Deep Soil Mixing (DSM) to strengthen soft soils in place
Deep Soil Mixing (DSM) improves soft or contaminated soils in place by mechanically mixing them with a cement, lime or blended binder, injected as a slurry (wet mixing) or as a powder (dry mixing) through hollow rotating mixing tools. The result is columns, overlapping walls or treated blocks of stiffened soil. It is used to support embankments and structures on soft ground, to form retaining or cut-off walls, and to stabilise or contain contaminated ground.
Suitable ground conditions and applications
Can be considered where…
- Soft clays, silts and organic soils beneath embankments, tanks and slabs.
- Cut-off and retaining walls of overlapping columns or panels.
- Stabilisation and containment of contaminated ground.
- Liquefaction mitigation in loose saturated sands.
Limitations
- Strength achieved depends on soil type, binder and mixing energy; a laboratory mix design and field trial are needed.
- Organic and highly plastic soils need more binder and give lower strength.
- Obstructions, boulders and stiff layers limit penetration.
- Treated soil is stiffer but not as uniform as concrete; variability is allowed for in design.
Suitability is confirmed from the site investigation and the performance requirement.
How it works
A rig with one or more hollow mixing shafts drills to the design depth. Binder is injected through the shafts while they rotate, during penetration, withdrawal or both, so the soil is broken up and blended with the binder.
Columns are placed on a grid for support, or overlapped to form walls or blocks. The treated soil cures over days to weeks and gains strength.
Design inputs
- Site investigation: soil types, moisture content, organic content, chemistry and groundwater.
- Laboratory mix design: binder type and dosage against target strength.
- Loads, settlement criteria and required strength.
- Column or panel layout, diameter and overlap.
- Spoil and surplus volume handling.
Trial and calibration
- Laboratory mix trials on site soils.
- Field trial columns with the proposed binder and mixing parameters.
- Cores or wet samples tested to confirm strength and uniformity before production.
Execution controls
- Binder dosage, injection rate and pressure recorded continuously.
- Penetration and withdrawal speed and mixing tool rotations per metre.
- Column position, depth and verticality.
- Wet samples taken at the agreed frequency.
- Spoil volume and condition observed.
Verification and acceptance
Design intent, production control and acceptance verification are kept separate, so it is always clear what was required, what was done and what was proven.
What is required
Strength, uniformity, permeability and overlap criteria agreed with the consultant.
What was done
Automated mixing records for each column.
What was proven
Unconfined compressive strength tests on cores or wet samples, and load tests where specified.
Deliverables
- Method statement and risk assessment
- Laboratory mix design report
- Column layout plan
- Trial report
- Mixing records
- Strength test results
Discuss whether Deep Soil Mixing (DSM) fits your ground conditions.
Share the site location, the investigation report and the performance requirement.