GROUNDWATER FLOW MODELLING AND SLOPE
STABILITY EVALUATION FOR DEEPENING OF
MAE MOH OPEN PIT LIGNITE MINE
by Anjula B. N. Dassanayake
Geotechnical and Geoenvironmental Engineering
School of Engineering and Technology
Examination Committee
: Dr. Noppadol Phien-Wej (Chairman)
: Dr. Pham Huy Giao (Co-advisor)
: Prof. Dennes T. Bergado
: Dr. Kyung-Ho Park
AIT Master Thesis Competition
17th May 2010
Asian Institute of Technology
May 2010
CONTENT
•
•
•
•
Introduction
Methodology
Results and Discussions
Conclusions and Recommendations
Asian Institute of Technology
May 2010
INTRODUCTION
N
Mae Moh mine
600km
Bangkok
Source-EGAT
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
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.
OBJECTIVES
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.
METHODOLOGY
GROUNDWATER FLOW MODELLING
Conceptual Groundwater model
development
Model development(Visual Modflow 2.7.2)
Calibration and verify model
Predictive simulations
SLOPE STABILITY EVALUATION
Study previous instabilities
Slope stability analysis
Identify critical locations within
study area
Evaluate stability associating
groundwater condition
REMEDIAL MEASURES FOR UNSTABLE CONDITIONS
CONCEPTUAL GROUNDWATER FLOW
MODEL
11km
11km
FINITE DIFFERENCE MODEL GRID OF
GROUNDWATER FLOW MODEL
11Km
143
No. of columns
137
No. of layers
5
Cell size
100mx100m
Cell size( refine)
50mx50m
Y-Northing
No. of rows
11
K
m
X-Easting
AQUIFER MATERIAL PARAMETERS
• Hydraulic conductivity and storage
Material
Type
1
2
3
4
5
6
7
8
9
•
•
Description
Huai Luang and Na Kheam formation
Basment formation – Argillite
Huai King formation
Basement formation – limestone – under NE pit –
Top
Basement formation – limestone East and West
basin margins – Top
Basement formation – Sandstone
Basement formation – limestone – under NE pit –
Basal
Basement formation – limestone East and West
basin margins – Basal
Basement formations – Deep, fresh rock
Hydraulic conductivity(m/s)
Kx
Ky
Kz
Ss(1/m)
Sy
5.787E-08
1.157E-08
4.629E-09
2.00E-07
0.0005
6.944E-09
2.314E-09
4.629E-09
5.00E-07
0.005
2.314E-07
4.629E-08
9.259E-09
6.00E-07
0.01
3.472E-05
8.101E-05
4.629E-05
1.00E-06
0.03
5.787E-06
1.736E-05
9.259E-06
1.00E-06
0.005
6.944E-09
2.314E-09
4.629E-09
1.00E-05
0.08
3.472E-06
8.101E-06
4.629E-06
3.30E-06
0.005
5.787E-06
1.157E-06
9.259E-07
3.30E-06
0.001
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)
HL & NK
Limestone-marginal (top)
Argillite
Sandstone
HK
Limestone NE pit (basal)
Limestone NE pit (top)
Limestone-marginal (basal)
Layer 1Layer2
Layer 3
Layer 4
Layer 5
HL & NK
Limestone-marginal (top)
Argillite
Sandstone
HK
Limestone NE pit (basal)
Limestone NE pit (top)
Limestone-marginal (basal)
MODEL SIMULATIONS
Steady State Calibration
Transient Calibration
Model Verification
Predictive run 2A (Transient simulation)
Predictive run 2B (Transient simulation)
Predictive run 3A (Steady state simulation)
Predictive run 3B (Steady state simulation)
Predictive run 4
potentiometric head distribution of 14 observation
wells during first half of 1994
The results of PA12B flow recession test with
discharge rate of 12,000m3/day for 176 days from
05th June 1995 to November 1995
PA12B flow recession test with 176 pumping period
and 175 recovery period
Discharge rate 3000m3/day
Production bore- PA12B, PA13B and OA64B for 7
years (2010-2017)
Same pumping schedule used by assigning surface
elevation in year 2010.
Discharge rate 3000m3/day
Production bore- PA12B, PA13B and OA64B
Assigning surface elevation of year 2010
Discharge rate 3000m3/day
Production bore- PA12B, PA13B and OA64B
(limestone formation)
Assigning surface elevation of year 2017
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)
scatter plot of calculated verses
observed head values
NRMS error =4.96%
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 formation
At the end of pumping rest(28th Nov 1995)
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin
Model Calibration -Transient state condition(5th June 1995 to 28th Nov. 1995)
Within Argillite
Within Sandstone
Within Limestone
Hydrographs of observed and calculated head distribution for 176 days
HYDRAULIC CONDUCTIVITY AND STORAGE
AFTER CALIBRATION
Material
Type
1
2
3
4
Description
Huai Luang and Na Kheam formation
Basment formation – Argillite
Huai King formation
Basement formation – limestone – under NE pit – Top
Hydraulic conductivity(m/s)
Kx(E-W)
Ky(N-S)
Kz
Ss(1/m)
Sy
5.787E-08
5.78704e-8
4.629E-08
4.00E-07
0.0005
6.944E-09
2.314E-09
4.62963e-8
5.00E-07
0.005
5e-7
5e-7
9.25926e-8
6.00E-07
0.01
0.0000035
0.0000035
4.629E-05
1.00E-06
0.03
5.8e-7
5.8e-8
0.0000093
3.30E-06
0.005
9.4444e-8
9.4444e-8
4.62963e-8
1.00E-05
0.08
3.5e-7
3.5e-7
0.0000046
3.30E-06
0.005
5.787E-06
1.157E-06
9.259E-07
3.30E-06
0.001
1.157E-10
1.15741e-9
1.15741e-10
2.00E-07
5.00E-04
Basement formation – limestone East and West basin margins –
Top
5
6
7
Basement formation – Sandstone
Basement formation – limestone – under NE pit – Basal
Basement formation – limestone East and West basin margins –
Basal
8
Basement formations – Deep, fresh rock
9
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)
(a)
Predicted head distribution (a) for 98 days (b)1271days
(b)
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin
Predictive simulations 2A(without mining)
(a)
Predicted drawdown (a) for 98 days (b)1271days
(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 2B(With mining)
Cross section
Row-60
N28.3(2830)
In 1994
In 2010
HL &
NK
Argillite
HK
Limestone NE
pit (top)
Limestonemarginal (top)
Sandsto
ne
Limestone NE pit
(basal)
Limestonemarginal (basal)
1.Groundwater flow Modeling of Mae Moh basin
Predictive simulations 2B(With mining)
(a)
(b)
(a) Predicted head distribution (b) Draw down distribution in Basement Formation after 2555days
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin
• 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
In 2010
In 2017
Cross section
Row-60
N28.3(2830)
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin
Predictive simulations 3
(a)
(b)
Head distribution resulted from the steady state simulation for (a) 2010 mine
configuration (b) 2017 mine configuration
Results and Discussions
1.Groundwater flow Modeling of Mae Moh basin
Predictive simulations 4
Predicted head distribution in Argillite Formation after 2555days
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
Surface in 2017
N24
Green clay
Faults
G4
G6
To prevent any potential
instability condition the
potentiometric water head
should be maintain below
176m,MSL
FS = 4.055821
FS =4.055821
Factor of Safety Vs Elevation of water Zw - Section N 24
5
Zw
148m,MSL
Factor of Safety
4
3
2
FS
11.5
0
-1
0
20
28m
Dry condition
40
60
80
100
120
Elevation of water Zw (m)
Stability with the variation of water level
Slope stability analysis-Cross section N 25
Surface in 2017
Green clay
N25
Faults
G4
G6
To prevent any potential
instability condition, the
potentiometric water head should
be maintain below 193.5m, MSL
Factor of Safety Vs Elevation of water Zw in Section N 25
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
Factor of Safety
FS = 4.055821
Zw
133.5m,MSL
FS
0
Dry condition
20
40
60
80
Elevation of water Zw (m)
100
120
Stability with the variation of water level
Mine Development
plan-Year 2017
Potential zone of
failure in west wall
of C1 pit
Source: EGAT
CONCLUSIONS AND
RECOMMENDATIONS
CONCLUSIONS
•
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.
RECOMMENDATIONS
•
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.