Groundwater flow modeling for pit wall stability and floor heave analyses, Mae Moh mine

Abstract

Mae Moh mine is a large open pit coal mine located in Mae Moh basin, in Lampang province, Thailand. The mine has been operated by Electricity Generating Authority of Thailand (EGAT) since 1955 and supplies up to 45,000 tons of lignite daily to feed EGAT’s power plant. Groundwater is the common problems for mine operation especially while developing the deep pit below groundwater level. Poorly controlling of groundwater movement will have negative impact on the safety, efficiency and economic situation of mining. This research is aimed to simulate groundwater system in mining area for groundwater management to help prevent the problems of pit wall instability and floor heave from groundwater pressure. The model uses the An application of 3-D GMS® software (Groundwater Modeling System) which is the finite difference groundwater flow model to simulate the groundwater system of basement formation aquifers in mining area. The geological, hydrological, hydrogeological data are collected and analyzed in order to formulate the conceptual model. After that, the numerical model was formulated with input data from the conceptual model such as boundary conditions, hydraulic properties of aquifers and other initial conditions.Transient state model was simulated using initial input data from the database such as hydraulic properties of the aquifer units, boundary conditions, recharge rates, geohydrological stress period and etc. Calibration of several simulations were run using trial and error process by varying of some parameters until the calculated and measured heads were compatible. Eventually water balance for particular times can be calculated with the root mean square of the simulation results. The parameter sensitivity analysis is also carried out. The calibration process and sensitivity analysis are performed until the piezometric levels from the model are consistent with the results from observation wells measurements. The slope stability in C1 west wall pit is analyzed using limit equilibrium method. The floor heave is evaluated as well. The instability of slope and floor heave conditions are defined. The calibrated model is used to predict the range of depletion of groundwater table. The planning for dewatering of each mine stage until the year 2049 is suggested. The results indicate that upon the completion of groundwater flow modelling simulations, it appears that the model is a realistic predictive method, to use for simulating aquifer depressurization. The calibration results show that the accuracy of groundwater flow model is 98.64%; the comparison of contour lines between measured heads and calculated heads provides good results, the groundwater levels and shape in contour maps are consistent. Flowing production wells located in the Basement Tr4 boundary will be the main method to achieve mine depressurization. The simulation results suggest that, in order to ensure safe mining from slope instability and floor heave, the dewatering requirements from 2021 to 2049 are at an average of 5,153 cubic meters per day, or 1.88 million cubic meters per year. With this prediction, the groundwater level will be depressed to be lower than the lowest pit floor, on a yearly basis, until the final stage plan in year 2049.