Kaimai Tunnel rail slab re-level innovation

Ground engineering and asset preservation specialist Mainmark filled voids and lifted the slab and re-levelled the railway line in the Kaimai Tunnel.

The Kaimai Tunnel first opened in 1978 and plays an important role as the only rail link from the Port of Tauranga. Its location within the North Island’s Kaimai Range creates unique conditions inside the tunnel, such as natural hot water vents and significant volumes of ground water, which had damaged sections of the subgrade under the tunnel floor.

After identifying the issue, KiwiRail worked with Mainmark to develop and implement a method to re-level sections of the slab and then repair the damaged subgrade.

The solution needed to be reliable, quick to apply, and suitable for the tunnel environment while enabling the Kaimai Tunnel to continue to operate as a vital thoroughfare for freight trains. Works could only be completed when the tunnel was closed for weekly maintenance.

Mainmark used its proprietary Teretek engineered resin injection to fill voids beneath the railway line, re-support the slab and lift the track back to its original level.  A bespoke grouting solution was then used to bond the subgrade and cracked base slab to the tunnel base rock.

Mainmark says its engineered resin injection technology is a unique two-in-one solution that can deliver both ground improvement and re-levelling, reducing the risk of voids forming in the future. Being impervious to water with no detrimental impact on the environment, Teretek has been used extensively to raise, re-level and re-support public road and rail infrastructure, it says.

The resin was applied via small injection points within the tunnel, using a process likened to keyhole surgery. Lifting was completed in controlled stages, with careful monitoring to help ensure the rail was lifted up to the original level.

The resin injection holes were then sealed, and other injection points were created so that the specially formulated grout could be applied below the base slab to bond the PaCT slab to the subgrade.

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