What exactly happened at Purley?  

While carrying out routine planned engineering works, engineers discovered several voids and moved quickly to close all lines in the affected section. This caused further lines to be suspended, bringing significant disruption to passengers travelling to and from Gatwick. 

The line closures happened immediately, highlighting how seriously Network Rail treats any signs of ground failure. However, it does raise a wider question. How many similar hidden voids are developing beneath the rail network? 

Why do voids form beneath railway infrastructure? 

Although the emergence of a void may appear suddenly, the underground issues that cause them usually develops gradually over several months or even years. Bridges, embankments and other railway infrastructure are more vulnerable due to the heavy loads they carry repeatedly.  

Several factors can cause the formation of underground voids including eroded soil or made-up ground. As water travels through the weak spots or fill material settles unevenly, small cavities begin to open. The ground beneath the track bed and nearby structures is put under constantly increasing pressure thanks to the weight of the rolling stock passing over it. Over time, this pressure is what turns a hidden void into a sudden collapse. 

Why Purley is not an isolated case 

The incident at Purley come not long after the emergence of the Godstone sinkhole in the neighbouring borough of Surrey. The sinkhole, which opened on Godstone High Street in February 2025, led to the evacuation of dozens of homes and the closure of the High Street for over a year.  

Mainmark have been involved in an increasing number of void fills within the last few years and continues to advocate for increased monitoring and preventative ground measures. 

Warning signs asset owners should watch for 

Even though voids form gradually before the surface above them finally gives way, there are several early warning signs asset owners should look out for. Not just beneath the tracks but in other critical infrastructure as well: 

• Dips or depressions in embankments, verges or track ballast.

• Cracks in retaining walls, bridges or platform edges.

• Tilting signage, fence posts or lineside structures.

• Water pooling in areas that had previously drained away.

Identifying the signs early, allows engineers to investigate the issue and intervene before a void causes a surface collapse. 

The importance of proactive ground monitoring 

The Purley incident highlights the importance of thorough ground investigations and that taking a proactive approach to managing settlement should be an essential part of asset maintenance for network operators. Identifying a void or a weakened section of the ground early on can be simpler and more cost-effective than extensive reactive repairs.  

Waiting to address ground issues until planned engineering works reveal them, or until a serious failure occurs, can cause major disruption for passengers and higher costs for asset owners. 

How Mainmark can help 

Mainmark specialises in fast, non-invasive solutions to railway settlement, helping to keep disruptions to the network to a minimum. 

Traditional methods often require weeks of excavating and heavy machinery on site, leading to high costs and disruption. Modern, non-invasive methods such as engineered resin injection works through small, targeted incisions, comparable to keyhole surgery. This technique strengthens and fills voids in the ground and can re-level the surface or structures where needed. Additionally, work can be carried out in stages, allowing engineers to treat sections at a time, which means the railway line can be restored and ready for use again almost immediately. 

“Asset owners or contractors should explore alternatives that preserve assets rather than, in the worst-case scenarios, require a complete rebuild,” said Mainmark General Manager UK, Ian Thornhill. “Targeted, efficient and less disruptive than traditional approaches, resin injection solutions can be used to strengthen and re-level infrastructure. Parts of the track or platform can be treated in stages, all while minimising the effect on daily operations.” 

If you’re concerned or have any questions about voids, settlement or other ground-related issues on your network, speak to Mainmark today.