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In the mix
Two European firms have joined forces to develop a new soil mixing technique for deep cut-off and retaining walls.

Conventional soil mixing involves combining soil with a cementitious material insitu to form columns of strengthened ground, reports Max Soudain.

In contrast, the new Cutter-Soil-Mixing (CSM) process is derived from the cutting wheels used in diaphragm walling. As such, the method is a viable alternative to diaphragm, bored pile or sheet pile techniques for forming cut-off or load-bearing retaining walls.

The technique is being developed and tested by Bauer Maschinen and Soletanche Bachy. The heart of the system is a mixing unit comprising two cutter gear drives each driving a standard cutter wheel. In contrast to “normal” cutter operation, the wheels can be driven in both directions.

As the cutter wheels break up the soil, bentonite slurry is pumped through a central nozzle between. This is mixed with the loosened soil by the rotating wheels to create a plastic bentonite-soil mixture, which is then forced up both sides of the mixing units. Once final depth is reached, the cutter wheels are reversed and cementitious slurry pumped in to replace the bentonite. The counter-rotation of the wheels thoroughly mixes the cement slurry with the soil.

The system can form walls up to 25m deep in panels a minimum of 2.2m long and 0.5m wide. Steel stanchions can then be inserted into the completed wall panels if required to create structurally load-bearing retaining walls.

Full-scale field trials of the system were carried out in December 2003 at Bauer Maschinen’s test facilities in Schrobenhausen in southern Germany. Twenty-eight panels were installed in sandy gravelly soil.

Fourteen wall panels were built during the first phase of the trial, with depths ranging from 6m to 20m. Penetration and mixing behaviour of different mix designs were also tested.

After the basic feasibility tests were finished, a 20m deep, 8m diameter “circular” wall in the shape of a polygon was formed. Strength tests carried out on hardened soil-cement showed the shaft could be excavated to a depth of 10m without causing distress to the surrounding ground.

The field trial also allowed the team to optimise the work sequence. It was found that because the CSM mixing unit is large and bulky, a “good-sized” trench has to be dug along the centreline of the wall before mixing begins. The pre-excavated trench retains the backflow of the bentonite-soil mixture. Excess material is pumped to a recycling plant and separated, with cleaned slurry reused.

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