The state of stress theory as developed by Rankine in 1860 for application
to earth pressures. He formulated that the pressure on a vertical
retaining wall restraining earth with a horizontal surface is (1 - sinphi
) / (1 + sinphi ) multiplied by the soil density for each foot depth of
soil retained, where phi is the angle of internal friction of the soil.
The value (1 - sinphi ) / (1 + sin#2f) is the coefficient of active earth
Dictionary of Mining, Mineral, and Related Terms