Civil Engineering April 2022 | Vol 30 No 3

14 April 2022 Civil Engineering As mentioned, the NorSand failure envelope exists in 3D, but for simplification is presented as a projection in 2D planes, in p’,q’ and p’,e space. The latter is provided in Figure 6. From these plots, it is evident that a significant increase in stress (facility height) would be required for the stress paths to intersect the failure locus if the prevailing conditions persisted. It should be noted that while the plot indicates a trend towards the CSL, the x-axis is logarithmic in scale, thus requiring a sub- stantial increase in mean effective stress to be of detrimental consequence. This analysis demonstrates that with an increase in loading under normal oper- ating conditions, the material state (stress and void ratio) changes do not regress to residual strengths, despite the material being contractive. INVESTIGATION INTO LOSS OF CONFINEMENT Since mean effective stress, which governs shear strength, is a function of confining pressure, removal of the latter would result in a reduction in shear strength. Confinement pressure could be lost due to deliberate removal, i.e., re-mining, or possibly shallow surface sloughing/toe failure brought about by uncontrolled circumstances. Drained and undrained analyses have been undertaken, generally resulting in a reduction in mean effective stress. Undrained load conditions are considered here as there is the possibility that the loss of confinement failure occurs rapidly, not allowing for dissipation of pore pressures. Figure 6 is an example of a set of stress paths in p’:q projection where grey dashed lines indicate the stress develop- ment during normal construction of the TSF and the solid coloured lines indicate stress change due to incremental loss of confinement. The sharp response of the stress paths towards the left indicates reduction in mean effective stress and consequential softening of the material. In some ele- ments this is followed by failure as the CSL is reached. In this instance loss of Void ratio 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 Overflow CSL Point 7 Mean effective stress 1 000 0 Intermediate CSL Point 1 Point 2 Point 3 Point 4 Point 5 Point 6 Point 8 Point 9 Point 10 Point 11 Point 12 Point 13 Point 14 Point 15 Point 16 Point 17 Point 18 Point 19 Point 20 Point 21 Point 22 Point 23 Point 24 Point 225 Figure 7 Void ratio versus mean effective stress Deviator stress 800 700 600 500 400 300 200 100 0 Mean effective stress 1 000 800 600 400 200 Point 1 Point 2 Point 3 Point 4 Point 5 Point 16 Point 17 Point 18 Point 19 Point 20 Point 25 Point 21 Point 22 Point 23 Point 24 Point 6 Point 7 Point 8 Point 9 Point 10 Point 15 Point 11 Point 12 Point 13 Point 14 Figure 8 Stress paths for queried points CSL Since mean effective stress, which governs shear strength, is a function of confining pressure, removal of the latter would result in a reduction in shear strength. Confinement pressure could be lost due to deliberate removal, i.e., re‑mining, or possibly shallow surface sloughing/ toe failure brought about by uncontrolled circumstances