Diaphragm Wall Grab Product

Overview

A diaphragm wall grab is a specialized excavation tool designed for constructing deep foundation walls in confined urban settings. Unlike open caisson methods, the grab works inside a continuous slurry-supported trench, allowing controlled excavation of clay, silt, and weak rock at depths exceeding 100 m. The equipment is integral to the diaphragm wall process: the slurry supports trench walls, the grab clamshell removes spoil, and the clamped material is raised to the surface for disposal or reuse.

Diaphragm wall construction dominates deep basement excavation for high-rise buildings, subway stations, and underground car parks. The grab attachment suspends from a tower crane or a fixed piling rig leader frame. Operators work from a control cabin, guiding the grab via proportional radio remote, achieving submeter placement accuracy. The hydraulic system must tolerate frequent cycling (300+ per day), contamination from wet clay slurry, and rapid pressure reversals.

The grab is built for ruggedness over finesse. Wear parts are easily replaceable—buckets, guide collars, rope, hoses—and the modular design allows quick field repair. A 3.2 m wide grab handling 600 tonnes per cycle is typical for new building basements in Singapore or Hong Kong; smaller 2.4 m grabs serve airport or tunnel works where access is more restricted.

How it works

The grab descends open under its own weight, guided by four vertical steel posts to keep the bucket halves parallel. A proportional hydraulic circuit operates two closing cylinders. When the operator pushes the joystick, the solenoid spool shifts, directing pump flow to the cylinders. Oil floods one end of each cylinder, driving the piston rod and pulling the bucket halves together via rigid linkage. Spring-loaded check valves prevent accidental opening under load.

Closing pressure builds to the system setting (210 bar) within 2–3 seconds. The operator feels resistance via proportional pressure feedback; with experience, an operator can "feel" whether the grab has bitten into hard clay or whether it is slipping on wet silty sand. Once closed, the grab clamps with force approaching 800 tonnes on a 3.2 m bucket, sufficient to hold dense spoil.

The hoist system features a four-part wire rope with mechanical advantage 2:1, equalizing load across four suspension points. As the grab closes, it becomes heavier; the rope system distributes this asymmetric load evenly to the crane hook. The sheave block uses bronze-lined sheaves on tapered roller bearings, rated for 5 million cycles per year.

Descent is controlled by opening the cylinders. The operator cracks the joystick backward; oil exhausts from the cylinder head end through a proportional throttle valve to tank. Descent rate (0.5–1.5 m/s) is set by orifice sizing and proportional valve opening. In slurry-filled trenches, the descending grab displaces slurry volume; operators monitor slurry level and consistency using a telescopic gauge or gamma probe.

On the surface, the grab discharges into a hopper or truck. The operator centers the grab over the hopper, then pulls the joystick to open. Cylinders extend, the buckets hinge apart, and spoil falls freely. Wet clay tends to stick; some grabs are fitted with a mechanical knocker or air vibrator to loosen it.

The Guide Frame frame is critical. Four posts spaced to match bucket width ensure the buckets stay parallel even as they wear. Misalignment causes spilling, loss of capacity, and rapid wear-ring failure. Guide collars are bronze-lined for low friction; during a 1000-tonne grab's closing cycle, friction forces exceed 100 kN.

The Hydraulic System system is open-center: pump flow returns to tank whenever the operator is idle. A proportional directional control valve with onboard Pressure Sensor provides load-sensing feedback. The Accumulator absorbs shock when the buckets slam to a halt after closing. Pressure spikes to 250 bar for 0.1 seconds; the accumulator, precharged to 190 bar, smooths the spike, extending cylinder seal life.

Rope wear is relentless in slurry. Slurry is abrasive, and the rope is in constant contact with the sheave block. The Wire Rope is inspected daily for broken wires; once the count exceeds 6 per strand, the rope is replaced. A 60 m length costs €800–1200 and takes 2 hours to swap.

Typical production rates: in medium clay, a well-practiced operator manages 30–50 grab cycles per hour, excavating 12–18 m depth per 8-hour shift. In harder clay or mixed materials (clay with cobbles), the rate drops to 20–30 cycles per hour. Hard rock requires specialist bits or smaller buckets, and production can halve.

Wear and maintenance

Bucket wear is accelerated by angular cobbles and cemented lenses. After 500 hours of service, buckets show 20–30 mm recession on cutting edges. Operator technique matters: plunging vertically at high speed shatters buckets; a controlled, rocking descent is gentler. Bucket replacement costs €12,000–18,000 per pair and requires a one-day shop visit.

The hydraulic system demands discipline. Slurry enters cylinders through micro-leaks in rod seals, contaminating oil. Many operators replace the full charge (2000 L) every 2000 operating hours, or quarterly, whichever comes sooner. A Seal Kit kit costs €2500 and includes rod seals, piston seals, and wear rings. Downtime for seal replacement is 3–4 hours per cylinder.

The Control System receiver module can fail in wet conditions despite IP65 rating. Operators often keep a spare radio system on site, swapped in < 1 hour. Battery-powered remote pendants must be recharged daily; typical runtime is 8–10 hours.

Standards and certification

Diaphragm wall grabs are classified as crane attachments under ISO 4413 (hydraulic safety), FEM 1.001 (cranes), and regional building codes. Most European and Asian sites require third-party certification (TÜV, DNV) and annual load testing to 150 % of rated capacity. Material test certificates for bucket steel and rope must be retained on file. Operators require a crane ticket (NCCCO or equivalent) plus grab-specific training; typical onboarding is 40–60 hours of supervised work.