Geotechnical Engineering is the branch
of civil engineering concerned with the engineering behavior of earth
materials. Geotechnical engineering includes investigating existing subsurface
conditions and materials; assessing risks posed by site conditions; designing
earthworks and structure foundations; and monitoring site conditions,
earthwork and foundation construction.
A typical geotechnical engineering project begins with a site investigation
of soil and bedrock on and below an area of interest to determine their
engineering properties including how they will interact with, on or in
a proposed construction. Site investigations are needed to gain an understanding
of the area in or on which the engineering will take place. Investigations
can include the assessment of the risk to humans, property and the environment
from natural hazards such as earthquakes, landslides, sinkholes, soil
liquefaction, debris flows and rock falls.
In geotechnical engineering, soils are considered a three-phase
material composed of: rock or mineral particles, water and air. The voids
of a soil, the spaces in between rock or mineral particles, contain the
water and air.
The engineering properties of soils are affected by four main factors:
the predominant size of the mineral particles, the type of mineral particles,
the grain size distribution, and the relative quantities of mineral, water
and air present in the soil matrix. Fine particles (fines) are defined
as particles less than 0.075 mm in diameter.
The following properties of soils are used by geotechnical engineers in
analysis of site conditions and design of earthworks, retaining structures,
Total unit weight: Cumulative
weight of the solid particles, water and air in the material per unit
volume. Note that the air phase is often assumed to be weightless.
Dry unit weight: Weight
of the solid particles of the soil per unit volume.
Saturated unit weight:
Weight of the soil when all voids are filled with water such that
no air is present per unit volume. Note that this is typically assumed
to occur below the water table.
Ratio of the volume of voids (containing air and/or water) in a soil
to the total volume of the soil expressed as a percentage. A porosity
of 0% implies that there is neither air nor water in the soil. Void
ratio is the ratio of the volume of voids to the volume of solid particles
in a soil. Void ratio is mathematically related to the porosity and
is more commonly used in geotechnical formulae than porosity.
A measure of the ability of water to flow through the soil, expressed
in units of velocity.
As a noun, the state of the soil with regards to prior loading conditions;
soils can be underconsolidated, normally consolidated or over-consolidated.
As a verb, the process by which water is forced out of a soil matrix
due to loading, causing the soil to deform, or decrease in volume,
Amount of shear stress a soil can resist without failing.
Liquid limit, plastic limit, and shrinkage limit, related to the plasticity
of a soil. Used in estimating other engineering properties of a soil
and in soil classification.