Page 112 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

3.6 THE WORK

FIGURE 3.4B Seismic recorder and readings.

TRACTION

Soils differ greatly in their ability to support and permit movement of vehicles. An important
characteristic is the amount of friction that exists between the ground surface and the drive tires
or tracks of a machine on it.
Tractive Efficiency. This is a measure of the proportion of the weight resting on drive wheels
or tracks that can be converted into movement of a machine. Soil characteristics and condition are
very important in determining it.
It is difficult to estimate and classify the tractive efficiency of ground surfaces, as wide varia-
tions may be caused by the shape of particles and their size gradation. In addition, the presence of
certain compounds that do not show up in a soil specification may increase traction by improving
packing qualities and even cementing particles together. Other substances may reduce traction by
lubricating grains so that they move freely on each other and on tire surfaces. For example, salt
improves packing qualities, lime acts as cement, and soapstone (serpentine, mostly H Mg Si O)
4 3 2
acts as a lubricant.
The amount of water present is an important variable. For any soil there is a certain moisture
content that gives best traction. In sandy or gravelly soils, less water is likely to allow them to
become loose and unsatisfactory, while more water has little effect. In fine-grained soils too little
water (once the soil is compacted) may not have any harmful effect except causing dust, but too
much water will make them first slippery and then soft. A soaking shower on the dusty loam sur-
face of a heavily traveled haul road may make it temporarily almost as slippery as ice.
Figure 3.6 gives a general guide to traction afforded by various surfaces, expressed as a decimal
fraction of the weight on drive wheels or tracks. The variations between low and high values are
sufficiently wide to take care of most differences. If the higher values are used, the possibility of
having to deal with lower ones should be kept in mind.
This table assigns much lower values to loose and wet soils than most references do.