July/August 2024 - 113

FEATURE ARTICLE
A Tale of Two Very Different MSE Wall Structures
Mechanically stabilized earth (MSE)
walls are composed of three main
elements: soil, reinforcement and a
facing. If any of these elements are not
functioning, its internal performance
would be unacceptable. In extreme
cases, it will result in a collapse. However,
if material characterization, design
and construction are properly done, MSE
walls will exhibit excellent long-term
performance while being economical. In
fact, when using multiple reinforcements,
redundancy is rendered, i.e.,
'overstressing' loads can be alleviated by
shedding it to understressed layers.
Common design of MSE walls with
respect to reinforcement and facing is
generally conservative. However, soil is
often poorly characterized. The result is
either an overly conservative structure
or worse, unacceptable structural performance.
Underestimation of backfill
strength is typical as often default shear
strength is used. For example, the
American Association of State Highway
and Transportation Officials (AASHTO)
specifies select reinforced soil, suggesting
a default friction angle of φ=34°
with an upper limit of 40°. For the select
backfill specified in AASHTO, this angle
is likely to be 45-55° implying that the
actual strength of the compacted
backfill could be as much as 50% larger.
However, if in construction the reinforced
soil is poorly compacted or
drained, conservatism due to underestimated
soil strength may vanish,
potentially leading to poor wall performance.
This is not different from
other geotechnical structures.
AUTHORS Dov Leshchinsky, Ph.D., University of Delaware and ADAMA Engineering
DEEP FOUNDATIONS * JULY/AUG 2024 * 113
The objective of this article is to
report failures of two dissimilar walls,
ending with costly remediations that
changed the characteristics of original
MSE structures. The cases reported are
not unusual and reflect similar problems
in other MSE wall failures the
author has investigated. The reported
cases are based on forensic studies
and, in a general sense, are structured
as such:
* A known outcome, e.g., excessive
settlement or collapse.
* An investigation process to separate
the chaff from the wheat, i.e., distinguish
between the " noise " typically
promoted by lawyers who are advocating
for a client.

July/August 2024

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