Role of Hydromechanical Modelling in Management of Large Open Pit Slopes

CIM Vancouver 2016
Mr Shane O'Neill (Principal Hydrogeologist - Schlumberger Water Services)
The movement of water through a porous rock modifies its mechanical behavior. Hydromechanical (HM) coupling is the physical interaction, between hydraulic and mechanical processes in fluid saturated geological porous media or fractures, due to a change in an external load and/or a change in the internal pore fluid pressure. Pore pressure can introduce a time dependency through the coupling of diffusion with deformation. HM coupling provides a framework to understand the relationship between stress and permeability for a given rock mass. HM modelling provides a more complete understanding of the role on pore pressure in pit slope stability rather than just applying a static pore pressure distribution and unchanging hydrogeological and geomechanical properties.
HM coupled models can predict fracture flow and account for deformation and permeability changes for both low and high fracture stiffness. Shear and normal stiffness, friction angle, dilation angle and magnitude of horizontal stress were found to be the most significant controls on the HM response of the rock mass.
Depressurization of a pit slope can be as effective in stabilizing a pit slope as dewatering it. This has important implications as it can be easier to depressurize a low permeability pit slope than to dewater it. HM models can be used to determine how much depressurization might be required to stabilize the slope. This too is important as it can predict in advance just how much depressurization is required and so minimize the amount of boreholes, drain holes and pumping that might be required over the life of the pit. Conventional groundwater flow models or geomechanical models would not be able to make such a prediction as can HM coupled models.
Assuming zero dilation will underestimate fracture transmissivity and not represent clustered fluid flow associated with critically stressed fractures in a slope. There is a general reduction in rock strength from both Mohr-Coulomb and Hoek-Brown for undrained conditions. This can have a direct effect on potential slope stability.
Fault normal displacement can be dependent on pressure increase and displacement of the surrounding less permeable discontinuities. There is a time lag effect between when pressure rises and a resultant displacement occurring. This means that when the pressure falls off, the displacement of the fault continues for a period of time afterwards. This is described as a distance-dilatancy effect and is due to the elastic behaviour of the system in response to a local pressure perturbation. A non-linear fracture deformation model should be considered as a linear model tends to underestimate flow.
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