Open Cut » Geotech
This project addressed the knowledge gap on the position of the groundwater surface, which is a key factor for reliable waste rock dump stability analysis.
The data and information assembled in this report suggests that water tables in in‐pit waste rock dumps depend on a host of site‐specific factors such as base slope, base width, base morphology, spoil hydraulic conductivity, available recharge from rainfall and bedrock seepage. Hence, “typical” values that are applicable in “most” situations cannot be recommended as an outcome of this work. The outcomes of this work do provide a better understanding of how these factors should be taken into account when making site specific estimates, and do indicate that water table depths are most likely within the range of 0 m to 15 m in most circumstances encountered in practice. The work adopted a range of different approaches to inform this discussion, including a literature search, direct measurements and numerical simulations, with subsidiary consideration given to the capacity of spoil dumps to admit water from rainfall, and to transmit and/or store water that has infiltrated.
A key parameter needed for reliable modelling of water movements in waste rock dumps is the hydraulic conductivity, which because of the anisotropic and heterogeneous nature of spoil dumps, may reasonably vary over as many as 5 orders of magnitude. Diverse sources including technical papers and reports, a recent study on hydraulic characteristics of longwall goaf rock and constant head permeameter tests were all used to derive hydraulic conductivity estimates for segregated and nonsegregated spoil, in both compressed (buried) and loose states. These were subsequently used in a series of numerical analyses, but are also reported and discussed for the future reference of others wanting to undertake groundwater simulations in waste rock dumps.
Numerical simulations suggest water tables in in‐pit waste rock dumps on flat, horizontal bases are unlikely to exceed 1 m in height, and very unlikely to exceed 3 m. Adoption of water depths of 3 m to 5 m would be conservative in most foreseeable cases.
Water tables in in‐pit waste rock dumps on uniformly sloping bases are more complex, and likely to be locally deeper, due to phenomena which concentrate the recharge/seepage at the down‐dip toe, where active dumps are continuously being extended. They are likely to be asymmetric, with the maximum depth occurring between 10% and 20% of the length of the base, upslope from the downdip toe. Based on the simulations performed, maximum depths of 10 m ‐ 12 m are possible, depending on the length of the sloping base up‐dip from the toe, and for very long sloping bases (>700 m), even greater depths may be plausible. Locally high water‐levels are likely to occur close to the down‐dip toe, as the result of cumulative concentration of upslope seepages. This behaviour is exacerbated as the base slope steepens from horizontal, however, it is expected that there is some optimum base slope that would maximise this concentration of water. The same concentration phenomenon could also cause peak water levels to occur in new lifts at the dump toe, soon after they are placed.
The numerical simulations here were limited to 2D sections with uniformly flat bases, although it is acknowledged that most pit floors are 3D with more complex morphology. This study reasonably considered that the accumulation and movement of groundwater in waste dumps is controlled by the underlying bedrock surface, but it cannot account for the specific morphological characteristics of that surface. It is important, when speculating on likely groundwater depths for specific dump situation, to take account of morphological structures such as basins and highs, as well as changes in slope and slope direction, which may have a profound effect on the groundwater behaviour.
Recharge rates were identified as an important controlling factor in groundwater table development in spoil dumps. It was noted that the large unsaturated volume of Zone 2 had a significant capacity to intercept and store water that passes through Zone 1, and that Zone 1 had the potential to capture more or less of the incipient rainfall, depending on the surface slopes, the development of cracks and extent of uncompacted irregular surfaces. Good management of the active dump surface (to promote runoff and reduce infiltration) should reduce the recharge rate from rainfall, and could possibly be used to justify lower estimates of groundwater depth.
For long term stability considerations, water levels related to annual average rainfall would certainly be appropriate. However, for specific, short term considerations of existing or newly constructed slopes, the consequences of any recent rainfall anomalies should be kept in mind. This is particularly pertinent given the seepage concentration phenomena which can potentially cause rapid rises to high water levels in the advancing toe of active dumps.