In an increasingly demanding world, cost – sustainable construction solutions are gaining more and more importance. This applies also to the deep foundation industry. The Cutter Soil Mixing technology, combines minimised environmental impact, high performance with small jobsite set ups and reduces costs for dewatering and waste material disposal. These features make it a technology which is competitive for a wide range of applications such as shafts or deep walls. The deep guide wall constructed for the FalCon Project in Canada shows the advantages of this method.
Cutter Soil Mixing (CSM) is an advanced deep soil mixing method and there are many successfully completed projects around the world. It has become a very popular methodology especially for ground improvement, cut-off walls and rehabilitation as well as upgrading of dams and dikes. State-of-the-art equipment allows the mixing of natural ground with a cementitious binder in order to install high quality vertical structures economically.
The cutter head is equipped with two gearboxes and four mixing wheels, both sides turn in opposite directions during the in-situ mixing process. As with other methods of deep soil mixing – it can be used for soil improvement, installation of cut-off walls for retaining walls if subsequently reinforced.
A compressive strength up to 15 MPa for retaining walls and permeability in the magnitude of 1 · 10–8 m/s for cut-off walls can be achieved within an economic.
product range. After positioning of the CSM tool at the specified panel location, penetration takes place to the final depth. For slow penetration speeds or large depths, bentonite slurry or water (in clayey soils) is used (- “two-phase system”). In case of higher penetration speeds and/or small depths, cementitious slurry can be used from the beginning ( “one-phase system”).
After reaching final depth the withdrawal process starts. Cementitious slurry is added and mixed with the soil. For the two-phase system, the total amount of cement is mixed into the soil in this step. For the one-phase system, only the difference between the calculated cement mass and the mass already introduced during penetration is added. In either case, the mix is further homogenized.
As a special application during an exploration at the FalCon Project in Canada 2018/2019, CSM panels were installed as guide wall collars for the trench cutter program. The CSM collars stabilized the upper loose sand strata, counteracted fluid fluctuations and guided the trench cutter in the in the surface area.
A CSM collar consisted of 7 panels with a width of 800 mm and a depth of approximately 18-20 m based on ground refusal. Ground refusal was defined as a combination of the penetration rate and hydraulic pressure in the cutter head.
All CSM elements required a strength of higher than 2 MPa. Cementitious slurry with a volume of 1.5 m³/m was injected into the ground to reach the strength. Each slurry batch was mixed with 473 kg/m³ of cement and 45 kg/m³ of bentonite using a water cement ratio of 1,6. Laboratory tests were carried out to control the strength of the slurry-soil mixture. Slurry samples from the mixed panels were retained after 7 days. The construction of one panel took 2 – 3 hours each.
The depths of CSM wall applications have been extended to 100 m by use of wire suspended systems. With the rope system, the penetration force is dictated by the weight of the CSM tool body which also provides directional stability.
The CSM tool is fitted with a set of instruments to display real-time information to monitor and control the orientation of the tool by mobile steering devices.
Alexander Blatt, BAUER Maschinen GmbH, Germany
Franz-Werner Gerressen, BAUER Maschinen GmbH, Germany
Stefan Schwank, BAUER Maschinen GmbH, Germany