pier. This unique method of installation results in significantly
increased horizontal stresses in the matrix soil.
Here’s what sets a RAP system apart when compared to
the stone column/aggregate pier process:
• Vertical ramming: Thin lifts of aggregate are rammed
to form a pier with undulating sides. Thin lifts allow for
better compaction of the aggregate and results in high
shear strength and stiffness. The tamper forces aggregate
into the RAP element sidewalls – providing excellent piersoil
coupling.
• Direct crowd force: Rather than being lowered off a
crane, the vertical ramming energy is augmented by
direct machine downward crowd pressure of 15 to 30
tonnes on each lift. Direct crowd combined with the
vertical ramming energy (not horizontal) massively
densifies the stone and drives up lateral stress in the
enclosing soil – crowd vertical ramming results in
higher performance.
• Alternative aggregates: The RAP construction method
is easily adaptable to use recycled concrete or cementtreated
aggregate. Alternative aggregates provide
flexibility to achieve more capacity in various soil types.
• Soil conditions: Geopier RAP elements can be installed
in almost all soil conditions. The crowd vertical ramming
process contributes to increasing the lateral stresses in
the matrix soils, which is especially important for soils
with silt or clay content that do not respond well to horizontal
vibrations.
• Greater capacity: The ramming process produces the
superior strength of a Geopier RAP and develops constructed
aggregate friction angles greater than 48 degrees
– almost 40 per cent greater than those reported for stone
columns. Higher friction angles provide greater pier
capacity, which means fewer Geopier elements versus
stone columns.
• Higher bearing pressure: Higher friction angles allow for
higher bearing capacities, which result in smaller footings
and lower foundation costs. Stone columns or generic
aggregate pier systems result in lower capacity piers and a
lower available bearing capacity.
• Better settlement control: The unique ramming process
creates piers that are two to nine times stiffer than stone
columns or aggregate piers. Lateral stress build-up in the
enclosing soil that happens in vertically crowd-rammed
RAP elements results in higher performance. Geopier
RAPs can be used on substantial structures including
buildings that are over 20 storeys.
• Proven advantages: Side by side comparisons demonstrate
significant performance differences between RAP
and traditional stone columns.
The orientation of installation forces is an important distinction
when considering ground improvement methods.
While some geologic regions in Canada are known for their
deposits of sand, non-sand soil such as silt and clay, are more
commonly encountered in Eastern Canada and project teams
must carefully consider the performance of stone column
(aggregate pier) elements in these soils. Fill is also a common
occurrence – particularly in major urban areas and the inherent
heterogeneity in undocumented fill is reason enough to
choose vertically rammed Geopier systems for these soil types.
To most people, the construction approach seems to
be generally the same: aggregate into a columnar shape
to improve the ground. It’s important to recognize that
maximizing performance requires the right installation
methodology. When working with soils other than straight
sand, an aggregate pier or stone column is just stone in the
ground. A RAP provides the performance and reinforcement
needed for those tougher soil conditions. While others may
want to put grout and concrete into their ground improvement
when faced with poor soils, simple yet effective RAP
systems are able to handle the most challenging soil conditions,
while maintaining a cost-effective structure.
Mark Tigchelaar is president and founder of GeoSolv Design/
Build Inc. Tigchelaar is a licensed engineer in both Ontario and
Alberta, chair of the Industry Advisory Board of York University
and the past chair of the Canadian Geotechnical Society-
Southern Ontario section. For inquiries about this article or
ground improvement, reach Tigchelaar at mark@geosolv.ca.
TECHNICAL
Geopier underground view of installation methods Geopier RAP being constructed
56 Q3 2020 www.pilingcanada.ca
/www.pilingcanada.ca
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