Forest Road Engineering Guidebook - Ministry of Forests

Appendix 1. Field identification of soils
Describing soils
Soil classification systems are based mainly on particle size, and these usually
fall into three main groups: coarse-grained soils, fine-grained soils, and
organic soils.
Coarse-grained soils
These contain particle sizes that are large enough to be visible to the naked
eye. They include gravels and sands and are generally referred to as cohesionless,
or non-cohesive, soils. Strictly defined, coarse-grained soils have
more than 50% of the dry weight larger than particle size 0.060 mm
(Table A1.1).
Fine-grained soils
Organic soils
These contain particle sizes that are not visible to the naked eye. They are
identified primarily on the basis of their behaviour in a number of simple
indicator tests. They include silts and clays, the latter of which are generally
referred to as cohesive soils. The term “cohesive” indicates stickiness in
soils. Strictly defined, fine-grained soils are soils having more than 50% of
the dry weight smaller than particle size 0.060 mm (Table A1.1).
These soils have a high (80%) natural organic content.
Forest Road Engineering Guidebook
These three main groups are further divided into a series of subgroups, each
determined by particle size divisions within the major groups.
In addition to particle size identification, soil classification also includes a
description of such properties as “consistency” of a cohesive soil and “density”
of a non-cohesive soil in its natural undisturbed state in the field. For
example, consistency is a term that is used to describe the degree of firmness
of cohesive soil and is indicated by such descriptive terms as soft, firm, or
hard. In practice, the term is used only in reference to the condition of the
cohesive fine-grained soils such as clayey silts, silty clays, and clays (i.e.,
those that are markedly affected by changes in moisture content). The term is
not usually applied to coarse-grained soils such as sands and gravels or to
non-cohesive silts.
As a cohesive soil changes consistency, its engineering properties change
also. The strength of a soil varies considerably with consistency. A clay at a
low moisture content and in a hard condition is obviously stronger than the
same clay at a high moisture content and in a soft condition. Thus, classifying
161