Mechanics of Tillage and Traction 3(2+1)

LESSON 28. Mechanical and hydraulic properties of compacted soil

The increase in soil strength following compaction is very desirable and even necessary in the construction of building foundations, roads and dams. However, it is not of benefit when soil must be subsequently excavated or tilled. Considerably more energy must be expended to cut a soil after it has been compacted, and the resultant structure of the remolded soil will most likely be different from an uncompacted soil which is excavated or worked. The moisture content of a soil affects its strength as well, generally by decreasing the cohesion and friction angle beyond the lower plastic limit. Nevertheless, at a fixed moisture content, a soil will have a higher strength at larger dry densities, which reflects the closer packing of solid particles. Figure 8.5 shows, for example, the effects of changes in both moisture content and soil dry density on the penetration resistance of a clay loam soil. It is evident that the penetration resistance, which was measured by a cone penetrometer in this case, cannot be determined by compacted density alone, but is also dependent on moisture content. The increased soil strength at higher densities will not only increase soil cutting forces and energy required, but will also impede the growth of plant roots. 

Table 2.1. Change in soil mechanical properties when soil density is increased at constant moisture content

Property	Soil type	Density range T m-3	Corresponding property range	Source
c	Clay	0.84 – 1.89	0 – 282 kPa	1
ϕ	Clay	0.84 – 1.89	0 – 37o	1
ϕ	Sand	1.74 – 2.11	18 – 55o	1
Penetration resistance	Clay	0.84 – 1.89	0 – 8250 kPa	1
ϕ	Sand	1.60 – 1.72	27 – 32o	2
Undrained C	Clay	1.02 – 1.25	5 – 40 kPa	3
Tensile strength	Loam	1.45 – 1.70	8 – 65 kPa	4
Stiffness kϕ	Yolo loam	1.26 – 1.55	982 – 5200 kPa	5
Stiffness kϕ	Sandy loam	1.48 – 1.73	2470-4800 kPa	5
Pulvarization energy (2.5 cm mean weight dia)	Loam	1.10 – 1.56	22 – 173 J/kgx10-2	6

1. Proctor (1948)

2. Taylor (1948)

3. Graecon (1960)

4. Vomocil et al. (1961)

5. Chancellor and Schmidt (1962)

6. Bateman et al. (1965)

 

Fig 2.1. The penetration resistance (cone index pressure) on a 12.8 mm cone (ASAE, 1984) in a clay loam soil at different dry densities and moisture contents by weight (Taylor et al. 1981)

28.2 Hydraulic properties

In a review of the effects of soil compaction on the content and transmission of water in soils, Warkentine (1971) noted that compaction alters the water content  and movement in soils by modifying the void size distribution. Large macropores in the soil fabric are the first to be reduced in volume, as shown in figure 8.6. This tends firstly to reduce the amount of water which is retained at low water suction pressures in the macropores, and secondly to reduce the saturated hydraulic conductivity of the soil.

 

 Fig 2.2. Schematic representation of large macro[pores in soilfabric before and after compaction

The reduction in saturated hydraulic conductivity of soils is generally logarithmic with changes in soil dry density, or with void ratio. Some of the changes of observed alterations in clay and loam soils are given in table 2.2.

Table 2.2. Observed changes in saturated hydraulic conductivity at different soil dry densities

Soil type	Range of dry density, t m-3	Range of saturated hydraulic conductivity, 10-6 cm s-1	Source
Beach sand	1.40 – 1.66	55000 - 20000-	1
Dam filter sand	1.66 – 1.83	13000 – 7000	2
Dike sand	1.24 – 1.61	1800 – 150	2
Ste-Rosalie clay	1.15 – 1.55	2700 – 100	3
Yolo silt loam	1.20 – 1.50	3000 – 30	4
North Carolina silt	1.43 – 1.66	60 – 0.6	1
Boston silt	1.27 – 1.51	1 – 0.002	2

1. Taylor (1948)

2. Lambe and Whitman (1969)

3. Rowles (1948)

4. Taylor and Henderson (1959)

As table 2.2 shows, the hydraulic conductivity of a saturated soil can change by one or two orders of magnitude when compaction occurs. The reduction in size of macropores, shown in figure 2.2 has other consequences for the water status in soils as well. The amount of water which can be stored and easily available for plant use will also be reduced, as well the quantity of air in the soil (of which about 10% of the total volume is needed for healthy plant roots). The drainage of a soil will also be slowed, which leads more readily to high degrees of saturation in rainy periods of the year, and an insufficient quantity of air for crop roots.

On the other hand, there is the possibility that a soil is not compacted enough. This arises in all soil types when the dry density is low, the volume of macropores is large and drainage may be too rapid. In such case, especially during periods of low precipitation, there may be insufficient water stored in the arable soil profile, and an excess of air. Plant roots then need to seek water stored in the micropores under high suction, and eventually dry the soil to the “wilting point” at which water content they can no longer extract moisture from soil fabric.

From the view point of soil hydraulic properties, there is actually no unique optimum dry density of a soil. The best structure is dependent on the type of crops growing on a field, their rooting depth and water extraction capability and the quantity of precipitation input, which varies from place to place, and even from year to year at a single location.