Irrigation and Its Methods in Civil Engineering Study

Irrigation and Its Methods for Engineering Purpose

Irrigation

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CHAPTER HIGHLIGHTS

☞ Introduction
☞ Types of irrigation
☞ Methods of irrigation
☞ Water requirements of crops
☞ Irrigation efficiencies
☞ Irrigation requirements of crops
☞ Crop seasons
☞ Water logging and drainage

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GENERAL UNDERSTANDING

The following are the main concerns on irrigation:-

1 How to apply

• i.e. what should be the method of irrigation: Border Flooding method, furrow irrigation method,sprinkler irrigation method, drip irrigation method etc.

2.How much apply

• i.e. how much moisture the soil can hold in its pores or Moisture holding capacity of Soil.

3.When to apply

• i.e. when has the soil moisture level depleted to 50 to 60% of moisture holding capacity and when is the time to irrigate. In other words, what should be the frequency of irrigation.

DEFINITION OF IRRIGATION

Irrigation may be defined as the science of artificial application of water to the land, in accordance with the crop requirements throughout the crop period for full-fledged nourishment of the crops.

CROP YIELD AND PRODUCTION IN IRRIGATION

The crop yield from irrigation is expressed as quintal/ha or tonnes/ha. The productivity of the crop is expressed as crop yield per mm of water applied.

Increase of yield or Productivity can be achieved by following methods in IRRIGATION

• Land shaping or land leveling
• Suitable crop rotation and crop planing
• Using high yielding varieties of seeds
• Using chemicals and Fertilizers
• Suitable methods of Irrigation adopted.
• Lining of canal and other bodies.
• Drainage of Irrigated land by surface and subsurface drainage.

ADVANTAGES AND DISADVANTAGES OF IRRIGATION

Advantages of Irrigation

Direct advantages of IRRIGATION

• Increase in food Production
• Protection against drought
• Revenue generation
• Mixed Cropping

Indirect advantages of IRRIGATION

• Power generation
• Transportation
• Ground water table
• Employment

Disadvantage of Irrigation

• Water logging due to excess irrigation
• Ground water pollution due to seepage of nitrates resent in soil as fertilizer etc.

Overall Benefits of Irrigation

• 1. Increase in food production
• 2. Protection from famine
• 3. Cultivation of cash crops
• 4. Eliminating mixed cropping
• 5. Addition to the wealth of the country
• 6. Generation of hydroelectric power
• 7. Domestic and industrial water supply
• 8. Inland navigation
• 9. Canal planting
• 10. Improvement of ground water storage

Effects of Irrigation

• 1. Breeding places of mosquitoes
• 2. Water logging
• 3. Damp climate

TYPES OF IRRIGATION PROJECTS

Irrigation projects	Irrigation Potential (CCA)	Cost of Project
Major	>10000 ha	>5 crores
Medium	2000 - 10000 ha	>.5 to 5 crores
Small	less than 2000 ha	>.25 to .5 crores

TYPES OF IRRIGATION

Types of Irrigation
Flow Irrigation					Lift Irrigation
Perennial Irriagation	Direct Irrigation	Inundation Irrigation	Storage Irrigation	Combined Storage and Diversion scheme	(well irrigation)
(The water required is supplied to the crop through out the year)	(diversion scheme) (or) (River canal irrigation) (Water is directly diverted to canal without storing)	(The irrigation is carried out by deep flooding)	(storage scheme) (or) (Tank irrigation) (Water is stored in dam (or) reservoir)	(Water is stored in dam (or) reservoir and then diverted to canal)	(Subsoil water is lifted to the Surface and conveyed to agricultural fields.)

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• 1. Surface irrigation.
• 2. Subsurface irrigation.

Surface irrigation

Surface irrigation can be further classified into 

a. Flow irrigation.- i.e. flow under the action of gravity
b. Lift irrigation.- i.e. water is lifted by pumps etc for supplying water.

Flow irrigation can be further sub-divide into

x. Perennial irrigation - water is supplied on whole base period or continuous required water supply.

y. Flood irrigation - uncontrolled irrigation or soil is kept submerged and thoroughly flooded with water.

Sub-surface irrigation

Sub-surface irrigation may be divided into two types.

a. Natural sub-irrigation - water leakage from channels etc.

b. Artificial sub-irrigation - by artificial mechanism.

METHODS OF IRRIGATION

• Free flooding method
• Border flooding method
• Check flooding method
• Basin flooding method
• Furrow irrigation method
• Sprinkler irrigation method
• Drip irrigation method

Free flooding or Ordinary flooding

• Also called wild flooding as the movement of water is not restricted.
• Initial cost of land preparation is low but labour requirement are usually high.
• Water application efficiency is also low
• Suitable for close growing crops, pastures, etc. particularly where land is steep.
• It may be used on rolling land or topography irregular where borders, checks, basins,furrows are not feasible.

Border flooding

• Land is divided into no of strips, separated by low levees called borders.
• Each strip is of 10 to 20 metres in width and 100 to 400 m in length.
• Water flows slowly toward the lower end and it infiltrates into the soil as it advances.
• When water reaches the lower end of the strip the supply is turned off.
• Size of supply ditch depends upon the infiltration rate of the soil and width of border strip.
• This method is most popular.

Check Flooding

• Similar to ordinary flooding except water is controlled by surrounding checked area with levees.
• Close growing crops such as jowar or paddy are preferred.
• Deep homogeneous loam or clay soils with medium infiltration rates are preferred
• Suitable for both more permeable and less permeable soils 
• less time required for highly permeable soil and vice-versa.

Basin flooding

• Special type of check flooding and adopted specially for orchard trees.
• One or more trees are generally placed in the basin and surface is flooded.
• Shape of basin can be square, rectangular, circular or it may be irregular.
• Flatter the land surface, easier it is to construct the basin
• Coarse sands are not suitable for basin irrigation Because of high percolation losses.
• Size and shape of basins are mainly determined by the land slope. the soil type, the available stream, the required depth of irrigation water to the applied.

Furrow Irrigation or Furrow method

• Water is applied to the land to be irrigated by series of furrows
• Furrows are small, parallel channels, made to carry water for irrigating the crops.
• Infiltrated water spreads laterally between furrows.
• The crops are usually grown on the ridges between the furrow.
• One half to one fifth area of land is wetted.
• Suitable for wide range of soil types, crops and land slopes.
• Preferred on uniformly flat or gentle slopes which should not exceed .5%.
• furrows can also be similar to long narrow basin.
• labor requirement and land preparation is reduced as compare to flooding.

Sprinkler irrigation method

• In the form of spray over crop through pipe system.
• Known as overhead irrigation.
• Used for all types of crops except rice and jute.
• Used for all types of soils except very heavy soils with low infiltration rates.
• Beset suited for very light soils as deep percolation losses are avoided.
• This suit undulating topography and hence land leveling is not necessary.
• This methods is used mainly by cultivation of tea coffee and vegetables in out country.

Notes :- 

• for rice and jute standing water is required
• light soils are sandy and silty with very little clay. generally easy to work, warm up quickly, dry out rapidly.

Drip irrigation method

• Latest method.
• Popular in areas with acute scarcity of irrigation water and salt problems.
• Water and fertilizer is slowly and directly applied to the root zone of the plants in order to reduce losses due to evaporation and percolation.
• Also known as Trickle irrigation
• Help of specially designed emitter and drippers.
• Centrifugal pump is best suited for this method.
• Best suited for row crops such as tomatoes, grapes,corn,citrus,melons,fruits,cauliflower,cabbage etc.

Water Requirements of Crops

• The water holding capacity of soil is the main characteristics which has to be taken into account for ideal irrigation. Thus the following topics deal with the water holding characteristics of soil and the parameters which help to measure it.

Classes of Soil Water

1. Saturation capacity:

• The amount of water required to fill the pore spaces between soil particles by replacing all air held in pore spaces. It is also called maximum moisture holding capacity or total capacity.

2. Field capacity:

• It is the moisture content of soil after free drainage has removed most of gravity water. It is the upper limit of water content available to plant roots.

3. Permanent wilting point:

• Plants can no longer extract sufficient water from the soil for its growth.This is also known as wilting coefficient. If the plant does not get sufficient water to meet its needs,it will wilt permanently. For most of the soils wilting coefficient is about 150% of hygroscopic water.

4. Temporary wilting:

• This will take place on a hot windy day but plant will recover in cooler day.

5. Ultimate wilting:

• At ultimate wilting point the plant will not regain its turgidity even after addition of sufficient water to the soil and the plant will die. It is similar to hygroscopic coefficient.
• Hygroscopic coefficient = 2/3(permanent wilting point)

6. Available moisture:

• Moisture content of soil between field capacity and permanent wilting point.

7. Readily available moisture:

• 75% of available moisture is known as readily available moisture. Readily available moisture depth, d w = S × d (Field capacity – Optimum moisture) = Sd (FC – OM)

8. Moisture equivalent

• = Field capacity = (1.8 to 2) × (Permanent wilting point) = 2.7 (Hygroscopic coefficient)

9. Available moisture depth

• = (d w ) = Sg × d × [F C – w C]
• Where
  S g = Apparent specific gravity of soil
  F c = Field capacity
  w c = Wilting coefficient.

10. Frequency of irrigation

• f= dw/Cu
• Where dw = Readily available moisture depth
• cu = Evapo-transpiration loss

11. Base period:

• Total time between first watering done for preparation of land for sowing of crop and last watering done before its harvesting is called base period.

12. Crop period:

• Total time elapsed between sowing of crop and its harvesting is called crop period.

13. Duty (D):

• It is the area of land in hectares which can be irrigated for growing any crop if one cumec of water is supplied continuously to the land for entire base period of crop.

14. Delta (∆):

• Total depth of water over the irrigated land required by a crop grown on it during the entire base period of the crop.
• Crop Average = Delta (cm)
• Rice = 120
• Wheat = 37.5
• Cotton = 45
• Tobacco = 60
• Sugarcane = 90

`Duty  = 8.64 × Base period / Delta`

B = Base period in days

∆ = delta in metres.

15. Consumptive use or evapotranspiration:

• It is thetotal loss of water due to plants transpiration and evaporation from the land.
• Lysimeter is used to measure Cu .One cumec day = 8.64 hectare metres, it is a volumetric unit.
• It is total volume of water supplied@ 1 cumec in a day.

Irrigation Efficiencies

1. Water conveyance efficiency ( η c ):

• It is the ratio of quantity of water delivered to the field to the quantity of water diverted into the canal system from reservoir.

2. Water application efficiency ( η a ):

• It is the ratio of quantity of water stored in the root zone of plants to the quantity of water delivered to the fields.

3. Water use efficiency ( η u ):

• It is the ratio of quantity of water used beneficially including the water required for leaching to the quantity of water delivered.

4. Water storage efficiency [ η s ]:

• Ratio of quantity of water stored in the root zone during irrigation to the quantity of water needed to bring water content of the soil to field capacity.

Irrigation Requirements of Crops

1. Consumptive irrigation requirements (CIR):

It is the amount of water required to meet the evapotranspiration needs of a crop CIR = Cu − Re

Re = Effective rainfall

2. Net irrigation requirement (NIR):

Amount of irrigation water required to be delivered at the field to meet evapotranspiration and other needs such as leaching NIR = Cu – Re + Le

Where, L e = leaching

3. Field irrigation requirement

`(FIR) = NIR/ ηa`

4. Gross irrigation requirement

`(GIR) = FIR / ηc`

5. Paleo irrigation:

• It is the watering done prior to sowing of crop.

6. Kor watering:

• The first watering after the plants have grown few cm high is known as kor watering

7. Outlet factor:

• Duty of water at canal outlet is known as outlet factor.

8. Gross command area (GCA):

• Total area which can be irrigated by canal system if unlimited quantity of water is available is known as gross command area.

9. Culturable command area (CCA):

• The portion of the GCA which is culturable or cultivable.
• CCA = GCA – Uncultivable area

10. Culturable cultivated area:

• That portion of CCA which is actually cultivated during a crop season.

11. Capacity factor:

• Ratio of mean discharge of canal for a certain duration to its maximum discharge capacity.

12. Time factor:

• Ratio of number of days the canal has actually run during a watering period to the total number of days of the watering period.

Crop Seasons

1. Kharif crops:

• These are the crops which are sown in the month of April and harvested in the month of September. Examples: Rice, maize.

2. Rabi crops:

• These are the crops which are sown in October and harvested in March. (Also called winter crops) Examples: Wheat, tobacco.

3. Perennial crops:

• These are the crops for which the water is supplied throughout the year. Example: Sugarcane

4. Hot weather crops:

• These are the crops which are grown between Kharif and Rabi season, i.e., from February to June.

5. Summer crops:

• The hot weather crops and Kharif crops are combinedly called as summer crops.

6. Dry crops:

• Crops grown without irrigation and depend only on rainfall for survival.

7. Wet crops:

• The crops which require irrigation are known as wet crops.

Water Logging and Drainage

 

Water Logging

• It is the condition in which there is excessive moisture in the soil making the land less productive.
• The depth of water table at which it tends to make the land, water logged, depends on the
• 1. height of capillary fringe and
• 2. type of crop.

 

Causes of Water Logging

• 1. Excessive rainfall in the area
• 2. Flat ground profile
• 3. Improper drainage of surface run-off
• 4. Excessive irrigation

 

Effects of Water Logging

• 1. Causes anaerobic conditions near roots of plants.
• 2. Causes salinity of soil.
• 3. Causes growth of wild aquatic plants.
• 4. Lowers the soil temperature which effects the activities of bacteria.
• 5. It makes cultivation difficult as the water logged areas cannot be easily cultivated.

 

Water Logging Control

• 1. By providing efficient under drainage
• 2. By preventing seepage from reservoirs
• 3. By introducing crop rotation
• 4. By improving natural drainage of area
• 5. By introducing lift irrigation

 

Drainage

• It is the means of preventing land from getting water logged as well as to receive the land already water logged.

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Self Purification of Stream - Environmental Engineering

Self Purification of Stream

A polluted stream undergoes self-purification

• Zone A – Zone of clear water
• Zone B – Zone of Degradation
• Zone C – Zone of Active decomposition
• Zone D – Zone of Recovery
• Zone E – Zone of recovery

Zone	Main Characteristics
Zone of Clear Water (1 & 5)	Clear water,  presence of fishes. 100% saturation
Zone of Degradation (2)	Dissolved oxygen level falls due to the decomposition of Organic Matter in this Zone. Dark and turbid water. Fishes may present Algae are absent. 40% saturation
Zone of Active Decomposition (3)	Zone of heavy pollution. Water is very dark and turbid Dissolved oxygen level falls to Zero. Critical Oxygen Deficient occurs. Formation of the dirty scum layer. Both fishes and algae absents 0% saturation
Zone of recovery (4)	Dissolved oxygen level increases and may reach to saturated value. Decomposition of organic matter takes place up to the Nitrate level. Both fishes and Algae reappear. BOD falls down. 40% saturation

Purification of sewage in river

When sewage or sewage effluents are discharged into the river, river gets polluted temporary, but the conditions do not remain so forever, because the natural forces  of purification such as dilution, sedimentation, oxidation, reduction in sunlight go on acting upon the pollutants  and bring back the water into its original condition. This process known as self purification and various forces responsible for this are described briefly below:

1. Dilution and Dispersion: 

• When sewage is discharged into large volume of river water, it gets rapidly dispersed and diluted. This results in reduction of concentration of organic matter in sewage.

2. Sedimentation: 

• The suspended solids in sewage gets settle down into the river bed by the action of gravity.

3. Oxidation: 

• The organic matter present in sewage gets oxidized by the microorganisms with the help of dissolved oxygen present in river and results in formation of stable products.

4. Reduction: 

• It occurs due to hydrolysis of organic matter settles down at the bottom of river bed either chemically or biologically.

The above natural forces of purification depend on various factors which are:

• 1. Temperature

• 2. Turbulence

• 3. Velocity of river stream

• 4. Availability of dissolved oxygen

• 5. Type and amount of organic matter present

• 6. Rate of re-aeration.

classification of secondary treatment units:

Important Terms

Hydrolysis:

• It is a chemical process in which a molecule is cleaved into two parts by the addition of a molecule of water. One fragment of the parent molecule gains a hydrogen ion (H+) from the additional water molecule. The other group collects the remaining hydroxyl group (OH-).

Pyrolysis

•  It is the process in which the most of organic matter upon heating in an oxygen free atmosphere splits through a combination of thermal cracking and condensation reactions into gaseous, liquid and solid fractions.
• The process typically occurs at temperatures above 430 °C (800 °F) and under pressure. It simultaneously involves the change of physical phase and chemical composition and is an irreversible process. So it is an endothermic process.

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Disposal of refuse - Environmental Engineering

Disposal of refuse and Sewage sickness of Land

Method of disposal of refuse

Landfill or Burying:

• Landfill refers to the disposal of waste material by burying it.
• It is an extended storage area for non-biodegradable waste.
• Landfill is an area, which prevent contamination from the waste entering the area surrounding by soil and water and it also helps to reduce odor and pests

Incineration: 

• This method involves burning of solid wastes in a furnace until the wastes are turned into ashes
• In this process, the combustible portion of the waste is combined with oxygen forming carbon dioxide and water, which are released into the atmosphere
• Incinerators are made in such a way that they do not give off extreme amounts of heat when burning solid wastes
• Suitable temperature and operating conditions are required to achieve for incineration
• It reduces the volume of waste up to 20 or 30% of the original volume
• This method of solid waste management can be done by individuals, municipalities and even institutions

• It involves of two stages involved such as drying and combustion. Drying and combustion may be accomplished either in separate units or successively in same units depending upon the temperature constraints or control parameter. It is an exothermic process.

Fluidized bed incineration:

• It is a combustion technology used to burn solid fuels. 
• In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone etc.) through which jets of air are blown to provide the oxygen required for combustion or gasification. 
• The resultant fast and intimate mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed

Pulverization 

• It refers to the action of crushing and grinding heavier solids into the lighter solids.

Composting 

• It is a natural biological process, carried out under controlled aerobic conditions (requires oxygen).
• In this process, various microorganisms, including bacteria and fungi, break down organic matter into simpler substances.
• The effectiveness of the composting process is dependent upon the environmental conditions present within the composting system i.e. oxygen, temperature, moisture, material disturbance, organic matter and the size and activity of microbial populations.

process of composting

Bangalore method

• It is an anaerobic method conventionally carried out in pits. 
• In the Bangalore method of composting, dry waste material of 25 cm thick is spread in a pit and a thick suspension of cow dung in water is sprinkled over for moistening.

Indore method

• It is an aerobic method. 
• The Indore method of composting in pits involves filling of alternate layers of similar thickness as in the Bangalore method.
• However, to ensure aerobic condition the material is turned at specific intervals for which a 60 cm wide strip on the longitudinal side of the pit is kept vacant.

There are mainly two methods (Indore and Bangalore method) adopted in India for the decomposition of Solid wastes generated. 

The main difference between Indore Method and Bangalore Method for decomposition of MSW is given below:

Indore Method	Bangalore Method
Decomposition of MSW by composting is done aerobically.	Decomposition of MSW by composting is done an-aerobically.
Decomposition takes 2 - 3 Months	Decomposition takes 5 - 6 Months
Mixing is ensured either Mechanically or manually	No mixing is done.

• In Composting with a combination of proper environmental conditions and adequate time, microorganisms turn raw putrescible organic matter into a stabilized product.

• Through composting, readily available nutrient and energy sources are transformed into carbon dioxide, water, and a complex form of organic matter compost. 

• Process management can be optimized for a number of criteria, including the rate of decomposition (to reduce residence time in reactors and thus minimize facility size requirements), pathogen control, and odour management.

• The key parameters are the available carbon to nitrogen (C: N) ratio, moisture, oxygen, and temperature.

• Decomposition slows dramatically in mixtures under 40 to 45 percent moisture, which can lead facility operators to prematurely assume compost is stabilized and ready to sell.

• A minimum moisture content of 50 to 55 percent is usually recommended for high rate composting of MSW.

• During the active composting phase, additional water usually needs to be added to prevent premature drying and incomplete stabilization. MSW compost mixtures usually start at about 52 percent moisture and dry to about 37 percent moisture prior to final screening and marketing.

C/N ratio is an important factor in composting

• since bacteria use nitrogen for their cell structure building and carbon for food.
•  The optimum composting C/N ratio should be in range (30 - 50)
•  So, If C/N ratio is more, Nitrogen is utilized prior to that of carbon, which results in incomplete digestion of organic matter.

gases produced from a land fill site

• Sanitary landfill involves dumping the waste in layers and compacting them after each layer, hence tractor can be used very effectively for this purpose.It is simple and economical.

• But there is continuous evolution of foul gases which may be explosive in nature. The major constituent gases produced from a land fill site are carbon dioxide and methane.

• It also causes leachate in landfills which pollutes the nearby ground water thereby impacting the ecology of the area.

Sewage sickness of  Land: 

• When sewage is applied continuously on a piece of land, the soil pores or voids may get filled up and clogged with sewage matter retained in them. This phenomenon of soil getting clogged is known as sewage sickness of the land.

The following preventive measures may be adopted to avoid sewage sickness:

Pre-treatment of sewage	Sewage should be given some pre-treatment before it is applied on land.
Rotation of crops	It is desirable to grow different types of crops on a piece of land instead of one single crop. Rotation of crops minimizes the chances of sewage sickness.
Drainage of soil	Subsoil drain pipes should be laid in sufficient number to collect the percolated effluent.
Depth of sewage	The depth of sewage on land should be carefully decided by keeping in view the climatic conditions, drainage facilities, nature of crops, and characteristics of the soil.
Alternative arrangement	There should be ample provision of extra land so that land with sewage sickness can be given the desired rest.
Intermittent application	Sewage should be applied on land at intervals. The period between successive applications depends on the general working of sewage farms and the permeability of the soil. Depending on the nature of the soil, this period between successive applications varies from few hours to few weeks.
Treatment to land	The land affected by sewage sickness should be properly treated before it is put up in use again. Clogged surfaces should be broken by suitable equipment.

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Environmental Engineering

for Civil Engineers

Water Supply and Sewage Treatment.

Water Supply Sewage Treatment water Distribution Definitions

Latest Article

Types Treatment unit and its Mechanism

for example - trickling filter, Oxidation pond, Septic tank and its nature like aerobic or anaerobic process More

Disposal of refuse.

Method of disposal of refuse, process of composting, C/N ratio in composting Read More

Self Purification of Stream

A polluted stream undergoes self-purification, Purification of sewage in river Read More .

Sewage Sickness of Land

When sewage is applied continuously on a piece of land, the soil pores or voids may get filled up and clogged with sewage matter retained in them. This phenomenon of soil getting clogged is known as sewage sickness of the land. Read More

Important Content

The density in kg/cum (in situ) of municipal solid waste

The density of municipal solid waste for a typical Indian city is 400 kg/cum to 600 kg/cum with a calorific value of 5000 to 6000 kJ/kg.

The density of municipal solid waste for a typical city of the USA is 100 kg/cum to 600 kg/cum with a calorific value of 15000 kJ/kg approximately.

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Popular in Civil Engineering

Important Theories.

Types of Footing and Settlement

Types of shear failure:

IS code specification for permissible settlement of Foundation

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Solid Waste

Important for Exams

• Pyrolysis
• Hydrolysis
• Purification of sewage in river
• Tunnels

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Important theories and its assumption on Soil Mechanics

Most Important theories

used in soil mechanics as engineer

Boussinesq Theory 

Westergaard Theory 

Coulombs Theory of Earth Pressure 

Terzaghi Theory

Rankine Theory of Earth Pressure

Boussinesq Theory: for stress strain relationship, due to a point load on surface:

• The Boussinesq method on Soil Mechanics uses the theory of elasticity to calculate the vertical stress under a point load in a homogeneous, semi-infinite half space its assumptions is very important for civil engineers to remember for solving many important civil engineering problems
• The stresses within a semi-infinite, homogeneous, isotropic mass, with a linear stress–
  strain relationship, due to a point load on the surface, were determined by Boussinesq
  in 1885
  

Assumptions consider on Boussinesq theory

• Soil is Homogeneous and Isotropic, Semi-infinite and elastic.
• Soil is Fully Saturated means Presence of zero air voids.
• Soil is Initially unstressed.
• Stress Distribution diagram about vertical axis is symmetrical.
• Any Change in volume of Soil due to Load is Neglected.
• Hooks law is Valid.
• Self weight of soil is neglected.

Equations

`\sigma=\frac{3Q}{2\pi z^2}\left[\frac1{1+\left(\frac rz\right)^2}\right]^\frac{5}{2}` 

 

The vertical, radial, circumferential and shear stresses at a depth z and a horizontal distance r from the point of application of the load in boussinesq theory on soil mechanics

Kb is known as influence factor

Westergaard Theory

Assumptions of Westergaard Theory for soil mass in civil engineering

• Soil is Homogeneous and Isotropic and Elastic.
• Soil is consider as Cohesive like- clay.
• Soil profile is Layered.
• Here, μ= Poisson's Ratio. If Poisson's Ratio is considered zero for all practical purposes

Equations

• `\sigma_z=\frac{Q}{\pi z^2}\left[\frac1{1+2\left(\frac rz\right)^2}\right]^\frac{3}{2}`
• `\sigma_z=\frac Q{z^2}I_w`
• Where,  Q is the point load and σz is the vertical stress due to the point load,
• `I_w=\frac1\pi\left[\frac1{1+2\left(\frac rz\right)^2}\right]^\frac3{2}`

\sigma=\frac{Q}{\pi z^2}\left[\frac1{1+2\left(\frac rz\right)^2}\right]^\frac{3}{2}`

Coulomb's Theory of Earth Pressure on Retaining wall

• Coulomb’s theory (1776) involves consideration of the stability, as a whole, of the wedge of soil between a retaining wall and a trial failure plane.
• The force between the wedge and the wall surface is determined by considering the equilibrium of forces acting on the wedge when it is on the point of sliding either up or down the failure plane, i.e. when the wedge is in a condition of limiting equilibrium
  

Assumptions of Coulomb's Theory of Earth Pressure for determining

• Soil mass is Homogeneous,Dry and Isotropic and ideally plastic.
• Back-fill is ideally plastic material and dry.
• Back-fill is cohesion-less like - Sand.
• Back of the wall surface is rough means frictional retaining wall and inclined
• The sliding Wedge itself act as a rigid body. i.e the equilibrium of the whole of the material.
• The Coulomb's theory is now interpreted as an upper bound plasticity solution

Terzaghi Theory or Terzaghi’s Bearing Capacity Theory

Assumptions

• Soil is Homogeneous and Isotropic.
• Soil is fully Saturated.
• Soil is considered as Hydrodynamic clay.
• No soil consolidation.
• Foundation is very Rigid.
• No applied Moment present.
• Soil is In-compressible.
• One dimensional flow.
• Darcy law is valid.
• Strain is small.
• No Sliding occur.

In other words of Terzaghi’s Bearing Capacity Theory

• 1. Footing base is rough and is laid at a shallow depth (i.e., D f < B).
• 2. The shear strength of soil above the base of footing is neglected. The soil above the base is replaced by a uniform surface,  g D f .
• 3. The load on the footing is vertical and uniformly distributed.
• 4. Footing is long, i.e., L/B ratio is infinite, where B is width and L is the length of the footing.
• 5. Shear strength of soil is governed by Mohr–Coulomb equation.

Zones: Zone I is elastic zone, zone II is radial shear zone and zone III is passive zone.

Rankine's Theory of Earth Pressure

    Rankine’s theory (1857) considers the state of stress in a soil mass when the condition of plastic equilibrium has been reached.i.e. when shear failure is on the point of occurring throughout the mass.

Assumptions

• Soil is Homogeneous and Isotropic and 
• semi-infinite mass of soil with a horizontal surface
  
• Consider Back of the wall is Friction-less or smooth.and vertical.
• in other word having a vertical boundary formed by a smooth wall surface extending to semi-infinite depth.
  
• Back-fill is dry and cohesion-less.
• Critical Shear surface is Plane.
• Back-plane is vertical and smooth like no friction.
• Wall is Infinitely Long.
• No strain condition occur.
• Failure type sliding failure.
• Resultant Force is not parallel to Back-fill Surface.
• When the horizontal stress becomes equal to the passive pressure the soil is said to be inthe passive Rankine state
• The Mohr circle representing the state of stress at failure in a two-dimensional element
  
• The soil element is in state of plastic equilibrium. i.e in the verge of failure.
•  The theory satisfies the conditions of a lower bound plasticity solution

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Foundation Engineering

Foundation

Quick revise your imp topics for exam 

Ultimate bearing capacity: It is the minimum pressure at the base of the foundation soil fails in shear.

Safe bearing capacity: It is the maximum pressure at which soil can carry without shear failure.

Net load intensity: It is the minimum net load at which shear failure of soil can occur.

Allowable bearing capacity: The net intensity of loading which the foundation will carry without undergoing settlement in excess or the permissible value for the structure under consideration but not exceeding net safe bearing capacity is termed as allowable bearing capacity.

Sheet Piles:

Sheet piles are similar to retaining walls which are constructed to retain earth, water or any other filling materials. These walls are thinner in section compared to masonry walls.

Sheet pile walls are generally used for water front structures, i.e. in building wharfs, quays and piers, building diversion dams, such as cofferdams, river bank protection and retaining the sides of cuts made in earth.

Based on the Assumptions, Terzaghi Theory is applicable for shallow foundation because side shear resistance and stressing of soil above the foundation is ignored whereas,

Meyerhoff considered stress zone extended up to G.L. Hence Meyershoff's theory is applicable for deep footing also.

Types of Footing and Settlement

Type of Footing and Soil	Settlement and contact pressure
Rigid footing and sand	Settlement : Uniform Contact Pressure:  Zero at edges and maximum at center
Rigid footing and Clay	Settlement : Uniform Contact Pressure:  maximum at edges and minimum at center.
Flexible footing and sand	Settlement:  Maximum at edges and minimum at center. Contact Pressure:   Uniform
Flexible footing and Clay	Settlement :  Minimum at edges and maximum at center Contact Pressure:  Uniform

Pile Group

Settlement of a pile group is more than the settlement of a single pile, even when the load is the same. This is because the pressure bulb of the pile group is deeper than that of individual piles, causing the compression of a larger volume of soil by the pile group.

Important Points:

• Pile group settlement for clayey soil can be computed from the principle of consolidation.

• Pile group settlement for sandy soil can be computed from the formula below:

• By Skempton’s Formula

• By Meyerhof’s Formula for square pile group only

• Where
• Sg = group settlement of pile
• Si = individual pile settlement
• B = Width of pile group
• r = number of rows in a pile group

The ultimate bearing capacity of a pile 

• The ultimate bearing capacity of a pile is the maximum load which it can carry without failure or excessive settlement of the ground. The bearing capacity also depends upon the method of installation.

• In Analytical method,
• Qup = Qeb + Qsf
• Qup = qb × Ab + qs × As
• Where,
• Qup = ultimate load on pile
• Qeb = end bearing capacity
• Qsf = skin friction
• qb = End bearing resistance of unit area, Ab = bearing Area
• qs = skin friction resistance of unit area, As = surface Area
• For circular footing, ultimate bearing capacity

• qu = 1.3 CNc + γDfNq + 0.3 γBNγ

• For square footing, ultimate bearing capacity

• qu = 1.3 CNc + γDfNq + 0.4 γBNγ

For clay soil

• Qup = Nc × C × Ab + α c̅ As;
• qb = Nc × C, qs = α c
• Where, α = adhesion factor
• c̅ = Average cohesion over depth of pile

‘Negative skin friction

• ‘Negative skin friction’ or ‘downward drag’ is a phenomenon which occurs when a soil layer surrounding a portion of the pile settles more than the pile. Such relative motion may occur when the clay stratum undergoes consolidation due to

• 1. A fill recently placed over the clay stratum.
• 2. Lowering of the ground water table.
• 3. Reconsolidation occurring due to disturbance caused by pile driving in sensitive clay stratum, etc.

• The axial capacity of a pile is a summation of upward reaction due to bearing at the base and net upward skin frictional resistance. As the negative skin friction (acting downward) lowers the net skin resistance, it in turn reduces the axial capacity of piles.

• Negative skin friction increases gradually as the consolidation of the clay layer proceeds since the effective overburden pressure gradually increases due to dissipation of excess pore pressure. 

According to IS 2911 : Part III 1973, the ratio of bearing resistance for double under- reamed pile to that of single under-reamed pile is 1.5 for sandy and clayey soils including black cotton soils.

Note:-

As per IS 2911: Part III (Some other recommendations)

• 1. For deep deposits of expansive soils the minimum length of piles, irrespective of any other considerations shall be 3.5 m below the ground level. For recently filled up grounds or other strata or poor bearing, the piles should pass through them and rest in good bearing strata.

• 2. The diameter of under-reamed piles may vary from 2 to 3 times the stem diameter depending upon the feasibility of construction. For Bored cast in situ under-reamed piles the bulb diameter normally be 2.5 times while for compaction piles it is 2 times.

• 3. For piles up to 30 cm diameter, spacing of bulbs should not be greater than 1.5 times the diameter of bulb. For piles of diameter greater than 30 cm, spacing can be reduced to 1.25 times the stem diameter.

• 4. The top most bulb should be at a minimum depth of 2 times the bulb diameter. In expansive soils, it should not be less than 1.75 m below ground level. The minimum clearance below underside of pile cap embedded in ground and the bulb should be a minimum 1.5 times the bulb diameter.

• 5. Under reamed piles with more than two bulbs are not advisable without ensuring their feasibility in strata needing stabilisation of boreholes by drilling mud. The number of bulbs in case of bored compaction piles should not exceed tow in such strata.

Retaining Wall

• Retaining wall is a structure that are designed and constructed to withstand lateral pressure of soil or hold back soil materials.

• The lateral pressure could be also due to earth filling, liquid pressure, sand, and other granular materials behind the retaining wall structure.

The empirical formula for determining the depth (d) of retaining wall is given by:

Where, ka is the active earth pressure coefficient and it is given by:

News Record formula is used to determine the ultimate load carrying capacity (Qup) of a pile embedded in sand.  It is given by:

Qap=WhFOS(S+C)  {For Drop hammer and Single acting steam hammer}

Qap=(W+ap)hFOS(S+C) {For Double acting steam hammer}

Where,

W = Weight if hammer in KN

FOS is Factor of safety which is generally taken ‘6’ for all type of hammers.

H = Height of fall in cm.

S = penetration of pile per blow in cm.

C = Constant for accounting elastic compression of pile and pile cap.

A = area of piston in m2 and p is steam pressure in kN/m2.

The values of S, C and FOS for different type of hammers are given below in tabulated form:

Single acting Hammer	Double acting Hammer	Drop Hammer
S  = Average value for last 25 blows	S  = Average value for last 25 blows	S  = Average value for last 5 blows
C = 0.25 cm	No defined value but generally taken C = 0.25 cm.	C = 2.5 cm
FOS = 6	FOS = 6	FOS = 6

Local shear failure:

This type of failure is seen in relatively loose sand and soft clay.

Some characteristics of local shear failure are:

• 1. Failure is not sudden and there is no tilting of footing.
• 2. Failure surface does not reach the ground surface and slight bulging of soil around the footing is observed
• 3. Failure surface is not well defined
• 4. Failure is progressive
• 5. In load-settlement curve, there is no well-defined peak
• 6. Failure is characterized by considerable settlement directly beneath the foundation
• 7. A significant compression of soil below the footing and partial development of plastic equilibrium is observed.
• 8. Well-defined wedge and slip surfaces only beneath the foundation.

Load-settlement curve:-

Note: In general shear failure, failure plane circular for cohesive soils and log spiral for sand and silts.

Types of shear failure:

General shear failure:

• It occurs in shallow foundations when placed on dense/stiff soil.
• At the time of failure, the foundation will get tilted and heaving will occur at the side.
• Before failure settlement will be small and negligible and the stress zone extends up to ground level. 

​Local shear failure:

• It occurs in loose sand and soft clays in case of shallow foundation.
• Before failure large settlement is recorded.
• The stress zone does not extend up to the ground level hence there may e little or no heaving at the sides.

Punching shear failure:

• It occurs in deep footing and pile which are placed on loose sand or soft clays.
• In this failure soil below the foundation gets cut off from adjacent soil by shearing and large settlement is recorded in the small-time period.
• The adjacent soil mass remains unstressed.

Foundtion Condition and Types of Failure

Foundation condition	Types of shear failure
Footings on the surface or at shallow depths in very dense sand	General shear failure
Footing on saturated and normally consolidated clay under undrained loading	General shear failure
Footings at deeper depth in dense sand	Punching shear failure
Footing on the surface or at shallow depths in loose sand	Punching shear failure
Footing on very dense sand loaded by transient dynamic load	Punching shear failure
Footings on very dense sand underlain by loose sand or soft clay	Punching shear failure
Footing on saturated and normally consolidated clay under drained loading	Punching/Local shear failure

Different IS codes and their use:

IS 456:2000 - Plain and reinforced concrete

IS 1080: 1985 - Design and construction of shallow foundation in soil (other than a raft, ring, and shell)

IS 1904:1986 - Design and construction of foundations in soils: General requirements

IS 2950: 1981 - Design and construction of raft foundation

IS code specification for permissible settlement:

(i) Total Permissible settlement:

• For isolated footing on clay = 65 mm
• For isolated footing on sand = 40 mm
• For raft footing on clay = 65-100 mm
• For raft footing on sand = 40-65 mm

(ii) Permissible Differential settlement:

• For isolated footing on clay = 40 mm
• For isolated footing on sand = 25 mm

(iii) Permissible angular settlement:

• For high framed structure < 1/500
• To prevent all type of minor damage < 1/1000

Note: 

For multi-storeyed buildings having isolated foundations on sand, the maximum permissible settlement is 60 mm 

[ For multistorey buildings having isolated foundations take the higher load as compare to single storey buildings having isolated foundations. So that deflection caused by multistorey building having isolated foundation higher than 40 mm (from the safer side)]

 

Foundtion and their Suitability

The different types of foundations and there suitability is specified in below in tabulated form:

Type of Foundation	Suitability
Spread footing foundation	This type of foundation can normally be used for three to four-storied buildings on common type of alluvial soils.
Stepped Foundation.	This type of foundation is provided on hilly places or in those situations where the ground is sloppy.
Pile Foundations	It is used in the following situations: When it is not economical to provide spread foundations and hard soil is at a greater depth. When it is very expensive to provide raft or grillage foundations. When heavy concentrated loads are to be taken up by the foundations. When the top soil is of compressible nature. When there is chances construction of irrigation canals in the near by area. In case of bridges when the scouring is more in the river bed. In marshy places.
Raft Foundations	This type of foundation is also recommended in such situations where the bearing capacity of the soil is very poor, the load of the structure is distributed over the whole floor area, or where a structure is subjected to constant shocks or jerks.
Well Foundations	This is generally provided for construction of bridge piers and the foundations are to be carried out in deep sandy soils of soft soils.

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