The paper presents an analysis of laboratory and field investigations of the anisotropy of natural early Permian claystone deformation behaviour. Current analytical solutions for determining the stress-strain state of anisotropic soil bases are overly complex and designed for simple loading schemes. The authors carried out pressuremeter tests, static plate tests of claystones and a number of laboratory oedometer tests of claystone and modern hard clay specimens cut parallel and normal to stratification, as well as numerical modeling of these tests. A number of the experiments made it possible to study the strain anisotropy of claystones in comparison with modern hard clay in the vertical and horizontal directions. It has been investigated experimentally under laboratory and field conditions that the anisotropic deformation properties of claystones and modern clays are observed.

1 Introduction

One of the basic assumptions of soil mechanics is that soil is an isotropic linearly deformable material. However, natural soils are not isotropic and they behave like linear elastic materials under relatively small loads. Therefore, when determining relative deformation the modulus of general deformation (hereinafter referred to as the modulus of deformation) is used. It considers both plastic and elastic soil deformations depends on the load.

Investigations of clay strain anisotropy have been carried out in many countries since the beginning of the last century. Zhiwei & Jidong (2012), Salager et al. (2013) noted that most natural clays were inherently transverse anisotropic due to the settlement accumulation process. Experiments showed that the properties of clays usually depended on the direction in which they were measured. The following foreign scientists, such as Barden (1963), Biarez (1961), Eftimie (1969) carried out research into soil deformability. Among Russian researchers Bugrov (1993) and Grechko et al. (1976) can be mentioned. Anisotropic deformability is clearly observed in soils and soft rocks having a layer structure. However, in the works of Grechko et al. (1976), it is pointed out that strain anisotropy is typical for homogeneous clays too. Biarez showed that the deformability of modern clays was more defined in the plane of stratification (in the horizontal plane). However, there are several papers pointing out that soft rocks have less compressibility in the plane of stratification (Grechko et al. 1976, Ponomaryov & Sychkina 2012). Zhang et al. (2012) others studied clayey rock deformability in oedometer under partly saturated conditions. It was shown that clayey rocks became more ductile and the elastic modulus decreased when the water content increased. Such effects were inherently related to the modification of clayey rock microstructure. For instance, the drying and wetting process could alter the distance among clay particles and lead to the change in mechanical properties of clay aggregates. At the present time, to determine the stress-strain state of bases, numerical methods are widely used.

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