Environmental Engineering Reference
In-Depth Information
4
densify by vibro flotation
or stone columns
1
1
(a)
2A
2B
2C
sa
n
d
rock
4
1
1
remove sand
from foundation
(b)
2A
2B
2C
rock
rock
4
add berm
add berm
1
1
(c)
2A
2B
2C
s
a
nd
rock
Figure 12.39.
Principles of ground improvement or treatment to reduce the likelihood of liquefaction in
dam foundations. (a) Densification by vibroflotation or stone columns. (b) Remove poten-
tially liquefiable soils. (c) Densification by vibroflotation or stone columns under a berm for
an existing dam.
dams and for remediation of existing dams. Densification is usually carried out by
vibro-flotation, dynamic consolidation, vibro piles or stone columns. Stone columns
have the added role of draining excess pore pressures developed by the cyclic loading
of the earthquake.
Filters, rockfill and other granular materials in the embankment should be
well compacted if they are likely to become saturated, so they will be dilatent and not
liquefy.
(e) Avoid founding the dam on the strain weakening clay soils, completely weathered
rock or weak rock with the potential to strain weaken. (In many older, smaller dams,
this was not done.) Post earthquake stability can be an issue if the earthquake causes
even relatively minor movements which can take the foundation strength from peak
to residual.
(f)
The foundation under the core should be shaped to avoid sharp changes in profile
across the valley. These are likely to make the core more susceptible to cracking
due to differential settlement under earthquake (and normal) loading. Details of
foundation slope modification are given in Section 17.6. ICOLD (1999) also suggest
that it is desirable to keep the slope in the upstream-downstream direction across the
core horizontal in the upper 30 m of the dam to reduce the likelihood of differential
settlements.
