What is an underground void? 

An underground void is a cavity or an empty hole below the ground surface, which can range in size from small, localised pockets to large caverns. When the void can no longer support the weight of the surface above it, the ground will sink and from a sinkhole. Voids are often invisible on the surface until a sudden failure occurs, which poses safety and operational risks.

How are underground voids formed? 

Underground voids are primarily formed by natural geological processes. However, they can also be caused by human activity. 

Karst process: acidic water (formed when rainwater absorbs carbon dioxide from the atmosphere and soil) travels through joints and fractures in the bedrock, slowly dissolving minerals such as limestone, chalk and gypsum. This process, known as karst, creates caves and sinkholes over time.  

Ageing or legacy infrastructure: abandoned mine workings, tunnels and culverts can deteriorate over time, creating underground voids and increasing the risk of collapse. 

Leaking pipes: voids can form following a burst water main or sewer pipe beneath a carriageway. Constant, leaking water erodes the surrounding soil over time, creating gaps beneath the surface, and directly beneath the road above. 

Poor compaction: loose or poorly compacted soil contains air pockets between particles, increasing the risk of gaps and voids. 

Risks to infrastructure: What asset owners need to know 

Roads and carriageways are particularly vulnerable to subsurface voiding. Unlike structures with significant depth or reinforcement, the surface layer of a carriageway is often thin. The road surface can act like a plug, concealing the loss of ground support taking place beneath it, until the plug suddenly gives way. This is when a hidden void becomes a visible, and potentially dangerous collapse.  

Even utilities such as water and gas pipes can be damaged by ground movement as a result of void formation, and the ongoing problem of leaking mains beneath roads is itself a leading cause of subsurface voiding.  

The issue is that voids are hidden and any problems that arise as a result of void formation happen gradually over time. Often asset owners will not be aware of deterioration until a sudden collapse occurs. This results in serious consequences such as emergency repair work, network disruption and concerns over risks to public safety.   

For highway authorities and infrastructure asset owners, the financial consequences of subsurface voiding extend beyond repair costs. Emergency road closures, traffic management measures, stakeholder disruption and reputational damage can significantly increase the overall impact of a collapse. Identifying and treating voids before a failure occurs can help reduce whole-life asset costs and improve network resilience. 

Asset owners are responsible for maintenance works, which means they can be held legally and financially accountable. For example, under the Highways Act 1980, local councils have a statutory duty to maintain roads and pavements in a safe condition. It’s essential that asset owners fully assess and manage any ground risks beneath their networks to ensure reliability and public safety. Detecting ground-related issues early is often more cost-effective than making reactive repairs following a collapse.

Addressing the problem 

If left untreated, subsurface voiding can lead to subsidence, weakened soils and water ingress. Whilst it’s difficult to detect underground voids from the surface, there are some visual indicators such as cracking or depressions in the carriageway surface, or unexpected settlement in the road. It’s vital to act on any early warning signs as soon as possible to prevent further structural damage and more expensive repairs.  

Mainmark help asset owners by assessing ground conditions, identifying potential risks and recommending the best treatment solutions. There are several treatment options available from ground improvement to re-levelling. Voids beneath carriageways can be filled with expanding resins or specialist grout to strengthen the ground and restore support. 

Not every case is the same, so it’s important that the treatment method is tailored to the specific ground conditions. 

How Mainmark can help 

Mainmark specialises in modern ground engineering techniques across a wide spectrum of infrastructure projects with particular expertise in carriageways, pavements, bridges and seawalls. 

Depending on project requirements, we offer non-invasive solutions such as computer-controlled grouting or resin injection to fill voids, support foundations and lift and re-level structures. These techniques allow projects to be turned around in days, with minimal disruption to road users and the wider network. 

Although subsurface voiding presents a significant challenge for the UK's highway network, proactively managing assets is essential in keeping carriageways safe and operational networks. Asset owners should seek professional advice before a void causes serious damage. This can prevent costly repairs and further structural issues. If you suspect a void beneath your carriageway or highway assets, or would like to learn more, speak to Mainmark today. speak to Mainmark today.