groundwater flow modelling and slope stability evaluation for deepening of mae moh open pit lignite...
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GROUNDWATER FLOW MODELLING AND SLOPE STABILITY EVALUATION FOR DEEPENING OF MAE MOH OPEN PIT LIGNITE MINE
by Anjula B. N. Dassanayake
Examination Committee : Dr. Noppadol Phien-Wej (Chairman)
: Dr. Pham Huy Giao (Co-advisor): Prof. Dennes T. Bergado: Dr. Kyung-Ho Park
AIT Master Thesis Competition17th May 2010
Asian Institute of Technology May 2010
Geotechnical and Geoenvironmental Engineering School of Engineering and Technology
CONTENT
• Introduction• Methodology• Results and Discussions• Conclusions and
Recommendations
Asian Institute of Technology May 2010
INTRODUCTION
Mae Moh Mine• Largest open pit lignite mine in
South East Asia• Location-Lampang province,
Northern Thailand– Latitude 18ᵒ18' 21" N– Longitude 99ᵒ 44' 02" E
Mae Moh mine
600km
Bangkok
Source-EGAT
N
STATEMENT OF THE PROBLEM
Unfavorable geological structure associated with the groundwater pressure, there could be a potential for slope instability mainly in the west wall of C1 pit.
Modeling groundwater flow behavior of Mae Moh mine area and depressurization requirement to confirm the
stability of west wall of C1 pit
Sub-objectives: Review of research works which have been done to date related to the topic.
Collect all necessary information, geological, geotechnical and hydrological data required and data compilation.
Groundwater modeling of the Mae Moh mine and predictive simulations to identify possible dewatering measures in the Argillite formation under the west wall of C1 pit area for stable mining.
Slope stability analysis of the selected critical locations of the West wall of the C1pit and stabilization measures.
OBJECTIVES
METHODOLOGY
SLOPE STABILITY EVALUATIONStudy previous instabilitiesSlope stability analysis
Identify critical locations within study area
Evaluate stability associating groundwater condition
GROUNDWATER FLOW MODELLING
Conceptual Groundwater model developmentModel development(Visual Modflow 2.7.2)Calibration and verify modelPredictive simulations
REMEDIAL MEASURES FOR UNSTABLE CONDITIONS
CONCEPTUAL GROUNDWATER FLOW MODEL
11km11km
FINITE DIFFERENCE MODEL GRID OF GROUNDWATER FLOW MODEL
No. of rows 143
No. of columns 137
No. of layers 5
Cell size 100mx100m
Cell size( refine) 50mx50m
11Km
11Km
Y-N
orth
ing
X-Easting
AQUIFER MATERIAL PARAMETERS
• Hydraulic conductivity and storage
Material Type
DescriptionHydraulic conductivity(m/s)
Ss(1/m) SyKx Ky Kz
1Huai Luang and Na Kheam formation
5.787E-08 1.157E-08 4.629E-09 2.00E-07 0.0005
2Basment formation – Argillite
6.944E-09 2.314E-09 4.629E-09 5.00E-07 0.005
3Huai King formation
2.314E-07 4.629E-08 9.259E-09 6.00E-07 0.01
4Basement formation – limestone – under NE pit – Top 3.472E-05 8.101E-05 4.629E-05 1.00E-06 0.03
5Basement formation – limestone East and West basin margins – Top 5.787E-06 1.736E-05 9.259E-06 1.00E-06 0.005
6Basement formation – Sandstone
6.944E-09 2.314E-09 4.629E-09 1.00E-05 0.08
7Basement formation – limestone – under NE pit – Basal 3.472E-06 8.101E-06 4.629E-06 3.30E-06 0.005
8Basement formation – limestone East and West basin margins – Basal 5.787E-06 1.157E-06 9.259E-07 3.30E-06 0.001
9Basement formations – Deep, fresh rock
1.157E-10 5.787E-09 1.157E-10 2.00E-07 5.00E-04• parameters were based on the results of test conducted by EGAT• Phase 1 & 2 investigation-1988-1993 Additional drilling program-1994-1996
GROUNDWATER FLOW MODEL
Cross section N70 (North boundary of the model)
Argillite
HL & NK
HK
Limestone NE pit (top)
Limestone-marginal (top)
Limestone NE pit (basal)
Limestone-marginal (basal)
Sandstone
Layer 1Layer2Layer 3
Layer 4Layer 5
Argillite
HL & NK
HK
Limestone NE pit (top)
Limestone-marginal (top)
Limestone NE pit (basal)
Limestone-marginal (basal)
Sandstone
MODEL SIMULATIONS
Steady State Calibrationpotentiometric head distribution of 14 observation wells during first half of 1994
Transient CalibrationThe results of PA12B flow recession test with discharge rate of 12,000m3/day for 176 days from 05th June 1995 to November 1995
Model VerificationPA12B flow recession test with 176 pumping period and 175 recovery period
Predictive run 2A (Transient simulation) Discharge rate 3000m3/day Production bore- PA12B, PA13B and OA64B for 7 years (2010-2017)
Predictive run 2B (Transient simulation)Same pumping schedule used by assigning surface elevation in year 2010.
Predictive run 3A (Steady state simulation) Discharge rate 3000m3/day Production bore- PA12B, PA13B and OA64B Assigning surface elevation of year 2010
Predictive run 3B (Steady state simulation)
Discharge rate 3000m3/day Production bore- PA12B, PA13B and OA64B (limestone formation)Assigning surface elevation of year 2017
Predictive run 4
Discharge rate 60m3/day Production bore P-ARG 1, P-ARG 2(Argillite formation).Assigning surface elevation of year 2017
RESULTS AND DISCUSSIONS
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin Model Calibration -steady state condition (For head distribution in 1994)
NRMS error =4.96%
scatter plot of calculated verses observed head values
Observed and modeled Potentiometric head distribution
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin Model Calibration -Transient state condition(5th June 1995 to 28th Nov. 1995)
(a)potentiometric head distribution and (b)Draw down of the Basement formationAt the end of pumping rest(28th Nov 1995)
Hydrographs of observed and calculated head distribution for 176 days
Within Limestone
Within Sandstone
Within Argillite
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin Model Calibration -Transient state condition(5th June 1995 to 28th Nov. 1995)
HYDRAULIC CONDUCTIVITY AND STORAGEAFTER CALIBRATION
Material Type
DescriptionHydraulic conductivity(m/s)
Ss(1/m) SyKx(E-W) Ky(N-S) Kz
1Huai Luang and Na Kheam formation
5.787E-08 5.78704e-8 4.629E-08 4.00E-07 0.0005
2Basment formation – Argillite
6.944E-09 2.314E-09 4.62963e-8 5.00E-07 0.005
3Huai King formation
5e-7 5e-7 9.25926e-8 6.00E-07 0.01
4Basement formation – limestone – under NE pit – Top
0.0000035 0.0000035 4.629E-05 1.00E-06 0.03
5
Basement formation – limestone East and West basin margins – Top
5.8e-7 5.8e-8 0.0000093 3.30E-06 0.005
6Basement formation – Sandstone
9.4444e-8 9.4444e-8 4.62963e-8 1.00E-05 0.08
7Basement formation – limestone – under NE pit – Basal
3.5e-7 3.5e-7 0.0000046 3.30E-06 0.005
8
Basement formation – limestone East and West basin margins – Basal
5.787E-06 1.157E-06 9.259E-07 3.30E-06 0.001
9
Basement formations – Deep, fresh rock
1.157E-10 1.15741e-9 1.15741e-10 2.00E-07 5.00E-04
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin Model verification
Hydrographs of observed and calculated head distribution for 176 days discharge and 175 days recovery period
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin Predictive simulations 2A(without mining)
Predicted head distribution (a) for 98 days (b)1271days
(a) (b)
Predicted drawdown (a) for 98 days (b)1271days
(a) (b)
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin Predictive simulations 2A(without mining)
(a) Predicted head distribution (b) Drawdown distribution in Basement Formation after 2555days
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin Predictive simulations 2A(without mining)
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin Predictive simulations 2B(With mining)
Cross sectionRow-60
N28.3(2830)
In 1994
In 2010
Argillite
HL & NK
HK
Limestone NE pit (top)
Limestone-marginal (top)
Limestone NE pit (basal)Limestone-marginal (basal)
Sandstone
(a) Predicted head distribution (b) Draw down distribution in Basement Formation after 2555days
1.Groundwater flow Modeling of Mae Moh basin Predictive simulations 2B(With mining)
(a) (b)
• Predictive simulations 3A - (Steady state simulation) 3000m3/day was pumped from each bore PA12B, PA13B and OA64B by assigning surface elevation in year 2010
• Predictive simulations 3B - (Steady state simulation) 3000m3/day was pumped from each bore PA12B, PA13B and OA64B by assigning surface elevation in year 2017
1.Groundwater flow Modeling of Mae Moh basin
Cross sectionRow-60
N28.3(2830)
In 2010 In 2017
Results and Discussions
Head distribution resulted from the steady state simulation for (a) 2010 mine configuration (b) 2017 mine configuration
(a) (b)
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin Predictive simulations 3
Predicted head distribution in Argillite Formation after 2555days
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin Predictive simulations 4
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin Predictive simulations 4
Predicted head distribution in Argillite Formation after 2555days
Slope stability analysis-Cross section N 24
G4
G6
N24
Green clay
Surface in 2017
Faults
FS =4.055821FS = 4.055821
Zw
148m,MSL
To prevent any potential instability condition the potentiometric water head should be maintain below 176m,MSL
0 20 40 60 80 100 120-0.50
0.51
1.52
2.53
3.54
4.5
Factor of Safety Vs Elevation of water Zw - Section N 24
FS
Elevation of water Zw (m)
Fact
or o
f Saf
ety
1.5
28m
Dry condition Stability with the variation of water level
Slope stability analysis-Cross section N 25
G4
G6
N25 Green clay
Surface in 2017
Faults
FS = 4.055821
Zw
133.5m,MSL0 20 40 60 80 100 120
00.5
11.5
22.5
33.5
44.5
Factor of Safety Vs Elevation of water Zw in Section N 25
FS
Elevation of water Zw (m)
Fact
or o
f Saf
ety
To prevent any potential instability condition, the potentiometric water head should be maintain below 193.5m, MSL
Dry condition Stability with the variation of water level
Mine Development plan-Year 2017
Source: EGAT
Potential zone of failure in west wall of C1 pit
CONCLUSIONS AND RECOMMENDATIONS
• Argillite shows a considerable draw down for long term groundwater discharging from limestone under NE pit.
• Installing pumping wells within argillite formation to lower the
potentiometric head distribution in argillite is not feasible. • Potentially unstable condition could be occurred in west wall of
C1 pit in year 2017 when weak green clay layers exposed in the slope face .The dip of the beddings of these layers are small (less than 10ᵒ) and slopes will be stable under dry condition. But it will become unstable with the presences of water. Hence it is essential to lower the potentiometric head below 170m, MSL to maintain a safe working environment.
CONCLUSIONS
• Long term pumping schedule should be initiated focusing the drawdown response of argillite formation within C1 pit.
• Refine the groundwater temperature in order to determine the potential effect on aquifer depressurization and groundwater movement in the basin using new temperature measurements.
• Groundwater chemistry within Mae Moh mine should be analyzed because; chemical gradient and movement of chemical constituent through the water can cause the flow of water.
• Investigation should be continued by using existing and new bore holes to clarify the structural geology and lithology in order to determine the precise hydrologeological condition within the basement formation.
RECOMMENDATIONS
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