Kaimai Tunnel Rail Slab Re-levelled and Bonded

Approximately 20 freight trains, move through the Kaimai Tunnel every day, transporting timber, and manufactured goods. In the 2018 financial period, more than 5 million tonnes of freight moved through the tunnel. This is the equivalent of 300,000 truck and trailer units.

The 8.8km tunnel was constructed between 1965 and 1976 through an old volcanic range and opened in September 1978. This creates unique conditions inside the tunnel, such as natural hot water vents and significant volumes of ground water. Over the last 40 years, the groundwater gradually damaged sections of the subgrade under the tunnel floor.

As New Zealand’s state-owned rail company, KiwiRail is responsible for maintaining 4,000km of rail corridor, bridges, tunnels, viaducts, overhead wires, signals and level crossings, including the Kaimai Tunnel.

Through proactive network monitoring, KiwiRail identified this concern early and worked with ground engineering specialists to develop a remediation solution.

In 2018, Mainmark was contracted by KiwiRail to remediate this section of the railway line as part of an infrastructure upgrade to prepare the tunnel for larger and heavier trains. Having previously worked with KiwiRail in 2012 and 2013 to rectify other incidences of subsidence, Mainmark already had a proven record for successfully delivering reliable remediation solutions within a short timeframe, and experience working within the tunnel.

During the 2018 Kaimai Tunnel upgrade, Mainmark used Teretek^® engineered resin injection technology to fill large voids that had formed beneath the railway slabs. TamPur, a specially formulated grout, was then used to bond the subgrade and cracked base slab and re-support the PaCT slab. This solution was successfully tested prior to project commencement.

Through early intervention, KiwiRail has been able to work with Mainmark to deliver a solution to prevent extensive damage to the tunnel floor which may have resulted in tunnel closure and a costly slab replacement.

Due to the unique requirements of working within an operational rail tunnel that is crucial to New Zealand’s export industry, it was vital for the solution to be reliable, quick to apply, and safe to use in the tunnel environment.

Works also needed to be carefully planned around the remote location of the Kaimai Tunnel, situated 2.5 hours from Auckland.

Strict restrictions had to be adhered to regarding tunnel access for personnel and vehicles, to ensure the Kaimai Tunnel could continue to operate as a vital thoroughfare for freight trains. Compulsory tunnel training had to be completed by Mainmark before technicians could enter the Kaimai Tunnel and commence works.

Teretek® is a unique two-in-one solution that can both fill voids and strengthen weak ground, reducing the risk of similar voids forming in the future. The solution is impervious to water and has no detrimental impact on the environment.

The engineered resin was applied via small injection points, using a process likened to keyhole surgery, to minimise disruption to operations. Lifting was completed in controlled stages, with careful monitoring of changes to ensure the rail was lifted up to the original level.

The resin injection holes were then sealed, and several injection points were created so that the specially formulated grout could be applied to a depth of 1500mm below the base slab to bond the PaCT slab to the subgrade.

Works could only be completed on the one day each week allocated to maintenance, when the tunnel was closed to train traffic for 12 hours.

All personnel, equipment and vehicles were transported to the site on the day prior to each session to allow time for maintenance vehicles to reach the work location within the tunnel from the single access point at the western portal. There were also restrictions on the number of vehicles that could be placed on the track at any given time, further impacting the productive time available during each 12-hour shift.

Works could only be completed on the one day each week allocated to maintenance, when the tunnel was closed to train traffic for 12 hours.

“Given the access limitations, we only had in the order of eight hours of productive time per shift. As a result, reliability and surety of product was key.”

“To achieve this, KiwiRail successfully worked with Mainmark to develop and implement a method to re-level sections of the slab and then inject structural grout into the damaged subgrade.”

“Mainmark was able to provide alternative grout products and international grout experts who worked directly with the local team, sharing their knowledge. This open, proactive team culture was key to successful delivery in this challenging tunnel environment.”

“Thank you Mainmark for your ongoing support of the Kaimai Project.”