Mine dewatering for pit slope stability
Concepts and Studies
By Dr Houcyne El Idrysy
Astana, Kazakhstan, 31 March 2014
Presentation Topics
1. Groundwater flow and pore pressure concepts
2. Hydrogeological conceptual models
3. Development of numerical groundwater models
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4. Optimisation of pit dewatering/depressurization and
input into mine design
5. Design of pit dewatering/depressurization and
monitoring
6. Conclusions
Take Away Statement
Relationship between pore water
pressure and rock strength
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• Effective stress (σ') acting at a point is calculated from two
parameters, total stress (σ) and pore water pressure (u) as follows
(Terzaghi, 1943):
' =  - u
(1)
• The relationship between the shear strength of a rock material and
pore pressure can be expressed as (Freeze and Cherry, 1979):
 = ( - u) tan + c
(2)
where
 is the shear strength on a potential failure surface,
σ the total normal stress,
u is pore water pressure,
c the cohesion available along the potential failure surface, and
φ is the angle of internal friction of the material on the potential failure surface.
In a saturated ore body, pore water pressure exerts a significant control on the
effective stress of the rock mass (in both porous and fractured media)
Dewatering leads to increased rock mass strength and hence more
stable and steep slopes in the mine.
Pore water pressure versus phreatic surface
Dewatering well
Dewatered formation
Drain
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Depressurised formation
Considering only a phreatic surface in the slope stability
analysis is not enough for the design of optimal pit slopes
Mine Dewatering/Depressurisation Study
for Slope Optimisation and Design
Objectives
• Estimate potential inflows into the mine
• Assess the ability and time to dewater/depressurise the pit
• Design dewatering system to achieve stable slope and
acceptable mining conditions
• Prepare surface water control and flood protection if needed
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Required tasks
•
•
•
•
Regional and local numerical groundwater modelling
Transient simulation of mine dewatering for the mine life
Optimisation and design of mine dewatering system
Surface water hydrology and flood risk assessment
Take Away Statement
Approach to groundwater modelling for
slope design input
• Advanced modelling of pore pressure is not usually
required for input to the analysis of a pit slope stability.
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• It is required only when pore water pressure and
groundwater regime are identified as controlling factors
due to the geotechnical setting of the pit
• Therefore, before embarking in an advanced modelling
and simulation of pore pressure, assess if this is a
controlling factor in the pit stability
• When required, the process of interaction between both
studies can be very complex but rewarding
Take Away Statement
Step 1: Conceptual Hydrogeological Model
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3D
View
Take Away Statement
Step 2: Numerical Groundwater Model (Example)
Criteria:
Scale
Layers
Resolution
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Boundaries
GMS
Numerical Groundwater Model: Criteria
• Select a suitable groundwater modelling software: MODFLOW,
MODFLOW-SURFACT, FEFLOW, MineDW
• Type and purpose of model: transient/steady state,
flow/salt/contaminant migration?
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• Model Parameters should be obtained from site specific
investigations and lab testing;
• Constrain the model calibration using groundwater level and
stream flow observations, if both available;
• If the data available not enough, consider not building a model
• Use pit shells (or the UG mine design) in the predictive model
to estimate inflows and optimise a dewatering system
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Model Calibration (mostly pre-mining conditions)
Observed
head (mAD)
Simulated
head (mAD)
Error /
Residual, m
GW1
262.18
261.5
0.69
GW2
262.4
261.58
0.82
GW3
260.74
260.82
-0.08
GW4
260.75
260.47
0.28
GW5
260.5
259.95
0.55
GW6
259.2
259.08
0.12
GW7
258.3
260.03
-1.73
GW8
256.17
256.38
-0.21
GW9
263.72
266.46
-2.74
GW10
263.51
260.06
3.45
GW11
260.39
266.12
-5.73
GW12
263.43
272.7
-9.27
GW13
264.85
273.15
-8.3
GW14
257.63
258.25
-0.62
GW15
259.65
262.01
-2.36
GW16
259.88
261.6
-1.72
GW17
258.04
258.26
-0.22
GW18
259.13
257.46
1.67
GW19
259.38
262.11
-2.73
GW20
260.43
261.61
-1.18
Comparison observed vs simulated heads
275
270
Simulated water level (masl)
BH ID
265
260
255
250
250
255
Other criteria to verify/check: Stream flows - Flow budget (balance)
260
265
Observed water level (masl)
270
275
Groundwater Model Sensitivity Analysis
38
Kh- Lay 1 and 2
Kh- Lay3
Average Head Change
38
Kh - Lay4
37
Kh - Lay5
Kh - Lay6
37
Kh - Lay7
36
Kh - Lay8
Kh - Lay9
36
Kh - Lay10
35
Kh - Lay11-13
35
Aquifer Recharge
Kz - Lay1 and 2
34
Kz - Lay3
Kz - Lay4
34
Kz - Lay 5
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33
Kz - Lay6
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Multiplier (of calibrated parameters)
Take Away Statement
1.4
1.5
Predictive Modelling: Dewatering Impact
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Local Model extent
Other possible
impact could be:
Loss in river or
spring flow
Regional Model extent
Predictive Modelling: Inflows into the Mine
Groundwater Inflow
depth of the Pit
350
500
450
300
250
350
300
200
250
150
200
Pit Depth (m)
Groundwater inflow (m3/h)
400
150
100
100
50
50
0
0
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0
2
4
6
8
10
12
Elapsed time since start of mining (years)
Other possible results could be:
– Prediction of variation of water quality over time (e.g. salinity)
– Prediction of ISL potential and design
– Groundwater rebound and pit lake formation after closure
Predictive Modelling: Pore Pressure Distribution
Hydraulic Head (masl)
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Pit floor
Take Away Statement
Drains
X section
Predictive Modelling: Achieved depressurization
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Pit floor
North
South
Pit floor
Diagram of Interaction between slope stability
analysis and mine dewatering optimisation
Optimisation of mine dewatering
Slope stability analysis
Initial analytical level assessment
Worse case scenario of pore water
pressure: no dewatering assumed
Simple phreatic surface is used in slope
stability. if this indicates instable slopes:
Regional analysis level
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Regional Numerical modelling carried out
and simplistic dewatering system assumed
The predicted pore water pressure still
indicate risk of potential slope failure
Detailed iterative analysis level
Refined numerical models and various
dewatering scenarios tested
Must provide optimal pore water pressure
for the required slope angles
Technologies for controlling pore water
pressure in pit slopes
• Vertical dewatering wells around and/or within the pit mine;
• Wick drains in low permeability materials;
• Passive vertical and sub-horizontal drains driven in to pit
slopes or from existing underground workings;
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• Drainage galleries installed below or behind the pit slope;
• Blasting can theoretically reduce pore pressure around the
blast hole, but the actual extent of this still remains unknown
• Sequential planning of mine dewatering is important and, in
cases, active dewatering may be required ahead of mining
Conclusions (I)
• The challenges of mine dewatering for the objective of pit slope stability
depends primarily on the geological structures, rock/soil mass properties and
hydrogeological setting;
• the level of detail of the groundwater modelling and optimisation for the
purpose of providing input to pore water pressure analysis varies dramatically
from one case to another
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• Pit slope stability is more critical in low strength, low permeability saturated
rock/soil masses in the proximity of the pit walls;
• When pore water pressure is a controlling factor in pit stability, very advanced
numerical modelling of pore pressure simulation and optimisation of
dewatering/depressurisation systems are required, and vice versa
• The optimisation of mine dewatering system must be carried out iteratively
with the slope stability analysis to achieve optimal pit slope angles
Conclusions (II)
Value should be added to a mine project by:
• Opting for international best practices that bring up to date
investigation and processing tools, and new design and analytical
approaches;
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• Accepting new design ideas: dewatering wells installation, flexible
and cost effective planning;
• Updating and Re-running coupled dewatering models and slope
stability analysis during the mine development; and
• Using monitoring data to keep close control on the dewatering
system performance, and updating the models accordingly
Take Away Statement
Thank you for your attention
© SRK Consulting (UK) Ltd 2011. All rights reserved.
Dr Houcyne El Idrysy
Senior Hydrogeologist
SRK Consulting
hidrysy@srk.co.uk
Take Away Statement