Classification of soils - The Mechanics of Soils and Foundations - page 56

predominates the soil is poorly graded. The grading of a soil often reflects its origin.
Soils deposited by rivers or wind tend to be poorly graded while boulder clays and tills
deposited from ice tend to be well graded with a wide distribution of sizes. Tests to
determine the grading of soils are described in Sec. 7.3.

5.4 Properties of fine-grained soils
The behaviour of a coarse-grained soil (i.e. silt-sized and coarser), is very like that of
an assembly of different sized marbles, but clays differ in two respects. Firstly, some
clay grains themselves may show significant volume changes as the loading and water
content changes; this accounts for clays tending to crack as they dry. Secondly, particle
surface effects become significant.
The surface of a soil grain carries a small electrical charge which depends on the soil
mineral and may be modified by an electrolyte in the pore water. These charges give
rise to forces between soil grains in addition to their self-weight. The magnitudes of
the interparticle forces are proportional to the surface areas of the grains, while self-
weight forces are proportional to the volumes of the grains. As particle sizes decrease
the surface forces diminish with the square of the effective diameter, whereas the self-
weight forces diminish with the cube; consequently the effects of surface forces are
relatively more important in fine-grained than in coarse-grained soils.
The relative importance of the surface and self-weight forces may be described by
the specific surface. This is defined as the total surface area of all grains in unit mass.
Table 5.1 lists typical values for the specific surface of three common clay minerals
and of clean sand; the differences in the values of specific surface for sand and clay are
very large.
In coarse-grained soils such as silt, sand and gravel, particle surface forces are
negligible compared to their self-weight forces, so that dry sand will run through
an hour-glass and form a cone at the base. Dry fine-grained materials, such as kitchen
flour, behave differently and if you squash a handful of flour in your hand it will
form a coherent lump. This is because, as the grains become densely packed and the
number of contacts in unit volume increases, the slight surface forces give rise to a
small cohesive strength; the lump is easily broken because the cohesive strength is very
small. We will see later that true cohesive strength in soils is usually negligible unless
they are cemented by other materials.

Table 5.1 Approximate values for the specific surface of some
common soil grains

Soil grain Specific surface

(m 2 /g)

Activity

Clay minerals
Montmorillonite

Up to 840 > 5

Illite 65-200 ≈ 0.9

Kaolinite 10-20 ≈ 0.4

2 × 10 − 4

Clean sand —