environmental impacts of groundwater control and dewatering
TRANSCRIPT
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ENVIRONMENTAL IMPACTS OF GROUNDWATER CONTROL SYSTEMS
Dr Martin PreenePreene Groundwater ConsultingJuly 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 materialMost commonly associated with poorly controlled sump pumping, particularly in soils with significant mobile fine particlesThis 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 DEWATERINGDewatering of Sizewell B Nuclear Power Station1987
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 PreenePreene Groundwater ConsultingJuly 2014