Laboratory experiments on undisturbed specimens of a large number of s
oft clay deposits, as well as previous measurements on clays and granu
lar soils, were used to examine and explain the magnitude and behavior
of the coefficient of earth pressure at rest, K(o): (i) after sedimen
tation - primary consolidation, (ii) during secondary compression agin
g, (iii) after active or passive preshearing away froin the laterally
constrained condition, (iv) during a decrease in effective vertical st
ress, and (v) during an increase in effective vertical stress in the r
ecompression or compression range, in terms of k, the slope of the eff
ective horizontal (sigma(h)') versus effective vertical (sigma(v)') st
ress path. The behavior of K(o) is explained using the concept of mobi
lized angle of friction in laterally constrained deformation condition
. The Jaky equation provides, in terms of the angle of internal fricti
on, a good estimate of K(o) of sedimented. normally consolidated young
clays and granular soils. as well as of K of presheared clays and san
ds, and of densified granular soils that are subjected to laterally co
nstrained compression from sigma(h)' = sigma(v)' = 0. Empirical equati
ons provide reasonable estimates of K(o) for clays and granular soils
after secondary-compression aging, after preconsolidation by unloading
, and for soft clay deposits that display a preconsolidation pressure
sigma(p)' greater than in situ effective vertical stress sigma(vo)'. P
roposed empirical equations and methods successfully predict K(o) of p
resheared clays.