Richard Hayes, past president of the Institute of Highway Engineers (IHE), writes about the current droughts the UK is facing and the threat that future rain may pose to the country's highway networks.
The call throughout the UK and especially in the south of England is for rain. However, for highway engineers, when long dry spells are followed by heavy rain, the risk of cracks, slips and costly repairs on road embankments and cuttings increases.
As the weather swings from drought to downpour, engineers are likely to see that some of the country's least visible road assets may be under growing strain. The slopes that support roads above surrounding land, and the cuttings carved through hillsides, can be weakened by the very sequence of weather that is becoming more familiar: weeks of dry heat followed by intense rainfall.
Road users usually notice the problem only when a lane is closed, a verge has slumped or a fresh crack appears along the carriageway. But the damage often starts much earlier, within the soil that holds the road in place. Many embankments and cuttings were built decades ago, before modern design and drainage standards, and their condition can change sharply when ground moisture rises or falls.
During a severe drought, clay soils can dry out, shrink and crack. Those cracks may open at the top or face of a slope, leaving hidden routes for water to enter later. The immediate signs can be small: uneven verges, fine cracks in the road surface or slight settlement at the edge of the pavement. Left unaddressed, however, they can become the starting point for more serious movement.
The risk can rise quickly when heavy rain arrives after a dry spell. Water can rush into drought cracks instead of soaking slowly through the ground. As the soil wets up, it can soften and lose strength. If water pressure builds within the slope, sections of an embankment or cutting may begin to slip, slump or erode.
Drainage is often the difference between a slope that copes and one that fails. Blocked ditches, damaged drains, silted culverts or uncontrolled runoff can concentrate water in exactly the wrong place. A storm that might otherwise pass without incident can then expose weaknesses created during the preceding dry period.
One dry summer or one wet winter may not be enough to cause a major failure. The greater concern is repetition. Dry ground shrinks and cracks; wet ground swells, softens and moves. Over years, this cycle can weaken older earthworks and make them more likely to need repairs after storms or heatwaves.
Vegetation can add to the challenge. Tree and shrub roots may help bind the surface, but they also draw water from clay soils during dry weather. In some locations, mature vegetation can increase drying around a slope and hide cracks or small movements until they are more advanced.
For road users, the consequences may appear as cracks, dips, uneven surfaces, temporary traffic lights, lane closures or emergency works after heavy rain. For councils and highway authorities, the challenge is more complex: identifying which slopes are most vulnerable before they fail, keeping drainage clear, and deciding where limited maintenance budgets should be spent first.
Warning signs include new cracks at the top of slopes, bulging ground, blocked drains, water seeping from a slope face, erosion channels, leaning fences or barriers, and repeated cracking at the edge of the road. After heatwaves and intense rainfall, inspections can help catch these problems while they are still manageable.
The most effective protection is often practical rather than dramatic: clean ditches, working culverts, well-maintained drains, controlled vegetation and careful monitoring of known weak spots. Where slopes are already moving, more substantial work may be needed, including regrading, reinforcement, new drainage or partial reconstruction.
As climate patterns shift, road maintenance is likely to become less about responding to single incidents and more about preparing for sequences of extreme weather. A road embankment weakened by drought may not fail until the rain comes. By then, the warning signs may already have been written into the ground.
