Geoscience Reference
In-Depth Information
not actually reflect a high shear strength due to the fact that the fibres are not always
solid and may be filled with water and gas. The presence of fibres will modify the
strength behaviour of peat, since the fibres can be considered as reinforcement and can
provide effective stress where there is none, inducing anisotropy.
The shear strength of peat can generally be found out in many ways.
In situ
methods such as the field vane shear test and the cone penetration test are very useful,
and these tests can be used to avoidmany of the problems associated with soil sampling.
However, these methods have some inherent limitations since the shear strength can
only be determined indirectly through correlation with laboratory results and also from
back calculation from the results of actual failures. Further, the variable nature of peat
and the difficulties in obtaining good representative samples from the field mean that
laboratory testing can only give indicative results (Culloch, 2006).
The most common laboratory test is a direct shear test to determine the drained
shear strength of fibrous peat. A triaxial test is frequently used to evaluate the shear
strength of peat in the laboratory under consolidated-undrained (CU) conditions. This
is due to the fact that the results of a triaxial test on fibrous peats are difficult to inter-
pret because the fibres often act as horizontal reinforcement; hence failure is seldom
obtained in a drained test because the triaxial test for peat with low permeability, if
performed under drained conditions, may take several days to complete.
Edil and Dhowian (1981) and Landva and La Rochelle (1983) showed that
the effective internal friction
ϕ
of peat is generally higher than that of inorganic
soil (e.g. undrained friction angle of amorphous peat and fibrous peat is in the range
27-32
◦
under a normal pressure of 3-50 kPa; on the other hand, for amorphous gran-
ular peat the effective internal friction is 50
◦
and for fibrous peat it is in the range
53-57
◦
). The undrained friction angle of peat in West Malaysia is in the range 3-25
◦
(Huat, 2004).
The determination of undrained shear strength is also important because peat
is always present below the groundwater level. This is usually done in situ because
sampling of fibrous peat for laboratory evaluation of undrained shear strength is almost
impossible. Some approaches that have been used for in situ testing for peat are: cone
penetration test, dilatometer test, vane shear test, pressure-meter test, plate load test
and screw plate load test (Edil, 2001). The vane shear test is the most commonly used;
however, the interpretation of the test results must be approached with caution. A
range of 3-15 kPa is obtained for the undrained shear strength of peat, which is much
lower than that of mineral soils. A correction factor of 0.5 is suggested by Noto (1991)
and Hartlen and Wolski (1996) for the test results on organic soil for which the liquid
limit is more than 200%.
The evaluation of shear strength of peat under undrained conditions is greatly
affected by disturbance of the soil sample. Shogaki and Kaneko (1994) stated that to
obtain and minimize the effect of sample disturbance, there is no need for sampling if
the undrained shear strength is obtained by field tests. The undrained shear strength
of the soil can also be measured by performing laboratory tests on specimens trimmed
from blocks or large undisturbed samples (Kazemian and Huat, 2009a; Kazemian
et al.
, 2012). In this condition, there could be nearly zero effective confining pressure
or even tension at failure in fibrous peat (Edil, 1997).
In fibrous peat, the force is taken by the fibres, which act as reinforcement if the
direction of the load is the same as that of the fibres. As a result of the sedimentation
