Civil Engineering Reference
In-Depth Information
of the soils tested, and Terzaghi therefore used the term
neutral stress
to describe u, instead of the now
more popular term pore water pressure.
Terzaghi concluded that only part of an applied stress system controls measurable changes in soil
behaviour and this is the balance between the applied stresses and the neutral stress. He called these
balancing stresses the effective stresses. He further explained that if a saturated soil fails by shear, the
normal stress on the plane of failure,
σ
, also consists of the neutral stress, u, and an effective stress,
σ
′
which led to the equation known to all soils engineers:
effective stress
=
total stress pore pressure
−
′
= −
σ
σ
u
where the prime represents 'effective stress'.
This equation is applicable to all saturated soils.
Example 3.1:
Total and effective stress
A 3 m layer of sand, of saturated unit weight 18 kN/m
3
, overlies a 4 m layer of clay, of
saturated unit weight 20 kN/m
3
. If the groundwater level occurs within the sand at 2 m
below the ground surface, determine the total and effective vertical stresses acting at
the centre of the clay layer. The sand above groundwater level may be assumed to be
saturated.
Solution:
For this sort of problem it is usually best to draw a diagram to represent the soil condi-
tions (see Fig.
3.6)
.
Total vertical stress at centre of clay
=
Total weight of so
il above
σ
v
=
2
m saturated clay
+
3
m saturated sand
= × + × =
2 20
3 18
94 kPa
Effective stress
=
Total stress Water pressure
−
′
= −
σ
v
94 9 81 2 1
.
(
+ =
)
64 6
.
kPa
Fig. 3.6
Example
3.1.
