Mine dewatering for pit slope stability

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

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 © SRK Consulting (UK) Ltd All rights reserved. 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. © SRK Consulting (UK) Ltd All rights reserved. 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 Drain Open pit Profile Dewatered formation Depressurised formation © SRK Consulting (UK) Ltd All rights reserved. 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 © SRK Consulting (UK) Ltd All rights reserved. Take Away Statement

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 © SRK Consulting (UK) Ltd All rights reserved. Take Away Statement

Step 1: Conceptual Hydrogeological Model

Step 2: Numerical Groundwater Model (Example)

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 © SRK Consulting (UK) Ltd All rights reserved.

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 © SRK Consulting (UK) Ltd All rights reserved. Other criteria to verify/check: Stream flows - Flow budget (balance)

Groundwater Model Sensitivity Analysis

Predictive Modelling: Dewatering Impact
Local Model extent Regional Model extent Other possible impact could be: Loss in river or spring flow © SRK Consulting (UK) Ltd All rights reserved.

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 © SRK Consulting (UK) Ltd All rights reserved.

Predictive Modelling: Pore Pressure Distribution

Predictive Modelling: Achieved depressurization

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 © SRK Consulting (UK) Ltd All rights reserved.

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 © SRK Consulting (UK) Ltd All rights reserved.

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 © SRK Consulting (UK) Ltd All rights reserved.

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 © SRK Consulting (UK) Ltd All rights reserved. Take Away Statement