The paper presents results from a laboratory investigation into the in¯uence of fast rates of displacement on the residual strength of soil. This in¯uence was studied in the ring shear apparatus. Shear zones were formed by slow drained shearing and then tested at alternately fast and slow rates of displacement. Results from a wide range of natural soils are presented. All soils show initially an increase in strength when resheared at fast rates of displacement. The strength then tends to decrease with fast displacement to a minimum valueÐthe fast residual strength. Three types of rate effects on the residual strength are identi®ed: a positive rate effect in soils showing a fast residual strength higher than the slow residual; a neutral rate effect in soils showing a constant fast residual strength, equal to the slow residual, irrespective of rate of displacement; and a negative rate effect in soils showing a signi®cant drop in fast residual strength below the slow residual when sheared at rates higher than a critical value. The relation between the types of displacement rate effect and the basic shearing modes of slow residual strength (turbulent, transitional and sliding) is considered and the causes of the negative rate effect and the in¯uence of the apparatus on it have been investigated. The practical implications of the results in the context of slope stability and displacement piles are also discussed.

The shear of clays of varying plasticity against interfaces of different materials and varying roughness has been studied in the ring shear apparatus. Results are compared with the shearing resistance of clay-on-clay. Particular attention has been paid to shearing resistance with large displacements, and the in¯uence of the interface in changing the shearing mode. The effect of corrosion of steel interfaces has been examined, as has the effect of fast shearing.

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The effect of curing time, content of grout solution and concentration of urea, calcium chloride and urease enzyme on the process of enzymatic calcium carbonate precipitation was analysed. Initially, the process was studied in test-tube experiments by evaluating the amount of calcium carbonate precipitated using X-ray diffraction tests. The method was then applied to stabilise a sandy soil to examine the strengthening effect using unconfined compressive strength tests, X-ray diffraction and scanning electron microscopy. The results showed the effectiveness of the method for improving the mechanical properties of a sandy soil. The soil strength and stiffness increased with increases in content of the grout solution and curing time, while an increase in the concentration of urease and urea–calcium chloride only had a positive effect for lower concentrations. The results also showed a relationship between unconfined compressive strength and calcium carbonate content and pH value.

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