Engineering Properties of Soils

© 2007 Donald G. McGahan (aka soilman) All Rights Reserved

Soil Strength

Soils can be stressed three ways and the soil resists these by compressive strength, tensile strength, and shear strength.

The two factors that determine soil strength are frictional resistance and cohesion. Soils without clay are cohesionless and with clay are cohesive. Cohesion between the water layers surrounding the soil particles is termed apparent cohesion because it is temporary.

Consistence is the resistance to soil deformation. This can be reshaping or rupturing. Moisture plays a role.

Atterberg Limits

Shrinkage Limit (SL)
No volume change with moisture loss.
Plastic Limit (PL)
The lowest moisture where soil behaves in a plastic manor.
Liquid Limit (LL)
Soil behaves as a viscous liquid.
Plasticity Index (PI)
The water content range at which the soil has plastic properties. This is between the liquid limit and plastic limit (LL-PL).

Coefficient of Linear Extensibility (COLE)

Typically three dimensional measurements are taken on clods taken from the site. The soil is coated to keep water from entering the pores when submerged. The clod is submerged to determine the volume when moist and when Oven Dried. This is a irregular three dimensional measurement.

COLE = 3moist volume / dry volume – 1

Another measurement is COLErod. A paste of the fine earth fraction is exuded into a ribbon on a greased tray. Its length, L, is measured moist, M, for LM, dried in an oven, and the length is measured dry, D, for LD.

COLErod =
LM - LD /  LM
X 100

COLE values and Hazards to Engineering Use Interpretations.
COLE Interpretation
<0.03 Low hazard
0.03–0.06 Moderate hazard
0.06–0.10 Severe hazard
0.06–0.10 Very severe hazard

Shrink Swell

Soil change in volume is related to the moisture content changes. The freedom of cations to enter into the interlayer of the clay unit cells is part of the story. The other factor is how tightly the cations are held. The looser held cations are free to add additional shells of water and effectively expand the interlayer space (swelling). When the water is removed the soil volume decreases.

The phyllosilicate clay content can be a mix of phyllosilicate group minerals. As the clay fraction increases, the potential for shrink swell increases. More importantly an increased amount of 2:1 layer type with lower charge increases the shrink swell potential.

Shrink swell by phyllosilicate clay type.
Phyllosilicate Group Layer Type Charge Interlayer Access Shrink Swell
Kandite 1:1 very very low No: H-bonded no apreciable
Smectite 2:1 very low loosely held cations high
Vermiculite 2:1 high tightly held cations less than smectite
Chlorite 2:1:1 very low No: filled with polymerized sheet similar to brucite or gibbsite doesn't