A

acceleration due to gravity (g)

g = 9.81 m/s² at sea level.

active earth pressure (s_ha , s'_ha)

The minimum horizontal stress exerted by a mass of soil on a retaining surface as the surface moves away from the soil, e.g. the pressure exerted behind an earth-retaining wall.

activity (A)

Typical values:
Kaolin clay 0.4-0.5
Illite clays 0.75-1.25
London clay 0.95
Montmorillonite clays >2.0

adhesion

The shear resistance between soil and another material (e.g. steel, concrete or timber; along a pile shaft or beneath a retaining wall). In physics, adhesion is described as ‘the force that holds together molecules or unlike particles within a substance'. (See also cohesion)

adsorbed water

Water held on the surface of flaky particles (e.g. clay) by electrostatic charge. The ions forming the platy surfaces of clay carry a negative charge and thus attract the positive end of bipolar water molecules.

ageing

Processes that occur with time, independent of changes in loading, and cause changes to the state of a soil, e.g. vibration, compaction, creep, cementing, weathering, changes in salinity.

air-voids content (A_v)

(Also air-void ratio or air content) The ratio of the volume of air to the total volume of a body of soil.

allowable bearing capacity (q_a)

The bearing pressure that can be allowed on a foundation soil, usually to limit settlements. Units: kPa

angle of dilation (y)

Defined by the ratio between the rate of volumetric strain and the rate of shear strain (note: the value is positive for dilation); also the direction of relative motion across a slip plane.

angle of friction (f')

The general Mohr-Coulomb failure criterion is given by:
t' = c' + s' tan f'
where f' is the angle of friction. Because soil can have different definitions of failure, it is necessary to identify the particular state described:
f´_p refers to the peak state
f´_c refers to the critical state
f´_r refers to the residual state

angle of ground slope (b, i)

The angle referred to horizontal of the ground surface.

angle of repose

The maximum angle of a just-stable slope of a heap of dry granular material.

angle of shearing resistance (f'_mob)

Defined by the ratio of effective shear and normal stresses mobilised at any state (e.g. prior to failure).
Also, f'_max is the angle of shearing resistance at the peak state.
t'_p = s' tan f'_max
f'_max = f'_c + y
where y is the angle of dilation.

angle of slip plane

The angle referred to horizontal of a plane or other surface along which a discontinuous slip or rupture will occur, e.g. behind or in front of a retaining wall.

angle of wall friction (d)

The angle of friction between soil and the surface of a structure (e.g. retaining wall, underside of foundation). The maximum resistance to sliding along the surface is N.tand, where N is the force normal to the surface.

angular distortion (D/L)

The ratio between the relative deflection (D) between two points in a foundation and the distance between them (L).

anisotropic

Not the same in all directions. In soils, properties in a horizontal direction may be different to those in a vertical direction, for example.

aquifer

A stratum of relatively high permeability; a water-bearing stratum of rock or soil.

artesian

Artesian conditions exist when the water table (piezometric surface) lies above ground level.

at-rest earth pressure (s_ho, s'_ho)

The horizontal stress developed in a mass of soil loaded in conditions of zero horizontal strain.

axial strain (e_a)

Direct strain measured along an axis, e.g. along the axis of a triaxial test sample.

axial stress (s_a, s'_a)

Total or effective stress acting along an axis, e.g. along the axis of a triaxial test sample.