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F

The ratio of a limiting value of a quantity to the design value of that quantity, e.g. the ultimate load divided by the design load. The quantity used can be a measure of force, moment, stress, displacement, size, or other design measure. The magnitude of a factor of safety deemed to be acceptable depends on the acceptable degree of risk, the probabilities associated with occurrence of limiting conditions, the quality of data, and the method of calculation.

The graph of the shear stress and normal effective stresses at which shear failure occurs. Because different failure states can be defined for soils, there are different failure envelopes for peak strength, critical state strength and residual strength.

A shear fracture in a rock mass along which movement has taken place.

Soils containing >35% particles smaller than 0.06 mm in size.

Soil grains smaller than 0.06 mm, e.g. CLAY, SILT.

The symbol D placed before a quantity symbol indicates a finite or large change in the quantity,
e.g. Ds = a finite change in stress.

A clay having an internal network of narrow joints or fissures, which tend to open upon drying and permit rapid entry of water when exposed to weathering, e.g. London clay.

Small, but often numerous, cracks, particularly in clay soils. (See also fissured clay)

(Also stream lines) The path followed by water particles in seepage flow; in a flow net, represent direction and path of flow.
N_f = number of flow lines in a flow net connecting the inflow and outflow boundaries.

A graphical representation of seepage flow in a body of soil, in which flow lines and equipotential lines intersect at right angles. Used in problems concerning groundwater seepage through embankments and earth dams, and under dams and cut-offs, to estimate flow quantities and pore pressures.

The total quantity of water flowing in a seepage problem (e.g. in flow net or permeability test). Units: m³, ml.

The rate of flow is the quantity flowing in unit time. Units: m³/s, litres/min.

(Also discharge velocity) The velocity of flow through soil.
v = q / A
where q is the quantity flowing in unit time and A is the total area of water and soil grains. So v is not the same as the velocity of a drop of water within the soil.

The action that tends to accelerate a body or to change its direction of movement. The SI unit of force is the newton (N) [after Sir Isaac Newton] and is the force required to accelerate 1.0kg by 9.81m/s². Weight is a force acting downward.

The depth below the ground surface of the base of a foundation.