| Injection Systems for Expansive
Soils |
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- Typical injection depths are 7 to 12 ft
deep under building foundations and up to 40 ft deep or greater
under railroad subgrades and landfills
- An economical, in situ stabilization system
with a 30 year history of treating cohesive soils. Injection
methods vary according to the type of material being injected
- Increasing in situ moisture contents
to the plastic limit plus 2 to 3 moisture points
- Reducing pocket penetrometer readings
to 3.0 tsf or less
- Reducing average swell to 1.0% or
less in the treated zone
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* Acceptance testing method is dictated by the material
being injected |
- Water and surfactant (surface active
agent) are injected into expansive clay to preswell the
clay. Each injection “pass” is performed on 5 ft centers.
Several passes are required to effectively preswell a site.
Preswelling is typically used for large (50K+) buildings and
large areas of pavement. Water injection is inexpensive,
fast and easy to use
- A surfactant is added to the water to
reduce water tension and increase the rate of adsorption of
the water by the clay
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Several conditions combine to
make water injection an effective technique.
- As water is added to the
clay by the injection process, it is adsorbed by
the clay particles due to the presence of
hydrating ions located in the spacing between
the particles of clay
- Eventually, the clay will
adsorb an amount of water that satisfies the
clay particle’s charge, at which point the
swelling process is complete
- A plane of shear develops
at the outer envelopes of the clay/water system
which corresponds to an overall reduction in
shear strength of the clay. For this reason,
measurements of shear strength, such as those
obtained with a pocket penetrometer, can be a
fast and effective way to determine the success
of injection
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- Additionally, the surface of the
building pad will be covered with lime slurry as a result of
the process
- This material is mixed with the soil
to form a working platform after injection
- Lime injection is typically followed
by water injection to preswell the clay and further
distribute the lime into the soil
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- Lime is injected to fill the
dessication pattern of expansive clay with slurry and
stabilize the surface of the pad for workability
- Lime injection is used for building
pads as well as streets, parking lots and runways
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- When the calcium
hydroxide is in contact with the clay surface in
the dessication pattern of the soil, it reacts
with the silica and alumina in the clay to form
calcium silica hydrates and calcium silica
aluminates
- These cementing compounds
represent new molecules and are non-expansive
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- Unlike preswelling, potassium
injection limits the amount of water a clay will adsorb. For
this reason, potassium injection also is a method of
arresting heave occurring in existing structures
- Typically, potassium injection is
limited to use in existing structures as well as those that
are highly sensitive to movement such as residences
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- Is mixed with ammonium
lignosulphonate and injected into expansive clays to
chemically treat clay and dramatically reduce its affinity
for water
- Is typically injected prior to
construction of highly sensitive structures as well as
through the floors of existing structures to reduce ongoing
heave
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Several conditions combine to
make water injection an effective technique.
- The
amount of water a clay will adsorb is dictated
by the predominant ion located in the spacing
between the clay particles
- By
changing the predominant ion, the clay/water
system’s behavior can be
altered
Potassium and ammonium are ions
that can satisfy the clay particle’s potential
energy and not have excessive hydration
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- In less reactive soils, Geocem is
also used to improve the strength of the soil
- Geocem is a blend of 80% fine ground
limestone and 20% Portland cement klinker. These two
materials are injected to improve conditions beneath
railroad subgrades, pavements, and landfills
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- Is used to treat lower strength clays
and silts to improve bearing capacity by lowering moisture
content and increasing dry density and shear strength
- Is used to treat railroad subgrade
problems in high fills. It is also used to improve subgrade
conditions in pavement structures such as runways and bridge
approaches
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- When
lime/fly ash is injected into a low strength
clay or silt, it displaces water which is
trapped in the soil
- The
material will then react chemically with the
soil resulting in further increases in shear
strength
- By injecting lime/fly ash
or Geocem it is possible to lower the moisture
content of the soil and improve the dry density
which both contribute to shear strength.
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