Geosynthetics August/September 2021 - 15

induced by the embankment fill and the
surcharge. This phenomenon is referred
to as soil arching by Terzaghi (1936).
Soil arching ratio, defined as the pressure
on the geosynthetic divided by
the average pressure from the embankment
load and the surcharge, is used
to describe the degree of soil arching.
The smaller soil arching ratio implies
more soil arching. When the geosynthetic
is under pressure, it deforms
and develops tension, T, the vertical
component of which carries the vertical
load from the embankment and reduces
the stress, σs, applied onto the soft soil.
This mechanism is often referred to as
the tensioned membrane effect. Stress
reduction ratio, defined as the stress
on the subsoil below the geosynthetic
divided by the average pressure from the
embankment load, is used to describe
the benefit of combined soil arching
and tensioned membrane effect. Since
the soft soil (often called the subsoil)
below the embankment settles more
than the columns, negative skin friction
(also called downdrag force), τd, develops
between the columns and the soil
so that the vertical stress in the subsoil
decreases with depth. The reduced vertical
stress in the subsoil minimizes its
compression. This is the mechanism of
column/subsoil interaction. The differential
settlement between the columns
and the subsoil depends on the relative
stiffness between the columns and the
subsoil, which affects the mobilization
of soil arching in the embankment fill
and the downdrag force in the subsoil.
Rigid columns promote soil arching
and downdrag force and therefore have
been increasingly used in recent years.
When the embankment is high enough,
an equal settlement plane can develop at
the critical height, Hcr. Van Eekelen and
Han (2020) provided a list of methods
to estimate this critical height, most of
which yield a similar result.
q0
Fill
Hcr
pc
ps
d
s
c
Column
a
Subsoil
s
Column
d
W
T
Geosynthetic
Reinforcement
pc
H
FIGURE 3 Load transfer mechanisms (modified from Han 1999)
In summar y, the mechanisms
involved in the GRCS embankment
include soil arching, tensioned membrane
and column/subsoil interaction.
The effects of these mechanisms depend
on column/subsoil relative stiffness.
Many studies have shown that soil arching
progressively evolves with normalized
displacement, defined as the differential
settlement, δ, divided by the clear spacing
of the columns, s-a (s is the centerto-center
spacing and a is the column
diameter). Iglesia et al. (1999) proposed a
ground reaction curve (GRC) to describe
the evolution of the soil arching ratio
with the normalized displacement. Han
et al. (2017) proposed a simplified GRC
using three straight lines to describe
the evolution of the soil arching ratio
with the normalized displacement: (1)
decreasing from 1.0 (no arching) to the
minimum arching ratio (i.e., the maximum
arching); (2) increasing from the
minimum ratio to the ultimate arching
ratio (i.e., stress recovery or soil arching
degradation); and (3) remaining constant
at the ultimate arching ratio.
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Geosynthetics August/September 2021

Table of Contents for the Digital Edition of Geosynthetics August/September 2021

Geosynthetics August/September 2021 - Cover1
Geosynthetics August/September 2021 - Cover2
Geosynthetics August/September 2021 - 1
Geosynthetics August/September 2021 - 2
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