Civil Engineering Reference
In-Depth Information
Fig. 12.22
Radial consolidation rates (after Barron,
1948)
.
(iii) Determine resultant percentage consolidation, U, from:
1
100
100
U
=
100
−
(
−
U
)(
100
−
U
)
z
r
Smear effects
The curves in Fig.
12.22
are for idealised drains, perfectly installed, clean and working correctly. Wells are
often installed by driving cased holes and then backfilling as the casing is withdrawn, a procedure that
causes distortion and remoulding in the adjacent soil. In varved clays (clays with sandwich type layers of
silt and sand within them) the finer and more impervious layers are dragged down and smear over the
more pervious layers to create a zone of reduced permeability around the perimeter of the drain. This
smeared zone reduces the rate of consolidation, and
in situ
measurements to check on the estimated
settlement rate are necessary on all but the smallest of jobs.
Effectiveness of sand drains
Sand drains are particularly suitable for soft clays but have little effect on soils with small primary but large
secondary effects, such as peat.
Example 12.7:
Sand drain system
A soft clay layer, m
v
=
2.5
×
10
−
4
m
2
/kN; c
v
=
0.187 m
2
/month, is 9.2 m thick and overlies
impervious shale. An embankment, to be constructed in six months, will subject the
centre of the layer to a pressure increase of 100 kPa. It is expected that a roadway will
be placed on top of the embankment one year after the start of construction and
maximum allowable settlement after this is to be 25 mm.
Determine a suitable sand drain system to achieve the requirements.
