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DIAPHRAGM & SLURRY WALL

DIAPHRAGM WALL

A diaphragm wall is a technique used to build reinforced concrete walls in areas of soft earth close to open water or with a high ground water table. This technique is typically used to build diaphragm (water-blocking) walls surrounding tunnels and open cuts, and to lay foundations.

A trench is excavated to create a form for each wall, then filled with bentonite slurry; it is kept full of slurry at all times. The slurry prevents the trench from collapsing by providing outward pressure which balances the inward hydraulic forces and prevents water flow into the trench. Reinforcement is then lowered in and the trench is filled with concrete, which displaces the slurry.

DIAPHRAGM WALL

Diaphragm walls are typically constructed by starting with a set of guide walls, typically 1 meter deep and 0.5 meter thick. The guide walls are constructed on the ground surface to outline the desired slurry trench(es) and guide excavation. Excavation is done using a special clamshell-shaped digger or a hydromill trench cutter. The excavator digs down to design depth, or bedrock, for the first cut. The excavator is then lifted and moved along the trench guide walls to continue the trench with successive cuts as needed. The trench is kept filled with slurry (usually a mixture of bentonite and water) at all times to prevent collapse.

DIAPHRAGM WALL

Once a particular length is reached, a reinforcing cage is lowered into the slurry-filled pit and the pit is filled with concrete from the bottom up using tremie pipes. The concrete displaces the bentonite slurry, which is pumped out and recycled.

Slurry walls are built to enclose the desired area, blocking water and softened earth from flowing into it. On completion of concreting, digging within the now concrete wall-enclosed area can proceed. To prevent the concrete wall from collapsing into the newly open area, temporary supports such as tiebacks or anchors are installed. When completed, the structure built within the walled-off area supports the wall, so that tiebacks and/or other temporary bracing may be removed.

CONSTRUCTION OF DIAPHRAGM WALL

Diaphragm Wall is generally reinforced concrete wall constructed in the ground using under slurry technique which was developed in Europe.

The technique involves excavating a narrow trench that is kept full of an engineered fluid of slurry.

Walls of thickness between 300 and 1200 mm can be formed in this way up to a depths of 45 meters.

DIAPHRAGM WALL-APPLICATION

Diaphragm wall is a continuous wall constructed in ground in to facilitate certain construction activities, such as: As a retaining wall

As a cut-off provision to support deep excavation

As the final wall for basement or other underground structure (e.g. tunnel and shaft)

As a separating structure between major underground facilities

As a form of foundation (barrette pile –rectangular pile)

DIAPHRAGM WALL-APPLICATION

Commonly used in congested areas.

Can be Installed in close proximity to existing structure.

Practically suited for deep basements.

Used in conjunction with “Top Down” construction technique.

ABOUT DIAPHRAGM WALL

Very unstable soil profiles below the water table.

Limited construction time where deeper than normal cantilever support may be required.

Can be Installed to considerable depth.

Formation of walls with substantial thickness.

Flexible system in plan layout

Easily incorporated into Permanent works.

ABOUT DIAPHRAGM WALL

Designable to carry vertical loads.

Construction time of basement can be lowered considerably.

Economic and positive solution for large deep basement in saturated and unstable soil profiles.

Can be used for seepage control in dams.

Noise levels limited to engine noise only.

No vibration during installation.

CONSTRUCTION PROCEDURE OF DIAPHRAGM WALL

Stage-1: Fixing of Alignment

Stage-2: Guide Wall Construction

Stage-3: Trenching

Stage-4: Trench Cleaning

Stage-5: Stop ends Fixing

Stage-6: Reinforcement Cage

Lowering

Stage-7: Placing of Concrete

Stage-8: Withdrawal of Stop Ends

CONSTRUCTION PROCEDURE OF DIAPHRAGM WALL

Stage-1: Fixing of Alignment Stage-2: Guide wall Construction

Stage-3: Trenching

Wire Operated Mechanical Grab

Hydraulic Grab

Stage-4: Trench Cleaning

The sides inside the trench cut can collapse easily. Bentonite slurry is used to protect the sides of soil. Bontonite is a specially selected fineclay, when added to water, forms an impervious cakelike slurry with very large viscosity. The slurry will produce a great lateral pressure sufficient enough to retain the vertical soil.

De-sanding equipment – the bontonite slurry carrying the soil and gravel re-circulate to this equipment with the bontonite slurry and the debris being removed. The debris will be collected in the storage tank for disposal later

Stage-5: Stop ends fixing Stage-6: Reinforcement Cage lowering

Reinforcement is inserted in the form of a steel cage, but may be required to lap a few sections in order to reach the required length.

Stage-7: Placing of Concrete Stage-8: Withdrawal of Stop ends

Placing of concrete is done using tremie pipes to avoid the segregation of concrete. As concrete being poured down, bontonite will be displaced due to its lower density than concrete. Bentonite is then collected and reused.

A slurry wall is a non-structural barrier built under ground to prevent the flow of groundwater. The initial application of slurry walls was first limited to civil construction projects namely hydraulic barriers used in and under dams, levees, and other similar structures.

More recently, however, slurry walls, cutoff walls, or slurry trenches are being widely used as a ground remediation tool for underground waste and groundwater containment. It is now common to use slurry walls in the construction and building of tunnels.

SLURRY WALL

The construction of a slurry wall begins with the “slurry excavation technique.” A narrow trench is excavated and filled with slurry, a clay-like mixture. The slurry mixture exerts hydraulic pressure against the trench walls during the excavation process essentially acting as support preventing the collapse of the wall even below groundwater.

Trenches for slurry wall construction are dug using hydraulic excavators and can be dug to a depth of 0.5 to 1.5 m. Trenches dug over a 50 m require the use of a special crane.

The trench is usually dug in a stratum that has low permeability such as clay or bedrock so that leakage can be minimized as much as possible.

SLURRY WALL

Slurry placed in the trench is typically a mixture of bentonite and water combined in a mixer. The slurry is pumped directly from the mixer through a pipe into the trench.

In addition to just stabilizing excavation, bentonite slurry forms what is called a “filter cake” on the slurry trench walls, reducing the wall’s final soil permeability.

Once excavation is completed, it is common the backfill the trench with a mixture of excavated soil, dry bentonite, and bentonite slurry. The backfill eventually has the consistency of wet concrete and is placed into the end of the slurry trench, displacing the other slurry. The process of excavation and backfill is carried out until the slurry wall is completed.

SLURRY WALL

Diaphragm wall as cut-off for basement excavation

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