An innovative process for the construction of cut-off and retaining walls for:

Deep excavations

The Cutter-Soil-Mixing or CSM technique is a specialist foundation construction process for the formation of stiffened soil elements by the introduction and mixing of fluid binders into the soil. The technique has been developed and tested by BAUER Maschinen GmbH of Schrobenhausen, Germany in co-operation with Soletanche-Bachy of Nanterre, France. This ground engineering technique was initially developed essentially as a method of soil mixing but with the development of more powerful cutters, the applications of the technique have been widened to encompass retaining wall construction, where it displays significant advantages over more established methods of retaining wall construction. In particular, this technique generally offers a significant cost saving, a superior final finish and simpler details for sealing slab/wall joints, when compared to many of the alternative/traditional shoring methods.

DSC_0715Traditional soil mixing processes comprise of the introduction and mixing of a grout suspension into the in-situ soils, with the mixing tools rotating about a vertical axis. The development of sets of multiple, overlapping mixing tools facilitated the production of panels instead of single columns. This process, based on the principle of the rotary drilling technique, produces an inferior finish from an aesthetic aspect and further lacks the precession of installation of the CSM technique.

In contrast, the CSM process is derived from the hydrofraise cutter diaphragm walling technique, whereby two counter rotating cutting wheels rotate about horizontal axes, but with several points of difference. During diaphragm wall construction the hydrofraise operates as a reverse circulation process for the removal of soil and its replacement by a stabilising fluid, usually bentonite suspension; whereas in the CSM method the soil is broken up by the twin wheels, again rotating about horizontal axes, and mixed with grout which is introduced at the level of the cutter wheels. This grout/soil combination is mixed in-situ by the rotating wheels to form a self hardening soil-cement mortar.

One notable benefit derived from the use of the CSM technique is the accuracy of the installation process, by virtue of the fine degree of adjustment provided by quality installation equipment, the rigidity of the kelly guide arrangement and the in-rig monitoring system.

The principal area of application of the CSM technique is in the construction of impermeable retaining and cut-off walls, as an alternative to conventional diaphragm walls, bored or CFA pile walls or sheet pile walls.

The CSM technique can, therefore, be deployed to construct deep foundations, retaining walls, including load bearing walls and hydraulic cut-off walls.

Description of the cutter head

The heart of the cutter system comprises two cutter gear drives, each driving a standard cutter wheel. In contrast to the hydrofraise cutter operation, the cutter wheels can be rotated in both directions. To achieve the soil mixing, the slurry is pumped through a central nozzle between the two cutter wheels and mixed with the loosened soil material by the rotating cutter wheels. These rotating cutter wheels, with vertical deflector bars mounted between the cutter teeth, have the effect of a compulsory mixer.

The mixed soil is fluidised and forced upwards at both sides of the kelly bar mounted mixing unit. The hollow stem of the kelly bar houses both the slurry supply pipe and the hydraulic hoses for supplying power to the wheel drive motors. The cutter unit is mounted on one of CF Group’s larger drilling rigs which has, in built, the power necessary to drive the cutter wheels . The plan area of the resulting mixed slurry-soil panel is 2.80 m x 0.60 m, however, different size wheels can be fitted to install panels of different thicknesses.

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Comparison with alternative techniques

Bauer RTG25S and BG30 piling - both CSM modes with BCM10 cutter wheels at East Village, Dasco, Zetland

The CSM technique offers significant advantages over conventional retaining wall or cut-off wall installation techniques.

Comparison with methods involving soil excavation (diaphragm wall, bored pile wall):

The in-situ soil is utilised as construction material using CSM

Minimal spoil removal using CSM.

The majority of surplus soil-cement mixture can be regenerated and recycled for further use.

The residual soil material can be removed off site as dry material.

Comparison with other soil mixing techniques (multiple auger or mixing paddle based).

Advantages of the CSM technique are as follows:

Wall depths of 25 m and daily outputs of 200 m2 can be achieved with base machines of operating weights ranging from 70 to 90 tonnes and power outputs of 260 to 300 kW.

Greater accuracy of installation reduces the risk of gaps at panel joints.

Better control of the verticality  of the wall panels is achieved with the counter rotation of the cutter wheels.

Clean and trouble-free joint construction facilitated by the cutter wheel principle for wall panels of different ages (e.g. after weekend rest periods or stoppages during the working process).

A conventional piling rig is used as the base machine.

The refit from standard drilling rig to CSM carrier rig is accomplished within a few hours.

Hard soil strata can be penetrated without difficulty by using the cutter wheels as a loosening and mixing tool.

The continuous display of installation parameters supports the rig operator in controlling the production process. This includes actual depth, speed and rotational direction of cutter wheels, hydraulic pressures at cutter wheel motors, ambient pressure at cutter head, the rate of grout delivery and the total volume of grout pumped.

Automatic recording of installation parameters as documentation of completed works.

Vibration-free and low-noise process.

Work sequence

Prior to the formation of the wall, a trench is excavated along the centre line of the wall alignment.

As the cutter penetrates the ground and the in-situ soil is broken down and mixed by the rotating wheels. A bentonite-cement slurry is introduced into the loosened soil material converting it to a fluidified plastic bentonite-cement-soil mixture.

The pre-excavated trench contains the backflow of the slurry-soil mixture.

On reaching the required depth, the supply of the bentonite cement slurry is stopped  and a cement grout is introduced as the CSM unit is raised, whilst continuing mixing. During gradual withdrawal of the mixing unit the cement slurry is mixed thoroughly with the soil material by the counter-rotating cutter wheels.

The insertion of steel beams into the completed wall panels, whilst the mixed soil remains fluid, enables the finished walls to perform the function of structural, load-bearing retaining walls.

Finished CSM wall at ManlySummary

This relatively new system for installing cut-off and shoring walls has proved to be effective and cost-efficient for projects throughout the world and is, where appropriate soil conditions exist, being preferred to more traditional systems, such as secant pile walls. This revolutionary system forms a major part of CF Group’s activities in providing complete shoring and foundation solutions.