The fall cone test in unsaturated soil and tailings pastes

Abstract

The fall cone test (FCT) is usually performed in saturated paste-like geomaterials to determine their undrained shear strength or liquid limit. Here the test is used to determine strength when a geomaterial is unsaturated and paste-like in its consistency. A mechanics-based interpretation is used, where the pore air pressure, pore water pressure and suction which prevail around the cone tip post-test are incorporated in strength and bearing capacity equations. The pore air and water pressures combine to form an apparent cohesion that dominates cone penetration resistance and can be determined from a FCT result. When the volumetric air content (va/v) is less than 0·15, samples become unvented and effectively sealed, and the FCT induces volumetric compression in the sample. This causes the apparent cohesion post-test to be smaller than the initial value. The ratio between the two is proportional to va/v for three tailings in a unique way. The initial apparent cohesion ranged from 0·6 kPa to 28 kPa. The post-test value was smaller by an amount depending on va/v. When va/v > 0·15, the pore air becomes connected to the atmosphere and very different pressure changes and volume changes occur around a penetrating cone, for which an alternative interpretation method is needed.