Dry soil mixing is a ground improvement technique that improves soft, high moisture clays, peats, and other weak cohesive soils, by mechanically mixing them with dry cementitious binder.

Keller rig performing dry soil mixing

Common uses

Applicable in soils that have a high moisture content, and often used in high groundwater conditions, to allow a chemical reaction of the soil and groundwater with stabilising binders injected in dry form
Increase bearing capacity
Decrease settlement
Fixate contaminants in situ
Increase global stability

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the binder is pumped pneumatically through the drill steel to the tool, where it is mixed with the soil as the tool is withdrawn.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.

Advantages

Quiet and vibration free
Environmentally-friendly as it produces almost no spoil and uses minimal binder
Almost no spoil
Operations can be done at low temperatures
Can replace more expensive deep foundation methods
Flexible in application
Reduces construction time as follow on works can proceed rapidly following installation

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, binder dosage, injection flow rate and pressure, total volume of slurry used, mixing tool velocities and rpm during penetration and withdrawal.

The mixing energy and binder content is calculated from this information to match design requirements.

Specimens of stabilised soils for testing are often obtained from the upper section of fresh columns with a push sampler.

Advanced core drilling and other field-testing methods can also be used in some circumstances 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.