Unsaturated flow modeling for exposed and covered tailings dams

Seepage through mining dams involves saturated and unsaturated flow. The latter is often neglected but should also be considered to accurately estimate the seepage flux and the corresponding position of the water table. Also, in some instances, the tailings may be reactive and produce acid mine drainage. In such cases, a layered cover can be installed on tailings dams (and on tailings in the pond) to control both water and oxygen fluxes. Again, to understand and predict the hydraulic behaviour of covered tailings dams, unsaturated flow has to be included in the analyses. This paper presents different calculations performed using the finite element method, to evaluate water motion and distribution in tailings dams, with and without covers. For exposed dams, different heights (from 5 to 50 m) were analysed numerically with a 2D FE code, for typical geometries. From the numerical results, simple mathematical expressions to predict the total seepage flow rate through dams are developed. In the case of dams with an inclined layered cover, numerical modeling was used to estimate water distribution in the cover, as a function of the system geometry. The influence of different parameters associated with the dam and the cover were investigated, including slope length and inclination, drainage time, and cover materials properties. The relationship between those parameters and the ability of a cover to limit gas migration is expressed through a simple equation.

[1]  Christopher E. Kees,et al.  Mixed finite element methods and higher order temporal approximations for variably saturated groundwater flow , 2003 .

[2]  B. Elberling,et al.  Evaluation of sulphide oxidation rates: a laboratory study comparing oxygen fluxes and rates of oxidation product release , 1994 .

[3]  D. Fredlund,et al.  Soil Mechanics for Unsaturated Soils , 1993 .

[4]  P. Huyakorn,et al.  Techniques for Making Finite Elements Competitve in Modeling Flow in Variably Saturated Porous Media , 1984 .

[5]  E. C. Childs.  Dynamics of fluids in Porous Media , 1973 .

[6]  Michel Aubertin,et al.  A predictive model for the water retention curve : application to tailings from hard-rock mines , 1998 .

[7]  E. C. Childs,et al.  The permeability of porous materials , 1950, Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences.

[8]  S. T. Yuster,et al.  Theoretical Considerations of Multiphase Flow in Idealized Capillary Systems , 1951 .

[9]  R. Gillham,et al.  Numerical simulations to investigate moisture-retention characteristics in the design of oxygen-limiting covers for reactive mine tailings , 1991 .

[10]  Fred J. Molz,et al.  A physically based, two-dimensional, finite-difference algorithm for modeling variably saturated flow , 1994 .

[11]  John A. Cherry,et al.  REDUCTION OF ACID GENERATION IN MINE TAILINGS THROUGH THE USE OF MOISTURE-RETAINING COVER LAYERS AS OXYGEN BARRIERS , 1989 .

[12]  P. E. Rijtema.  An analysis of actual evapotranspiration , 1965 .

[13]  Ernest K. Yanful,et al.  Water Flow through Cover Soils Using Modeling and Experimental Methods , 2000 .

[14]  Feike J. Leij,et al.  Closed‐Form Expressions for Water Retention and Conductivity Data , 1997 .

[15]  L. A. Richards.  Capillary conduction of liquids through porous mediums , 1931 .

[16]  D. Fredlund,et al.  Predicting the permeability function for unsaturated soils using the soil-water characteristic curve , 1994 .

[17]  R. J. Millington,et al.  Diffusion in Aggregated Porous Media , 1971 .

[18]  E. Leong,et al.  REVIEW OF SOIL-WATER CHARACTERISTIC CURVE EQUATIONS , 1997 .

[19]  W. R. Gardner.  SOME STEADY‐STATE SOLUTIONS OF THE UNSATURATED MOISTURE FLOW EQUATION WITH APPLICATION TO EVAPORATION FROM A WATER TABLE , 1958 .

[20]  R. Srivastava,et al.  Numerical Simulation of the Wicking Effect in Liner Systems , 1994 .

[21]  P. Huyakorn,et al.  A three‐dimensional finite‐element model for simulating water flow in variably saturated porous media , 1986 .

[22]  M. Aubertin,et al.  A user's approach to assess numerical codes for saturated and unsaturated seepage conditions , 2001 .

[23]  Modelling and field measurements of water percolation through an experimental soil cover on mine tailings , 1995 .

[24]  M. Mbonimpa,et al.  LABORATORY MEASUREMENTS AND PREDICTIVE EQUATIONS FOR GAS DIFFUSION COEFFICIENT OF UNSATURATED SOILS , 2002 .

[25]  Y. Mualem,et al.  Hydraulic Conductivity of Unsaturated Soils: Prediction and Formulas , 2018, SSSA Book Series.

[26]  N. T. Burdine.  Relative Permeability Calculations From Pore Size Distribution Data , 1953 .

[27]  M. Aubertin,et al.  Development of a model to predict the water retention curve using basic geotechnical properties , 2003 .

[28]  M. Aubertin,et al.  A simplified method to estimate saturated and unsaturated seepage through dikes under steady-state conditions , 2001 .

[29]  A. Rasmuson,et al.  A Comparison of Gas Diffusivity Models for Unsaturated Porous Media , 1988 .

[30]  H. Saunders,et al.  Finite element procedures in engineering analysis , 1982 .

[31]  R. Green,et al.  Calculation of Hydraulic Conductivity: A further Evaluation of some Predictive Methods , 1971 .

[32]  Van Genuchten,et al.  A closed-form equation for predicting the hydraulic conductivity of unsaturated soils , 1980 .

[33]  Clinton N Dawson,et al.  FLOTRAN, a three-dimensional ground water model, with comparisons to analytical solutions and other models , 2000 .

[34]  M. Aubertin,et al.  The behavior of inclined covers used as oxygen barriers , 2003 .

[35]  M. Aubertin,et al.  OXYGEN DIFFUSION AND CONSUMPTION IN UNSATURATED COVER MATERIALS , 2002 .

[36]  Ernest K. Yanful,et al.  Measurement of evaporative fluxes from candidate cover soils , 1997 .

[37]  Y. Mualem.  A New Model for Predicting the Hydraulic Conductivity , 1976 .

[38]  Ernest K. Yanful,et al.  A column study of static nonequilibrium fluid pressures in sand during prolonged drainage , 1994 .

[39]  R. Healy,et al.  Simulation of solute transport in variably saturated porous media with supplemental information on modifications to the US Geological Survey's computer program VS2D , 1990 .

[40]  S. P. Neuman,et al.  SATURATED-UNSATURATED SEEPAGE BY FINITE ELEMENTS , 1973 .

[41]  C. Dirksen,et al.  Hydraulic Conductivity and Diffusivity: Laboratory Methods , 2018, SSSA Book Series.

[42]  Michel Aubertin,et al.  A model to predict the water retention curve from basic geotechnical properties , 2003 .