Civil Engineering April 2021 | Vol 29 No 3

Civil Engineering April 2021 23 of application owing to the difficulties associated with measuring wetting curves. However, recent modifications by Vandoorne (2021) have extended the capabilities at UP to measure wetting curves continuously. Figure 2 illustrates the drying and wetting curves of silica flour using this equipment. A monotonic drying curve is also presented for comparison. Figure 2 illustrates how the wetting and drying cycles were stringently controlled between suction values of 5 kPa and 150 kPa. The new SWRC apparatus allows for automatic wetting and drying cycles to be performed within specific suction ranges (provided this range falls within the capacity of the tensiometer being used). Finally, acquisition of a new cyclic tri- axial apparatus at UP has allowed for the mechanical properties of both saturated and unsaturated soils to be assessed under conditions which more closely represent that of traffic loading. Some results from this newly acquired equipment, with spe- cific reference to railway formation mate- rials, have recently been demonstrated by Mpye (2020). IN-SITU MEASUREMENTS Whereas laboratory measurements of SWRCs may be useful in the design phase, in-situ measurement of soil suction can provide a real-time indication of the performance of a structure. As stated by Toll et al. (2011), pore water pressure is the most reliable indicator of slope failure and can therefore be an extremely valuable parameter to monitor and use as a trigger value for early warning systems. Since the publication by Gaspar & Jacobsz (2019), UP has been involved with the instrumentation of several sites around South Africa, measuring both soil suction and water content. Monitoring systems either incorpo- rated UP’s in-house low-cost tensiometers (Jacobsz, 2018) or commercially available fixed-matrix porous sensors (Tripathy, 2016; Metergroup, n.d.). The benefit of tensiometer measurements is that they can record changes in suction as small as 1 kPa almost instantaneously. Such sensors do however need to be properly saturated prior to use and may require re-saturation if cavitation of the sensor occurs due to suctions exceeding the capacity of the sensor (up to 1.7 MPa (Jacobsz, 2019)). The benefits of fixed-matrix porous sensors are that they do not require any preparation before being used on site and require no maintenance after installation. Interestingly, preliminary tests have shown no significant difference in the response time of these sensors versus tensiometers. However, laboratory tests have shown that these sensors are not as accurate as tensiometers, and show appreciable hysteresis (Vandoorne, 2021). Furthermore, the fixed-matrix porous sensors suffer in low suction environ- ments as they cannot measure suctions less than approximately 9 kPa. Unlike tensiometers, fixed-matrix porous sensors are also not able to measure positive pore water pressure. Despite these limitations, these sensors can offer valuable informa- tion on seasonal changes in suction. Figures 3 and 4 illustrate the mea- surement of soil suction within tailings storage facilities in two regions of South Africa, using tensiometers. From both figures it can be seen that tensiometer measurements respond sharply to tailings depositions and rainfall events. What is perhaps more important in this result is the longevity of the low-cost tensiometers used. Figure 4 illustrates that after a period of two years, the tensiometers continue to function. Such a result is com- parable with some of the longest studies involving the measurement of suction in the field (Mendes et al . (2008); Toll et al. (2011); Toll et al. (2012)). It is worth noting that at no point during the measurement period did pore water pressure move into the positive range. Soil suctions can contribute positively to slope stability but are hardly ever measured. Consequently, their variation over time is poorly understood. Recognising this observation, it may be worth considering the routine use of ten- siometers on tailings dams, augmenting or even replacing more routinely used instrumentation such as piezometers. Replacing piezometers with tensiometers is, in principle, feasible as tensiometers can also measure positive pore pres- sures. The addition of tensiometers to tailings dam monitoring systems would provide significant additional data on the pore pressure regime at very little additional cost. In addition to measurements on tailings storage facilities presented in Figures 3 and 4, Figure 5 illustrates mea- surements beneath a railway formation using both tensiometers and fixed-matrix porous sensors. A three-month period is Figure 2 Drying and wetting SWRCs measured on silica flour Degree of saturation (%) 100 90 80 70 60 50 40 30 20 10 0 Suction (kPa) 1 000 100 10 1 0.1 Monotonic drying Repeated drying and wetting Whereas laboratory measurements of SWRCs may be useful in the design phase, in-situ measurement of soil suction can provide a real-time indication of the performance of a structure