Believing be change and the soil capacity cannot resist

Believing on the importance of soil in any project, there are various
values need to be check for sub-soil that will hold the whole structure. The
values are the internal friction angle and cohesion which need to be founded
using different field and laboratory tests. These values will give indication
about how the soil is strong in facing loads due to different sources like the
structure loads or filling loads.

 

However, to start any project, the soil characteristic must be compute
using different soil laboratory tests. Then, the geotechnical report will be
ready for the responsible engineer to decide if stabilization of soil is needed
or not. In fact, these works are concentrate on the bearing capacity of soil
whether it can carry the loads from the structure and foundations or not. If
the site location cannot be change and the soil capacity cannot resist the
load, then stabilization of soil is needed. Otherwise the site need to be
changed before design process and this issue need to be checked for the new
site also.

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Stabilization is a process applied on soil to improve its capacity in
resisting loads by enhancing the physical properties of it. The stabilization
has different effect on the soil and its strength which can be done using
various methods. One of them is mechanical stabilization and the other is
chemical method. In both methods, there can be different results on soil
improvement which depend on the percentage of chemicals or effort done using
mechanical equipment.

 

 

 

 

 

 

 

 

 

 

 

 

 

1      
Effect of various binders on
stabilized soil 1:

Stabilization of soft
soils by deep mixing with binders is the most frequently used method of ground
improvement. The binders react with water to form cementitious composite
materials. The most common binders employed are cement and lime, but a variety
of other binders may also be used for stabilization of soils. This section
contains a summary of the performance of lime, cement, fly ash and blast
furnace slag.

 

1.1      Cement:

 

Its considered as primary
stabilizing agent or hydraulic binder because it can be used alone to bring
about the stabilizing action required. Hydration process is a process under
which cement reaction takes place. The process starts when cement is mixed with
water and other components for a desired application resulting into hardening phenomena.
The hardening (setting) of cement will enclose soil as glue, but it will not
change the structure of soil.

This process can be affected by:

 

presence of foreign matters or impurities
water-cement ratio
curing temperature
presence of additives
specific surface of the mixture.

 

Depending on factor(s)
involved, the ultimate effect on setting and gain in strength of cement
stabilized soil may vary.

However, Cement
stabilized soils have the following improved properties:

decreased cohesiveness (Plasticity)
decreased volume expansion or compressibility
increased strength

 

1.2      Lime:

 

The two primary types of
lime used in construction today are quicklime (calcium oxide) and hydrated lime
(calcium hydroxide).  Heating limestone
at elevated temperatures produces quicklime and the addition of water to
quicklime produces hydrated lime. For soil stabilization with lime, soil
conditions and mineralogical properties have a significant effect on the
long-term strength gain. Introduction of calcium hydroxide increases the pH,
causing the silica and alumina in the clay particles to become soluble and
interact with the calcium in a pozzolanic reaction.  A pozzolanic reaction between silica or
alumina in the clay particles and calcium from the lime can form a cemented
structure that increases the strength of the stabilized soil.

 

1.3      Fly Ash:

 

Fly ash is an industrial
by product generated at coal-fired electricity generating power plants that
contains silica, alumina, and calcium-based minerals. it has little
cementitious properties compared to lime and cement. Most of the fly ashes
belong to secondary binders; these binders cannot produce the desired effect on
their own. However, in the presence of a small amount of activator, it can
react chemically to form cementitious compound that contributes to improved
strength of soft soil. Fly ashes are readily available, cheaper and
environmental friendly. 

 

1.4      Blast Furnace Slags:

 

These are the by-product
in pig iron production. The chemical compositions are similar to that of cement.
It is however, not cementitious compound by itself, but it possesses latent
hydraulic properties which upon addition of lime or alkaline material the
hydraulic properties can develop.

 

 

 

 

Itemized slag in
three forms, namely:

 

1.   Air-cooled
slag :
Hot slag after leaving the blast furnace may be slowly cooled in open
air, resulting into crystallized slag which can be crushed and used as
aggregate.

2.    
Granulated or Pelletised slag:
The granulated blast furnace slag is a result of use of water during
quenching process, while, the use of air in the process of quenching may result
into formation of pelletised slag.

3.   Expanded
slag :
Under certain conditions, steam produced during cooling of hot slag may
give rise to expanded slag.

 

2       stabilizations
Methods

The main purpose of the
soil stabilizations is to make the soil incompressible and impermeable.
Impermeability can by supported mainly by chemical methods of stabilization,
but incompressibility can be achieved mainly by mechanical methods of
stabilization.

First, the
mechanical methods of stabilization will be explain, and then the chemical
methods.

 

2.1      Mechanical Methods of Soil Stabilizations:

These methods
of soil stabilization include physical measures for soil improvement. The most
common mechanical methods are:

  A-Compression Method.

  B-Drainage Method.

The most
common mechanical method is compression method, so it will be explained first.

 

 

2.1.1       
 Compression Method:

It is the
most obvious and simple way of increasing the stability and supporting capacity
of soil (improving the physical properties of soil). Soil compaction is the
process of increasing the unit weight of soil by forcing solids into a tighter
state and reducing air. The compaction will reduce the voids in the soil and
thus will increase the strength of the soil, which means having higher bearing
capacity. By compacting the soil, the air volume will decreases until a point
which more compaction cannot apply to prevent damaging the grain structure.

In the field,
there are four different mechanisms of compression: vibration, impact, kneading
and pressure. These different types of compaction are found in two principle
types of force: static and vibratory. Static compaction uses deadweight of the
machine as downward force on the soil surface. To change the intensity of
compaction force, weight of the machine has to be decreased or increased.
Static compression is limited to upper soil layers. Vibratory compaction uses
engine-driven mechanism for applying downward force in addition to the
machine’s static weight.

Different types of compaction are best suited for different soil types
and conditions. This is because of the underlying density and moisture that
different soil types are able to retain. Soil types are classified in three
soil groups, with consideration to grain sizes.

These types are:

Cohesive: In cohesive soils, such as clay, the particles contain
characteristics that make them easily stick together so compaction can be
achieved by high impact, which forces the air out of the particles,
pushing them together.
Granular: granular soils include sand, gravel, and other particles that
typically range in size from 0.003 to 0.08 inches (0.008 to 0.2 cm).
Because granular soils have good water-draining properties, they are able
to obtain high density when fully dry or saturated. Granular is best
compacted by shaking or vibrating the particles. Any type of vibratory
equipment is best suited for this type. Depending on the type of granular
soil, different degrees of vibration are required.
Granular and Cohesive: often, soils are a mixture of both granular and cohesive,
requiring more precise compaction equipment. Equipment should be chosen on
the basis of the soil in the mix that is present in the highest
percentage. Some materials, such as asphalt, require both vibration and
static pressure to be compacted effectively. Machinery uses frequency and
amplitude to apply a force for compaction. Frequency is the measure of the
speed of the eccentric shaft rotation, or of the jumping of the machine,
quantifiable by vibrations per minute (vpm). Amplitude measures the
maximum movement of a vibrating body from its axis in one direction

From Soil
Mechanics Course (CENG341), we took our results about the compaction experiment
for normal hammer using different water content, and then plot the values and
get the max dry density of the soil with the optimum water content should be
used to compact the soil in order to get high strength.

Here are the
results and plot:

No. of blows: 27, No. of layers: 3, Wt of hammer =2.5kg               

Mold dimensions: D=10.1cm, H=11.638cm

Mold Volume=?(10.1)2(11.638)/4=932.42 cm3                                  

 

Table 1 Density Calculations for 2.5 kg Rammer Method

Determination No.

1

2

3

4

Wt of mold+ compacted soil (g)

6006

6055

6115

6135

Wt of mold (g)

4326

Wt of compacted soil (g)

1680

1729

1789

1809

Wet density ? (g/cm3)

1.802

1.85

1.92

1.94

Dry density ?d (g/cm3)

1.66

1.65

1.67

1.64

 

 

 

Table 2 Water Content Determination for 2.5 kg Rammer Method

Determination No.

1

2

3

4

Container No.

A-1

A-2

A-3

A-4

Wt container + wet soil (g)

110.49

103.96

85.78

131.45

Wt container + dry soil (g)

102.34

93.91

75.68

112.27

Wt water, Ww (g)

8.15

10.05

10.1

19.18

Wt container (g)

9.32

9.30

8.88

9.33

Wt dry soil, Ws (g)

93.02

84.61

66.8

102.94

Water content w%

8.76%

11.88%

15.12%

18.63%

 

Sample of calculation:

Wt of compacted soil=(Wt of mold+ compacted soil)–(Wt of mold)=6006 –
4326=     1680g

Wet density= Wt of compacted soil / volume of mold= 1680/932.42= 1.802
g/cm3

Dry density= wet density /(1+w)= 1.802/(1+0.0876)= 1.66 g/cm3

Wt water, Ww =(Wt container + wet soil)–(Wt container + dry
soil)=110.49–102.34=8.15g

Wt dry soil, Ws=(Wt container + dry soil)–(Wt container)=102.34 –
9.32=93.02g

Water content W%= (Ww/Ws)*100= (8.15/93.02)*100= 8.76%

Zero Air Voids :  ?d = Gs/(1+w
Gs)

 

Table 3 Calculation of Zero Air Voids for 2.5kg Rammer Method

Water content W

8.76

11.88

15.12

18.63

Dry density ?d

2.15

2.015

1.89

1.77

 

Figure 1 Water Content vs Dry Density for 2.5 kg Rammer Method

 

After plotting water
content vs dry density of the soil sample and the zero air void sample the
value of the maximum water content is calculated using the derivative of the
curve equation: Y= -0.0005×2+0.0119x+1.5882 , Y?= -0.0005(2)x+0.0119, , Y?= 0       0= – 0.001x+0.0119 so x=11.9%= 12% of
water content to get the maximum dry density equal  Y= -0.0005×2+0.0119x+1.5882 = -0.0005(12)2
+0.0119(12) +1.5882 =1.66g/cm3  with
error equal (1.66 – 1.67)/1.66= 0.006 where the 1.67 is the experimental dry
density.

Compaction Energy CE= No.
of blows*Wt of hammer*Hdrop / volume                  =(27*3*2.5*0.3)/(9.3242*10-4)
=65153 kJ

According to
Graph-1,While the water content at the beginning increased the dry density of
the soil is increased too and that’s because the water is lubricating the soil
grains so it be easier to compact, after reach the maximum density a drop on it
will occur and that’s happened because when add more and more water the sample
will resist the compaction because of the huge amount of water and the water is
incompressible fluid.

2.1.2       
Drainage Method:

Drainage involves underground systems of
pipes and pumps to extract excessive water from soil. Common drainage methods
are well-point systems, deep-well drainage, vacuum dewatering, dewatering by
electro-osmosis, etc.

A well point
system consists of a number of well points spaced along a trench or around an
excavation site, all connected to a common header which is attached to one or
more well point pumps. Well point assemblies-made up of a well point, screen,
riser pipe, and swing joint with tuning-are generally installed by jetting.
They provide for entry of water into the system by creation of a partial
vacuum. The water is then pumped off through the header pipe. A well point pump
is a combination of two pumps, one of which pumps water from the header and the
other of which is a vacuum pump to remove air which enters the system. Control
of air is important, as excessive air causes cavitation which reduces pump efficiency.
The dewatering pumps used are normally designed specifically for the dewatering
function. They are available in sizes from 4″ to 10″ with handling capacity up
to 500m3/hour.

 

 

 

                    

 

2.2     
Chemical Methods of
Stabilizations of Soil 4:

 

There are many chemicals
can be used for chemical stabilization. The type of chemical can be selected
depend on the soil type and condition of construction site. For example, Lime
is not good stabilizer for silts, granular materials and soils with sulphate
contents greater than 0.3 percent. Actually, lime is helpful material for
clay-bearing and highly cohesive soil whereas fly ash is being used for
granular or poorly cohesive soil.  Many
materials can be used like, lime , cement, fly ash and asphalt. these material
can improve strength, compressibility, hydraulic conductivity, swelling
potential and volume change properties with different results depending on the
type of material ant its action with soil.

 

2.2.1       
Soil Stabilization with Lime:

Lime is a white caustic
alkaline substance consisting of calcium oxide “CaO”, which is obtained by
heating limestone and which combines with water with the production of much
heat. It is very effective chemical in modifying high plastic clayey soil which
can be used alone or by mixing it with another chemical component like cement,
fly ash or bitumen. It is used mainly for stabilizing subgrade.

Lime stabilization will be done by changing the nature of adsorbed
layer and providing pozzolanic action. As a result, the high plasticity index
of the soil will be reduced. Also, the optimum water content will be increased
and decreasing in the maximum compacted density of the soil. Therefore, less
compaction will lead to the maximum compacted density.

It found that lime can
increase the soil strength and durability when it added to the soil as ( Fig. )
shows. There is an advisable percentage of lime to gain that good improvement
from it, from 5-8% for plastic soil.

2.2.2       
Soil Stabilization with Cement 5:

The soil stabilized with cement is known as soil cement which done due
to cementing action. Cementing is the result of chemical reactions of cement
with siliceous soil during hydration reaction. There are some factor affecting
soil-cement such as nature of soil content, conditions of mixing, compaction,
curing and admixtures used. In addition, depending on the soil type, the amount
of cement will differ as follow:

Gravels – 5 to 10%         Sands – 7 to 12% 

 Silts – 12 to 15%,   and       
Clays – 12 – 20%

In general , Cement stabilization give better strength and improve
quality of soil. Other additives can be mixed with cement to enhance the soil
like Lime, calcium chloride, sodium carbonate, sodium sulphate and fly ash. In
fact, the type of cement which used for stabilization usually is ordinary
Portland cement. calculation of required cement amount is discussed following:

If the layer of soil having surface area of A (m2), thickness H (cm)
and dry density rd (tonnes/m3), has to be stabilized with p percentage of
cement by weight on the basis of dry soil, cement mixture will be =
100*P/(100+p)

and, the amount of cement
required for soil stabilization is given by

Amount of cement
required, in tonnes =   

 

2.2.3       
Soil Stabilization with Bitumen:

 

Asphalt is a dark bituminous
substance that is found in natural beds and is also obtained as a residue in
petroleum refining and that consists chiefly of hydrocarbons. Asphalts and tars
are bituminous materials which are used for stabilization of soil. This
stabilization method can be defined as the treatment of naturally occurring
neoplastic or moderately plastic soil with liquid asphalt at normal
temperatures to improve the load-bearing qualities of the soil.

That Bituminous materials
can impart cohesion and reduced water absorption when added to a soil.

 

3       Stabilization Tools:

There are many machines
and tools can be used in stabilization. Choosing the approperiate machine
depends on the method adopted, type of the soil and the soil area to be
adjusted. Here the tools will be classified according to the method used:

 

3.1.1       
Machines for Mechanical Methods 6:

 

1.    Rammers: Rammers are
used for compacting small areas by providing impact load to the soil. This
equipment is light and can be hand or machine operated. The base size of rammers
can be 15cm x 15cm or 20cm x 20cm or more.

Figure 8 RAMMERS

 

 

2.       Vibrating Plate Compactors: Vibrating
plate compactors are used for compaction of coarse soils with 4 to 8% fines.
These equipments are used for small areas. The usual weights of these machines
vary from 100 kg to 2 tonne with plate areas between 0.16 m2 and 1.6 m2.

Figure 9 Vibrating Plate
Compactors

 

3.    Smooth
Wheeled Rollers:
a. Static smooth wheeled rollers.
b. Vibrating smooth wheeled rollers.
The most suitable soils for these roller type are well graded sand, gravel,
crushed rock, asphalt etc. where crushing is required. These are used on soils
which does not require great pressure for compaction. These rollers are
generally used for finishing the upper surface of the soil. These roller are
not used for compaction of uniform sands.
In case of vibrating smooth wheeled rollers, the drums are made to vibrate by
employing rotating or reciprocating mass.

4.     
Sheepsfoot Roller: Sheepsfoot
rollers are used for compacting fine grained soils such as heavy clays and
silty clays. Sheepsfoot rollers are used for compaction of soils in dams,
embankments, subgrade layers in pavements and rail road construction projects.

5.     
Pneumatic
Tyred Rollers: Pneumatic tyred rollers are also called as rubber
tyred rollers. These rollers are used for compaction of coarse grained soils
with some fines. These rollers are least suitable for uniform coarse soils and
rocks. Generally pneumatic tyred rollers are used in pavement subgrade works
both earthwork and bituminous works. Pneumatic rollers have wheels on both
axles. These wheels are staggered for compaction of soil layers with uniform
pressure throughout the width of the roller.

6.     
Grid Rollers: Grid rollers
are used for compaction of weathered rocks, well graded coarse soils. These
rollers are not suitable for clayey soils, silty clays and uniform soils. The
main use of these rollers are in subgrade and sub-base in road constructions.
As the name suggests, these rollers have a cylindrical heavy steel surface
consisting of a network of steel bars forming a grid with squire holes. The
weight of this roller can be increased by ballasting with concrete blocks.

 

 

 

 

 

 

 

 

 

 

 

3.1.2    
Machines for Chemical Methods 7:

 

Here are some steps that
explain which machines should be used in this method:

1.     
Pulverize the
area to be stabilized

2.     
Uniformly
spread stabilizing material

3.     
Blend with
the Asphalt Zipper machine

4.     
Then
homogenous mixture will appear

5.     
Reshape

6.     
Compact

7.     
Pave

Figures below shows the
machines with steps clearly:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4      
CONCLUSION

The consideration of soil
properties is very important stage before any other stages can take place in
the projects. If the geotechnical engineer found the soil is weak, which have
low bearing capacity and cannot resist the loads coming from the structure,
then as a solution for that soil stabilizations can made. After checking the
effects of various binders on stabilized soil, two main methods can do to make
the soil stabilized. The most common binders employed are cement and lime, but
a variety of other binders may also be used for stabilization of soils such as
fly ash and blast furnace slag. The main purpose of the stabilizations is to
make the soil incompressible and impermeable. Impermeability can by supported
mainly by chemical methods of stabilization, but incompressibility can be
achieved mainly by mechanical methods of stabilization. For the mechanical
methods, there are two common sub methods, the first one is compression or
compacting method and the other is drainage method. The compaction can
effectively improve the strength of the soil, but to do that an optimum water
volume should be spread on the surface of the soil to be as lubrication agent
to ease the compaction. Also, each type of soil has its own way to compact
using different machines. The proper machine should be used to get the maximum
possible soil strength. The effective stress of the soil will be decreased
while the ground water table increased, so to solve this issue the drainage
method will be useful to remove the water from the site where the structures
should build. The other method of soil stabilization is the chemical methods
which have many ways to do it, but the three more used ways explained in this report
like chemical stabilization using lime, cement and bitumen. Both methods,
mechanical and chemical are effective, but they need to be made by the proper
machine or equipment by the exact procedure as explained in this report.
Finally, the site of the project will have a high bearing capacity soil after
it stabilized. Therefore, it can resist the applied load coming from the
structure without bearing failure or excessive settlement.