Geoscience Reference
In-Depth Information
land. Peat land can be identified on aerial photos if it is not covered by mineral soils. It
typically has a flat topography (except for raised bogs) that coincides with low areas,
plants requiring a great amount of water, and wetlands, coastal lowlands and isolated
river marshes. Mapping peat land is particularly important in the selection of highway
routes.
A soil investigation aims at delineating not only the aerial distribution but also
the subsurface stratigraphy. In peat areas, determination of the depth of soft organic
substrata is important. This is best accomplished by sounding. Several methods of
sounding are available, from simple to more sophisticated. The practice of sounding
varies regionally. In Scandinavian countries, the Light Dynamic ProbingMethod (LDP)
and the Weight Sounding Test (WST) are two of the simpler sounding methods. A
variety of dynamic probing methods are also used in Germany and the Netherlands,
especially for sounding deposits containing hard or gravelly layers. In Japan, a single-
tube cone penetrometer was developed for peat deposits in 1952 and a double-tube
cone penetrometer in 1958 (Noto, 1991). However, currently the Dutch cone is the
most common method of sounding. More sophisticated methods of sounding include
the cone penetration test (CPT), which is widely used in Europe and Japan and to
a limited extent in the USA, and the standard penetration test (SPT), widely used in
the USA. In addition to sounding, CPT provides strength information and SPT soil
samples.
Ground-penetrating radar can be used to determine the thickness of strata with
highly contrasting electromagnetic properties (i.e. dielectric constant) from the surface
down to a depth of 10 m. During investigation, the antenna is moved over the ground
surface, while the reflection of the electromagnetic waves is recorded. The method is
quick and effective in delineating bottom layers. This method is gaining greater interest
and much development is under way. Down-hole radar provides a three-dimensional
distribution of soil strata.
Sampling, i.e. extraction of soil samples, is a necessary complement to other inves-
tigation methods, both for the identification of soils and for laboratory investigation.
For the case of peat, collection of an undisturbed organic soil core is certainly a more
difficult task compared with soft mineral soils such clays and silts. Conventional thin-
wall sampling tubes (typically 76.2mm in diameter) used for taking 'undisturbed'
samples of soft clays may not be suitable for sampling peat. Sample size is important
with respect to both sampling disturbance and representative sample volume. Peat
samplers 100 to 250mm in diameter have been developed, including a block sampler
(Landva
et al
., 1983).
There are a number of different tools for taking soil samples in different types of
soil. However, the quality of the samples will vary between the samplers and between
the types of soil. When choosing sampling quality and tool, it is necessary to con-
sider the subsequent laboratory investigation. For soil identification only, disturbed
or remoulded samples can be used. If the deformation and strength characteristics of
the soil are to be investigated in the laboratory, it is necessary to obtain an undis-
turbed sample. In 1981, the ISSMFE subcommittee on soil sampling presented an
international manual on soil sampling of soft cohesive soils that describes the defini-
tion of undisturbed sample as 'undisturbed samples, the soil retains the same fabric,
type and proportion of constituents and physical and mechanical properties as in
the field'.
