Railway tunnels in Porirua City

This page desribes the construction details of the tunnels between Pukerua Bay and Paekakariki on the main trunk line.

History of the tunnel construction

The most technically difficult section of the W&MRC was the construction of six tunnels in five kilometres through the challenging coastal cliffs between Pukerua Bay Station and Paekakariki (Contract 15); the line also dropped fairly steeply at 61 metres in eight kilometres to the flats at Paekakariki.

The tunnels were Pukerua (No. 8, 152.69 metres), St. Kilda (No. 9, 278.02 metres), Seaview (No. 10, 186.48 metres), Brighton (No. 11, 244.22 metres) and the shortest, Neptune (No. 13, 58.74 metres). The sixth tunnel (No. 12, 270.66 metres) was between Brighton and Neptune. The contractor for this work was Samuel Brown from Wellington.

The works locomotive Belmont being lowered down the construction tram to the temporary tracks on the foreshore.
Tunnel construction in progress, ca. 1885. “The works locomotive Belmont is being lowered down the construction tram to the temporary tracks on the foreshore, Pukerua Bay beach settlement below.”
Reproduced from An Uncommon Carrier, p.33

The tunnels were driven through almost vertical cliffs above the sea and the one-sided pressure on the tunnels and poor rock conditions made them particularly vulnerable during earthquakes, which were not infrequent. While the first tunnel was being constructed an earthquake caused a slip, caving in the tunnel and resulting in the deaths of three men. After this disaster, work on the first tunnel progressed slowly and it took six months to complete.

The relatively soft nature of the rock can be inferred from the rapid pace of construction of the tunnels. For example, the 186 m Seaview tunnel only took 10 weeks to complete.

The No. 12 tunnel proved to be most problematic with earthquake shatter belts encountered inside it, making the excavation difficult and risky. A lawsuit following an incident in 1900, when a piece of brick dislodged from the tunnel roof, broke a cab window and caused a piece of glass to pierce the eye of a driver, ultimately resulted in the tunnel being abandoned and the line was re-routed around the hillside. [See Ref. viii]

Access to the tunnel sites was particularly difficult. Brown used Pukerua Bay as a landing post and the materials, including all the bricks, rails and sleepers were brought to the beach in lighters from ships moored further off-shore. An incline tramline was constructed at the north end of the beach settlement, running from the rail alignment down to the beach and the construction materials were run up the tramway from the beach; this process is shown in the photograph above.

Initially brickworks were set up at Paekakariki and Muri to produce materials for the tunnels, however, these bricks proved of poor quality. [See Ref. ix] Ultimately five Wellington brick-makers, as well as the brickworks at Mount Cook Prison supplied bricks for the tunnels. There is also a small site near the No. 8 tunnel reputed to have been a brick manufactory. [See Ref. x] Some of the tunnel bricks have the prison arrows on them (like those that can be seen on the former Mt Cook Police Station wall in Tasman Street, Newtown). Bricks were used to line all the tunnels except Neptune (No. 13 tunnel), which was lined with concrete.

On 4 October 1886 the tunnels and tracks were finished and a small ceremony was held to mark the occasion. The Chairman of the W&MRC, J. E. Nathan, ordered a special train for the occasion and at No 12 tunnel a marble plaque bearing the names of the directors, engineers and contractors was placed on the wall of the northern portal of the tunnel.

According to Cassells birch sleepers were originally used but these subsequently rotted out and were replaced with iron bark and heart totara. [See Ref. xi]

All the tunnels remain in use today, except for the decommissioned No. 12, which was largely dismantled except for the northern portal, and the commemorative plaque removed. No. 11 tunnel has had a protective wooden ramp added to its southern portal to protect it from a stream that falls down the hill at this point. Other modest changes have been made to the tunnels over the years to accommodate different rolling stock and electrification, and the tunnels can accommodate the largest rolling stock in normal use.

Design of the tunnels

The six tunnels between Pukerua Bay and Paekakariki are one of the most distinctive features of the rail line.

The tunnels were designed to the same standard W&MRC pattern – much the same as the contemporary NZR pattern – with a horse-shoe shaped paraboloid pointed-arched cross section lined in brick (concrete on No. 13, which must have been especially unstable). The shape was a compromise between structural efficiency, minimising the volume of excavation and providing clearances to the roofs of the rolling stock. This compromise was of absolute importance when tunnelling was mostly carried out by hand excavation, but the tunnel dimensions today restrict the size of rolling stock that can be used on this section of the line.

The outside faces were finished with plain plastered portals at either end. These portals have a flush face extending into the rock on either side and are capped with a horizontal cornice. Over the years, the tunnel floors have been progressively lowered to fit larger rolling stock through, retaining walls and rock-fall guards have been put in place, and waterfalls and streams diverted over or around the tunnels.

References

viii New Zealand Railway Tunnels at 31st March 1975  http://www.teara.govt.nz/en/1966/tunnels-rail-and-road
ix Hoy, Douglas George. West of the Tararuas - An illustrated history of the Wellington Manawatu Railway Co. Wellington : Southern Press 1972
x Hoy, D. G. Rails Out of the Capital, Wellington : New Zealand Railway and Locomotive Society, Wellington Branch, 1970 diagram p120
xi Cassells, K. R. Uncommon Carrier - The History of the Wellington and Manawatu Railway Company 1882-1908 Wellington, 1994 p. 88

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