This presentation reviews the dewatering and groundwater control measures that can be used for open pit mines and surface quarries

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DEWATERING OF OPEN PIT MINES AND
QUARRIES
Dr Martin Preene
Preene Groundwater Consulting
August 2015

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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
Definition
Groundwater 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

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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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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 adits
and 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 water
in 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 system
Image 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 Preene
Preene Groundwater Consulting
August 2015
Email: mp@preene.com