Page 107 - Moving the Earth_ The Workbook of Excavation

Source: MOVING THE EARTH

CHAPTER 3

ROCK, SOIL, AND MUD

SOIL AND ROCK

Soil is loose surface material. Rock is the hard crust of the earth, which underlies and often pro-
jects through the soil cover. There is no clear distinction between soil and rock. Geologically, all
soils are considered to be rock formations. In ordinary usage, rock is something hard, firm, and
stable.
A contractor’s definition is that rock is any material which cannot be dug or loosened by avail-
able machinery, but this distinction from soil may depend more on the power, size, and digging
efficiency of the machinery than on the material itself.
Material to be excavated can also be roughly divided into three classes: rock, hard digging, and
easy digging.
Rock is anything that needs blasting or ripping for efficient digging by most machines. Hard
digging is compacted, cemented, or rocky dirt, clay, soft shale, and rotten rock, that can be dug
directly by heavy machinery, or loosened readily by rippers. Easy to medium digging is any soft
or fine, firm or loose deposit.
Soil. Soil is composed of particles of various sizes and chemical compositions. It can be ana-
lyzed as to sizes by sifting a dried and weighed sample through a set of testing sieves, such as are
shown in Fig. 3.1, and weighing the material retained on each screen.
If further analysis is required for particles passing the smallest (200-mesh) sieve, it is done by
hydrometer. This process is based on the fact that the speed of settlement of such particles is pro-
portional to their size.
Figure 3.2A indicates the size particles which are included in the common soil classifications.
There are several scales, in which the boundaries between different classes may vary. The differ-
ences among them are not important to the average contractor. Figure 3.2B shows another means
of classifying soils.
1
A plastic soil is one which can be rolled as between the hands, into strings ⁄ 8 inch in diameter
without falling apart. Plasticity is a function of soil character and of moisture content. The minimum
amount of water in terms of percent of oven-dry weight of the soil which will make it plastic is
defined as the plastic limit of the soil. If no amount of water will allow it to roll into strings, it is
called nonplastic, with the symbol NP.
The liquid limit is minimum moisture content, in terms of percent of oven-dry weight, which
will cause the soil to flow if jarred slightly.
The plasticity index is the difference between the plastic limit and the liquid limit; that is to
say, the range of moisture content in which the soil is plastic.
Soils or other particles may also be classified as to grain shape or hardness, and mineral and
organic content. These factors will affect their resistance to weather, stability under load, wear on
digging parts, and internal friction.
Most soils are inorganic, and are made up of products of decay and breaking up of rock. Organic
soils and organic material in soils are largely humus, which is formed by decay of vegetation and

3.1