The $55 million Wellington Inner City Bypass (WICB), a 1.2km-long arterial state highway road system between the Terrace Tunnel and the Basin Reserve, became fully operational in June 2007.

The project’s design complexities and innovative construction methods are gaining increased recognition. Drawing attention, as one of the most striking elements of the project is the 450m northbound trench section immediately south of the Terrace Tunnel, which relied heavily on concrete solutions.

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The WICB project involved the construction of a pair of two-lane one-way streets to provide access to and from the southern and southeastern parts of the city. The WICB project was a partnership between Transit New Zealand and Wellington City Council, with Opus International Consultants responsible for design and construction supervision, and Fulton Hogan as head contractor. 

A variety of design options were considered for the 450m long trench section of the route. As groundwater conditions and the high seismicity of the area meant that gravity walls were not practical, a combination of soil nailed walls and concrete U-shaped trough structure were chosen.

The trough solution comprises a 150m-long cast in situ 1m to 1.2m-thick reinforced concrete floor slab and walls forming a 13m-wide trough up to 7m high. The trough walls are propped at the top with 760mm diameter steel pipes at 5m centres to assist with transmitting the earth and seismic loads from one side to the other.

Allied Concrete supplied Remote Temperature Matched Curing (RTMC) data loggers to be utilised on the wall sections of the trough. This system was used to determine the earliest form stripping time, and thus reduce the time between wall pours. Each 10m long floor slab section used 150m3 to 190m3 of concrete and took up to eight hours to complete. Two cranes utilising two 2m3 skips were used to service the six agitators being used on the site.

External formwork vibrators were used to reduce the amount of time workers were required to be inside the formwork manually vibrating the concrete. Each 5m-long wall pour contained approximately 30m3 of concrete.

Soil nailed walls (SNW) up to 8m high support the western side of the trench, at each end of the trough. SNWs are a cost-effective top-down construction method, with the cut face being formed in sections and stabilised by drilling sub-horizontal holes and grouting in reinforcement bars on a regular grid, and finishing off with a reinforced shotcrete facing. The 200m length of soil nailed walls represents one of the largest such walls in New Zealand.

The soil nail reinforcement used 25mm diameter deformed Reid Bars (RB25), with associated couplers and washer, head plate and nuts. A larger 200mm square plate was used at the heads to transfer the shear loads. The SNWs have a 150mm thick mesh reinforced shotcrete to support the slope and transfer loads from the nail heads. The shotcrete is a 30MPa 13mm aggregate mix with specialised aggregates and plasticizers in order to ensure pumpability.

To provide an aesthetically pleasing appearance, the trough, soil nailed wall facing panels and acoustic barriers have fluted concrete finishes. The walls with their fluted facing required some innovative formwork.

Precast concrete acoustic barriers were provided at the top of the trough and SNWs, while EnviroPavers were used around tree pits to permit rainwater to permeate. In addition, a new single span, 10.5m-wide bridge was required to carry traffic over the trench. The deck consists of eight 1150mm wide precast double hollow core units. The WICB project also provided an opportunity to upgrade the area’s stormwater system. This work included 750m of precast concrete pipes, up to 2.5m in diameter, installed up to 6.4m below ground surface. Five large concrete chambers were constructed at the junctions of the new Te Aro Stormwater Diversion.

Through the use of innovative concrete solutions, the project was delivered to design, with minimal disruption, ahead of schedule and within budget. Furthermore, the trench and trough components have become a pleasing addition to the city’s landscape; appreciated for their functional design and intriguing aesthetic.