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The southern approach to the Wainui Saddle, the highest point on the new Transmission Gully motorway – extensive earthworks have been required, with benching to stabilise the slopes

Major milestone for Transmission Gully's highest point

One of the most active worksites on the Transmission Gully motorway project is the Wainui Saddle, the highest point on the new road. And while it is one of the shortest sections – just 800m of the 27km of new motorway – it is one of the most complex earthworks challenges of any project in New Zealand.

Transmission Gully is renowned for its steep-sided, isolated valleys. Named from the 110,000-volt transmission line that used to run through it, it is the location for one of New Zealand’s newest motorways, running approximately north-south between the Kapiti Coast and Tawa through the hills east of Porirua.

The four-lane motorway will feature four new interchanges, two new link roads, 25 structures (bridges and major culverts), 534 ha of ecological mitigation and 27 km of stream remediation, and will provide a route from Wellington to the lower and central North Island that is more resilient to extreme weather and earthquakes.

The highest point of the motorway is the Wainui Saddle which marks the boundary between Porirua City and Kapiti Coast District. Due to the scale of the work, the steepness of the terrain, and the Ohariu fault line that runs right through the saddle, this section of road poses one of the most complex challenges for the CPB HEB joint venture design and construction team.

Early access and stabilisation works

Just getting to the saddle was difficult in the early days of the project, a team spokesperson says. “It’s a remote location and access was initially only available via a farm track. We started constructing access tracks into the saddle in February 2016 from the Paekakariki (northern) end. It was an incredible and challenging path and work had to be done in stages.”

The team were able to start clearing vegetation from the saddle slopes in May 2016, and the first cut and slope stabilisation work was completed in August that year. Four years on, the manner in which the cut has had to be constructed has made it one of the most challenging road engineering projects in the country.

“Work in the saddle has been non-stop as this section is on the critical path of delivering the Transmission Gully project,” says the spokesperson. “It took 18 months of nightworks during the middle stages of construction to progress some of the bulk earthworks in the saddle, with our teams working in shifts, 24 hours a day.”

Cuts had to start from the top down. It was a detailed earthworks operation to construct the cut benches. The slope stabilisation had to be sorted before cuts could begin. The benches are 10 m high and in the deepest part of the cut there are six benches. These benches were generally cut in 4 to 5 m drops to allow slope/face stabilisation works to be completed prior to the next cut. The cuts are generally at a 65-degree gradient.

“One of the things that made stabilisation so difficult was the steep slopes above the initial cuts,” explains the team spokesperson. “Some sections had slopes that were over 150 m high above the initial cut faces. A particularly challenging aspect of this was managing stormwater runoff through the winters of 2016 and 2017. Stabilising the numerous valleys above the initial cut benches was a real challenge with limited access for machinery and plant.”

Water, water everywhere

When you want to do earthworks, water is not your friend. The big surprise for the project team was that initial boreholes didn’t indicate the level of groundwater that was actually found – the amount of groundwater expected was a lot lower. Not only did workers have to deal with stormwater issues, but a fair amount of water was seeping out of the cut faces.

Horizontal bored drains were inserted, varying from 20 to 40 m, into the cut to relieve the water pressure and make it stable. Drainage pipes 50 mm in diameter were installed into bored holes 100 mm wide. Due to the topography, the team found it really hard just to get clean water diversions installed.

“Just opening up the area was a real challenge,” the team spokesperson says. “Due to the constrained nature of the gully on both sides of the saddle, it was a difficult cut, the fill sites were difficult, and it was difficult material to place.” The saddle is the main fill source for the road fill embankments for both the Te Puka and Horokiri valleys to the north and south. “Add to that the streams that the team were working over, and it just made everything really hard and slow-going, with limited space for efficient earthworks.”

Some nervous moments

The Ohariu fault line extends northeastward from offshore of the Wellington south coast near Tongue Point through Porirua towards Paraparaumu. It runs right through the Wainui Saddle. As a result, the underlying rock material is fractured due to earth movement along the fault line. Inconsistent weathering of the material in the upper parts of the saddle further complicated cut and fill operations.

There were numerous health and safety risks. The project team had to deal with the difficult issue of rock falls and unstable slopes during the excavation. Specialised instrumentation was installed, and continuous monitoring regimes implemented to ensure the cut faces were stable during the construction phase.

The project leaders selected very highly skilled excavator operators to work the initial access tracks to the top. “There were some nervous moments when they cut that initial track up. Normally, operators have a big bench that they’re sitting on with a bund (a temporary ridge of compacted earth) that stops the excavator going over the edge, but the excavator operators that pioneered up to the top of the saddle had to work at the limit of what the plant could do up there,” the spokesperson says. “It took weeks of planning and meticulous reviews of our safety controls to ensure this was done without any incidents.”

One of the main purposes of the new motorway is to provide resilience for Wellington in the event of a large earthquake, so no shortcuts were taken. There are stabilisation bolts going into the cut faces up to
8.5 m deep. “With the slope stabilisation works and cut bench configuration, the public can be assured that the Wainui Saddle will be a much safer stretch of road to be on in the event of an earthquake than the coastal route along State Highway 1,” the spokesperson adds.

Completion

The earthworks on the Wainui Saddle are nearly complete, with just 15,000 cu m left to go (as of early October) before pavement works and roadside drainage can begin. When the pavement is complete, the northern and southern approaches to the saddle will be at gradients of 8% and 6.5% respectively. An additional uphill ‘crawler’ lane is provided on the steep gradients.

The Wainui Saddle is a big job and it’s not finished yet, but strong ‘bones’ are now in place for this challenging section of road that will take traffic between Pauatahanui and Mackays Crossing at Queen Elizabeth Park.

The Transmission Gully motorway is scheduled to open to traffic by September 2021.

Watch a timelapse video of the work around the clock on the Wainui Saddle here: youtu.be/ChOQhtw2vGI


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