P

particle size characteristics (d_n)

(Also Hazen’s characteristics, after Allan Hazen, 1868-1930) Used in particle size analyses and grading: stated as the maximum particle size of a specified percentage (smallest) of the total content of a soil graded by mass.
d₁₀ = maximum size of the smallest 10% of the particle content (also referred to as effective size)
d₆₀ = maximum size of the smallest 60% of the particle content

pascal (Pa)

(After Blaise Pascal, 1623-62) The derived SI unit of pressure or stress equal to 1 newton per square metre:
1 Pa = 1 N/m²
1 kPa = 1000 Pa

passive earth pressure (s_hp, s'_hp)

The maximum horizontal stress exerted by a mass of soil on a retaining surface as the surface moves toward the soil.

peak shear strength

The maximum shear strength of a soil at a given normal effective stress and water content; this occurs at relatively small strains.

permeability

The property which allows the flow of water through a soil.

See also coefficient of permeability.

pF index

A measure of soil suction: pF = log₁₀ (suction head in cm),
i.e. for a suction of 100 kPa, pF = 2.0
[Range for soils is pF = 0 to 7]

pH value

A measure of acidity or alkalinity of groundwater or soil water extract based on the hydrogen ion content:
pH = -log₁₀(hydrogen ion content)
pH < 7.0 indicates acidity.
pH > 7.0 indicates alkalinity.

piezometer

An instrument used to measure in situ pore pressures; may be an open standpipe or an enclosed electronic pressure transducer.

piezometric surface

An imaginary surface corresponding to the hydrostatic water level of a confined body of groundwater; the notional level to which artesian pressure would raise water in a (real or imaginary) standpipe.

pile spacing (s)

The distance from centre to centre of piles in a group.

plane strain

A two-dimensional stress state, where the out-of-plane strain (i.e. the strain normal to the plane being considered, e_z ) is zero. An example of a plane strain situation would be on a cross-section through a long structure being loaded in the x-y plane, such as an embankment dam.

plastic deformation

The flow or distortion resulting in a permanent and irrecoverable change in shape or volume.

plastic limit (w_p)

(Also PL) The moisture content above which a soil will have a plastic consistency, but below which it crumbles.

plastic strain

Deformation or strain that is not recovered upon unloading.

plasticity

1. The property of a soil (or other material) which allows it to deform continuously.
2. Plasticity theory is used to calculate plastic (irreversible) deformations.

plasticity index (I_p)

(Also PI) The difference between the liquid limit and plastic limit.

I_p = w_L - w_P

Poisson's ratio (n, n')

(After Simeon Poisson 1781-1840)
The ratio of the change in strain perpendicular to the direction of loading to the change in strain caused in the same direction.

n´ = - de_y / de_{x    (for} to loading or unloading in the x-direction).

 

For undrained loading of saturated soil,

n_u = 0.5

For drained loading or unsaturated soil,

n´ = 0.2 - 0.5

pore air pressure (u_a)

The pressure of air in a partially saturated soil; not necessarily the same as pore water pressure due to the surface tension on air-water interfaces within the voids.

pore pressure (u)

The pressure exerted by the fluid within the pores or voids in a porous material; in saturated soil the pore pressure is the pore water pressure.

pore pressure coefficient (A)

The ratio of the change in pore pressure to the change in deviator stress, e.g. in an undrained triaxial test; the value of A varies with strain and the overconsolidation ratio.

pore pressure coefficient (B)

The ratio of the change in pore pressure to the change in isotropic stress in undrained loading.

For saturated soils, B = 1.

pore pressure force (U)

The resultant force due to pore pressure acting on a given area.

pore pressure ratio (r_u)

At a given point in a body of soil, the ratio of the porewater pressure to the vertical overburden pressure.

pore water pressure (u_w)

(See also pore pressure) In partially saturated soils the pressure exerted by the water in the voids may not be the same as the pore air pressure, due to the surface tension on air-water interfaces.

porosity (n)

The ratio of void volume to total volume:
n = V_v / V
where
V_v = volume of voids
V = total volume

potential function (F)

A function introduced in the solution of the Laplace equations defining two-dimensional seepage flow. (See also stream function.)

pressure head (h_w)

(Also head) The height of a column of water required to develop a given pressure u at a given point.
h_w = u / g_w

See also total head

pressure in tension crack (p_w)

The horizontal pressure exerted in a slope or against a retaining wall due to hydrostatic water pressure in tension cracks.

principal axes

A set of orthogonal axes perpendicular to which the shear stresses and shear strains are zero and normal stresses and strains are referred to as principal stresses and principal strains.

principal strains (e₁, e₂, e₃)

Normal stresses acting in the direction of principal axes of stress. Note: the principal axes of stress and strain may not coincide.