 25 chains could be dicided upon with a steady gradient at1 in 33 descending from the east portal at Arthurs pass towards the west.

Tenders for the tunnel were called in 1907. Five years were to be allowed for its completion, in fact it took 15. The contract was awarded to John McLean & Sons, the most capable and experienced contractors in New Zealand at that time, for the sum of ₤599,794. As events were to prove the final cost of the tunnel would be over twice that.

Labour troubles bedevilled the job – union leaders of the day, Bob Semple, Paddy Webb and Tim Armstrong – promoted strikes and persuaded union members to work elsewhere than at Otira. This contributed to a chronic shortage of suitable labour which in its turn brought poor productivity, a rise in accidents, and not least, the rise of the parliamentary Labour Party.

McLean and Sons, who had been highly thought of both as responsible contractors and as good employers, petitioned Parliament to be released from their obligations from which they were freed towards the end of 1912. The company went out of business soon after, financially destroyed by the Otira Tunnel contract.

The Public Works Department had to step in and take over. World War 1 intervened and although work continued it was not pushed to any great extent. Eventually on July 20, 1918 the tunnel was holed through and the surveyors’ centre lines produced from each end were found to vary in line and level by only 0.75 inch (19 mm) for line and 1.25 inch (29 mm) for level. Excellent accuracy before the days of lasers. But much remained to be done – the headings had to be enlarged to full tunnel profile and the concrete lining set in place – so it was not until September 1921 that the tunnel was complete – as a tunnel only but not as a working railway.

When first projected the Otira Tunnel was to have been traversed by steam locomotives, railway electrification being still in its experimental stages. During the tunnel construction electric railway traction showed itself to be practical and economic proposition particularly in long tunnels with their inherent problems of smoke and furnace gases leading to undesirable and often dangerous situations. Tenders were called in 1920 for a steam generating plant at Otira, six electric locomotives and 8.5 miles (14 km) of electrified line. The successful tender was English Electric of London.

The Midland railway was then finally opened for through traffic in August 1923 after almost 40 years in the building. The railway company formed to build it had come and gone. The largest New Zealand contracting company had been destroyed – but the railway was complete.

Design: Public Work Depaprtment
Contractors: J. McLean and & Sons 1908 –1912, New Zealand Government 1912 – 1923.
Owner: Tranz Rail Ltd

Location
From Otira on West Coast to Arthur's Pass in Canterbury

Region/s
Canterbury, West Coast/Buller

Access Info
While the tunnell itself is not accesible, the portals can be viewed at either end. The daily Tranz Alpine train from Christchurch to Greymouth and return passes through the tunnel.

Nature of Engineering
Rail Transportation