Geoscience Reference
In-Depth Information
Figure 5.2
Scanning electron microphotograph of peat: (a) vertical section and (b) horizontal section
(
after
Mesri and Ajlouni, 2007).
Field experience on earth-filling projects on several sites on peat shows that peat
exhibits a short rapid primary consolidation stage, followed by a prolonged and slow
secondary/creep compression (Wong, 2003).
The calculation of settlement requires evaluation of soil parameters from the com-
pression curves usually obtained in oedometer tests. The results of incremental loading
oedometer tests are usually presented as the relationship between void ratio,
e
, and
effective vertical stress,
σ
v
σ
. The vertical effective stress may be plotted on a linear
scale to determine the coefficient of volume change,
m
v
, and oedometer modulus,
M
,
or on a logarithmic scale to determine the compression index
c
c
. Figure 5.4 shows a
typical sketch of an oedometer.
Kirov (2003) found that when testing soft organic soils, the loading schedule used
in the oedometer tests has a considerable influence on the results obtained. He sug-
gested that it is better to use an individual loading schedule, as close as possible to
the loading conditions expected
in situ
. If other (i.e. different from
in situ
and hence,
unsuitable) schedules are used, misleading values for the coefficient of volume com-
pressibility may be obtained. This can subsequently lead to mistakes in the calculation
of the settlement of project structures designed for site in question.
5.2 COMPRESSIBILITY PARAMETERS OF PEAT
A peat can consolidate, compress and settle in two ways when loaded:
(a) Slowly, with gradual consolidation and compression allowing time for the peat
mass to respond to the load. This is the desired method for constructing a road
on peat and allows time for the peat to improve its strength and bearing capacity.
(b) Rapidly, without a change in volume, with rapid spread and shear of the peat
causing failure. Peat is highly vulnerable to 'shear overstress' and loadings need
to be carefully controlled to keep stresses within the available strength.
In the normal course of events the consolidation, compression and settlement of
peat may be considered as taking the form of three phases, instantaneous settlement
