lecture 8&9: construction dewatering · pdf fileconstruction dewatering overview...
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Arab Academy for Science, Technology & Maritime Transport
Colleague of Engineering & Technology
Construction & Building Engineering
CB 523 Methods and Equipment for Construction 1
Lecture 8&9:
Construction Dewatering
Instructor: Dr. Ahmed Elyamany
Courtesy of Dr. Ahmed Alhakeem & Dr. Ahmed Alhady
Construction DewateringOverview
• Soil dewatering means the removal of water from thesoil.
• Construction dewatering means Control of thesubsurface and surface water environment in order toallow construction to proceed.
• Techniques for dealing with the problems that resultdepend on the
• excavation dimensions,
• soil type,
• groundwater control requirements
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Construction DewateringOverview
•The simplest dewatering operations are carriedout with little planning.
•Major operations in difficult conditions requireadvanced engineering and construction methods.
•(An aquifer is a permeable geological stratum orformation that can both store and transmit waterin significant quantities).
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Construction DewateringIntroduction
• Construction dewatering can become a costly issue ifoverlooked during project planning.
• In most contracts, dewatering is the responsibility of thecontractor.
• The contractor selects the dewatering method and isresponsible for its design and operation.
• The purpose of construction dewatering is to controlthe surface and subsurface hydrologic environment insuch a way as to permit the structure to be constructed“in the dry.”
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Construction DewateringIntroduction
• If ground water issues are addressed appropriately atthe investigation and design stage, constructiondewatering is rarely a problem.
• Construction dewatering has existed as a specialtyindustry for a long time.
• Consequently, a number of well-established techniqueshave been developed to lower the ground water tableduring excavation.
• The geology, ground water conditions, and type ofexcavation all influence the selection of dewateringtechnology.
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Construction DewateringIntroduction
The most common methods for dewatering includesumps, wells and wellpoints.
• Sumps provide localized, very shallow dewatering (lessthan 3 feet) and consist of pumping from perforateddrums or casings in a gravel-filled backhoe pit. Sumpswork best in tight, fine grained soils, or very coarse,bouldery deposits.
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Construction DewateringIntroduction
• Wells are large-diameter (greater than 6 inches) holes,drilled relatively deep (greater than 10 feet), and containslotted casings and downhole pumps. Wells work best insoils consisting of sand, or sand and gravel mixtures, andcan dewater large areas to great depths.
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Construction DewateringIntroduction
• Wellpoints are small-diameter (less than 6 inches),shallow wells, and are closely spaced (2 to 10 feetapart).
• effectively dewater coarse sands and gravels, or silts and clays.
• They have a wide range of applications.
• Use a vacuum system and their depth is limited to about 25feet.
• systems generally cost more than either sumps or wells, andrequire near-continual maintenance.
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Construction DewateringIntroduction
• Other dewatering techniques include:
oground freezing,
o Electroosmosis,
oVertical Sand Drains,
oWick Drains,
oGrouting, etc.
• However, such techniques are very costly and used only for particularly difficult dewatering applications.
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Construction DewateringUnderwater Excavations
• In special cases where the soil is very pervious or when it is notpossible or desirable to lower the groundwater table, underwaterexcavations can be considered.
• If underwater excavation is to be performed, the work area mustbe enclosed with an impervious structure. Once the imperviousstructure is in place, the excavation is performed within thestructure.
• Once the desired excavation level is achieved within the structure,it is sealed with an impervious layer, such as concrete, in order toprevent water from sipping into the work area.
• After the impervious seal has been constructed, the waterremaining within the structure is pumped out and construction iscompleted.
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Construction DewateringCaissons
• Caisson is a structurethat is constructed atlocation if the project siteis on land, but if theproject site is offshore, itis constructed on landand then floated to thesite offshore.
• In the caisson method ofconstruction, theexcavation is performedfrom within thepermanent structure.
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Construction DewateringCaissons
• After the caisson is in position, excavation from within the caissonstructure begins.
• As the excavation is carried out, the caisson structure starts tosink by its own weight, or if necessary, by imposed loads.
• This procedure continues until the desired foundation level isachieved. Figure 1 shows this process schematically.
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Construction DewateringCaissons
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Construction DewateringCaissons
• By injecting bentonite clay slurry at the soil-structureinterface, adding weight, or in case of cohesive soilsusing jetting, the frictional resistance between thecaisson and the surrounding ground may be significantlyreduced.
• When a pile, or in this case caisson, must be driventhrough dense and hard materials, several driving aidshave been developed.
•The principal function of these driving aids is to speedthe driving operation and to prevent damage to thestructure that results from heavy driving.
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Construction DewateringCaissons
• Jetting is applicable to those situations where structuremust be driven through cohesive soil materials togreater depths.
• Water jets can be used to displace granular soils frombeneath the toe of a pile or caisson.
• Jetting is accomplished by pumping water through pipesattached to the side or center of the structure as it isdriven. The flow of water creates a “quick” conditionand thereby reduces skin friction along the sides of thedriven structure. The result is that the structure drivesmore easily.
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Construction DewateringCaissons
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Construction DewateringCaissons
• During unwatering (pumping the water to outside ofcaisson) a caisson in cohesive soils, the upward flowfrom the surrounding groundwater induces a quickcondition, which results in loss of strength at the bottomof excavation.
• In other words, if the flow is upward then the waterpressure tends to lift the soil element. If the upwardwater pressure is high enough the effective stresses inthe soil disappear, no frictional strength can bemobilized and the soil behaves as a fluid.
• This is the quick condition and is associated with pipinginstabilities around excavations.
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Construction DewateringCaissons
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Quick condition is shown in Figure 3
Construction DewateringCaissons
• To prevent quick condition, the head difference causingflow, i.e. the difference between the groundwater tablelevel and the standing water level within the caisson,should be kept low.
• Caissons should not be used in the case of existingstructures that can be damaged due to loss of groundfrom beneath their foundations.
• At the desired excavation level, an impervious seal isplaced, usually by using tremie concrete.
• Once the Tremie concrete seal is in place, thedewatering of the caisson can begin.
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Construction DewateringCaissons
Construction DewateringCaissons
The tremie concreteplacement method uses a vertical or near vertical pipe, through which concrete is placed by gravity feed below water level.
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What information is neededto properly select and design
a dewatering program?
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Construction DewateringDewatering Analysis
• Understand the objective of the dewatering program:• Dimensions of excavation
• Adjacent structures
• Construction sequence
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Construction DewateringDewatering Analysis
• Soil Data
• Layering
• Soil Properties (permeability and density)
• Rock locations and formation
• Water chemistry (corrosivity)
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Construction DewateringDewatering Analysis
• Hydrology
• Water quantities have two components:• Storage: Water needed to be removed to lower ground water to intended
level (can be up to 2/3 of total pumping volume) …(Impact on schedule!!!)
• Steady-state recharge
• Sequence and duration of work:
• Is uplift a consideration?
• Requirements for pressure relief as the structure is being built
• Must dewatering be relocated as construction proceeds?
• Must portions of the system be installed inside the excavation?
• How / Can we remove the dewatering system afterconstruction? 25
Construction DewateringDewatering Analysis
• Typically water needs to be pumped out of the soil through theuse of various types of pumps
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Construction DewateringDesign of Dewatering Analysis
• Un-confined Aquifer (Water Table Aquifer)
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Q: Quantity of water
pumped
K: Soil permeability
H: Height of ground water
table above impervious layer
h: Height of reduced
ground water table
Ro: Radius of influence
r: Radius of well
Construction DewateringDesign of Dewatering Analysis
• Confined Aquifer
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Construction DewateringDesign of Dewatering Analysis
• Soil Permeability
• The ability of water to flow through a soil
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Construction DewateringDesign of Dewatering Analysis
•Self-Practice Example
In a construction dewatering job in anunconfined aquifer of depth 20m you haveinstalled a 30 m3/h pump. The radius ofinfluence extends for 10m around the well.What is the expected drawdown?
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Construction DewateringDesign of Dewatering Analysis
• Multi-layered Aquifers
In the case where we have different soil layers that make up theaquifer we can calculate the effective permeability of the aquiferas follows:
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where K is the effective
permeability; Ki is the permeability
of layer i; Bi is the thickness of layer
i; and B is the overall thickness of
aquifer.
Construction DewateringDesign of Dewatering Analysis
• Multi-well Analysis
• How can we calculate the effect of multiple wells?
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If the drawdown in the aquifer is a small percentage (about 10–20%)
of the aquifer thickness, the effect of each well can be superimposed
on the other to determine the cumulative effect.
M is the number of pump wells; H is the original water table level; K is the effective
permeability; B is the layer thickness; Qj is the pumping rate of well j; Rj is the
influence radius of well j; and rj is the distance between pumping well j and the
observation point.
Unconfined Aquifer Confined Aquifer
Construction DewateringDesign of Dewatering Analysis
•Self-Practice Example
In the shown 20x50mexcavation three dewateringwells are installed at an offsetof 4m from the edge of theexcavation.
The depth of the excavationis 5m and the ground watertable is 2m below the naturalground level. The groundwater is in an unconfinedaquifer of 25m depth
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Construction DewateringDesign of Dewatering Analysis
• Site Pumping Tests
• In order to determine actual well performance, a site pumpingtest is commonly performed prior to the final design of thedewatering system.
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� This will help determine the actual
performance and soil
permeability.
� Observation wells / peizometers
are installed to monitor the actual
drawdown
Construction DewateringDesign of Dewatering Analysis
• A trench is excavatedaround the area to bedewatered.
• Surface pumps with hosesare used to pump waterfrom the ditches.
• Practical to be used withrelatively small quantitiesof water and small waterheads.
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Construction Dewatering - TechniquesSumps, Ditches and Trenches
• Small pipes (up to 2.5” diameter)connected to screens at thebottom and a vacuum header pipeat the surface.
• Screens prevent soil particles frombeing pumped away with thewater
• A centrifugal / vacum pump isconnected to the manifold
• Wellpoint systems are constrainedby the maximum possible suctionhead
• Common dewatering heightsrange from 4.5 - 7.5m 36
Construction Dewatering - TechniquesWell point system
• Construction Steps
1. The wellpoints are jetted into the ground;
2. The annulars void is filled with filter media;
3. The wellpoints are connected to a header pipe by means of a riser;
4. The header pipe is connected to suction pumps for pumping
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Construction Dewatering - TechniquesWell point system
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Construction Dewatering - TechniquesWell point system
• What if we need to lower the ground water table morethan the well point can handle?
• 2-stage well point system
• Deep wells
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Construction Dewatering - TechniquesWell point system
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Construction Dewatering - TechniquesWell point system
• Consists of a borehole fitted with a slottedliner and an electric submersible pump.
• As the pump is submersible, there is nosuction head limitation as for well points.
• Deep wells work best in soils with specificpermeability profiles (k=1x10-5cm/s to1x10-7cm/s) and the amount of drawdownthat a well can achieve is limited only bythe size of the pump.
• Pumps are engineered to withstand theaggressive corrosion factors associatedwith use over sustained periods of time ina saline environment.
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Construction Dewatering - TechniquesDeep Well
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Construction Dewatering - TechniquesDeep Well system
• Advantages:
• Ability to penetrate strata impervious to the jettingmethod of wellpoint systems.
• Installation of up to 100 feet deep or more in a singlestage.
• Capable of pumping tens to thousands of gallons perminute per well.
• Deep Wells can be effective when placed outside of thejobsite work area.
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Construction Dewatering - TechniquesDeep Well system
• Provides both awater cutoff and astructuralcofferdam.
• Widely used inEurope.
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Construction Dewatering - TechniquesGround Freezing
• Freeze pipe are installed along theline of the wall to be frozen every1m. Pipe consists of an externalcasing and an internal pipe.
• A refrigerated brine solution iscirculated through the system andthe freeze pipes.
• At the freeze plant a heatexchanger transfers the heat tothe ground. 45
Construction Dewatering - TechniquesGround Freezing
• Important Considerations:
• Natural ground water velocities.
• Prevent formation of wall.
• Presence of external heat source.
(e.g. adjacent pipelines).
• Soft clay will creep when frozen.
• Ground movement.
• This technique requires continuoustemperature monitoring.
• Not economic in hot climates.
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Construction Dewatering - TechniquesGround Freezing
Construction DewateringOther considerations during dewatering
• Power supply:
• Provision of electricity source.
• Safe electrical wiring on site.
• Provision of backup power supply.
• Environmental considerations during dewatering:
• Disposal of water.
• Impact on ground water aquifers.
• Impact on surrounding structures.
• Conducting pumping tests to verify actual site performance
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Thank You
Questions ?