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MODULE 1. Introduction to mechanics of tillage tools
MODULE 2. Engineering properties of soil, principl...
MODULE 3. Design of tillage tools, principles of s...
MODULE 4. Deign equation, Force analysis
MODULE 5. Application of dimensional analysis in s...
Module 6. Introduction to traction and mechanics, ...
Module 7. Traction model, traction improvement, tr...
Module 8.Soil compaction and plant growth, variabi...
LESSON 27.SOIL COMPACTION
28.1. Soil compaction
The compaction of soil can be defined as an increase in its dry density, and the closer packing of solid particles or reduction in porosity. Compaction can result from natural causes, including rainfall impact, soaking, internal water tension and the like. Artificial compaction occurs under the downward forces of machines which are usually of short duration in the case of moving vehicles. The mechanical analysis of compaction under wheels and tracks is not simple, owing to the non-uniform nature of stress distribution, both normal and shear, in the ground. Figure 1.1, for example, shows the patterns of porosity under a rigid plunger in a Yolo sandy loam soil at 14.7 percent moisture content by weight. The greatest change in porosity is not at the plunger surface, where it is expected that the pressure is highest, but rather at the apex of a 45o isosceles triangle under the plunger face, where there is a concentration of normal stress plus shear.
A similar soil density change was observed by Gill and Reaves (1956) under smooth tires, and by Raghavan et al. (1976) in field tests using a tractor with lugged tires trailing a smooth tired sprayer. These latter results are shown in figure 1.2 as a distribution of change in soil dry density on a cross section under the tires, for different numbers of repeated passes of the machines in the same track.
It is evident in figure 1.2 that the number of times that a load is repeated affects not only the magnitude of change in soil density, but also the volume of soil which is affected. After 10 and 15 passes of the tractor and sprayer, the volume of compacted soil was seen to be progressively deeper and wider than after one or five passes. Other field tests on a clay soil at 38% surface moisture content by weight also demonstrated the increase in compacted density with higher contact pressures and number of repeated loadings, in a pattern very similar to that in figure 1.2. Figure 1.3 shows the maximum increase in dry density measured under two different tires at various average contact pressure, plotted against the number of repeated passes on a logarithmic scale. It appears that the number of repeated loadings has a similar role to the increase in contact pressure of a tire, and Raghavan et al. (1977a) suggested the following equation to describe maximum density changes, for moisture contents below the “optimum” moisture content for soil compaction.
------------------------------- (1.1)
Where,
\[\gamma\]d = soil dry density (mass of solids per unit soil volume)
A, B, C = soil constants
N = number of repeated passes of a TIRE
W = soil moisture content by weight (%), below the optimum.
Equation 1.1 includes the influence of soil moisture content on the degree of soil com pactionfor a given applied load. In fact, the humidity of the soil is very important factor in the change of dens ity undr surface pressures.
Equation 1.1 includes the influence of soil moisture content on the degree of soil com pactionfor a given applied load. In fact, the humidity of the soil is very important factor in the change of dens ity undr surface pressures.
Figure 1.4 shows the final levels of dry density in a sandy loam soil, compared to the uncompacted state, after applying various loadings by tractors of different masses and numbers of repeated passes on the surface. The standard Proctor compaction test results for this soil (Lambe, 1951) are also given in figure 1.4.
It is evident from figure 1.4 that the moisture content of this type of soil is an important factor controlling the degree to which compaction will occur under a particular load. The 15 repeated passes of a tractor with 41 kPa contact pressure, for instance, increased the orginal soil density by an amount about double that of a single pass at the same loading pressure. However, all of the traffic levels at the “optimum” soil moisture content for compaction, which was close to 15% by weight in this soil, produced between four and five times as much increase in soil density as they would in a dry soil state, below 5% moisture content.
Last modified: Wednesday, 19 March 2014, 12:24 PM