The construction methodology and excavation options for a diaphragm wall are explained in the following sequences so read the full article and put your comment about this
Diaphragm Wall Construction Methodology
A shallow guide wall is constructed first, along the alignment of the proposed wall, the guide wall serves to locate the excavation equipment within acceptable tolerances before the retaining wall construction commences
The guide wall is made from reinforced concrete typically 1 to 1.5 metres deep walls are constructed to tight verticality tolerance excavation of the diaphragm wall is carried out in panels
A typical panel is excavated in three bites a left bite, a right bite and lastly removal of the middle dumpling bite widths depend on the grab type being used but 2 to 2.8 meters wide is typical at the same time as the soil is removed from the panel
Bentonite mud is pumped into the excavation to support the surrounding soil preventing a collapse into the excavated trench
Excavation continues until the bite reaches the design depth bentonite mud or inappropriate circumstances polymer is bashed in an on-site plant allowed to hydrate to gain full gelling properties and is then used for temporary excavation support
Typically bentonite powder in bulk or bagged form is added to water and rapidly mixed in a colloidal mixer to produce the desired concentration of bentonite mud
The mud is usually pumped to a storage tank where it hydrates for up to 12 hours the bentonite particles swell and absorb the water before it's used to support the excavation
When required the hydrated mud or recycled mud or a mix of the two is pumped to the excavation trench to replace the excavated soil maintaining a constant head of the mud above the prevailing groundwater levels
Excavation of the panel continues generally completing left and right bites before removing the centre bite
Then the centre bite is excavated down to the required panel toe level steel or precast concrete stop ends are placed at the edges of the excavated panel
The stop end may contain a rubber water bar that later becomes embedded into the concrete the water bar minimizes water ingress between panel joints once the earth is excavated inside the diaphragm wall
A prefabricated reinforcement cage is then lowered into the excavation between the stop ends block outs for floor slab connections ground anchors or other penetrations can be formed in the reinforcement cage
Concrete is poured into the excavation using the tremie method this is achieved by the use of a hopper and steel tube extending to the base of the excavation
Typically two hoppers are used for a 6 meter wide panel the hoppers and tubes are charged with concrete then the tubes are lifted off the bottom of the excavation so that the concrete flows out displacing the lighter bentonite mud upwards
The bentonite is pumped back to the recycling plant as it's displaced as the concrete level rises in the panel the tube lengths are shortened to ensure that fresh concrete is always closest to the concrete bentonite interface
When the concrete rises to the required level the tremie pipes are removed and the concrete is allowed to set
The first panel constructed in a run of the wall is called the primary panel
During concreting, the excavation grab and the base crane is moved to another location a safe distance from the concreted panel and excavation recommenced on another primary panel
The next excavated panel is completed and concreted in the same procedure as just described subsequent panels excavated adjacent to a completed panel are called secondary all running panels
The final panel concreted in a run of all is termed the closing panel as this panel closes up the length of the wall the stop end is removed from one panel during excavation of the adjacent panel
Diaphragm wall panels can be excavated using several types of equipment including hydraulic cutters these are powerful machines capable of excavating into hard rock but require large bentonite plants and large cranes to hold the cutter steering mechanisms are built into the ground body to provide a higher degree of verticality control
The hydraulic grab – This type of grab has hydraulically operated closing jaws and so is able to exert considerable closing force to excavate dense sands, stiff clay or weak rock material these grab bodies can also include steering mechanisms
The cable grab this grab uses only pulley forces to close the grab jaws and so is more suited to the softer ground and the Calla grab
This grab type is rigidly suspended from the base machine whereas the three previous types are rope suspended the Kelly grab can provide reasonable excavation force and verticality control but may not be as fast as suspended grab excavation
Typical diaphragm wall widths are 600 millimetres, 800 millimetres, 1200 millimetres and 1500 millimetres though other sizes can be used for special requirements piling contractors think safe work safe home safe
