Bachelor’s project points to new potential in Danish clay – and challenges current testing practices

Today, when a new road is being built, we excavate clay, transport it away and bring in gravel.

Transporting many thousands of tonnes of surplus excavated soil is costly and has a significant climate impact. At the same time, it is becoming increasingly difficult and expensive to source virgin aggregates such as sand and gravel to replace the excavated clay.

The construction industry is therefore looking for new ways to make use of surplus soil.

Three civil engineering students from the Department of Civil and Architectural Engineering have investigated whether part of the solution may already be beneath our feet.

“If the existing clay can instead be reused directly on the construction site, we can reduce the need for virgin aggregates and save CO₂,” says Arthur Kraglund Stès.

Together with fellow students Patrick Strandgaard Laursen and Rasmus Clement Bødtker, he investigated how lime stabilisation affects the frost susceptibility of the clay.

“We investigated whether the material could achieve the strength required for use in the sub-base, which is traditionally constructed using aggregates,” says Patrick Strandgaard Laursen.

“But also, and in particular, how the material behaves under freezing conditions.”

If the material is frost-susceptible, the pavement structure can undergo movement, potentially causing cracking and deformation.

Although lime stabilisation is already used in several places, there is still a lack of documentation on how Danish moraine clay behaves under Danish frost conditions.

That is why several people in the industry are following the students’ work closely, explains Associate Professor Bent Grønskov Jensen, who supervised the project.

“There is considerable interest in this area because there is a desire to replace the traditional materials used in the sub-base, but there are very few specialists in Denmark working in this field. So it is important to generate this knowledge,” he says.

New results challenge the existing testing method

An earlier project by COWI and the Danish Road Directorate raised questions about the material’s frost susceptibility. The three students therefore set out to investigate what happens when the material is allowed to cure for a longer period.

“That is the property that determines whether the material can actually perform in practice through many years of winters – and that is why it is important,” says Patrick Strandgaard Laursen.

The three civil engineering students conducted a series of extensive tests in which moraine clay with different amounts of lime was examined over curing periods of up to 140 days.

The tests measured, among other things, the material’s strength development, chemical development and behaviour under frost exposure.

Their results showed that the material continued to gain strength throughout the four-month testing period, while frost heave decreased significantly.

“After 91 days of curing, the frost heave was actually almost at the same level as that of conventional sub-base sand,” says Arthur Kraglund Stès.

But the experiments also produced an unexpected result.

One of the project’s most important findings concerns not the material itself, but the way it is tested.

“We investigated whether the methods that have traditionally been used to assess frost susceptibility actually provide a representative picture,” explains Arthur Kraglund Stès.

“But we were unable to reproduce the data that formed the basis for the capillary rise tests. We did not get any coherent data at all.”

This suggests that the traditional tests used to date do not, on their own, provide a sufficient basis for assessing the material’s frost susceptibility.

The assessment is therefore that direct frost-heave testing should play a greater role when the material is evaluated.

Results that matter to the industry

The results are significant because they provide new evidence in an area where knowledge under Danish conditions has so far been limited.

“There is considerable potential in taking the results they have produced further,” says Bent Grønskov Jensen.

“The results are relevant both to material optimisation and resource savings. That matters to the industry, both economically and in terms of climate impact.”

The researchers stress that further studies and full-scale trials are still needed. Nevertheless, the results indicate that lime-stabilised Danish moraine clay could become an important material in future pavement construction.

“We believe in the material and think it has potential,” says Patrick Strandgaard Laursen.

“But the way we assess and document frost susceptibility in Denmark could be improved. What we need are national guidelines for how frost susceptibility should be evaluated.”

The bachelor’s project therefore contributes new knowledge about lime-stabilised moraine clay under Danish conditions. At the same time, the results point to a need for national guidelines and a stronger technical basis for assessing and documenting the material’s frost susceptibility.