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 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

• • 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 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 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.

• 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 3D View Take Away Statement

Step 2: Numerical Groundwater Model (Example) Criteria: Scale Layers Resolution 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?

• 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

Model Calibration (mostly pre-mining conditions)

BH ID GW13 GW14 GW15 GW16 GW17 GW18 GW19 GW20 GW1 GW2 GW3 GW4 GW5 GW6 GW7 GW8 GW9 GW10 GW11 GW12 Observed head (mAD) Simulated head (mAD) 262.18

262.4

260.74

260.75

260.5

259.2

258.3

256.17

263.72

263.51

260.39

263.43

264.85

257.63

259.65

259.88

258.04

259.13

259.38

260.43

261.5

261.58

260.82

260.47

259.95

259.08

260.03

256.38

266.46

260.06

266.12

272.7

273.15

258.25

262.01

261.6

258.26

257.46

262.11

261.61

Error / Residual, m -8.3

-0.62

-2.36

-1.72

-0.22

1.67

-2.73

-1.18

0.69

0.82

-0.08

0.28

0.55

0.12

-1.73

-0.21

-2.74

3.45

-5.73

-9.27

275 270 265 260 255 250 250

Comparison observed vs simulated heads

255 260 265

Observed water level (masl) Other criteria to verify/check: Stream flows - Flow budget (balance)

270 275

Groundwater Model Sensitivity Analysis 38 38 37 37 36 36 35 35 34 34 33 0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Multiplier (of calibrated parameters) 1.3

1.4

1.5

Kh- Lay 1 and 2 Kh- Lay3 Kh - Lay4 Kh - Lay5 Kh - Lay6 Kh - Lay7 Kh - Lay8 Kh - Lay9 Kh - Lay10 Kh - Lay11-13 Aquifer Recharge Kz - Lay1 and 2 Kz - Lay3 Kz - Lay4 Kz - Lay 5 Kz - Lay6 Take Away Statement

Predictive Modelling: Dewatering Impact Local Model extent Regional Model extent

Other possible impact could be:

Loss in river or spring flow

Predictive Modelling: Inflows into the Mine Groundwater Inflow depth of the Pit 350 300 250 200 150 100 50 0 0 2 4 6 8

Elapsed time since start of mining (years)

10 300 250 200 150 100 50 12 0 500 450 400 350

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)

Pit floor

Drains Take Away Statement

Predictive Modelling: Achieved depressurization

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 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; 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 • 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; • 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

Dr Houcyne El Idrysy Senior Hydrogeologist SRK Consulting [email protected]

Take Away Statement