applies. The phreatic surfaceis defined as the datum level at which the
porewater pressure within a soil mass is zero, i.e. atmospheric. The stable
water level attained in a standpipe is termed the piezometric level. The
level of the water table (WT) or groundwater level (GWL) undergoes sea-
sonal fluctuations and may also change as a direct result of construction
operations. Below GWL the soil is assumed to be fully saturated, but it
may contain small volumes of entrapped air. Above GWL, soil moisture
may be held by capillary forces. The silty soils and clays frequently
employed in embankment fills are generally non-saturated when first com-
pacted, i.e. some pore space is filled with compressible pore air. The fill
will progress to a saturated state, on which much of soil mechanics practice
is predicated, as the seepage front advances through the embankment.
The vertical section through a soil mass generating a vertical total
stress,, and static porewater pressure, uw, on the horizontal plane X–X at
depthzis shown in Fig. 2.3(a). Positive porewater pressure below the
water table decreases interparticle contact pressure and the intergranular,
i.e. effective, stress, , transmitted through the soil particles is less than
the total stress by an amount equivalent to the porewater pressure, i.e.
the effective pressure or stress is given by
uw (2.1)
as illustrated in Fig. 2.3(b).
This, the effective stress relationship, is at the core of much of geo-
technical practice since it is effective stress level which determines the
46 EMBANKMENT DAM ENGINEERING
Fig. 2.3 Porewater pressures and vertical geostatic stresses: static
groundwater case
