Specialist ground engineering firm Geobear has completed major stabilisation and water-sealing works to restore Blackheath Tunnel to full rail traffic capacity. These essential repairs constituted part of a £10 million maintenance project on south east London’s rail network between Blackheath and Charlton. The project was completed on July 25, carried out over five weeks during a scheduled ten-week line closure, ensuring the tunnel’s long-term stability and protection against water ingress.

 The mile-long tunnel, first opened in 1849, had suffered progressive lining degradation over decades, leading to water ingress that washed out fine sediments from behind the brickwork.

 Persistent leakage damaged the brick face and overhead lines, while pooling water on the track corroded rails. These issues contributed to around 1,000 minutes of delays in the last year for Southeastern passengers.

VolkerFitzpatrick, the project’s tier-one contractor, considered traditional methods, such as grouting or concrete lining, but dismissed them due to their high water requirements and the logistical challenges of transporting water inside the confined tunnel environment.

The contractor approached Geobear in January 2025 for its expertise in water sealing with geopolymer technology. Unlike cement-based grouts, geopolymer requires no added water, is five times lighter than traditional materials, and technicians can deploy it twice as fast.

Civil engineering contractor Kilnbridge played a critical role in the preparation phase, drilling 1,000 core holes over 900 metres of tunnel lining to allow the installation of 4,000 injection tubes, a total of 13 km of pipework.

Engineers worked in coordinated day and night shifts to complete the injection phase within five weeks of the blockade injecting a total on 55 tonnes of geopolymer. It was critical a five-week deadline was set to allow for finalised works to follow for final completion within the ten-week blockade.

The works also included diverting water into controlled drainage channels through weep holes connected to drainage tracks.

“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,” said Paul Garthwaite, Geobear infrastructure director.

Blackheath Tunnel remains an essential rail link for London and the Southeast. These works ensure that this 175-year-old structure can continue to serve passengers well into the future, while minimising environmental impact compared to traditional repair methods.