SH21 Narrows Bridge repairs progressing


Work to stabilise the Waikato River banks adjacent to Narrows Bridge, just south of Hamilton on State Highway 21 (Airport Road), is progressing well.

Work is currently being carried out by the NZ Transport Agency beneath the Narrows Bridge to strengthen and stabilise the adjacent Waikato River banks using a technique called soil nailing. This involves drilling around 75 holes in each bank, 100mm in diameter, to a depth of 18 metres. Once the holes are drilled a reinforcing bar is installed in each one and the hole is then filled with cement grout. The soil nailing of the first bank is expected to be completed before Christmas.

The soil nailing work is being carried out using drilling rigs suspended on ropes. The use of the drilling rigs has minimised the impact of the work on the Waikato River and river users, and avoids the challenges of using heavy machinery on a barge with rising and falling river levels.

Once the soil nailing is completed on both banks, the faces of the river banks will be covered with concrete to protect them and prevent further erosion as river levels rise and fall.

NZTA State Highway Manager Kaye Clark says these works should give the bridge a good 30 years more life.

Mrs Clark says, “The Narrows Bridge was constructed in 1939 and is still structurally sound. However, ever since the bridge was built the Waikato River banks have been gradually eroding to the point where we became concerned about the stability of the bridge’s foundations. The soil nailing fixes this but is taking a little longer than we hoped because of ground conditions that have made drilling some of the holes quite challenging. We hope to have all the works finished within the next few months.”

The Narrows Bridge is currently operating as one lane, carrying westbound traffic heading to Hamilton airport, with the temporary bailey bridge, immediately upstream of the Narrows Bridge, carrying eastbound traffic. The provision of one lane of traffic on each bridge has restored the highway to two lanes, minimising the impact on traffic flows.

Both bridges have a 30km/h speed restriction in place and are open to all legal loads, including Class 1 heavy vehicles. Cyclists are also able to use both bridges.

The duel bridge operation will remain in place until the erosion protection works have been completed and the Narrows Bridge can be restored to full two-lane operation.

Mrs Clark says there is a possibility that some further brief closures of the Narrows Bridge may be required over the next few months while certain works are carried out. During these closures traffic will be reduced back to one lane over the bailey bridge, and controlled by traffic signals.