Elsevier

Engineering Geology

Volume 66, Issues 3–4, November 2002, Pages 197-209
Engineering Geology

Particle orientation and its influence on the mechanical behaviour of isotropically consolidated reconstituted clay

https://doi.org/10.1016/S0013-7952(02)00040-6Get rights and content

Abstract

The behaviour of naturally occurring geological materials such as clay and sand depends on many factors. For example, stresses, strains, previous stress history, mineralogy and the depositional environment all contribute in some degree to a characteristic that all natural soils share, namely “structure”. The structure of clay, or more generally, the microstructure of microscopically sized clay mineral particles, is just as important as the many other parameters that are used to quantify the performance of clays. This paper examines the microstructure that results from the particle arrangement brought about during reconstitution in the laboratory and considers its relevance to the resulting stress–strain behaviour.

Samples of reconstituted kaolin clay were produced using two different procedures. In the first series of tests, kaolin slurry was simply isotropically compressed in one increment. In the second series, the slurry was first isotropically compressed to a low pressure and then completely remoulded. This was followed by isotropic compression to the same pressure as the other series. Specimens were taken from the two series of samples, reconsolidated at various isotropic pressures, and sheared under undrained conditions.

Scanning Electron Microscope (SEM) images indicated that the monotonically compressed samples (Series 1) exhibited an anisotropic microstructure that was distinct from the remoulded (Series 2) samples. Significant differences were also found in the consolidation and stress–strain characteristics of the samples produced in the two series.

Introduction

The structure or fabric of clay is an important aspect of the understanding of mechanical behaviour of fine materials. The words “structure” and “fabric” appears to be synonymous Lapierre et al., 1990, Delage and Lefebvre, 1984, Locate et al., 1996, Tanaka and Locate, 1999. Within the context of this paper, the term “structure” is preferred. There are many forms of structure in clays which are inherited for various reasons (Collins and McGown, 1974). One of the forms is the arrangement of the particles, which is further influenced by several factors, for example, amount of loading, pore water chemistry, etc. (Delage and Lefebvre, 1984, Anson and Hawkin, 1998). In some cases, particles are randomly oriented, whereas in others they are arranged in particular forms that may be oriented, or dispersed. The presence of bi-modal pore size distributions consisting of aggregates of particles exhibiting both intra-aggregate and inter-aggregate porosities has been recognised. The occurrence of structures such as laminated or varved, found in many glacial deposits, reflects the environmental conditions at the time of deposition. Stress relief creates fissures which considerably weaken the soil. There has been considerable research interest in the structure of soils for many years and several conceptual models have been developed. A detailed description of the models is presented in Collins and McGown (1974).

Many of the models are associated with particular depositional environments. The factors which influence the resulting structure include particle size, shape, grading, clay mineralogy, exchangeable cations, acidity, organic content, concentration of sediments, state of agitation, depth of water, seasonal variation, chemistry of the water, etc. Also, the arrangements of particles and their structure can be influenced by the way the materials are loaded after deposition. Leroueil and Vaughan (1990) examined the structure that results from the arrangement of particles in reconstituted clays. Since clay mineral particles are often sub-microscopic in size, it is common to call the physical arrangement of the particles ‘microstructure’. The loading after deposition can create anisotropy in soils. Delage and Lefebvre (1984) carried out a study on the evolution of structure of sensitive Champlain clay using SEM and Mercury Intrusion Porosimetry Technique. Tests were carried out on natural clays as well as reconstituted clay, consolidated to various vertical pressures. The results produced clear evidence to support a strong anisotropy in the microstructure whereby particles were generally oriented in horizontal planes identified by platy surfaces and edges of particles observed in SEM images taken on horizontal and vertical directions, respectively. The reported work in this paper focuses on the anisotropy of the soils' microstructure and examines the evolution of microstructure resulting from two different reconstitution processes, followed by isotropic loading.

Most research on the stress–strain behaviour of clay soils (for example the elastic–plastic framework developed at Cambridge University and subsequently modified by Roscoe and Burland, 1968) has been carried out on reconstituted specimens produced from slurry. The underlying assumption in these studies was that reconstituted samples would have isotropic properties. Though the framework developed on this basis has been a valuable tool in understanding clay behaviour, it has always been accepted that the results of such research may not represent the true behaviour of natural clays. This is due to the anisotropic structure which most natural clays possess (Graham and Lau, 1988). The importance of structure has rarely been discussed explicitly in connection with comparative laboratory tests for stress–strain behaviour. In order to explain its relevance, it is important to first understand the characteristics of those soils whose natural structure has been completely destroyed. The work reported in this paper examines the characteristics of such materials.

Reconstituted samples are generally produced from clay slurry prepared at a water content 1–1.5 times the liquid limit (Burland, 1990). The slurry is then consolidated one dimensionally in a rigid cylinder. Once the clay has attained sufficient strength, the sample is transferred to a triaxial cell for further loading. It is likely that since the clay is initially loaded one dimensionally, it will be inherently anisotropic and particles will be highly oriented (Delage and Lefebvre, 1984). Despite this, it is standard practice to consolidate the samples isotropically in the triaxial cell in the hope of erasing the structure created during one-dimensional loading. The research described in this paper examines how successful this approach actually is.

Section snippets

Experiments

Commercially available powdered kaolin was used for making specimens for testing. The liquid limit of the kaolin was 72% and the plastic limit 36%. Samples were prepared by consolidating kaolin slurry prepared at 1.5 times the liquid limit. Fig. 1 shows the compression system used for consolidating the slurry isotropically. In this system, a 100-mm-diameter rubber membrane was placed around the pedestal and sealed on to the base of the cell by two O-rings. A membrane stretcher was placed around

SEM images

Fig. 2a and b shows SEM photographs taken on Series 1 specimens for horizontal and vertical specimens, respectively. In this case, specimens were room-dried before the images were taken. The magnification of the images is 10,000×. The images indicate significant differences between the particle arrangements for specimens trimmed horizontally and vertically. In the horizontal specimen, the particles are generally oriented in a single direction, with the plate surfaces generally horizontal. That

Conclusions

One-dimensional consolidation is generally believed to generate a degree of anisotropy in the microstructure. However, this paper suggests that anisotropy can also result from isotropic loading. Scanning Electron Microscope images of (a) specimens that were consolidated isotropically from a slurry and (b) consolidated after remoulding show differences in microstructure. Specimens consolidated from slurry appear to have retained evidence of anisotropic microstructure, while in remoulded

Acknowledgements

Funding for the work was provided by P.J. Carey (Contractors) Limited, Carey House, Great Central Way, Wembley Middlesex, United Kingdom. SEM images were taken in the Electron Microscopic Unit of Queen's University of Belfast with the help of Mrs. Jacqueline Patrick.

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