The UK road network is deteriorating due to ageing infrastructure, climate change and increasing traffic. Due to community and economic reliance on a functioning road system, there is an urgent requirement to build resilience. The roads of south Lincolnshire have high susceptibility to ground movement due to the underlying geology. Deposits such as peat, tidal flat deposits and alluvium have a high susceptibility to compress, particularly when loaded or through loss of water content driven by climate change or lowering of water in drainage channels. The shallow foundations of Lincolnshire's rural evolved roads, originating from old mud tracks, are poorly constructed, increasing their vulnerability to movement. Types of damage include longitudinal cracking, edge failure and uneven long-section profiles. Through knowledge exchange, data sharing and collaboration between the British Geological Survey and Lincolnshire County Council, a direct relationship between road condition and geohazard susceptibility has been demonstrated; showing compressible ground has a greater correlation with road damage than originally considered. This suggests improved understanding of the relationships between the geological, climatic and anthropogenic driving forces on ground movement and road damage enables more informed repair prioritization, decision support and improved bespoke road repair practices, increasing future resilience of road networks.

Thematic collection: This article is part of the Climate change and resilience in Engineering Geology and Hydrogeology collection available at: https://www.lyellcollection.org/topic/collections/climate-change-and-resilience-in-engineering-geology-and-hydrogeology

The national road network's resilience to degradation is a major challenge due to its age, a changing climate and ever-increasing traffic density, particularly from loading by heavy vehicles. UK motor vehicle traffic increased by 1134% from 1949 to 2019 (Department for Transport 2021). In the county of Lincolnshire, UK, annual road traffic rose from 2.8 billion vehicle miles (bvm) in 1993 to 4.3 bvm in 2019 (Department for Transport 2023a). Even post-COVID in 2021, it remained high in the county at 3.9 bvm. Post-COVID, there has been an increase in ordinary goods vehicles (OGVs) and heavy goods vehicles (HGVs) on the roads (Department of Transport 2023b), partly fuelled by the rise of online deliveries. The UK is gradually falling down the international ranks for road quality (World Economic Forum 2021), ranking 14th globally in 2006 but dropping to 37th by 2019. However, the importance of resilient existing infrastructure to a country's sustainable development has been highlighted within the United Nations Sustainable Development Goals (Bricker et al. 2021; Lagesse et al. 2022; United Nations 2022). Berg et al. (2015) concur, highlighting that ‘roads are the arteries through which the economy pulses; linking producers to markets, workers to jobs, students to school, and the sick to hospitals, roads are vital to any development agenda’. The Fenlands, which stretch into the south of Lincolnshire, are known as the breadbasket of England (Natural England 2023). The clay-, silt- and peat-rich soils are a major resource of national importance for agriculture, leading to the roads being vital for the transfer of agricultural products to the rest of the UK, and internationally.

In 2021, the United Nations Climate Change Conference (COP26) acknowledged that communities will continue to feel the inevitable impacts of a changing climate, and stated its aims to limit the rise in global temperatures and drive action globally on mitigation, adaptation, finance and collaboration (COP26 2021).

In 2022, the Climate Change Committee published its third Independent Assessment of UK Climate Risk (Climate Change Committee 2022). The importance of the road network was emphasized, stating ‘transport is fundamental to day-to-day life, but regularly faces climate challenges from flooding, heat, erosion, subsidence, and extreme weather. As the climate continues to change, the severity of these risks is projected to increase’. Committee members go on to declare that ‘in the absence of additional adaptation, the impacts of climate change … will lead to costly disruption or loss of service’ (UK Climate Risk 2022, p. 3). They also underline the long lifespan of transport infrastructure, and therefore the importance that climate risks are considered for all existing, retrofitted and new assets, and gaps are identified in planning by many of the country's critical infrastructure providers.

Nationally, Great Britain's minor, or non-strategic roads, are maintained by local authorities and represent 87% of the country's overall road network (Department for Transport 2022). These are all A, B, C and unclassified (U) roads. Within Lincolnshire, 87% of roads are minor (7866 km), aligning with the national average, but the percentage of rural C and U roads is higher at 66% (5969 km) compared to 45% nationally. These roads, generally, have little or no foundations (Pritchard et al. 2015) and are therefore vulnerable to natural shallow processes that cause ground movement.

Over the last decade, Lincolnshire has experienced significant increases in road damage, including undulations, ridges, cracking and subsequent weaknesses in the road surface that are attributable to the conflation of natural and anthropogenic processes (Pritchard et al. 2014). These have contributed to major traffic disruptions such as road closures and road traffic incidents. There are several well-identified geohazards in the county, driven by near-surface soil water content changes, which cause ground movement, including clay shrink–swell and compression. The main lithologies affected are the fine-grained deposits, such as tidal flat deposits and organic soils (i.e. peats).

The road network of Lincolnshire is adapted to its drainage system, which has developed since Roman times, draining low-lying areas to make them suitable for cultivation. There are deep dykes and ditches, often running parallel only 1–2 m from the road. The drainage channels have influenced where roads have been built, and controlled the height of the water table, road construction methods and consequently ongoing road conditions. Despite the county providing approximately 70% of the UK's food produce processed in the country, its limited road network is mostly single lane. The increasingly high volume of heavy haulage traffic transporting agricultural products leads to loading that the roads were not designed for and, therefore, increased potential for carriageway deterioration. ‘Other Goods Vehicles 2’ (OGV 2s), have the biggest impact on Lincolnshire's network in terms of damage and wear. The volume of ‘Heavy Goods Vehicles’ (HGVs), which includes OGV2s, has increased recently on the main A roads in the county. An increase of 23 and 35% was recorded on the A17 and A15, respectively, between 2011 and 2021 (Department for Transport 2023a).

Future projections, made by the British Geological Survey (BGS) and based on United Kingdom Climate Projections (UKCP) climate scenarios, indicate that large areas of Lincolnshire could experience greater road degradation during the twenty-first century (Harrison et al. 2020) due to increased susceptibility of the geology to shrink–swell and compression due to more extreme changes in water content. Through knowledge exchange and data sharing between the BGS and Lincolnshire County Council's Highways department, another increasingly important hazard in the county has been recognized as impacting the local road network, volume reduction of deposits classified as highly compressible. Consolidation, due to compression from static and dynamic loading from infrastructure and heavy traffic, and shrinkage, caused by decreasing water content, is affecting road condition in the county. In addition, once cracks in the surface course occur, ingress of water leads to freeze–thaw processes that cause further damage. Not only are the roads exposed to ground movement due to their shallow construction but previous conventional short-term repair methods, such as adding overlay, can increase loading and result in further consolidation of the ground below.

The County Council, as the local highway authority, recognizes the need for improved understanding of future vulnerability and risk to the road system. This requires relevant information to support the decision-making process to mitigate and adapt to the projected changes in both road traffic and climate change. Initially, the highway authority's priority was susceptibility to ground movement from clay shrink–swell but the increased thickness of subgrade material on compressible deposits poses additional, previously unforeseen, challenges.

The BGS has developed six national GeoSure datasets to provide geological information about potential ground movement due to shrink–swell, compression, collapsible soils, landslides, running sand and soluble rocks (Lee and Diaz Doce 2018). The BGS GeoClimate clay shrink–swell dataset utilizes GeoSure shrink–swell and UKCP09 and UKCP18 climate projection data. GeoClimate provides datasets that can be used as part of a decision support tool to address changes in susceptibility to shrink–swell due to climate change in the UK (Harrison et al. 2019, 2020). Lincolnshire County Council now includes BGS GeoClimate Open data, as well as their existing road network data, in their decision-making process for highway maintenance to establish which specific treatments highway engineers can use to improve damaged road sections, especially in areas where major failures have occurred.

This paper introduces four case study sites that demonstrate the most common problems experienced in the local road network of Lincolnshire. It then presents county-level temporal road-damage data, and identifies the relationships between road condition and remaining life, and the underlying geological units. Finally, it proposes more appropriate, and cost-effective, mitigating road construction and maintenance strategies. The findings are also relevant to global areas with similar land use and geological properties, particularly reclaimed land and large river flood plains.

The administrative county of Lincolnshire is situated in mid-eastern England, spanning from the Humber Estuary in the north to The Wash in the south (Fig. 1). The county's relief is predominately low lying (0–50 m above sea level) and covered by Quaternary deposits, except for the Lincolnshire Wolds in the central-northern area of the county where the Chalk Group crops out, and the Lincolnshire Limestone Formation escarpment, running north–south, in the western part of Lincolnshire on which lies the historical part of Lincoln. These hills range from 50 to 200 m above sea level. Much of the southern part of the county is low lying, has fertile soil and the land use is predominantly high-intensity agriculture.

Historical land use

It is likely that prehistoric farming of the rich soils for agriculture and, in particular, horticulture was limited due to regular inundation by the sea, resulting in saline ground conditions. The first attempts to drain the Fens were made by the Romans, who were also the first to construct roads in the county. Notably, they built the Fen Causeway, which avoids much of the soft, compressible soils of the Fens where possible and, in places, raises it above the marshland using a gravel base. There is also limited evidence of drainage during the Middle Ages. During the seventeenth century, after King James I considered the land to be a ‘waste and unprofitable’, large-scale drainage works constructed major drainage channels including the Old and New Basford rivers in the central Fens to the SW of Lincolnshire. However, drainage resulted in lowering of the land surface and so flood protection was also required. Eventually, in the 1820s, large-scale drainage and flood protection of the Fenlands was successfully undertaken, utilizing pumping stations. This has led to the geomorphology that can be seen today, that of a lower surface with rivers and dykes draining the area, often situated alongside the road network, and rigid roads remaining relatively higher than the surrounding farmland.

Quaternary deposits

Quaternary deposits overlie much of the county's bedrock geology (Fig. 2; Table 1), dominating the north as sheets of till and the southeastern areas as unconsolidated Holocene sediments. The Quaternary period began 2.6 myr ago (Head 2019), and consists of cycles of cold ‘glacial’ periods interspersed by warmer ‘interglacial’ periods. Ice sheets advanced over parts of Lincolnshire at least three times during these glacial periods. The Anglian glaciation occurred c. 450 kyr ago. It was by far the largest glaciation and ice covered most of the country, reaching as far south as the River Thames. Anglian till deposits would have covered much of Lincolnshire. These have since been eroded by Devensian glacial activity, when ice lobes advanced at least twice into the north of the area and along the east coast, once around 40 kyr ago and again during the Last Glacial Maximum, c. 20 kyr ago (Straw 2008). Tills are identified by their fine-grained matrix with suspended mixed rock fragments up to boulder size.

In contrast, SE Lincolnshire is dominated by The Fenlands. These span inland from The Wash, with the northern tip reaching into Lincolnshire. The Fens are a low-lying marshy plain, underlain by thick sequences of interbedded peats, silts and clays with coarser-grained old river channels – evidence of the variations in sea level over the last 18 kyr. The cycle of marine inundation and retreat supported the development of mineral- and organic-rich soils. The Fenlands can be clearly identified by the large swathe of alluvium deposits in the 625 k scale generalized geological map (Fig. 2), with alluvium and tidal flat deposits classified as Alluvium at this scale.

Roddons are the deposits of old rivers and tidal creek systems that meandered through the Fenland landscape during the mid–late Holocene (Smith et al. 2010), and can be found throughout this part of the county. They comprise silt, clay and sand, sometimes gravel, but little organic matter, having a lower water content locally in comparison to the surrounding tidal flat deposits and peat. After deposition, the component sediments underwent much less consolidation due to water loss than other deposits, meaning that the roddons now form a slightly raised dendritic network of positive topographical features within the landscape.

Bedrock geology

The bedrock of Lincolnshire comprises an easterly-dipping succession of Triassic and Jurassic mudstones and clays to the west, with younger Cretaceous chalks to the far east, with a NNW–SSE sub-crop (Fig. 3). The bedrock is largely concealed beneath a thick cover of Quaternary deposits, which control the ground movement susceptibility within the area, with the underlying bedrock having little or no direct impact.

Lincolnshire's road network is more at risk than other counties due to a greater exposure to more susceptible geology and a greater vulnerability due the shallow foundations of the evolved road network, which originates from old mud tracks. Figure 4 demonstrates common road structure and thickness in the county. At Chalk Lane, Withern (543327, 382202), the thin surface course of 25 mm has no underlying foundations; as seen in Church Road, Friskney (546188, 355195), if repair work is required these courses have been overlaid and the thickness gradually built up, reaching a depth of 200 mm.

Geological susceptibility to ground movement

The main geological controlling factors of susceptibility to subsidence in the county are the presence, thickness and heterogeneity of shrinkable and compressible deposits. In Lincolnshire, these are primarily tidal flat deposits and peats, which were deposited with a high water content. As they are relatively recent deposits – and have not been loaded by more recent deposits that would facilitate natural consolidation – they have maintained their high water content. This high water content, and their low density, is more likely as these deposits generally have some organic content, which tends to retain water.

If subsidence susceptibility exists at a site, local factors contribute to the increase in likelihood of ground deformation. These include: road construction type; vegetation such as trees and large shrubs in the vicinity; proximity of dykes and other excavations; agri-industry practices such as water management, including water pumping from the dykes and irrigation; and type of road traffic and use of HGVs. The potential amount of volume change is dependent on the water content of a material.

Climate change

The county currently has a mild temperate climate and a relatively low mean annual rainfall of around 619 mm a−1 (Met Office 2022a). Looking to the future, the UK Climate Projections 2018 scenarios (UKCP18) project a greater chance of hotter, drier summers, as well as warmer, wetter winters (Murphy et al. 2018). This, coupled with the county's underlying geology, will continue to result in some of the highest soil moisture deficits (SMDs) in the UK. Abnormal summers like 2003, one of the hottest on record, when the county experienced much higher than average damage to many of its roads could occur regularly by the year 2050 because of climate change (Met Office 2022b). Agricultural practices also play a part and need to be considered; during dry weather the farmland is irrigated using water from the drainage channels, further lowering groundwater levels and reducing near-surface water content.

Flooding resulting in embankment and ditch slope failures has increased in occurrence recently due to more frequent high-intensity rainfall events. For example, a bank failure occurred in November 2019 at Fodderdyke Bank, following a high-intensity rainfall event, causing the bank to become quickly saturated. This caused the adjacent highway to become unstable as the minimal subgrade to the carriageway slipped away as part of the bank failure (Fig. 5).

Irrigation practices

Lincolnshire's drainage system is often closely aligned with the adopted road network, with deep ditches running parallel to roads that are often only a few metres away. Thus, they have a strong influence on the water levels below the road network.

The Internal Drainage Board (IDB) manages water levels in the county's dykes. During the last decade, it has seen increased irrigation in the summer months due to the warmer, drier summers. This has led to the IDB meeting levels at the lower end of their range during the summer months due to the larger volumes of water required for irrigation. This is in comparison to the rainier winter months when levels are kept even lower. This is to meet increasing field drainage requirements due to generally wetter winters, assisting in keeping the fields dry to ensure early planting of crops.

In addition, climate change is leading to higher sea levels, which is affecting tidal rivers in the area. This leads to a greater hydraulic gradient between the managed drainage regime and the surrounding tidal rivers, thus increasing the pressure on the IDB to irrigate and retain the required water levels.

Road maintenance practices

Road condition and underlying geology are not considered within the central government funding distribution, which is informed by condition assessment, asset management and carriageway length. Therefore, Lincolnshire County Council must plan and prioritize repair and maintenance work in the county as there is a higher percentage of vulnerable roads requiring attention compared to many other counties. In the past, the local highway authority would repair road surfaces as and when the damage occurred. However, although a short-term fix, this patching work adds further load and compounds the compression of susceptible geological units, leading to the need for further repair work in coming years.

Increased traffic

As the county provides a high percentage of the country's food production, a high number of HGVs travel on the ‘evolved roads’, with no ‘main’ road alternative. There are no other options such as major railway routes in the county. This has led to an increasing load from heavy vehicles such as farming machinery and those transporting agricultural products to the rest of the country.

Vegetation

It is also noted in all case study sites that there is a relationship with the proximity of vegetation (trees and large shrubs) to the carriageway edge and accelerated deformation/subsidence. Trees and large shrubs adjacent to the carriageway remove water from the ground locally through evapotranspiration, leading to a variation in water content across the carriageway and, therefore, differential ground movement, causing road damage. The depth and extent of the removal of ground water is dependent on the species and size of the plant. However, vegetation also provides shelter, reducing the risk of frost in winter and preventing road surface bleeding in summer, and provides wind breaks, along with other environmental benefits such as supporting animal habitats. Removal of vegetation can also have detrimental effects as the root systems can assist stabilization of banks. If the trees are removed, then there may be swelling of the unsaturated soil layers, and roots may rot and cause further settlement as they change volume.

Four case study sites have been selected to demonstrate the geological, climatic and anthropogenic controls that affect vulnerability to road damage in Lincolnshire, and the impacts observed on the road network (Table 2; Fig. 6).

Site 1: Fodderdyke Bank – increased loading due to HGV use

Fodderdyke (537955, 356956) is a 5 km stretch of road near Midville, north of Boston, and is a cut-through road from the A16 to the A52. It is flanked by steep-sided drainage dykes and is raised relative to its surroundings. It is a ‘C’ class part of the network and has relatively modest traffic flows but includes large agricultural vehicles and HGVs.

Movement history

During the last decade, splitting along the longitudinal section (Fig. 7), undulations, deformation including localized shears, slope movement and other subsequent highway failures have developed. Due to the worsening state of the road, it became relatively unsafe for travelling on at speed. In 2015 a road traffic incident took place; tragically, this resulted in a fatality, and a lower speed limit was enforced until repair work could be completed. Mitigation work was successfully completed by the local authority during 2018. Although overlays had been the typical repair method, a different approach was used to increase the life of the road and reduce future maintenance requirements. This entailed recycling of the existing road surface; therefore, not introducing additional load. Due to the length of this road this is a rolling programme with works being undertaken each year. It is still ongoing, giving the highway authority the opportunity to review, monitor and make fine adjustments to the design, whilst continuing its research.

Geological background

A depth of 5–10 m of peat underlies the western stretch of the highway, and tidal flat deposits lie beneath the road to the east.

Potential causes of movement at the site and future climate change impacts

The Environment Agency (EA) LiDAR scan data (Fig. 7) show the lower ground to the north of Fodderdyke, which coincides with the highly compressible peat deposits. These are topographically similar to the tidal flat deposits to the south, which are also susceptible to consolidation due to loading and shrinkage due to a reduction in water, although this movement is less than found in the peat as they have a lower natural water content and show less ground movement. The roddons cross the area from north and south, raising the ground surface, appearing as ‘inverted channels’. North of the road, compression of the peat deposits has led to the land surface lowering, and therefore a lowering of the overlying roddons. Due to less movement in the tidal flat deposits to the south of the road, the roddons have a comparably slightly higher elevation to the south compared to the north. However, in all locations, roddons provide a positive topographical feature. The presence of deposits with different lithology and characteristics along the road leads to large variation in the amount of movement seen.

Considering the increasing SMDs projected for the area, and the higher subsidence susceptibility of the peat deposits, the difference in elevation is expected to increase, causing further road damage where peat deposits meet the road and differential movement occurs. The presence of roddons leads to further heterogeneity in the underlying ground conditions and increased undulations along the road.

The road surface is impermeable and restricts ingress of water directly beneath it. The presence of ditches either side of the road means that a larger surface area is exposed, driving enhanced changes in water content. As the adjacent ground experiences annual fluctuations in water content, and resulting volume changes, the soil water content beneath the road remains relatively constant, sealed off from external variations. This leads to a soil water content gradient from the edge to the centre of the road, and a gradual drying out of the outer edges of the road over time. This drying causes a decrease in soil water content and pressure, leading to volume reduction and longitudinal deformation parallel to the edge of the road, as can be seen in Figure 7. This process is further exacerbated by heavy loading from vehicles travelling along the road, and the carriageway being informally widened over time, so load is being applied nearer to the break in slope. Once deformation has occurred, these weak zones are broken up by deformation cracks that allow water ingress, causing further damage through localized changes in water content and freeze–thaw processes. Acting locally, within a few metres, the presence of vegetation, such as trees and large bushes, exacerbates these processes due to evapotranspiration.

Site 2: Brandy Wharf – peat compression

The site is a 400 m section of the B1205, situated 3 km east of Waddingham village (501820, 397087), in the hamlet of Brandy Wharf Spa. The road is relatively low lying with a shallow drainage ditch running along the northern edge.

Movement history

The road has developed large undulations in the surface, as well as edge cracking.

Geological background

The site is in an area of peat, underlain by glaciolacustrine deposits (silts and sands). The New River Ancholme is at the western edge of the site. In the seventeenth century, the River Ancholme was canalized from Bishopbridge to Ferriby, passing through Brandy Wharf, with this work taking more than 200 years to complete. This New River Ancholme now carries most of the water but the old course still meanders, reduced nearly entirely to a ditch. The Environment Agency (EA) LiDAR data (Fig. 8) show the meandering trace of the old river channel running north–south in the centre of the LiDAR map, in darker blue, and marks the eastern edge of the site. The damaged part of the road is along the short stretch that passes over the peat.

In September 2020, Lincolnshire County Council undertook windowless sample holes, drilling to a maximum depth of 4.5 m, at four points along the stretch of road (Fig. 8). The road surface had a consistent 40 cm of asphalt, underlain by c. 60 cm of fill and sub-base. The depth of peat in the boreholes beneath the road ranges from 1 to 3 m, showing large variability between each of the sites; with WS4 recording much shallower peat that WS1–WS3 (Figs 912). The deposits underlying the peat at all four sample holes are clayey peats and clays with rare organics, considered to be glaciolacustrine deposits. These are much thicker in WS4 than at the other three locations. Sand with flint gravel is at depth in all four holes, located at around 2 m depth at WS1–WS3 and at almost 4 m at WS4. The LiDAR image (Fig. 8) indicates the presence of a roddon at WS1 and the old River Ancholme passing through WS4. However, disturbance due to the construction of the new river channel, and the road construction, could have disturbed deposits laid down by shallow river channels (roddons) in the location of borehole WS1.

Potential causes of movement at the site and future climate change

At sites WS1–WS3, the peat is deeper, and the road has experienced more subsidence and damage than at WS4, due to the high compressibility of the peat. In addition, the underlying glaciolacustrine deposits, with a varying organic content, increase in depth towards the east (WS4), leading to further variations in compressibility along the short stretch of road. With the presence of roddons and the old river course passing over the site, the variable and complex history of the site leads to large differences in susceptibility along the short section of road.

Site 3: Witham Bank – alluvium with vegetation

Witham Bank (518837, 359977) is an evolved road with no foundations, running parallel to the River Witham.

Movement history

Lateral variation in water content across the carriageway has led to subsidence and the generation of a longitudinal crack in the road surface (Fig. 13), commonly up to 20 cm wide. A likely cause of this variation is the increase in vegetation along the western edge during the last decade, as seen in Figure 13 (May 2018), which withdraws water from the ground on the western side. Other potential processes could be compounding changes in water content at the site. These include: the western bank potentially being steeper than the eastern bank; the road having been historically widened on the west rather than the east; the parallel ditch having been recently overdug; or the water table dropping due to pumping to the west.

Geological background

The road is underlain by highly compressible alluvium. In 2020, the authority undertook a small ground investigation to 6 m below the carriageway on Witham Bank. Below 3 m of made ground, fibrous peat or peaty clay was found. The peat was continuous to 6 m depth. The material was laminated and denser than typical spongy fibrous peat deposits. It is considered that the increased density was due to compression of the peat from the loading of the embankment and possibly the lowering of the groundwater level.

Potential causes of movement at the site and future climate change

Recently, there has been an increase in dynamic loading caused by an increase in traffic, as well as increased static loading due to repair work with the addition of overlays. These add to the load, leading to compression and thus exacerbating the cracking.

It is anticipated that projected changes in climate will further intensify the cracking due to lower water levels and drying out.

Site 4: Amber Hill – tidal flat deposits with deep drainage ditch

North of Amber Hill village, Sutterton Drove (522349, 347593) is an evolved road that runs parallel to a large drainage ditch.

Movement history

Longitudinal cracking has occurred down the centre of the carriage way, as can be seen in Figure 14.

Geological background

Geological maps and borehole logs indicate tidal flat deposits with a thickness of c. 5 m, underlain by thick gravels.

Potential causes of movement at the site and future climate change

Changes in water management, causing fluctuations in pump volumes in the winter due to changes in precipitation and changes in agricultural practice, including field irrigation during dry winters, has led to lower water levels in ditches and drying out of the embankment, much more so than in the recent past. As seen in Figure 14, trees are established adjacent to the drainage ditch. As water levels decrease in the ditch, the trees could be taking additional water from the road side of the embankment. Longitudinal cracking has occurred down the centre of the carriage way due to the high soil water gradient between the embankment and beneath the road surface.

 Lincolnshire County Council and the BGS started exchanging knowledge and data after meeting in 2015 at a stakeholder event that contributed to the design and development of the GeoClimate shrink–swell UKCP09 data product (Harrison et al. 2020). This collaboration has continued and developed to sharing lessons learnt with national infrastructure managers through conferences and meetings (e.g. Harrison 2018; LCRIG 2022).

BGS GeoSure and Lincolnshire County Council residual life dataset

Lincolnshire County Council monitor the road network using several methods to enable them to prioritize the maintenance work and utilize funds efficiently. Deflectogram surveys (Highways England 2020) provide an estimate of residual road life (RL) for the principle and strategic road network in the county. The RL is calculated from the maximum deflection of the road surface, when a vehicle of a known weight and speed passes over it, at a spacing of c. 4 m intervals. The estimated RL is the number of years remaining before the onset of ‘investigatory’ conditions, meaning that more intensive monitoring is required of the road's structural condition. Deflectogram surveys are completed regularly, providing annual data for many roads within the county.

Deflectogram data for 3 years, which includes a period of drought conditions (2010–2012), were analysed against BGS datasets. The data do not correspond with the case study sites due to the deflectogram surveys being limited to the county's principal and strategic road network. The decrease in RL value was calculated to provide a measure of road deterioration. BGS GeoSure compressibility and clay shrink–swell datasets were used to compare susceptibility to ground movement of the underlying geology with road deterioration (GeoSure compressibility is shown in Fig. 15) to assess whether it is related to the underlying geological units and/or transitions between units with different lithological characteristics.

Table 3 provides an explanation of the GeoSure compressibility and clay shrink–swell values. Table 4 compares the relationship of average annual deterioration with the shrink–swell and compressibility GeoSure ratings of the underlying geological units. This comparison demonstrates a strong inverse relationship between RL and the GeoSure compressibility rating (Fig. 16). Roads within areas where the compressibility of the underlying strata is classed as high or very high (D and E) experience much more rapid deterioration rate in RL than those with lower compression susceptibility. Roads within areas where the compressibility of the underlying strata is classed as high or very high (D and E) experience much more rapid deterioration, and therefore decrease in RL, than those in areas with lower compression susceptibility.

In contrast, there is a weak relationship with GeoSure clay shrink–swell values, with the average deterioration rate remaining relatively static for all clay shrink–swell values, with the average RL deterioration rate for A and D being very similar (1.10 and 1.15 years per year). This suggests that clay shrink–swell is not the primary driving factor for carriageway deterioration on principle roads in Lincolnshire, and other intersecting processes are more influential.

When analysing these datasets further and removing all A roads, a much faster road deterioration, and therefore decrease in RL, is shown on roads underlain by geology with a very significant compressibility GeoSure rating (E), increasing from a deterioration rate of just 3 years to 17 years annually. Areas with compressibility GeoSure rating D, also see a large increase in deterioration; from 1.1 years per year including A roads to 3.2 years per years without. Similarly, removing the A roads in the analysis of areas with higher clay shrink–swell susceptibility also leads to an increase in deterioration of the roads. This demonstrates that when A roads are removed, the rate of road deterioration increases in areas of high susceptibility to ground movement, indicating that engineered A roads, with deeper foundations, are more robust and less influenced by natural ground conditions.

BGS GeoSure and Coarse Visualization Inspections

 Lincolnshire County Council also carry out Coarse Visualization Inspections (CVIs) of their unclassified road network; generally, the roads that have much shallower foundations. The CVI is carried out on a slow-moving vehicle, or by foot, by a trained surveyor, who records cracking, rutting and defects in the road surface. This provides information on the structural condition of the road, in the form of the Structural Condition Index (CI), which is expressed as a value between 0 and 93, with 0 being very good condition and 93 being very poor condition. Table 5 and Figure 17 show a direct relationship between higher CI values (worse structural condition) and the compressibility of the underlying geology. Although there are no data for roads on geology with a GeoSure compressibility value of C, the higher CI values with worse structural condition are in areas with very high susceptibility to ground movement due to compression.

BGS GeoSure and Lincolnshire County Council drought-damage data

In 2003, when the UK experienced one of its hottest summers on record, Lincolnshire reported a greater deterioration in the condition of its highways than in previous, non-drought, years. Since 2003, all 10 years recording the highest annual temperature since 1884, when records began, have occurred (Met Office 2022c). In 2011, the dry period led to Lincolnshire making a bid to central government for road repairs related to what was assumed to be drought-related subsidence. The roads affected, mostly unclassified and C roads, are shown in red in Figures 18 and 19. Most of the damaged roads are underlain by superficial deposits, primarily high water content, compressible tidal flat deposits (represented by alluvium in the 625 k scale geological map) and peat. The four case study site locations are highlighted for context.

A comparison of Lincolnshire County Council drought-damage data and BGS GeoSure clay shrink–swell compression datasets (Fig. 19) showed that clay shrink–swell was not the county's only likely cause of road damage, and that compressible ground had a greater correlation with road damage than was originally considered (Pritchard et al. 2015).

BGS GeoClimate and road-damage projections

The GeoClimate datasets are national datasets showing potential changes in clay shrink–swell subsidence susceptibility due to changes in climate (Harrison et al. 2019, 2020). They have been developed by considering climate projections and the associated changes in SMD, alongside hydrogeological modelling and the geotechnical properties of the ground (Tables 6 and 7) to provide long-term, modelled analysis for resilience assessments. They provide information on the potential for clay shrink–swell to occur at a given location during a given future time period. GeoClimate projections for clay shrink–swell susceptibility for Lincolnshire for the coming century are shown in Figure 20.

The GeoClimate projections shown in Figure 20 (Harrison et al. 2019) are partially driven by the Met Office UKCP09 climate projections (Murphy et al. 2009). The SMD increases that are projected for the county are reflected in the values calculated for each case study site (Table 8). These are based on the UKCP18 Met Office climate projections (Murphy et al. 2018) modelled using the UK national-scale recharge model (Mansour et al. 2018). The SMDs at each site increase from modelled historical values (1979–2000) and are projected for 2060–2080. These indicate an increased likelihood of drier ground conditions at the case study sites and in Lincolnshire in the coming century. The natural change in water content, due to changes in climate over the coming century, will impact shrinkable and compressible deposits across the country, and cause changes in volume over the short, medium and long term. Compressible ground will also be affected due to loading and anthropogenic practices such as surface water management.

Based on an increased knowledge of variability in geological hazard susceptibility across the county, Lincolnshire County Council has developed new road repair and maintenance practices, founded on recycling and reinforcement techniques. The new regime is not only more cost-effective but the authority has found that it also further extends the road's life in comparison to previous practices and reduces the carbon footprint of the work. The practices included ‘in situ (re-tread)’, recycling the existing road top surface, and reinforcement of the road surface with wire mesh, dependent on the road conditions and underlying geology. Lincolnshire County Council is increasingly focusing on lower carbon solutions. This research has driven how maintenance techniques are now being considered, taking into account carbon reduction.

Following extensive investigation and review of previous trials undertaken by Lincolnshire County Council, the recommendation for repair work at Fodderdyke (Case Study Site 1) was:

  • to pulverize the existing road to a depth of c. 250 mm;

  • shape and compact the pulverized material to make a new and increased thickness sub-base;

  • lay 60 mm of AC20 binder course material (asphaltic concrete with a maximum aggregate size of 20 mm),

  • lay and pin a steel reinforcing grid over the whole width, including overlaps at the joints;

  • lay 60 mm of AC20 binder course material; ans

  • lay 40 mm of 40 mm surface course.

Remedial work at Fodderdyke was carried out during 2018 (Figs 2123), reflecting the knowledge of the underlying road conditions, their geotechnical properties and the impact of projected climate scenarios on susceptibility to movement in the future. Recognizing the addition of added weight would cause more compression, the existing road surface was recycled. A mesh was included, covering the width of the road, enabling it to move as a single entity and thus experience less differential movement, resulting in less surface cracking. Although this initial repair work was costlier and took longer than the traditional patching, this informed decision-making was targeted and defendable. It alleviates the requirement of abundant future repair work and associated road closures with their resultant economic costs, demonstrating improvements both financially for the authority and for the local community. The authority has shared an online overview of the work carried out at Fodderdyke (LincolnshireCC 2018).

Benefits of this type of construction have been the non-removal of tar-bound material off site, which can be classed as a hazardous material with high disposal costs, and the associated carbon savings. Keeping this material onsite has helped to regulate and reshape these deformed roads. The use of the steel mesh ensures that the road ‘holds together’, preventing failures beneath the road surface potentially causing major collapse overnight.

Reshaping keeps material onsite and has been efficient in terms of reconstruction speed, and, hence, opening roads quicker, which is especially important in rural fenland areas where diversion routes can be many miles. Although the technique has proven successful, it is expensive and can prove problematic when future resurfacing is required or if a utility company wishes to undertake works on this road network.

Looking to the future, work and research is being undertaken on this type of treatment and how the authority can continue to maintain its highway network. In terms of new techniques, the authority is developing and working with industry and academia, examining climate change and carbon calculations for the designs of schemes. Where possible, the authority is using thinner materials, with similar strength characteristics and flexibility. New proposals and trials are being constantly considered to overcome the cost and carbon issues that the industry is seeing, whilst simultaneously ensuring they are informed and prepared for any future changes in legislation.

Lincolnshire roads in the south of the county have a high susceptibility to ground movement due to the underlying geology, accentuated by additional causal factors, to varying degrees, including the presence of vegetation, road maintenance regimes and drainage practices. The superficial peat, tidal flat deposits and alluvium all have a high susceptibility to compression when loaded or due to loss of water content because of drought or ground water lowering through managed drainage and irrigation. The shallow foundations of the county's evolved road network increases its vulnerability to these fluctuations, meaning that ground movement often causes road damage. This includes longitudinal cracking and edge failure, and uneven long-section profiles due to localized failures. Knowledge exchange and data sharing between Lincolnshire County Council and the BGS has enabled the direct relationship between unclassified/evolved road residual life (RL) and structural condition, as well as the compressibility of the underlying geological formations, to be demonstrated.

Historically, repair work by the authority has included sprays and overlays, adding a new road surface on top of the existing one, adding weight and compounding the issue, and causing further damage in future years. Additional anthropogenic pressures on the network include increased road traffic (Lincs Laboratory pers. comm. 16 March 2023), including heavier farm vehicles and lorries transporting goods, and decreases in water levels in the dykes managed by the Internal Drainage Board (IDB). The modifications in water management are necessitated by irrigation of crops due to changes in climate. Hotter drier summers, and warmer wetter winters, lead to naturally higher annual SMDs than the road network has experienced. These are projected to increase into the future, leading to the need to either prevent or mitigate against the consequences. This will involve collaboration between geotechnical asset managers and engineering geologists to make informed decisions about maintenance and repair based on an understanding of geological properties and processes, and how these may change in future.

This paper has presented examples of changes in maintenance and repair work instigated by Lincolnshire County Council as an example of the successful application of this new regime. It has also assisted the local highway authority in starting to reduce its carbon footprint with the utilization of recycling techniques (reduction of materials to landfill) and the limited usage of virgin materials, together with thinner layers of materials.

The limited budgets for highway maintenance are well documented by local authorities, and the entire spectrum of uncertainties of new Parliamentary Acts and the consequences of implementing such Acts have not been fully researched. This collaborative research undertaken by the BGS and Lincolnshire County Council demonstrates that further work on climate change and highway construction needs to continue to ensure the longevity of this vital UK asset. Future monitoring, such as remote sensing and regular condition surveys, would demonstrate the precise efficacy of new repair techniques, and the resilience of the network to climate change.

A.M. Harrison and D.C. Entwisle publish with the permission of the executive director of the British Geological Survey. The authors would like to thank the reviewers, Dr Ursula Lawrence and Dr Oliver Pritchard, for improving the paper. The Editors and Manager from QJEGH are also thanked for their inputs.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

AMH: data curation (lead), formal analysis (lead), methodology (lead), writing – original draft (lead), writing – review & editing (equal); MH: conceptualization (supporting), data curation (supporting), methodology (equal), resources (supporting), validation (supporting), writing – original draft (supporting), writing – review & editing (equal); DCE: formal analysis (supporting), methodology (supporting), writing – original draft (supporting), writing – review & editing (lead).

This work was supported by the British Geological Survey via NERC national capability funding.

The data that support the findings of this study are available from British Geological Survey and Lincolnshire County Council but restrictions apply to the availability of these data, which were used under licence for the current study and so are not publicly available. Data are, however, available from the authors upon reasonable request and with permission of British Geological Survey and Lincolnshire County Council.

This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/)