Civil Engineering Reference
In-Depth Information
Fig. 2.10
Upward flow through a soil sample.
Upward forces
=
γ
h A
w
i.e.
G
−
+
1
s
h A
γ
=
γ
e
Al
w
w
1
or when
h
l
G
−
+
1
s
=
=
i
c
1
e
This particular value of hydraulic gradient is known as the critical hydraulic gradient and has an average
value of about unity for most soils. It makes a material a quicksand, which is not a type of soil but a flow
condition within the soil. Generally quicksand conditions occur in fine sands when the upward flow condi-
tions achieve this state, but there is no theoretical reason why they should not occur in gravels (or any
granular material) provided that the quantity of flow and the head are large enough. Other terms used
to describe this condition are 'piping' or 'boiling', but piping will not occur in fine silts and clays due to
cohesive forces holding the particles together; instead there can be a heave of a large mass of soil if the
upward forces are large enough.
2.11.2 Seepage forces
Whenever water flows through a soil a seepage force is exerted (as in quicksands). In Fig.
2.10
the excess
head h is used up in forcing water through the soil voids over a length of l; this head dissipation is caused
by friction and, because of the energy loss, a drag or force is exerted in the direction of flow.
The upward force h
γ
w
A represents the seepage force, and in the case of uniform flow conditions it can
be assumed to spread uniformly throughout the volume of the soil:
Seepage force
Unit volume of soil
h A
Al
γ
w
=
=
i
γ
w
This means that in an isotropic soil, the seepage force acts in the direction of flow and has a magni-
tude
=
i
γ
w
per unit volume.
