Lincolnshire County Council has been using soil data provided by the University of Cranfield to take a more targeted approach to road maintenance.
The ubiquity of local roads means that their important function, particularly in rural areas, is often overlooked.
Local roads represent 98% of the overall UK highway network and comprise the non-strategic A, B, C and U (unclassified) road categories.
They are owned, operated and maintained by local authorities. As a subclass, the C and U categories of road represent 60% of the overall network and are predominantly evolved. This means that C and U roads have little, if any, engineering foundation.
As a result, the road surface often sits directly on, or in close proximity to, the natural soil surface. Soil therefore exerts a significant impact on the roads’ physical condition. One such soil-related impact is the volumetric shrinkage and swelling of clay susceptible soils.
During periods of extensive drought, clay-related shrinkage can result in significant deterioration of the local road network. Several East Anglian counties including Lincolnshire, Cambridgeshire and Norfolk have reported widespread deterioration of their respective road networks in recent years. They consider this to be a result of clay-related subsidence, resulting in millions of pound worth of damage. This has ultimately led local authorities to seek emergency funding from the Department for Transport (DfT) to clear maintenance backlogs. The ICE has also classified the local road network as “at risk”.
Soil and road link
Despite the intimate link between soil and road infrastructure, soil survey data has seldom been used in highway asset management to date. Therefore, Cranfield University has been undertaking research, through funding from the Engineering and Physical Sciences Research Council (EPSRC) and the Infrastructure Research Transitions Consortium (ITRC), to help Lincolnshire County Council identify areas at risk of clay-related subsidence. LCC has reported that approximately 150 road sections were badly damaged during the 2010-11 period, resulting in an estimated £6M of damage.
Cranfield University holds the responsibility, on behalf of the Department for Environment, Food and Rural Affairs, for the national soil map, held in the national Land Information System, LandIS. Since the early 1990s, Cranfield University has been developing a series of thematic geohazard models, through reinterpretation of the National Soil Map of England and Wales.
Lincolnshire County Council provided Cranfield researchers with a temporal spread (2007-2014) of road assessment data for its unclassified road network. This represents 66% (5,800km) of Lincolnshire’s overall network. Road condition data from the unclassified network has been intersected with Cranfield’s thematic geohazard maps within a geographic information system to understand the relationship between subsidence risk and road condition. Analysis has revealed that areas at higher risk of subsidence have suffered significant road deterioration.
Rethinking surfacing approach
This has since prompted Lincolnshire highways engineers to rethink their approach to resurfacing unclassified roads. Adaptation options such as increasing the roads’ sub-base depth, or placing wire mesh reinforcement over the entire network have proved effective in mitigating subsidence hazards. But they are unfeasible considering network extent and current road-funding mechanisms. Instead, Lincolnshire County Council has trialled a method of resurfacing known as “in-situ retread” targeted at areas of high risk.
The “retread” process involves using a milling machine to pulverise the material on the road’s existing top surface. The material is then mixed, harrowed and graded. The surface is sprayed with a thick bitumen emulsion and a 10mm to 14mm-sized aggregate is used to fill in holes before rolling. A double application of a binder and up to 6mm surface dressing is used.
This is not only proving cheaper by saving on material costs, but also reduces carbon emission outputs. Furthermore, road material won from other sites, for example during A-road planning and resurfacing, is being reincorporated into the retread process, thus deepening road foundations and saving on hazardous (bitumen-containing) waste disposal costs. With an acceptance that the road will ultimately fail again, the lower cost of resurfacing means that the road can be treated more often.
Alongside this work, Cranfield University researchers have also been developing a novel model of geohazard risk for England, Scotland and Wales which incorporates 5km resolution UKCP09 climate projections. This allows for probabilistic estimations of clay-related subsidence risk for 2030 and 2050 time period scenarios. This has served as a further tool to help Lincolnshire County Council’s estimates of where it may face particular issues going forward, allowing it to prepare its network for future drought events.
Ultimately, both aspects of the research will help mitigate the impact of clay-related subsidence and make Lincolnshire’s local road network more resilient to potential climate change. These approaches may now be extended to other areas of the country facing similar issues.
- Stephen Hallett is principal research fellow for environmental informatics at Cranfield University