Page 116 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

3.10 THE WORK

FIGURE 3.7 Load-bearing capacity.

Other Factors. For repeated crossings, ground should have two or more times the minimum bear-
ing capacity needed for a static load as above, as impact shock during travel greatly increases
effective weight. Once ground begins to yield, truck wheels dropping into holes may deliver
blows several hundred percent greater than static weight.
Bearing qualities cannot be measured entirely in square inches. A country road of native soil
may easily support a 200-pound person balancing on a 1-inch metal cube, but break down rapidly
under 30-ton trucks having only one-third as much ground pressure per square inch.
This may be explained by the formation of a bulb of pressure that is built up underground by
the sum of individual surface points or areas of pressure. Figure 3.8 shows a cross section of soil
under a rubber tire roller, where each tire produces its own zone of compaction and their com-
bined weights produce another and larger pressure bulb.
On hard ground a high-pressure tire does less flexing and has a smaller area of ground contact,
and therefore less rolling resistance than a low-pressure tire. However, if the ground is soft
enough for the tire to sink in at all, the low-pressure tire will develop less resistance.

FIGURE 3.8 Pressure bulb.