This paper presents some experimental results on desiccation cracking tests conducted on thin layers of clay soils. Observation of the evolution of cracking patterns was examined to clarify the transient mechanisms of the crack formation of clay soils. Laboratory experimentation on desiccation cracking was carried out to examine experimentally the quantitative relationships between the characteristics of soil cracks and the prevailing controlling conditions. Five desiccation cracking tests for slurried clay soils were carried out using shrinkage moulds in a humidity chamber, which was capable of controlling relative humidity and temperature. The soil used in the experimental studies was residual basaltic clay and was classified as a highly reactive soil. In order to provide simple conditions for theoretical modelling, the tests were conducted in perspex and metal moulds with rectangular crosssections. The lengths of the moulds were considerably larger than their widths so that parallel cracking were generated in thin layers. In each cracking test, several rectangular moulds of different thicknesses and widths were used. Some of these tests were used for observation, crack initiation and evolution, and others for moisture content measurement during desiccation. The test results include evolution of the cracking pattern, influences of speed of desiccation and typical crack spacing to depth ratios for soil layers.

This paper elucidates some of the controlling factors governing soil desiccation. The desiccation tests were conducted on three materials -clay, potato starch and milled quartz sand -all three featuring similar fracture energy. Two controlling factors were identified in desiccation cracking, regardless of the material. The first is the tensile stress and strain energy development within the material when the material is restrained against shrinkage. The distribution of the tensile stress will depend on the boundary conditions and material stiffness, and will dictate where cracks are likely to originate. The second factor is that the exact positions of crack initiations will be controlled by the flaws and/or pores within the material. For materials such as clay, with very fine particles, the cracking mechanism is governed by flaws, since the desaturation of fine pores would require very high suction stress, and this requirement leads to sequential cracking and orthogonal crack patterns. If the material has particles giving relatively large and uniform pore sizes with high moisture diffusivity leading to high shrinkage energy prior to cracking, then the fracture energy balance indicates that cracking can occur in near hexagonal patterns with 1208 crack initiations, which occur predominantly simultaneously. However, even for materials with lower moisture diffusivity, such as for clay, high desiccation rates can give rise to an 'effective layer' over which high suctions and strain energy develop, leading to almost simultaneous dense cracking.

This paper presents reviews of studies on properties of coal pertinent to carbon dioxide (CO 2 ) sequestration in coal with specific reference to Victorian brown coals. The coal basins in Victoria, Australia have been identified as one of the largest brown coal resources in the world and so far few studies have been conducted on CO 2 sequestration in this particular type of coals. The feasibility of CO 2 sequestration depends on three main factors: (1) coal mass properties (chemical, physical and microscopic properties), (2) seam permeability, and (3) gas sorption properties of the coal. Firstly, the coal mass properties of Victorian brown coal are presented, and then the general variations of the coal mass properties with rank, for all types of coal, are discussed. Subsequently, coal gas permeability and gas sorption are considered, and the physical factors which affect them are examined. In addition, existing models for coal gas permeability and gas sorption in coal are reviewed and the possibilities of further development of these models are discussed. According to the previous studies, coal mass properties and permeability and gas sorption characteristics of coals are different for different ranks: lignite to medium volatile bituminous coals and medium volatile bituminous to anthracite coals. This is important for the development of mathematical models for gas permeability and sorption behavior. Furthermore, the models have to take into account volume effect which can be significant under high pressure and temperature conditions. Also, the viscosity and density of supercritical CO 2 close to the critical point can undergo large and rapid changes. To date, few studies have been conducted on CO 2 sequestration in Victorian brown coal, and for all types of coal, very few studies have been conducted on CO 2 sequestration under high pressure and temperature conditions.

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SUMMARYThe ability to model and predict the formation of desiccation cracks is potentially beneficial in many applications such as clay liner design, earth dam construction, and crop science, etc. However, most studies have focused on statistical analysis of crack patterns and qualitative study of contributing factors to crack development rather than prediction. Because it is exceedingly difficult to capture the nonlinear processes during desiccation in analytical modelling, most such models handle crack formation without considering variation of material properties with time, and are unattractive to use in realistic modelling. The data obtained from laboratory experiments on clay soil desiccating in moulds were used as a basis to develop a more refined model of desiccation cracking. In this study, the properties, such as matric suction, stiffness and tensile strength of soil, and base adhesion, could be expressed approximately as functions of moisture content. The initial conditions and the development of suction due to desiccation and the varying material properties were inputted to UDEC, a distinct element code, using its internal programming language FISH. The model was able to capture some essential physical aspects of crack evolution in soil contained in moulds with varying lengths, heights, and materials of construction. Extension of this methodology is potentially beneficial not only for modelling desiccation cracking in clay, but also in other systems with evolving material properties such as concrete structures and road pavements.