Highway Maintenance -Overlay Design

Overlay can be of many types:

(1) flexible overlay over a exiting flexible pavement

(2) flexible overlay over a rigid pavement.

(3) Rigid overlay  over a flexible pavement.

(4) Rigid overlay over a rigid pavement.

(1) Flexible overlay over flexible pavement by convention method of design:

h0 = hd - he

ho= thickness of the flexible overlay over flexible pavement .

hd= Design thickness of the flexible pavement

he= thickness of the existing pavement.

Analysis of Data:

Standard deviation of deflection:

Characteristic Deflection :

Overlay Thickness Design: 

ho= R/0.343 log10(Dc/Da)    ....Ruiz's equation.

ho= Thickness of bituminous overlay in cm.

R= Deflection reduction factor depending on the overlay material

As per IRC: 

ho=550log10(Dc/Da)

ho= Thickness of granular or WBM overlay in mm.

D = D(mean) +6, after applying the correction for the pavement temperature and sub grade moisture.

Da= 1.00, 1.25 & 1.50mm if the projected design traffic A is 1500 to 4500, 450 to 1500 & 150 to 450 respectively.

Rigid Overlay over Rigid Pavement:

Flexible overlay over Rigid Pavement:

hf =2.5(Fhd - he)

where, hf= Flexible overlay thickness

            he= Existing rigid pavement thickness

            hd= Design thickness of rigid pavement

     f= Factor which depend upon modulus of existing pavement.

hb= 1.66(Fhd - he)

where, hb= thickness of the bituminous overlay

Intelligent Transportation System(ITS)

Intelligent Transportation Engineering(ITS) is a system which uses the new innovations and technology to control and regulate the traffic. This system is also used to provide the users the required information.
The new technologies like, Automatic Traffic signals, CCTV, wireless communication devices in the hands of the traffic police, digital methods to calculate the traffic  volume etc. are the technologies which come under ITS.
Transportation users are kept informed of the various transportation services as per their requirements. Suppose you have reserved your seat in a bus, and you are waiting at the bus station for the bus. If due to some reason the bus is late, you will get the information about the bus being late so you can plan your time accordingly. All the updated information about the arriving and departures of the buses can be displayed with the electronic devices so one is always informed about all the schedule.
The traffic volume can be calculated using the GPS techniques, or some other sensing technique in which each vehicle is sensed with an unique identification number.  The floating car method is best method to calculate the amount of traffic.
Traffic control is now much easier, because their are various new useful technologies available in this sector. Their are speed sensors, which can detect if you are over speeding on the highways. In some countries they have developed a technique to avoid the collision. Their is this indicator, which will warn you if their is any other vehicle coming at speed from the another side. Their are dynamic speed limit indicators. 

Tunneling (Transportation Engineering)

Hi there,

 A tunnel is needed when an obstacle in the form of a hill or rising ground is met within the alignment of a highway or railway track. The first tunnel , about 4000 years ago of a cross section of 3.6 m * 4.5 m; 910 meter long, was built in Babylon connecting two buildings.

In Europe the first tunnel was built by Roman Emperor Claudius in 54 AD for carrying spring water through Apennines.  The cross of the tunnel was 3.0 m * 1.8 m and its length was 5.8 Km. It was completed in 12 years by 30, 000 laborers.

This article covers the following aspects of Engineering related with the Tunneling:

1. Sections of tunnels: advantages, limitations and suitability of each section
2. Shaft, pilot tunnel.
3. Driving tunnel in rocks, sequence of construction operations
4. Full face method, heading and bench method, drift method
5. Driving tunnels in soft ground, sequence of construction operations, needle beam method
6. Shield tunneling, compressed air tunneling.

• Sections of tunnels, suitability of each section:

In general you will find the four types of tunnel sections which are famous in tunneling:

1. Circular shaped tunnels
2. Elliptical
3. Horse-Shoe shape tunnels
4. Vertical walls with arched roof(D-Shaped tunnels).

The shape of the cross section of a cement concrete lined tunnel will depend on the pressure of the ground which the lining must be able to resist and the purpose for which the tunnel is to be constructed.

If the ground is solid rock, then any shape may be adopted but for soft ground such as soft clay or sand, the pressure from the sides as well from the top is to be resisted. In such difficult situations, circular cross section is best suited if it is able to serve the purpose of the tunnel.

A circular tunnel, in general is not suitable for the highways because highways need a flat surfaces. Circular sections are more suitable for aqua-ducts.

D-shaped tunnels have a horizontal base( flat invert), two vertical sides and an arched roof. Such tunnels are best suited as the vehicular tunnels.

 The semicircular top acts as an arch, and thus takes and shifts the top soil load to the vertical sides and thus to the bed. If the ground bed is broken, subjected to the horizontal pressure, vertical wall sections of a vehicular tunnel may be replaced with a horse shoe section as it can resist the external pressure better.

Circular and elliptical tunnels are popular for water(irrigation) and sewage conduits while the horse shoe and vertical wall sections (D-Shaped) sections are popular for vehicular tunnels depending upon the condition and type of ground.

• Shaft, Pilot Tunnel:

Shafts are the vertical tunnels, generally circular in section. In case of the hydro projects you have to construct the surge shafts to prevent the water hemorrhage. In the highway projects surge shafts are constructed from the top to reach down to the main tunnel and provides the access path to the main tunnels.

A numbers of shafts may be constructed at places more than one in a long tunnel project, and work may be started from those numbers of places. Diameter of a shaft depends upon the purpose of the shaft, if a TBM is to be lowered to the main tunnel than it is necessary to make the shaft of the required size.

Similar to a shaft Pilot tunnels serves as the access tunnels to the main tunnels. The cross section of a pilot tunnel is usually 240 cm or a little bigger and are driven parallel to the main tunnel. The pilot tunnel is first driven to the full length of the tunnel and is connected to the center line of the main tunnel at many points. From these points, the work of the main tunnel may be started and also they make is easy to take out the muck.  Uses of the pilot tunnels may be summarized in the following points:

1. It helps in providing proper ventilation to the main tunnel.
2. It helps in removing the muck from the main tunnel quickly.
3. It helps in providing proper lighting in the main tunnel.

Pilot tunnels also offers a path to reach to the main tunnel so that you can access it to go for the further construction. Pilot tunnels are constructed generally parallel to the main tunnel, and when in connects to the main tunnel path, you get two faces/two directions to excavate your main tunnel.

• Driving Tunnel in rocks, sequence of construction operations:

Actual sequence of the tunneling operations, will depend upon the site conditions, size of the tunnel  and method of construction. However, the general sequence of the operations carried out during construction of a tunnel is as given below:

1. Setting up and drilling
2. Loading holes with explosives and firing them.
3. Ventilation and removing the dust after the explosion.
4. Loading and hauling muck.
5. Removing the ground water if necessity arises.
6. Erecting supports for sides and roofs in necessity arises.
7. Placing reinforcement.
8.  Placing concrete lining.

There are several methods of tunneling, the method to be selected for a particular site will depend upon the size of the bore, the equipment available, the condition of the formation and the extent to which the timbering is required. Tunneling may be grouped into two groups:

1. Tunneling in hard rocks
2. Tunneling in soft rocks.

Tunneling in hard rocks is carried by one the following methods:

1. Full face method
2. Heading and benching method
3. Drift Method
4. Pilot tunnel method
5. Perimeter method

Here we will discuss the first three methods in details.

• Full Face Method

This method of tunneling is adopted when the length of the tunnel is more than 3 meters. Large sized tunnels in rocks are always driven by this method. With the development of drill carriage this method is becoming more and more popular. In this method vertical columns are fixed to the face of the tunnel to which a large number of drills may be mounted or fixed at any suitable height as shown in the figure below. A series of drill holes are drilled at about 120 cm center to center in any number of desired rows, preferably in two rows. The size of the holes may vary from 10 to 40 mm. These holes are then charged with explosives and ignited. The muck is removed before the next operation of drilling holes.

• Heading and Benching Method

Tunnel cross section is divided into two parts, the top portion of the tunnel is known as the heading and the bottom portion as bench. Usually this method is adopted for railway tunnels. In this method of tunneling, top portion or heading will be about 3.70 to 9.6 m ahead of the bottom portion as shown in the figure below. In hard rock which may permit the roof to withstand without supports, the top heading  generally is advanced by one round of bottom portion. If the rock is broken then heading may be driven well ahead of the bottom portion and after giving proper support to the roof, the bottom portion is completed. In hard rock the heading is bored first and the holes are driven for the bench portion at the same time as the removal of the muck. This required less explosive than the full face method, but due to the development of the drill carriage or jumbo, the use of this method is decreasing.

Heading and Benching Method (Tunneling)

• Drift Method:

Drift is a small tunnel, usually its size is 3m*3m. In driving a large tunnel it has been found advantageous to drive a drift first through the full length or in a portion of the length of the tunnel prior to the excavating the full bore.

Drift Method (tunneling)

The drift may be provided at the center, sides, bottom or top as desired. In this method after driving the drift, the drill holes are drilled all round the drift in the entire cross section of the tunnel, filled with explosives and ignited. The rock shatters, the muck removed and the tunnel expanded to the full cross section.

Driving tunnels in soft ground

 While tunneling in soft grounds, explosives are not used and tunneling is done with the hand tools such as pick-axes, shovels etc. During excavation operation supports for soil are required immediately depending upon the type of soil. In the old days, timber was the only material used for supporting soft ground till the introduction of the steel liner plates few years ago. As heavy supporting system is needed to support the roof and sides, there is more obstruction in the movement inside the tunnel, which reduces the progress of the work. Care should taken to ensure that all struts should be sufficiently strong to bear the pressure coming on them. The method to be adopted in the soft ground tunneling depends upon the type of ground.

• Needle beam method, sequence of construction operations 

This method is useful for tunneling in the soft ground whose roof soil can stand without support for few minutes. In this method 5 to 6 meters long R.S. joist or timber beams are required in addition to other timber boards and struts. This method requires large number of jacks which cause obstruction in the efficient working of the laborers. For tunneling in soft ground it is more economical than other methods.

• Sequence of Working: 

1. First of all a small drift of size of about 1*1 m is prepared on the working face of the tunnel.
2. The needle beam consisting of two I girders, bolted together with a wooden block at the center, is inserted in the drift and its roof is supported on lagging carried on the wooden segment as shown in the figure below. These segments are supported by jacks resting on the needle beam.

   

   Needle beam method- 1

3. As shown in the figure below, the needle beam is placed horizontally, whose front end rests on the drift itself and the rear end is supported on the vertical stout post, resting on the lining of the tunnel.

   

   Needle beam -2

4. The jack is placed on the top of the beam(Needle Beam) to support the roof with lagging and then drift is widened side ways and the whole section is excavated. After excavating lining may be provided. 

• Compressed Air Tunneling Method:

This method is considered as most modern method of tunneling in soft grounds having water bearing stratum. A compressed air is forced into the enclosed space to prevent the collapse of the roof and sides of the tunnel. 

Usually air is used in conjunction with a shield and air-tight locks. However, numerous small tunnels have been driven using only linear plates or wood cants only. This method can be safely adopted if the air pressure is approximately 1 kg/cm^2. If the pressure is more than 1 kg/cm^2, the working hours should be reduced considerably which will increase the cost of tunneling.

Application of the air pressure to the tunneling is not so simple due to the following reasons:

1. The earth pressure varies from the top of the tunnel to the bottom of the tunnel.
2. As the pressure in the floor of the tunnel depends upon the nature of the strata, it is difficult to ascertain it theoretically.
3. The value of pressure varies with the moisture content in different strata, which is difficult to ascertain. 
4. The compressed air will escape through the pores of the soil, hence air pressure will diminish continuously. Thus the value of air pressure will have to vary from time to time to get a balanced value and the determination of this value depends more on experience than theoretical considerations. 

This method is ideally suitable for clay formations which do not contain large number of pores and the pressure does not vary much from top of the tunnel to its bottom. 

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Highway Geometrics- Short introduction

Hi,
You must know the various highway cross section elements if you are a highway engineer. You might have the introduction with the cross section elements like friction, camber, formation width, road width, super elevation and sight distance etc. To design a safe and economic road you must consider all these elements.
For safe and efficient running of the traffic it must have the required formation width. For the durability of the roads, it must have the required camber. Super elevation is required at the horizontal curves to keep the fast moving vehicles on the track. Sight distance is again very important in terms of safety and therefore for effective traffic movement. You must consider the Codal specifications to look for the standard values. In India Indian Roads Congress is an active organization which provides the specifications for design. In USA it is AASTHO.
If you are a highway engineer, then remember that you have the responsibility of the safety of the vehicles and the lives in terms of the road serviceability. Do your best and keep rest up to God.
If you have any suggestions to improve the article please leave a comment.
Thank you for reading.

Tips for Preparation for GATE (M. Tech & PSUs)examination.(Inspirational)

Hi,
I hope you are ready for it.

If you clicked on this link, that means you are someone, who is appearing for the GATE 2014, and you are planning to crack the GATE with the 99 percentile, so that you can get the entrance to the M.Tech. courses in the IITs or the entrance to the top PSU companies.
The bad news is that it is not only you, there are a numbers of students, or graduates, who are planning to get the same out of the GATE 2014.
Now all can not get the 99 percentile score, but it will be the only those who perform better than the other 99% of the appearing. What are the resources at the present moment?
(1) Time(everybody has the same 24 hours per day)
(2) Books
(3) Mind.
So these are three resources which everybody have right now, and only those will get into the top % who know how to use these three resources in a best way. The first resource i.e. time is same with everyone, and it is 24 hours in a day.

The difference arises in the use of these 24 hours by different people in different way. Now to get into the top 1% use your time you have with your books, by applying the mind in a right manner.
Become your self observer, and evaluate yourself. Ask your self if you are using these resources to your full potential? Keep asking until you get the honest answer "yes".

1.Time:  Time is equal for all. What is needed is a proper utilization of it. There is a need of a proper time table in which you have to give time for every essential activities along with the time for the preparation for the GATE exam. Preparing the time table is one thing and then following it regularly is the other part, which makes this resource fruitful for you.

2. Books: If the time is enough for the preparation, then you should read the text books in details, from chapter to chapter to get the deep understanding of the concepts. In the last days it is advised to revise your syllabus by performing some test on your self. For all this you need the books. Get the latest books, of the known publishers and read them.
Remember a proper,deep understanding is needed to crack the GATE. I am not going to refer any books here, all you have to do is, go to the nearby engineering  books shop, and then search for the book for yourself.

3. Mind: It is our mind which has to store all the knowledge and the data, and then to process it to give the right answers for the questions you have to face. So it is important to keep your mind alert.

I will suggest the following:
(1) Eat healthy food for your mind.(Search internet for the foods which affect the functioning of the mind.)

(2) Do the regular meditation or exercise to get the concentration.

So when your mind is okay, you can perform well with your preparation, which will give you the required result.  "A sound mind lives in a sound body".


I have bought a hand book prepared by this coaching institute named as Made-easy, established in Dellhi. It contains all the subject matter in short and can be helpful to you.

Cement concrete pavements, pre-stressed pavements

Cement concrete pavements: 

1. Cement concrete pavements comes under the category of the rigid pavement, due to the higher flexure strength of the concrete slab. 
2. CC pavements have the longer life, because of the higher durability and high load carrying capacity.
3. CC pavements require very less maintenance, so the maintenance cost is reduced.
4. CC pavements have very good aesthetic appearance.

There are some disadvantages of the CC pavements,

1. Initial cost of construction of the CC pavements is very high. 
2. The provision of the joints is mandatory.

Pre-stressed concrete pavements:

Pre-stressed concrete pavements are constructed by using the pre-stressing techniques. The lower layer of the concrete slab is compressed from its normal state, with the help of the high tension cables.

The cable are laid on the position and are applied with the high tensile stresses on both the sides. When the bar is in the tension state, the concrete is poured in and is allowed to set. 

When the tension arrangement of the bars is removed the bars tries to regain its original shape, and it results in the induction of the bond stress between the concrete and the bars. So ultimately the concrete is subjected the compressible stress. When the loading is applied in the transverse direction from the top, the lower layers which are subjected to the tensile stresses, the tensile stresses are neutralized with the induced compressible stresses. So the cracks are not formed due to flexure.

So it is possible to construct a slab of length up to 120m without the transverse joints.

Construction of WBM roads, WMM - Wet mix macadam - Transportation Engineering

Hi, 

Here we are going to discuss the construction of the WBM and WMM roads in brief. 

WBM and WMM roads

• (I) - WBM (Water Bound macadam)  roads construction: 

The water bound macadam road construction technique was given by the John Macadam. This technique in present day is used as given below.

For WBM construction we use three materials:

1. Aggregates
2. Screeners
3. Binders.

Aggregates:

We use the aggregates of different grades. IRC(Indian Roads Congress) has classified the coarse aggregates into 9 grades, according to their size.

For the construction of the WBM roads aggregates are used in the sub-base, base and surface course and so the aggregates are divided into 3 grades according to their size.

Grade 1 - particles of size 90 mm to 40 mm.

Grade 2 - particles of size 63 to 40 mm.

Grade 3 - particles of size 50 to 20 mm.

The grade 1 aggregates having size of 90 mm to 40 mm are preferred for the sub-base material and grade 2 for the base and grade 1 for the surface course. However, if we only use the WBM as the surface course, it gets deteriorated fast due to abrasion with the traffic so, bituminous surfacing over the WBM is general practice.

Screeners are the aggregates of the smaller sizes, generally 12.5 mm or 10 mm, for grade A and grade B.  They are of the same chemical composition as of the coarse aggregates. 

For economic considerations IRC has suggested non plastic materials such as, crushed over burnt bricks, moorum, gravels,  etc. provided the liquid limit of the material is less than 20%, plasticity index is less than 6.0% and the portion of fines passing 0.075 mm sieve is less than 10%. 
However if crush-able type of aggregates are used, use of the screeners may be disposed off.

Binders:  

Binders, are the layers of materials which are laid after the compaction of the aggregates and the screening materials one after the another. Kankar dust or lime stone dust may be utilized if locally available. 

The binding material with plasticity index value of 4% to 9% is used in surface course construction; the plasticity index of binding course material should be less than 6% in the case of the WBM layers used as base course or sub-base course, with bituminous surfacing. 

However if the screening used are of crushable material like moorum or soft gravel, there is no need to apply binding material, unless the plasticity index value is low.

• (II) - WMM(Wet mix macadam) road construction:

Aggregates used are of the smaller sizes, varies between the 4.75 mm to 20 mm sizes and the binders(stone dust or quarry dust having PI(Plasticity Index) not less than 6%) are premixed in a batching plant or in a mixing machine. Then they are brought to the site for overlaying and compaction.

The PI(plasticity Index) of the binding material is kept low because it should be a sound and non plastic material. If the plasticity index is more then there are the chances of the swelling and more water retention properties. So this value should be kept in mind.

• Comparison of the WBM and WMM road construction:

Although the cost of construction of the WMM is said to be more than that of the WBM sub-base and bases but the advantages given below will compensate for that. Here are the points of difference:

1. The WMM roads are said to be more durable.
2.  The WMM roads gets dry sooner and can be opened for traffic withing less time as compare to the WBM roads which take about one month for getting dry.
3. WMM roads are soon ready to be black topped with the Bituminous layers.
4. WMM roads are constructed at the faster rate.
5. The consumption of the water is less in case of the WMM roads.
6. Stone aggregates used in WBM is larger in size which varies from 90 mm to 20 mm depending upon the grade but in case of the WMM size varies from 4.75 mm to 20 mm.
7. In case of WBM, stone aggregates, screenings and binders are laid one after another in layers while in WMM, aggregates and binders are premixed in the batching plants and then brought to the site for overlaying and compacting.
8. Materials used in the WBM are the stone aggregates, screenings and binder material(Stone dust with water) while in WMM material used are only stone aggregates and binders.
9. Quantity of the WBM is generally measured in cubic meters while that of the WMM in square meters.

References: Highway Engineering by S.K. Khanna and C.E.G.Justo

                & Wiki answers.

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