Causes of highway subsidence and how to treat it
Subsidence can cause major disruption to roads, resulting in fracture, unevenness and, in the worst cases, sinkholes. Being aware of the causes of highway subsidence, as well as the most effective ways of preventing and treating it, should be key considerations for local authorities and contractors.
Movement in the concrete base
The deterioration of the road surface is usually due to the concrete base underneath experiencing movement. Changes in the soil and its ability to support the concrete and asphalt structure of the road above can mean that concrete slabs subside, causing bumps, cracks and other issues with the road surface.
There are a number of different reasons why the subsoil may become unstable or form voids.
If the subsoil underneath the road is a cohesive soil, such as clay or silt, it will be prone to swelling and shrinkage. The types of soil retain a lot of water, meaning that during periods of heavy rainfall they will swell and become larger. Accordingly, during hot weather, water is lost from the soil and it contracts. It is in this situation that the contracting soil can cause concrete slabs on the highway to sink.
Highways, therefore, need to be constantly monitored for any signs of subsidence, especially following periods of extreme weather.
This can often be a problem in roads across the UK, as clay soils are very common. You can check the risk of subsidence in different areas using this subsidence map.
Washout tends to affect sand and gravel-based soils, which can easily be washed away if there is a sudden influx of water. Fractured drainpipes, nearby water sources such as rivers and reservoirs, and flooding can all cause washout. When this happens, the soil is no longer able to support the weight of the concrete slabs above.
Rainwater ingress over a prolonged period can cause similar issues. Fill material can be washed away and joints and bridge abutments weakened.
This is another factor to be vigilant about. Animals in the local area such as badgers and rabbits can burrow deep underground. If this is underneath a road, or even nearby, it can cause shifts in subsoils that can lead to more significant problems of subsidence.
Highways in the UK are subject to massive amounts of vehicle traffic each year. The accumulation of all of these vehicles travelling at high speeds can result in subsidence, as the constant vibrations from cars, vans, lorries and busses can cause movement in the soil below.
When evidence of subsidence begins to appear on highways, authorities may be hesitant to take decisive action right away, for fear or road closures and traffic disruption. However, there are non-disruptive solutions available that mean the problem can be sorted speedily and efficiently.
The traditional solutions
There are typically a number of alternatives to tackle this issue, either dig out and replace the section of road; if it’s a slab issue it could be lifted and replacement fill installed, deep recycling could be used whereby existing materials are re-used in-situ. Or the final alternative would be the patchwork approach whereby only the worst sections or areas are treated. Needless to say, all of these solutions can be a lengthy process with great expense involved.
Geopolymer injection – the non-disruptive solution
This is the modern solution to treating highway subsidence. It’s a popular choice, as no excavation of the road is required and it can be carried out during quieter times, such as at night.
The geopolymer injection method, patented by Geobear, creates small, strategically-placed in holes in the ground, through which a special geopolymer resin is injected. Underground, the resin follows the path of least resistance to fill voids in the soil and close up any gaps. The resin expands to compact the soil and, where required relevel the road surfaces. It then hardens to create a strong, solid base for the road and treat the subsidence issue.
This BBC report shows footage of the geopolymer resin injection technique being used to successfully treat subsidence on the M11.
As mentioned, this solution benefits from being extremely non-disruptive, non-invasive and efficient. While traditional-style underpinning can mean roads having to be closed for weeks or months on end, this solution can be performed in just a fraction of the time.
Thus, there is no need to delay treatment of highway subsidence. If there is suspected subsidence, a survey should be carried out as soon as possible to determine the existence, extent and cause. If subsidence is indeed present, geopolymer resin injection can solve the issue quickly and effectively, preventing any further damage to the road or avoidable accidents.