July/August 2024 - 123

Letting Purpose Determine Risk
Rigid Inclusions (RIs) are columns that
are significantly stiffer than the
surrounding soil (typically made of
concrete or mortar) used to reduce
settlement and increase bearing
capacity. Over the last 30 years, their
use has evolved. Initially, RIs were
designed for low axial compressive
stresses (approximately 4 MPa or
600 Psi) and were not considered to
impact the footing's bearing capacity or
the structural design of the RI-supported
foundation. The RIs mitigated
set-tlement and allowed for the use of
shallow foundations and slab-on-grade
in place of pile caps, grade beams and
structural slabs. Load transfer platforms
(LTPs), which separated the RIs
from the structure, were carefully
designed and constructed. The result
was a highly redundant system with
lightly loaded elements.
As the industry became more comfortable
with RIs, we recognized that
they could be designed more strategically
and with higher axial compressive
stresses (7 MPa or 1,000
Psi ).
Owners and developers
saw an opportunity to
reduce costs by loading
RIs to higher stresses and
footings to higher bearing
pressures, using fewer
RIs and building smaller
spread footings. We also
began to use RIs to provide
bearing capacity on
sites where the soil alone
couldn't meet the bearing
capacity required. With
high applied loads and
the addition of a lifesafety
condition, the beacknowledge
what impact their place
on the spectrum has on the resulting
RI system.
RIs for projects on the left end of the
A regular column
from the DFI Risk and
Contracts Committee
By Sonia Sorabella
Swift, P.E., Director of
Engineering, Menard
USA
havior of the RIs becomes more
ambiguous, and design considerations
need to be more rigorous.
This evolut ion describes the
spectrum of RIs, which have been, can
be and should be used anywhere along
the spectrum, provided that all parties
As the industry became more comfortable with
RIs, we recognized that they could be designed
more strategically and with higher axial
compressive stresses (7 MPa or 1,000 Psi).
spectrum will benefit from some
redundancy due to load sharing with
the soil and the load supported by the
RIs will be significantly less than if the
RIs were designed for 100% of the
applied loads, like piles. RIs for projects
on the right end of the spectrum will
behave similarly to deep foundations
and the number of RIs may be similar to
the number of required piles, though
RIs may still meet the performance
criteria and offer cost savings. The large
range of behavior and requirements on
projects drives the need for an RI
designer that has the appropriate
experience and design rigor to evaluate
the system thoroughly and responsibly.
The load factors used for geotechnical
capacity of the RI, the bearing capacity
of the footing and the determination of
the test load must consider the purpose
of the rigid inclusion. Additional design
considerations include service versus
ultimate limit state performance
criteria, the
interaction between the
RIs and the foundations
they support, the LTP
thickness and composition,
and constructability.
There are many
factors but there's no
one-size-fits-all design
method for RIs, rather
understanding thei r
intended behavior on a
particular project is the
key to their responsible
design and application.
The rigid inclusion spectrum
DEEP FOUNDATIONS * JULY/AUG 2024 * 123
RISK CORNER
July/August 2024

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