Diaphragm wall is method of creating cast in-situ reinforced concrete retaining wall using slurry supported trench method, Hence they are also known as slurry walls.
These walls provide rigid, cost effective solution for permanent retaining wall and shafts with less construction joints.
Construction Procedure of Diaphragm Wall
1. 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.
2. Pretrenching :
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.
3. 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.
4. Panel Excavation (Vertical Segment) :
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 are 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.
5. Endstop Placement :
Endstops are used to control concrete placement so that secondly adjacent panels are not excavating monolithic concrete. Endstop may be permanent or removed after concrete placement. Permanent Endstops are typically wide flange shaped. Removal Endstops can be pipe/Special keyway Endstops.
6. 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.
7. Reinforcing Cage Placement :
Reinforcing cage is inserted into panel excavation concrete is placed around reinforcing cage using tremie methods to form concrete panel.
8. Tremie Concrete :
Tremie pipes are placed in panel at bottom concrete with 8-10 inch slump is then tremied into panel. Concrete mix provide 4000-6000 psi strength with high slump and contain fairly high cement content often other pozzolona, plasticizers and chemicals.
Application of Diaphragm Wall
- In areas with dense and historic urban infrastructure.
- Where very rigid earth retention system is required.
- They are used where noise and vibrations must be limited.
- Where dewatering is not possible.
- Where geology and ground water precludes use of conventional earth retention system.
Compared to other wall types, Diaphragm walls are stiff with respect to ground movement control. Diaphragm wall are often attractive in granular soils with high ground water table, When low permeability layer underlies granular soil. Diaphragm walls are terminated in underlying low permeability layer which consist of soil/rock keying into low permeability layer reduce ground water seepage below wall.