Geoscience Reference
In-Depth Information
application 13.4
A soft clay site is to be prepared for development by applying a surcharge sand
fill of 2m thick. Thickness soft clay is 6m. Groundwater level: 1m below surface.
The wet unit weight soft clay is 14 kN/m
3
, the dry unit weight sand: 16 kN/m
3
, and
porosity is 40%. Determine the required vacuum to be applied to prefabricated
vertical drains to obtain 80% consolidation in 4 months. With what means could
such a vacuum be applied? Drain distance 2
R
= 1.4 m; drain diameter 2
r
0
= 0.07 m.
Coefficient of consolidation
c
v
= 2x10
-7
m
2
/sec.
application 13.5
Prefabricated vertical drains with vacuum are applied to drain a site with a very
low
c
v
. Determine the required vacuum to be applied to the vertical drains to obtain
80% consolidation in 120 days. Surface load is 32 kPa. Coefficient of
consolidation:
c
v
= 2x10
-8
m
2
/s. Drain distance 2
R
= 1.4 m. Drain diameter 2
r
0
=
0.07 m
application 13.6
Fig 13.8 shows settlement stages during preloading. Given
C
c
/
(1+
e
0
) = 0.5,
C
/
(1+
e
0
) = 0.025,
t
0
= 180 days,
'
II
= 120 kPa,
'
III
= 80 kPa, the most optimistic
estimate of the period of grace is given. This is too optimistic, as explained in the
notes. Find a more realistic estimate by assuming the unloading is only effective
for 50%.
application 13.7
A fill, which imposes a load of 50 kN/m
2
, compresses a clay layer of 5 m thick.
Vertical drains are installed to accelerate the consolidation process. The drain
width is 0.1 m, drain thickness is 0.01 m.
Determine an equivalent drain radius
r
0
, based on surface similarity.
Calculate the degree of consolidation
U
after 6 month, given
c
v
= 2x10
-7
m
2
/s,
drain spacing 2
R
= 2 m (see Fig 12.6).
Calculate the time required to obtain the same degree of consolidation if vertical
drains are not used, and only vertical consolidation occurs to the ground surface.
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