Shales exhibit high anisotropic swelling strains. A model is developed to quantify this highly anisotropic swelling behaviour. The model considers the effects of particle double-layer swelling, fabric structure and particle force interaction on the swelling behaviour. Model evaluation with experimental data published in literature demonstrates that the model provides a better framework to characterize the anisotropic swelling. The model also indicates that the repulsive-attractive (R-A) stresses induced by the physico-chemical change in pore fluid chemistry are anisotropic depending on fabric structure and confining stresses.
Mineralogical tests indicate that swelling behaviour is related to the soil composition and fabric. In general, the orientations of mineral particles are randomly distributed in a mass of soil. However, under highly anisotropic consolidation stresses a strong preferred orientation of particles in shales could be developed (e. g., Czurda et al. 1973; Lo et al. 1978). Thus, models developed on the basis of mineral fabric may be more appropriate to quantify the anisotropic swelling behaviour of shales. Zhong (1994) proposed a simple anisotropic linear swelling model for shales with free axial-radial swelling strain ratios less than 2. This paper presents a model for highly anisotropic swelling shales, along with model verification. Further development of the model for analysis of time-dependent swelling problems are also addressed.
For dense shale, it is possible to envisage the fabric structure as shown in Fig. 1 (Bennett and Hulbert 1986). There are three basic microfabric features: elementary particle clusters, particle assemblages and pore spaces. The elementary particle clusters are made up of clay particles or minerals in a parallel configuration. The particle assemblages are formed by arrays of elementary particle clusters, and they are described as matrices. The pore space is made up of intramatrix pores existing between the elementary particle clusters. Hydration swelling will occur when there is a physio-chemical change in pore fluid chemistry (exposure to water). Water is attracted to clay particles or minerals in several mechanisms (Low 1961). Water interacts with the negatively charged silicate surfaces of clay minerals and forms hydrogen bonds with silicate oxygens. Also water interacts with cations attracted to negatively charged clay surfaces. Hydration interaction of water with the silicate surfaces and cations reduces the free energy of water and thus provides a driving force to cause spontaneous water adsorption and swelling until the activity or chemical potential of the associated water becomes equal to that of the free water. Movement of water by osmosis is another possible mechanism. Because of this increased concentration and the restriction on diffusion of ions from the vicinity of the surface, as a result of electrostatic attraction, water molecules tend to diffuse toward the surface in an attempt to equalize concentrations. For swelling deformation analysis, there are two levels of soil structures required to be considered: (i) a microstructural level that corresponds to the active clay minerals and its vicinity, and (ii) a macrostructural level that accounts for the entire matrix of the structure.