ENGINEERING CHARACTERISTICS OF SOILS

The major engineering characteristics of the main soil groups discussed in the previous section as related to foundation design are summarized as follows.  A discussion on the practical aspects of the engineering characteristics are presented for granular and fine-grained soils following these summaries.

  Engineering Characteristics of Coarse-Grained Soils (Sands and Gravels)

•  Generally, very good foundation material for supporting structures and roads.

•  Generally, very good embankment material.

•  Generally, the best backfill material for retaining walls.

•  Might settle under vibratory loads or blasts.

•  Dewatering may be difficult in open-graded gravels due to high permeability.

•  Generally, not frost susceptible.

Engineering Characteristics of Fine-Grained Soils (Inorganic Clays)

•  Generally, possess low shear strength.

•  Plastic and compressible.

•  Can lose part of shear strength upon wetting.

•  Can lose part of shear strength upon disturbance.

•  Can shrink upon drying and expand upon wetting.

•  Generally, very poor material for backfill.

•  Generally poor material for embankments.

•  Can be practically impervious.

•  Clay slopes are prone to landslides.

Engineering Characteristics of Fine-Grained Soils (Inorganic Silts)

•  Relatively low shear strength.

•  High capillarity and frost susceptibility.

•  Relatively low permeability.

•  Frost heaving susceptibility

•  Difficult to compact.

 Engineering Characteristics of Organic Soils

The term organic designates those soils, other than topsoil, that contain an appreciable amount of vegetative matter and occasionally animal organisms in various states of decomposition.  Any soil containing a sufficient amount of organic matter to influence its engineering properties is called an organic soil.   The organic matter is objectionable for three main reasons:

1.  Reduces load carrying capacity of soil.

2.  Increases compressibility considerably.

3.  Frequently contains toxic gasses that are released during the excavation process.

Generally organic soils, whether peat, organic clays, organic silts, or even organic sands, are not used as construction materials.