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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