Engineering geology

Soil Strength

Properties of a soil depend on the grain size, mineralogy and water content, all of which are interrelated.
Clay minerals can hold a high water content; for fine grained soils, the critical concept is consistency when it is related to water content.


With varying water content, a soil may be solid, plastic or liquid. Most natural clays are plastic.
Water content (w) = weight of water as % of dry weight. Consistency limits (Atterberg limits) are defined as:
Plastic limit (PL) = minimum moisture content where a soil can be rolled into a cylinder 3 mm in diameter. Disturbed soil at PL has shear strength around 100 kPa.
Liquid limit (LL) = minimum moisture content at which soil flows under its own weight. Disturbed soil at LL has shear strength around 1 kPa.
Plasticity index (Pl) = LL–PL. This refers to the soil itself and is the change in water content required to increase its strength 100 times; it is the range of water content in which the soil is plastic or sticky. High Pl soils are less stable, with large swelling potential.
Liquidity index (Ll) = (w–PL)/Pl. This is a measure of soil consistency and strength at a given water content.


Soils are classified on grain size and consistency limits. A-line distinguishes visually similar clays and silts. More subdivisions exist in a full soil classification.


Plasticity and properties of clay soils depend on amount and type of clay minerals.
Soils with <25% clay minerals are generally stronger, with low Pl, and φ < 20%.
Activity of clay = Pl / % fines (<0·002 mm diameter). Soils with high clay fraction and high activity can retain high water content, giving them low strength; and these also have low permeability.
Activity is mainly due to clay mineral type; smectite (montmorillonite) clays are the most unstable.


All soils fail in shear.
Shear strength is a combination of cohesion and internal friction; expressed by Coulomb failure envelope.
Cohesion (c) derives from interparticle bonds; it is significant in clays, but zero in pure sands.
Angle of internal friction (φ) is due to structural rough ness; higher in sand than in clay.
Shear strength = cohesion + normal stress x tan φ Normal stress is critical to shear strength but pore water pressure (pwp) carries part of overburden load on soil, thereby reducing normal stress.


Drainage progress of a loaded clay is critical, as any increase of pore water pressure may lead to failure; significant in new excavations and embankments.
Peak strength declines to residual strength due to restructuring, notably alignment of mineral plates, during dislocation along a plane. Change is due to almost total loss of cohesion and also reduction in friction angle.
Significant in all clays, notably those with higher Pl. Brittleness = % decline from peak strength.

Sensitive clays lose great proportion of their strength on restructuring of entire mass; they have high Ll and small grain size, so cannot drain rapidly and load is taken by pwp; shear strength approaches zero.
Sensitivity = ratio of undisturbed : disturbed strengths, and relates to undrained brittleness.

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