Numerical modelling of unsaturated flow in vertical and inclined waste rock layers using the seep/w model
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Date
2003-06-16
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Degree Level
Masters
Abstract
Conventional disposal of waste rock results in the construction of benches with interbedded fine and coarse layers dipping at the angle of repose. The waste rock benches are typically 20-meters in height and are constructed in a vertical sequence to form waste rock dumps commonly greater than 100-meters high. The interbedded structure influences the flow pathways for infiltration water within the waste rock profile. Preferential flow pathways develop when one material becomes more conductive than the surrounding material. The flow of meteoric waters through the interbedded waste rock structure is difficult to describe since the dumps are constructed above natural topography and are generally unsaturated.
Two previous research studies were undertaken at the University of Saskatchewan to study end dumped waste rock piles and the relationship to preferential flow for unsaturated conditions. The first study was conducted during the excavation of a large waste rock pile at Golden Sunlight Mine in Montana (Herasymuik, 1996). Field observations showed that the waste rock pile consisted of steeply dipping fine and coarse-grained layers. The results of further laboratory analysis indicated the potential for preferential flow through the fine-grained material under conditions with negative pore-water pressures and unsaturated flow.
The second study investigated the mechanism for preferential flow in vertically layered, unsaturated soil systems (Newman, 1999). The investigation included a vertical two-layer column study and a subsequent numerical modelling program showing that water prefers to flow in the finer-grained material. The preferential flow path was determined to be a function of the applied surface flux rates and the unsaturated hydraulic conductivity of the fine-grained material layer.
A numerical modelling program to evaluate preferential flow was conducted for the present study in an inclined four-layer system consisting of alternating fine and coarse-grained waste rock. The numerical modelling program was undertaken using the commercial seepage software package, Seep/W, that is commonly used by geotechnical engineers. The result obtained using Seep/W showed preferential flow to occur in the fine-grained layer. However, difficulties with respect to convergence under low flow conditions with steep hydraulic conductivity functions were encountered.
A comprehensive sensitivity analysis was completed to investigate the factors that influence convergence in the Seep/W model including: convergence criteria, mesh design and material properties. It was found that the hydraulic conductivity function used for the coarse-grained material was the most important factor. The problem of the steep slope for the hydraulic conductivity function specified for the coarse-grained material was solved by progressively decreasing the slope of the hydraulic conductivity function at 10-8 m/s (for applied fluxes of 10-7 m/s or less). The sensitivity analysis showed that the manipulation of the hydraulic conductivity function had insignificant changes in the flux distribution between the waste rock layers and great significance for achieving convergence. Based on the discoveries of the sensitivity analysis, a 20-meter high multi-layer waste rock profile inclined at 50º with an applied flux of 7.7e10-9 m/s equal to the annual precipitation at the Golden Sunlight Mine was successfully simulated. A parametric study was subsequently conducted for an applied flux rate of 10-5 m/s for slope heights of 1-meter to 20 meters with slope angles varying between 45º and 90º. The parametric study demonstrated that flow in a multi-layered waste rock dump is a function of inclination, contact length between the layers, and the coarse and fine-grained hydraulic properties for the waste rock. An alternative numerical modelling technique based on a modified Kisch solution was also used to investigate preferential flow. The Kisch method helped to verify and simplify the numerical problem as well as to illustrate the mechanics of preferential flow in a two-layered system.
In general, commercial seepage modeling packages are powerful and useful tools that are designed to adequately accommodate a wide range of geotechnical problems. The results of this research study indicate that Seep/W may not be the best-suited tool to analyze unsaturated seepage through sloped waste rock layers. However, numerical modelling is a process and working through the process helps to enhance engineering judgment. The Seep/W model provided an adequate solution for a simplified simulation of unsaturated seepage through waste rock layers. The modified Kisch solution independently verified the solution and provided additional confidence for the results of Seep/W model.
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Keywords
Unsaturated flow, Numerical Modelling, waste rock
Citation
Degree
Master of Science (M.Sc.)
Department
Civil Engineering
Program
Civil Engineering