01 Taylor Slurry Wall
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Transcript of 01 Taylor Slurry Wall
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Introduction to diaphragm walls
Slurry wall CommitteeHouston WorkshopOct 16th 2012
What are diaphragm walls?
• Structural retaining walls excavated under bentonite slurry.
• Bentonite slurry maintains the panels open during excavation and allows placement of structural reinforcement and concrete.
• Diaphragm walls can be constructed both in soil and rock to depths exceeding 300 ft.
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Designed and patented in Italy in the 1940s, where engineers were experimenting with the process of filling rectangular panels with bentoniteslurry for stability during excavation.
Over the next several decades, diaphragm wall technology would continue to be refined while it was adopted throughout the world.
History of diaphragm walls
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‐ Cut‐off wall is a waterproof barrier in the ground. Excavated and backfilled with bentonite slurry.
‐ Slurry wall is also used for cut‐off walls excavated in trenches with backhoes with long sticks.
‐ Diaphragm wall. The term was historically used for cut‐off walls but is now used for structural walls too.
=> Our focus today is structural walls excavated in panels under a slurry bentonite and backfilled with concrete.
Diaphragm walls/slurry walls/cut‐off walls
When are slurry walls used?
• Deep excavations
• Difficult soil conditions
• Rigid support of excavation
• Top‐down construction
• To become part of the permanent structure
• Self‐supporting shafts
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Example of Slurry Wall Construction Site
Manitowoc 12000
• Hydromill Rig
• 120 Ton Crane
• 90’ Boom
Liebherr LR 1200
• Support Rig
• 275 Ton Crane
• 184’ Boom
2 Liebherr HS 855
• Clamshell Rigs
• 110 Ton Crane
• 85’ Boom
Construction Phases
1. Site preparation & pre‐trenching
2. Guidewall construction
3. Excavation of the slurry wall panels
4. Desanding of the bentonite slurry
5. Placement of the reinforcing cage
6. Placement of the concrete
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Typical Diaphragm Wall Sequence
1‐ Guide wall construction2‐ Panel excavation under slurry3‐ Rebar cage placement4‐ Cast concrete under slurry
Guide wall
Bentonite slurry
joints
1. Site Preparation & Pre‐Trenching
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2. Guide Wall Construction
Guidewall on the inside only for tight sites
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3. Slurry Wall Panel Excavation
Slurry wall excavation is performed in panels (typically 10’ to 25’ long) with the following equipment:
• Mechanical clamshells
• Chisels
• Hydraulic rigs and clamshells
• Hydromill Equipment
• Trench is filled with bentonite slurry
How does bentonite slurry work?
It is a colloidal dispersion of powdered bentonite clay powder in fresh water. When introduced into a trench, it tends to permeate the soil, forming an impervious “filter cake” on the face of the excavation. The head of slurry inside the trench keeps the excavation stable.
5 ft min
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Mechanical Clamshells
Percussion chisels
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Hydraulic Rig & Clamshell
> The guides of the clamshell can be directedin any direction to control front and back deviation tendencies
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Excavation Equipment
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Hydromill/Hydrofraise/Cutter Equipment
How does the hydromill work?– Reverse mud circulation rig
– Vibration and shock free equipment
– Sized according to site/depth
– Cutting drums equipped with tungsten‐carbide tipped teeth
– Drill mud is constantly screened and de‐sanded during excavation
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Hydromill Desanding Unit
Hydromill slurry wall
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• Verticality measurement and correction
• Measurement of bentonite slurry properties
• Documentation (excavation field reports, panel reports)
QUALITY CONTROL DURING CLAMSHELL ? EXCAVATION
Clamshell ExcavationVerticality Measurement and Correction
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Hydromill – Real Time Verticality Control
4. De-sanding Operation
• It is critical for the success of the slurry wall !
• Sand content needs to be reduced below 5% or less, before concrete placement
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5. Slurry Wall Reinforcement
Slurry walls can be reinforced by:
• Rebar cages
• Rebar cages with post‐tensioned cables
• Soldier piles (SPTC walls)
Rebar Cage Pick
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Setting of Reinforcing Cage
SPTC Walls Reinforcement
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6. Concrete Placement by Tremie Method
The following is required:
• Low sand content in the bentonite slurry (5%, or lower)
• High slump (8” to 10”), small aggregate, cohesive concrete able to flow through the rebar cage
• Sufficient clearance between rebars (at least 6 times max. size of aggregate)
• Good service from concrete plant
Typical Concrete Placement
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Slurry Wall Panel Joints
Forming the joint between panels can be done in different ways:
• By using tubes
• By using soldier piles that remain in place
• By using removable endstops and extracting them:• vertically
• laterally and water‐stops can be installed at the joint
• By grinding a portion of the previously concreted panel (hydromill method)
Circular end‐stops (tubes‐joints)
> Grinding of adjacent panel concrete (hydromill)
> Flat end‐stop joint CWS (with waterstop blade)
Primary Primary
Secondary
> Panel end‐stop = panel joint
> Soldier beam endstops
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Soldier piles used as permanent end‐stops
Vertically extracted endstops
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• PVC waterstops– Installed in primary and follow‐
up panels
– Allows waterstop insertion in panel joints
– Guides clamshell during adjacent panel excavation
Laterally extracted endstops
Laterally extracted endstops
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Hydromill Panel Joint
Slurry Wall Construction Excavation the First 10‐20 feet with Clamshell
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Slurry Wall Construction Excavation by Hydromill
Slurry Wall Construction Cage Installation
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Slurry Wall Construction Concreting Operation
Challenges ?
• Difficult soil conditions
• Boulders and man‐made obstructions
• Proximity to buildings
• Confined work areas
• Low headroom conditions
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Boulders
Existing Foundations & Man‐made Obstructions
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Proximity to Buildings
Confined work areas
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Low headroom Conditions
How Are Slurry Walls Supported During Mass Excavation?
• Internal bracing
• Tiebacks
• Concrete slabs (Top‐down construction)
• Self‐supporting (Circular structures)
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Internal Bracing
Bracing Trusses
CA/T Ramp D
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Tiebacks
Slurry Wall Supported by Tiebacks & Internal Bracing
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Top‐Down Construction Concept
1. Slurry wall installation
2. Drilled shafts or LBE’s (usually single bite element) installation. The drilled shafts / LBE’s are typically poured to subgrade and a structural steel columns is embedded in the concrete placed below subgrade.
3. The ground floor slab is constructed.
4. The erection of the superstructure takes place, while at the same time excavation of the underground basement can start.
Self‐Supporting Structures
Circle
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Self‐Supporting Structures
Ellipse
Self‐Supporting Structures
Peanut
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Conclusions Why Using Slurry Walls?
• Slurry walls can be constructed in a confined urban environment where there are difficult soils conditions and rock
• They can become part of the permanent structure and allow top‐down excavation
• They are versatile structures that can be used in a variety of challenging applications