Environmental Engineering Reference
In-Depth Information
Normally to slightly overconsolidated clays have no frictional component (
φ
0) and the strength is expressed as the undrained strength
s
u
, as will be discussed
later.
●
Applicable Field Conditions
Initial phases of slope excavations or retaining wall construction in slowly drain-
ing soils, where time is too short to permit drainage. (Results are conservative if
drainage occurs.)
●
Sudden drawdown conditions in the upstream slope of a dam embankment, or
when flood waters recede rapidly from stream banks and the soils remain satu-
rated.
●
Rapid placement of embankments or loading of storage tanks over soft clays, or
high live loads applied to slow-draining soils.
●
Effective Stresses
Saturated Soils
If the soil is permitted to drain as load is applied, the stress is initially carried partly by the
pore water, but as drainage occurs the load is transferred to the soil grains, and the effec-
tive stresses (intergranular stresses) are mobilized. The effective stress is equal to the total
normal stress
σ
n
minus the pore-water pressure
u
, and the peak “drained” shear strength
is expressed as
s
(
σ
n
u
)tan
φ
σ
n
tan
φ
(3.25)
As shown in
Figures 3.26
and
3.27,
values based on effective stresses are often higher
than those based on total stresses as long as
u
is positive. In dense sands,
u
is negative and
the undrained strength is higher, i.e.,
φ
is higher. If pore pressures are measured during
undrained
loadings, effective stresses can be computed.
For soils with a cohesion intercept (
Figure 3.28)
,
the drained strength is expressed by the
Coulomb-Terzaghi equation as
φ
s
c
(
σ
n
u
)tan
φ
c
σ
n
tan
φ
(3.26)
Applicable Field Conditions
Drained strength prevails in the field under relatively slow loading conditions during
which pore pressures can dissipate. Such conditions generally exist in:
Most foundations, except for cases involving rapid load application.
●
Natural slopes, except for the sudden drawdown case.
●
Cut slopes, embankments, and retaining structures some time after construction
completion.
●
Partially Saturated Soils
In partially saturated soils, strength is controlled by effective stresses, but the effective
stress concept cannot be applied directly because of pressures in the air or gas in the par-
tially saturated voids. Strength should be estimated from tests performed to duplicate
in
situ
conditions as closely as possible in terms of percent saturation, total stress, and pres-
sure on the liquid phase (Lambe and Whitman, 1969).
