Civil Engineering Reference
In-Depth Information
Drainage blanket
: after drains are installed a blanket of gravel and sand from 0.33 to 1.0 m thick, is
spread over the entire area to provide lateral drainage at the base of the fill.
Overfill or surcharge
: often used in conjunction with sand drains. It consists of extra fill material placed
above the permanent fill to accelerate consolidation. Once piezometer measurements indicate that
consolidation has become slow this surcharge is removed.
Strain effects
: although there is lateral drainage, lateral strain effects are assumed to be negligible. Hence
the consolidation of a soil layer in which sand drains are placed is still obtained from the expression:
ρ
c
=
m dp H
2
Consolidation theory
The three-dimensional consolidation equation is:
2
2
∂
∂
u
t
∂
∂
1
∂
∂
u
∂
∂
u
u
c
+
c
=
+
h
v
2
2
r
r
z
r
where c
h
=
coefficient of consolidation for horizontal drainage (when it can be measured: otherwise
use c
v
).
The various co-ordinate directions of the equation are shown in Fig.
12.20.
The equation can be solved
by finite differences.
Equivalent radius
The effect of each sand drain extends to the end of its equivalent radius, which differs for square and
triangular arrangements (see Fig.
12.19)
.
For a square system
:
Area of square enclosed by grid
=
a
2
Area of equivalent circle of radius R
=
a
2
2
2
i i.e. R
. .
π
=
a or R
=
0 564
.
a
.
For a triangular system
:
A hexagon is formed by bisecting the various grid lines joining adjacent drains (Fig.
12.21)
. A typical
hexagon is shown in the figure from which it is seen that the base of triangle ABC, i.e. the line AB,
=
a/2.
Now
a
a
AC AB
tan
CBA
tan
30
=
∠
=
° =
2
2 3
Fig. 12.20
Coordinate directions.
