CHARACTERISTICS OF SOILS - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 28

D STRUCTURE OF GRANULAR SOIL
One distinguishes sedimentary soil, being transported by water, air, ice, gravity
or organisms, changing properties during that process and finally deposited, and
residual soil, resulting from rock weathering and accumulated in place. Peat soils
either accumulate in place (sedentary) or are dispersed by erosion and deposited,
often together with mineral soil. Soils may change their constitution due to stress
(gravity), time (ageing), water, environment (salty/fresh) and other disturbance
(cementation, bioactivity), see Fig 2.4. A notorious example of the effect of
chemical change is the cause of the Rissa landslide in April 1978. The structure of
soil depends on its origin, history and environment, and can be classified by
compaction and consistency.

Compaction and consistency
Grain texture (fabric), shape and the surface roughness (or roundness) are
important for internal friction and compaction features. The compaction of sand
also depends on the relative density D r , i.e. the particle distribution and the packing
(voids between large particles may be filled with smaller particles: well-graded).
The consistency of cohesive soil (clay) depends on water content, flocculation and
electrical forces, and is expressed in terms of undrained shear strength c u . The
corresponding qualitative classification is shown in Table 2.4.

TABLE 2.4 CLASSIFICATION OF SAND AND CLAY

compaction of granular soils consistency of cohesive soils

classification D r classification c u [kPa]

very loose
loose
medium dense
dense
very dense

0 - 15
15 - 35
35 - 65
65 - 85
85 - 100

very soft
soft
medium firm
stiff
hard

< 10
10 - 25
25 - 50
50 - 100
> 100

Peat
Peat differs from sand and clay by its high organic content OC , in the form of an
open fibre structure with little inorganic material ('ash'). In the botanic origin of
the fibres one distinguishes between water plants, reed, sedge, wood, moss, rushes
and heather. The fibres may contribute substantially to the shear resistance,
depending on their orientation, weathering and length (usually between 0.25 and
4.0 mm). The fibre orientation may give a directional dependent friction and
cohesion value (anisotropy).

Clay
Most common clays are kaolinite, illite and montmorillonite. Kaolinite or
Chinese clay is the result of erosion of minerals of granite. The structure is a two
layered system of silica and gibbsite. It shows little swell and has a relatively low
plasticity. Illite is structured according to a three layered system of silica-gibbsite-
silica. It is slightly weaker than kaolinite and shows little swell. Montmorillonite is
most common and is a product of further weathering of illite. It is an important