Discrete element modelling of clay

The aim of this project is to use the Discrete Element Method to explain the particle-scale origins of the mechanical behaviour of clay. Clays, like all soils, are granular materials composed of solid particles and fluids. Yet clays exhibit the most complex behaviour and remain the least understood. The Critical State Soil Mechanics framework has been used to describe and predict the general behaviour of soils for around 60 years, but the origins of this behaviour have rarely been investigated. Within this guiding framework, all soils (i.e. both sands and clays) exhibit the same general patterns of behaviour. For example, when a soil is sheared, it will dilate or contract, depending on the stress level and how dense or loose the initial soil is. If a soil is compressed under increasing isotropic stress, after a high enough stress is reached, a permanent decrease in volume occurs. For sands, this is known to be due to particle crushing. In fact, the normal compression line, which is a line in volume-stress space, which a sample of soil follows when subject to compression, has been shown recently by McDowell and de Bono (2013) to be solely a function of the particle strengths (specifically, the rate at which the average particle crushing strength increases with decreasing particle size). This therefore provides a micro mechanical explanation for a well-known and fundamental feature of soil behaviour.

For clays on the other hand, the underlying mechanisms which control the bulk behaviour remain unknown. This is due to difficulty in observing or measuring particle interactions due to their small size. The individual particles in clays are too small to be seen with the naked eye, and are so small that the interactions between these particles are controlled by molecular forces rather than mechanical forces. Clay particles also have more complicated shapes when compared with sand, such as hexagonal platelets or cylindrical tubes. The inter-particle forces acting between clay particles are highly dependent on the environmental conditions (e.g. pH, salinity, etc.). These forces can be attractive or repulsive, and different forces may exist between the different parts of clay particles (e.g. the 'edges' and 'faces'). The variety of different combinations of forces between particles leads to many different geometrical arrangements of particles in real clays. Just how the particles interact during engineering applications (i.e. loading/unloading) and how their geometrical arrangement changes or controls the macroscopic behaviour remain speculative, and the particles are presently impossible to observe experimentally due to the small size.

There is therefore no fundamental understanding as to what causes or leads to observed phenomena such as a decrease in volume when subjected to increasing (e.g. isotropic) stress, or volume change during shearing. These phenomena will be explained by using the Discrete Element Method (DEM) to model and investigate the behaviour of clays with varying inter-particle forces. DEM is a numerical tool which computes the interactions and motion of a large number of discrete particles. By default, the majority of DEM simulations are typically only concerned with mechanical contacts between entities, which are easily calculated; yet it is possible to implement any number of custom, more complex interaction laws. DEM simulations have been typically limited by the computational hardware available, and to date clay has rarely been modelled. This ambitious project will use DEM to 'look inside' a numerical clay sample with a large number of particles and realistic particle interactions as it undergoes a variety of stress path tests, changing the way we understand (and teach) clay behaviour.

Revealing the underlying origins of clay behaviour will allow engineers and researchers to develop more accurate models and ultimately will lead to safer, more economic designs of foundations and underground structures.

The end result of this project will be that those in the geotechnical community, and ground engineering industry in general, will have a microscopic understanding of clay behaviour, and therefore the most important micro properties which influence the bulk behaviour. With this greater and fundamental understanding of how the micro mechanics govern the macroscopic constitutive behaviour, it will be possible to re-write critical state soil mechanics for clay, with constitutive equations based on the micro properties of the soil. This will allow more informed predictions of soil behaviour and therefore improved geotechnical designs. In the long term, this will lead to more durable, sustainable, safer and more economical infrastructure.

This is a fundamental proposal which aims to explain all the salient features of clay behaviour within the Critical State Soil Mechanics framework, which has been central to all of soil mechanics and geotechnical engineering for the last 60 years. The impact is therefore potentially very large, global and of great significance. Almost every soil prediction or geotechnical design has Critical State Soil Mechanics at its core, but with no micro mechanical basis. No one really knows why the behaviour of clay is similar in many ways to a sand (albeit at different stress levels), yet a clay can be endlessly reconstituted and a sand cannot. This project aims to answer this question. The outcome will be a step change in the understanding of mechanical behaviour of clay and a major breakthrough for soil mechanics as a subject.