Page 109 - Moving the Earth_ The Workbook of Excavation
P. 109
ROCK, SOIL, AND MUD
ROCK, SOIL, AND MUD 3.3
FIGURE 3.2A Soil classification.
Digging Resistance. The resistance which must be overcome to dig a formation will be made up
largely of hardness, coarseness, friction, adhesion, cohesion, and weight.
In digging, hardness is resistance to penetration. It is increased by close packing of soil, or filling
of voids with finer particles, or lime or other natural cements. Clay soils are hard when dry, and
soft when wet.
Cobbles, boulders, or hard lumps increase the power requirement for penetration. They are most
troublesome when they are oversize for the machine, or packed so firmly in place that they can-
not slide or rotate away from the cutting edge.
As the digging edge penetrates, friction absorbs an increasing proportion of its force. It is
affected by particle size and hardness, by the amount of moisture, and the presence or absence of
natural lubricants such as humus or soft clay.
