DFI Magazine September/October 2020 - 83

S3 2020: A Virtual Success
Benjamin Turner, Ph.D., P.E., G.E., senior
engineer, Dan Brown and Associates
Postponement to 2021 of the in-person
activities for DFI's ongoing conference
series, S3: Slopes, Slides, and Stabilization,
provided a unique opportunity to leverage
virtual meeting tools for the fourth Software
Discussion that has become a staple of S3.
With planning for the Software Discussion
well underway by the time it became
apparent that an in-person event would not
be possible this year, the organizing
committee, led by Program Chair Roberto
Lopez, P.E., of Malcolm Drilling, and
Conference Co-Chairs Christopher Ramsey,
P.E., of Wood Environment & Infrastructure Solutions (Deep Foundations for
Landslides/Slope Stabilization Committee
Chair) and Edward Laczynski, P.E., of GA &
FC Wagman and Jeff Segar, P.E., S.E., of
Braun Intertec (DFI/ADSC Anchored Earth
Retention Committee Co-Chairs), made
the decision to still hold the event in a
virtual format.
With behind-the-scenes coordination
from DFI's technical staff - building off of
their recent experience from delivering
SuperPile 2020 as a virtual event - the
panel members, organizing committee and
over 200 audience members were seamlessly connected for an engaging event. The
Software Discussion consisted of technical
presentations by representatives of four
software companies followed by a panel
discussion with Q&A. Given the technical
and detailed nature of the presentations,
feedback from many audience members
indicated that they actually preferred being
able to view the slides "up close" on their

Quick poll at S3

own computers rather than on a projector
screen in a conference room setting.
The virtual format also allowed the event
to begin with an audience poll, which
provided interesting insights into the state of
the profession and a stimulating precursor
to the presentations. Poll results indicated
that almost 80% of the audience use limit
equilibrium method (LEM) software as their
primary tool for performing slope stability
analysis (SSA), and that 95% of the SSA of
any kind is performed in 2D as opposed to
3D. In response to the question, "For LEM
SSA, how do you represent the resistance
provided by deep foundation elements in
the SSA model?" 52% of the audience
responded that they simply "assign the shear
strength of structural material properties"
from the available answers, which included
options of performing separate, laterally
loaded pile analyses to define the resistance
or collaborating with a structural engineer.
This was a perfect lead in to the presentations, which were developed around the
theme, What is the appropriate method for

incorporating resistance of deep foundation
elements in slope stability analysis?
The motivation for this year's theme is
the observation that in many instances,
engineers are assigning a resisting force in
SSA without explicitly analyzing the
behavior of the deep foundation under
lateral loading. Under many circumstances, the deep foundation would reach a
strength limit state other than shear failure,
such as flexural failure or geotechnical
"pushover" or "soil flow-around" failure,
prior to the full shear strength of the
foundation element being mobilized. This
means that assigning the structural shear
strength as the resisting force could
significantly overrepresent the available
foundation resistance, artificially inflating
the factor of safety (FOS). This is an
example of a true soil-structure interaction
problem that requires a working knowledge of both soil and structural mechanics, yet more than half of the audience
indicated they are sidestepping an important component of the analysis.
The presentations began with an
introduction of fundamentals by the event
moderator Benjamin Turner, Ph.D., P.E.,
G.E., of Dan Brown and Associates, who
illustrated the different failure limit states
and provided guidance on how to perform
combined SSA and laterally loaded pile
analyses to determine the controlling
mechanism. This was followed by an
overview of the problem assigned to the
software providers, based on the 2003
Trunk Highway 2 Landslide in Crookston,
Minnesota. The site was chosen because it
is well characterized in a report by Barr

Poll at Virtual S3

DEEP FOUNDATIONS * SEPT/OCT 2020 * 83
DFI Magazine September/October 2020

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