Wednesday, 5 January 2011

State Highway 2 Matahorua Realignment Nears Completion (Part 1)

The $30 million realignment of part of State Highway 2 at Matahorua Stream is nearing completion and NZTA has invited the public to preview the new section of highway prior to its February opening. The section of highway is directly adjacent to the Palmerston North Gisborne Line railway at the Matahorua Viaduct, which itself crosses over the old highway. In addition the old highway section incorporates two additional grade separated crossings of the railway line. These three crossings are replaced by a single new bridge over the railway. The existing highway bridge over the Matahorua Stream about 100 metres downstream of the railway viaduct will be replaced by the new concrete viaduct just upstream from the railway.
Refer now to the map below for a detailed description of the old and new routes.

View Larger Map
As is the customary practice on these maps, specific roads are shown as black lines while the railway in this case is a red line. Symbols are used to highlight specific features.
Starting from the bottom left corner, the old highway (still open at the time of writing) takes a sharp S bend across the railway via an overbridge which required drivers to reduce their speed to 45 km/h (quite significant when the open road speed limit is 100 km/h, although the bridge grade would also encourage speed retardation for most drivers regardless). The bridge itself was probably originally a grade separation project of earlier years when curvature radius was not such an important consideration, as up and down the country many such overbridges with similarly restrictive approaches dating from the first half of the 20th century are gradually being replaced (such as the one at Waipukurau last year). Travellers continuing north-east soon encounter a further succession of highway condition warnings, including one for falling rocks in the gorge where the highway passes under the railway. (Presumably since the demise of passenger and stock trains on the line the risk of being hit by falling materials from the train itself is lessened these days).
Since old and new highways cross over in the gorge, I’ll now contrast the new highway up to the point where the routes cross over. Once the new section is opened, traffic from the south-west will simply continue straight on without crossing the railway, executing a gentle right hand turn that probably will not require any speed reduction and will not have the severe grade changes of the current route. As it crosses the gorge at its top the unstable rockface alongside which the current route runs is bypassed. A new concrete viaduct will give travellers a sidelong view of the Matahorua rail viaduct where previously they would have seen it from underneath at a considerable height difference. Before it gets to the viaduct, the old route descends into and runs through the gorge of the tributary Kahika Stream and at the confluence it crosses the Matahorua and makes use of its gorge for a time before climbing out. It is the use of these gorges that makes this a challenging section of highway in many ways and eliminating them is the obvious goal of the highway project.
Back on the old highway and after crossing a curved 1930s style concrete bridge we are going under the Matahorua railway viaduct built around the same time. At this point travellers would also have seen the new highway viaduct in the last year or two. There have been several highway closures in recent months to allow sections or components of this viaduct to be lifted into place. After this the highway is climbing out of the gorge over the next kilometre. The opportunity to gradualise this over the whole length, as would be done today, again seems to have been lost on road builders of the era, and consequently there is another short sharp shock at the exit of the gorge, incorporating a passing lane for uphill traffic. As the end of this lane is reached, the rail overbridge with its height restriction warning is encountered. This bridge is fitted with protection beams as some other bridges have been, which suggests it may have been hit by overheight road vehicles in the past. The highway continues its long sharp rightward curvature before reaching a reverse curve taking it past the point where the new highway will join.
On the other hand, travellers on the new highway, after crossing the new Matahorua viaduct, will  take a gentle curve left and pass over the railway line on a new concrete overbridge. I would presume the railway was closed to traffic for periods to enable the parts of this bridge to be lifted into place at various times. The new highway will then curve gradually to the right in order to join the present highway. We can immediately see the compelling nature of these improvements and why this work has been a high priority for improving the highway between Napier and Wairoa, eliminating a narrow twisting road in a gorge with many and severe changes of grade, sharp curvature, visibility restrictions and falling debris from unstable rockfaces as well as the risk of damage to the railway bridge. I expect most of the old route will be closed as there is little land along it that requires public access from it. However, as there is a fairly solid concrete bridge that would have to be demolished, it’s entirely possible the route may be abandoned rather than demolished, or converted to some other function.