Execution of diaphragm walls with hydrofraise
Definition of hydrofraise
Definition of hydrofraise
A hydrofraise is a reverse circulation excavation tool which consists of a heavy steel frame with two drive gears attached to its bottom end, which rotate in opposite direction round horizontal axes. Cutter wheels area mounted onto the drive gears. As they rotate, the soil beneath the cutter is continuously removed, broken up, mixed with the bentonite slurry in the trench and moved towards the opening of the suction box. The slurry charged with soil or rock particles is pumped by a centrifugal pump which is located right above the cutter wheels through a ring main of hose pipes to a desanding plant where it is cleaned and returned into the trench.
The verticality of the cutter and the trench is measured in the panel axis and perpendicular to the panel axis by means of two independent inclinometers. The data provided by these instruments is processed by the on-board computer and displayed so that the operator can continuously monitor and if necessary correct the verticality of the cutter. Adjustment of the verticality in two directions can be achieved with a system of steering plates. Throughout the correction process the rig operator is constantly guided by measures proposed on the computer screen.
The hydrofraise is also called hydromill and trench cutter (or just cutter).
Main applications of the hydrofraise
The hydrofraise is used for the construction of diaphragm and cut-off walls in difficult conditions: excavation of rock and hard layers of soil, deep panels and strict tolerances
The equipment required for the execution of diaphragm walls with hydrofraise is composed by the excavation machine (hydrofraise) and the slurry plant; its main functions are:
- Excavation of the soil.
- Pumping of the mix soil-support slurry from the trench to the slurry plant.
- Cleaning of the bottom of the excavation.
- Replacement of the support slurry before concreting.
- Production and storage of the support slurry.
- Pumping of the Support slurry from the slurry plant to the trench.
- Separation of the mix soil-support slurry created by the hydrofraise.
- Treatment of the waste slurry.
Hydrofraise slurry plant
The execution of diaphragm walls with hydrofraise is based on the reverse circulation technique. The excavation is carried out by milling soil by means of two drums fitted with tungsten carbide cutters, rotating in opposite directions. Particles of soil, suspended in the support fluid, are pumped off the trench by utilizing a suction pump and delivered to the desanding plant through the pipelines. At the plant the particles of soil are removed from the bentonite suspension, which is recycled into the storage tanks.
The construction phases are as follows:
- The support slurry is prepared in the slurry plant. Bentonite powder (1) is mixed with water in a high turbulence mixer (2) to produce the support slurry, which is pumped (3) to the storage tanks (5).
- The support slurry is pumped from the storage tanks to the trench my means of a centrifugal pump (6).
- The hydrofraise (8) cuts the soil, mixes it with the support slurry and pump it back (8) to the plant, where the soil is separated from the slurry with a desanding unit (4).
- Waste slurry is pumped to the waste slurry storage tank (9) and then treated with a filter press (10).