Soil mechanics

The study of soil mechanics helps to understand the underlying mechanism for application in civil engineering. Karl von Terzaghi, who worked on a rational approach to soil engineering, is known as the Father of Soil Mechanics.

A civil engineer has many diverse and important encounters with soil. Soil is used as a foundation to support structures and embankments, as a construction material; structures must be designed to retain soil from excavations and underground openings. The flow or 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 Bearing capacity

5.1 Stability of slopes

6 Seepage

6.1 Ground investigation

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

Shear strength

strength of soil against any deformation should be strength against failure (not deformation)

Lateral earth pressure

Bearing capacity

Stability of slopes

Seepage

This is the flow of a fluid (typically water) through the soil pores. Seepage under dams and sheet pile walls is often estimated using the simple graphical flownet method.