Types of Canal Lining
The following are the different types of canal lining which are generally recommended according to the various site conditions.
1. Cement concrete lining
2. Pre-cast concrete lining
3. Cement mortar lining
4. Lime concrete lining
5. Brick lining
6. Boulder lining
7. Shot Crete lining
8. Asphalt lining
9. Bentonite and clay lining
10. Soil-cement lining
Cement concrete lining
This lining is recommended for the canal in full banking. The cement concrete lining ( cast in-situ) is widely accepted is as the best impervious lining. It can resist the effect of scouring and erosion very efficiently. The velocity of flow may be kept above 2.5 m/sec. It can eliminate completely growth of weeds. The lining is done by the following steps –
(a) Preparation of sub-grade
The sub-grade is prepared by ramming the surface properly with a layer of sand (about 15 cm). Then, a slurry of cement and sand (1: 3)is spread uniformly over the prepared bed.
(b) Laying of concrete
The cement concrete of grade M15 is spread uniformly according to the desired thickness (generally, the thickness varies from 100 mm to 150 mm). After laying, the concrete is tapped gently until the slurry comes on the top. The curing is done for 2 weeks. As the concrete is liable to get damaged by the change of temperature, the expansion joints are provided at appropriate places. Normally no reinforcement is required for this cement concrete. But in special cases, a network of 6 mm diameter rods may be provided with spacing 10 cm centre to centre.
Pre-cast concrete lining
This lining is recommended for the canal in full banking. It consists of pre-cast concrete slabs of size 60 cm x 60 cm x 5 cm which is set along the canal bank and bed with cement mortar(1:6). A network of 6 mm diameter rod is provided in the slab with spacing 10 cm centre of the centre. The proportion of the concrete is recommended as 1:2:4. Rebates are provided on all the four sides of the slab so that proper joints may be obtained when they are placed side by side. The joints are finished with cement mortar (1:3). Expansion joints are provided at a suitable interval. The slabs are set in the following sequence –
(a) The sub-grade is prepared by properly ramming the soil with a layer of sand. The bed is levelled so that the slabs can be placed easily.
(b) The slabs are stacked as per estimate along the course of the canal. The slabs are placed with cement mortar(1:6) by setting the rebates properly. The joints are finished with cement mortar (1:3).
(c) The curing is done for a week
Cement mortar lining
This type of lining is recommended for the canal fully in cutting where hard soil or clayey soil is available. The thickness of the cement mortar (1:4) is generally 2.5 cm. The sub-grade is prepared by ramming the soil after cutting. Then, over the compacted sub-grade, the cement mortar is laid uniformly and the surface is finished with neat cement polish. This lining is impervious but is not durable. The curing should be done properly.
Lime concrete lining
When hydraulic lime, Surkhi and brick ballast are available in plenty along the course of the canal or in the vicinity of the irrigation project, then the lining of the canal may be made by the lime concrete of proportion 1:1:6. The procedure of laying this concrete is the same as that of the cement concrete lining. Here, the thickness of concrete varies from 150 mm to 225 mm and the curing should be done for a longer period. This lining is less durable than the cement concrete lining. However, it is recommended because of the availability of the materials and also because of economics.
This lining is prepared by the double-layer brick flat soling laid with cement mortar (1:6) over the compacted sub-grade. The first class bricks should be recommended for the work. The surface of the lining is finished with cement plaster (1:3). The curing should be done perfectly.
This lining is always preferred for the following reasons –
(a) This lining is economical.
(b)Work can be done very quickly.
(c) Expansion joints are not required.
(d)Repair works can be done easily.
(e) Bricks can be manufactured from the excavated earth near the site.
However, this lining has certain disadvantages –
(a) It is not completely impervious.
(b) It has low resistance against erosion.
(c) it is not so much durable.
In hilly areas where the boulders are available in plenty, this type of lining is generally recommended. The boulders are laid in a single or double-layer maintaining the slope of the banks and the bed Label of the canal. The joints of the boulders are grouted with cement mortar(1:6). The surface is finished with cement mortar (1:3). Curing is necessary for this lining too. This lining is very durable and impervious. But the transporting cost of the material is very high. So, it cannot be recommended for all cases.
Shot Crete lining
In this system, the cement mortar (1:4) is directly applied to the subgrade by the equipment known as cement gun. The mortar is termed as shot create and the lining is known as shotcrete lining. The process is also known as the guniting, as a gun is used for laying the mortar. sometimes, this lining is known as gunited lining. The lining is done in two ways –
(a) By dry mix
In this method, a mixture of cement and moist sand is prepared and loaded in the cement gun. Then it is forced through the nozzle of the gun with the help of compressed air. The mortar spreads over the sub-grade to a thickness which varies from 2.5 cm to 5 cm.
(b) By wet mix
In this process, the mixture of cement, sand and water is prepared according to the approved consistency. The mixture is loaded in the gun and forced on the sub-grade.
This type of lining is very costly and it is not durable. It is suitable for resurfacing the old cement concrete lining.
This lining is prepared by spraying asphalt (i.e. bitumen) at a very high temperature ( about 150-degree Celsius) on the subgrade to a thickness varies from 3 mm to 6 mm. The hot asphalt when becomes cold forms a waterproof membrane over the sub-grade. This membrane is covered with a layer of earth and gravel. The lining is very cheap and can control the seepage of water very effectively but it cannot control the growth of weeds.
Bentonite and clay lining
In this lining, a mixture of bentonite and clay are mixed thoroughly to form a sticky mass. This mass is spread over the sub-grade to form an impervious membrane which is effective in controlling the seepage of water. but it cannot control the growth of weeds. This lining is generally recommended for small channels.
This lining is prepared with a mixture of soil and cement. The usual quantity of cement is 10% of the weight of dry soil. The soil and cement are thoroughly mixed to get a uniform texture. The mixture is laid on the sub-grade and it is made thoroughly compact. The lining is efficient to control the seepage of water, but it cannot control the growth of weeds, So, this is recommended for small channels only.
Book – Irrigation Engineering, Writer – N N Basak
When water flows under gravitational pull through the artificial canal towards the agricultural land, it is known as Flow irrigation. In this system, the head of the canal should always be at a higher elevation than the land to be irrigated. The Flow irrigation can be of two types, Inundation irrigation and Perennial irrigation.
1. Inundation irrigation
In Inundation irrigation, the canals are excavated from the bank of the inundation river (i.e .the river which overflows in the rainy season but nearly dried up in summer and winter).
In this case, water flows to the agricultural land in rainy season only. There is no regulator at the head of the canal to control the flow of water.[Fig.(a)]
The bed level of the canal is fixed at such level that the water can flow through the canal only when the water level of the river rises above the canal bed. [Fig.(b)]Again, the flow of water through the canal stops automatically when the water level of the river falls below the canal bed.[Fig.(c)]
So, this system of irrigation depends completely on the water level of the river.
As there is no regulator at the head of the canal, over-irrigation is possible resulting in damaging the crops.
2. Perennial irrigation
In Perennial irrigation, either a weir or a barrage is constructed across the perennial river(i.e., the river which flows throughout the year in its full capacity) to raise the water level on the upstream side or a dam is constructed to form a storage reservoir. Then the main canal is constructed on either or both the banks of the river. The regulator is constructed at the head of the canal to control the flow of water through the canal towards the agricultural land. This system is reliable as water is available throughout the year. The perennial system of irrigation may be of the following types:
(a) Direct irrigation system
In this system, a weir is generally constructed across a perennial river to raise the water level on the upstream side up to a certain limit so that the water can flow through the canal. Here, the water level on the upstream side will remain at a constant height and the excess water flows over the weir. Sometimes, a barrage is constructed, in place of a weir, to regulate the water level on the upstream side. The hydraulic structure which is constructed in a direct irrigation system is known as Diversion Head Works.[Fig.(1.2)] (b) Storage irrigation system
In this system, a dam is constructed across a river valley to form a storage reservoir.[Fig.(1.3)]The main canals may be taken from both sides of the dam. The flow of water through the canal is controlled by head regulator. This storage reservoir is also known as a multipurpose reservoir as it serves the following purposes:
(b) Water supply
(c) Hydro-electric power generation
(e) Flood control
Book – Irrigation Engineering, Writer – N N Basak
Definition of Irrigation
The process of artificial application of water to the soil for the growth of agricultural crops is termed as irrigation. It is practically a science of planning and designing a water supply system for the agricultural land to protect the crops from the bad effect of drought or low rainfall. It includes the construction of weirs, dams, barrages and canal systems for the regular supply of water to the culturable (i.e. cultivable) lands.
Necessity of Irrigation
Throughout the crop period, adequate quantities of water are required near the root zone of the plants for their growth. At times during the crop period, the rainfall may not be adequate to fulfil the water requirement. The intensity of rainfall is practically uncertain and beyond the control of human power and it may not be well distributed throughout the crop season or the culturable area. So, irrigation becomes absolutely necessary to fulfil the water requirement of crops. The following are the factors which govern the necessity of irrigation.
When the seasonal rainfall is less than the minimum requirement for the satisfactory growth of crops, the irrigation system is essential.
(b) Uneven distribution of rainfall
When the rainfall is not evenly distributed during the crop period or throughout the culturable area, the irrigation is extremely necessary.
(c) Improvement of perennial crops
Some perennial crops like sugarcane, cotton, etc. require water throughout the major part of the year. But the rainfall may fulfil the water requirement in rainy season only. So, for the remaining part of the year, irrigation becomes necessary.
(d) Development of agriculture in a desert area
In a desert area where the rainfall is very scanty, irrigation is required for the development of agriculture.
Benefits of irrigation
The following are the important benefits of irrigation:
(a) Yield of crops
In the period of low rainfall or drought, the yield of a crop may be increased by the irrigation system.
(b) Protection from famine
The food production of a country can be improved by ensuring the growth of crops by availing the irrigation facilities. This helps a country to prevent famine situation.
(c) Improvement of cash crops
Irrigation helps to improve the cultivation of cash crops like vegetables, fruits, tobacco, etc.
(d) Prosperity of farmers
When the supply of irrigation water is assured, the farmers can grow two or more crops in a year on the same land. Thus the farmers may earn more money and improve their living standard.
(e) Source of revenue
When irrigation water is supplied to the cultivators in lieu of some taxes, it helps to earn revenue which may be spent on other development schemes.
The irrigation canals may be utilised for inland navigation which is further useful for communication and transportation of agricultural goods.
(g) Hydroelectric power generation
In some river valley projects, multipurpose reservoirs are formed by constructing high dams where hydroelectric power may be generated along with the irrigation system.
(h) Water supply
The irrigation canals may be the source of the water supply for domestic and industrial purposes.
(i) General communication
The inspection road along the canal banks may serve as a communication link with the otherwise remote villages.
(k) Development of fishery
The reservoir and the canals can be utilised for the development of fisher projects.
ILL-effects of irrigation
The following are the ill-effects of irrigation:
(a) Rising of the water tables
Due to the excessive seepage of water through the bed and banks of canals, the water table in the surrounding area may be raised which may constantly saturate the root zone of the crops and the soil may develop alkaline property which is harmful to the crops.
(b) Formation of marshy land
Excessive seepage and leakage of water from the irrigation canals may lead to the formation of marshy lands along the course of the canals. These marshy lands form the colonies of mosquitos which may be responsible for diseases.
(c) Dampness in weather
The temperature of the commanded area of an irrigation project may be lowered considerably and the area may become damp. Due to dampness, the people residing around the area may suffer from cold, cough and other such diseases originating from dampness.
(d) Loss of valuable lands
Valuable land may get submerged when storage reservoirs are formed by constructing barrages or dams and, it also may be lost while constructing irrigation canals.
Book – Irrigation Engineering, Writer – N N Basak
When water is lifted from surface sources or underground sources by man or animal power, mechanical or electrical power and directly supplied to the agricultural land, then it is known as Lift Irrigation. In this method, isolated small areas can be irrigated. The vast areas cannot be included in this system. lift irrigation can be divided into two groups.
(a) Lifting of water by man or animal power.
(b) Lifting of water by mechanical or electrical power.
When mechanical or electrical powers are not available in the villages or the economic condition of the farmers is not good enough to afford this expensive method, the lifting of water is done by the following method from the surface sources( like ponds, lakes, rivers, etc):
(3) Persian wheel
(4) swinging busket
(6) Rati or pulley
(7) wind lass
When mechanical or electrical power is available in villages or the farmers can afford the expenditure for the installation of the same, the underground water is lifted by pumps( diesel pump set or electrical pump set) and directly supplied to the agricultural land. The underground water may be available from the following sources:
(1) Open well
(2) Shallow tube well
(3) Deep tube well
Nowadays, the pumping system of lift irrigation from Shallow or deep tube well is widely practised.
Advantages of Lift irrigation
The following are the advantages of lift irrigation
(1)The farmers can supply water to their fields according to the requirement, and hence there is no possibility of over – irrigation.
(2) The water table is lowered when water is lifted from the wells thereby reducing chances of waterlogging in the area.
(3) As water is supplied directly to the fields, there is no water loss due to conveyance.
(4) The initial cost is low as there is no necessity of constructing hydraulic structures.
(5) As the loss of water is low, the duty of water is high.
(6) The maintenance cost is low.
(7) More than one crop can be grown in a year on the same land.
(8) Loss of valuable land is prevented as there is no necessity of constructing the network of canals.
(9)The water of the well is cooler in the hot season and warmer in the cold season.
This phenomenon is favourable for the crops.
Disadvantages of Lift Irrigation
The following are the disadvantages of lift irrigation.
(1) In summer the surface water may be dried up and the water table may go down below the suction head. Hence, the lift irrigation from the surface source and from the shallow tube well May fail in summer.
(2) If the lifting mechanism(i.e. pump) fails due to mechanical or electrical failure, then water cannot be supplied until the mechanism is restored.
(3) The well water has no silt content. The yield of a crop, therefore, depends on chemical fertilizer, which is costly.
(4) The lift irrigation is a consideration with the yield of the crop is not cost-effective.
Book – Irrigation Engineering, Writer – N N Basak
Tender is a return offer submitted by contractors in response to tender call notice, to execute certain work or supply of specified materials or transport of materials at certain rates, within a fixed time with the terms and conditions laid down in the tender documents. The tender duly filled in is placed in the locked tender box kept in the room of the officer inviting the tender on or before the specified date and hours.
A Tender form is a printed standard form of contract giving terms and conditions of tenders.
It is obtained from the office of the officer inviting tender on payment. The amount of payment called the ‘cost of the tender form’ is decided by the department concerned inviting the tender. In the tender form, there is also a memorandum forgiving (1) general description of work (2) estimated cost, (3) time allowed for the work from date of written order to commence, (4) estimated cost, (5) earnest money, (6) columns for signature of the officer by whom accepted and signature of contractor before submission of tender, signature of witness to contractor’s signature. It is a part of a tender document.
Before inviting tenders, the following documents are carefully prepared:
(1) Complete set of drawings, showing full details of work to be executed.
(2) Detailed specifications of each item of work, along with the schedule of quantities is notified in tender call notice. If the reference to P.W.D specifications is to be made, it should be a specifically mentioned against each item.
(3) List of materials to be issued by the department, giving rates of each item and their place of issue.
(4) Conditions of the contract are printed on the form. The additional set of conditions required for the specific work are also notified along with general conditions.
(5) If it is decided to deduct security at a rate less than 10% from runnings bills of contractors, it should be notified in the tender call notice.
(6) Authority to reject any or all tenders without the assignment of any reason, should always be reserved and notified in tender call notice.
Documents mentioned above are prepared by the Head Clerk and checked by Head Draughtsman and Divisional Accountant and got approved from the competent authority, before actual issue of the tender.
Tender notice or Notice inviting tender (N.I.T)
Following instructions should be followed:
* (1) As soon as Administrative approval, Technical sanction and Appropriation, are conveyed and contract documents are approved, sealed tenders are invited from approved P.W.D contractors. N.I.T. is prepared on standard forms by the Head clerk.
* (2) Before N.I.T is actually issued. The Divisional accountant shall verify that the amount of earnest money to accompany the tenders is correct as per rules and schedule of quantities and specifications of work are as per sanctioned estimate.
* (3) Wide publicity is given to the tender call notice as per procedure is given below:
(a)Works costing rupees 10, 00, 000
(1) Three weeks clear notice is given to contractors.
(2) Tender notice is advertised in leading dailies.
(3) Letters are issued individually to the eligible contractors as per rules of the department.
(4) Copies of tender call notice are sent to all divisions of the department for exhibiting on their notice boards.
(b) Works costing over rupees 1,00,0001 and up to rupees 10,00,000.
(1) Two weeks clear notice is given to contractors.
(2) Notice is not advertised in the press. Remaining instructions mentioned above shall be followed.
(c) Work costing up to rupees 1,00,000
(1) Ten days clear notice is given to contractors.
(2) Copies of notice are sent to all divisions and subdivisions of the concerned circle for displaying on notice boards.
* (4) N.I.T. should clearly state the “Name of work” as given in sanctioned estimate.
* (5) The tender notice should specify the place where, and the time when, contract document can be seen, and the place where blank forms of tenders are available and cost of each form.
* (6) N.I.T. should specify the place and last date for receipt of tenders and place, date and time of the opening of tenders.
* (7) The amount of earnest money (with the mode of payment) to a company the tenders.
* (8) The tenders are required to specify the period during which they agree to complete the work.
* (9) Tenders should be informed that they shall initial corrections if any, as regards percentage,
time etc. otherwise their tenders are liable to be rejected.
For very big and specialised job or supply of any item, Global tenders are sometimes invited by the concerned authority throughout the global to get competitive offers from a specialised firm throughout the world. In the notice inviting global tender, the word global tender notice is written on the heading of the notice. The particulars and contents of the tender notice are same as that of ordinary tender notice. The Global tender notice is circulated to the leading newspaper which covers most of the countries or to the local newspaper of outside countries. It is also circulated through the embassies of important countries.
Opening of tenders
The tenders are opened by the Executive Engineer at the time and place already notified, in the presence of head clerk, divisional accountant and tenderers or their representatives who may like to be present. Particulars of each tender in the tender register by the Executive Engineer. The officer opening the tenders has to read out the rates offered in each tender for information of all those present. The tenders received without the required amount of the earnest money should be rejected and a record of such cases should be kept in the tender register.
The Following procedure should be followed by the Executive Engineer, while opening tenders, to avoid tampering of original tender documents:
(1) All corrections in the contract document should be initialled and dated by the Executive Engineer.
(2) The officer concerned should mark all corrections and overwriting and number them and attest them in the red link. Separate numbers should be allotted for corrections and overwritings. The total number of corrections should be mentioned at the end of the concerned page initialled. Omissions if any, and ambiguity in the words or figures of rates quoted, should be clearly pointed out on each page of the schedule, attached to the tender concerned. Where rates have been quoted in rupees and no ‘P’ mentioned, the word only should be added after rupees, corrections initialled and remarks given at the end.
Normally, no conditional tender should be accepted. But where no financial implication is involved or the tender remains lowest even after taking into account financial implication, it may be considered on merit.
A tender is said to be informal:
(1) When it is not supported by the requisite earnest money in the prescribed manner.
(2) When it is not submitted in the tender form sold by the department or by the due date and time notified in the notice inviting tender.
(3) When the tender form is not properly filled in or signed by the contractor.
In item rate tender, contractors quote their rates for each and every individual item. But on the basis of information from any source regarding the item of work as which are likely to be increased and which are likely to be decreased, a contractor quote the high rate for the items likely to be increased and less rate for items likely to be decreased.
If the information to the contractor proves to be correct the tender becomes unbalanced and the department lose heavily if the work is awarded to the contractor. On the other hand if the information proves to be wrong the contractor would stand to lose. For item rate tenders unbalanced tender may occur.
An agreement which is enforceable by law is a contract. It is an undertaking given by a person (not being a Govt. servant), Syndicate or firm, for the execution of any work or supply of materials or performance of any service in this connection.
This term is applied normally to a person (other than a Govt. servant), Syndicate or firm, giving an undertaking for the execution of any work or supply of materials or performance of any service in that connection.
Qualification of contractor
The following categories of contractors are generally enlisted with P.W.D:
1.For building and roads
2.For sanitary installations and water supply
3.For electrical, and Air conditioning
Each of these categories of contractors should be competent enough to deal with the work entrusted to them and have an engineering organisation to do the work. For 2 and 3 of works, contractors should have valid licence to perform the work. Each contractor is expected to have the required equipment and machinery for the job. The qualification of contractors are further adjudged from (a) their financial resources (b) their professional ability to understand and implement the contractual obligation (c) their ability to control labour (d) the income tax clearance certificate.
After the Gujarat earthquake, Govt. has introduced a bill to change the qualification required for building contractor. If the contractor is an individual then he must have civil Engg. Degree and if it a firm, then they must have civil engineers with them.
Essentials of contracts
The essential elements of a contract are:
- Minimum two parties: Minimum of two parties are necessary to form a contract because one person cannot enter into a contract with himself. In order to form a contract, one party has to make an offer and the other must accept it. The person who makes the offer is called the ‘promisor’ and the person who accepts the offer is called the ‘promisee’.
- Offer and acceptance: There must be an ‘offer’ and ‘acceptance’ to the offer in any contract. Both ‘offer’ and ‘acceptance’ should be lawful.
- Competent parties: The parties to a contract must be competent i.e., they are of the age of majority, of sound mind and not disqualified form contracting by any law to which they are subjected.
- Free consent: To form a valid contract, all parties should give their free consent. Consent means that the parties must agree upon the same thing in the same sense, consent should be free unless it is induced by undue influence, fraud, misrepresentation and mistake.
- Intention to create legal obligation: The parties must have the intention to create a legal obligation. In a commercial agreement, an intention to create legal relations is presumed, unless the parties have expressly agreed to otherwise.
- Lawful object: The object of an agreement is said to be lawful if it is not fraudulent or illegal or immoral or opposed to public policy and also it should not imply injury to the person or property of another.
- Not expressly declared void: An agreement expressly declared to be void under any law, is not enforceable and is, thus, not a contract.
Different types of contract
Contracts are of the following types:
(A) Item rate contract, including
(1) percentage rate contract
(2) Schedule rate contract
(3) Labour rate basis
(4) Through rate basis
(B) Lumpsum contract
(C) Combination of both item rate and lumpsum contract.
(A) Item rate contract
In this type of contractor, the contractors have to quote the rate of each item of the work without reference to any schedule of rates. This method is adopted when a reasonably complete schedule of rates is not available for the place where the work is to be carried out or when the number of non-schedule items in the work is more.
- Percentage rate contract
In this type of contract, the rates for various items are fixed by the department and the contractor agrees to do the works at a percentage above or below fixed rates. This method is adopted for works at places where a reasonable and complete schedule of rates is available.
- Schedule rate contract
In this type of contract, the work is allotted at fixed rates for different items and the payment depends upon the quantity and kind of work done or supply made.
- Labour rate basis
In some cases, the contractor quotes the labour rate at some percentage above or below the rates prevailing in the area for various items of works.
- Through rate basis
In some cases, the contractor quotes the rates for doing work some percentage above or below the through rates.
- Lumpsum contract
In this type of contract, the contractors have to quote a lumpsum figure for completing the work according to the given specification, drawings etc. This type of contracts is given only in exceptional cases. The contracts must include the necessary conditions to safeguard and protect the interest of the government.
- Combination of both
In this contract, a fixed sum is agreed upon for the completion of a particular work by the contractor. In case of any addition or alterations, the payment is made or deducted on the basis of a schedule of rates.
Advantages and Disadvantages
Item rate contract
- This form of contract ensures a more detailed analysis of cost by the contractor and is more scientific.
- The contractor is required to quote rates for individual items of work and hence it is not easy to form a ring during submission of tender and to allot a work to one of the contractors without competition.
- This form of contract helps in smooth progress and timely completion of a work because the unworkable rated tender may be avoided.
4 Intermediate payment to contractors for the completed item is easier.
- In this type of contract, there is always a loophole for a contractor to correct a rate in order to be the lowest tenderer.
- This type of contract is not always advantageous when the quantities of certain items are to be increased or decreased. If by anticipation or by some other source a contractor comes to known the items whose quantities are likely to be increased may quote a high price for these items and similarly can quote low prices for items likely to be decreased. In that case, the department would stand to lose heavily due to unbalanced tender.
- Preparation to a comparative statement of this type of tender requires more time and through scrutiny required for this type of tenders.
- Chances of rejection of such type of tenders are more because of human error like overwriting and cutting and not doing the initial to their overwriting and cutting in the quoted rates of items by the contractor.
- It requires accurate measurements of all the items of work.
- The cost of the work is known before hand.
- Detailed measurements of the work done or not required except in case of any addition and alteration.
- Since there is difficulty in making intermediate payment to contractors in this type of contract, every contractor has a tendency to complete the work quickly, resulting in the early completion of work.
- Proper planning and efficient management for the execution of work are more convenient.
- Making an intermediate payment to contractors is not so easy.
- Detailed drawing with full information and specification is essential to avoid any dispute.
- The materials stored in Government are utilised.
- Delay of work due to non-availability of materials in the open market is avoided. Thus better progress with standard quality of materials can be maintained.
- The increase in the cost of work is checked in spite of any rise in the price of materials in the market.
- There may be a delay in the obtaining materials from the department stores resulting in the delay in completion of the work.
- A large storage area is required by the department to store the materials.
- Theft of materials from a store, shortage of material and damage of material are constant worries for a department.
- Reconciliation of material is essential in this type of contract.
Book- Civil estimating & Costing, Writer- A.K. Upadhyay
Types of flow
Fluid flows are classified as:
- Steady and unsteady flow
- Uniform and non-uniform flow
- Laminar and turbulent flow
- Compressible and incompressible flow
- Rotational and irrotational flow
- Ideal and real flow
- One, two and three-dimensional flow
Steady and unsteady flow
Steady flow is that type of flow in which fluid parameters (velocity, pressure, density etc.) at any point in the flow field do not change with time. This means that the fluid particles passing through a fixed point have the same flow parameters like velocity, pressure, surface tension etc. The parameters may be different at the different cross-section of the flow passage.
Mathematically, a steady flow is defined as
Unsteady flow is that type of flow in which fluid parameters (velocity, pressure, density etc.) at a point changes with time.
Mathematically, an unsteady flow is defined as
Uniform and Non-uniform flow
Uniform flow is defined as that type of flow in which the velocity of flow of a fluid at any instant does not change with respect to space. In other words, it is the flow in which the velocity of flow remains constant throughout the flow field at any given time.
Mathematically, for uniform flow
where ∂ Ʋ = change in velocity
∂ s = length of flow in a direction, s.
Non-uniform flow is defined as that type of flow in which the velocity of flow changes with respect to space at any given time.
In other words, it is the flow in which the velocity of flow is different for a different section in the path of flow.
Mathematically, for non-uniform flow
Laminar and Turbulent flow
Laminar flow is defined as that type of flow in which each fluid particle has a definite path and paths of individual particles do not cross each other.
Laminar flow is also called streamline or viscous flow. This type of flow occurs in smooth pipes having the low velocity of flow. It also occurs in liquids having high viscosity.
Turbulent flow is defined as that type of flow in which each fluid particle does not have a definite path and the paths of individual particles cross each other.
In other words, it is the flow in which fluid particles move in a zigzag path.
When a fluid is flowing in a pipe, the type of flow is determined by a non-dimensional number, called Reynold’s number.
For laminar flow, Reynold number ˂ 2000
For turbulent flow, Reynold number ˃ 4000
Compressible and Incompressible flow
The flow in which the density of fluid changes, due to pressure and temperature variations, from point to point during the flow is called compressible flow.
In other words, it is the flow in which the density of a fluid is not constant during the flow.
Mathematically, for compressible flow
P ≠ constant
The flow in which the density of fluid does not change during the flow is called incompressible flow.
In other words, it is the flow in which the density of a fluid is constant during the flow.
Mathematically, for incompressible flow
P = constant
Liquids are generally incompressible which means that pressure and temperature changes have a very little effect on their volume. Gases are compressible fluids.
Rotational and Irrotational flow
Rotational flow is that type of flow in which fluid particles also rotate about their own axes while flowing along a streamline.
Irrotational flow is that type of flow in which fluid particles do not rotate about their own axes while flowing.
Ideal and Real flow
An ideal flow is the flow of a non-viscous fluid. In the ideal flow, no shear stress exists between two adjacent layers or between the fluid layer and boundary, only normal stresses can exist in ideal flows.
The flow of real (viscous) fluids is called real flow. In real flow, shear stress exists between to adjacent fluid layers. These stresses oppose the sliding of one layer over another.
One, Two and Three-dimensional flow
One dimensional flow is the flow in which parameters (velocity, pressure, density, viscosity and temperature) vary only in one direction and the flow is a function of only one co-ordinate Axis and time. The flow field is represented by streamlines which are straight and parallel.
Mathematically, for one-dimensional flow
Two-dimensional flow is the flow in which fluid parameters vary along two directions and the flow is the function of two rectangular space coordinates (x and y-axis) and time. The flow field is represented by streamlines which are curves.
Mathematically, for two-dimensional flow
Three-dimensional flow is the flow in which flow parameters change in all the three directions and the flow is the functions of three mutually perpendicular co-ordinate Axis (x, y, z-axis) and time. The streamlines are space curves.
Mathematically, for three-dimensional flow
Book- Fluid Mechanics(Hydraulics), Writer- A.K. Upadhyay
Classification of channels
Channels can be classified as:
1.Prismatic channels and non-prismatic channels
2.Natural and Artificial channels
3.Rigid boundary and mobile boundary channel
A channel is said to be prismatic channel if the cross-section and bed slope does not change along the length of the channel.
Example: A laboratory flume laid at a constant bed slope and uniform cross-section.
A channel is said to be non-prismatic channel if there is a change in cross-section and bed slope along the length of the channel.
Example: A river with varying cross-section and bed slope.
Rivers and streams are natural channels. Natural channels have the irregular cross-section. So it is difficult to define the flow depth for natural channels.
Human-made channels are called artificial channels. Irrigation Canals, sewage pipes, aqueducts etc. are examples of artificial channels. Artificial channels have regular cross-sections like triangular, trapezoidal, rectangular, circular etc.
Rigid Boundary channel
A channel is said to be a rigid boundary channel if the material on the bed and sides of a channel is not easily movable.
Example: concrete lined canal, a metal flume.
Mobile Boundary channel
A channel is said to be a mobile boundary channel if the material on the bed and sides of a channel is loose and easily movable due to the flow of water. Analysis of mobile boundary channel is more complicated due to sediment erosion, deposition and resulting in additional resistance to flow.
Book- Fluid Mechanics(Hydraulics), Writer- A.K. Upadhyay
The part of the road which is finished with a hard smooth surface is known as a pavement. It helps in making them durable and able to withstand traffic.
Types of road pavement
(a) Flexible pavements
The pavements which have a very low flexural strength and are flexible in their structural behaviour under the wheel loads are called flexible pavements. Because of the low flexural strength, the pavement deforms if the sub-grade deforms, in flexible pavments the vertical load of traffic is transmitted to the lower layers by grain to grain transfer. A well compacted granular structure forms a good flexible pavement layer.
The load transferring ability of the flexible pavement depends on the type of material and thickness of payment. The pavement thickness is so designed that the stress on the sub-grade soil is kept within its bearing capacity and the sub-grade is prevented from excessive deformations. In flexible pavement the sub-grade plays an important role as it carries the vehicle load transmitted through the pavement. The strength and smoothness of the pavement depend to a great extent on the deformation suffered by the sub-grade.
All bituminous pavements, Water Bound Macadam (W.B.M), gravel pavements etc., are the example of flexible pavements.
(b) Rigid pavements
Rigid pavements are those which possess a considerable flexural strength of flexural rigidity.
A Rijid payment derives its capacity to withstand loads from flexural strength. The rigid pavement has the slab action and is capable of transmitting the loads to a wider area. The Rigid pavment does not get deformed to the shape of the lower surface as it can bridge the minor variations of the lower layer.
Cement concrete pavement is the best example of the Rigid pavement.
(c) Semi-rigid pavement
Semi-rigid pavements represent an intermediate state between the Flexible and the Rigid pavement. The flexural strength of such type of pavment is much lower than the concrete slab, but it derives support by the lateral distribution of loads through the pavement depth as in flexible pavement.
Lean cement concrete, soil cement and lime-puzzolanic concrete construction are examples of semi-rigid pavements.
Book- Highway Engineering, Writer- A.K. Upadhyay
WEIR, BARRAGE AND DAM
An impervious barrier which is constructed across a river to raise the water level on the upstream side is known as Weir. Here the water level is raised up to the required height and the surplus water is allowed to flow over the Weir. Generally, it is constructed across an inundation river.
When adjustable gates are installed over a weir to maintain the water surface at different levels at different times, then it is known as Barrage. The water level is adjusted by operating the adjustable gates or shutters. The gates are placed at different tiers and these are operated by cables from the cabin. The gates are supported on the piers at both ends. The distance between pier to pier is known as Bay.
The high impervious barrier constructed across a river valley to form a deep storage Reservoir is known as Dam. The surplus water is not allowed to flow over the dam, but it flows through the spillways provided at some designed level.
Book- Irrigation Engineering, Writer- N N Basak