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 paper presents results from ring shear tests, carried out on two samples from the slip surface of the Vaiont landslide in north Italy. The tests were carried out at slow and fast rates of shearing in order to determine values of the coef®-cient of friction appropriate to static and dynamic aspects of the landslide. Both samples showed a signi®cant loss of strength (negative rate effect), up to 60% below the slow residual strength, when sheared at rates of shearing greater than 100 mmamin. The observed loss of strength may explain the high acquired velocity of the landslide, or alternatively some combination of this and other mechanisms may have taken place.

This article presents results from a laboratory investigation into the dynamic characteristics of reconstituted and undisturbed cohesive soils by means of resonant-column tests. In particular, results showing the influence of various soil parameters, such as confining stress, overconsolidation ratio, void ratio, plasticity index, calcium carbonate content, and time of confinement on shear modulus and damping ratio at small and high shear strains are presented and then discussed. Relationships for the small-strain shear modulus, the degradation of shear modulus at high strains, and the increase of damping ratio at high strains over its small-strain value are proposed. Finally, the practical implications of the results in the context of seismic site response analysis are discussed.

No abstract

This paper presents a laboratory investigation into passive stabilisation of liquefiable sands by means of colloidal silica (CS). In order to examine the improvement of the mechanical behaviour of liquefiable sands stabilised with CS, an extensive laboratory testing programme comprising unconfined compressive strength and Brazilian tensile strength tests, as well as undrained monotonic and cyclic triaxial tests, was performed on a clean quartz sand treated with CS. The test results are compared to the corresponding results for the untreated sand. The effects of CS concentration, curing time, density and confining stress on the monotonic and cyclic response of the treated sand are examined, among others. It is shown that stabilisation of the tested sand with CS significantly improves both the undrained monotonic and cyclic strength. The influence of CS concentration on the monotonic response is reduced with increasing mean effective stress. Cyclic resistance, however, is shown to be practically unaffected by CS concentration. Furthermore, cyclic straining at 5 to 10% of double-amplitude axial strain does not influence the undrained shear strength of the stabilised sand.