Deep soil mixing (DSM) is a ground improvement technique that improves soft, high moisture clays, peats, and other weak soils, by mechanically mixing them with dry cementitious binder.
Process
To construct the columns, a high-speed drill advances a drill rod with radial mixing paddles near the bottom of the drill string into the ground. During penetration, the tool shears the soils to prepare them for mixing. After the tool reaches the design depth, the binder is pumped pneumatically through the drill steel to the tool, where it is mixed with the soil as the tool is withdrawn.
The dry soil mixing process constructs individual columns, rows of overlapping columns, or 100% mass stabilisation, all with a designed strength and stiffness.
Common uses
- Applicable in soils that have a high moisture content, and often used in high groundwater conditions, to allow chemical reaction of the soil and groundwater with stabilising binders injected in dry form
- Increase bearing capacity
- Decrease settlement
- Mitigate liquefaction
- Fixate contaminants in situ
- Increase global stability
Advantages
- Quiet and vibration free
- Environmentally-friendly as it uses readily available materials
- Almost no spoil
- Operations can be done at low temperatures
- Economical
- Can replace more expensive deep foundation methods
- Flexible in application
- Reduces construction time
Quality assurance
Quality is controlled and assured through column installation protocols and the results of relevant laboratory and field-verification tests. Each column is provided with a chart-log, which typically comprises: element identification, mixing tool details, mixing depth, mixing time, slurry specification, injection flow rate and pressure, total volume of slurry used, mixing tool velocities and rpm during penetration and withdrawal and torque of the shaft.
The mixing energy and binder content is calculated from this information to match design requirements.
Specimens of stabilised soils for testing are usually obtained from fresh columns with a wet grab.
Advanced core drilling and other field-testing methods can also be used to obtain specimens and to inspect the continuity, uniformity and stiffness of deep soil mixing columns. The selection of suitable verification methods depends on their relevance, accuracy and applicability in relation to the purpose and pattern of soil treatment and design properties of stabilised soil.