ABSTRACT:
Soft ground conditions such as those reported in the Alberta oil sands of Northern Canada give rise to extensive operational and maintenance problems experienced by large mobile mining equipment, especially in the ultra class category. Under static conditions with no cyclic activity, these units are barely able to float on the ground surface, having maximized the available footprint area accounting for possible interference with adjacent equipment. The operating action of shovels effect a series of unloading – loading cycles that peak and relax ground loading pressures from approximately 100 psi to 0 psi by duty cycle. The nominal static balance for these units is roughly around 30 psi, one third of the peak value being cycled on the ground surface. In a given footprint position, a shovel may cycle the same ground for 25 to 50 cycles before only partially repositioning at the face. This paper looks at the findings of dynamic plate load tests, effectively mimicking the action of a shovel track pad loading and unloading oil sand of a known fluid content at a given set of adverse environmental conditions. The outcome shows that after frequent cycles, regardless of ground relaxation, rebound and recovery intervals between cycles, the pressure stiffness, expressed as the footprint normalized stiffness in units of pressure per unit deformation, converges on a plateau value. This information was used to predict and compare to the actual deformation experienced for an ultra class shovel operating on similar oil sand ground conditions. Oil sand has unique properties exhibiting performance akin to sandstone in winter and weak clay in summer periods. Joseph et al. (2003) reported that after only a few cycles, truck and shovels operating on soft-ground will become less stable.
