Groundwater Control for Construction

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A overview of design and practice for dewatering and groundwater control for construction projects

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Groundwater Control for Construction

  1. 1. GROUNDWATER CONTROL FOR www.preene.com CONSTRUCTION Dr Martin Preene Preene Groundwater Consulting June 2014
  2. 2. www.preene.com GROUNDWATER CONTROL Definition Groundwater 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)
  3. 3. www.preene.com GROUNDWATER CONTROL Synopsis • Definitions • Approaches to groundwater control: – by exclusion – by pumping • Approaches to design • A bit of history and dewatering philosophy
  4. 4. www.preene.com 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)
  5. 5. HOW DO WE GET GOOD DESIGN? www.preene.com Data Information Knowledge Wisdom
  6. 6. HOW DO WE GET GOOD DESIGN? www.preene.com Data Information Knowledge Wisdom Theory Projects Screw ups Good dewatering design
  7. 7. www.preene.com GROUNDWATER
  8. 8. www.preene.com GROUNDWATER CONTROL Two main philosophies of groundwater control • Exclusion: Physical cut-off walls • Pumping: Arrays of wells or sumps (construction dewatering)
  9. 9. EXCLUSION: VERTICAL CUT-OFF WALLS www.preene.com Cut-off walls penetrate into underlying low permeability stratum
  10. 10. EXCLUSION: CUT-OFF WALLS AND PUMPED WELLS www.preene.com Cut-off walls do not reach deep impermeable stratum: dewatering wells are needed
  11. 11. EXCLUSION: VERTICAL CUT-OFF AND HORIZONTAL BARRIERS www.preene.com Cut-off walls do not reach deep impermeable stratum: horizontal barrier is used to exclude groundwater from base
  12. 12. www.preene.com 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)
  13. 13. www.preene.com STEEL SHEET-PILING Circular sheet-pile cofferdam with concrete walings
  14. 14. CONCRETE DIAPHRAGM WALLS www.preene.com Circular concrete diaphragm wall
  15. 15. CONCRETE DIAPHRAGM WALLS www.preene.com Rope operated diaphragm wall grab Construction sequence for diaphragm walls from Woodward (2005): An Introduction to Geotechnical Processes Source: Bachy Soletanche Rockmill diaphragm wall cutter Source: Cementation Skanska
  16. 16. www.preene.com BORED PILE WALLS Secant pile wall exposed showing unreinforced female piles and reinforced male piles (Source: Bachy Soletanche)
  17. 17. www.preene.com BENTONITE SLURRY WALLS Bentonite slurry wall constructed by long reach excavator Source: Arup
  18. 18. www.preene.com BENTONITE SLURRY WALLS Bentonite-cement slurry wall constructed by long reach excavator Common European practice Soil-bentonite slurry wall constructed by long reach excavator Common North American practice
  19. 19. www.preene.com PERMEATION GROUTING • Cement-based grouts in coarse soils and fissured rocks • Micro-fine cement grouts and chemical grouts (gels) in lower permeability soils
  20. 20. www.preene.com JET GROUTING
  21. 21. ARTIFICIAL GROUND FREEZING www.preene.com
  22. 22. ARTIFICIAL GROUND FREEZING Artificial ground freezing system around a shaft Source: British Drilling and Freezing Co. Ltd www.preene.com
  23. 23. ARTIFICIAL GROUND FREEZING (BRINE) www.preene.com AGF using brine circulation Brine freeze plant Source: British Drilling and Freezing Co. Ltd
  24. 24. ARTIFICIAL GROUND FREEZING (LN) Schematic diagram of liquid nitrogen (LN) freezing system www.preene.com
  25. 25. GROUNDWATER CONTROL BY PUMPING www.preene.com
  26. 26. www.preene.com SURFACE WATER CONTROL Groundwater control alone cannot keep an excavation dry. Surface water must also be controlled Poor control of surface water
  27. 27. www.preene.com 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
  28. 28. SURFACE WATER CONTROL METHODS www.preene.com • 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)
  29. 29. GROUNDWATER CONTROL BY PUMPING www.preene.com Available Techniques • Sump pumping • Wellpoints • Deepwells • Ejector wells
  30. 30. www.preene.com SUMPING APPLICATIONS Drainage trench used to feed water to sump formed from concrete manhole ring Source: WJ Groundwater
  31. 31. www.preene.com WELLPOINTS Excavation in sand using wellpoint dewatering Source: WJ Groundwater
  32. 32. www.preene.com DEEPWELLS From CIRIA Report C515 (2000): Groundwater Control: Design and Practice
  33. 33. www.preene.com EJECTOR WELLS From CIRIA Report C515 (2000): Groundwater Control: Design and Practice
  34. 34. RANGE OF APPLICATION OF METHODS www.preene.com Amount of lowering of groundwater level Low permeability (silts) High permeability (gravels) From CIRIA Report C515 (2000): Groundwater Control: Design and Practice
  35. 35. WHAT IS DEWATERING DESIGN? www.preene.com 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)
  36. 36. UNDERSTANDING THE PROBLEM www.preene.com • 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?
  37. 37. DEFINING PERFORMANCE TARGETS • Are we trying to reduce groundwater levels or to reduce www.preene.com 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
  38. 38. www.preene.com 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
  39. 39. www.preene.com 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
  40. 40. www.preene.com ACCURACY OF DESIGN
  41. 41. www.preene.com APPROACHES TO DESIGN • Empirical • Analytical • Numerical • Observational
  42. 42. www.preene.com 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
  43. 43. www.preene.com 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
  44. 44. www.preene.com 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
  45. 45. 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 www.preene.com
  46. 46. www.preene.com 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
  47. 47. 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, www.preene.com 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
  48. 48. 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 www.preene.com 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
  49. 49. www.preene.com 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.
  50. 50. GROUNDWATER CONTROL FOR www.preene.com CONSTRUCTION Dr Martin Preene Preene Groundwater Consulting June 2014

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