1. Mine dewatering for pit slope stability
Concepts and Studies
By Dr Houcyne El Idrysy
Astana, Kazakhstan, 31 March 2014

2. Presentation Topics Groundwater flow and pore pressure concepts
Hydrogeological conceptual models
Development of numerical groundwater models
Optimisation of pit dewatering/depressurization and input into mine design
Design of pit dewatering/depressurization and monitoring
Conclusions
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Take Away Statement

3. Relationship between pore water pressure and rock strength
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.
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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.

4. Pore water pressure versus phreatic surface
Dewatering well
Drain
Open pit Profile
Dewatered formation
Depressurised formation
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Considering only a phreatic surface in the slope stability analysis is not enough for the design of optimal pit slopes

5. 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
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
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Take Away Statement

6. 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.
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
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Take Away Statement

7. Step 1: Conceptual Hydrogeological Model
3D View
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Take Away Statement

8. Step 2: Numerical Groundwater Model (Example)
Criteria:
Scale
Layers
Resolution
Boundaries
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GMS

9. 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?
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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10. Model Calibration (mostly pre-mining conditions)
BH ID
Observed
head (mAD)
Simulated
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
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Other criteria to verify/check: Stream flows - Flow budget (balance)

11. Groundwater Model Sensitivity Analysis
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Take Away Statement

12. Predictive Modelling: Dewatering Impact
Local Model extent
Regional Model extent
Other possible impact could be:
Loss in river or spring flow
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13. Predictive Modelling: Inflows into the Mine
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
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14. Predictive Modelling: Pore Pressure Distribution
Hydraulic Head (masl)
Pit floor
Drains
Drains
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Take Away Statement

15. Predictive Modelling: Achieved depressurization
X section
Pit floor
North
South
Open pit Profile
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Pit floor

16. 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
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
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17. 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;
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
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18. 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
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
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19. 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;
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
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Take Away Statement

20. Thank you for your attention
Dr Houcyne El Idrysy
Senior Hydrogeologist
SRK Consulting
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Take Away Statement