Geoscience Reference
In-Depth Information
The compression behaviour of peat varies from the compression behaviour of other
types of soil in two ways. First, the compression of peat is much greater than that of
other soils. Secondly, the creep portion of settlement plays a more significant role in
determining the total settlement of peat than for other soil types.
The primary consolidation of fibrous peat takes place very rapidly. A large sec-
ondary compression, and even tertiary compression, is also observed to take place
(Kazemian and Huat, 2009a).
The dominant factors controlling the compressibility characteristics of peat include
the fibre content, natural water content, void ratio, initial permeability, nature and
arrangement of soil particles, and inter-particle chemical bonding in some of the soils
(Mesri and Ajlouni, 2007). Determination of the compressibility of fibrous peat is
usually based on the standard consolidation test.
The
in situ
void ratio of fibrous peats is very high because of the fact that very
compressible and bendable hollow cellular fibres form an open entangled network of
particles, giving a high initial water content. During both primary and secondary com-
pression, water is expelled simultaneously from within and among the peat particles
(Mesri and Ajlouni, 2007). Therefore the e-log
p
curves show a steep slope indicating
a high value of compression index (
C
c
). The compression index of peat soil ranges
from 2 to 15. Furthermore, there is a possibility that secondary compression will
start before the dissipation of excess pore water pressure is completed (Leonards and
Girault, 1961).
As mentioned earlier, the unit weight of peat is close to that of water; thus the
in situ
effective stress (
σ
) is very small and sometimes cannot be detected from the
results of a consolidation test (Mesri
et al
., 1997). It is also very difficult to obtain
the beginning of secondary compression (tp) from the consolidation curve because the
preliminary consolidation occurs rapidly (Yulindasari, 2006).
Compression of fibrous peat continues at a gradually decreasing rate under con-
stant effective stress, called secondary compression. The secondary compression of
peat is due to the further decomposition of fibers which is assumed to occur at a
slower rate after the primary consolidation is over (Mesri
et al
., 1997). The slope of
the final part of the graph of void ratio versus logarithm of time curve (
C
) is defined as
the rate of secondary compression. This estimate is based on the assumptions that
C
is
independent of time, thickness of compressible layer and applied pressure. The ratio
C
/
C
c
has been widely used to study the behaviour of peat (Dhowian and Edil, 1980)
and a range between 0.05 and 0.07 for
C
/
C
c
is reported by Mesri
et al
. (1997).
Although the rate of primary consolidation of fibrous peat is very high, it decreases
with the application of consolidation pressure. According to Lea and Brawner (1963),
there will be a significant decrease in the rate of coefficient of consolidation (
C
v
) during
application of pressure from 10 to 100 kPa. The significant reduction factor of 5-100
is attributed to the reduction of permeability due to the increasing pressure.
As with mineral soils (silt and clay), the settlement parameters of peat (i.e. con-
solidation settlement) may also be determined from standard incremental oedometer
(one-dimensional compression) tests, as shown in Figure 5.4. The parameters are inter-
preted from traditional e-log and
σ
v
plots. There may be differences in the magnitudes
of various quantities measured, but the general shape of the consolidation curves
appears reasonably similar and the formulation developed for clay compression can
be used to predict the magnitude and rate of settlement (Edil, 1997).
