Civil Engineering April 2022 | Vol 30 No 3

Civil Engineering April 2022 13 normal development of a facility, and under possible “abnormal” loading conditions that could potentially trigger liquefaction failure at any time. Among the numerous potential trig- gering mechanisms that may be consid- ered, the four potential triggers set out in Table 1 are expounded upon. Post processing of the stability analysis data from “queried elements” (Figure 5) allows illustration of the stress path in relation to critical state. In addition to this, material behaviour can be monitored to ascertain whether liquefaction and/or failure have occurred. Each of the above triggers, as applied to upstream on-wall cycloned TSFs, are briefly discussed below. NORMAL CONSTRUCTION AS TRIGGERING MECHANISM This analysis involves “building” the facility in stages with the state at repre- sentative queried elements being altered by the addition of each subsequent tailings layer. Here, it is generally assumed that drained conditions prevail, justified by the absence of any excess pore water pres- sures in CPTu and piezometer readings. Stress state of elements in the satu- rated zone near the perimeter of a TSF (as shown in Figure 5) are tracked during the normal development of the facility. These are plotted as stress paths in Figure 6, with the CSL included. It can be seen from these stress paths that the mean effective stress (p’) increases at a greater rate than the deviatoric stress (q) in the early stages of deposition. This results in the stress path diverging from the critical state line, indicating an increase in strength. However, as deposition continues, the rate of increase of deviatoric stress ac- celerates and eventually exceeds that of mean effective stress and the path begins to trend towards the critical state line, signifying a decrease in stability. This is to say that the tailings material strength is increasing slower than applied load in the latter stages of modelling. With this said, the stress state does not near the CSL at the time of attainment of current facility height. Table 1 Potential triggering mechanisms considered Liquefaction type Triggering mechanism Static Trigger by normal construction/development of the facility Loss of confinement causing decrease in effective stress Raised phreatic surface causing decrease in effective stress Dynamic Seismic event Figure 5 Typical cross section of a facility with locations of queried points indicated Point 1 Point 2 Point 3 Point 4 Point 5 Point 9 Point 8 Point 10 Point 16 Point 11 Point 6 Point 17 Point 7 Point 18 Point 20 Point 21 Point 12 Point 13 Point 19 Point 22 Point 24 Point 23 Point 25 Point 14 Point 15 Deviator stress 800 700 600 500 400 300 200 100 0 Mean effective stress 1 000 800 600 400 200 CSL 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 6 Typical stress paths in p:q space