Geoscience Reference
In-Depth Information
Figure 5.19
c
α
vs. initial moisture content (
after
Hobbs, 1986).
There are insufficient results to confirm the trend noted by Hobbs (1986) of a rela-
tively constant value of
c
α
above a moisture content of 250%, as shown in Figure 5.19.
Hobbs also noted that
c
α
would reach limiting values of between 0.02 and 0.04 for
highly organic soils.
O'Loughlin and Lehane (2003) found that
c
α
increases with increasing organic
content.
c
α
evidently also depends on
w
o
and consequently
e
o
. They also suggested a
relationship between
c
c
and
c
α
as shown in Figure 5.20.
According to Mesri
et al
. (1997), reliable data suggest that
c
α
/
c
c
for peat lies
within the range 0
.
06
0
.
01. However, studies carried out by Paikowsky
et al
. (2003)
on Cranberry bog peat, Massachusetts, showed that
c
α
/
c
c
is not constant, but varies
as shown in Figure 5.21.
Compression of fibrous peat continues at a gradually decreasing rate under
constant effective stress, and this is termed secondary compression. The secondary
compression of peat is thought to be due to further decomposition of the fibres, which
is conveniently assumed to occur at a slower rate after the end of primary consolidation
(Mesri
et al
., 1997). Kazemian and Huat (2009a) presented the results of the ratio of
compressibility (
c
α
/
c
c
) versus consolidation pressure of fibrous peat (Figure 5.22). As
expected, this ratio increased with increasing consolidation pressure.
±
5.2.4 Tertiary compression
Secondary compression carries with it the widespread connotation of constant rate
with the logarithm of time, which means a decreasing true settlement rate with time.
There is some evidence that the coefficient of secondary compression (
c
α
) may change
with time, even for clays (Mesri and Godlewski, 1977). For fibrous peat, there is a
significant deviation from constant
c
α
in most cases of stress application (Edil, 2003).
Edil and Dhowin (1979) reported the steepening of the log
t
versus compression
curve in oedometer tests on Wisconsin peat (Figure 5.23), and defined a tertiary com-
pression phase after the secondary compression phase. Dhowian and Edil (1980) first
introduced the term, finding rather strong effects in fibrous and hemic Wisconsin peat.
They also proposed to redefine tertiary compression as the steepening of natural strain
