Diaphragm Wall Equipment

Diaphragm wall equipment is the group of machines used to excavate deep, narrow trenches in the ground and fill them with reinforced concrete, building a continuous underground wall. The main excavating tools are grabs, which dig by closing a pair of jaws, and trench cutters, which mill the soil or rock with rotating cutting wheels. They are carried by heavy crawler base machines and supported by plants that mix, pump and clean the fluid that keeps the trench open during excavation.

Diaphragm walls, also called slurry walls, retain soil and groundwater around deep excavations, form the permanent walls of basements, underground stations and tunnel shafts, and act as cut-off walls that stop seepage under dams and around contaminated sites. Because the walls can reach great depths with little noise and vibration, the equipment is central to underground construction in dense cities.

Brief History of Diaphragm Wall Equipment

The diaphragm wall method grew out of the use of bentonite drilling fluids, borrowed from oil well drilling, to support excavations. Italian engineers and contractors were among the pioneers in the late 1940s and 1950s, and the technique spread through Europe in the following decades for metro systems, dams and deep basements. Early walls were dug with cable-operated clamshell grabs hung from cranes, and chisels were dropped to break hard layers.

Hydraulic grabs, guided by a rigid frame or a Kelly bar, improved verticality and control. The next major step was the trench cutter, or hydromill. Soletanche developed its Hydrofraise in the 1970s, and Bauer began building trench cutters in the 1980s. Cutters mill the ground continuously and pump the cuttings out with the fluid, allowing deeper and more accurate panels and excavation into rock. Since then, both grabs and cutters have gained electronic verticality measurement and steering, and compact cutters have appeared for low headroom and soil mixing work.

How Diaphragm Wall Equipment Works

Guide walls. Two shallow concrete guide walls are built along the line of the diaphragm wall. They set the position and width of the trench and support the top of the excavation.
Support fluid. The trench is kept full of bentonite or polymer slurry. Its pressure holds the sides of the trench stable while the soil is removed.
Excavation. A grab or cutter digs the trench in panels, usually a few metres long. The grab lifts the soil out bite by bite, while a cutter mills it and pumps it to the surface mixed with the fluid.
Desanding. On cutter jobs, a separation plant with screens, hydrocyclones and sometimes centrifuges removes the soil from the fluid, which is then returned to the trench.
Reinforcement and concreting. Steel reinforcement cages are lowered into the panel and concrete is placed through tremie pipes from the bottom up, displacing the lighter fluid, which is recovered.
Joints. Stop-end elements or, with cutters, overcutting into the neighbouring panel create a watertight joint between panels.

Types of Diaphragm Wall Equipment

Mechanical Cable Grabs

Grabs hung from a crane or base machine on wire ropes and closed by a rope or by their own weight. They are simple and robust, and remain in use for many soil walls.

Hydraulic Grabs

Grabs closed by hydraulic cylinders, either hanging on ropes with a hose reel or guided by a Kelly bar. They offer strong closing force and better control of verticality, and many are fitted with inclinometers and steering plates.

Trench Cutters

Heavy frames with two pairs of cutting wheels at the bottom, driven by hydraulic or electric motors, and a pump that lifts the cuttings with the slurry. Cutters can excavate hard soils and rock, reach very large depths and keep accurate verticality.

Compact and Low Headroom Cutters

Smaller cutters mounted on shortened masts or excavator-type carriers, used under bridges, power lines or inside buildings, and for shallower walls.

Cutter Soil Mixing Equipment

Derived from the trench cutter, these machines mix the soil in place with cement slurry instead of removing it, producing soil-cement walls for cut-offs and retaining structures.

Support Plants

Bentonite mixing plants, silos, storage tanks, pumps and desanding units, which are essential parts of any diaphragm wall site.

Size Classes

Equipment	Typical wall thickness	Typical depth range
Hydraulic and cable grabs	about 0.5 to 1.5 m	up to about 50 to 70 m
Compact cutters	about 0.5 to 1.2 m	up to about 30 to 40 m
Standard trench cutters	about 0.6 to 1.5 m	up to about 80 m
Heavy trench cutters	up to about 3 m	100 m and more

Figures are indicative and depend on ground, equipment and project requirements.

Main Manufacturers

Bauer, with trench cutters, hydraulic grabs, base carriers and desanding plants.
Liebherr, with diaphragm wall grabs, trench cutter systems and duty cycle crawler cranes.
Soilmec and Casagrande, Italian makers of hydraulic grabs, cutters and base machines.
Soletanche Bachy, the specialist contractor that developed the Hydrofraise and builds equipment for its own use.

Main Uses of Diaphragm Walls

Retaining walls for deep basements and underground car parks.
Metro stations, cut-and-cover tunnels and underpasses.
Launch and reception shafts for tunnel boring machines.
Cut-off walls in dams, levees and embankments.
Containment walls around contaminated land.
Load-bearing wall elements, called barrettes, used as deep foundations.

Grab or Cutter?

Ground. Grabs work well in soft to medium soils. Cutters are preferred in dense soils, gravels with cobbles and rock.
Depth and accuracy. Cutters reach greater depths with tighter verticality tolerances, which matters for deep, watertight walls.
Cost and setup. Grabs need less support equipment. Cutters require a desanding plant, more space and a higher initial investment.
Joints. Cutters can overcut the previous panel for a clean joint, while grab walls usually rely on stop-end elements.
Combined use. On many jobs a grab excavates the upper soft layers and a cutter finishes the deeper or harder part of the panel.

Site Layout and Logistics

A diaphragm wall site needs more space than the wall itself suggests. Besides the base machine with its grab or cutter, the site usually includes a crane for reinforcement cages, a cage assembly area, slurry mixing and storage tanks, a desanding plant on cutter jobs, pumps and pipelines, and access for concrete trucks and spoil removal. Panels are normally excavated in an alternating sequence, with primary panels first and secondary panels between them, so the plan must allow machines to move along the wall line without damaging completed work. On tight urban sites, compact equipment and careful sequencing are often as important as the choice of excavating tool.

Safety

Diaphragm wall sites combine heavy base machines, open trenches full of fluid, suspended loads and large reinforcement cages. Trenches must be protected by guide walls and barriers so that people and machines cannot fall in, and fluid levels must be kept high enough to prevent collapse. Base machines need firm working platforms because of their height and heavy tools. Lifting cages requires careful planning, often with two cranes, and exclusion zones. Workers handling hoses, slurry and tremie pipes face slipping and crushing risks, and bentonite and cement require suitable protective equipment.

Trends and Technology

Current equipment gives operators real-time displays of the position and inclination of the grab or cutter in the trench, with automatic steering to correct deviations. Data from each panel is recorded for quality control. Electric drives for cutters and base machines reduce noise and emissions in city centres. Improved slurry treatment reduces waste volumes, and cutter soil mixing has become a common alternative to conventional walls where it suits the ground and design.

Research into new joint systems, better slurry recycling and lower-carbon concrete for permanent walls is also shaping how diaphragm walls are designed and built.

Key Terms

Slurry: bentonite or polymer fluid that supports the trench walls during excavation.
Panel: one section of diaphragm wall excavated and concreted as a unit.
Guide wall: shallow concrete walls that align the trench and support its top.
Hydromill: another name for a trench cutter.
Tremie pipe: a pipe used to place concrete from the bottom of the trench upward.
Barrette: a rectangular deep foundation element built with diaphragm wall equipment.

Frequently Asked Questions

What is the difference between a diaphragm wall and a secant pile wall? A diaphragm wall is built in rectangular panels with grabs or cutters. A secant wall is made of overlapping bored piles. Diaphragm walls are generally used for deeper and more heavily loaded excavations.

Why is bentonite used? It forms a dense fluid and a thin filter cake on the trench sides, which holds the soil in place until concrete replaces the fluid.

Can a trench cutter excavate rock? Yes. With suitable cutting wheels and teeth, cutters can excavate many types of rock, which is one of their main advantages over grabs.

How long is a diaphragm wall panel? Panels are typically a few metres long, often set by the opening of the grab or the width of the cutter and by the stability of the trench.