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ENVIRONMENTAL IMPACTS OF
GROUNDWATER CONTROL SYSTEMS
Dr Martin Preene
Preene Groundwater Consulting
July 2014

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SYNOPSIS
• Methods of groundwater control
• Indicative factors for potential impacts from
groundwater control
• Categories of potential impacts
• Monitoring and mitigation
• Case history examples

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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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GROUNDWATER CONTROL
Two main approaches to groundwater control
• Exclusion: Physical cut-off walls
• Pumping: Arrays of wells or sumps (construction
dewatering)

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GROUNDWATER CONTROL BY EXCLUSION
Cut-off walls penetrate into
underlying low permeability
stratum

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GROUNDWATER CONTROL BY PUMPING

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INDICATIVE FACTORS FOR IMPACTS
• It is difficult to provide general indicators of potential impacts
• In reality the potential for impacts is largely controlled by the site
setting and is hydrogeology dependent
• Scale and duration of dewatering activities is not necessarily a good
indicator, although the ‘zone of influence’ can be a useful indicator
• Lack of pumping does not mean there is no potential for impacts
• Need to know what a ‘receptor’ looks like (so we can identify if any
are present and where they are) and how it may be affected
• Therefore need to categorise impacts

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POTENTIAL CATEGORIES OF IMPACTS
• Several different ways to categorise potential impacts from
groundwater control activities:
– Geotechnical impacts
– Contamination impacts
– Water dependent feature impacts
– Water resource impacts
– Water discharge impacts

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GEOTECHNICAL IMPACTS
• Definition: Impacts where the geotechnical properties or state
of the ground are changed by groundwater control
• Ground settlement – Effective stress increases
• Ground settlement – Loss of material

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GEOTECHNICAL IMPACTS
Ground settlement – increase in effective stress, and hence ground
settlement, will occur every time groundwater levels are lowered

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GEOTECHNICAL IMPACTS
Ground settlement – effective stress increases
In the great majority of cases, movements are so small that no
distortion or damage is apparent in nearby structures
• There will be some circumstances when the risk of damaging
settlements may be significant
The principal relevant factors include:
• Ground conditions (e.g. soft alluvial soils)
• Depth of drawdown
• Period of dewatering
• Distance to nearby structures
• Sensitivity of structures

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GEOTECHNICAL IMPACTS
Ground settlement – loss of material
Most commonly associated with poorly controlled sump pumping, particularly in
soils with significant mobile fine particles
This settlement risk is not inevitable and can be avoided
Image source:
Cashman and Preene (2012)

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GEOTECHNICAL IMPACTS
Ground settlement – loss of material. Can give erratic and unpredictable
ground settlement

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CONTAMINATION IMPACTS
• Definition: Impacts where pre-existing ground or groundwater
contamination is mobilised, transported and/or where transmission
pathways are created
Horizontal migration of contaminants from neighbouring sites

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CONTAMINATION IMPACTS
• Vertical migration can be a problem as well as horizontal migration
Image source:
Cashman and Preene (2012)
Poor well design – screens and filters
cross aquifers without seals
Wells and excavations penetrate confining
layers and create pathways from surface
to aquifer at depth

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WATER DEPENDENT FEATURE IMPACTS
• Definition: Impacts where groundwater flows, levels and/or quality
are affected in water dependent features (natural or artificial)
Image source:
Cashman and Preene (2012)
Depletion of ponds or wetlands
Reduction in yield
of springs

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WATER DEPENDENT FEATURE IMPACTS
• The barrier effect of cut-off walls can also cause impacts
Image source:
Cashman and Preene (2012)

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WATER RESOURCE IMPACTS
• Definition: Impacts where water availability or water quality (including
saline intrusion) are affected either at defined abstraction points (wells or
springs) or in known water resource units (aquifers)
Image source:
Cashman and Preene (2012)

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WATER RESOURCE IMPACTS
• Large structures (e.g. metro stations) or groups of structures can
also cause barrier impacts over wide areas, by blocking
groundwater flow or reducing the aquifer cross-sectional area
Image source:
Cashman and Preene (2012)

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WATER DISCHARGE IMPACTS
• Definition: Impacts where the discharge of water from pumping
systems impacts on the receiving environment (surface water or
groundwater, where recharge wells are used)
Photo: Toby Roberts

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WATER DISCHARGE IMPACTS
• Water treatment may be needed prior to discharge, most
commonly for removal of suspended solids
Photo: Siltbuster Limited

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MONITORING AND MITIGATION
• Monitoring and mitigation of potential impacts are closely linked
• They require a solid understanding of the conceptual hydrogeological model for
the site and its environs. The conceptual model should define:
– The aquifer types and potential vulnerability to groundwater impacts
– The depth and extent of the excavation and the proposed method of groundwater
control, and the duration of pumping where relevant
– The presence of any nearby sensitive groundwater receptors (wetlands, third party
water wells, etc.)
– The geotechnical properties at the site (compressible strata, etc.)
– The presence of any groundwater contamination in the vicinity of the site (not only in
the stratum being dewatered, but also in any strata above or below)
• The conceptual model should allow identification of the most significant potential
groundwater impacts which could result from the proposed dewatering. This
should be used to direct the design of the groundwater control system and the
associated mitigation and monitoring measures

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MONITORING
Baseline (pre-construction) monitoring
• Monitoring planning should be based on the site investigation, including a desk study, to
allow hydrogeological conditions and environmental receptors to be identified
• It is prudent to have pre-construction monitoring of groundwater levels, spring flows, ground
levels, etc. to determine baseline conditions against which any impacts can be assessed. This
requires early access to site, or sourcing of third party data
• If settlement damage to structures is a concern, pre-construction building condition surveys
may be appropriate within the predicted zone of influence
Operational monitoring regime
• Monitoring of groundwater levels and pumped flow rates is a routine and necessary part of
any groundwater control scheme
• However, where environmental impacts are assessed to be of concern then operational
monitoring assumes even greater importance. Additional monitoring may include:
– surveying of ground levels
– regular inspection of structures at risk of settlement
– water quality monitoring (to assess migration of groundwater contamination)
– monitoring of conditions in water-dependent features such as rivers and wetlands

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MITIGATION
• Mitigation measures are intended to avoid, reduce or ‘compensate
for’ the impacts of dewatering
• Mitigation should actually begin with the selection of the
dewatering approach and/or technology
• For example:
– Exclusion methods to reduce or avoid pumping could be used if there
is concern that groundwater levels may be widely lowered, third party
wells affected or groundwater resources depleted
– Conversely, the barrier effect when cut-off walls of large lateral extent
act to dam groundwater flow may militate against the use of the
exclusion technique in some circumstances

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MITIGATION
• The mitigation measures must be developed on a site-specific basis, but can
include:
– Artificial recharge: Groundwater from the pumped discharge can be re-
injected or re-infiltrated back into the ground, either to prevent lowering of
groundwater levels and corresponding ground settlement, or to prevent
depletion of groundwater resources
Image source:
Cashman and Preene (2012)

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MITIGATION
• The mitigation measures must be developed on a site-specific basis, but can
include:
– Targeted groundwater cut-off walls: Where there is a specific receptor to be
protected, such as a wetland or sensitive structure, it may be possible to
install a targeted section of cut-off wall or grout curtain between the
dewatering system and the receptor, to reduce the drawdown at the receptor
Image source:
Cashman and Preene (2012)

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MITIGATION
• The mitigation measures must be developed on a site-specific basis, but can
include:
– Temporary cut-off walls: If there is a concern that permanent cut-off walls will
affect the long term groundwater flow regime, due to the barrier effect, then
it may be possible to use temporary cut-off techniques. For example, steel
sheet-piles that can be withdrawn at the end of the project, or artificial
ground freezing, which will eventually thaw and allow groundwater flow to
pass
– Protection of individual receptors: If there are only a small number of isolated
receptors, it may be more cost effective to simply fix or prevent the problem
directly at the receptor, for example by underpinning the foundations of a
sensitive structure, or by providing a new piped water supply to replace a
residential water supply well where lowering of water levels has reduced the
yield

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EFFECTIVE STRESS SETTLEMENT
 Concerns over building damage due to
effective stress increases are often
voiced by project teams
 In many cases the risk is low, but a
rational approach is needed to assess
the risk. It may not always involve
elaborate analysis or modelling
 The zone of influence is the area around the dewatering system where
groundwater levels are significantly lowered
 The zone of influence could extend for a few tens of metres or several hundred
metres, and settlement occurs only within the zone of influence
 By setting trigger levels of settlement for slight, moderate and severe damage
categories, risk zones can be delineated

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GROUND SETTLEMENT – EFFECTIVE STRESS
Simple risk zones for
radial flow to a small
dewatering system
Risk zones modified in
light of geological
mapping from desk
study

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GROUND SETTLEMENT – EFFECTIVE STRESS
• Need to consider time-dependent
consolidation and different effective
stress changes with depth
– ‘Highly permeable’ strata (sands, gravels,
fissured rocks) respond effectively
instantaneously to drawdown – ‘complete’
settlement will occur during even short
duration projects
– ‘Low to moderately permeable’ strata (silts,
clays) will respond slower to drawdown,
based on consolidation characteristics –
‘complete’ settlement may not occur even
during long duration dewatering
• It is very easy to over-estimate
dewatering-related settlements

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GROUND SETTLEMENT – EFFECTIVE STRESS
• On major projects, settlement is best calculated by dividing the soil or rock
sequence into a series of horizontal layers, and calculating effective stress
and soil/rock stiffness individually

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POWER STATION DEWATERING
• The UK has had three phases of construction of nuclear power
stations
– 1st phase: 1950s – 1970s
– 2nd phase: 1985 – 1995
– 3rd phase: 2011 onwards
• Sites are largely at coastal or estuarine locations (for cooling water
purposes)
• 2nd and 3rd phase sites will be very close neighbours to existing
nuclear power stations (either generating or decommissioned) so
that power transmission infrastructure can be shared or re-used
• The coastal location and depth of foundations typically requires
significant dewatering. Managing the impacts of dewatering is a key
aspect of construction

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POWER STATION DEWATERING
Dewatering of Sizewell B Nuclear
Power Station
1987
Photos: Andrew Hawes

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PERIMETER DIAPHRAGM WALL
Image source: Howden and Crawley (1995)

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DEWATERING MONITORING
Image source: Howden and Crawley (1995)

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SUMMARY
• It is important to realise that groundwater control (even if pumping
is not involved) can cause a range of environmental impacts
• It can be useful to categorise the impacts to help identify sites and
projects which may be impacted. Suggested categories include:
– Geotechnical impacts
– Contamination impacts
– Water dependent feature impacts
– Water resource impacts
– Water discharge impacts
• Monitoring and mitigation measures may be needed, and should be
based on a sound hydrogeological conceptual model

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ENVIRONMENTAL IMPACTS OF
GROUNDWATER CONTROL SYSTEMS
Dr Martin Preene
Preene Groundwater Consulting
July 2014