Effects of soil structure on soil behaviour: Illustrated with loess, glacially loaded clay and simulated flaser bedding examples

Soil structure is considered to be a systematic homogeneously repetitive heterogeneity of properties and conditions of soil. For upscaling and homogenisation of properties and conditions it is necessary to understand the ways in which soil structures occur, and to obtain understanding of the ways in which it influences soil behaviour. Using three case studies, different aspects of the effects of soil structure in soil mechanics are discussed. The very loose grain scale packing of loess deposits, held together by weak structural bonds between particles, results in dramatic propagating failure when the strength of the bonds is exceeded by deformations, as illustrated in the first case. Nonhorizontal stresses induced in the subsurface below moving continental ice sheets result in rotated stress fields within the affected soil. The rotated stress field is stored in the structure of the soil, as is shown in the second case, and influences present day properties and behaviour of these clays. It is shown in the third case that there is a large difference in the behaviour of clayey sand depending on whether the clay occurs in the form of the common clay flasers or whether the clay is homogeneously dispersed in the sand. The deformation and stress propagation in the case of sand with clay flasers results in local failures and shear planes due to the large contrast in the properties of the materials making up the sedimentary structure. The result is that many of the properties of sand with flasers are rather more like the clay of the flasers than the sand. Incorporation in the engineering practice of effects of common types of soil structure is made possible with the widespread availability of techniques such as numerical modelling. These techniques enable the development of appropriate tools for practical application.