Services > Ground Improvement > Soil Mixing (Wet-Method)
Soil Mixing-Wet Method

Soil Mixing - By The Single-Axis Tool Method
Soil Mixing, also known as the Deep Mixing Method, is the mechanical blending of the in situ soil with cementitious materials (reagent binder) using a hollow stem auger and paddle arrangement. The intent of the soil mixing program is to achieve improved character, generally a design compressive strength or shear strength and/or permeability. Soil mixing can also be used to immobilize and/or fixate contaminants as well as a treatment system for chemical reduction to a more ‘friendly’ substrate.


Reagent Binder
Typically, the reagent is delivered in a slurry form (i.e. combined with water), although dry delivery is also possible. Depending on the soil to be mixed, the volume of slurry necessary ranges from 20 to 30 percent by volume. Can be a variety of materials including:
  • Cement (Type I through V)
  • Fly ash
  • Ground Blast Furnace Slag
  • Lime
  • Additives
  • Combination of the above
Equipment
 
Single Axis Mix Tool:
No single tool will be the best for all soil types and, for this reason, mix tools are often developed for individual projects. Considerations include:
  • Soil type and available turning equipment.
  • Often designed for particular site conditions
  • Size ranges from 1.6 to 11.5-ft diameter.
  • Can be a combination of partial flighting, mix blades, injection ports and nozzles, and shear blades.
Kelly and Swivel:
  • The Kelly rod transfers the torque and feed pressure to the tool.
  • The swivel seals the connection to the Kelly for delivery of the binder during rotation.
  • The Kelly can be single annulus or dual annulus to inject slurries at different locations on the mix tool.
Mix Machine:
  • Drilling system to drive the tool. Size varies from conventional hydraulic drill heads to dual-motor, crane-mounted turntables.
  • Torque requirements range from 30,000 ft-lbs to 300,000 ft-lbs.
Binder Delivery:
  • Batching system – computer controlled colloidal shear mixer to in-line jet mixing system.
  • Temporary storage (usually agitated).
  • Pumping system – trash pumps, moyno progressive cavity pumps, triplex piston positive displacement pumps.
Soil Mixing Process

As the soil mix tool advances into the soil, the hollow stem is used as a conduit to pump the binder and mix it with the soil in contact with the paddle. Single columns or integrated walls are created as the augers are worked in overlapping configurations. Mixing energy (penetration and rotation rates) is combined with binder dosage to obtain desired soil-cement product.

Soil Mixing Applications
Initially, the market was expected to be for environmental applications, or civil applications with environmental overtones. Continuing technology advancement has reduced costs to a point where soil mixing is finding a greater market in civil work for:
  • In Situ Walling
  • Excavation support
  • Port Development
  • Tunneling Support
  • Foundation Support
  • Liquefaction Mitigation
Soil Types and Treatment Depth
  • Soft and wet soil
  • Treatment is possible up to depths of 100 feet
Cohesionless soils are typically easier to mix and blend than cohesive soils. Cohesive soils vary widely in their ability to be mixed, depending on the soil type, strength, water content, stratigraphy and texture. Organic soils may require significant quantities of binder or pretreatment, and laboratory testing prior to production is recommended for all projects.

Important Geotechnical Considerations
In situ soil mixing can treat a wide range of soil types, and this treatment can vary depending on applied mix energy and reagent binder type. Soft cohesive soils are usually targeted as other technologies can treat other soil types more economically. This system does create a spoil material (when the reagent binder is injected in slurry form) consisting of a mix of soil and slurry binder. This spoil will solidify and can usually be used as a fill material elsewhere on the project.

Mixed Soil Strengths
A range of unconfined compressive strengths between 10 psi and 500 psi is possible, depending on the native soil and the binder content of the mixed soil. The character of mixed soil is usually brittle, of low confining pressures, with maximum strength at about one to two percent strain. Many factors influence the soil-cement produce including method of installation and soil and binder types.

Quality Assurance / Quality Control
  • Pre-production laboratory testing to prescribe mix methodology, energy and binder slurry system
  • Production documentation, to include:
  • Column identification
  • Column diameter
  • Working grade
  • Mixing depth
  • Start/time at bottom/finish time
  • Mixing duration
  • Grout mix details
  • Grout injection rate and pressure
  • Total grout injected
  • Tool rmp on penetration/withdrawal
From this information, the mixing energy and binder content can be calculated to match laboratory and/or test columns
  • Wet sample retrieval (in some soils) for laboratory testing
  • Pre-and post-construction CPTs
  • In large diameter mixing, visual inspection by drilling a shaft into overlapping columns
  • Post-construction field measurement
  • FLAC Analysis of the soil mixed structure with applied loading
Advantages of Soil Mixing
  • Soil mixing has the ability to strengthen soft and wet cohesive soils in a very short time period to permit many types of construction projects.
  • Soil mixing can be utilized as ground improvement of a site, to individually treat foundation locations, or provide retaining walls.

 
©2003 Hayward Baker, All Rights Reserved. Privacy Policy
Site Design by HindSite Interactive, Inc.