IMWA 21 Sydney, Nova Scotia Mine Water & Innovative Thinking HYDROGEOLOGICAL ASSESSMENT OF THE THAR LIGNITE PROSPECT 1. R.N. Singh and L. R. Stace, The University of Nottingham, UK 2. A.S. Atkins Staffordshire University, UK 3. A. G. Pathan, Mehran University of Engineering and Technology, Sindh, Pakistan 4. F Doulati Ardejani, Shahrood University of Technology, SHAHROOD, IRAN 1 Paper is concerned with Geological and hydro-geological studies of Thar coal field Bara formation-depth 13-25m In a mining prospect of 4 Km 2 Total reserves 9 billion Tonnes. Recoverable 3 billion Tonnes Thickness of lignite 5-6m Lignite contains- 42-49% moisture 2 Paper Presents Results of pumping out tests Ground water model to simulate de-pressuring of aquifers Results show that 22 pumping out wells are required Period of 1 years Overall pumping rate of 53 L/sec 3 Problems of lignite mining 1. Lignite deposits are under-laid and overlaid by aquifers presenting problems: Inflow of water causing safety, operational and environmental problems. 2. Confined aquifers below the deposit cause: In-pit flooding, Erosion of toe of high-wall, High-wall instability. 4 Background The paper presents; Hydrogeological appraisal of proposed mining operations in Thar Lignite prospect, Sindh, Pakistan Thar coalfield -9 km2, Estimated reserves- 192 Bt lignite; Depth of seams- 13 to 25m, Seam thickness- between 7.5m to 36m, Maximum thickness of individual seam 23m, Eight blocks have been explored which have; 9 Bt proven reserves. Coal seams surrounded by three aquifers. 5 Main Approach In brief, the paper addresses two questions: 1. Dewatering Schemes for top, intermediate and bottom aquifers, 2. Estimation of inflow quantities from each aquifer using; (a)equivalent well approach (b)seep/w Finite Element Software Package 6 1
IMWA 21 Sydney, Nova Scotia Mine Water & Innovative Thinking Thar Lignite Field, Sindh, Pakistan 7 th largest in the world discovered in 1994. 4 Km. East of Karachi. Total area of 9 km 2 Estimated resources of 193 Bt. Mining Area 4 km 2 9 Bt reserves Average Composition of Lignite Moisture 46.77 % Fixed carbon 16.66% Volatile matter 23.46% Ash 6.24% Sulphur 1.16% Heating value 1,898 Btu/lbs 7 8 Eight areas being prospected. Thar prospects Geology of the Thar prospect Proven coal reserves in 25 were over 9 Bt in eight Prospects Dune Sand 48m Sub Recent- 77m Bara formation Total lignite resources in whole coalfield - 193 Bt 9 Base Aquifer 5-6m thick Granite basement 1 Hydrogeology 1. Top aquifer At the base of dune sand Permeability 3 x 1-7 m/s Water column up to 5m thick Water table 1-12m, above mean sea level 2. Intermediate aquifer Scattered lenses of sand Permeability 1-5 to 1-7 m/s 3. Bottom aquifer Beneath the coal formation down to the granite base Fine to coarse sand grains, non-homogeneous permeability 5-6m thick aquifer Piezometric surface 25m above mean sea level Aquifer covers an area of 15 Km 2 11 Q14 L/s Δs1.35m (draw down in one log cycle) Evaluation of Pumping Test Pumping out well RE51; Observation well RE12P 12 2
IMWA 21 Sydney, Nova Scotia Mine Water & Innovative Thinking Pumping out test on RE-52 & Observation well RE 22 Recovery test on RE 51 and Observation well RE-12 Q13L/s Piezometer distance 3m; s Draw down in one log cycle.3 m T 2.3x.13/4x3.14x.3 7.9x1-3 m2/s Δs 1.38m in 1 log cycle 14 FE of Pumping out test Pumping Out Test in Well Results 1. RE51-RE12P Δs 1.35m; 1 35m; Q.13 Q 13 m3/s to43s; r25m 2. RE51-RE12P Δs 1.38; Q.14 m3/s 3. RE52-RE22 Δs.3m; Q.13m3/s Results 258 nodes, 5 elements in single 3m layer, model length-2m, 8-noded elements k6.3 x 1-5 m/s S2 x1-44 S2.x1 F E model of well RE-51 intersecting the Bottom aquifer 1.No flow U & L Boundaries. 2. Head Boundary on RHS 3LHS FLUX Boundary 4. k 7.3 x 1-5 m/s k8.x 1-5 m/s S8x 1-4 k 2.63 x 1-4 m/s S 2.7 x 1-3 16 t s 159 PUMPING OUT TESTS t6 s t12 s 157 t3 s t6 s 156 t95 s 155 t19 s t4 s 154 t6 s t8 s 153 S 2.9x 1-4 Initial Head 16m t1 s 152 5 1 15 2 Distance from pumping well (m) Hydraulic head vs. draw down of RE-51 well at different radial distances for 24 time steps Transient flow condition 16 15 Comparison of field and predicted results RE51 test t6 s 158 Finite element model of well RE-52 Intersecting the Bottom aquifer Comparison of model and field results 258 nodes, 51 elements 6m thick layer -2m length; 8nodded elements FE model of RE52 intersecting bottom aquifer 16 159.9 H yd rau lic h ead (m ) 159.8 Residual draw down RE12P vs. time t s t3 s 159.7 159.6 t2 s 159.5 t1 s 159.4 t4 s t6 s 159.3 t1 s 159.2 159.1 159 5 1 15 2 25 Distance from pumping well (m) Close agreement Hydraulic head vs distance from well 17 Draw down vs time Showing Reasonable agreement 18 3
IMWA 21 Sydney, Nova Scotia Mine Water & Innovative Thinking Road to North from Mithi Surface Landscape 19 Factors affecting inflow to the Bottom Aquifer 2 Factors affecting pumping out tests 16 155 145 14 135 13 (k8e-5 m /s ) m odel (k6e-5 m /s ) (k4e-5 m /s ) (k3e-5 m /s ) (k2e-5 m /s ) 125 16m head k k8.x 8 1-5 1 5 m/s / S2.7 x 1-3 Q.14 m3/s 12 5 1 15 2 25 3 35 25 3 35 Time (days) 253 nodes,5 eight nodded elements, 5m long, 16 t1 hour t5 hours t1 hours 155 t1 day t1 month t6 months t1 year 15 t2 years t5 years Hydraulic head vs Elapsed time intervals for various rates of pumping 15 16 Distance from the pumping well 15 Hydraulic head vs. time for various permeabilities FE of pumping well in the base aquifer 14 13 12 11 t1 years 145 1 2 3 4 5 1 Distance from pumping well (m) 5 (Q4 l/s ) (Q6 l/s ) (Q8 l/s ) (Q1 l/s ) (Q12 l/s ) (Q14 l/s ) (Q16 l/s ) (Q18 l/s ) (Q2 l/s ) 1 15 2 Time (days) 21 22 Mithi district after rain Hydrology Main rainfall is in July or August when it rains 1 mm/h and it may stop working of two coal benches in the open cut mine for two days 23 24 4
IMWA 21 Sydney, Nova Scotia Mine Water & Innovative Thinking After Rain Mithi District After Rain, Thar Desert, Sindh 25 Total water inflow due to rainfall 26 Aquifers Dewatering in The Thar Prospect Q 2.78 K A I 2.78 x 463.77 x.58 x 1 7.5 x 1 4 litres/s Where, Q Peak flow in litres/s A Catchment area in hectares 463.77 hectares K run-off co-efficient in decimal.58 I rainfall intensity 1 mm/h 27 28 Dewatering Second Aquifer Dewatering First Aquifer Aquifer Characteristics Thickness of aquifer L 5m Drawdown require D 2m Radius at which draw down required 21m Radius of influence, R 3m k 3 x 1-7 m/s.259 m/d T transmissivity.259 x 5.13 m2/d h 12m H 2 Pumping Calculations Aquifer Characteristics Unconfined steady state linear aquifer Modified Dupuit (1865) Equation: Scattered lenses Thickness of aquifer L 1m Drawdown required 89+21m Radius at draw down 15m Radius of influence, R 25m k 1-6 m/s.86 m/d n.5 π k (H ) 2 h2 R ln r 3. 14 x. 259 2 2 12 2 3 ln 21 58 m 3 / d Q ( 29 ) Pumping Calculations 2π k L D ( Peterson Equation) R n ln r 2 2 π x.86 x 1 x1 25.5 ln 15 2 5.4 x 1 14.69 m 3 / d.367 Q 3 5
IMWA 21 Sydney, Nova Scotia Mine Water & Innovative Thinking INFLOW FROM BASE AQUIFER TO VARIOUS SIZED PITS Dewatering Base Aquifer Aquifer Characteristics Pumping Calculations Thickness of aquifer L 55m Peterson equation Drawdown required 15+25 175m 2π k L D Q Radius at draw down 75m R n ln r 2 Radius of influence, R 25m 2 x 3.14 x 1 x.219 x 6 x 6 24 k 1.3 x 1-4 m/s 11.23 m/d 25.5 ln n.5 75 2 42777.95 5737.27 m 3 / d 1.5.25 Results- Pumping 3.5 m3/s for two years Rheinbraun Consultants estimated that 22 pumping out wells are required (i)period of 1 years (ii)overall pumping rate of 53 l/sec 32 31 Inflow simulation to top aquifer for partially penetrating pit 9 days 18 days 36 days.4 72 days Inflow vs time in advancing pit.3 Inflow (m^3/s) 133 nodes 896 elements 14m thick 5m long Simulation of inflow by top aquifer.2.1 2 4 6 8 33 Simulated inflow from intermediate aquifer FE inflow simulation from the base aquifer k5x 1-5 m/s S 2.7x1-3 H265 m right H48.5m left 92 nodes, 45 elements; 1m,5m thick layer 246 nodes nodes,12 12 rect. rect Elements;6m thick & 1m long aquifer 2 t1 s 1 t5 s t1 s 15 t2 s t1 hour t5 hours t1 hours 1 t1 day t5 days 5 t1 days t1 month Inflow vs. time 14 steps of iterations Inflow(m ^3/s) Hydraulic head vs Distance For various time periods 34 FE mesh of the Base Aquifer Finite element mesh of intermediate aquifer k 5x1-6 m/s S2x -4 h 2 h2m Q.3 m3/s 1 Time (day) 1 1 1 1 1 2 3 4 5 Distance (m) 4 8 12 16 2 24 28 32 36 Time (day) 35 36 6
IMWA 21 Sydney, Nova Scotia Mine Water & Innovative Thinking Steady state flow in top unconfined aquifer Intermediate Confined aquifer Constant Head2m RADIAL DISTANCE 13m r11m Constant head at pit12m K3.x 1-7 m/s Nodes 132nodes, 11 Elements 1 Layers of 3m thick Inflow k to fully penetrating top aquifer is 112 m3/d Finite Element grid, velocity vectors and water table of the top unconfined aquifer 55 Elements, 116 nodes,in single layer 1m thick and 145m long Aquifer thickness 1m K 1. x 1-6 m/s D1m Inflow quantity 141 m3/d Modified k of unconfined aquifer 37 38 FE Grid of bottom confined aquifer Inflow Rate m 3 /d Comparison of analytical and numerical Inflow Results Analytical solution m 3 /d Numerical solution m 3 /d % Error Top Unconfined Aquifer Modified Dupuit Eq. Q116 m 3 /d 112 m 3 /d 3.4% Intermediate Con. Aquifer Peterson EQ. 147 m 3 /d 141 m 3 /d 4.1% Axis symmetrical grid 246 nodes, 12 rectangular elements 55m thick single layer k 2.19x 1-3 m/s D26m ; r 75m ; radius of influence25m Base Aquifer Confined k2.19 x 1-5 m/s k 1.3 x 1-4 m/s Peterson Eq. Q 2.25 x 1 5 m 3 /d Q1.34 x1 5 m 3 /d 2.34x1 5 m 3 /d 1.4x 1 5 m 3 /d 6.4% 4.43% 39 4 CONCLUSIONS The paper uses the SEEP/W software to analyse pumping out data in RE-51 and RE-52 wells in an infinite confined aquifer in the Thar Lignite prospect. The pumping test simulation results were close to the analytical results and field data. Conclusions (Continued) A model simulation of a hypothetical pumping out well carried out a sensitivity analysis of various factors affecting ground water inflow. It was indicated that the model is sensitive to permeability of the aquifer as an input data. 41 42 7
IMWA 21 Sydney, Nova Scotia Mine Water & Innovative Thinking Conclusions (Cont.) The model was then used to predict ground water inflow during the open cut mine advancement at various time periods and inflow into fully penetrating pit into the three aquifers using the steady state flow condition. The results of inflow provide significant information for the design of an effective dewatering system for all stages of mining 43 Thank You Very Much for Your Attention Raghu N. Singh University of Nottingham 44 8