Dewatering for open pit mines and quarries
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www.preene.c om DEWATERING OF OPEN PIT MINES AND QUARRIES Dr Martin Preene Preene Groundwater Consulting August 2015
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Transcript of Dewatering for open pit mines and quarries
www.preene.com
DEWATERING OF OPEN PIT MINES AND QUARRIES
Dr Martin PreenePreene Groundwater ConsultingAugust 2015
www.preene.com
MINE AND QUARRY DEWATERING
Synopsis
• Background and definitions
• Groundwater control techniques:
– by pumping
– by exclusion
• Fundamentals of dewatering
• Some thoughts on the future
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PRACTICE PROFILE
Preene Groundwater Consulting is the Professional Practice of Dr Martin Preene and provides specialist advice and design services in the fields of dewatering, groundwater engineering and hydrogeology to clients worldwide
Dr Martin Preene has more than 25 years’ experience on projects worldwide in the investigation, design, installation and operation of groundwater control and dewatering systems. He is widely published on dewatering and groundwater control and is the author of the UK industry guidance on dewatering (CIRIA Report C515 Groundwater Control Design and Practice) as well as a dewatering text book (Groundwater Lowering in Construction: A Practical Guide to Dewatering)
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DEFINITIONS
DefinitionGroundwater Control“The process of temporarily dealing with groundwater, to allow
excavations to be made in workably dry and stable conditions below natural groundwater level”
May be known as Dewatering or Groundwater Lowering
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BENEFITS OF MINE DEWATERING
Workably dry conditions – avoid being flooded out and provide more efficient operational conditions• Improved trafficability and digging• Better blasting conditions• Reduced moisture content of ore/product and waste
Stable conditions – geotechnical stability• Reduction in pore water pressures• Allows steeper side slopes and increased factors of safety• Avoid base heave where confined aquifers are below working level• Reduced risk of erosion and piping of weak zones in slopes
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MINE DEWATERING
There are three aspects to the design and implementation of mine dewatering
An understanding of:
• Hydrogeology
• Dewatering technology
• Environmental sensitivities and regulation
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MINE DEWATERING
There are three aspects to the design and implementation of mine dewatering
An understanding of:
• Hydrogeology
• Dewatering technology
• Environmental sensitivities and regulation
www.preene.com
WATER MANAGEMENT FOR MINING
Water Management
Treatment if necessary
PumpingExclusion
Groundwater controlSurface water control
Diversion Pumping
Disposal to Waste
Environmental Mitigation
Beneficial use
On site Off site
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WATER MANAGEMENT FOR MINING
Water Management
Treatment if necessary
PumpingExclusion
Groundwater controlSurface water control
Diversion Pumping
Disposal to Waste
Environmental Mitigation
Beneficial use
On site Off site
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MINE DEWATERING
Two main approaches to mine dewatering:
• Pumping: In-pit pumping from sumps or external pumping from arrays of wells
• Exclusion: Physical cut-off walls, grouting and artificial ground freezing to reduce groundwater inflows
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IN-PIT PUMPING
In-pit pumping is used to pump from sump areas within the pit
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IN-PIT PUMPING
In-pit pumping is used to pump from sump areas within the pit
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PERIMETER DEWATERING WELLS
Perimeter dewatering wells are used to intercept lateral groundwater flow into the pit and lower groundwater levels in advance of mining
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PERIMETER DEWATERING WELLS
Perimeter dewatering wells are used to intercept lateral groundwater flow into the pit and lower groundwater levels in advance of mining
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SUB-HORIZONTAL SLOPE DRAINS
Where perimeter dewatering wells are used to dewater the pit, the pore water pressures in the slope may drain more slowly. Sub-horizontal drains can be used to depressurise pit slopes
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ARTIFICIAL RECHARGE
Artificial recharge (via recharge wells or trenches) may be used to dispose of water as part of environmental mitigation strategies
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LOCALISED SLOPE DEWATERING
Wellpoint or ejectors systems may be used to stabilise granular overburden or sands and gravels in pit slopes
First stage wellpoints
Second stage wellpoints
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DRAINAGE TUNNELS AND ADITS
Drainage aditsand tunnels may be used to provide large scale drainage behind rock slopes
Pumped drainage adit with drain holes
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EXCLUSION METHODS
Low permeability barriers or cut-off walls may be used to exclude waterin key strata or in zones where seepage is a concern (e.g. when mining next to rivers or lakes or when the mine has to pass through a highly permeable superficial deposit)
Cut off wall
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EXCLUSION METHODS
Cut-off walls – physical barriers of lower permeability materials driven or excavated into the ground
One of the common methods is a bentonite slurry wall, where a narrow trench is excavated by backhoe or grab and is then backfilled with cement-bentonite or a soil-bentonite mixture
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EXCLUSION METHODS
Grouting and artificial ground freezing – modifying the properties of the in-situ ground to reduce hydraulic conductivity and exclude groundwater from alluvial or drift deposits or to seal off preferential flow along permeable strata or fissured zones
Rock grouting Fluid grout is pumped to the ground to fill rock fissures with little or no disturbance of the rock fissures
Artificial ground freezing for sinking of mine shaft: Image source: British Drilling and Freezing Co. Ltd
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FUNDAMENTALS OF DEWATERING
• Dewatering is typically a ‘distress purchase’ – it is only done when necessary
• There is therefore a focus on cost minimisation– CAPEX
• Pump and equipment capital cost• Installation and commissioning
– OPEX• Power cost• Maintenance and replacement equipment• Monitoring
• But it should be remembered that there are trade offs between dewatering costs and potential benefits to wider mining costs
• Monitoring (e.g. of pumped flow rates and groundwater levels) is vital for effective management of dewatering
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FUNDAMENTALS OF DEWATERING
• Energy costs are a key part of OPEX of pumping systems
• Energy costs (and associated carbon emissions) can be reduced by:– Reducing flow rate Q– Reducing total head H– Reducing hours run t– Improving pump efficiency η
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POSSIBLE FUTURE TRENDS
Three areas for discussion where mine dewatering may change in the future :
• Improved efficiency
• Technology transfer for new techniques
• Alternative business models to procure dewatering
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IMPROVED EFFICIENCY
• This can be an economic reality on almost any site
• 4G connection can make hard wired data connections obsolete
• Investment in inverter (variable speed) drives can allow significant ‘automation’ of systems to give more efficient operations
• Initial investment costs are reducing as technology develops, payback is in the form of reduced energy costs, reduced carbon emissions and increased equipment life
Example of remote interface for control and monitoring of dewatering systemImage source: Holscher Wasserbau
Real Time Remote Monitoring and Control
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IMPROVED EFFICIENCY
Pump Optimisation and Improved Materials
Image source: Weir Minerals
Image source: Angus Flexible Pipelines
Highwall dewatering pump
Flexible rising main for borehole pumps
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TECHNOLOGY TRANSFER
HDD (Horizontal Directional Drilling) of Dewatering Wells
Image source: Golder Associates
• Not new technology• Has been used to stabilise
polders in Holland• Has been used to extract
leachate from beneath closed landfills
• Used or proposed on a small number of open pit mines to date
• There are practical challenges with well installation and development
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TECHNOLOGY TRANSFER
Removal of Suspended Solids from Dewatering Water• Technology in wide use in industrial processes, water supply
and construction industry
Image source: Cornelsen Limited
Image source: SLD Pumps & Power
Image source: Siltbuster Limited
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ALTERNATIVE BUSINESS MODELS
• The traditional business model for pumped dewatering is sale/lease/hire of pumps and ancillary equipment and then maintenance either by the mine operator site team or under a maintenance contract
• This is not really a ‘dewatering service’
• A mine or quarry wants to produce product economically (maximise output, minimise unit cost), they don’t care about the pumps per se
• Can we move to a dewatering service more focused on what is more important for the mine or quarry operator?
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ALTERNATIVE BUSINESS MODELS
• In the airline industry Rolls Royce, Pratt & Whitney and GE no longer sell engines – they offer a service based on ‘thrust hours’ – focusing on what the client needs – Rolls Royce has trademarked the term ‘Power by the Hour’
• Generically known as ‘performance-based logistics’
• Made possible by long term agreements and remote monitoring and control, contractual arrangements where the service provider has vested interest in reducing energy usage, and improving efficiency
• Client shifts the main investment from CAPEX to OPEX and gains certainty of cost over mine life, and confidence that pumping availability will not affect mining
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A FINAL THOUGHT
It is possible that there are some unheralded technologies, apparently far removed from mining and quarrying that will have a significant impact on mine dewatering in the future.
History shows that any consideration of the future needs to be tempered by Amara’s law, which states:
We tend to overestimate the effect of a technology in the short run and underestimate the effect in the long run.
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DEWATERING OF OPEN PIT MINES AND QUARRIES
Dr Martin PreenePreene Groundwater ConsultingAugust 2015
Email: [email protected]
