FIELD OF THE INVENTION
[001] The present invention relates to the field of mitigation of liquefaction
at a site constituting pond ash. The present invention in particular relates to
a process to mitigate the liquefaction of a site constituting pond ash deposit
underlain with alluvium for providing safe foundation system for power
plant and allied structures.
BACKGROUND OF THE INVENTION
[002] In India, the major portion of electricity is being generated from
coal fired thermal power stations. In order to manufacture huge thermal
power stations, huge area of land is a prerequisite. This becomes a major
problem due to scarcity on land availability.
[003] Today vast areas of land in the form of abandoned ash pond are
available but unutilized. So need is to identify and make use of existing
unserviceable/unutilized land/areas. Ash is waste produced in any coal
fired thermal power station and is produced in huge quantities. Ash pond
is an area wherein ash is dumped/disposed.
[004] Generally abandoned ash pond sites are not selected/preferred for
putting up major industrial projects including power stations due to lack
of engineering expertise/experience/data on putting up safe foundation on
such deposits/fills. Also, the fact that the ash pond sites are generally

very unstable and it is not safe to put up major industrial projects such as
power stations etc directly on such fills.
[005] In such a scenario, it is very important and the need of the hour to
develop expertise on determining the stability of the ground under ash
pond and also to mitigate the liquefaction of these ash pond sites for
setting up new power stations/industrial parks.
OBJECTS OF THE INVENTION
[006] It is therefore an object of the invention to develop ground
improvement method to mitigate liquefactionof ash pond site.
[007] A further object of the invention is to establish the liquefaction
potential of the site to put up safe foundation.
SUMMARY OF THE INVENTION
[008] According to the present invention, a process to mitigate the
liquefaction of ash pond site is disclosed. Said process comprises the
steps of: detailed geotechnical investigation of the site followed by
installation of compaction piles using the following steps:
1. Compaction pile is installed by driving a 500mm diameter mild
steel casing pipe with a detachable conical shoe at the bottom using a
drop hammer of 5 MT at the desired locations up to the termination depth
of 12m. The height of fall of the drop hammer is 1 to 1.5 m. During the
process huge vibrations were developed and propagates in the

surrounding area due to which reduction in volume of voids occurred
there by densifying the ground.
2. After driving the mild steel casing pipe with detachable conical
shoe up to the termination depth of 12m, the backfill material of 1 sand: 2
coarse aggregate is filled for 1000 mm depth and withdrawn the casing
pipe for 800 mm from the bottom.
3. After withdrawal of the casing (for 800 mm), the backfill material
inside the hole is thoroughly compacted by means of dynamic
compaction. Dynamic compaction is done with a tamper.
4. Once the first layer (bottom layer) is compacted, the backfill
material is filled for another 1000 mm height and the casing is withdrawn
for another 800mm. The backfill material is compacted by means of
dynamic compaction.
5. The procedure mentioned above is repeated in layers and
compacted till reaching the ground level.
6. Quality tests viz standard penetration test and plate load test were
carried out on the improved ground.
7. The ground improvement (densification) is thus achieved with the
induced vibration during driving of casing with detachable shoe aswell as
with the compaction of fill material.
BRIEF DESCRIPTION OF DRAWNGS
[009] The proposed invention will be better understood by the following
description with reference to the accompanying drawings:

Figure 1 discloses the process according to the present invention.
Figure 2a and 2b shows Standard Penetration Test
Figure 3 shows load settlement plot after ground improvement using 4D
Figure 4 shows load settlement plot after ground improvement using 5D
DETAIL DESCRIPTION OF THE INVENTION
[0010] The present invention is described in detail below.
[0011] A process to mitigate the liquefaction of a site constituting pond
ash deposit underlain with alluvium, according to the present invention
comprising of the following steps:
1. Detailed geotechnical investigation of the site is conducted.
2. The pattern and depth of ground improvement by compaction pile
is designed considering the geotechnical parameters of the site.
3. Based on the pattern, compaction piles are installed in the
following steps:
a. Compaction pile is installed by driving a 500mm diameter mild steel
casing pipe with a detachable conical shoe at the bottom using a drop
hammer of 5 MT at the desired locations up to the termination depth of
12m. The height of fall of the drop hammer is 1 to 1.5 m. During the
process huge vibrations are developed and propagate in the surrounding

area due to which reduction in volume of voids occur there by densifying
the ground.
b. After driving the mild steel casing pipe with detachable conical
shoe up to the termination depth of 12m, the backfill material of 1 sand: 2
coarse aggregate is filled for 1000 mm depth and withdrawn the casing
pipe for 800 mm from the bottom.
c. After withdrawal of the casing (for 800 mm), the backfill material
inside the hole is thoroughly compacted by means of dynamic
compaction. Dynamic compaction was done with a tamper.
d. Once the first layer (bottom layer) is compacted, the backfill
material is filled for another 1000 mm height and the casing is withdrawn
for another 800mm. The backfill material is again compacted by means
of dynamic compaction.
e. The procedure mentioned above is repeated in layers and
compacted till reaching the ground level.
f. Quality tests viz standard penetration test and plate load test are
carried out on the improved ground.
g. The ground improvement (densification) is thus achieved with the
induced vibration during driving of casing with detachable shoe as well
as with the compaction of fill material.

[0012] Compaction piles are used to densify the ground i) in order to
mitigate the liquefaction of the in-situ soil/pond ash deposit and ii) in
order to increase thebearing capacity of liquefiable soil to 15 t/sqm.
[0013] Initially the design (pattern/configuration/depth) of compaction
piles is done just to mitigate the liquefaction of site. Accordingly the
centre to centre spacing of 500mm diameter compaction piles worked out
to be 6 times the diameter of compactionpiles (6D) i.e. 3000mm. Further,
while carrying out the test patches at site thepattern/configuration was
tried at 5D as well as with 4D so as to get bearing capacityof 15 t/sqm.
[0014] 6D/5D configuration was developed for the structure where in pile
foundation wereprovided. This is to mitigate the liquefaction of site.
[0015] 4D/5D configuration was developed for the structure where in
shallow foundation is provided. This is to increase the bearing capacity as
15 t/sqm after mitigationof liquefaction.
[0016] The invention has been described in an illustrative manner, and it
is to be understood that the terminology that has been used is intended to
be in the nature of words of description rather than of limitation.
Obviously, many modifications and variations of the present invention
are possible in light of the above teachings. It is therefore to be
understood that within the specification, the reference numerals are
merely for convenience, and are not to be in any way limiting, the
invention may be practiced otherwise than is specifically described.

WE CLAIM:
1. A process to mitigate the liquefaction of a site constituting pond
ash deposit underlain with alluvium,according to the present
invention comprising of:
Geotechnical investigation of the said site to determine
geotechnical parameters;
Installing compaction piles by driving a 500mm diameter mild
steel casing pipe with a detachable conical shoe at the bottom
usinga drop hammer of 5 MT at the desired locations up to the
terminationdepth of 12 M;
a) a backfill material of sand and coarse aggregate is filled up to
1000 mm from the bottom in said mild steel casing pipe and
said mild steel casing pipe is partially withdrawn till 800 mm
from the bottom;
b) backfill material is compacted using dynamic compaction with
the help of a tamper through a hole in said mild steel casing
pipe; and
steps (a) and (b) are repeated till the ground level is reached.
2. The process to mitigate the liquefaction of a site constituting pond
ash deposit underlain with alluviumas claimed in claim 1, wherein
the height of fall of the drop hammer is between 1 to 1.5 m.

3. The process to mitigate the liquefaction of a site constituting pond
ash deposit underlain with alluvium as claimed in claim 1, wherein
the ratio of sand and coarse aggregate in backfill material is 1:2.
4. The process to mitigate the liquefaction of a site constituting pond
ash deposit underlain with alluvium as claimed in claim 1, wherein
the bearing capacity of densified pond ash deposit is in range 13-
16 t/sqm and preferably 15 t/sqm.
5. The process to mitigate the liquefaction of a site constituting pond
ash deposit underlain with alluvium as claimed in claim 1, wherein
6 test patches of with different configuration viz 4D and 5D
spacing of compaction piles having diameter of 500 mm each, are
used to mitigate the liquefaction of the soil.
6. The process to mitigate the liquefaction of a site constituting pond
ash deposit underlain with alluvium as claimed in claim 1, wherein
3 sets of compaction piles having diameter of 500 mm each, 4D
spacing are used and tested to increase the bearing capacity of
liquefiable soil after mitigation of liquefaction.