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A STUDY ON ADVANCES IN GROUND IMPROVEMENT TECHNIQUES
Conference Paper · April 2016
DOI: 10.13140/RG.2.1.4865.4965
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National Taiwan University of Science and Technology
Indian Institute of Science
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A STUDY ON ADVANCES IN GROUND IMPROVEMENT TECHNIQUES
Mohammad Bilal
Department of Civil Engineering, AMU, Aligarh. Email: mdbilal@zhcet.ac.in
Abdullah Talib
Department of Civil Engineering, IIT Delhi, New Delhi.
ABSTRACT
In a construction project, there are numerous foundation problems that are encountered during
the execution phase. Soil in its natural form, at a construction site, is not always suitable to
completely bear heavy structural loads. For such situations, the soil needs to be improved to
enhance its bearing capacity and decrease the expected settlement. There are certain techniques
for ground improvement which are often used to improve sub-soil properties in terms of their
bearing capacity, shear strength, settlement characteristics, drainage, etc. These techniques have
a wide range of applicability from coarse grained soils to fine grained soils. Depending upon the
loading conditions and nature of soil, a suitable technique which is also economical needs to be
adopted. This paper gives the overview and concept of recent major ground improvement
techniques and discusses their practical applications.
Keywords: Ground improvement, soil nailing, vibrofloatation, jet grouting, pre compression
1. INTRODUCTION
Ground improvement, is the modification of soil in foundation so as to provide better efficiency
under design and/or operational loading conditions at the construction site. Ground improvement
changes soil characteristics thereby permitting different types of construction operations. These
characteristics may be shear strength, swelling and shrinkage characteristics and bearing
capacity. There is an increasing use of these techniques in the construction industry where the
soils are having poor subsurface conditions. The ground improvement has been of great concern
since early times. Different technologies started to develop since 17 th century AD. Today, use of
modern methods have made soil improvement relatively easier for the experts in the construction
industry.
In this paper, some of the major recent and conventional technologies are discussed with their
applications in the field and their advantages and disadvantages.
2. METHODS OF GROUND IMPROVEMENT
There are numerous techniques for soil stabilization. These methods mainly depend on the nature
of strata and the purpose of improvement. Techniques for soil stabilization can be broadly
classified as
1.
2.
3.
4.
5.
Soil improvement using additives
Soil improvement using mechanical methods
Soil improvement without using admixtures
Soil improvement using thermal methods
Other methods
3. SOIL IMPROVEMENT USING ADDITIVES
Certain additives such as lime, bitumen, fly ash and cement etc. are added onto the soil at site to
improve its characteristics. These may be classified as following.
3.1 Soil Improvement Using Chemicals
Some of the chemicals like lime, fly ash and cement are used as additives for soil improvement.
3.1.1 Lime Stabilization
This technique came into picture more than half a century ago. Lime can be used to treat soils in
order to improve their workability and load bearing characteristics in a no. of situations.
Quicklime delays the reaction time with soil by about 1.25 times the time taken by slaked lime.
Use of lime as a stabilizer enhances the long-term permanent strength, stability and stiffness
particularly with respect to the action of water and frost especially in fine grained soils and
sometimes in in fine grained fractions of granular soils too. Once the soil has been cured using
lime, important works such as creating embankments or subgrade of structures can be done with
them, hence avoiding the expensive works like excavation and transport. Generally 2-8% of lime
may be required for coarse grained soils and 5 to 8% of lime may be required for plastic soils.
3.1.2 Cement Stabilization
Cement has been one of the oldest binder in the soil stabilization technique. Soil reacts with
cement and the hard mixture obtained from the reaction of pulverized soil, Portland cement and
water is known as soil-cement. This is done at site by using special equipment.
The cementing action is
said to be the result of chemical reactions of cement with siliceous soil during
hydration reaction. Nature of soil content, mixing conditions, compaction, curing and type of
admixtures used are some of the factors that affect the properties of soil cement. This technique
is used in shallow depth stabilization in the case of highways and embankment material and in
the stabilization of weak soils at a greater depth such as soft soils and peaty soils. The recent
developments have taken place mainly in the optimization of tools and process for mass
production.
3.1.3 Fly Ash Stabilization
Fly ash, being a waste product from the thermal power plants is generally used in a variety of
operations. Around 15% of the fly ash is utilized in the manufacturing of bricks and cement and
the remaining is stored as slurries in lagoons. Hence despite having lesser cementitious
properties than in lime and cement, the abundance of fly ash has made it an increasingly popular
alternative during recent years. The fly ash is used potentially as a subgrade stabilizer and in land
reclamation.
3.2 Jet Grouting
This is a costly method for soil stabilization. In this method, External stabilizers are injected into
the soil. Jet grouting is used across wide range of soils. In this technique in situ geometries of
soilcrete (grouted soil) are created, using a grouting monitor attached to the end of a drill stem.
Hydraulic Rotary drill is used to reach the design depth. The jets erode and mix the in situ soil as
the
drill
stem
and
jet
grout
monitor
are
rotated
and
raised.
This method is suitable for stabilizing buried zones of relatively limited extent
and
is
not useful for clayey soils because of their low permeability.
There are three traditional grout systems, namely single, double and triple fluid systems.
In a single-fluid jet grouting system (Figure 1.a.), a high-velocity cement slurry grout is used for
eroding and mixing the soil mass. With this grout, functions such as breaking down of soil
matrix, removal of excess material and mixing with soil are easily fulfilled. This method is most
commonly used for silts and clays.
Whereas in the double-fluid jet grouting system (Figure 1.b.), a two-phase internal fluid system
with a coaxial air-jet supply line around the grout-jet supply which increases the erosion
efficiency. This system is more effective in cohesive soils than the single-fluid system.
In the triple-fluid jet grouting system (Figure 1.c.), water jet is surrounded by an air jet, with a
lower grout jet to inject cement slurry at a lower pressure. To erode the soil structure, coaxial air
and high-velocity water are used with an additional improvement through partial substitution of
the finest soil particles.
(a) Single Jet Grouting system
(b) Double Jet Grouting system
(c) Triple Jet Grouting system
Figure 1: Types of Jet Grouting systems
4. SOIL IMPROVEMENT USING MECHANICAL METHODS
In this method soil is being densified using rollers and vibrators by applying a compressive force
on the given soil. These techniques are further classified below.
4.1 Stone Column
Though this technique was used first in France in 1830s, the wide range of use of this technique
spread especially in Europe since 1950s. In this method, the columns consist of compacted
gravel or crushed stone arranged by a vibrator. Stone column technique decreases the
compressibility of soft and loose fine graded soils leading to increase in strength, accelerates
consolidation effect and reduce the liquefaction potential of soils. Stone columns are more
preferable than sand drains because of their granular nature which provides additional shear
strength to the surrounding soils. They are mainly used for stabilization of soft soils such as soft
clays, silts and silty-sands. The geo-synthetic-reinforced fill and stone column system can
provide an economic and effective solution for structures constructed on clay soil. The use of
geo-synthetic reinforcement transfers the stress from soil to stone columns due to stiffness
difference between the stone columns and soil, and this may prevent large displacement and
reduce the total as well as differential settlement. Stone columns are installed using either top- or
bottom-feed systems, either with or without jetted water. Most widely used methods for
installation of stone columns are: Vibro-Replacement (Wet, Top Feed Method) and VibroDisplacement (Dry, Top and Bottom Feed Method). (Figure 2.)
Figure 2: Commonly used methods of stone column technique
4.2 Vibro Floatation
Vibro-compaction, sometimes referred to as Vibro-floation, is the rearrangement of soil particles
into a denser configuration by the use of powerful depth vibration. Particles of granular soil can
achieve the effective depth of surface compactor and vibratory roller is limited to a few meters
below ground level and the larger depths can be reached by deep compaction methods using
depth vibrators. Combined action of vibration and water saturation by jetting rearranges loose
sand grains into a more compact state. Vibro compaction is performed with specially-designed
vibrating probes. Both horizontal and vertical modes of vibration have been used in the past. The
probe is first inserted into the ground by both jetting and vibration. After the probe reaches the
required depth of compaction, granular material, usually sand, is added from the ground surface
to fill the void space created by the vibrator. A compacted radial zone of granular material is
created. Its applications are reduction of foundation settlements, reduction of risk of liquefaction
due to seismic activity, permit construction on granular fills.
Vibro compaction may be used as a ground improvement technique to support all type of
structures from embankments to chemical plants .Vibro compaction is used to increase the
bearing capacity of foundations and to reduce their settlements. Another application is the
densification of sand to mitigate the liquefaction potential in earthquake prone zones. Vibro
compaction method is not effective for soil having a percent finer more than about 15 to 20 %.
4.3 Micro Piles
Micro piles are deep foundation elements constructed using high-strength, small-diameter steel
casing and/or threaded bar. Micro piles were first used in Italy in the early 1950s for
underpinning of those monuments and historic buildings that were getting damaged with time.
Micro-piles have a small diameter (up to 300 mm), and have a high load bearing capacity (up to
5000 KN in compression). They can be installed through virtually any ground condition,
obstruction and foundation and at any inclination and ensure minimum vibration or other damage
to foundation and subsoil. Micro piles be installed in as little headroom as 6' and close to existing
walls. The advantage of micro piles is that they can resist compressive, tensile or lateral loads, or
even combinations of all the three loads. Micro piles can be designed as soil frictional piles and
rock socketed piles either under tension and compression. Micro piles can be used as a
foundation for new structures or repair / replacement of existing foundations. Soil strengthening,
protection and Arresting / Prevention of movement Embankment are also some of the common
applications of micro piles
4.4 Soil Nailing
Soil nailing is a method of earth retention which uses grouted tension-resisting steel elements
(nails) designed for permanent or temporary support. The fundamental concept of soil nailing
consists of reinforcing the ground by passive inclusions, closely spaced, to create in-situ soil and
restrain its displacements.
Soil nailing is normally used for stabilizing existing slopes or excavations where top to bottom
construction is beneficial as compared to other retaining wall systems. For certain conditions,
soil nailing offers a feasible alternate from the viewpoint of technical viability, construction costs
and duration when compared to ground anchor walls, which is another popular top-to bottom
retaining system. Soil nailing technique has proved well in excavation applications for ground
conditions that require vertical or near-vertical cuts. The other field of application of soil nailing
is in railway and roadway cut excavations (Figure 3), road widening under an existing bridge
end, repair and reconstruction of existing retaining structures, and temporary/permanent
excavations in an urban environment. Excavation retaining structures in urban areas for high-rise
buildings and underground facilities and construction and retrofitting of bridge abutments with
complex boundaries involving wall support under piled foundations are also sometimes done
using this technique.
Figure 3: Soil nailing in railway construction
5
SOIL IMPROVEMENT WITHOUT USING ADMIXTURES
Some methods do not require use of any admixtures for soil stabilization. Some of these
techniques are described below.
5.1 Soil Replacement
Where the soil is soft and of limited depth and thickness, removal of unsuitable material and
replacement with well compacted suitable fill may be carried out. The removal and replacement
required to be carried where the naturally occurring soils were found to be of a low shear
strength and high moisture content. Subsurface drainage may have to be introduced in most of
these areas.
5.2 Vertical Drains
Vertical drains (Figure 4.) are used for speeding up the consolidation and thus increasing the
shear strength and bearing capacity of fine grained soils (or say impervious soils), as they
provide a shorter distance for water to travel through the permeable vertical drains out of soil.
This limits the long term settlement. They are also known as wick drain or band drain. There are
two common types of vertical drains namely, sand drains and prefabricated vertical drains
(PVD).
Figure 4: Cross section of vertical drains
5.2.1 Sand Drains
These are the vertical drains in which holes are drilled using rotary drilling and the hole is filled
with sand (Figure 5.) which is highly permeable. It is a process of radial consolidation which
increase rate of drainage and allows faster consolidation of fine grained soil.
Figure 5: Sand drain with surcharge
5.2.2 Pre-Fabricated Vertical Drains (PVDs)
Prefabricated vertical drains also known as wick drains consist of channeled synthetics core
wrapped in geotextile fabric (Figure 6). They are flexible, durable, and inexpensive and have an
advantage over sand drains is that they don't need drilling. The installation of prefabricated
vertical drains is done by a mandrel and it is a displacement installation. The dimensions of the
prefabricated drains are much smaller compared to sand drains and subsequently are the
dimensions of the mandrel. Thus, the degree of soil disturbance caused by the size of the mandrel
during installations is lower.
Figure 6: Typical mandrel and shape of a prefabricated drain
5.3 Preloading or Pre Compression
Preloading is the process of compressing the soil under applied vertical stress prior to
construction and placement of the final construction load. The two common preloading
techniques are conventional preloading and vacuum induced preloading.
5.3.1 Conventional Preloading
A preload is applied, e. g. by means of an embankment. When the load is placed on the soft soil,
it is initially carried by the pore water and this pore water pressure decreases gradually as the
pore water flows away very slowly in vertical direction (Figure 7). In order not to create any
stability problems, the load must mostly be placed in two or more stages. If the temporary load
exceeds the final construction load, the excess refers to as surcharge load.
Figure 7: Preloading of soil
5.3.2 Vacuum Preloading
When the soil is soft and so weak that even a common 1.5 m embankment can cause stability
problems, then it is suitable to use the method of vacuum preloading. This method is introduced
in 1952 by Kjellman to accelerate consolidation.
In this method the surcharge load is replaced by atmospheric pressure. It consists of a system of
vertical sand drain and a drainage layer (sand) on top and is sealed from atmosphere by an
impervious membrane (Figure 8). Horizontal drains are installed in the drainage layer and
connected to a vacuum pump. Negative pressure is created in the drainage layer by means of the
vacuum pump. The applied negative pressure generates negative pore water pressures, resulting
in an increase in effective stress in the soil, which in turn is leading to an accelerated
consolidation.
Figure 8: Vacuum consolidation system
5. SOIL IMPROVEMENT USING THERMAL METHODS
By applying thermal treatment on soil its strength related properties can be influenced. There are
two methods of thermal treatment; soil heating and soil freezing. These methods seem to be
effective but its use is limited because of its high cost.
5.1 Soil Heating
The increase in temperature of especially fine soil can cause significant increase in its strength,
by reducing electric repulsion between the grains and also flow of pore water takes place due to
thermal gradient and a reduction in moisture content because of increasing evaporation rate.
5.2 Soil Freezing
Lowering the temperature of soil causes its pore water or moisture to freeze down and thus
increase in volume of water and this acts as a cementing agent between the soil particles thereby
increasing the shear strength of soil and decreasing its permeability. The process typically
involves installing double walled pipes in the soil. A coolant is circulated through a closed
circuit. A refrigeration plant is used to maintain the coolant’s temperature. Soil contaminated
with radioactive elements that leaked from Japan's Fukushima Daiichi nuclear power plant was
contained through ground freezing.
6. CONCLUSIONS
Room for improvement in the traditional techniques has become difficult. Also, some methods
such as thermal improvements are costlier whereas methods such as vertical drains require
skilled labor. On the other hand waste disposal has also become a great problem for the
policymakers. Therefore, researchers are now shifting towards utilization of waste products such
as construction demolition wastes, pozzolanic materials, silica fume, etc. in the soil stabilization
processes. Many different researches have been carried out using different waste materials.
Recently, different combinations of waste materials are being tested for soil stability and these
types of waste products can provide better characteristics and are environmental friendly.
7. REFERENCES
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Materials," 2009.
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2010.
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