July 11, 2013
The hydrology of a proposed cover system, under varying drainage layer conditions, for a mine site rehabilitation project located in Southeastern Manitoba was analyzed with the SoilVision Systems Ltd., SVFlux™ software. The proposed 1 m thick engineered soil cover included a 0.2 m thick highly permeable basal granular drainage layer, which was replaced with a man-made synthetic drainage system (DRAINTUBE™).
Four different levels of potential clogging of the man-made drainage layer were analyzed, equivalent to 99%, 90%, 50% and 0% clogging of the original material. The purpose of the analysis was to observe the flow dynamics and change trends in the hydrology of the engineered soil cover system located above the drainage layer, over a period of 10 years, and from there, estimate future trends in the cover system hydrology and its impacts.
The SVFlux™ finite element modeling software was the main numerical tool used to model the hydrologic regime and complete the analyses. The software results include, among other, net percolation, flux, and saturation in each of the layers within the cover system. The plots were used for analysis and interpretation. Analyses took into consideration both external (climatic) and internal (soil properties), and the numerical method including transient conditions.
The hydrology of the soil cover was simulated with four different flow systems in the DRAINTUBE™ layer through varying the hydraulic conductivity in the DRAINTUBE™ layer to simulate a progressive clogging geotextile. The four cases below were drawn and modeled in SVFlux™ .
- Case a) Assuming drainage layer is a clean granular drainage layer: this simulation would be equivalent to a 0% clogging of the pipes
- Case b) Fine sand drainage layer – equivalent to 50% of the pipes clogged
- Case c) Silt drainage layer – equivalent to 90% of the pipes clogged
- Case d) Sandy clay drainage layer – equivalent to 100% of the pipes clogged
The software presented various hydrological plots as outputs including a summary of the water balance, net percolation into the soil cover, saturation in each layer, and flux along the soil boundaries. Below is an example of the first case scenario plots showing, a hydrologic summary, saturation, flux, and net percolation versus time.
The modeling performed for each given case with SVFlux™ showed increasing flow trends over time as the hydraulic conductivity was reduced. The software results illustrated that the least desirable case and most extreme case showed that if the drainage system were to fail by becoming completely clogged in the long term; more than 50% of the drainage layer would be saturated and an increasing trend over 20 years could increase water percolation rates into the tailings. However, the system has some level of flexibility in terms of drainage layer clogging. Even with a 50% clogging of the drainage layer, the cover system would still be performing as designed, with a minor increase in saturation levels and limited increase in percolation rates. From this perspective, the system, as designed, has a built-in safety factor that allows good long term performance and should perform according to original requirements such as limiting erosion, limiting direct human exposure to tailings, shedding water away and supporting the vegetative cover.
I utilized the SVFlux™ package for the analysis of a one-dimensional seepage model for my thesis. I modeled a clogging drainage layer at a mine site closure. The software was easy to learn, quick to run, and results displayed comprehensive plots with all the information I needed for the analysis. I would highly recommend this software to other students completing their thesis.
Alexandra Nan, Student, Geological Engineering, University of British Columbia