Diaphragm Wall

DIAPHRAGM WALL

Diaphragm wall is method of creating cast in-situ reinforced concrete retaining wall using slurry supported trench method. They are also known as slurry walls. These walls provide rigid, cost effective solution for permanent retaining wall and shafts with less construction joints. Diaphragm wall construction methods are relatively quiet and cause little or no vibration. Diaphragm wall panels are also used in deep, load-bearing soil layers as foundation elements to carry concentrated structural load in the same way as large drilled piles do. These foundation elements are known as "Barrettes".

TYPES OF CONSTRUCTION METHOD

The construction of diaphragm wall may be of the following two types:

❖ Diaphragm Wall (grabbed): In this method, two different grab systems, suitable for wall thicknesses of 600 mm to 1,500 mm, are available for trench excavation. The grab is lowered into the trench and the soil is discontinuously transported. Larger blocks or sections of rock are punched through, excavated or displaced using modified grabs or heavy-duty chisels. Measuring systems can be installed to verify verticality. On mechanical grabs, a roller system operates the jaws. To increase the closure force, the closing cable is reeved five to six times. Hydraulic grabs work with a hydraulic cylinder. They have very high closure forces and work with lower noise and fewer vibrations than mechanical grabs.

❖ Diaphragm Wall (cut): Trench cutters are used to produce diaphragm walls in thicknesses of 500 mm to 3,200 mm. As they transport the spoil continuously, they are particularly suitable for depths of more than 40 m. Trench cutters operate with two opposite-running cutter wheels which, depending on configuration, are suitable for a wide variety of strata, including even ultra-hard rock. The soil material is cut by the rotation of the wheels, crushed, mixed with slurry and transported to the surface by a suction pump. Hydraulic steering flaps control the cutter in both horizontal directions. The penetration depth and speed are regulated by controlled activation of the cutter weight.

PROCEDURE FOR CONSTRUCTING DIAPHRAGM WALL:

❖ Site Logistic and Slurry Plant Setup: Diaphragm wall installation requires sufficient work area to setup slurry plant and to assemble reinforcing cages prior to placement in wall. This work may be difficult in congested sites. To reduce area requirement of site cage, prefabrication is possible. Slurry plant includes slurry mixer, storage tanks and descending units. Sufficient storage tanks must be used for bentonite slurry hydration.

❖ Pre-trenching: It is performed to remove shallow obstructions and provide stable support for guide walls. This is performed as open excavation backfilled with flowfill / excavated under self hardening slurry.

❖Guide Wall Construction: Guide walls provide template for wall excavation panel layout, support top of trench, restrain end slopes, serves as platform to hang reinforcement provide reference elevation for inserts, support tremie pipes, hold down cage during concreting and provide reaction for jacking out some types of end slopes. Guide walls are reinforced concrete whose top should be atleast 4 feet above ground water table to allow construction in dry condition.

❖ Panel Excavation: Special clamshell also known as grabs/buckets are rectangular shaped and used to excavate vertical slots are known as panels. Digging mechanism may be cable or hydraulic operated. The excavation is performed in panels which are in vertical segments. Trench stability is mostly provided by fluid weight of bentonite and arching action of soil around trench. Bentonite slurry is placed in trench added to maintain atleast 3 feet above ground water table and within 2 feet of top of guide wall.

❖ End-stop Placement: End-stops are used to control concrete placement so that secondly adjacent panels are not excavating monolithic concrete. End- stop may be permanent or removed after concrete placement. Permanent End-stops are typically wide flange shaped. Removal End-stops can be pipe/Special keyway End-stops.

❖ Panel Descending: Panel may be descended to remove excess sand in slurry and bottom panel. Removal of sand from slurry decreases density of slurry so that tremie concrete doesn't mix with slurry or trap pockets of sand.

❖ Reinforcing Cage Placement: Reinforcing cage is inserted into panel excavation concrete is placed around reinforcing cage using tremie methods to form concrete panel.

❖ Tremie Concrete: Tremie pipes are placed in panel at bottom concrete with 200-250 mm slump is then allowed into the panel. Concrete mix with high slump and fairly high cement content often other pozzolona, plasticizers and chemicals are used.

APPLICATIONS OF DIAPHRAGM WALLS

Diaphragm wall has many applications to facilitate certain construction activities, as follows:

❖ As a retaining wall and cut-off provision to support deep excavation in areas with dense and historic urban infrastructure.

❖ Used as a final wall for basement or other underground structure like tunnel and shaft.

❖ For a separating structure between major underground facilities and sheet piles in hydraulic structures.

❖ It is intended to take up high vertical loads from above ground structures during construction

❖ It is preferred, where geology and ground water precludes use of conventional earth retention system

ADVANTAGES AND DISADVANTAGES OF DIAPHRAGM WALLS

Advantages:

❖ Facilitate excavations below groundwater while eliminating dewatering.

❖ Provides underpinning and minimizes settlement of adjacent buildings.

❖ Provide fairly watertight walls.

❖ It can be installed before excavation commences.

❖ Accommodate connections to structures.

❖ Easily adapted to both anchors and internal structural bracing systems.

❖ Top-down basement construction gives significant advantages.

Disadvantages:

❖ High Cost

❖ Requires special equipment

❖ Technical skilled labour required