Diaphragm wall is a technique of constructing a continuous underground wall from the ground level. Diaphragm walls provide structural support and water tightness. These reinforced concrete diaphragm walls are also called Slurry trench walls due to the reference given to the construction technique where excavation is made possible by filling and keeping the wall cavity full with bentonite-water mixture during excavation to prevent collapse of vertical excavated surfaces. These retaining structures find following applications: earth retention walls for deep excavations; basements, and tunnels; High capacity vertical foundation elements; Retaining wall foundations; water control. These are also used as a permanent basement walls for facilitating Top-down construction method.
Typical wall thickness varies between 0.6 to 1.1m. The wall is constructed panel by panel in full depth. Panel width varies from 2.5m to about 7m. Short widths of 2.5m are selected in less stable soils, under very high surcharge or for very deep walls. Different panel shapes other than the conventional straight section like T, L are possible to form and used for special purposes. Traditionally, panel excavation is carried out using cable supported Grab. Hydraulic grabs with Kelley arrangement have recently been introduced on large Infrastructural projects. More recently developed hydraulic cutter type machines are being used in several projects. Slurry wall technique is a specialized technique and apart from the crane mounted grab, other equipment involved are cranes, pumps, tanks, desanding equipment, air lifts, mixers etc.
Cable mounted hydraulic grab
Kelly mounted hydraulic grab
Hydrofraise mounted on crane
Applications of diaphragm wall
Diaphragm walls are most commonly used:
• In areas with dense and historic urban infrastructure,
• Where a very rigid earth retention system is required,
• Where noise and vibration must be limited,
• Where the geology and groundwater preclude the use of conventional earth retention systems
• And/or where dewatering is not practical
Compared to other wall types, diaphragm walls are considered to be very stiff with respect to ground movement control.
Diaphragm walls are often attractive in granular soils with a high groundwater level, especially when a low permeability layer underlies the granular soils. The diaphragm walls are typically terminated in the underlying low-permeability layer which can consist of soil or rock. Keying into this low permeability layer reduces groundwater seepage below the wall.
Projects that have used these walls include:
• Below grade parking/ deep basements
• Cut and cover subway tunnels
• Highways as cut and cover tunnel walls and for underpasses
• Shafts for deep sewers
• Dam appurtenances
Diaphragm wall in retaining structure
Application of diaphragm wall in metro station construction (Omran Ista with partnership of Kayson)
Cut& cover tunnel
Cut off wall
UG tanks, water station, …
Diaphragm wall in oil tanks
Benefits of diaphragm wall
Diaphragm walls can:
• be formed to depths of several hundred feet, through virtually all soil types and through rock, and with great control over geometry and continuity
• facilitate excavations below groundwater while eliminating dewatering
• provide fairly watertight walls
• provide structural stiffness which reduces ground movements and adjacent settlements
• during excavation
• be load bearing transferring loads to the underlying layer be reinforced to allow
• incorporation of many structural configurations,
• accommodate connections to structures
• be easily adapted to both anchors and internal structural bracing systems
• be constructed in relatively low headroom and in areas of restricted access
• be installed before excavation commences
• provide economic solutions in cases where temporary and permanent support can be integrated or redesigned into one retaining structure
• Diaphragm walls combine into a single foundation unit the functions of temporary shoring, permanent basement walls, hydraulic (groundwater) cutoff, and vertical support elements.
Because of this combination, they have proven to be an economical alternative in many circumstances.
The trench excavation is performed using slurry for support. The slurry is typically bentonite and water or polymer and water.
Diaphragm walls are constructed in the following steps:
• Pretrenching to remove obstructions
• Guidewall construction
• Panel (vertical segments) excavation
• Endstop placement
• Panel desanding
• Reinforcing cage placement
• Tremie concrete
• End stop removal (if temporary)
Construction Sequence of diaphragm wall
1- Site logistics and Slurry plant setup
It is important to note that diaphragm wall installation requires sufficient work area to set up the slurry plant and to assemble the reinforcing cages prior to placement in the wall. This work may be difficult on congested sites. To reduce site area requirements, offsite cage fabrication is possible.
The cage fabrication area is dependent on the number of rigs and production schedule. The plant area is dependent on number of tanks. The slurry plant includes a slurry mixer, storage tanks, and desanding units. Sufficient storage tanks must be used for bentonite slurry hydration, several panels of bentonite and recycled bentonite.
Pretrenching is often performed to remove shallow obstructions and provide stable support for the guidewalls (next step). This pretrenching may be performed as open excavation backfilled with flowfill or excavated under self hardening slurry.
Guidewalls provide a template for wall excavation and panel layout, support the top of the trench, restrain the endstops, serve as a platform to hang the reinforcement, provide a reference elevation for inserts ( anchors, slabs, etc.), support the tremie pipes, hold down the cage during concreting, and provide reaction for jacking out some types of endstops.
Guidewalls are reinforced concrete and constructed similar to the figure and photo below.
The top of the guidewalls should be at least four feet above the groundwater table to allow for construction in the dry and to allow for slurry level to be three feet above groundwater table.
4- Panel (vertical segments) excavation
Special clamshells also known as grabs or buckets are rectangular shaped (see photos) and used to excavate vertical slots known as panels. These clamshells may be cable hug or Kelly mounted, and the digging mechanics may be cable or hydraulic operated.
The excavation is performed in “panels” which are vertical slots. Trench stability is mostly provided by the fluid weight of the bentonite and the arching action of the soil around the trench. Calculations on trench stability often do not show that successfully excavated trenches should stay open which indicates conservatism and effects that have not been considered. The bentonite slurry is placed in the trench after a few buckets have been excavated and continuously added to maintain at least 3 feet above groundwater level and within 2 feet of the top of the guidewall.
Panel lengths are typically 20 to 24 feet governed by the geometry of the project and the size of contractors’ special clamshells. The panel width is governed by the contractor’s clamshells. Various widths can be accommodated by reinforcing design including shear and bending reinforcement.
5- Endstop placement
Endstops are used to control the concrete placement so that adjacent secondary panels are not excavating monolithic concrete. Endstops may be permanent or removed after concrete placement. Permanent endstops are typically wide flange shapes. Removable endstops can be pipe or special keyway end stops.
6- Panel desanding
The panel must be de-sanded to remove excess sand in the slurry and bottom of panel. The removal of sand from the slurry decreases the density of the slurry so that tremie concrete does not mix with the slurry or trap pockets of sand.
7-Reinforcing cage placement
Carefully fabricated three-dimensional reinforcing cage are then inserted into the panel excavation. The reinforcing cage may also support future structural or utility connections using “knockouts” that are pre-set in the wall. Concrete is then placed around the reinforcing cage using tremie methods to form each concrete panel.
8- Tremie concrete
Tremie pipes are placed in the panel to within a foot of the bottom. Typically two tremie pipes are used for full size panels and one tremie pipe is used for single bite panels. Concrete with 8 to 10 inch slump is then tremied into the panel.
The concrete mix is special to provide 4000 to 6000 psi strength with high slump and contains fairly high cement content, often other pozzolans, plasictizers and often other chemicals.
The concrete level is sounded after each load and records maintained on actual versus theoretical concrete take. Tremie pipe sections are removed as the concrete level rises but maintained 10 feet into the concrete. While the concrete is being placed, the bentonite slurry is pumped back to storage tanks for treatment and reuse.
9- End stop removal (if temporary)
As the concrete is setting typically four hours after placement at a given depth, temporary endstops are removed by crane or jacks. This often means late nights and overtime.
Diaphragm wall after construction
The design analyses for excavation support systems can range from relatively simple empirical analyses to more complex computer analyses, where typically all stages of the excavation sequence are evaluated. The design considerations should include not only the stresses and loads on the support system, but also the effects of construction movements on the response of adjacent structures. The level of effort for the evaluation often depends on the stage of the project, proximity of structures, contractor’s methods of construction, and known local practice.