Lecture 8 Elements of Soil Mechanics

Phase relationships of earth materials
Effective stress
Shear strength
Settlement and consolidation

Soil mechanics concerns with the action of forces on soil masses. It involves the understanding and prediction of soil behaviors for engineering designs or uses as construction materials. Soil mechanics is a major component of  the field of study known as Geotechnical Engineering. Here we deal only with a few basic elements of it. Examples and derivations are given in the lectures.

Phase relationships of earth materials (pp 106-113)

Earth materials have a three-phase system, including solid, water, and air. Water and air occupy voids between solid particles. For soils, the physical weight volume relationship between these phases are particularly important. The relationship can be examined using a schematic solid-water-air phase diagram.

Effective stress (pp. 129-132)

Effective stress is an important concept, which has many applications in Geology. Consider a layer saturated with water beneath the surface. The total stress produced by the overlying layers is opposed from below by both the stress transmitted through the solid grains of the layer and the hydrostatic pore fluid pressure. Thus the stress acting between the solid grains (called effective stress) in effect is the total stress subtracted by the pore pressure.

Shear strength of soils (pp. 133-139)

The strength of a soil indicates the stress that the soil can sustain until failure (displacement or excessive deformation). In soil mechanics, the shear strength is the main concern because most failures result from excessive shear stresses. The shear strength of a soil determines its ability to support the load of a structure or remain stable on a slope.

Settlement and consolidation (pp. 143-145)

It is not very common for external loads to overcome the strength of the soils to cause failures. But soils do deform under external loads. The usual response  is a volume decrease beneath the foundation. Consolidation is the reduction in volume of a soil under load as water drains from the pores. Settlement is the vertical subsidence of the building as the soil is compressed. Excessive settlement, particularly when it is unevenly distributed, can result in serious damage to the structure.

Typically, the void ratio is plotted against the log of the effective consolidation pressure. The slope of this compression curve is called compression index of the soil.