Geoscience Reference
In-Depth Information
Figure 3.8
Atterberg limit.
The plastic limit test procedures involve carefully rolling the soil samples into
threads. As this rolling process continues, the thread becomes thinner and eventually
breaks. If the soil is dry, it breaks at a large diameter. If it is wet, it breaks at a much
smaller diameter. By definition the soil is at its plastic limit when it breaks at a diameter
of 1/8 of an inch (3mm) (Figure 3.9 (c)).
For peat, the liquid limit depends on type of plant detritus contained, degree of
humification and the proportion of clay soil present. For temperate peat, the liquid
limit of fen peat ranges from 200-600%, and for bog peat from 800-1500% (Hobbs,
1986). Skempton and Petley (1970) put the boundary at approximately H3 of the von
Post scale for the liquid limit and H5 for the plastic limit. In any case, according to
Hobbs (1986), there is little point in performing plastic limit tests on peat soils since
the plasticity gives little indication of their character. Table 3.4 shows the Atterberg
limits of various soil deposits.
3.11 DENSITY AND SPECIFIC GRAVITY
Density or unit weight is another useful soil parameter for a geotechnical engineer.
Density (
ρ
) is simply defined as total mass (
M
) over total volume (
V
):
ρ
=
M/V
(3.5)
Unit weight (
γ
) is total weight (
W
) over total volume (
V
). Weight (often expressed as
kN) is equal to mass (kg) times acceleration due to gravity,
g
(
g
9.81m s
−
2
).
Peat's unit weight is both low and variable compared with mineral soils, being
related to the organic content, mineral content, water content and degree of saturation.
The average unit weight of peat is typically slightly higher than water. Amorphous peat
=
