Geoscience Reference
In-Depth Information
process and compaction, the main direction is usually horizontal, but it is possible
for a section of peat to have a vertical orientation. The effect of organic matter and
stiffness of soils depends largely on whether the organic matter is decomposed or
consists of fibres which can act as reinforcement (Arman, 1969; Landva
et al
., 1983).
In general, fibrous peat has higher shear strength than other groups of peat, such as
hemic and sapric peats. The shear strength behaviour of peat is observed to be highly
anisotropic (Hanzawa
et al
., 1994). The shear strength of a soil is not only a function
of the material itself, but also of the stress applied and the manner in which it is
applied.
Shear strength parameters always play a vital role when engineering decisions have
to be made about any soils that include peat. Shear strength is a concern both during
construction (to support construction equipment) and at the end of construction (to
support the structure). The low shear strength and high compressibility of peat soils
confines them to the 'problematic' category. Accuracy in determining the shear strength
of these soils is associated with several variables, as described earlier.
For peat, the presence of fibres modifies our concepts of strength behaviour in sev-
eral ways. It can provide effective stress where there is none and it induces anisotropy. It
also results in reduced
K
o
values compared to clays, as will be explained later. Finally,
shear resistance may continue to develop at high strains without a significant peak
behaviour.
Early research on peat strength indicated some confusion as to whether peat should
be treated as a frictional material like sand or cohesive like clay. Commonly, surficial
peats are encountered as submerged surficial deposits. Because of their low unit weight
and submergence, such deposits develop very low vertical effective stresses for consol-
idation and the associated peats exhibit high porosities and hydraulic conductivities
comparable to those of fine sand or silty sand (Dhowian and Edil, 1980). Such a mate-
rial can be expected to behave 'drained' like sand when subjected to shear loading.
However, with consolidation porosity decreases rapidly and hydraulic conductivity
becomes comparable to that of clay. For example, the time for the end of primary con-
solidation as defined by full dissipation of the base pore pressure in a singly drained
consolidation test on Middleton peat increased 10 times under the second load incre-
ment, from about 2min to 20min, and to 200min after several load increments (Edil
et al
. 1991, 1994). There is a rapid transition immediately from a well-drained material
to an 'undrained' material.
4.2 LABORATORYTESTING
Several methods can be used to determine the drained and undrained shear strength in
the laboratory, namely the triaxial test, shear box test, ring shear or direct shear, and
vane shear test. Figure 4.1 illustrates some of the equipment normally used for shear
strength testing in the laboratory.
As mentioned above, unlike mineral soils, the presence of fibres in peat and their
varying interaction within the shearing mode imposed by the particular testing pro-
cedure creates difficulties in assessing the true operating strength value. The use of
drained strength (usually obtained from a triaxial test) with estimated pore pressure
