The reopening of Godstone High Street after an 18-month repair programme has brought one of the UK’s most complex recent ground collapse projects to a close. However, Mainmark UK warns that ageing infrastructure, water leakage and historic mine workings continue to contribute to ground movement and ground collapse risks across the country.
Final works have been completed in this Surrey village following the major collapse in February 2025, which created a hole 20 metres long, five metres wide and five metres deep. The collapse was initially linked to sandstone erosion caused by water from a burst water main. However, investigations later uncovered an underground mine network eight metres below the village, requiring extensive ground strengthening before the road could be reconstructed.
Mainmark worked as part of a multidisciplinary team appointed by Surrey County Council, carrying out resin injection works to strengthen weak and disturbed ground beneath the affected roads. Following remediation of the deeper mine workings by the wider project team, Mainmark carried out targeted resin injection between one and four metres below ground level to strengthen weakened and disturbed ground and address potential pathways for future void migration. Mainmark installed approximately 600 resin injection points across High Street and Salisbury Road during the three-week programme of ground improvement works.
Ground-strengthening techniques such as resin injection can be used to improve weak or disturbed ground while minimising disruption to surrounding infrastructure and communities. The works were supported by pre and post treatment Dynamic Penetrometer Testing, which confirmed measurable improvements in ground strength across the treated areas.
David Hedley, Infrastructure Lead at Mainmark UK, said: “Ground collapse events rarely have a straightforward cause. While damaged utilities can contribute to the formation of underground voids, investigations at Godstone also identified historic sand mine workings beneath the site, creating a more complex remediation challenge that required multiple ground engineering techniques.”
Mainmark has been involved in a growing number of projects across the UK where ageing buried infrastructure, leaking utilities and historic underground workings have created ground movement and collapse risks for highways, commercial assets and residential communities. “The challenge is that many of these risks develop silently over years or decades. By the time cracking, settlement or a road collapse becomes visible, the underlying problem may already be extensive.”
When a buried utility leaks, water gradually removes fine particles from the surrounding soil, creating a hidden cavity. Collapse occurs when the remaining ground can no longer support its own weight or the loads above it.
“Before repairs can begin, engineers must identify the root cause, assess ground conditions, understand any risks to nearby structures and protect live utilities. Depending on the complexity of the site, this process can sometimes lead to extended traffic restrictions while investigations, design work and remediation plans are completed.”
“The true cost of a sinkhole is not only the physical repair, but the disruption caused while communities wait for action,” David added. “The longer a void remains untreated, the greater the risk of further movement affecting roads, utilities and foundations.”
The Godstone project demonstrated how complex ground collapse incidents often require multiple remediation techniques working together. A layered ground engineering approach was adopted, with different techniques used at different depths to address specific ground conditions. Deep mine workings required remediation, while Mainmark’s resin injection strengthened the shallower layers beneath the road and helped improve the condition of disturbed ground above.
David said: “There is rarely a single standard solution. The priority is to identify the cause quickly and select the least disruptive, technically appropriate combination of treatments. “As underground assets age, local authorities and contractors must place greater emphasis on early investigation, accurate records and rapid ground-strengthening measures. That is how we protect homes, secure highways and help communities move again sooner.”
Established in the UK in 2016, Mainmark Ground Engineering (UK) Ltd is a subsidiary of the Mainmark group of companies.
