Lal R., Shukla M.K. Principles of Soil Physics
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FIGURE 7.13 The vehicle load can be
distributed over a large area by using
dual tires. The soil compaction hazard
is less when the load is distributed over
a large area.
FIGURE 7.14 Similar to dual tires,
wide tires can also distribute the load
over a large area. Nonetheless, the
Principles of soil physics 204
strategy to minimize soil compaction is
to decrease the frequency of heavy
vehicular traffic.
FIGURE 7.15 Subsoiling by chisel
plow can decrease bulk density and
reduce soil strength temporarily. The
long-term goal is to create stable
biochannels in the subsoil.
compaction-preventive technologies for subsequent farm operations. Biological
measures, which create biopores through worm holes (Fig. 7.16), or root channels (Fig.
7.17), or macrofauna (Fig. 7.18) are better options. Mulch farming techniques (Fig. 7.19)
minimize risks of soil compaction.
7.8 SOIL CONSOLIDATION
Soil consolidation refers to the densification process when reduction in volume occurs
due to expulsion of water under saturated conditions. Soil may be either initially saturated
or compacted to attain saturation. The process of soil consolidation occurs at much
slower pace than that of soil compaction because water is several orders of magnitude
more viscous than air. Therefore, soil consolidation has more application in foundation
engineering than in agriculture. The rate of consolidation is quicker in dense sandy than
in porous clayey soils, but the magnitude of consolidation is more in clayey than in sandy
Soil strength and compaction 205
soils. Terzaghi (1953) developed the theory of consolidation, which is available in
standard texts on soil mechanisms (Wu, 1982).
Principles of soil physics 206
FIGURE 7.16 (a) Earthworm channels
or stable biopores enhance infiltration
rate and promote root growth and
proliferation, (b) The presence of
biopores improves gaseous exchange
and enhances infiltration even in a
compacted subsoil of platy or massive
structure, (c) Dr. William Edwards,
soil scientist at Coshocton, Ohio,
demonstrates earthworm channels in
the soil managed with a no-till system
and manure application for a long
period of time.
Soil strength and compaction 207
FIGURE 7.17 Biochannels created by
tap roots of a tree or a woody perennial
can be exploited by fibrous roots of an
annual crop such as corn.
FIGURE 7.18 Large macropores of 2
to 5 cm in diameter are created by the
burrowing activity of rodents and other
animals that inhabit soil.
Principles of soil physics 208
FIGURE 7.19 Crop residue mulch in a
no-till farming system buffers the
impact of heavy vehicles and
minimizes the risk of soil compaction.
PROBLEMS
1. How many times is the wheel rut deeper in a tire with doubling of the tire pressure?
2. What is soil compactability? List and briefly describe processes, causes, and
consequences of soil compaction on agricultural lands (1–2 pages).
3. Consider the following data from a Proctor test.
Gravimetric moisture
content (%)
Wet density
(ρ′
b
, g/cm
3
)
5 1.4
10 1.6
Soil strength and compaction 209
15 1.8
20 2.0
25 1.8
30 1.4
35 1.2
Determine the range of moisture for the optimum density.
Assuming it to be a mineral soil, calculate the degree of saturation corresponding to
each moisture content.
4. What are mechanical means of alleviating soil compaction.
5. What are environmental impacts of soil compaction?
6. How does soil compaction impact eutrophication of surface water?
7. Calculate the void ratio (e) of a soil whose total porosity is 0.40.
8. Calculate the modulus of rupture of a briquette 4-cm wide and 1-cm thick, with a
distance between two supports of 5 cm when the normal force applied is 200 g.
9. Soil porosity was measured with a mercury injection porosimeter. Calculate pore
size corresponding with injection pressure of 100 cm, 1000 cm, and 10,000 cm(r=2γ cos
Θ/ρPg), γ for mercury is 430 dynes/cm, ρ of mercury is 13.6 Mg/m
3
, and g is 980 cm/s2).
10. Calculate porosity of a soil sample whose void ratio is 0.7 and particle density is
2.6 Mg/m
3
.
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