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How is Soil Mixed?

Published in Soil Improvement Techniques 4 mins read

Soil is mixed primarily through mechanical processes, most commonly by using specially designed augers or special tools to mechanically mix the soil with an in-situ binder. This technique, often referred to as soil mixing or deep soil mixing (DSM), is a ground improvement method used to enhance the engineering properties of soil.

The Core Process of Soil Mixing

At its essence, soil mixing involves the mechanical blending of existing soil with various binding agents directly within the ground (in-situ). This transforms the soil into a more stable and stronger material.

Key Components of the Mixing Process:

  • Specialized Equipment: The process utilizes specially designed augers or special tools. These tools can be single-axis, double-axis, or multi-axis augers, designed to penetrate the ground and thoroughly mix the soil. For deeper applications, specialized mixing heads or cutting tools are attached to drilling rigs.
  • Mechanical Mixing: The core of the operation is the mechanical mixing action. As the auger or tool rotates and penetrates, it churns the soil, creating a homogeneous blend. This ensures that the binder is uniformly distributed throughout the soil matrix.
  • In-Situ Binders: An in-situ binder is introduced during the mixing process. These binders react with the soil and groundwater to form a solidified, improved material. Common binders include:
    • Cement: Often used for strength and stiffness.
    • Lime: Effective for treating plastic clays by reducing plasticity and increasing workability.
    • Slag: A by-product of steel production, used for its pozzolanic properties.
    • Bentonite: Used for creating low-permeability barriers.
    • Fly Ash: Another industrial by-product, contributing to strength and reducing permeability.

Why is Soil Mixed?

Soil mixing is a versatile ground improvement technique employed for various purposes, including:

  • Soil Stabilization: Enhancing the strength, stiffness, and bearing capacity of soft or weak soils.
  • Permeability Reduction: Creating low-permeability barriers for groundwater control or containment of contaminants.
  • Liquefaction Mitigation: Increasing the resistance of loose, saturated sands to liquefaction during seismic events.
  • Contaminant Solidification/Stabilization: Encapsulating or immobilizing hazardous contaminants within the soil matrix to prevent their migration.
  • Excavation Support: Creating stable walls for deep excavations.

Common Types of Soil Mixing Techniques

The method of soil mixing can vary depending on the depth, soil type, and project requirements. Here's a brief overview:

Technique Description Typical Depth Applications
Wet Soil Mixing Binder slurry (cement, water, additives) is injected through hollow augers while mixing. Shallow to Deep Soft soil stabilization, liquefaction mitigation, retaining walls
Dry Soil Mixing Dry binder (cement, lime) is injected pneumatically through hollow augers, reacting with in-situ water. Shallow to Medium Stabilizing highly saturated soft clays, reducing water content
Mass Mixing Large-scale mixing for large volumes of soil, often using excavators with specialized mixing attachments. Shallow Embankment stabilization, landfill caps, general ground improvement
Jet Grouting High-pressure jets cut and mix the soil with binder slurry, creating columns or panels. Deep Underpinning, excavation support, permeability cut-off walls

Practical Insights and Examples

  • Infrastructure Projects: Soil mixing is frequently used in road construction, railway embankments, and bridge foundations to create stable subgrades in areas with poor soil conditions.
  • Environmental Remediation: For sites with contaminated soil, soil mixing can encapsulate pollutants like heavy metals or organic compounds, preventing their leaching into groundwater. For example, a contaminated former industrial site might use soil mixing to solidify lead-contaminated soils, turning them into a stable, inert mass.
  • Urban Development: In densely populated areas with limited space for traditional excavation and replacement, soil mixing offers a viable solution for ground improvement beneath new buildings or structures.
  • Dam and Levee Construction: To reduce seepage and enhance stability, soil-cement columns created through soil mixing can be used as cutoff walls or internal buttresses.

By mechanically integrating binders with existing soil, the properties of the ground are significantly improved, making it suitable for a wide range of engineering and environmental applications.