Finland-based private company Geobear has announced that it has completed major stabilisation and water-sealing works on the Blackheath Tunnel in south-east London.

The company, which specialises in ground engineering, carried out the repair work as part of a critical maintenance project costing £ 10 million on the railway between Blackheath and Charlton.

The project involved the tunnel closing for ten weeks from May this year, with the work finishing in late July.

Blackheath Tunnel is a mile in length and originally opened in 1849. The North Kent line passes through the tunnel immediately after the junction with the Bexleyheath line, and the tunnel remains an essential rail link for London and the Southeast.

Over the last few decades, the tunnel lining had degraded, and as a result, water ingress washed away fine sediments behind the tunnel’s brickwork. Water leaks persisted, damaging the brick face and overhead lines. In addition, water pooled on the track, causing rails to corrode.

The problems with the tunnel contributed to around one thousand minutes (nearly seventeen hours) of delays for passengers on Southeastern services in the last year.

VolkerFitzpatrick, the project’s tier-one contractor, contacted Geobear in January of this year, asking the firm to carry out water sealing work on the tunnel. Geobear uses geopolymer technology, which, unlike cement-based grouts, does not need added water. It is five times lighter than traditional materials, and can be fitted in half the time.

Preparation involved engineers drilling 1,000 core holes over 900 metres of tunnel lining. This enabled them to go on to install 4,000 injection tubes, consisting of 13 km of pipework. Engineers then spent five weeks injecting a total of 55 tonnes of geopolymer.

The repair work also involved diverting water through weep holes into controlled drainage channels.

“This was a large, complex project that required multiple companies working together to protect the railway. VolkerFitzpatrick did an excellent job coordinating the delivery, ensuring the tunnel is safeguarded for decades to come.”

Paul Garthwaite, Geobear infrastructure director.