Knowledge of high-plasticity clays may prolong design life of structures

Tuesday 05 Mar 19

Contact

Varvara Zania
Associate Professor
DTU Civil Engineering
+45 45 25 50 92

Contact

Irene Rocchi
Assistant Professor
DTU Civil Engineering
+45 45 25 19 16

High-plasticity clay

High-plasticity clays are fat, red, green, or grey clays that can be easily moulded. High-plasticity clays were deposited in the Paleogene era 54-38 million years ago. High-plasticity clays are found in Denmark at Røsnæs, near and under the Little Belt and the Fehmarn Belt.
In the field of geotechnical engineering, the properties of high-plasticity clays are of great importance, since changing the load conditions along with water absorption can result in settlements and possibly damage in buildings founded on high-plasticity clay.
If the load over high-plasticity clay reduces, it can absorb water and swell. The forces in clay, so-called swelling pressure, can cause serious damage to a building if the building weighs less than the amount of soil removed.
Researchers at DTU Civil Engineering are testing high-plasticity clays from the seabed under the Fehmarn Belt and exploring new methods for treating them.

Research in high-plasticity clay under structures such as bridges and immersed tunnels can be an investment in climate-friendly and durable solutions. That’s the opinion of Associate Professor Varvara Zania, who - together with a team of researchers at DTU Civil Engineering - is analysing data on high-plasticity clays from the Fehmarn Belt area. In the new laboratories for geology and geotechnical engineering, the team is working on experimental investigation of the clay material and is testing methods that can improve the mechanical behaviour of clay.

“Our research may lead to alternative and better solutions than using more cement and more steel to reinforce and secure structural foundations. By gaining greater insight into the mineral composition of clay, its microstructure, and its deformation behaviour under different loading conditions, we get the opportunity to predict the interaction between structures and soil. This may lead to designs with reduced risk due to reduced uncertainty and possibly new more sustainable solutions,” says Varvara Zania.

The researchers at DTU Civil Engineering review and process data on high-plasticity clay originating from soil studies under the Fehmarn Belt. This information forms part of new experiments that will provide new knowledge and be used for a detailed characteristic of how high-plasticity clay deforms. Knowledge that in the long-term can be used to develop engineering tools and new methods of handling high-plasticity clay. The researchers will, among other things, investigate whether it is possible to limit and control the swelling of plastic clay which swells when in direct contact with water.

Varvara Zania points out that there is a need to develop new methods because the traditional solutions lead to rigid structures. This means that the structure cannot follow the ground, and it is thus prone to increased stresses and damage when large deformations occur in the soil, as is the case in high-plasticity clays.

Bridge settled 70 centimetres

The new methods and new knowledge of plastic clay is currently in demand in the Lighthouse project in Aarhus, where Geo has set a local record with a 120-metre geotechnical drilling to clarify the foundation conditions. Previously, damage due to settlements to structures in high-plasticity clay has resulted in costly repairs or reinforcements.

This was the case in 2014, for example, when Banedanmark reinforced the Little Belt Bridge at a cost of DKK 120 million and poured 125,000 m3 of stones down around four critical bridge piers. The reinforcement was necessary because the Little Belt Bridge had settled a total of 70 centimetres, far more than the 10 centimetres that were calculated at the inauguration in 1935.

The research at DTU Civil Engineering takes place in two research projects under the supervision of Varvara Zania and Assistant Professor Irene Rocchi. One is carried out in collaboration with COWI, which has financed PhD student Emil Mejlhede Kinslev and his research, entitled ‘Efficient performance of large infrastructure: a geomechanical approach towards sustainable design’.

The research is based on laboratory experiments and analytical models, where researchers investigate how high-plasticity clay behaves under varying loads in order to use them to predict construction phases in infrastructure or buildings. The observations are mapped into a material model that contains information on the load history, duration, and magnitude of different designs so that they can be used to improve the design of, e.g., immersed tunnels and high-rise buildings.

The second research project is carried out by PhD student Giorgia Di Remigio, entitled ‘Multi-scale assessment of swelling and compressibility of fine-grained geomaterials’, focuses on the mechanisms that occur at the micro and nano-scale in the clay. Using various experimental techniques, the researchers study how the clay mineralogy and microstructure influence the clay’s reaction to changes in pressure or when, for example, it is exposed and absorbs water.

The two research projects contribute to state-of-art knowledge, and the plan is to develop tools that will enhance the current capacity in designing on complex soils. The projects will be completed at the end of 2020 and 2021, respectively.