IS STRAIN SOFTENING RESPONSIBLE FOR SAKUNGEN FEATURES?

Sackungen features are enormous slope failures which appear to occur via mechanisms of gravity-induced creep. Classic ridge spreading has defied analysis using conventional kinematic methods, such as the Mohr-Coulomb failure criterion. Computerized limit equilibrium methods, such as OMS, BMM, UTEXAS3 and XSTABL, are 2D methods intended for application to normally consolidated soils, which are strain hardening. Some workers have attempted to back-analyze sackungen features by adjusting the pore pressures upward until a failure occurs. Others have ascribed movement to the likely pre-existence of low strength bounding discontinuities, such as faults, which then entrap sufficient pore pressure to reduce their frictional resistance to exceedingly low values.

Gravity deformation is omnipresent and constant. In part, this is ascribable to creep, and a progressive degradation of material strength parameters due to a number of factors. These degrading factors include: unloading and dilation, the residual stress field, release of stored elastic strain energy (rebound), cyclic loading due to Earth tides and earthquakes, thermal expansion and contraction, stress concentrations along valley walls and beneath valley floors, and both chemical and physical weathering. Micaceous and smectitic shales exhibit profound strength loss upon unloading and progressive absorption of moisture. In some cases, the degradation of shale strength parameters may be sufficient to demonstrate the observed geometry of some suckungen features.

In confined canyons, another complicating factor may be pore pressure imbalance. Lava dams and landslide dams back up large pools of water, but eventually breach, sometimes catastrophically. A rapid release of water could destabilize upstream slopes through extreme pore pressure imbalance caused by rapid drawdown. This mechanism may account for gravity faulting features long recognized to exist in the Grand Canyon.