Chapter 3: Traffic and Soil Mechanics

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18 Ιουλ 2012 (πριν από 6 χρόνια και 3 μήνες)

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For the tires of a wheeled vehicle, the path of an individual soil element also exhibits
shear and compressive displacement. Viewing the movement of a particle near the soil
surface indicates how the soil is deformed as both the front and back wheels pass over the
soil area of interest. With approach of the front wheel, the individual soil particle is
displaced upward and forward from its initial location in the soil. When the wheel passes
vertically over the particle, it is displaced downward. There is finally some backward and
upward rebound of the particle as the front wheel continues beyond the affected soil
volume. These steps are subsequently repeated and magnified with passage of the heavier
rear wheel. Compaction is realized by the difference in the downward displacement of the
particle between its initial and final location in the soil.

Finally, foot traffic acting on the soil can produce a differential degree of compression
and shear deformations depending on the specific activity. Standing and walking generate

In this figure, the x-axis is the increasing percent clay in these native soils. Low clay
content for a native soil generally yields a sandy soil since in nature pure silts are not
commonly found. A high clay content soil is generally devoid of appreciable sand since
often silts and clays are found together. The y-axis is this figure is the 'compression
index’ that is calculated as the slope of a curve relating soil bulk density to applied stress.
The steeper this slope, the greater the bulk density for a given applied stress and
consequently, the more compactable a soil is. Thus, a large compression index represents
a soil with less strength and being more prone to compaction while a small index
represents a soil that resist compaction due to it’s greater compressive strength.

The maximum compression index for these soils occurs at about 50% clay with the
remaining 50% either sand or silt (likely more silt than sand as noted above). Above 50%
clay the compression index declines slightly. That is, soils approaching pure clay also
tend to somewhat resist compaction. Thus, it is not clay alone that controls soil
compaction but rather silt and clay combined.

While the average particle size of a soil influences its compactability, the distribution of
particle sizes around this average also affects a soil's response to an external stress. Thus,
consider two sand samples both with an average particle size of 0.75-mm diameter. One
sample is very uniform in its particle size distribution having essentially all particles in
the coarse fraction range (0.5 to 1.0 mm). The other sample is not uniform and has an
appreciable number of particles in the fine (0.1 to 0.25 mm), medium (0.25 to 0.5), coarse
and very coarse (1.0 to 2.0 mm) fractions. The uniform sample resists compaction due to
the stable bridging of the similar sized particles. The non-uniform sample is more prone
to compaction since the external stress tends to pack the smaller particles into the voids
between the larger particles, thus compacting the soil. Note that the void space between
uniform spherical particles has a diameter approximately 40% of the diameter of the
particles themselves. This is true regardless of whether the particles are sand sized or silt
sized; the ratio remains the same. Thus, for particles 0.75 mm diameter, a particle 0.3 mm
diameter or smaller will generally fit within the void space of the larger particle. If there

The clay loam soil used in this example is expected to be rather prone to compaction due
to its fine texture. With increasing levels of applied stress, therefore, the percent of the
total pore volume due to pores > 15 µm (the first 2 columns combined) experiences the
greatest decline (on average a 68% reduction). Correspondingly, pores between 1 and 15
µm increase slightly and pores < 1 µm double as a percent of the total. For comparison
purposes, the four levels of increasing applied stress correspond to 0.84, 2.15, 6.5, and
16.5 psi, respectively.

Soil pores are often classified according to their size and hydraulic function. Macropores
are the very large soil pores that mainly serve as routes for water infiltration, water
drainage and soil aeration or gas exchange. You may think of macropores as the route for
mass movement of water and air through a soil body. As in flow through pipes, to move
large quantities of air and water through a soil, there is a need for large and highly
conductive pores. Mesopores are the middle-sized pores that conduct water more slowly.
When short distances are involved or flow occurs more slowly, these pores can be quite
useful in the normal functioning of a soil. Thus, mesopores provide for capillary water
movement to roots and for moisture redistribution (or wicking) within the soil profile.
Micorpores, because of their small size do not readily transmit water through the soil but
rather serve to hold water within the soil body and serve as a storage reservoir.

By reducing the proportion of macropores, the soil’s potential for drainage and aeration
are reduced. For this reason, compaction is probably the most serious damage that can
occur in recreational turf soils. Compaction results in a downward spiraling of soil
hydraulic properties and the ability of a soil to support turf and play. Consider an area of
a fairway with a cohesive soil of adequate macroporosity for aeration and drainage. Foot
traffic on this area shortly after a rainfall results in a slight degree of compaction and
closure of some soil macropores. This results in a slightly reduced drainage potential for
this soil so after the next rain the soil remains wet longer. Thus, the soil has a longer
window where it is prone to compaction by traffic. With repeated events, the soil
becomes progressively compacted; experiences further reduced drainage and is