Department of Civil engineering
Civil Engineering Project
Project Report
On
‘ HIGHWAY NETWORK SYSTEM ’
1.0 INTRODUCTION
A highway is a
public road, especially a major road connecting two or more destinations. Any
interconnected set of highways can be variously referred to as a "highway
system", a "highway network",
or a "highway transportation system”. The history of highway engineering
gives us an idea about the roads of ancient times. Roads in Rome were
constructed in a large scale and it radiated in many directions helping them in
military operations. Thus they are considered to be pioneers in road
construction.
The modern roads by and large follow Macadam's construction
method, use of bituminous concrete and cement concrete are the most important
developments. Various advanced and cost-effective construction technologies are
used. Developments of new equipments help in the faster construction of roads.
Many easily and locally available materials are tested in the laboratories and
then implemented on roads for making economical and durable pavements.
Scope of transportation system has developed very largely.
Population of the country is increasing day by. The life style of people began
to change. The need for travel to various places at faster speeds also
increased. This increasing demand led to the emergence of other modes of
transportation like railways and travel by air. While the above development in
public transport sector was taking place, the development in private transport
was at a much faster rate mainly because of its advantages like accessibility,
privacy, flexibility, convenience and comfort. This led to the increase in vehicular
traffic especially in private transport network. Thus road space available was
becoming insufficient to meet the growing demand of traffic and congestion
started. In addition, chances for accidents also increased. This has led to the
increased attention towards control of vehicles so that the transport
infrastructure was optimally used. Various control measures like traffic
signals, providing Roundabouts and medians, limiting the speed of vehicle at specific
zones etc. were implemented.
With the advancement of better roads and efficient control, more
and more investments were made in the road sector especially after the World
wars. These were large projects requiring large investment. For optimal utilization
of funds, one should know the travel pattern and travel behaviour. This has led
to the emergence of transportation planning and demand management.
2.0 MODERN SOIL STABILIZATION TECHNIQUES
The stabilization of
naturally-occurring or native soil has been performed by millennia. The Mesopotamians
and Romans separately discovered that it was possible to improve the ability of
pathways to carry traffic by mixing the weak soils with a stabilizing agent
like pulverized lime stone or calcium. This was the first chemical
stabilization of weak soils to improve their load-carrying ability.
Successful
modern soil stabilization techniques are necessary to assure adequate subgrade
stability, especially for weaker and wetter soils. It is widely recognized that
selection between cementitious stabilizing agents cement and lime is based on
the Plasticity Index (P I) of the primary soil type being improved.
2.1 STABILIZATION
WITH CEMENT
2.1.1 CTB
(CEMENT TREATED BASE)
According to the PCA (Portland Cement Association), CTB (Cement
Treated Base) has provided economical, long lasting pavement foundation. These
structures have combined soil and/or aggregate with cement and water which
compacted to high density. The advantages of cement stabilization are several:
1.
Cement
stabilization increases the base material strength and stiffness, which reduces
deflection due to the traffic loads. This delays surface distresses such as
fatigue, cracking and extends pavement structure life.
2.
Cement
stabilization provides uniform and strong support, which results in reduced
stresses to the sub-grade. Testing indicates a thinner cement-stabilized layer can
reduce stresses more effectively than a thicker un-stabilized layer of
aggregate. This reduces sub-grade failure, pot-hole formation and rough
pavement surface.
3.
Cement
stabilized base has greater moisture resistance to keep water out; this
maintains the higher strength of the structure.
4.
Cement
stabilization reduces the potential for pumping of sub-grade fines.
5.
Cement
stabilized base spread loads and reduces sub-grade stress.
FIG.1.
Load Distribution
2.1.1.1 COMPOSITION AND CONSTRUCTION
The mixture shall be composed of existing sub-grade,
base course and surface course materials, and/or an imported soil aggregate,
with Portland cement and water added. The mixture shall contain not less than
4% cement by volume of compacted mixture, 1420 kg (94 pounds) of cement being
considered as 1 cu m (1 cubic foot). At least 30 days before the beginning of
stabilizing operations, adequate quantities of soil and cement shall be
supplied to the Materials Division for determination of cement requirements.
The Engineer will specify, based on laboratory tests, the exact percentage of
cement to be used. Specimens of soil aggregate, cement, and water shall develop
a compressive strength of a least 2.7 M Pa (400 psi) in 7 days.
FIG.2. Mixing of soil-cement for sub-grade stabilization
MATERIALS
The materials used shall comply with the following requirements:
(a) WATER
Water used in mixing or curing shall be
clean and free from injurious amounts of oil, salt, or other deleterious substances.
Where the source of water is relatively shallow, it shall be maintained at such
a depth and the intake so enclosed as to exclude grass, vegetable matter, or
other foreign materials.
(b) CEMENT
Fly
ash may be used as a partial replacement for the cement. Replacement amounts,
not exceeding 25% by weight, shall be determined through trial batch
investigations using the specific materials proposed for the project. Mixtures
with fly ash shall meet the same requirements as mixtures without fly ash. All
trial batches required by this specification shall be accomplished by the Contractor,
observed by the Engineer, and approved by the Engineer of Materials. Fly ash
will not be allowed as a substitute for high early strength or blended cements.
For
in-place stabilization, the fly ash and cement shall be blended to form a
homogeneous mixture before application on the roadway.
The
use of cement salvaged from used or discarded sacks will not be allowed. Cement
placed in storage shall be suitably protected. Any loss of quality occurring
during the storage period will be cause for rejection. If the cement furnished
shows erratic behaviour under the field conditions incident to the mixing and
placing of the mixture, or in the time of the initial or final set, the
Contractor will at once, without notice from the Engineer, cease the use of
that brand of cement and furnish material of such properties as to ensure quality
work conforming to these specifications.
2.1.1.2 CONSTRUCTION
REQUIREMENTS
Sufficient equipment
shall be available so that the work may proceed in proper sequence to completion
without unnecessary delay. Equipment, tools, and machinery used shall be
maintained in a satisfactory working condition.
The application of cement and mixing of the cement
and soil aggregate will be allowed only on an approved sub-grade, free of excess
moisture. No work will be allowed on a frozen sub-grade.
The
operations shall be such as to prevent the drifting of cement or dust off the
right-of-way.
(a) PREPARATION
OF THE ROADBED
Prior to
other construction operations, the existing roadbed, including the shoulders,
shall be brought to line and grade and shaped to the typical cross section of
the completed roadbed and compacted to sufficient density to prevent rutting
under normal operations of construction equipment. All soft areas shall be
corrected to provide uniform stability.
(b) PULVERIZING
After shaping and compacting the
roadbed, the material to be processed shall be scarified and pulverized before application
of cement. Pulverizing shall continue during mixing operations until a minimum
of 80% by weight of the material, exclusive of coarse aggregate, will pass a
4.75 mm (#4) sieve. Material retained on a 75 mm sieve and other unsuitable material
shall be removed.
(c) APPLICATION AND MIXING OF CEMENT
The application and mixing
of cement with the aggregate material shall be performed according to one of
the following methods:
(1) TRAVEL PLANT METHOD
The specified
quantity of cement shall be applied uniformly on the material to be processed,
and shall not exceed that which can be processed the same working day. When
bulk cement is used the equipment shall be capable of handling and spreading
the cement in the required amount. The moisture content of the material to be
processed shall be sufficiently low to permit a uniform and intimate mixture of
the aggregate material and cement. Mixing shall be accomplished by means of a
self-propelled or self-powered machine equipped with a mechanical rotor or
other approved type of mixer that will thoroughly blend the aggregate with the
cement. Mixing equipment shall be so constructed as to assure positive depth
control. Care shall be exercised to prevent cement from being mixed below the
depth specified. Machines designed to process less than the full width of base
at a single pass shall be operated so that the full width of base can be compacted
and finished in one operation. Water shall be uniformly added and incorporated
in the mixture. The water supply and distribution equipment shall be capable of
supplying the total required amount of water to the section being processed within
3 hours. If more than one pass of the mixer is required, at least one pass
shall be made before water is added. Mixing shall continue after all water has
been applied until a uniform mixture of aggregate, cement, and water has been
obtained for the full depth of the course.
The
aggregate and cement mixture that has not been compacted and remains
undisturbed more than 30 minutes shall be remixed. In the event of rain adding
excessive moisture to the uncompacted material, the entire section shall be
reworked. Should the Contractor be unable to finish the section within the same
day, the section shall be reconstructed and an amount equal to 50% of the
original amount of cement added to the mixture at no cost to the Department.
(2) CENTRAL PLANT METHOD
When a central plant is used, the soil
aggregate, cement, and water shall be mixed in a pug mill either of the batch or
continuous flow type. The plant shall be equipped with feeding and metering
devices that will add the soil aggregate, cement, and water into the mixer in
accurately proportioned amounts as determined by the laboratory design. Aggregate
and cement shall be dry-mixed sufficiently to prevent cement balls from forming
when water is added. Mixing shall continue until a uniform mixture of
aggregate, cement, and water has been obtained. The mixture shall be hauled to
the roadway in trucks equipped with protective covers. Immediately before
spreading the mixture, the sub-grade or foundation course shall be moistened and
kept moist, but not excessively wet, until covered by the mixture. The mixture
shall be placed on the roadbed in a uniform layer by an approved spreader or
spreaders. No more than 60 minutes shall elapse between adjacent spreader runs
and not more than 60 minutes shall elapse between the time of mixing and the
beginning of compaction. The layer shall be uniform in depth, and in such
quantity that the completed base will conform to the required grade and cross
section. Dumping of the mixture in piles or windrows will not be permitted.
(a) COMPACTION AND SURFACE FINISH
The moisture content of the mixture
during compaction shall not vary more than ±5% from the optimum moisture. The
surface of the treated roadway shall be reshaped to the required lines, grade,
and cross section after the mixture has been compacted. It shall be scarified
lightly to loosen any imprints left by the compacting or shaping equipment and
rolled thoroughly. The operation of final rolling shall include the use of
pneumatic tired rollers. The rolling shall be done in such manner as to produce
a smooth, closely knit surface, free of cracks, ridges, or loose material, and
conforming to the crown, grade, and line shown on the plans.
The
density, surface compaction, and finishing operation shall not require more
than two hours.
Water
shall be added, if necessary, during the finishing operation to maintain the
mixture at the proper moisture content for securing the desired surface.
Areas
inaccessible to rollers or finishing and shaping equipment shall be thoroughly
compacted to the required density by other approved compacting methods and
shaped and finished as specified.
(b) JOINTS
As soon as final compaction and
finishing of a section has been completed, the base shall be cut back
perpendicular to the centre line to a point where uniform cement content with
proper density has been attained and where the vertical face conforms to the typical
section shown on the plans. When the road mix method is used, a header shall be
placed against the vertical face of the finished section and securely staked in
place. This header shall be left in place until all mixing operations on the
adjoining section have been completed, after which the header shall be removed
and the trench backfilled with processed material. This material shall be compacted
so that a well-sealed joint is formed and a smooth riding surface is obtained.
As an
alternate to using a header, the subsequent day's operation may be started by
cutting back into the previously placed course to the extent necessary to
obtain uniform grade and compaction.
(c) SURFACE TEST
The finished surface
of the treated base course shall conform to the general surface provided for by
the plans. It shall not vary more than 6 mm (¼") from a 3 m (10')
straightedge applied to the surface parallel to the centre line of the roadway,
nor more than 12 mm (½") from a template conforming to the cross section
shown on the plans. Excess material shall be disposed of as directed.
(d) PROTECTION AND COVER
Immediately after
the rolling and shaping has been completed, the surface of the treated base
course shall be covered by a protective coating of asphalt to prevent loss of moisture
during the curing period and to serve as a prime coat for the later application
of wearing course. The asphalt shall comply with the requirements listed herein
and shall be applied by means of an approved pressure distributor at the rate
of 0.4 to 1.1 L/sq m to provide complete
coverage without excessive runoff. The actual rate of application will be
determined by the Engineer. When used, emulsified asphalt shall be diluted with
an equal amount of water before application. At the time of application, the
base shall be in a moist condition. The protective coating of asphalt shall be
maintained until the wearing surface is placed. If the condition of the
protective coating is satisfactory, no additional prime coat will be required
at the time of placement of the wearing surface.
Furnishing
and placing asphalt will not be paid for separately, but full compensation therefore
will be considered included in the contract unit price bid for Processing
Cement Treated Base Course.
Finished
portions of the roadway adjacent to construction that is travelled by equipment
used in constructing an adjoining section shall be protected by means satisfactory
to the Engineer. If earth covering is used on fresh bases, straw, hay, building
paper or similar material shall be placed under the earth so that the covering may
be removed without damage to the base.
2.2 STABILIZATION WITH
BITUMEN
The basic principles
in bituminous stabilization are water proofing and binding. By water proofing
the inherent strength and other properties of the soil could be retained. In
case of the cohesion less soils the binding action is also important. Generally
both binding and water proofing actions are provided to soil.
In
granular soil the coarse grains may be individually coated and stuck together
by a thin film of bituminous materials. But in fine grained soils bituminous
material plugs up the voids between small soil clods, thus water proofing the
compacted soil-bitumen.
FIG.3.Soil stabilization with bitumen
The mechanics of
asphaltic soil stabilization are discussed based upon the major four factors
for any given soil material: (1) soil status, (2) asphaltic material, (3)
mixing, and (4) compaction and curing. A method of bituminous stabilization of
soils is presented as related to soils developing appreciable degrees of
cohesiveness when moist and which may be stabilized by the principle of
waterproofing. this method is based upon the theory that soil, water, and
bituminous material, including asphalt, may be placed in such independent
relative positions within a compacted mass of mixture so that a definite system
exists or tends to predominate. The system consists essentially of soil-water
mixtures which are waterproofed by bituminous films held or absorbed on their
surfaces. Stabilization by waterproofing may be accomplished with: (1)
relatively small quantities of bitumen, (2) a minimum of mixer work and time,
(3) utilization of the economies accruing from intermediate soil moisture
contents during mixing and compaction, and (4) the more complete utilization of
soils in situ due to the greater range of soils which may be successfully
treated. The bituminous stabilization of soil utilizing supplementary
admixtures was investigated by the use of Portland cement, lime, and aqueous
solutions of certain heavy metal salts. data presented indicate that
stabilization of soil with materials as cement, consist of two separable and
distinguishable functions, one an alteration of soil character reducing the
sensitiveness of the soil to physical changes induced by water, the other a
cementation of the altered particles of soil into a water- tight coherent mass.
The first function may be produced by small quantities of cement, and the
second by bitumen, yielding a dual or composite form of stabilization
possessing high strength, flexibility, and high immunity to action of water and
temperature. These principles were applied in two processes, one a
pre-treatment of soil with cement which included mixing, wetting, curing, and repulverization,
while the other method consisted of mixing, in consecutive order, the materials
soil, cement, water, and bitumen, forming a mixture capable of being
immediately laid and compacted. The effects of different types of cement upon
the changes induced in soil were discussed. The character of the reactions
induced in soil by both cement and lime were discussed. The efficiency of lime
as an admixture material for bituminous stabilization was studied. The economic
practicality of the use of cement and lime as bituminous stabilization adjuncts
was discussed with attention to the method of soil dilution by aggregate as an
alternative.
2.3 FLY ASH IN SOIL STABILIZATION
Soil
stabilization is the permanent physical and chemical alteration of soils to
enhance their physical properties. Stabilization can increase the shear
strength of a soil and/or control the shrink-swell properties of a soil, thus
improving the load bearing capacity of a sub-grade to support pavements and
foundations. Stabilization can be used to treat a wide range of sub-grade
materials from expansive clays to granular materials. Stabilization can be
achieved with a variety of chemical additives including lime, fly ash, and
portland cement, as well as by-products such as lime-kiln dust (LKD) and
cement-kiln dust (CKD). Proper design and testing is an important component of
any stabilization project. This allows for the establishment of design criteria
as well as the determination of the proper chemical additive and admixture rate
to be used to achieve the desired engineering properties. Benefits of the
stabilization process can include: Higher resistance (R) values, Reduction in
plasticity, Lower permeability, Reduction of pavement thickness, Elimination of
excavation - material hauling/handling - and base importation, Aids compaction,
Provides “all-weather” access onto and within projects sites. Another form of
soil treatment closely related to soil stabilization is soil modification,
sometimes referred to as “mud drying” or soil conditioning. Although some
stabilization inherently occurs in soil modification, the distinction is that
soil modification is merely a means to reduce the moisture content of a soil to
expedite construction, whereas stabilization can substantially increase the
shear strength of a material such that it can be incorporated into the
project’s structural design. The determining factors associated with soil
modification vs. soil
stabilization may be the existing moisture content, the end use of the soil
structure and ultimately the cost benefit provided. Equipment for the
stabilization and modification processes include: chemical additive spreaders,
soil mixers, portable pneumatic storage containers, water trucks, deep lift
compactors, motor graders.
High-calcium and
low-calcium class C
fly ashes from the Soma and Tuncbilek thermal power
plants, respectively, in Turkey, were used for stabilization of an
expansive soil. An evaluation of the expansive soil-lime, expansive soil-cement,
and expansive soil-fly ash systems is presented. Lime and cement
were added to the expansive soil at 0–8% to establish baseline values. Soma fly
ash and Tuncbilek fly ash were
added to the expansive soil at 0–25%. Test specimens were
subjected to chemical composition, grain size distribution, consistency limits, and
free swell tests. Specimens with fly ash were cured for 7
days and 28 days, after which they were subjected
to free swell tests. Based on the favourable results
obtained, it can be concluded that the expansive soil can be successfully
stabilized by fly ashes.
2.4 STABILIZATION OF BLACK COTTON SOIL
Modification of black cotton soils by
chemical admixtures is a common method for stabilizing the swell-shrink
tendency of expansive soils. Advantages of chemical stabilization are that they
reduce the swell-shrink tendency of the expansive soils and also render the
soils less plastic. Among the chemical stabilization methods for expansive
soils, lime stabilization is most widely adopted method for improving the
swell-shrink characteristics of expansive soils. Lime stabilization of clays in
field is achieved by shallow mixing of lime and soil or by deep stabilization
technique. Shallow stabilization involves scarifying the soil to the required
depth and lime in powder or slurry form is spread and mixed with the soil using
a rotovator. The use of lime as deep stabilizer has been mainly restricted to
improve the engineering behaviour of soft clays Deep stabilization using lime
can be divided in three main groups: lime columns, lime piles and lime slurry
injection. Lime columns refer to creation of deep vertical columns of lime
stabilized material. Lime piles are usually holes in the ground filled with
lime. Lime slurry pressure injection, as the name suggests, involves the
introduction of lime slurry into the ground under pressure. Literature review
brings out that lime stabilization of expansive clays in field is mainly
performed by mixing of lime and soil up to shallow depths. The use of lime as
deep stabilizer has been mainly restricted to improve the engineering behaviour
of soft clays. Use of lime in deep stabilization of expansive soils however has
not been given due attention. There exists a definite need to examine methods
for deep stabilization of expansive soils to prevent the deeper soil layers
from causing distress to the structures in response to the seasonal climatic
variations. In addition, there exists a need for in-situ soil stabilization
using lime in case of distressed structures founded on expansive soil deposits.
The physical mixing of lime and soil in shallow stabilization method ensures efficient
contact between lime and clay particles of the soil. It however has limitation
in terms of application as it is only suited for stabilization of expansive
soils to relatively shallow depths. Studies available have not compared the
relative efficiency of the lime pile technique and lime-soil mixing method in
altering the physico-chemical, index and engineering properties of expansive
black cotton soils.
2.5 STABILIZATION OF DESERT SAND
There are large deposits of the
desert sand in the regions of Rajasthan and other places in India. It is really
a great problem to construct roads across the desert mainly because of non
availability of other suitable materials. There is also acute scarcity of water
in the desert regions. Hence a suitable stabilization technique seems to be the
only economical solution.
The desert sand deposits
consist of fine grained uniformly graded sand with, rounded particles. This
renders the desert sand with poor stability. The cement requirement for
satisfactory stabilization is also very high in such soil. Due to scarcity of
water, soil-cement stabilization is all the more difficult as considerable
water is needed for soil cement base course construction.
Use of hot sand bitumen would
result in satisfactory mix, provided some material including filler can be
added to give a proper gradation of the mix. In this connection mixing of
locally available kankar dust has been found to give satisfactory result.
However use of hot sand bitumen mix is not economical for sub base and base
course construction. If cut back is to be used, the requirement of mixing water
content would be considerable.
The most promising bituminous
material in desert region seems to be the emulsion. As the emulsion contains
about 50% of water content, the additional quantity of water needed for mixing
would be very less. During curing the water evaporates, the emulsion breaks
down and the bitumen stabilizes the sand. The stability of the mix could be
improved by the addition of kankar powder, and other material to improve the
gradation.
3.0 USE
OF FLY ASH IN CONCRETE
Fly ash is one of the residues generated in combustion,
and comprises the fine particles that rise with the flue gases. Ash which does
not rise is termed bottom ash. In
an industrial context, fly ash usually refers to ash produced during combustion
of coal.
Fly ash is generally captured by electrostatic precipitators
or other particle filtration equipments before the flue gases reach the
chimneys of coal-fired power plants, and
together with bottom ash
removed from the bottom of the furnace is in this case jointly known as coal
ash. Depending upon the source and makeup of the coal being burned, the
components of fly ash vary considerably, but all fly ash includes substantial
amounts of silicon
dioxide (SiO2) (both amorphous
and crystalline) and calcium oxide
(CaO), both being endemic ingredients in many coal-bearing rock strata.
FIG.4.FLY
ASH IN CONCRETE
Toxic
constituents depend upon the specific coal bed makeup,
but may include one or more of the following elements or substances in
quantities from trace amounts to several percent: arsenic, beryllium,
boron,
cadmium,
chromium,
chromium VI, cobalt, lead,
manganese,
mercury, molybdenum, selenium,
strontium,
thallium,
and vanadium,
along with dioxins and PAH compounds.
Fly ash has been used as a pozzolanic
admixture in concrete for more than 50 years. Earlier uses were largely
confined concrete for more than 50 years. Earlier uses were largely confined to
low-calcium ashes from hard bituminous or anthracite coals. However, increased
demand for fly ash coupled with the declining availability of suitable
low-calcium ashes has attracted a wider variety of fly ashes to the marketplace
in recent years. Some of these ashes are characterized by very high calcium
contents (for example. >25% CaO) and such materials affect the properties of
concrete in a different manner than traditional fly ashes. The latest Canadian
Standard covering fly ash for use in concrete divides fly ash into three
categories strictly on the basis of its calcium content. This paper provides a
rationale for this change in concept.
4.0 RETAINING WALLS
A retaining
wall is a structure designed and constructed to resist the lateral pressure
of soil when there is a desired change in ground elevation that exceeds the
angle of repose of the soil. The active pressure increases on the retaining
wall proportionally from zero at the upper grade level to a maximum value at
the lowest depth of the wall. The total pressure or thrust may be assumed to be
acting through the center of the triangular distribution pattern, one-third
above the base of the wall. Retaining walls serve to retain the
lateral pressure of soil. The basement
wall is thus one form of retaining wall. However, the term is most often used
to refer to a cantilever retaining wall, which is a freestanding structure
without lateral support at its top.
Typically retaining walls are cantilevered
from a footing extending up beyond the grade on one side and retaining a higher
level grade on the opposite side. The walls must resist the lateral pressures
generated by loose soils or, in some cases, water pressures.
FIG.5. RETAINING WALL
The most important consideration in proper
design and installation of retaining walls is to recognize and counteract the
fact that the retained material is attempting to move forward and down slope
due to gravity. This creates lateral earth
pressure
behind the wall which depends on the angle of internal friction (phi) and the cohesive strength (c)
of the retained material, as well as the direction and magnitude of movement
the retaining structure undergoes.
Lateral earth pressures are typically
smallest at the top of the wall and increase toward the bottom. Earth pressures
will push the wall forward or overturn it if not properly addressed. Also, any groundwater behind the wall that is not
dissipated by a drainage system causes an additional
horizontal hydrostatic pressure on the wall.
4.1 TYPES OF RETAINING WALLS
FIG.6 TYPES OF RETAINING WALL
(a)GRAVITY
WALLS:
Gravity
walls depend on the weight of their mass (stone, concrete or other heavy
material) to resist pressures from behind and will often have a slight 'batter' setback, to improve
stability by leaning back into the retained soil. For short landscaping walls,
they are often made from mortar
less stone or segmental concrete units (masonry units).
Dry-stacked gravity walls are somewhat flexible and do not require a rigid
footing in frost
areas. Home owners who build larger gravity walls that do require a rigid
concrete footing can make use of the services of a professional excavator,
which will make digging a trench for the base of the gravity wall much easier.
Earlier in the 20th century, taller
retaining walls were often gravity walls made from large masses of concrete or
stone. Today, taller retaining walls are increasingly built as composite
gravity walls such as: geosynthetic or with precast facing; gabions (stacked steel wire baskets filled with
rocks); crib walls (cells built up log cabin style from precast concrete or
timber and filled with soil); or soil-nailed walls (soil reinforced in place
with steel and concrete rods).
(b)CANTILEVERED WALLS:
Cantilevered
retaining walls are made from an internal stem of steel-reinforced,
cast-in-place concrete or mortared masonry (often in the shape of an inverted
T). These walls cantilever loads (like a beam)
to a large, structural footing, converting horizontal pressures from behind the
wall to vertical pressures on the ground below. Sometimes cantilevered walls
are buttressed on the front, or include a counter fort on the back, to improve
their strength resisting high loads. Buttresses are short wing walls at right angles to the main
trend of the wall. These walls require rigid concrete footings below seasonal
frost depth. This type of wall uses much less material than a traditional
gravity wall.
(c)SHEET PILING WALLS:
Sheet
pile retaining walls are usually used in soft soils and tight spaces. Sheet
pile walls are made out of steel, vinyl or wood planks which are driven into
the ground. For a quick estimate the material is usually driven 1/3 above
ground, 2/3 below ground, but this may be altered depending on the environment.
Taller sheet pile walls will need a tie-back anchor, or "dead-man"
placed in the soil a distance behind the face of the wall, that is tied to the
wall, usually by a cable or a rod. Anchors are placed behind the potential
failure plane in the soil.
(d)ANCHORED WALLS:
An
anchored retaining wall can be constructed in any of the aforementioned styles
but also includes additional strength using cables or other stays anchored in
the rock or soil behind it. Usually driven into the material with boring,
anchors are then expanded at the end of the cable, either by mechanical means
or often by injecting pressurized concrete, which expands to form a bulb in the
soil. Technically complex, this method is very useful where high loads are
expected, or where the wall itself has to be slender and would otherwise be too
weak.
5.0 NEW TECHNIQUES IN PAVEMENT
Chip seals are applied in a three-part
process. The asphalt emulsion binder is first sprayed onto the pavement. This
is followed immediately by an application of rock chips. Finally, the rocks are
pressed into the asphalt binder using a heavy roller. This process is more
appropriate for use on roads than on parking lots. Service life is usually 5 to
7 years. The road takes on more of the colour of the rock used in the chip
layer since it's not mixed together with the asphalt binder, so use of lighter
coloured aggregate here can make more of a difference in cooling the road
surface.
Emulsion sealcoats are the familiar
pre-mixed products often seen in shopping center parking lots or on driveways.
They consist of a fine aggregate (rocks of small size) in emulsion (suspended
in water) with an asphalt binder. Emulsion sealcoats are brushed on over
existing pavements to seal small cracks and protect the surface. When used properly
they're expected to last 3 to 5 years. These products are usually black but are
occasionally made in gray or tan with the addition of zinc oxide, although this
may cost a bit extra.
Slurry seals combine an asphalt emulsion
with graded aggregate (rocks of special, even sizes). This mixture is then
applied to existing pavement using a squeegee-like drag. Slurry seals are
expected to last 3 to 5 years. Like the emulsion sealcoat, slurry seals are
usually black but can be made gray or tan with the addition of zinc oxide.
Asphalt surface coatings are painted or
sprayed directly over clean asphalt. These coatings are decorative, while also
serving to protect the asphalt underneath. They come in many colours, but the
lightest colours have the highest solar reflectivity and stay coolest.
Pavement texturing is a process that uses
standard asphalt to produce a decorative pavement in a variety of colours and
patterns. These pavements are used in street paving, traffic calming,
pedestrian areas, medians & boulevards, parking lots, playgrounds, and
other applications. These pavements are less labour-intensive to install, with
the additional advantage of having no joints where water can infiltrate and
weeds can grow. The construction process consists of first laying the asphalt,
compacting it into a patterned form, and then finishing it with a polymerized
cement coating. The resulting pavement can withstand extreme weather and
traffic loading by combining the strength of concrete with the flexibility of
asphalt. The choice of a lighter coloured coating is needed to make the surface
more reflective and keep it cooler.
5.6 ROLLER COMPACTED CONCRETE AND SOIL-CEMENT
PAVEMENT
Roller Compacted Concrete (RCC) combines
cement with natural or graded aggregate to create a pavement suitable for heavy
loads at low speeds, such as warehouses or airport taxiways. Soil-cement pavements
combine cement with sand or alluvium material to construct pavement suitable
for low-speed, low volume uses like hiking trails and bike paths. Both RCC and
soil-cement pavements have a natural appearance, taking on the colour of the
added aggregate or sand. Choice of lighter colours can keep the pavement
cooler.
5.7 WHITE-TOPPING
This is a technique of covering existing
asphalt pavement with a layer of concrete. Traditional white-topping with
concrete added a 4 to 8 inch thick layer of concrete over an existing asphalt
base. New concrete mixtures with fiber reinforcement, called ultra-thin
white-topping, mean you now need only apply a 2 to 4 inch overlay of concrete
to withstand normal loads on residential and low-volume roads. Special mixtures
with higher cement content can also be used on surfaces that must be cured and
ready for traffic within 24 hours.
The white-topping construction process
consists of four steps: 1) coring the existing asphalt to determine its depth,
type and condition, 2) preparing the road surface by water or abrasive
blasting, or milling and cleaning, 3) placing the concrete, and 4) finishing
and texturing the surface, and curing and sawing its joints. The proper joint
spacing is critical to control cracking of the concrete surface.
Concrete pavements have a 1.5 to 2 time’s
greater service life than asphalt pavements. Concrete pavements are naturally
light gray in colour and need no further lightening. Concrete pavements can be
periodically pressure-washed to remove dirt and stains and to help retain its
reflective qualities.
6.0 PRESTRESSED CONCRETE PAVEMENT
The prestressing technique has been
applied to the highway pavements in recent years. The prestressed pavement can
be built in continuous length up to 120 m without joints. Elimination of joints
without inducing cracks in the pavement could be considered advantageous, in
view of the maintenance problems associated with the joints. To accommodate
higher loads, there is obvious tendency of increasing the thickness. It may be
realized that an increase in the thickness gives rise to a great temperature
differential of the slab and also greater frictional resistance. A thick slab
is therefore undesirable as well as costly. By providing a residual compressive
stress to the slab by use of tendons etc, the total tensile stress can fairly
be neutralized and thus same unit thickness of prestressed concrete pavement
could support heavier loads than plain concrete pavement and can be built for
longer without joints.
Following
are few observations for the design:
a)
Length: A length up to about 120 m can be prestressed
for the pavement.
b)
Width: A width of 3.6 m for prestressed pavement is
desirable and a longitudinal a joint therefore should be provided.
c)
Thickness: Because of the need to provide a required
cover for tendons, the minimum recommended thickness is 15 cm.
d)
Stress magnitude: A minimum value of 22 kg/cm2
of prestress is recommended for 120 m long prestressed pavement slabs. A
transverse prestress if required should be of 3 to 4 kg/cm2.
FIG.7. Prestressing
Concrete Process
Prestressing is applied either by
pre-tensioning or by post-tensioning. For highway pavement, post-tensioning
system has been used. Most of systems employ wire of 7.0 mm diameter with ultimate tensile
strength of 142-173 kg/cm2.
The construction
of prestressed concrete pavement is difficult job and needs a skilled team. Due
to the long length of tendons, there is a great amount of energy stored in it
and any failure of anchor could be very severe.
7.0
CONCLUSIONS
Traditionally highways were used by people on foot
or on horses. Later they also accommodated carriages, bicycles
and eventually motor cars,
facilitated by advancements in road
construction. In the 1920s and 1930s many nations began
investing heavily in progressively more modern highway systems to spur commerce
and bolster national defense.
India has an extensive road network of more than 3 million kms
which is the second largest in the world, Roads carry about 60% of
the freight and nearly 85% of the passenger traffic, Highways/Expressways
constitute about 66,000 kms. The Government of India spends about Rs.18000
crores (US $ 4 billion) annually on road development.These new
trends are initiative in the highway improvements. Now highways are well
stabilized and more secure. The costs in the construction as well as in
maintenance are reduced. These new trends are eco friendly because the use of
fly ash is used as an important material and it is a residual of thermal power
stations and in Free State, it is very harmful for the environment. So there is
a great hope for the further improvement in these techniques.
8.0 REFERENCES:
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