Depending on the engineering field of application we differentiate between:
- structural diaphragm walls such as containment or foundation works;
- diaphragm walls with hydraulic impermeability (plastic barriers) or drainage (drainage channels) functions.
Structural diaphragm walls in their turn are divided into the following types depending on their execution:
- reinforced concrete diaphragm walls cast on site;
- prefabricated reinforced concrete diaphragm walls;
- self-hardening reinforced mud diaphragm walls.
Impermeable diaphragm walls in their turn are divided into:
- self-hardening mud diaphragm walls sometimes with impermeable membranes;
- diaphragm walls in plasticized concrete.
Depending on the method of excavation we differentiate between:
- excavation with inactive mud;
- excavation with active mud;
- dry excavation without the use of mud.
Usual diaphragm wall thickness in reinforced concrete: 50-60-80-100-120 cm.
The equipment used is:
- boring with inactive mud (grab) : excavation with a grab is used on loose ground where crushing of the lithoidal elements is not required.The grab can be hung on a cable (cable grab) or connected to a rigidly rigged boom (grab controlled by a kelly or telescopic kelly to reach greater depths).
- boring with active mud – inverse circulation (hydraulic cutter): used for rocky ground or in loose ground at great depth.On a metal frame with the same panel section to excavate are mounted two rotary cutters equipped with suitable teeth, these rotate in opposite directions, in order to remove and break up the ground.A pump, situated within the core of the cutters, pushes the detritus suspended in the mud to a treatment and separation station.
Construction materials and elements
a. metal reinforcement: composed of prefabricated cages in Fe B 44 k steel, the crosswise reinforcement rods are clamped on the outside of the lengthways rods positioned in the form of stringers attached to appropriate strengthening diagonals.The reinforcement rods of two adjoining cage elements are joined by overlapped binding material.
b. concrete: generally concrete has a water to cement ratio of W/C< 0.5 with R’ck at least equal to 25 MPa .
Method used for a reinforced concrete diaphragm wall cast on-site
a. laying of the stringcourses : with dimensions of 50 to 80 cm and a height of 40 to 60 cm in order to provide an alignment permanently formed on the ground.
b. excavation carried out with bentonite mud with the possible desanding in the case of ground with a particularly high sand or lime content.
c. cleaning of the excavation and desanding.
d. positioning of the metal reinforcement rods: of the above type, paying attention not to place the cages on the floor of the excavation.
e. casting of the concrete: using the above method through a suitable shoot or tube (casting tube) from the base upwards. The tube is fitted with a loading hopper at the upper end. Attention must be given to stop its footing 25 to 50 cm from the bottom of the excavation.
f. finishing : of the top of the diaphragm wall.
g. checks during the work: are the checks of the laying and preparation, checks of the excavation procedure and vertical trueness, checks of the drilling mud, checks of the reinforcement rods, checks of the fresh concrete, checks of the casting of the concrete.
h. final checks: are the checks of load resistance (load tests), non-destructive tests of barrier integrity (tests of mechanical tolerance), non-destructive tests of barrier integrity (ultrasound tests).