At the surface of clay minerals
The physical and mechanical properties of clay minerals are often influenced by their specific surface area. Researchers claim to have found a simplified way of predicting this to determine their impact on fine-grained soil characteristics.
This method could also be used to predict engineering properties. Professor Bojana Dolinar at the University of Maribor, Slovenia, has been leading research into how the mechanical properties of soils change due to water content and mineral composition, and how they are influenced by environmental factors, such as temperature, texture of soils, chemical composition of pore water, quantity and type of organic substances, and geological pressure.
Dolinar claims that much research into this field so far has focused on specific case studies, that ‘establish a relationship between selected physical properties of soils and some mineralogical characteristics, yet all these dependencies are only approximate and valid for the investigated soils.’
She adds that while these field or lab-based methods accurately reflect the actual state of the environment, they offer no help when it comes to trying to predict the effects of changed geotechnical conditions. ‘This prediction, however, is of utmost importance when building structures,’ she says.
In an effort to solve this, Dolinar and her team have conducted studies into which clay minerals influence water content and the different mechanical properties of finegrained soils.
They have analysed five samples of non-swelling clay minerals such as the kaolinite and illite groups originating from different regions in the United States, as well as swelling minerals in the montmorillonite group, the surface areas of which are usually determined experimentally.
Traditional methods include measuring the external surface area by adsorping simple molecules such as nitrogen at low temperatures, which can be time-consuming and expensive.
Instead, Dolinar's team used the Atterberg limits to estimate the specific surface area, then selected five heterogeneous soil samples and measured the liquid and plastic limits as well as the grain-size distribution.
Dolinar claims that the research found interdependence between the geomechanical properties of fine-grained soils and water content. ‘Fine-grained soils contain both clay minerals and associated minerals, and the interactions between the clay minerals and the water affect the soil’s water-holding capacity,’ she says. ‘Different geomechanical properties, such as Atterberg limits, strength, compressibility and soil suction, actually depend on the quantity of water contained, but these relationships are not so simple.’
She continues, ‘When we found out that the mechanical properties of fine-grained soils depend on the quantity of free pore and adsorbed water on the external surfaces of clay minerals, and that these quantities of water depend on the external specific surface area, we were able to express different geomechanical properties.’
According to Dolinar, the findings will help to understand how different minerals in soil composition influence the water content and geomechanical properties. The established relationship between the Atterberg limits and soil’s surface area enables them to estimate the external specific surface area using the liquid limit value.