Engineering Of Ground Modification CE-505
Assignment 1
1. Need and objectives of soil modification?
Soil modification: the process of adding and mixing some chemical agents in to the soil to not
only dry the soil, improving workability, but also reducing the plasticity index and shrink swell
potential.
These improvements often result in a slight increase in the shear strength of the soil.
The soil in the ground usually comes in a state which is harder for construction purposes.
Sometimes the problem is found before the construction of an embankment and sometimes later
which results in the instability of the structure. Therefore to avoid such issues, certain techniques
are used to modify or improve the soil underneath.
Need of soil modification:
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For construction in treating soft soils, collapsible soils etc.
Changes soil’s mechanical properties for better use.
It refine the bearing capacity of soil.
To use a soil again, it must be modified.
To strengthen the subgrade and reduce plasticity.
To reduce the amount of weather-related delays.
To get soil resistant to swelling and shrinkage cycles.
Objective:
 Increasing stiffness, durability, strength and stability.
 Mitigating or Reducing undesirable properties such as shrinking or swelling, compressibility
 Modifying permeability
 Attaining or improving efficiency and productivity by using techniques that save time and
expenses.
2. Vibrocompaction method?
Vibrocompaction is a technique which is used to compact granular soils and rearranges the soil
particulars into a denser state. Depth vibrators are used for compaction which are typically
suspended from a crane or mounted on piling equipment. Vibrator along with water jetting
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helps in decreasing the inner-granular forces between the soil particles and helps in increasing
the soil density.
Natural or manmade deposits of sand and gravel are frequently not dense enough to allow a
proposed structure to be reliably and safely founded. Therefore under such circumstances depth
vibrators increases the soil density and homogenizes it independently from the groundwater
table.
The four phases of Vibrocompaction treatment:
1. PENETRATION: The PTC Vibrolance penetrates through the ground because of vibrations
released by vibrator and because of Vibrolance own weight.
Generally, this penetration is performed with water jetting system at the end of the
Vibrolance. Water is used in the process because it decreases the interstitial pressure
between the soil particles and therefore, reduces the friction between them.
2. COMPACTION: Vibrolance at depth performs compactions in intervals. During
compaction, the vibration occurred by vibrolance makes the soil particles to rearrange
and form a denser compacted state.
If the depth of the penetration is more than 20m use water jetting. When the penetration
depth crosses 20m, Side water jetting may be used to ease the extraction of the
Vibrolance.
3. BACKFILLING: The densification of the soil moves the surface level down and and the soil
forms a hole at the top of vibrolance insertion point. This hole is then backfilled by using
in-situ material.
Both the compaction and the backfilling process are performed again and again till the
Vibrolance reaches the surface and is removed from the ground.
4. FINISHING: After the filling, Vibrolance reaches the surface and the compacted
ground makes a cylindrical surface. This phase take place after the soil reaches the
surface.
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Benefits of using this method:
 Fast, economical and reliable.
 No extra time required for earthworks or deep foundations.
 Especially used for economical design of foundations and floor slabs
Application:
 For Off-shore compaction of sea bed
 For Deification of embankment zones and excavation bases to reduce permeability
 In Liquefaction mitigation
3. Preloading by vertical drains?
Preloading: It means compressing the soil by applying vertical stresses. They are normally done
before the construction or before placing the final construction load.
Vertical drains: This method is generally used to reduce the path of the excess pore water and
to quicken the consolidation process.
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The purpose of using preloading with vertical drain is because it increases the shear strength of
the soil and reduces its compressibility and permeability before construction. It also helps in
averting settlements and damages in the structures.
Sometimes, using Preloading technique alone may not work because of the thick clay layer or
its low permeability. As a result, the time required for pre-compression is very long and not
practical. To deal with this type of problem, vertical drains are installed together with preloading
either by an embankment or by means of vacuum pressure.
During the consolidation of the clay, the pore water gets squeezed out due to the hydraulic
gradients created by the preloading. This water can flow with higher speed in a horizontal
direction towards the vertical drains which is helpful as most clay deposits exhibit a higher
horizontal permeability compared to the vertical. These pore water can also flow freely along the
vertical drains vertically towards the permeable layers. Therefore, the vertical drain installation
accelerates the consolidation process, reduces the length of the drainage path and allows the
clay to gain a rapid increase in strength, to carry the new load by its own.
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4. Methods of grouting and applications?
Grouting : Injecting pumpable materials into a soil or rock formation to change its physical
characteristics. The injected material (generally composed of water, cement, sand, often color
tint and sometimes fine gravel) is referred to as the grout.
Methods of Grouting:
1. PERMEATION GROUNTING:
It is the filling of the voids or cracks within a soil or rock mass and using water and air
from the voids and replacing it with grout, without disturbing the soil particles or
widening the existing fissures in the rock. It includes: Injecting the soil with thin grouts.
 After the soil cures, it becomes a solid mass.
 Usually the process is completed using chemical grouts.
 Used for making groundwater barriers.
APPLICATIONS:
 It is used for the stoppage of seepage through joints of underground structures
such as tunnel lining, basement wall, etc.
 It is mainly used for making vertical seepage barriers beneath hydraulic structures.
 It is used for the stabilization of soil around shafts and tunnels.
2. DISPLACEMENT-COMPACTION GROUTING: In this method, a thick grout is injected into a
homogeneous mass without penetrating into the soil voids. When the grout mass starts
expanding, it displaces the surrounding soil and densifies it.
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When low- slump compaction is injected into granular soils, grout bulbs are formed that
displace and densify the surrounding loose soils.
APPLICATIONS:
 It is used to repair excessive settlement structures.
 It is used to improve soil bearing capacity.
 It is used to control structural leveling.
 It is used to densify soil following the passage of a tunneling machine and to
strengthen the foundation of soil against sinkhole formations.
3. DISPLACEENT-SOIL FRACTURE GROUTING: In this technique a cement slurry of soil and
water is injected with high pressure to fracture the soil and form root like grout material
in the soil mass. Densification of the soil mass is caused as the grout material spreads all
around the grout hole, along with an increase in its macroscopic strength.
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APPLICATIONS:
 It is used to level structures again.
 It is used to stabilize overlying structures during tunneling.
 It is used to fix the shape of a tilted building.
4. JET OR REPLACEMENT-DISPLACEMENT GROUTING: In this method, a grout with high
speed water jets is injected to erode and excavate the soil. As the monitor is drawn out,
strong, impervious columns are formed by mixing grout with the remaining soil.
APPLICATIONS:
 It is used for Grouted columns
 It provides excavation support and seals the bottom of the planned excavation.
 It is used for Underpin foundations.
 Used as jet grout raft to reinforce cofferdam to reduce its deflection and thus the
settlement caused by the excavation works.
5. ROCK FISSURE GROUTING: This technique uses a hole drilled through the fissures and
joints of a rock mass to allow grout to be injected at close centers vertically and reinjecting if required. In rock grouting, the grout is injected under pressure through the
grout hole drilled into the rock mass to be treated.
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APPLICATIONS:
 It is used to seal the rock mass present underneath and at ends of dams, to prevent
seepage or leaking of the reservoirs.
 It is used to prevent water seepage into the excavated tunnel by sealing rock mass
above and underneath a rock tunnel.
 It is used to cement the fractured rock mass.
 Mainly used in the field of water stops, especially in the tunnel excavation project.
6. TUBE-A-MANCHETTES(TAM) GROUTING: In this method, a grout containing sleeped
perforated pipe is used so that it can be injected at close centres. It is applicable for
grouting in soil formation only, the pores in between the soil particles are filled with
grout under pressure with a partial or complete displacement of in-filling groundwater.
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APPLICATIONS:
 It is used to seal soil mass above and underneath a tunnel, excavated in soil under
compressed air condition.
 It is also used to seal soil mass behind the soldier pile wall, pipe pile wall etc.
 Required for sealing “windows” in cofferdams.
 Used for sealing underlying dams soil.
 Used for consolidation of the loose soil mass.
5. Note on Atleast 3 admixtures used for ground improvement
The use of additives and admixtures, such as lime, cement, bitumen and chemicals, is the oldest
and most widespread methods of soil modification. The main objective of using such a mixture is
to strengthen a locally available soil fill to construct a low-cost road base.
1. Bituminous Stabilization: When bitumen is mixed with soil, it imparts binding property as well
as helps it to become water-resistant. Even in the presence of water, this property helps the
soil to impart and regain its strength. In the case of fine-grained soils, bituminous materials
seal the voids between fine soils and keep it away from coming in direct contact with water,
and thus inherent properties of the soil are retained. In the case of sand and gravel, individual
particles get coated with a very thin film of bituminous material, and thus the binding
property is gained by the soil. Water absorption increases in the first stage at very low
bitumen content but then starts decreasing. The maximum dry density of the compacted layer
is decreased.
2. Lime Stabilization: When stabilization of soil is achieved by mixing lime in proper proportion,
the process is known as soil lime stabilization. Lime is generally used for short term
modification of soil properties. It can modify almost all fine-grained soils but the most
affected change occurs in clay soils of moderate to high plasticity. Modification occurs when
calcium captions provided by the hydrated lime replaces the surface captions of the clay
mineral. Soil stabilization occurs when lime is added to a reactive soil that uses a pozzolanic
reaction to generate long term strength. The way of getting pozzolanic reactivity and
stabilization is by having a reactive soil, a good mix design, and good construction practices.
Benefits of soil-lime stabilization include:
 Over time the mixture continue gaining strength, even after environmental or load damage
 Due to its addition, improvements in shear strength is also observed
 For the long term, it provides performance benefits that reduce maintenance costs.
 It is not difficult to work upon. After proper mix design and testing is completed, in-place
mixing is used to add the appropriate amount of lime to the soil, mixed to an appropriate
depth
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3. Cement Stabilization: Soil-cement is a mixture of pulverized soil and measured amount of
cement and water, which are compacted to the desired density and healed. Cement is used to
improve the engineering properties of available soil such as strength compressibility,
permeability, swelling potential, frost susceptibility, and sensitivity to changes in moisture
content. Depending on the type of soil and amount of cement used, soil-cement materials
range from semi-flexible to semi-rigid. The mixture approaches a rigid behavior when granular
soils are used and the concentration of cement is increased. In rigid pavements contraction,
expansion, construction, and longitudinal joints are used. For low-maintenance costs and
smooth pace, reduction or elimination of the joints is useful. The thermal properties of soils
such as specific heat, thermal resistivity and thermal diffusivity are lower as compared to that
of concrete. The thermal properties of soil-cement particularly for fine-grain soils are
generally expected to be lower than that of concrete. Thus results in lower warping and lesser
interior stresses in pavements of soil-cement. Reinforced may be used to improve the load
carrying capacity of seal.
6. Soil reinforcement
Soil reinforcement is a technique which uses geo-engineering methods to improve the stiffness
and strength of the soil. In the past, natural fibers were used to reinforce the soil but the
technique did not have a high yield and required a lot of time for the soil to recover. In
geotechnical engineering, soil is restored and reinforced with the distribution of minerals and soil
nutrients. Soil reinforcement is necessary in lands with high erosion. It is particularly useful in
areas with soft soil as it fails to provide support to the construction of the building. This type of
soil is also highly susceptible to various environmental and natural factors such as high
compressibility, poor shear strength, temperature changes, etc.
How it’s performed:
Soil reinforcement is initiated by placing tensile elements in the soil which magnifies its natural
stability and strength. This is made possible by bringing reinforcement elements in contact with
surfaces in the aggregate and sub-base of soil mass. When pressure on the soil mass causes a
strain on the reinforcements, it generates a tensile load which can counter soil movement and
supplies increased strength for additional support. This way, a soil-reinforcement system is
designed to provide shear strength higher than the soil mass alone.
Principle of Reinforced Soil:
An introduced material creates a tensile restraining force that helps in decreasing the lateral
stress which is necessary for maintaining the equilibrium of a loaded soil.
Under vertical stress, it undergoes lateral deformation when the soil element is compressed.
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The soil element will be restrained against lateral deformation when the reinforcement is added
to the soil in the form of horizontal layers, as it is acted by a lateral force. It is important to
know that the tensile force only occurs when there is a lateral strain in the soil.
Advantages of reinforced soil:
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With this technique, construction on soft ground can be achieved
The need for smaller quantities of earth fill
Steeper embankment slopes reduce the land take required, therefore reinforced soil
technique is used to increase its strength
Since it’s a part of the placing and compaction of the earth fill process, this technique helps
structures to be built more quickly than using conventional methods.
Limitation of reinforced soil:
Reinforced soil relies upon deformation for its use as the tensile resistance only develops when
the soil strain is transferred to the reinforcement. There are some issues regarding the
durability and long time performance of the reinforcing material as severe and rapid corrosion
of steel reinforcement is possible.
Polymeric materials degrade when left under ultra-violet rays and can be damaged by rough
handling or by sharp stones infill. When constructing soft soils care has to be taken to ensure not
to overstress the reinforcement.
7. Ground anchors and crib walls
GROUND ANCHORS
Ground anchors are used for the stabilization of steep slopes or soft soil slopes, and also for the
improvement of embankment or foundation soil capacity. They prevent excessive erosion and
landslides. It consists of cables or rods connected to a bearing plate. Often the use of steel
ground anchors is restricted because of the overall durability during placement (due to weight),
and the difficulty in maintaining tension levels in the anchor.
Composite ground anchors have three parts:
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1. Firstly, the anchorage which is basically a stainless steel sheath with an anchor nut/plate
through which the composite cable is run. The anchorage is usually poured with a non-shrink
expansive cement mortar that ensures fixity and no slippage. It is also used to fasten the
system to the outside structure.
2. The cable can have multiple rods, that can be separate or braided together, or a single rod.
3. A sheath or sleeve made from polyethylene or PVC that is contoured around the free anchor
length of the cables.
Types of Ground Anchors:
 This anchor type is not suitable for soils but is generally used for rocks, where the rock
strata are stable. These are often called Rock Anchors.
In this type a percussive drill rig is used while if collapsible soils overlying rock are
encountered, a rotary percussive rig is normally used.
 This type of anchor is useful for both cohesive and cohesionless soils.
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In this type rotary or rotary percussive rig can be used to drill anchor hole. Bit on hollow
rods working within an outer casing is widely used.
 This type of anchor is applied with clay strata.
In this type, the load carrying capacity of the anchor depends on the strength of the clay
available at the anchor/clay interface.
Application of ground anchors:
 Even with poor ground conditions, high load can be obtained
 Driven anchors can be used for soil conditions
 Permanent anchors are used in ports and harbor developments, road schemes, dam
refurbishments and for the tensile support
 It is generally used to ensure ‘active’ forces in the structures
Crib Walls
Crib walls are usually constructed by interlocking precast units (parallel to and at right angles to
the line of the wall) that are built to form a series of hollow boxes. The boxes are filled with
selected granular material to form a retaining wall. The individual units are generally small which
makes it easier to be installed manually on site.
Walls can be readily curved in plan and can be of varying height as the maximum length of a unit
is in the region of 1 m.
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Crib walls are commonly used for residential purposes such as stabilizing building platforms and
driveway access. They can be of any shape such as straight, curved or angled if required.
Installing such walls is an easy process because they can be made even n a complex curvature
and can be planted with climbing vines. Dying the concrete in a brown or tan earthen color will
also help to improve the final result. Its failure can be caused by differential settlement.
Advantages of using Crib walls:
 Easy to perform the construction to any form such as gentle curves, slopes and terrains.
 It is a Low cost construction.
 Crib walls are really malleable.
8. Functions and applications of Geosynthetics
There are six main functions for Geosynthetics:
 Barrier
The geosynthetic performs as a relatively impermeable barrier to fluids or gases. For
example, geomembranes, thin film geotextile composites, geosynthetic clay liners
(GCLs) and field-coated geotextiles are used as fluid barriers to disrupt flow of liquid or
gas. This function is also practiced in asphalt pavement overlays, encapsulation of
swelling soils and waste containment.
 Drainage
The geosynthetic acts as a drain to transfer fluid flows through less permeable soils. For
example, geotextiles are used to disperse pore water pressures at the base of roadway
embankments. Geocomposite drains have been used for higher flows. These materials
have been used as pavement edge drains, abutment, etc. Prefabricated vertical drains
(PVDs) have been used to escalate consolidation of soft cohesive foundation soils below
embankments and preload fills.
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 Filtration
the geosynthetic acts similar to a sand filter by allowing water to move through the soil
while holding all upstream soil particles. Geotextiles are used to prevent soils from
migrating into drainage aggregate or pipes by flowing through the system. Geotextiles
are also used below rip rap and other armor materials in coastal and river bank
protection systems to prevent soil erosion.
 Protection
Geotextile or a geotextile-related product helps to prevent or limit any local damage to
a given element.
 Reinforcement
the geosynthetic acts as a reinforcement element which helps the soil mass in
forming a composite that ensures better strength deformation properties over the
unreinforced soil. For example, geogrids are used to add tensile strength to a soil
mass in order to create vertical or near-vertical changes in grade.
Reinforcement helps embankment construction over soft foundation or slopes at
steeper angles. Geosynthetics have also been used to cover voids by forming a
bridge over it.
 Separation
the geosynthetic acts to separate two layers of soils that have different particle size
distributions. For example, geotextiles are used to prevent road base materials from
entering the sub which helps in maintaining roads design thickness. Separators also help
to prevent the sub-grade soils to enter into the road base material.
 Surface Erosion Control
the geosynthetic helps in reducing soil erosion which occurs because of rainfall and
surface water runoff.
For example, temporary geosynthetic blankets and permanent lightweight
geosynthetic mats areused over soil erosion surface to eliminate it. Geotextile silt
fences are also used to remove suspended particles from sediment-laden runoff water.
Some of these mats are manufactured using biodegradable fibers.
Applications:
 Majorly used for Ground stabilization /improvement
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 Helps in Pavements processes: Roads, Parking Bays, Hard Standings, Runways,
Aprons and Taxiways
 Heavy duty pavements: Ports and Harbors
 Erosion control
 Helpful for Retaining, Re walls and Bridge abutments
 Geosynthetic systems reduce the use of natural resources and the environmental
damage associated quarrying, trucking, and other material handling activities which
is a great step for nature
 Geosynthetics can be installed quickly and can be used even during short
construction seasons.
9. Electro-kinetic dewatering
Removing water from slurries and sludge’s is a requirement of a large number of processes
and activities. This process requires four broad stages:
1. Sedimentation or settling – usually achieved by active or passive settling of solids
through a liquid column.
2. Thickening – taking a liquid mixture and removing water from it to produce a higher
density liquid or paste.
3. Dewatering - taking a sludge or paste and removing water to effectively create a phase
transition wherein the behavior of the material which has been dewatered resembles more
of a solid than that of a liquid. The transition between the phases is often hard to notice.
4. Drying – this dried paste is then used to produce a friable granular or free flowing
material.
With each stage there consist various methods and approaches that can be adopted.
Removal of water is a difficult task for for fine-grained materials as sometimes the soil
particles settle very slowly in the ground through water and other times water flows very
slowly in between particles.
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Electrokinetic has developed different material which helps in increasing the performance
of dewatering by joining electro-osmosis and filtration process together. Because of this,
spped of dewatering improves, making the process easier.
Understanding that electrokinetic dewatering is aimed at mixtures of water and fine
grained solids, some applications for EKG dewatering methods are mentioned:
EKG dewatering bags
Filtration bags which helps in joining electro-osmosis and hydraulic filtration results in allowing
dewatering of materials in hanging bags.
Applications include:
(i) Dewatering of small volumes of industrial waste which may be associated with food
production.
(ii) Dewatering the arising of drilling boreholes e.g. for ground source heat boreholes.
(iii) Dewatering arising from roadside gully operations.
EKG In situ dewatering
Dewatering is challenging process where the materials are very soft and fine grained because
the materials are often in a condition, described as “too thin to shovel but too thick to pump”.
Electrokinetic has made a method to join electro-osmosis with conventional together to
form a technology which allows an easy and improved dewatering method.
10. Thermal modification and ground freezing
Thermal Modification
It is a ground improvement technique. It has been seen that heating and cooling change
some properties of soil. Many types of research were conducted which showed
impressive results that is useful for soil stabilization. Therefore, heating and cooling have
been used as soil improvement techniques.
A heat flow analysis can also be done just like seepage or consolidation analysis of soil.
The transfer of heat in soil occurs by convection, conduction and radiation. The most
preferred heat transfer is through conduction, which takes place in three constituents of
soil that are soil solids, water (which may be in the form of a liquid, ice or vapor) and pore
air. Heat conduction in soil is influenced by soil thermal properties such as latent heat of
fusion, thermal conductivity, heat of vaporization of soil water, etc.
The behavior of heat flow in soil is mainly governed by the latent heat of fusion of water
on freezing and heat of vaporization of water on heating above 100 0C.
The latent heat of fusion can be defined as the heat amount that must be added to the
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unit mass of a substance to change it from liquid to solid or solid to liquid without
temperature change. It is seen that a higher heat input per mass of the soil have greater
effects on the properties. If you increase the temperature, the electric repulsion between
the soil particles decreases and as a result the strength of a fine-grained soil increases.
Due to the change in thermal gradient flow of the pore water takes place. Therefore, it is
seen that this method is more feasible and technically possible to perform.
APPLICATION:
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Used for landslide stabilization
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Helpful in improving soil collapsing
Used for forming vitrified piles in place 
Used for reducing lateral stresses acting on retaining walls
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Ground freezing
It is a ground improvement technique in which a soil mass of certain geometry is frozen
using a refrigeration process involving a coolant, either chilled brine or liquid nitrogen,
which is circulated through freeze pipes embedded in the ground. It is a process of making
water-bearing strata temporarily water-resistant and to increase their compressive and
shear strength by changing them into ice.
Freezing is normally provided for structural underpinning; temporary support for an
excavation or to prevent ground water flow into an excavated area. It can be used for any
size, shape or depth of excavation and the can be reused for other jobs.
Principles of freezing: The efficacy of freezing depends on the water to be iced, particles
cementing and increasing the ground strength.
 If the ground is saturated it will help to make it water-resistant.
 It will supposedly add water, if the moisture doesn’t fill pores.
 The strength acquire depends on three things namely freezing temperature,
moisture content and the nature of the soil.
 On freezing, water expands in volume and if the volume increases further to a point
where it gets confined it causes stress and strain in the soil.
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Cooling causes a small loss of strength of clayey soils because of increasing inter-particle
repulsion. However, if the temperature is reduced to the freezing point, the pure water
freezes and the soil is stabilized. Ice formed here, acts as a cementing agent. The strength
of the soil increases as more and more water freezes. This method of stabilization is very
costly.
Freezing may be
 By injecting the coolant directly into the ground.
 By using secondary coolants which circulate in the ground through tubes.
 Direct, by circulation of the primary refrigerant fluid
Application:
 Temporary underpinning of adjacent structure and support during permanent
underpinning.
 Tunneling through mixed ground.
 Shaft sinking through water–bearing ground.
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