Earth pressure coefficient, K, is a key parameter. It is defined as the ratio between the horizontal and vertical effective (principal) stresses. It is a required parameter in numerous analytical solutions developed to estimate the stresses in backfilled openings such as retaining walls, silos, trenches and mine stopes. Some researchers proposed to use the Jaky’s at-rest earth pressure coefficient K0 as long as the confining walls are immobilized while others proposed to use the Rankine’s active earth pressure coefficient Ka even though the confining walls are immobile. Li and coworkers have further indicated that the immobilization of confining walls is a necessary and sufficient condition for the soil to be remain in an at-rest state only when the soil is initially at an at-rest state. When a confining structure exists or is built first, the state of the backfill later placed is unknown. It can be in an at-rest or active (yield) state. An active state is totally possible even though the confining walls remain immobile during and after the placement of the backfill, depending on the values of friction angle and Poisson’s ratio and the considered position. As the Poisson’s ratio of granular material is very difficult to measure and generally unknown, the state of a backfill placed in a confining structure remains unknown. In this paper, a few results of measured values of K will be presented and discussed.

Tailings storage facilities can be designed with retaining dikes built using three different disposal techniques: the upstream, downstream and centerline methods. The upstream-raised method is the most common due to its simplicity and low cost. However, the stability of such impoundments is often fragile, and these have been observed to be prone to dramatic failures, especially in high seismicity regions. To improve the stability of tailings impoundments with upstream-raised dikes, the use of waste rock inclusions was proposed. This method of co-disposal of tailings and waste rock consists of placing rows of waste rock within the impoundment prior to each dike raising. Recent studies on the effects of waste rock inclusions on the stability of tailings impoundments have indicated that this co-disposal method may improve the geotechnical behavior of the impoundments under both static and dynamic conditions. This paper presents finite difference analyses of the seismic behavior of simplified tailings impoundments reinforced with waste rock inclusions. The effects of the downstream slope and height of the impoundment on the seismic stability were investigated through a parametric study. The effect of each parameter was investigated individually by assessing its effect on various indicators such as the critically displaced volume of tailings, the deformation of the impoundment crest, and the permanent displacement of the downstream slope. Simplified design equations and charts are provided to estimate the permanent displacement of the downstream slope of tailings impoundments as a function of waste rock inclusion’s configuration, and the geometry of the impoundment.

The determination of the flow rate in the underground excavation is a very important parameter in the design of the structures. Among all parameters that have an impact on the inflow rate to the tunnel, the hydraulic gradient is one of the most effective one that, according to Darcy’s law, controls the tunnel inflow rate. An empirical-numerical equation is proposed for the determination of the vertical hydraulic gradient in the wall of a tunnel excavated below a water table. The horizontal hydraulic gradient is not supposed to have a significant impact on the tunnel inflow rate as its value is very low. By contrast, the existing vertical hydraulic gradient is among the most effective parameters. On the other hand, in the case of an underground excavation, no equation exists for the determination of the hydraulic gradient that considers more than one parameter, i.e., the depth. Using the results of the numerical simulations, it was deduced that the depth of the tunnel, the ratio between principal hydraulic conductivities, and their relevant directions are the most effective parameters that have a significant influence on the hydraulic gradient and inflow rate to the underground tunnels. The resultant hydraulic gradients in the vicinity of the wall of the tunnel were obtained using the RS2 Roc Science software. The mathematical relationship between the input data, i.e., the depth (z), ratio between hydraulic gradients (a) and their relevant directions (α), and the result of the simulation, i.e., the hydraulic gradient (iz), have been derived by curve fitting. Finally, for each orientation of the principal hydraulic conductivity, an equation is proposed for the calculation of the hydraulic gradient.

Mine operations typically generate two primary types of solid wastes: waste rocks and tailings. Coarse grained waste rocks mainly result from the blasting activities performed to access the ore zones in underground or open pit mines; these are usually disposed in waste rock piles on the surface. Tailings are fine grained materials produced by the milling treatment of extracted ore; they are generally disposed hydraulically as slurry in surface impoundments confined by retaining dikes. The design of surface impoundment raises various geotechnical issues, including the risk of static instability of the external slope. Waste rock inclusions (WRI) can be used to improve the geotechnical response of tailings impoundments. This method consists of placing waste rock strategically inside the impoundment to improve drainage and reinforce the retaining system.
In this paper, numerical results will be presented to illustrate the effect of inclusions on stability of tailings dikes under static loading conditions, based on the characteristics of typical tailings and impoundments. The calculations are conducted with GeoStudio and FLAC for cases with and without waste rock inclusions. The numerical modelling calculations, simulating the behavior of dikes during and shortly after deposition of tailing in the tailing impoundment, illustrate how WRI can enhance the safety factor of dikes.
The results are part of a global research program aimed at developing an optimization strategy for the use of waste rock inclusions inside tailings impoundments for increasing the stability against static and dynamic loading.

The Alberta oil sands contain approximately 27 billion m3 of bitumen at depths sufficiently shallow to be commercially surface-mined. These operations require removal of the mine wastes and ores, which form pit walls, and use earth dams either ex-pit or in-pit to store process water for bitumen extraction and to contain tailings after the oil is removed. These large earth structures are typically designed using general geological information, discrete subsurface boreholes, piezometric data and laboratory testing on selected samples to characterize foundation conditions and material properties. Significant investments are necessary for site investigation and design, notwithstanding reliance on the observational approach to manage uncertainty associated with geologic, geometric and operational variabilities. The observational approach promotes efficiency while managing risk, provided the strategy is implemented appropriately.

This paper presents two cases studies, in which the observational approach was successfully applied to the designs and construction of the mine structures at Imperial’s Kearl oil sands mine.

The first case study demonstrates efficiencies by embracing a 3D limit equilibrium method for the mine pit walls where 2D analysis did not meet the target Factor of Safety. As part of observational approach, the 3D analysis method was validated using instrumentation and visual performance observations. As mitigation, a mine-replace strategy was adopted to allow mining to progress and confirm 3D design results.

The second case study relied on the observational approach in dealing with potential inflow from Basal or Devonian aquifers under an in-pit tailings dyke, in which shutdown-recovery tests were conducted to confirm whether or not potential conduits or hydraulic cracks exist in Devonian aquitard or in the Lower McMurray Muds. Significant cost savings were achieved by removing the requirement for sand blanket installation and long term operational costs of depressurization wells based on the shutdown-recovery test results.

Understanding the storage and flow of granular materials in hoppers and silos is relevant to a wide range of industrial activities. However, common problems encountered with these activities are erratic flow, blockage and dead zones. Mechanical interaction between the particles can also give rise to asymmetrical distributions of dynamic stresses and anomalous stresses damaging silos, e.g. Jenike's ‘switch stresses’. Unfortunately, the key factors needed for optimum design of storage silos based on the physical characteristics of the contained granular materials, silo quasi-static yield and rapid flow are still poorly understood. Hence the applicability of current experimental and computational tools is generally limited. Furthermore, most non-intrusive experimental measurements evaluate only bulk-flow parameters at the outlet of the silo, boundaries of the flow field, or dimensions that are orders of magnitude smaller than those in ‘field’ applications.

The classic Beverloo equation defines the flow of granular solids through an orifice, where mass outflow-rate is a function of orifice diameter, material bulk density, average particle size and silo geometry. The study described in this paper has investigated the outflow behaviour of different scaled physical model granular storage hoppers containing fine-grained silica sands. The model geometries include classical vertical-sided silos, cones and hourglass-shapes, with varying diameter, depth, relative orifice and particle diameters; and gravity effect in geotechnical-centrifuge.

This approach is designed to provide a better understanding of the governing phenomena in falling and constant head orifice flow problems and assess the effectiveness of the geotechnical centrifuge as an experimental tool for investigate the behaviour of storage silos. Granular deformations were investigated using particle image velocimetry (PIV) to analyse propagation of strain localizations and flow regimes during outflow. The results were used to calibrate the Beverloo flow equation for the different models and check the model scaling laws for these structures and granular flow phenomena.