Hydraulic, Thermal and Electrical Conductivities of Composite Soils

Composite or intermediate soils, that is soils that are formed of more than one category of soils, are commonly encountered either as natural soils on which engineered earth structures are built, or as reconstituted materials used for the processes of filling and stabilization in many engineering problems, e.g. in engineered liners and environmental barriers. These soils are often difficult to sample and test when using standard site investigation methods. The basic concept of composite soils is known, but, studies performed on these materials are limited. This research focuses on experimentally investigating the hydraulic, thermal and electrical conductivity of a wide range of composite sand-clay mixtures. The ultimate goal is to increase the understanding of the composite soils and to establish a coherent framework for supporting the design and construction of conductivity related projects by establishing correlations between soil’s compositional and physical factors, and its conductivity. Two new pieces of equipment were designed for the purposes of the study. The first piece of equipment was designed to comply with the assumptions of Terzaghi for one-dimensional consolidation to determine the hydraulic conductivity indirectly. The second one was designed to determine the hydraulic, thermal and electrical conductivities directly following the principles of Darcy, Fourier, and Ohm. Their design criteria, principles, and test methods are described in detail. Both sets of equipment were shown to be simple and effective and provided repeatable results which could be compared to data obtained from other laboratory investigations and published models. Soils of known composition formed of five clay minerals and three types of sands were used to investigate the effect of compositional (e.g. clay/ sand content, clay mineralogy, particle size distribution, pore water composition) and physical (e.g. water content, porosity, dry density, confining stress) properties on the soil’s conductivity. The results on the composite soils reveal that such soils can be divided into matrix dominated soils in which the electro-chemical inter particle relationships of the clay content dominate the behaviour and clast dominated soils in which the inter particle contact forces of the sand content dominate the behaviour. The transition zone between them depends on the sand to clay content ratio, the initial conditions, the confining stress at the point at which the property was determined, clay mineralogy, particle size distribution and shape. Under a consolidation pressure of up to 1280 kPa, the transition zone occurs when the sand content is between 58% and 85%. In hydraulic conductivity, it happens when the clay content is between 20 - 35 %. Data on the thermal and electrical conductivity shows that the transition behavior is not so well defined but it is the sand particles that dominate the mass behavior when its content exceeds 65 – 78%. The concept of matrix and intergranular void ratios can be used effectively to describe the consolidation and hydraulic conductivity of composite soils. The results of the consolidation tests on sixty sand-clay mixtures show that increasing the clay content increases the compression index and reduces the hydraulic conductivity. There is a relationship between hydraulic conductivity of composite soils and void ratio. For matrix dominated soils, the relationship is a function of clay type (expressed by activity) and matrix void ratio; for clast dominated soils, it is a function of particle size and intergranular void ratio. However, the concept of the matrix and intergranular void ratios cannot be used to describe the thermal and electrical behavior of composite soils as there is no simple relationship with thermal and electrical conductivity. Test results on thirty two saturated samples of composite soils show that the bulk thermal conductivity of soils can be determined from the soil’s constituents (water, clay, sand) using the thermal conductivity of the individual solid particles and their volumetric fractions present in the whole soil based on the geometrical mean method. The results also indicate that the bulk thermal conductivity increases when the sand content increase, dry density increases, and water content decreases. At a constant heat flux, the increase in temperature in sands generally exceeded that for clays and the time needed to reach the maximum temperatures was shorter for sands than for clays which depend on the clay mineralogy and sand particle size. The interpretation of the electrical conductivity of composite sand-clay soils demonstrates that the overall electrical conductivity in these soils can be modelled as a parallel function of two main components; bulk pore fluid conductance and clay minerals conductance within the soil that are dependent on the conductivity of fluid and clay particle surfaces, respectively, and their volumetric fractions. Test data on thirty seven samples of composite sand-clay soils prepared with tap water of low electrical conductivity shows that, for the majority of the data, the clay conductance exceeds that of bulk water indicating the importance of clay in the process of current transfer through soils. A soil mixture prepared with tap water of low electrical conductivity having higher cation exchange capacity (CEC) and specific surface area (As) (i.e. bentonite) will display a higher electrical conductivity than a soil with lower CEC and As (i.e. kaolinite, sand) at the same porosity. The results also show that as the sand content increases the overall electrical conductivity decreases and there is either a direct or inverse correlation between the electrical conductivity and soil’s porosity or water content that depends mainly on the interplay between the clay and water conductance. By testing a wide range of soils of known composition (known clay, sand and water content and type), it has been shown that it is quite possible to correlate conductivity with the physical properties of the soil constituents and establish new conduction models based on the soil’s compositional and physical properties.