Anders Samstad Gylland

Experimental Study of the Deformation Pattern around a Penetrating Coned Tip

Improved knowledge on the deformation pattern around the tip during cone penetration tests is needed. In the present experimental work, the observations and measurements of real soil behavior around a penetrating tip in laboratory tests are presented. Two experimental setups are described: one involving freezing the sample after penetration to preserve deformation patterns and failure features, and one involving computerized tomography. Similar failure patterns were observed in both tests regarding compression directly under the tip and shear structures along the shaft that might be related to a process of accumulation and release of friction during penetration.

The Effect of Deformation Rate in Progressive Slope Failure

For understanding the processes acting during the triggering and evolution of a landslide in sensitive clay, it is of importance to account for the material response in the failed state. This includes the rate of strain softening and the level of residual resistance. Through a review of new experimental data it is illustrated here that these properties depend on the displacement rate in the case of shear band formation. Higher rate increases the brittleness both in terms of a steeper softening response and a lower residual level. These observations are attributed to processes of local drainage and dissipation of pore pressure gradients in the vicinity of the shear band. Viscous effects, which yield higher resistance for higher rates of deformation, are also apparent. These aspects of material failure in sensitive clay are utilized in the context of progressive slope failure in order to explain observations from case records and to quantify the impact of the rate dependent softening response on the resistance of a slope. The strong brittleness of sensitive clay at high displacement rates under shear band formation motivates a recommendation of using a design approach close to a first yield criterion if the construction situation might include such high rates.