Soil mechanics

Soil mechanics is a discipline that applies the principles of Engineering mechanics to predict the mechanical behavior of soil. Karl von Terzaghi, who worked on a rational approach to soil engineering, is known as the Father of Soil Mechanics.

Soil mechanics is an important discipline for many branches of engineering, such as Civil engineering, Geotechnical engineering and Engineering geology. It is used in the design of foundations to support structures, embankments, retaining walls, earthworks and underground openings.

The percolation of water through soils is of importance in the construction of tunnels and deep foundations, and obviously water-related structures like bridge pilings and dams.

Contents

1 Effective Stress σ '

1.1 Total Stress σ
1.2 Pore water pressure μ

2 Consolidation theory

3 Shear strength

4 Lateral earth pressure

5 Seepage

6 References

7 See also

Effective Stress σ '

The concept of effective stress is central to understanding behaviour of soils under different conditions. Effective stress is a measurement of the load borne by the soil skeleton. This pressure determines the ability of soil to resist shear stress. If the effective stress in a soil is reduced to zero, quick condition is said to occur (see quicksand).

Effective stress (σ ' ) of a soil is calculated from two easily measured parameters, total stress (σ) and pore water pressure (μ) according to:

σ' = σ - μ

where all three terms have units of pressure.

Total Stress σ

The total stress σ is equal to the overburden pressure, it is simply the weight of everything which rests on the soil, including the soil above. Total stress always increases with increasing depth.

Pore water pressure μ

The pore water pressure μ can be calculated as the hydrostatic pressure of water according to fluid statics if it is assumed that the flow of water through soil is slow. This assumption is valid under most conditions (quick condition being a notable exception). Pore water pressure can be estimated as zero above the water table and increases linearly with increasing depth below the water table.

Consolidation theory

When water flows into or out of a soil mass without causing the volume to change, the flow is known as seepage. If, on the other hand, the flow of water within a soil mass induces a volume change, then the flow is referred to as transient. The process of volume change triggered by a transient flow is known as consolidation. It is related to the change in effective stresses within the soil matrix due to a surface loading (or unloading) or variation in the water table. The excess porewater pressure (i.e. load-induced porewater pressure) generated in both cases causes the water to be either squeezed out of the soil mass (positive pore water pressure) or sucked into the soil matrix (negative porewater pressure). This movement of water continues at a changing rate until all excess pressure has dissipated, and the equilibrium of stresses has been restored according to the effective stress principle.

If at some stage during its geological history the soil has been subjected to unloading, e.g. disappearance of an ice cover or a severe erosion, then the present pressure do to the overburden pressure (self weight) is smaller than that which existed before the onset of the unloading process, and the soil is known as overconsolidated. If, on the other hand, the soil has not been subjected to any unloading during its entire geological history, then the present overburden pressure, constitutes the largest pressure that the soil has ever experienced, and the soil is referred as normally consolidated.

Shear strength

It is the maximum resistance a soil can offer before the occurrence of shear failure along a specific failure plain. THe shear strength is related to the soil type, thus, the response of a granular soil to an applied load depends to a large extent on its density, whereas a cohesive overconsolidated soil exhibits a markedly different behaviour to that of a normally consolidated soil.

Lateral earth pressure

Rataining structures are subjected, apart from their self weight, to lateral thrusts whose intensity and direction depend on the movement (or lack of it) of the structure itself. The type of thrust is examined using the coeficient of earth pressure defined as:

$K= \frac{\sigma'_h}{\sigma'_v}$

If the wall does not move at all then $K$ is referred as the coefficient of earth pressure at rest $K o$ .
If the wall is pushed into the soil then at failure, the coefficient $K$ reaches its maximum value known as the coefficient of passive earth pressure $K p$ .
If the wall is moved away from the soil it supports, then at failure the ratio $K$ reaches its minimum value, known as the coefficient of active earth pressure $K a$ .

Seepage

This is the flow of a fluid (typically water) through the soil pores, without volume change of the soil mass. Seepage under dams and sheet pile walls is often estimated using the simple graphical flownet method.

References

Azizi, F., Applied Analyses in Geotechnics, (2000), E & FN SPON.