Geosynthetics August/September 2021 - 14

Geosynthetic-reinforced column-supported embankments: Improving practice with better theory
Bridge
Bump
Settlement
Fill
Settlement
Soft soil
Piles
(a) Problem
Bridge
Load transfer platform
Fill
Geosynthetic
2a
Soft soil
Piles
Columns
(b) Solution
FIGURES 2a and 2b Problem (2a)
and solution (2b)
2b
soils. This problem becomes even more
critical when there is a transition from
soft soils to rigid supports-for example,
bridge approach embankments as
shown in Figures 2a and 2b. When the
bridge abutment is supported by piles, it
has minimum to nearly no settlement.
However, when the approach embankment
is constructed on the soft soil
(Figure 2a), large deformation can occur,
resulting in large differential settlement
between the abutment and the embankment.
This differential settlement at
the intersection of the bridge abutment
and the approach embankment is often
referred to as a " bump, " a driving hazard
to the public. Geosynthetic-reinforced
column-supported embankments have
been increasingly used to solve this bump
problem because of their advantages of
fast construction and effectiveness. In
this technology, as shown in Figure 2b,
columns (e.g., concrete columns, deepmixed
columns and stone columns) are
used to increase bearing capacity, reduce
settlement and enhance global stability.
Geosynthetics (one or multiple layers)
placed in the load transfer platform above
the columns help transfer more vertical
embankment loads to columns and carry
lateral thrusts from the embankment
so that lower vertical loads are applied
on the soft soil and lower lateral loads
are transferred to the columns. In addition,
geosynthetics can reduce differential
settlement between columns. This technology
has also been used to solve differential
settlement problems for roadway
widening, retaining walls, box culverts,
storage tanks, buildings and buried pipes.
The early application of this techBridge
Load
transfer platform
Fill
Geosynthetic
Soft soil
Piles
Columns
(b) Solution
nology can be traced back to a project
in 1972, in which vertical timber piles
and three layers of 2.9 ounces per square
yard (100 g/m2) multifilament woven
polyester geotextiles were used to support
an approach embankment (Holtz
and Massarsch 1976). Applications of
14
Geosynthetics | August September 2021
this technology increased since the 1990s
after the important research by Hewlett
and Randolph (1988) and the publication
of the design guidelines (BS8006) by the
British Standards Institution (1995). One
of the earliest uses of this technology in
the U.S. was the construction of a storage
tank on a geogrid-reinforced load
transfer platform on concrete columns in
Philadelphia, Pa., in 1994 (ASCE 1997).
With the increased application of this
technology, GRCS embankments have
become a hot research topic worldwide
in the past 20 years. Many research projects
and papers on this subject have been
published, which have helped better
understand the mechanisms involved in
this system. At the same time, different
theories were proposed, and different
design methods were developed, which
have enabled engineers to design such a
system. Unfortunately, different design
methods have resulted in different
design requirements and performance
predictions to be discussed in the following
section.
Theory
Mechanisms
The mechanisms for GRCS embankments
depend on the location of the columns
in the embankment (i.e., center or
edge). This article focuses on the mechanisms
associated with the columns in the
center of the embankment. Figure 3 presents
the load transfer mechanisms of one
unit cell from the GRCS embankment.
Since the soft soil of lower modulus
deforms more than the columns of
higher modulus under the embankment
load, it induces shear stresses, τs, along
the slip surfaces in the embankment so
that the pressure applied on the geosynthetic,
ps, is lower than the average
pressure (γH + q0, γ is the unit weight of
the embankment fill, H is the embankment
height, and q0 is the surcharge)

Geosynthetics August/September 2021

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