| Soil Mixing-Dry Method |
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| 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
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| 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.
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- 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.
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- 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.
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- 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.
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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. |
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| 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
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- Soft and wet soil
- Treatment is possible up to depths
of 100 feet
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| 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. |
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| 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. |
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| 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. |
- 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
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| 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
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- 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.
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