Civil Engineering Reference
In-Depth Information
●
Overconsolidated clay
:
Clay which, during its history, has been subjected to pressures greater than its
existing overburden pressure. One cause of overconsolidation is the erosion of material that once
existed above the clay layer. Boulder clays are overconsolidated, as the many tons of pressure exerted
by the mass of ice above them has been removed.
●
Preconsolidation pressure
:
The maximum value of pressure exerted on an overconsolidated clay before
the pressure was relieved.
●
Overconsolidation ratio
:
The ratio of the value of the effective preconsolidation pressure to the value
of the presently existing effective overburden pressure. A normally consolidated clay has an OCR
=
1.0
whilst an overconsolidated clay has an OCR
>
1.0.
4.12.1 Undrained shear
The shear strength of a soil, if expressed in terms of total stress, corresponds to Coulomb's Law, i.e.
= +
tan
τ
σ
φ
f
u
u
where
c
u
=
unit cohesion of the soil, with respect to total stress
φ
u
=
angle of shearing resistance of soil, with respect to total stress (
=
0)
σ
=
total normal stress on plane of failure.
For saturated cohesive soils tested in undrained shear it is generally found that
τ
f
has a constant value
being independent of the value of the cell pressure
σ
3
(see Fig.
4.30)
. The main exception to this finding
is a fissured clay.
Hence, we can say that
φ
u
=
0 when a saturated cohesive soil is subjected to undrained shear. Hence:
1
2
τ
= =
c
u
(
σ
−
σ
)
1
3
Because of this, the term c
u
is referred to as the
undrained shear strength
of the soil. As will be seen
later, the value of c
u
is used in slope stability analyses when it can be assumed that
φ
u
=
0 and the value
of c
u
can be obtained on site by the simple and economical unconfined compression set.
Fig. 4.30
Strength envelope for a saturated cohesive soil subjected to an undrained shear test.
