Regulations for Groundwater Dewatering during Construction ...
Groundwater Control for Construction
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www.preene.c om GROUNDWATER CONTROL FOR CONSTRUCTION Dr Martin Preene Preene Groundwater Consulting June 2014
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A overview of design and practice for dewatering and groundwater control for construction projects
Transcript of Groundwater Control for Construction
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GROUNDWATER CONTROL FOR CONSTRUCTION
Dr Martin PreenePreene Groundwater ConsultingJune 2014
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GROUNDWATER CONTROL
DefinitionGroundwater Control“The process of temporarily dealing with groundwater, to allow excavations
to be made in dry and stable conditions below natural groundwater level”
May be known as Dewatering or Construction Dewatering or Groundwater Lowering
Additional definition:Permeability = coefficient of permeability = hydraulic conductivity (expressed in m/s)
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GROUNDWATER CONTROL
Synopsis
• Definitions
• Approaches to groundwater control:
– by exclusion
– by pumping
• Approaches to design
• A bit of history and dewatering philosophy
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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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HOW DO WE GET GOOD DESIGN?
Data
Information
Knowledge
Wisdom
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HOW DO WE GET GOOD DESIGN?
Data
Information
Knowledge
Wisdom
Theory
Projects
Screw ups
Good dewatering design
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GROUNDWATER
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GROUNDWATER CONTROL
Two main philosophies of groundwater control
• Exclusion: Physical cut-off walls
• Pumping: Arrays of wells or sumps (construction dewatering)
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EXCLUSION: VERTICAL CUT-OFF WALLS
Cut-off walls penetrate into underlying low permeability stratum
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EXCLUSION: CUT-OFF WALLS AND PUMPED WELLS
Cut-off walls do not reach deep impermeable stratum: dewateringwells are needed
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EXCLUSION: VERTICAL CUT-OFF AND HORIZONTAL BARRIERS
Cut-off walls do not reach deep impermeable stratum: horizontal barrier is used to exclude groundwater from base
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EXCLUSION TECHNIQUES
• Displacement barriers– Steel sheet-piles
• Excavated barriers– Concrete diaphragm walls– Bored pile walls (secant pile walls and contiguous pile walls)– Bentonite slurry walls and trenches
• Injected barriers– Permeation grouting– Rock grouting– Jet grouting– Mix-in-place methods
• Artificial ground freezing
• Compressed air (for tunnels and shafts)
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STEEL SHEET-PILING
Circular sheet-pile cofferdam with concrete walings
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CONCRETE DIAPHRAGM WALLS
Circular concrete diaphragm wall
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CONCRETE DIAPHRAGM WALLSRope operated diaphragm wall grab
Construction sequence for diaphragm wallsfrom Woodward (2005): An Introduction to Geotechnical Processes
Source: Bachy Soletanche
Rockmill diaphragm wall cutter
Source: Cementation Skanska
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BORED PILE WALLS
Secant pile wall exposed showing unreinforced female piles and reinforced male piles
(Source: Bachy Soletanche)
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BENTONITE SLURRY WALLS
Bentonite slurry wall constructed by long reach excavatorSource: Arup
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BENTONITE SLURRY WALLS
Bentonite-cement slurry wall constructed by long reach excavatorCommon European practice
Soil-bentonite slurry wall constructed by long reach excavatorCommon North American practice
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PERMEATION GROUTING
• Cement-based grouts in coarse soils and fissured rocks
• Micro-fine cement grouts and chemical grouts (gels) in lower permeability soils
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JET GROUTING
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ARTIFICIAL GROUND FREEZING
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ARTIFICIAL GROUND FREEZING
Artificial ground freezing system around a shaft
Source: British Drilling and Freezing Co. Ltd
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ARTIFICIAL GROUND FREEZING (BRINE)
AGF using brine circulation Brine freeze plantSource: British Drilling and Freezing Co. Ltd
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ARTIFICIAL GROUND FREEZING (LN)
Schematic diagram ofliquid nitrogen (LN) freezingsystem
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GROUNDWATER CONTROL BY PUMPING
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SURFACE WATER CONTROL
Groundwater control alone cannot keep an excavation dry. Surface water must also be controlled
Poor control of surface water
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SURFACE WATER CONTROL
Groundwater control alone cannot keep an excavation dry. Surface water must also be controlled
Poor control of surface water Adequate control of surface water
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SURFACE WATER CONTROL METHODS
• Source control- intercept run-off before it reaches the excavation- prevent unnecessary generation of water in the excavation- collect water as soon as it reaches the work area (or before!)
• Water collection - French drains to intercept run off- collector drains and sumps- pumping systems (keep it simple!)
• Water treatment- solids removal (settlement tanks, Siltbusters)
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GROUNDWATER CONTROL BY PUMPING
Available Techniques
• Sump pumping• Wellpoints• Deepwells• Ejector wells
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SUMPING APPLICATIONS
Drainage trench used to feed water to sump formed from concrete manhole ring
Source: WJ Groundwater
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WELLPOINTS
Excavation in sand using wellpoint dewatering
Source: WJ Groundwater
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DEEPWELLS
From CIRIA Report C515 (2000): Groundwater Control: Design and Practice
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EJECTOR WELLS
From CIRIA Report C515 (2000): Groundwater Control: Design and Practice
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RANGE OF APPLICATION OF METHODS
Amount of lowering of groundwater level
Low permeability (silts) High permeability (gravels)
From CIRIA Report C515 (2000): Groundwater Control: Design and Practice
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WHAT IS DEWATERING DESIGN?
Dewatering design is NOT about:
• Developing excessively complex models and concepts
• Trying to ‘simulate’ reality
The objective of design should be to:
• Allow engineering and commercial decisions to be made
• Focus on appropriate level of detail relevant to key issues for your problem (e.g. technology selection, flow rate estimate, environmental impacts)
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UNDERSTANDING THE PROBLEM
• Site investigation (field and desk studies)
• Why is groundwater control required – what are the objectives?
• What are the aquifer conditions?
• What is the likely range of permeability?
• What are the practical and environmental constraints?
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DEFINING PERFORMANCE TARGETS
• Are we trying to reduce groundwater levels or to reduce pore water pressures?
• What are the target groundwater levels or pressures, and where (plan location and stratum) do we need to achieve these effects?
• Required timescale?
• Need for standby/back up systems?
• Any specified environmental mitigation?
• Flow rate is NOT normally a performance target
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SELECTING TECHNOLOGIES
Technologies may not be interchangeable
Amount of lowering of groundwater level
Low permeability (silts) High permeability (gravels)
From CIRIA Report C515 (2000): Groundwater Control: Design and Practice
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ESTIMATING FLOW RATE
• The pumped flow rate from a dewatering system is often the key parameter to be estimated
• It will influence the capacity of the dewatering system
• It may control selection of the technology
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ACCURACY OF DESIGN
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APPROACHES TO DESIGN
• Empirical
• Analytical
• Numerical
• Observational
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EMPIRICAL METHODS
• A great many dewatering systems are not ‘designed’ in the formal sense
• They are selected based on established ‘rules of thumb’
• A key issue is that site conditions must appropriate to the assumptions behind the rule of thumb
• Problems occur if the rules of thumb are applied (knowingly or unknowingly) in inappropriate conditions
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ANALYTICAL METHODS
• Use of ‘textbook’ equations on paper or by spreadsheet
• A wide variety of analytical equations are available. It is important that the appropriate one(s) are selected for site conditions
• Need to have a conceptual model FIRST in order to be able to select the appropriate analytical method
• Use of inappropriate analytical equations will give gross errors
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NUMERICAL METHODS
• Use of numerical models (e.g. finite difference and finite element models)
• Two-dimensional or three-dimensional models• Steady state or transient models can be
constructed• Proprietary software packages are used
(MODFLOW, FEFLOW, SEEP/W, etc)• Need a conceptual model first• Used when there is very good (or sometimes very
bad!) data
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OBSERVATIONAL METHODS
• An application of the established geotechnical Observational Method which is sometimes called ‘design as you go’
• Is a formalised, step by step approach• Develop a conceptual model and make design
predictions (by analytical or numerical methods)• Have defined monitoring programme with ‘trigger
levels’• If trigger levels are reached, defined additional
dewatering measures are put into place
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KEY FACTORS IN DESIGN
• Good design is all about getting the right conceptual model at an early stage
• This will allow better selection of appropriate design methods
• And allow the selection of appropriate methods and technologies
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A BIT OF HISTORY AND PHILOSOPHY
• Some of the basic theory used in dewatering design pre-dates the birth of soil mechanics theory– Darcy’s law (1856) – dynamics of laminar groundwater flow– Dupuit equations (1863) – well equations– Hazen’s rule (1892) – hydraulic conductivity of uniform sands
• An understanding of the theory of groundwater flow is essential, but is not enough
• Getting the conceptual model right is fundamental• An understanding of the capability and limitations of pumping and
exclusion technologies is also required – you have to get the technology right
• A little bit of local experience goes a long way
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A BIT OF HISTORY AND PHILOSOPHY
• In general dewatering design is not ‘codified’
• There are few prescriptive design and practice rules to be followed
• Eurocode 7 (BS EN 1997-1: 2004) includes a short (1 page) section on dewatering, but this is not prescriptive, and only gives generic guidance on good practice.
• I am not aware of any English language prescriptive dewatering design codes (those that exist are generic)
• One of the main existing design documents is CIRIA Report C515 Groundwater Control: Design and Practice (2000). The original document was written in 1997 and is currently being updated by CIRIA for 2014
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AND FINALLY….
Hugh Golder on dewatering
“The limitations of the different systems which are available are practical rather than theoretical and the design of a system is no task for an optimist. A sound engineer with a melancholy outlook, whose life has been a series of unhappy trials, is the best man to plan a water-lowering system.”
H. Q. Golder and J. L. Seychuk“Soil Problems in Subway Construction”3rd Pan-American Conference on Soil Mechanics and Foundation Engineering, Caracas, Venezuela, July 1967, pp. 203-240.
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GROUNDWATER CONTROL FOR CONSTRUCTION
Dr Martin PreenePreene Groundwater ConsultingJune 2014
