The direct simple shear (DSS) test is one of the most popular testing techniques for measuring the shear strength of soils and mine waste tailings. However, uncertainties remain regarding the suitable sample diameter and whether a DSS sample should be saturated or can be tested without flushing with water. Various designs and configurations of shearing caps are also incorporated in different DSS equipment with little information on their performance and comparison soil shearing behavior with different caps. This study examines the monotonic shearing behavior, static liquefaction and instability, and post-liquefaction strength of a coarse oil tailings sand in extensive series of monotonic DSS tests on two different specimen diameters of 50 mm and 70 mm. Moist-tamped samples are reconstituted with and without flushing with water and sheared using top and bottom caps with concentric wedges and projecting pins. These are examined across a wide range of consolidation vertical stress, and for three different stress paths corresponding to undrained (CV), drained (CVS), and constant-shear unloading (CSU) shearing paths. Static liquefaction and instability were triggered in the CV and the CSU tests at the emergences of undrained strength reduction and volumetric collapse, respectively. The results show little effects of sample flushing and diameter on the static liquefaction triggering and post-liquefaction shear strengths of the tailings sand. The effect of sample diameter was primarily observed on the one-dimensional compressibility and volumetric strain of samples. The smaller diameter specimens underwent smaller volume changes during one-dimensional compression and drained shearing compared with the larger D = 70 specimens.

The assumption of seepage parallel to an infinite slope is realistic and indeed typical of most hillslope failures, to which one-dimensional infinite slope analysis may be applied. In this study, however, the general case of an ideal infinite slope of homogeneous isotropic saturated granular soil affected by uniform steady seepage with a vertical upward component was considered. Stability analysis was carried out in view of i) Mohr-Coulomb shear failure, the main result being the preparation of a stability chart for an infinite slope acted upon throughout by seepage in an arbitrary direction; ii) hydraulic instability in the guise of hydraulic heave failure. This occurs when the seepage gradient, at which the upward seepage forces transmitted to the soil exceed the gravitational forces, is the critical hydraulic gradient , for which a simple, albeit general, equation was derived. Subsequent comparison of these two types of failure showed that Mohr-Coulomb failure precedes hydraulic heave failure, except in one particular case, i.e. horizontal ground and vertical upward flow, where the failures are simultaneous. The study also considered iii) the phenomenon of static liquefaction resulting from undrained monotonic shear of saturated contractive loose soils, which generates a build-up of excess pore-water pressure triangle u. This leads to a sudden substantial or total loss of shear strength, i.e. the phenomenon of static liquefaction which, in turn, can produce catastrophic failures,even in gentle slopes. Lastly, in relation to the above mentioned excess pore-water pressure , an equation that enables us to estimate its value was easily obtained.

Loose saturated granular materials are particularly susceptible to instability, resulting in deviatoric strain softening, and static liquefaction. When instability occurs in the context of a landslide, the consequences in terms of the mobility of the debris and risk to life and property can be catastrophic. Physical model landslides initiated in a geotechnical centrifuge under rising groundwater conditions were used to trigger instability and static liquefaction. Four experiments with a loose contractile soil and two experiments with a dense dilative soil were performed. The velocity of propagation of the liquefaction front within the loose granular soils at the base of a landslide was quantified using a dense sensor network of pore water pressure sensors and high-speed imaging. On triggering of a landslide, a localized toe failure was observed to shear and liquefy the soil at the base of the landslide. However, the velocity of this initial failure (0.5 m/s) was an order of magnitude slower than the subsequent 4.2 m/s propagation velocity of the liquefaction front. These experiments demonstrated and quantified how a localized failure onto a liquefiable deposit may propagate liquefaction much farther than simply the runout of the localized failure, and highlight the potential implications and consequences of such an occurrence.

Tailings are waste materials of mining operations, consisting of a mixture of clay, silt, sand with a high content of unrecoverable metals, process water, and chemical reagents. They are usually discharged as slurry into the storage area retained by dams or earth embankments. Poor knowledge of the hydro -mechanical behaviour of tailings has often resulted in a high rate of failures in which static liquefaction has been widely recognized as one of the major causes of dam collapse. Many studies have dealt with the static liquefaction of coarse soils in saturated conditions. This research provides an extension to the case of silty tailings in unsaturated conditions. The static liquefaction resistance was evaluated in terms of stress -strain behavior by means of monotonic triaxial tests. Its dependency on the preparation method, the volumetric water content, the void ratio, and the degree of saturation was studied and compared with literature data. The static liquefaction response was proved to be dependent mainly on the preparation technique and degree of saturation that, in turn, controls the excess of pore pressure whose leading role is investigated by means of the relationship between the -B Skempton parameter and the degree of saturation. A preliminary interpretation of the static liquefaction response of Stava tailings is also provided within the Critical State framework.