2. GROUNDIMPROVEMENT
Defined as the
“the process of enhancing the quality of soil.”
Ground improvement mainly refers to the
improvement of soil layers
The ground improvement techniques applied
are tools used by the geotechnical engineer for
"fixing” the problems of poor ground.
3. When a poor ground existed at the project site,
for instance, the early builder was faced with
the following questions:
replaced with a more suitable material?
Should the weak ground be bypassed laterally
or vertically ?
Should the design be changed to reflect the
ground’s limitation?
4. With the development of ground improvement,
the new questions facing the current builders
are:
Should the problematic ground at the project
site be fixed instead of bypassed?
What are the critical issues that influence the
successful application of a specific fixing tool?
Which fixing tool should be used for improving
the soil conditions?
5. Why do we choose “STONE COLUMN” out of so
many “Ground Improvement Techniques”
available?
They act as vertical drains.
They also mitigate the potential for Liquefaction.
They have negligible demerits which can be easily
overcome by suitable techniques.
6. Significance Of Stone-Columns in INDIA
Method of installing stone column (RAMMING)
does not require any skilled labour-any layman can
do the job.
Its installation is economically very feasible-no
high cost is required to do the execution.
India has significant deposits of soft clays.
7. What are basically stone columns??
STONE COLUMNS are vertical columns of
compacted aggregate are formed through the
soils to be improved These columns result in
considerable vertical load carrying capacity and
improved shear resistance in the soil mass.
8. What Soils are Suitable for installation of
STONE COLUMNS?
Any soil type that does not respond to vibration
alone is a candidate for stone columns. These
soils include silty and clayey sands, silts, clays,
and some layered soils where damping of
vibrations occurs.
9.
10. Fines content more than 12 % and/or clay
content of more than 2 % cannot be compacted
by Vibro Compaction.
Zone C silty sand with 12 % to 20 % fines is
treated with Stone Columns.
Top Feed Stone Columns were not “invented”
but rather evolved from the Vibro Compaction
process.
Zone D contains the silts and clays-stone
11. Areas of Application of Stone Columns
To improve the stability of embankments and natural
slopes
To increase the bearing capacity of a site to make it
possible to use shallow foundation on the soil
Reduction of total and differential settlements.
Reduction of liquefaction potential of cohesionless
soil.
To increase the time rate of settlement.
Permits constructions on fills.
12. Benefits of Vibroflotation
Stone Columns are a technical and potentially economical
alternatives to deep foundation
Stone Columns are more economical than the removal and
replacement of deep poor bearing soil on a large site
Very useful where infrastructure does not permit high vibration
technique such as dynamic compaction, deep blasting or piling
Where time is critical to project start-up site improvement can
be achieved quicker by vibroflotation than by preloading the
soils.
Stone Columns provide a vertical drainage path for excess
pore water pressure dissipation
With Vibroflotation differential settlements are often in the order
of 10%to 15% of total settlement
13. Installation methods of stone columns
VIBRO-COMPACTION METHOD.
Wet, Top Feed Method (Replacement and
Displacement)
Dry, Bottom Feed Method (Displacement)
RAMMING (INDEGENEOUS METHOD).
14. Vibro-replacement
This refers to the wet, top
feed process in which water
is used to aid the
penetration of the poker
vibrator into the ground.
Part of the in-situ soil is
washed away due to the
jetting action of the water
thereby creating a long hole.
The soil that is washed away
is replaced with compacted
15. Vibro-displacement (Top Fed)
This refers to the dry,
top or bottom feed
process
No water is used.
As vibrating Poker is
inserted, in-situ soil is
displaced and laterally
compacted.
The drilled hole is filled
with compacted gravel.
17. Relative advantages and
disadvantages
VIBROREPLACEMEN
T
Faster and usually less
costly.
Achieves better results
in silty sands above
ground water.
Furnishes better
VIBRODISPLACEMEN
T
Reduces the effort of
sludge handling with
clayey soils.
Ideal for contaminated
soils where the
contaminants shall
remain at depth.
18. RAMMING :
Ramming is an indigenous
method. It can described as
the
“METHOD IN WHICH
DENSIFICATION OF SOIL
MASS IS ACHIEVED
MANUALLY WITHOUT ANY
SKILLED LABOUR”
19. SIGNIFICANCE OF RAMMING
No skilled labour required.
No big initial investment required- it’s highly
feasible economically.
Easy to follow the steps, no qualification
required.
Lastly, this method got its birth in INDIA, so can
be indigenously followed.
20. DESIGN OF STONE COLUMNS
Necessary Information:
Soil investigation data: Bore logs in-situ tests
results Nature of soil, soil profiles etc.,
Ground water level and its condition.
Layout of the structure: Foundation system,
loading pattern and intensity as determined by
structural analysis.
Sufficient information of structures existing
21. IMPORTANT FEATURES OF STONE COLUMN TREATMENT
Influence of soil type: Suitable for loose sandy soils
including silty or clayey sands.(7 to 50kpa). Not
suitable for sensitive clays & silts. (sensitivity>4).
Influence of construction Methodology.
Treatment depth.
Area of treatment.
Termination.
24. Equivalent Diameter
The equivalent circle has
an effective diameter (De)
which is given by the
equation:
De= 1.05 S for an
equivalent triangular
pattern.
=1.13 S for a square
pattern
Replacement Ratio (as)
as=As/A=As/(As+Ag)
Where
As= Area of stone
column.
Ag=Area of ground
surrounding the
column.
A=Total area within the
cell.
25. Failure Modes of Stone Columns
Four Basic Failure Modes of Stone Columns are:
General shear failure.
Local shear failure.
Bulging failure.
Failure by sliding.
26. The modes of failure of Stone Columns depend
upon
the following parameters:
Type of Stone Column (End-bearing or Free
Floating).
Type of Loading on columns.
Passive resistance of tributary clay.
27.
28.
29.
30. DESIGN CONCIDERATIONS
GENERAL
ADJASCENT STRUCTURE
ULTIMATE LOAD CARRING
CAPACITY
ENVIRONMENTAL FACTORS
LOAD TEST RESULTS
FACTOR OF SAFETY
