In-situ soil mixing and the choice between dry mixing and wet mixing

Soil Mixing (SM) for Ground and Environmental Enhancement

Soil mixing (SM) has seen widespread and growing application since its inception for improving the engineering and environmental properties of soft or contaminated ground. The popularity of SM is primarily due to its high flexibility, which makes it adaptable to the specific demands of complex projects and variable ground conditions.

Another key advantage is the cost-performance ratio, which is often more favorable compared to alternative geotechnical solutions.

In this method, in-situ soil is mixed with stabilizing binders that chemically react with soil or groundwater. The resulting stabilized soil, particularly when used in excavation support, exhibits higher strength, reduced permeability, and lower compressibility compared to its original state.

Soil improvement is based on:

• Cation exchange in clay minerals

• Particle bonding

• Pore-filling from reaction by-products

The most commonly used binders are cement and lime, although materials like blast furnace slag, gypsum, fly ash, and other composite binders are also used. In environmental remediation, binders are often replaced with chemically reactive but non-toxic materials, such as chemical oxidizers.

Two Main Categories of Soil Mixing

Soil mixing techniques are generally classified into:

• Deep Soil Mixing (DSM)

• Shallow (Surface) Soil Mixing

Both categories include a variety of specialized equipment and systems.

Choosing Between Wet and Dry Mixing Processes

As with foundation techniques and other ground improvement technologies, selecting the most suitable soil mixing process requires engineering judgment. Both technical and economic factors must be considered in the context of the structure type, soil conditions, and available equipment.

While the choice between wet and dry processes is often project-specific, identifying precise execution limitations and validating selection criteria can be challenging.

Engineering Considerations:

To achieve high strength in the wet mixing method, cement dosage must be increased—which is manageable during construction, albeit with higher waste generation.

However, for both wet and dry methods, higher strength requirements generally mean increased binder content. In this context, in-situ mixing conditions play a critical role in the stirring process. Wet mixing allows easier and more uniform distribution of cement grout compared to dry cement powder, especially when:

• The natural soil has low water content

• The soil exhibits high shear strength

In dry mixing, complete recovery of air injected into the soil is a major concern. Reduced efficiency in air recovery can compromise the uniformity of soil columns, directly affecting the strength and stiffness of the treated ground.