10.0 Other important soil properties

The following sections briefly define a number of other soil properties important for planning the construction of earthen dikes, dams and canals. Some examples for the construction of dikes and dams in specific soil groups are listed in the last six columns of Table 26.

10.1 Permeability of compacted soil

The degree of permeability of compacted soil relates to the rate at which water moves through soil after it has been compacted. If a coarse-grained soil, after compaction* contains large continuous pores, it transmits water rapidly and is said to have a high permeability. Fine-grained soils contain very small continuous pores and a compacted fine-grained soil will transmit water very slowly and have low permeability.

10.2 Compaction characteristics

Compaction characteristics of a soil indicate the relative response of that soil to compactive effort (consolidation). Soils with good compaction characteristics can be compacted to a high degree with minimum effort. Soil material with a plasticity index close to 16 percent has the best compaction characteristics.

For each soil, there is an optimum moisture content that will permit the soil to be compacted to the maximum degree with the least effort and allow the compacted soil to attain its lowest permeability. Optimum moisture content for various types of soils are as follows:

Optimum moisture content for compaction (range in %)
Clayey sands, sand-clay mix
11 - 10
Sand-silt-clay mix with plastic, silt + clay fraction
15 - 11
Inorganic silt, clayey silt
24 - 12
Inorganic clay
24 - 12
Organic silt
33 - 21
Inorganic clay, highly plastic
36 - 19
Organic clay
45 - 21

Note: the optimum moisture content is usually 2-3 percent less than the plastic limit of the soil.

10.3 Compressibility

Compressibility is the degree to which a soil mass decreases in volume when supporting a load. Compressibility is lowest in coarse-grained soils where particles are in contact with each other. It increases as the proportion of small particles increases and becomes highest in fine-grained soils which contain organic matter. The following are some examples of compressibility for various soils:

  • Gravels and sands are practically incompressible. If a moist mass of these materials is subjected to compression, there is no significant change in their volume;
  • Clays are compressible. If a moist mass of clay is subjected to compression, moisture and air may be expelled, resulting in volume reduction which is not immediately recovered when the load is removed.

Fine-grained soils which contain at least 50 percent of silt + clay may be listed in three classes of compressibility on the basis of their liquid limit . They are as follows:

  • Low compressibility: LL smaller than 30;
  • Medium compressibility: LL from 30 to 50;
  • High compressibility: LL greater than 50.

In general, compressibility is approximately proportional to the Plasticity index (see Section 8.5). The greater the PI, the greater the compressibility of the soil.

10.4 Shrink-swell potential

The shrink-swell behaviour of a soil is that quality which determines its volume change under varying moisture conditions.Some soils will shrink when dry and swell when wet.

The volume change of the soil mass is influenced by the amount of moisture change as well as by the amount and kind of clay present in the soil. The following are some examples of shrink-swell potential for various soils:

  • Low shrink-swell potential: loamy sand, sand and kaolinite clay;
  • High shrink-swell potential: montmorillonite* clay.

10.5 Shear strength

The shear strength of a soil means the relative resistance of that soil to sliding when supporting a load. The highest resistance to sliding occurs in soils that are composed of clean gravel with less than 5 percent silt + clay. Shear strength of soils decreases as the proportion of fine particles increases. It is lowest in fine-grained organic soils. For example, when building a dam, it is important to remove all of the organic soil to decrease the possibility of sliding.

10.6 Susceptibility to piping

The susceptibility of a soil to piping means the degree of internal erosion which takes place when water moves through the pores or cracks of that soil. Soils with a high susceptibility to piping are those which have large pores through which water moves very rapidly, but in which soil particles are fine enough and sufficiently lacking in coherence for the individual particles to move readily. The most susceptible materials are fine sands and non- plastic silts with a plasticity index of less than 5. Although coarse sands and gravel may also transmit water rapidly, they resist internal movement better because they consist of large individual particles. Other soils which have a low susceptibility to piping are the fine-grained, cohesive plastic soils which transmit water very slowly and resist internal erosion well.