Temperature effects on clay soils

Temperature changes occur in soils in a number of ways, e. g. landfill liners, around buried services and during sampling. An experimental programme was conducted to examine the effect of temperature (between 10 to 80 °C) on the volume change and shear behaviour of saturated clays. Testing included Liquid Limit (cone penetrometer), residual shear strength (modified Bromhead Ring Shear), laboratory vane shear ( at moisture contents between the Liquid and Plastic Limits) and oedometer tests. An extensive literature survey indicated that kaolinites and smectites would show extremes of thermal behaviour. To examine this two artificially pure clays were tested: English China Clay (a well crystallised kaolinite) and Wyoming Bentonite (a monovalent smectite). To supplement this four British soils were tested: Keuper Marl, Lower Lias Clay, London Clay and Oxford Clay. Full material data were obtained coupled with careful control of stress and thermal histories. It is concluded that two types of extreme thermal response exists: a thermomechanical and a thermo-physicochemical change exhibited by kaolinite and smectite respectively. The temperature sensitivity of clays relative to a particular parameter is positively related to its specific surface area. A quick and repeatable method to qualitatively assess this has been developed: the LUT method. Its advantages include that no temperature calibrations are needed and it has a relatively large operating temperature range, 10 to 80 °C having been successfully used. The consolidation pressure (in the oedometer) needed to change the nature of a soil's thermal response is negatively related to its specific surface area. This, it is postulated, occurs at the same 'critical' contact stress for all clays, i. e. the interparticle threshold stress at which a thermo-physicochemical response changes to a thermo-mechanical one. This threshold stress occurs at a anisotropic consolidation pressure of 60 kPa for a well crystallised kaolinite , at 250 kPa for reconstituted London Clay and at 480 kPa for a mono-valent smectite. Furthermore, greater parallel particle alignment or reconstituting a sample enhances a soil's temperature sensitivity in the oedometer. The thermal changes to consolidation and permeability coefficients can be typically predicted by the corresponding change to the dynamic viscosity of water. Deviations occur with smectites at normal stresses greater than 480 kPa, while for Keuper Marl this occurred at normal stresses of 50 kPa and greater than 850 kPa. Keuper Marl exhibits a greater temperature sensitivity of different parameters than predicted by index tests. This is strongly dependent on consolidation pressure and temperature. At elevated temperatures (>40 °C) and under increasing consolidation pressure, ped units tend to collapse, but once the pressure is removed ped reformation occurs. Thus knowledge of thermal and stress histories, coupled with full material data, is essential to effectively predict temperature effects on the engineering behaviour of soils with any degree of confidence

Extensible Automated Constraint Modelling

In constraint solving, a critical bottleneck is the formulationof an effective constraint model of a given problem. The CONJURE system described in this paper, a substantial step forward over prototype versions of CONJURE previously reported, makes a valuable contribution to the automation of constraint modelling by automatically producing constraint models from their specifications in the abstract constraint specification language ESSENCE. A set of rules is used to refine an abstract specification into a concrete constraint model. We demonstrate that this set of rules is readily extensible to increase the space of possible constraint models CONJURE can produce. Our empirical results confirm that CONJURE can reproduce successfully the kernels of the constraint models of 32 benchmark problems found in the literature