DFI Magazine September/October 2020 - 103

interaction during seismic events is
complex, simplified methods of analysis
are available that may suffice for many RI
projects. In addition, fully coupled, 3-D
numerical analysis methods are available
for critical projects.
Transient free-field soil displacement
can be estimated by performing site
response analyses, and permanent soil
displacements can be evaluated using
seismic slope stability methods. These freefield soil displacement distributions can be
applied as a pseudostatic kinematic load to
a beam on a nonlinear Winkler foundation
analysis (for example L-Pile). The structural response of the RI to the pseudostatic
deflection would be investigated and
moment capacity versus demand would
be checked. Locations with significant

Figure 3. Horizontal shear with
RI-supported embankment

moment demand typically occur at
inflection points in the soil displacement
profile (interfaces between soft and stiff
soils) and in zones of higher curvature (see
red circles in fig. 4).
The performance of RIs subjected to
seismic inertial and kinematic loading must
be evaluated for both capacity and
serviceability objectives. If moment
demand exceeds capacity, there is a
potential for plastic hinges to
develop. That situation has potential engineering consequences and
impacts on system performance,
such as decreased system bearing
capacity and additional settlement,
and must be addressed.

a steel cage. Typical design scenarios that
require reinforcement of RIs include: when
RIs need to resist tensile forces; when RIs are
subjected to large lateral forces or bending
moments (typically due to lateral movement
of walls or embankments); when RIs are
used to resist seismic forces (shear); and,
when constructability concerns exist, such
as: RIs being closely spaced and requiring
installation in two passes; expected site
heave during RI installation; follow-on work
that involves significant excavations; or
when vibrations occur near installed RIs.
Reinforcement used for constructability
reasons consists of a single steel center bar
installed over the top 10 ft to 20 ft (3 m to
6.1 m) of the RIs. The purpose of the steel is
to provide the RIs with more capacity and to
maintain continuity in the event of cracking.
RIs are also occasionally used to resist tensile
forces or to support embankments over soft
soils that experience large lateral movements during construction. In these cases,
the RIs are reinforced with steel cages to
resist lateral forces.
The design of steel-reinforced RIs varies
drastically depending on the reason for the
reinforcement. Typically, no reinforcement
design is required when RIs are reinforced
solely for constructability concerns. The bar
is typically sized based on construction
practices and material availability, and is
most often a No. 8 bar, Gr. 60 steel. In these
cases, the RI is installed and then the steel
bar is pushed into the center of the RI. When
the reinforcement is required for tensile
forces, the RI is designed to structurally resist
the tensile force. Then, the geotechnical

Steel Reinforcement
RIs are typically unreinforced, fully
grouted elements that are designed
to resist axial compressive forces.
However, there are some conditions
in which RIs are reinforced, either
with a single center steel bar or with

Figure 4. RI-soil interactions from seismically
induced loadings

capacity of the soil surrounding the reinforced portion of the RI is checked, since
that capacity generally dictates the length to
which the steel reinforcement needs to
extend. For RIs resisting large lateral forces,
bending moments, or that are subjected to
seismic forces, the design generally follows
guidelines similar to those used for augercast piles.

Conclusions
RIs can be used for a variety of ground
improvement applications ranging from
settlement control to liquefaction mitigation. Design of RIs needs to account for
proper load-sharing considerations, including inertial and kinematic loading under
seismic conditions. Steel reinforcement is
generally not used, but is required when RIs
are subjected to uplift, bending and lateral
loading. A robust QA/QC program is
essential to confirming the effectiveness of
RIs for the applications discussed here.
José Clemente, Ph.D., P.E., D.GE, is the manager of
geotechnical engineering for Bechtel. He is also a
Bechtel Fellow and vice chair of the Geo-Institute's
Soil Improvement Committee for the American
Society of Civil Engineers.
Tanner Blackburn, Ph.D., P.E., is vice president of
engineering for Keller North America. He has over
15 years of experience in geotechnical engineering
design and construction.
Jie Han, Ph.D., is the Glenn L. Parker Professor of
Geotechnical Engineering at the University of
Kansas and chair of the Geo-Institute's Soil
Improvement Committee for the American Society
of Civil Engineers.
Roberto Lopez, P.E., is chief engineer for Malcolm
Drilling. He has over 30 years of experience and
oversees the technical aspects, design and engineering
of projects in the United States.
Morgan NeSmith, P.E., is Berkel's director of
engineering and the immediate past chair of the DFI
Augered Cast-In-Place and Drilled Displacement
Pile Committee as well as a current member of the
DFI Board of Trustees.
Tim Siegel, P.E., G.E., D.GE, is CEO and a senior
principal engineer for Dan Brown and Associates
and chair of DFI's Ground Improvement Committee. He has over 25 years of experience in
foundations and ground improvement.
Sonia Swift is the design manager at Menard USA
and has 14 years of experience in the industry. She
focuses on ground improvement design and
construction.
DEEP FOUNDATIONS * SEPT/OCT 2020 * 103
DFI Magazine September/October 2020

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