The in-situ soil is used as a construction material
Little generation of spoil
No vibrations induced during construction
Extended depths (up to 80 m) can be reached – when using rope-suspended units
Construction sequence
Preparation: Excavation of a guide trench for collecting surplus slurry
Step 1:
Positioning of the cutter head in wall axis.
Step 2:
The mixing tool is driven into the ground. At the same time self-hardening slurry or bentonite slurry is added. The soil matrix is broken up by the cutting wheels and liquified by adding slurry. The rotating wheels and cutting teeth push the soil particles through vertically mounted shear plates that have the effect of a compulsory mixer.
Step 3:
After reaching the final depth the mixing tool is extracted while self-hardening slurry is added.
Step 4:
Reinforcing elements required for structural purposes can be inserted into the mixed wall. Therefore, steel beams (usually H-beams) can be installed into the not yet hardened fresh mixed panels. The spacing of the beams and beam cross sections are designed on the basis of the applied loads.
A continuous wall is formed in a series of overlapping primary and secondary panels. Overcutting into fresh adjacent panels is called the „fresh-in-fresh method“.
The cutter technique also allows the “hard-in-hard method”, whereby secondary panels are cut into the already hardened primary panels.
The cutting and mixing procedure can be executed in two ways - in the one-phase or the two-phase system.
One-phase system
With the one-phase system binder slurry is introduced during both the downstroke and upstroke processes. During the penetration (downstroke) phase, cutting, mixing, liquifying and homogenising is performed while pumping the binder slurry and compressed air into the soil.
In the upstroke phase the remaining volume of binder slurry is blended into the soil. The speed of extraction can be higher as the majority of the binder slurry has already been mixed with the soil in the downstroke phase.
Advantages of the one-phase system:
No auxiliary desanding circuit required
Higher speed of extraction
Preferred application in easy and uniform soils, depth range < 20 m
Two-phase system
The soil is liquified and homogenised in the downstroke phase by pumping bentonite slurry or water (in case of clay) into the soil. The backflow can be pumped to a desanding plant, where the sand is separated from the slurry which is then pumped back to the rig.
After reaching the final depth, the injection of bentonite slurry or water is stopped and replaced by binder. During the upstroke process cement slurry is mixed thoroughly with the liquified soil. The speed of extraction and flow
of binder are adjusted to ensure that the total calculated quantity of binder is blended with the soil.
Advantages of the two-phase system:
No risk of getting stuck when working at extended depths or when the working process is interrupted, due to hardening bentonite slurry above the mixing tool
Wear and tear on the cutting wheels is reduced by using binders
Preferred application in difficult soil conditions and for extended depth
