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A non linear incremental finite element program for the analysis of shafts and tunnels in oilsands

University of British Columbia

A method for analysing the deformation behaviour of oilsand adjacent to shafts and tunnels is presented. Oilsand is comprised of a dense sand matrix with its pore spaces filled with bitumen, water and free or dissolved gases. The Engineering behaviour of oilsand is governed by the stresses in the sand matrix. The bitumen does not contribute directly to the strength of the sand. However, indirectly the presence of bitumen may greatly affect its behaviour. This is because the presence of bitumen reduces the effective permeability of the oilsand and very often undrained conditions occur. Then the pressure of the pore gases remain high reducing the effective stresses for unloading conditions. A nonlinear incremental finite element model is used to analyse the oilsand skeleton behaviour. Dilation or shear induced volume change is an important characteristic of a dense sand and this is included in the analysis using a modified form of Rowe's stress dilatancy theory. The unloading condition at the face of a tunnel or shaft can lead to a violation of the failure criterion and this condition is rectified by a stress redistribution technique. The compressibilities of the oil and water phases are neglected in comparison with that of the gas phase and pore pressure changes are predicted by the ideal gas laws. Under undrained conditions the pore pressure is coupled into the skeleton stresses by maintaining volumetric strain compatibility between the skeleton and pore fluid phases. The results have been checked with drained and undrained closed form solutions. The solution for the unloading of a tunnel in oilsand is presented and it shows that the limiting support pressures can be reduced by venting elements to a reasonable distance from the tunnel. It is also found that the effects of shear dilation are significant only when the limiting support pressure is approached.

Text

2010-03-26

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http://hdl.handle.net/2429/22791

Civil Engineering

University of British Columbia

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