Geoscience Reference
In-Depth Information
Shear strength of natural peat
4.1 INTRODUCTION
As mentioned before, peat and organic soils commonly occur as extremely soft, wet,
unconsolidated surficial deposits that are integral parts of wetland systems. They may
also occur as strata beneath other surficial deposits. These problematic soils are known
for their high compressibility and low shear strength. Access to these surficial deposits
is usually very difficult, as the water table will be at, near or above the ground surface.
Undoubtedly, this is the cause of the tendency to either avoid construction and building
on these soils, or when this is not possible, to simply remove, replace or displace them.
In some instances this may lead to possibly uneconomical design and construction
alternatives. However, in many countries this material covers a substantial area. The
pressure on land use from industry, housing and infrastructure is leading to more
frequent utilization of such marginal ground. It is therefore necessary to be able to
obtain suitable design parameters for strength and compressibility, as well as to find
suitable construction techniques on these materials.
Shear strength is one of the most important parameters in engineering design when
dealing with soil, especially during the pre- and post-construction periods, since it used
to evaluate the foundation and slope stability of soil. If the ultimate shear strength is
exceeded, the soil will fail or deform. The failure criterion is developed using the stress-
strain relationship and elasticity theory. The magnitude of the strain in soil depends
on parameters such as the magnitude of the applied load, the composition of the soil,
past stress history and void ratio, and also on the manner in which the stress is applied
(Anggraini, 2006). Peat usually has very low shear strength and the determination
of shear strength is a difficult job in geotechnical engineering because it depends on
factors such as the origin of the soil, its water content, organic matter and the degree
of humification. During the sampling stage, disturbance of the sample will also affect
the evaluation of shear strength.
The increase in shear strength upon consolidation of peat may be significant.
According to Munro (2004), the greater the moisture content and decomposition,
the lower the shear strength; in addition, a higher mineral content causes higher shear
strength. In general, shallow peat, due to its more fibrous nature, is likely to have
greater strength than more humified peat at depth (Culloch, 2006).
Peat is mostly considered a frictional or non-cohesive material due to its fibre
content and the spatial orientation of the fibres. The high friction angle of peat will
