Snow in roadside ditches can make roads both stronger and more vulnerable

How snow is managed in roadside ditches can have a major impact on traffic safety, road durability and winter maintenance costs. New research from Luleå University of Technology shows that snow in ditches does not only protect roads from cold, but can also contribute to cracking and frost damage when its insulating effect is uneven.

Snow that accumulates in roadside ditches can either protect a road or contribute to cracking and frost damage. In her doctoral thesis in Soil Mechanics, Zahra Motamedi investigates how ditch snow influences the temperature of road structures and the risk of frost-related damage in cold climates.

Through detailed field measurements outside Luleå and advanced modelling, she demonstrates how snow acts as an uneven insulating blanket. This can make certain parts of the road embankment freeze deeper than others, increasing the risk of deformations.

Snow in the ditch – protection or risk?

In the thesis, Zahra Motamedi studies how the snowpack and ground temperatures evolve throughout the winter at two experimental field sites. One represents natural, undisturbed snow, while the other is a roadside ditch where the snow consists of a mix of natural and ploughed snow.

"Snow in the ditch acts as an insulating layer, but it does not insulate evenly. This can create large temperature differences within the road embankment and contribute to cracks and frost damage," says Zahra Motamedi.

She has measured snow temperature at different heights, weather conditions and ground temperature down to two metres depth. The results reveal how the snow structure changes during winter and how this affects the snow’s thermal conductivity.

"We observed that natural, undisturbed snow provides stronger insulation than the more mixed snow in roadside ditches. This means that the same snow depth does not necessarily offer the same protection against frost," she says.

New tools for winter maintenance

A key outcome of the thesis is an empirical relation describing how the thermal conductivity of snow in roadside ditches evolves over time. It can be used in similar climates to better estimate the insulating effect of ditch snow.

Zahra Motamedi has also developed and validated a numerical model that simulates the temperature profile within the road. The model includes weather, cloud cover and traffic loading – all of which strongly influence how quickly the pavement cools during winter.

"With a more realistic model of pavement temperatures, we can better predict when frost penetrates the structure and schedule maintenance measures in time. It is about road safety, but also about using maintenance resources more efficiently," she says.

The findings show that ditch snow can be managed more strategically. By adapting how and where snow is ploughed or compacted, road authorities can influence road temperatures and reduce the risk of freeze-related damage.

"If we understand the true thermal behaviour of snow in ditches, we can move from simply pushing snow aside to using it consciously in road design and winter maintenance," she says.

• Research topic: Soil Mechanics