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