available that can easily increase the bearing capacity of
even the worst undocumented fill soils by up to five times
while leaving poor soils in place and not disturbing them.
High bearing capacity offered by these techniques means
smaller foundations are needed, thereby lowering overall
costs of the project. By stabilizing the existing soils, you can
avoid having to completely remove and replace the problem
materials, allowing for ease of construction, significant time
savings and relief from the regulatory burdens. Also, ground
improvement does not require the same structural foundation
treatment as piles. With ground improvement, simple
spread footings and slab on grade are all that is required.
One of the innovative technologies available to help mitigate
these challenges is called the Geopier GeoConcrete®
Column System (GCC). GCC elements are used for extremely
high loads in very poor soils, such as the waterfront soils
described earlier. GCC elements are considered a rigidinclusion
approach to ground improvement. Rigid inclusion
solutions are often used for soil stabilization when a load
is being transferred through very soft soils to stiffer soil or
rock, increasing load carrying capacity and reducing compressibility
of the poor soils. Rigid inclusions consist of
cement-treated aggregate, grouted aggregate or concreted
columns that are used to transfer the stress from the foundation
or embankment loads to more competent soil/rock
layers. While rigid inclusions can be used as a singular solution
for a project, they can also be used in combination with
a variety of ground improvement methods to provide an optimized
support solution.
GCC elements use quality-controlled, ready-mix concrete
to quickly build expanded base rigid inclusions with very high
capacities. A special mandrel pressure vessel is used with
a concrete pumping inlet, air valve and gauge, and a stone
valve at the bottom end. While pumping in ready-mix concrete,
and with the stone valve closed, the vertically ramming
mandrel is forced into the ground to the design depth, which
is typically a stiff-to-hard layer. The mandrel is then raised
and redriven repeatedly with full crowd and vertical ramming
energy, extruding an expanded base while improving
the base soils below and around the expanded concrete bulb.
The mandrel is then pulled up, while extruding a column of
concrete right to the surface.
Factors considered during installation include the pressure
on the concrete, the pressure in the air gap, the volume
of concrete inserted and the rate of mandrel retraction. A
load transfer cushion is then placed over groups of GCC elements
and a regular spread footing can be designed for high
bearing on that load transfer cushion, even in very poor soils.
GCC elements can typically treat very poor soils up to depths
of 16 metres using 300mm to 500mm diameter elements to
obtain very high capacity, while allowing for simplified foundation
elements (spread footings at up to 450 kPa SLS) on a
load transfer cushion.
Upon first glance, GCC elements appear somewhat similar
to belled caissons. While constructed from concrete,
there are several aspects of this system that set it in a different
class from traditional deep foundations such as caissons
(drilled shafts). The GCC elements do not require hard rock
or traditional “safe” end-bearing soils like deep foundations
TECHNICAL
Advantages of Geopier
GeoConcrete® Columns
Strong and stiff: The high modulus, rigid concrete
element with an expanded bottom bulb, transfer heavy
loads through weak soil layers and into a suitable
dense bearing layer to control settlement.
High quality: The GCC system is built from controlled readymix
concrete and the robust quality control monitoring during
the construction process, which results in the installation
of a high-quality ground improvement product.
Depth: Treatable depths greater than 15 metres.
Quick: Rapid installation process means
shortened project schedules.
Efficient: Optimized geotechnical and structural
capacity tailored to the soil conditions.
Economical: Often results in significant cost savings
compared to traditional foundation alternatives.
do. GCC elements can safely bear onto “good” soils such as
medium-dense glacial tills, often found in Ontario, for example,
at elevations well above bedrock. These tills are often not
considered high-capacity end bearing by expert engineering
consultants, and, as such, caissons that are designed to found
on tills can often be large and inefficient, with the structural
capacity of the caisson shaft far outshining the geotechnical
capacity of the caisson at the base on the till; resulting in
very large caisson diameters. Therefore, caissons often must
extend well into bedrock layers to ensure high capacity and a
reasonable diameter.
In contrast, GCC elements are rammed down onto the till
soils, improving the till and belling out the GCC element to
maximize the geotechnical capacity of the GCC such that the
geotechnical capacity at the base is well matched with the
structural capacity of the shaft. Further, unlike caissons, the
GCC elements are unreinforced, are small diameter and are
tightly grouped together. The most striking difference is the
ability to design a high capacity footing on GCC elements,
through the use of a specially-designed, yet simple, load
transfer cushion.
PILING CANADA 73