Environmental Engineering Reference
In-Depth Information
volume of sliding mass under water
unit weight of water
volume of sliding mass
r
u
×
unit weight of soil
or, since the unit weight of water is approximately equal to onehalf the unit soil weight, it
can be expressed approximately by
cross-sectional area of sliding mass under water
2
r
u
×
total cross-sectional area of sliding mass
Applied Stresses
Applied stresses cause an increase in pore pressures. Loading a clayey soil causes the
process of consolidation (see
Section 3.5.4).
The pore water first carries the load, then, as
the water drains from the soil, pore pressures dissipate, the voids become smaller, and the
load is transferred to the soil skeleton. If the load is applied rapidly, however, the soil has
no time to drain, friction is not mobilized, and the substantially lower undrained shear
strength prevails. Even if drainage occurs, the frictional component of strength will be
reduced by the amount of pore pressure.
The pressure in the pore water is referred to as the neutral stress because it does not con-
tribute either to compression or to an increase in shearing resistance.
Piezometers
These devices, described in
Section 4.4.2,
are installed in the field to measure water tables
and pore pressures. Several conditions are illustrated in
Figure 8.27.
Seepage
Velocity
The average seepage velocity
v
s
of water flowing through the pores of a saturated
soil mass is equal to the discharge velocity (
v
d
k
i) times the ratio (2
e
)/
e
, where
e
FIGURE 8.27
Various possible conditions of pore water pressures as measured by piezometers:
P-1 — Installed in a sand stratum, measures the static water table.
P-2 — Installed in a desiccated, overconsolidated, and still only partially saturated clay, reflects negative pore
pressures (pressures lower than the static water level).
P-3 — Installed in an artesian aquifer, shows excess hydrostatic pressure (pressure higher than the static water
level).
P-4 — Installed in a clay below the depth of desiccation beneath a newly constructed storage tank, shows
excess hydrostatic pressure because the clay is still consolidating under the applied load.
