Concrete Diaphragm Walls
黃安斌
2003年春季
The history
The idea of using slurry as a stabilizing agents dates back to early 1900’s, noticed by oil engineers when drilling deep boreholesSignificant progress in slurry preparation and control of properties started in 1940’s
Contiguous drilled pier walls
Adapted from oil-well drillingStarted in 1950’s, 30 years after the introduction of bored piles
Slurry trench cutoff walls
45 ft deep cutoff wall was built at Terminal Island near Long Beach, CA, 1948
Continuous diaphragm wall
Idea of simplified contiguous drilled pier walls by the use of grabs for linear excavation under slurry was conceived by Veder in 1938.The first continuous diaphragm wall was built in 1950’s.
What is a concrete diaphragm wall
Continuous concrete wall built from the ground surfaceMay consist of precast or cast-in-place concrete panels or contiguous bored concrete piles
Construction of concrete diaphragm wall
Guide wallsExcavating 3 to 6m wide panels under slurryTremie concreteDiaphragm wall thickness 50 to over 150 cm
AdvantagesSaves time – feasible for top-down or under-the-roof construction, whereby excavation of basement and eraction of superstructure proceed simultaneouslyFlexible in construction procedure, low in noise and vibrationEasy to control ground movement
Disadvantages
Finished surface may be roughObstructions may cause concrete blistersDifficult in sloped hard formations, may cause deflection in excavation
Slurry trench stabilityFluid pressure + arching in the groundLocal penetration of slurry into pervious soil –imparts cohesion to the soil and prevents spallingBentonite slurry is kept higher than the groundwater tableHydrostatic pressure, osmotic pressure, electrolytic properties of the colloid, membrane or “mudcake” forms against the walls of the trench
Stability analysis
Fluid pressure + archingAnalysis mostly based on experience
Stability of unsupported trenches
Stability of slurry - filled trenches in clay
Stability of slurry - filled trenches in dry sand
f
f
γγ
γγα
2tan
−=
f
fFγγ
φγγ
−=
tan2
Stability of slurry - filled trenches in sand w/ water
f
fFγγ
φγγ
′−′
′′′=
tan2waf K γγγ +′= For F = 1
ArchingLoad concentrates at the ends of excavated panel, thus relieving the stress condition near the center and improving stability (horizontal arching)Guide wall restraints lateral movement at the ground surface, develops arching in the vertical planeAt great depth, the stress is high, arching effects in the horizontal plane is minimal
Arching effects on short trenches in sand
hw = depth of ground water
Arching effects on short trenches in sand
Arching effects, the Schneebeli theory
Effects of mud(filter)cake in sand
w/o mudcake, seepage force only w/ mudcake
r = pore radius
τf = gel strength
Excavation in clay – shallow circular cuts
Depth to diameter ratio < 12
Excavation in clay – deep circular cuts
Depth to diameter ratio > 12
Properties of bentoniteMontmorillonite mixed with waterBentonite slurry forms a gel and develop shear resistance if left undisturbed - thixotropicBentonite slurry displays plastic viscosity, develops additional shear resistance depending on the rate of shearing rate – Bingham fluid
MudcakeK > 10-1 cm/sec – no mudcake can developK between 10-2 and 10-1 cm/sec – some time lag before mudcake can developK < 10-2 cm/sec mudcake develops with no time lag
Pressure of slurry fluid
At least at 4ft above the groundwater tableBentonite concentration – 4 to 6% by weight with specific gravity at 1.023 to 1.034
Other contributing factors to trench stability
Electro-osmotic phenomenon –migration of colloidal particles to the trench wall by electrical potential at the slurry – soil interfacePenetration of slurry into cohesionless soil – forming of the mudcake by seepage force – affected by depth and permeability of the soil
Quality control (assurance)Slurry level – sufficiently above the ground water level – maybe necessary to raise the height of the guide wallControl of bentonite quality –contamination controlTrench stability and concrete quality
Bentonite contamination
Detritus contaminationSand, clay and silt particles build up in the slurryIncreases the density of slurryAdversely affects concrete placement
Bentonite contamination
Calcium (Ca+) contaminationCalcium in cement or building debris causes flocculation of bentoniteThe slurry becomes thick and more difficult to circulateForms thick and more permeable mudcake
Bentonite contamination
Salt (Na+) contaminationSame effects as the calcium contamination
The slurry mix
Dense enough to provide stabilityThin enough to allow circulation and concretingAgents may be added to assure quality and prevent chemical contamination
Slurry mix agents
To prevent flocculation - dispersing agents, known as the mud thinner, decreases the viscosity of slurry
Field quality control
DensityPHviscosity
Excavation equipment
Concreting and construction
Joints
Keyed and water-stop joints
Construction planning
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