Volume Changes in Soils

Introduction
The expression a p - S = U is used in a method of calculating the equilibrium moisture distribution in soils, where
a = a compressibility factor;
S = the suction characteristic of the soil;
U = the pore water pressure relative to the position
of the water-table;and
P = the pressure due to the overburden above the zone under consideration. The value p can also include any surface loading n due to a road, runway, or light structure, i.e.,
p = h Dw, + n, where h = depth from the surface to the centre of the zone:
Dw = Dd (100 + m)/ 100 = bulk density of the soil,and Dd is the dry density: m = a value of moisture content assumed as a first approximation (this may be adjusted when the equilibrium moisture contents have been estimated once, by choosing the new value
of m to give a better estimate of Dw).

E. LL. Morgan