Diaphragm walls are wall panels which are constructed by excavating the existing soil and replacing it with construction material.
An essential construction process-specific characteristic of cutter diaphragm and cut-off walls is the process of excavation of the soil material by a single insertion of the cutter into an unobstructed trench supported by bentonite slurry.
The excavated soil material (cuttings or spoil) is continuously conveyed to the surface by a mud pump as a mixture of spoil or cuttings suspended in the support slurry. The soil particles and cuttings are then separated from the slurry in the desanding plant.
The treated slurry is returned into the trench.
Applications
In-Situ Concrete Diaphragm Walls
The in-situ concrete diaphragm wall constructed by the two-phase process is generally used as a
structural wall element for excavation support.
The first phase, generally bentonite slurry, is used as support fluid to facilitate the construction of the
trench and is subsequently replaced by the second phase which is generally concrete. If required for
structural reasons, a reinforcement cage is inserted into the slurry-supported trench prior to placing the concrete.
Whilst the alternating construction sequence with overcutting of joints applies to the cutter, the grabspecific
construction technique uses stopends with water bars. Joints are formed by concreting water
bars into adjacent panels with the help of tubular stopends, precast sections or removable stopend
sections.
The in-situ concrete diaphragm wall is only occasionally used as a foundation element for buildings or
bridge abutments.
Single-Phase Self-Hardening Cut-Off Walls
The main difference between the single-phase self-hardening cut-off wall and the in-situ concrete diaphragm wall is that the construction material for the cut-off wall remains in the trench after completion of the excavation process and then hardens to form the barrier. Depending on the type of cut-off material used, compressive strengths ranging between 0.5 N/mm² and 3.0 N/mm² can be achieved.
These walls are generally used for sealing purposes, often in connection with the encapsulation of
contaminated sites.
The walls are unable to withstand any form of structural loading without additional measures being
taken. As a special measure in connection with encapsulations, an HDPE geomembrane is occasionally
incorporated into the slurry cut-off wall as an additional barrier.
Single-Phase Cut-Off Wall with Inserted Sheet Piles
By the addition of an "inserted sheet pile wall", the simple slurry trench cut-off wall is transformed into a structural retaining wall. The construction principle is essentially equivalent.
However, the sheet piles are inserted into the slurry-filled trench immediately after completion of the excavation and before the self-hardening slurry has hardened.
This type of retaining wall can provide a cost-effective alternative to an in-situ concrete diaphragm wall, if a deep wall with structural loading of comparatively low intensity is required, for instance in the case of a shallow excavation where the natural or artificial impermeable soil formation is found at a considerable depth below the base of the excavation. The sheet pile wall can then be reduced to the structurally required length by being suspended from the guide wall. The cut-off wall itself must be constructed to the full depth required to achieve a proper seal.
