23-04-2014, 04:55 PM
GROUND IMPROVEMENT TECHNIQUES – AN OVERVIEW
INTRODUCTION
Though the term Ground Improvement has been familiar to Civil Engineers, the
design approach is still empirical, mostly based on past experience. Well defined
design procedure, constructions procedure and codal provisions are yet to be developed.
In the absence of above, execution of the same is difficult and sometimes lead to
contract disputes.
On the other hand, adequacy of Ground Improvement for supporting even large
structures has been proved beyond doubts. Application of Ground Improvement is not
only cheaper but reduces the construction time significantly. In last two decades several
major projects have been successfully built in the country adopting some of the ground
improvement techniques. For each project, field trials and ground monitoring has been
carried out. This has generated large amount of field data and built the confidence of
geotechnical designer for adopting ground improvement techniques.
In this lecture note, aspects such as occurrence of weak deposits, typical
prosperities weak deposits, methods of ground improvement, etc. have been discussed.
In addition, three case studies are described were one of the ground improvement
techniques has been adopted for a large project.
Vertical Drains and Preloading
This method is suitable for deep deposit of soft clay. The natural moisture
content in this stratum can be brought down substantially by installing vertical drains
with a preloading. The presence of vertical drains reduces the drainage path of water in
the pores of soil and thereby reduces the time required for consolidation. The spacing
of drains depends on the speed at which required improvement is to be achieved.
In earlier days, such vertical drains were installed by driving a close ended steel
pipe of 100-200 mm diameter upto the full thickness of such soft clay deposit. The pipe
is then filled with sand and withdrawn in stages to form a vertical sand drain. The pipe
is generally refused for installing other drains.
In the recent past, there has been number of different materials developed to
replace the sand drains. These are basically flexible plastic sections having thickness
varying from 5 to 10 mm and width from 100 to150 mm. The section has channels to
permit flow of water. The perimeter of the section is covered with a layer of geo-textile
to prevent the entry of soil particles into the channel. The advantage of such drain is
that it results in minimum remoulding of surrounding soil during installation. The
process of installation is also very fast. The machine is mounted on a crane and typical
drain upto 10 m depth can be installed in a period varying from 1-2 minutes including
the time for shifting the machine to the new location.
Vacuum Dewatering
In vacuum dewatering technique also the layer is provided with vertical drains
as mentioned above. The surface is then covered with a layer of sand blanket 150-300
mm thick and covered with a thin plastic sheet which is suitably anchored along the
perimeter. Vacuum is then applied within the sand blanket which creates a suction
pressure upto 50-60 mm height of mercury and forces the pore water out from the
deposit through vertical drains. The area is also subjected to atmospheric pressure
which then acts as a preload. The vacuum pressure is maintained till the required
improvement is achieved. The period of vacuum application can be predicted in similar
way as discussed for the vertical drains.
Stone Column
Stone columns are cylindrical columns made below ground level which
comprises of granular material of large size varying from 25 to 100 mm. A hole is
made in the soft deposit by different techniques and then filled with stones in layers and
compacted to form the complete column. When a structure is placed over the area
treated by stone columns, majority of the load (80-90%) is transmitted to the stone
column because of their higher stiffness. Balance 10-20% of the load is taken by clay
deposit. With the help of this 10% of surcharge load, the soft clay is able to provide
adequate confinement to the cylindrical column. The maximum permissible actual
stress on the columns can be predicted from the known theory.
In-situ Deep Mixing
In-situ deep mixing using hydraulically operated helical blade augers penetrates
the ground to the required depth. The hollow stem of the auger is used to inject cement
slurry/lime/any other stabilising compound into the ground. A pair of 2 or 3 augers is
operated simultaneously. After injecting the stabiliser into the ground, the augers are
rotated such that the soil along with the stabiliser is churned in-situ and mixed the
stabiliser thoroughly with the soil without necessity of taking out the soil. The auger
configuration can be chosen such that either a stabilised wall can be formed (to act as
barrier) or the entire area can be stabilised.
Thermal Treatment
In thermal treatment, the moisture from the soft clay is driven out by raising the
temperature of the deposit. This is done by passing hot gasses through the soil.
Inclined bore holes of suitable diameter are drilled along the slope and a fire is created
near the bottom of the holes. The bore holes act as a chimney and carry the hot flues.
This reduces the water content considerably and stabilises the slope. The deposit could
also be heated by electrical energy, but it works out to be more expensive. Thermal
treatment is used more for temporary purpose like stabilisation of excavation slope
during construction.