Pressure

When a fluid (either liquid or gas) is at rest, it exerts a force perpendicular to any surface in contact with it, such as a container wall or a body immersed in the fluid. While the fluid as a whole is at rest, the molecules that makes up the fluid are in motion, the force exerted by the fluid is due to molecules colliding with their surroundings. A fluid always has pressure as a result of innumerable molecular collisions. Pressure at any part of the fluid must experience forces exerted on it by adjoining fluid or by adjoining solid boundaries. If, therefore, part of the fluid is arbitrarily divided from the rest by an imaginary plane, there will be forces that may be considered as acting at that plane. As shown in Fig. 1, pressure occurs when a force is applied to an area. Fluid pressure is the force exerted by the fluid per unit area. Fluid pressure is transmitted with equal intensity in all directions and acts normal to any plane. In the same horizontal plane, the pressure intensities in a liquid are equal.

Fig.1 Force Producing a Pressure

The relationship between force, pressure and area is

Using the base units of Newton (N) for force and square metres (m²) for area, the unit of pressure is N/m², which for convenience is called a pascal (Pa). Because the pascal is an extremely small unit of pressure (car tyre pressure are around 200 000 Pa) the more commonly used unit is the kilopascal (kPa) or the megapascal (MPa). The pascal unit is used for the low pressures that occur in fans or in ventilation ducts. Kilopascals are used for normal gas and liquid pressures. The pressure in an oil hydraulic system would be measured in megapascals. The other multiple of the pascal that may b;e used on the weather forecast is the hectopascal (hPa), being used for the barometric pressure. A hectopascal is 10² pascals. A typical barometric pressure is 1013 hPa.

Pressures of large magnitude are often expressed in atmospheres (abbreviated to atm). For precise definition, one atmosphere is taken as 1.01325 × 10⁵ Pa. A pressure of 10⁵ Pa is called 1 bar. The thousand^th part of this unit, called a millibar (abbreviated to mbar), is commonly used by meteorologists. It should be noted that, although they are widely used, neither the atmosphere nor the bar are accepted for use with SI units. For pressures less than that of the atmosphere the units normally used are millimetres of mercury vacuum. These units refer to the difference between the height of a vertical column of mercury supported by the pressure considered and the height of one supported by the atmosphere. In the absence of shear forces, the direction of the plane over which the force due to the pressure acts has no effect on the magnitude of the pressure at a point. The fluid may even be accelerating in a particular direction provided that shear forces are absent – a condition that requires no relative motion between different particles of fluid. Many other pressure units are commonly encountered and their conversions are detailed below.

            1 bar =10⁵ N/m²

            1 atmosphere = 101325 N/m²

1 psi (1bf/in² - not SI unit) = 6895 N/m²

1 Torr = 133.3 N/m²

Pressure is determined from a calculation of the form (force divided by area), and so has the dimensions [F]/[L²] = [MLT⁻²]/[L²] = [ML⁻¹T⁻²]. Now although the force has direction, the pressure has not. The direction of the force also specifies the direction of the imaginary plane surface, since the latter is defined by the direction of a line perpendicular to, or normal to, the surface. Here, then, the force and the surface have the same direction and so in the equation Force = Pressure × Area of plane surface pressure must be a scalar quantity. Pressure is a property of the fluid at the point in question. Similarly, temperature and density are properties of the fluid and it is just as illogical to speak of ‘downward pressure’, for example, as of ‘downward temperature’ or ‘downward density’. To say that pressure acts in any direction, or even in all directions, is meaningless; pressure is a scalar quantity.

Terms commonly used in static pressure analysis include the following.

Pressure Head

The pressure intensity at the base of a column of homogenous fluid of a given height in metres.

Vacuum

A perfect vacuum is a completely empty space in which, therefore the pressure is zero.

Atmospheric Pressure

It is the pressure of earth's atmosphere. This changes with weather and elevation. The pressure at the surface of the earth due to the head of air above the surface is called atmospheric pressure. At sea level the atmospheric pressure is about 101.325 kN/m² (i.e. one atmosphere = 101.325 kN/m² is used as units of pressure).

Gauge Pressure

The pressure measured above or below atmospheric pressure is called Gauge pressure. Pressure cannot be measured directly; all instruments said to measure it in fact indicate a difference of pressure. This difference is frequently that between the pressure of the fluid under consideration and the pressure of the surrounding atmosphere. The pressure of the atmosphere is therefore commonly used as the reference or datum pressure that is the starting point of the scale of measurement. The difference in pressure recorded by the measuring instrument is then termed the gauge pressure.

Gauge pressure = Absolute pressure – Atmospheric pressure

Absolute Pressure

The pressure measured above absolute zero or vacuum is called Absolute pressure. The absolute pressure, that is the pressure considered relative to that of a perfect vacuum, is then given by

                        Absolute Pressure = Gauge Pressure + Atmospheric Pressure

pabs = pgauge +patm

The pressure of the atmosphere is not constant. For many engineering purposes the variation of atmospheric pressure (and therefore the variation of absolute pressure for a given gauge pressure, or vice versa) is of no consequence. In other cases, especially for the flow of gases – it is necessary to consider absolute pressures rather than gauge pressures and a knowledge of the pressure of the atmosphere is then required.

Positive and Negative Pressures

Because we are subjected to an atmospheric pressure, the pressure indicated on a gauge can be either positive (pressure) or negative (vacuum). Above atmospheric pressure is positive and called a gauge pressure for clarity. A typical pressure gauge would be calibrated in kPa. Below atmospheric pressure is negative and called a vacuum or a negative pressure. Fig.2 indicates the relationship between the pressure and vacuum ranges and introduces the concept of one pressure range starting from absolute zero pressure and called the absolute pressure range.

Fig. 2 Pressure/Vacuum Relationships

We normally express pressures in terms of gauge pressure and before these values may be used in calculations regarding the change of state of a gas, the gauge pressure must be changed into an absolute pressure. Changing to absolute values is done by adding the accepted value for atmospheric pressure, nominally 101.3 kPa.

Absolute Pressure (kPa) = Gauge Pressure (kPa) + 101.3 kPa

It is important, when specifying pressures or using pressures in a calculation, to determine if the values given are in terms of gauge pressure (sometimes written `kPa g’ or `kPa gauge’) or absolute pressure (sometimes written `kPa abs’).

Compressibility

A parameter describing the relationship between pressure and change in volume for a fluid. A compressible fluid is one which changes its volume appreciably under the application of pressure. Therefore, liquids are virtually incompressible whereas gases are easily compressed. The compressibility of a fluid is expressed by the bulk modulus of elasticity (K), which is the ratio of the change in unit pressure to the corresponding volume change per unit volume.

Vapour Pressure

When evaporation of a liquid having a free surface takes place within an enclosed space, the partial pressure created by the vapour molecules is called the vapour pressure. Vapour pressure of a liquid is the partial pressure of the vapour in contact with the saturated liquid at a given temperature. Vapour pressure increases with temperature.

All liquids possess a tendency to evaporate or vaporize i.e., to change from the liquid to the gaseous state. Such vaporization occurs because of continuous escaping of the molecules through the free liquid surface. When the liquid is confined in a closed vessel, the ejected vapour molecules get accumulated in the space between the free liquid surface and the top of the vessel. This accumulated vapour of the liquid exerts a partial pressure on the liquid surface which is known as vapour pressure of the liquid. As molecular activity increases with temperature, vapour pressure of the liquid also increases with temperature. If the external absolute pressure imposed on the liquid is reduced by some means to such an extent that it becomes equal to or less than the vapour pressure of the liquid, the boiling of the liquid starts, whatever be the temperature. Thus a liquid may boil even at ordinary temperature if the pressure above the liquid surface is reduced so as to be equal to or less than the vapour pressure of the liquid at that temperature.

If in any flow system the pressure at any point in the liquid approaches the vapour pressure, vaporization of liquid starts, resulting in the pockets of dissolved gases and vapours. The bubbles of vapour thus formed are carried by the flowing liquid into a region of high pressure where they collapse, giving rise to high impact pressure. The pressure developed by the collapsing bubbles is so high that the material from the adjoining boundaries gets eroded and cavities are formed on them. This phenomenon is known as cavitation. When the liquid pressure is dropped below the vapour pressure due to the flow phenomenon, we call the process cavitation. Mercury has a very low vapour pressure and hence it is an excellent fluid to be used in a barometer. On the contrary various volatile liquids like benzene etc., have high vapour pressure. Cavitation can cause serious problems, since the flow of liquid can sweep this cloud of bubbles on into an area of higher pressure where the bubbles will collapse suddenly. If this should occur in contact with a solid surface, very serious damage can result due to the very large force with which the liquid hits the surface. Cavitation can affect the performance of hydraulic machinery such as pumps, turbines and propellers, and the impact of collapsing bubbles can cause local erosion of metal surfaces.

Variation in Pressure with Depth

If the weight of the fluid can be neglected, the pressure in a fluid is the same throughout its volume. But often the fluid's weight is not negligible and under such condition pressure increases with increasing depth below the surface.

Let us now derive a general relation between the pressure ‘P’ at any point in a fluid at rest and the elevation ‘y’ of that point. We will assume that the density ′ρ′ and the acceleration due to gravity ‘g’ are same throughout the fluid. If the fluid is in equilibrium, every volume element is in equilibrium.

Consider a thin element of fluid with height ‘dy’. The bottom and top surfaces each have area ‘A’ and they are at elevations y and (y + dy) above some reference level where y = 0. The weight of the fluid element is

                                     dW = (volume) (density) (g)

                                            = (A dy) (ρ) (g)

             or                  dW = ρ g A dy

The pressure at the bottom surface P, the total y component of upward force is PA. The pressure at the top surface is P + dP and the total y-component of downward force on the top surface is (P + dP)A. The fluid element is in equilibrium, so the total y component of force including the weight and the forces at the bottom and top surfaces must be zero.

               Σ Fy = 0

                                                            PA – (P + dP)A – ρ g A dy = 0

This equation shows that when y increases, P decreases, i.e., as we move upward in the fluid pressure decreases.

If P1 and P2 be the pressures at elevations y1 and y2 and if ρ and g are constant, then integration of Equation (1), we get

                                   or                                     P2 – P1 = – ρ g (y2 – y1) (2)

It's often convenient to express equation (2) in terms of the depth below the surface of a fluid. Take point 1 at depth h below the surface of fluid and let P represents pressure at this point. Take point 2 at the surface of the fluid, where the pressure is P0 (for zero depth). The depth of point 1 below the surface is,

                                   h = y2 – y1

    and equation (2) becomes

                                           P0 – P = – ρ g (y2 – y1) = – ρgh

                                                      P = P0 + ρ gh                     (3)

Thus pressure increases linearly with depth, if ρ and g are uniform. A graph between P and h is shown below.

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Newtonian and Non-Newtonian Fluids

Fluids in which shear stress is directly proportional to the rate of deformation are “Newtonian fluids. Most common fluids such as water, air and gasoline are Newtonian under normal conditions. If the fluid is Newtonian, then

The constant of proportionality in Eq. (1) is the absolute (or dynamic) viscosity, μ. The Newton's law of viscosity is given for one-dimensional flow by

Note that, since the dimensions of ′τ′ are [F/L²] and the dimensions of dv/dy are [1/t], μ has dimensions [Ft/L²]. Since the dimensions of force, F, mass, m, length, L, and time, t, are related by Newton's second law of motion, the dimensions of μ, can also be expressed as [M/Lt]. In the British Gravitational system, the units of viscosity are lbf.s/ft² or slug/(ft .s). In the Absolute Metric system, the basic unit of viscosity is called a poise [1 poise = 1 g/(cm .s)]; in the SI system the units of viscosity are kg/(m. s) or Pa. s (1 Pa. s = 1 N. s/m²).

In fluid mechanics the ratio of absolute viscosity, μ, to density, 𝜌, often arises. This ratio is given the name kinematic viscosity and is represented by the symbol ′𝜗′. Since density has dimensions [M/L³], the dimensions of 𝜗 are [L²/t]. In the Absolute Metric system of units, the unit for 𝜗 is a stoke (1 stoke = 1 cm²/s). For gases, viscosity increases with temperature, whereas for liquids, viscosity decreases with increasing temperature. If one considers the deformation of two different Newtonian fluids, say Glycerine and water, one recognizes that they will deform at different rates under the action of same applied stress. Glycerine exhibits much more resistance to deformation than water. Thus we say it is more viscous.

Non-Newtonian Fluids

Fluids in which shear stress is not directly proportional to deformation rate are non- Newtonian. Many common fluids exhibit non-Newtonian behaviour. The familiar example is toothpaste. Toothpaste behaves as a "fluid" when squeezed from the tube. However, it does not run out by itself when the cap is removed. There is a threshold or yield stress below which toothpaste behaves as a solid. Non-Newtonian fluids commonly are classified as having time-independent or time-dependent behaviour.

Numerous empirical equations have been proposed to model the observed relations between τ and dv/dy for time-independent fluids. They may be adequately represented for many engineering applications by the power law model, which for one-dimensional flow becomes

where the exponent, n, is called the flow behaviour index and the coefficient, k, the consistency index. This equation reduces to Newton's law of viscosity for n = 1 with k = μ. To ensure that τ has the same sign as dv/dy, Eq. (2) is rewritten in the form

The idea behind Eq. (3) is that we end up with a viscosity 𝜂 that is used in a formula that is the same form as Eq. (2), in which the Newtonian viscosity μ is used. The big difference is that while μ is constant (except for temperature effects), 𝜂 depends on the shear rate. Most non-Newtonian fluids have apparent viscosities that are relatively high compared with the viscosity of water.

Fluids in which the apparent viscosity decreases with increasing deformation rate (n < 1) are called pseudo plastic (or shear thinning) fluids. Most non-Newtonian fluids fall into this group; examples include polymer solutions, colloidal suspensions, and paper pulp in water. If the apparent viscosity increases with increasing deformation rate (n > 1) the fluid is termed dilatant (or shear thickening). Suspensions of starch and of sand are examples of dilatant fluids.

A "fluid" that behaves as a solid until a minimum yield stress, τ, is exceeded and subsequently exhibits a linear relation between stress and rate of deformation is referred to as an ideal or Bingham plastic. Clay suspensions, drilling muds, and toothpaste are examples of substances exhibiting this behaviour. The study of non-Newtonian fluids is further complicated by the fact that the apparent viscosity may be time-dependent. Thixotropic fluids show a decrease in 𝜂 with time under a constant applied shear stress; many paints are thixotropic. Rheopectic fluids show an increase in 𝜂 with time. After deformation some fluids partially return to their original shape when the applied stress is released; such fluids are called viscoelastic.

Fig. 1 Variation of Shear Stress with Velocity Gradient

Non-Newtonian Liquids

For most fluids the dynamic viscosity is independent of the velocity gradient in straight and parallel flow, so Newton’s hypothesis is fulfilled. A graph of stress against rate of shear is a straight line through the origin with slope equal to μ. There is a fairly large category of liquids for which the viscosity is not independent of the rate of shear, and these liquids are referred to as non-Newtonian. Solutions (particularly of colloids) often have a reduced viscosity when the rate of shear is large, and such liquids are said to be pseudo-plastic. Gelatine, clay, milk, blood and liquid cement come in this category.

A few liquids exhibit the converse property of dilatancy; that is, their effective viscosity increases with increasing rate of shear. Concentrated solutions of sugar in water and aqueous suspensions of rice starch (in certain concentrations) are examples. Additional types of non-Newtonian behaviour may arise if the apparent viscosity changes with the time for which the shearing forces are applied. Liquids for which the apparent viscosity increases with the duration of the stress are termed rheopectic; those for which the apparent viscosity decreases with the duration are termed thixotropic.

A number of materials have the property of plasticity. Metals when strained beyond their elastic limit or when close to their melting points can deform continuously under the action of a constant force and thus in some degree behave like liquids of high viscosity. Their behaviour, is non-Newtonian, and most of the methods of mechanics of fluids are therefore inapplicable to them.

Viscoelastic materials possess both viscous and elastic properties; bitumen, nylon and flour dough are examples. In steady flow, that is, flow not changing with time, the   of shear is constant and may well be given by τ/μ where μ represents a constant dynamic viscosity as in a Newtonian fluid. Elasticity becomes evident when the shear stress is changed. A rapid increaseof stress from τ to τ +δτ causes the material to be sheared through an additional angle δτ /G where G represents an elastic modulus; the corresponding rate of shear is (1/G)∂τ/∂t so the total rate of shear in the material is (τ/μ) + (1/G)∂τ/∂t.

The fluids with which engineers most often have to deal are Newtonian, that is, their viscosity is not dependent on either the rate of shear or its duration, and the term mechanics of fluids is generally regarded as referring only to Newtonian fluids. The study of non-Newtonian liquids is termed rheology.

Inviscid Fluid

An important field of theoretical fluid mechanics involves the investigation of the motion of a hypothetical fluid having zero viscosity. Such a fluid is sometimes referred to as an ideal fluid. Although commonly adopted in the past, the use of this term is now discouraged as imprecise. A more meaningful term for a fluid of zero viscosity is inviscid fluid.

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Continuum Hypothesis and Transport Phenomena

Continuum Hypothesis 

A fluid, or any other substance for that matter, is composed of a large number of molecules in constant motion and undergoing collisions with each other. Matter is therefore discontinuous or discrete at microscopic scales. In principle, it is possible to study the mechanics of a fluid by studying the motion of the molecules themselves, as is done in kinetic theory or statistical mechanics. However, we are generally interested in the gross behaviour of the fluid, that is, in the average manifestation of the molecular motion. For example, forces are exerted on the boundaries of a container due to the constant bombardment of the molecules; the statistical average of this force per unit area is called pressure, a macroscopic property. So long as we are not interested in the mechanism of the origin of pressure, we can ignore the molecular motion and think of pressure as simply “force per unit area.” 

It is thus possible to ignore the discrete molecular structure of matter and replace it by a continuous distribution, called a continuum. For the continuum or macroscopic approach to be valid, the size of the flow system (characterized, for example, by the size of the body around which flow is taking place) must be much larger than the mean free path or the molecules. For ordinary cases, this is not a great restriction, since the mean free path is usually very small. For example, the mean free path for standard atmospheric air is ≈ 5 x 10^-8 m. In special situations, the mean free path of the molecules can be quite large and the continuum approach breaks down. In the upper altitudes of the atmosphere, for example, the mean free path of the molecules may be of the order of a matter, a kinetic theory approach is necessary for studying the dynamics of these rarefied gases. 

Transport Phenomena 

Consider a surface area AB within a mixture of two gases, say nitrogen and oxygen (Fig. 1), and assume that the concentration C of nitrogen (kilograms of nitrogen per cubic metre of mixture) varies across AB. 

Fig. 1 Mass flux qm due to concentration variation C(y) across AB

Random migration of molecules across AB in both directions will result in a net flux or nitrogen across AB, from the region  of higher C towards the region of lower C. Experiments show that, to a good approximation, the flux of one constituent in a mixture is proportional to its concentration gradient and it is given by 

                                       qm = - km Δ C                                  (1) 

Here the vector qm is the mass flux (kg m^-2 s^-1) of the constituent, Δ C is the concentration gradient of that constituent, and km is a constant of proportionality that depends on the particular pair of constituents in the mixture and the thermodynamic state. For example, km, for diffusion of nitrogen in a mixture with oxygen is different than km, for diffusion of nitrogen in a mixture with carbon dioxide. The linear relation (1) for mass diffusion is generally known as Fick's law. 

Relations like these are based on empirical evidence and are called phenomenological laws. Statistical mechanics can sometimes be used to derive such laws, but only for simple situations. The analogous relation for heat transport due to temperature gradient is Fourier's law and it is given by 

                                         q = - k ΔT                                         (2) 

where q is the heat flux (J m^-2 s^-1), ΔT is the temperature gradient, and k is the thermal conductivity of the material.

Fig. 2 Shear stress ‘τ’ on surface AB. Diffusion tends to decrease velocity gradients, so that the continuous line tends toward the dashed line

Next, consider the effect of velocity gradient du/dy (Fig. 2). It is clear that the macroscopic fluid velocity ‘u’ will tend to become uniform due to the random motion of the molecules, because of intermolecular collisions and the consequent exchange of molecular momentum. Imagine two railroad trains traveling on parallel . tracks at different speeds and workers shovelling coal from one train to the other. On the average, the impact of particles of coal going horn the slower to the faster train will tend to slow down the faster train, and similarly the coal going from the faster to the slower train will tend to speed up the latter. The net effect is a tendency to equalize the speeds of the two trains. An analogous process takes place in the fluid flow problem of Fig. 2. The velocity distribution here tends toward the dashed line, which can be described by saying that the x-momentum (determined by its “concentration” u) is being transferred downward. Such a momentum flux is equivalent to the existence of a shear stress in the fluid, just as the drag experienced by the two trains results from the momentum exchange through the transfer or coal particles. The fluid above AB tends to push the fluid underneath forward, whereas the fluid below AB tends to drag the upper fluid backward. Experiments show that the magnitude of the shear stress ‘τ’ along a surface such as AB is, to a good approximation, related to the velocity gradient by the linear relation 

which is called Newton’s law of friction. Here, the constant of proportionality ′µ′ (whose unit is kg m^-1 s^-l) is known as the dynamic viscosity, which is a strong function of temperature T. For ideal gases the random thermal speed is roughly proportional to √𝑇, the momentum transport, and consequently µ, also vary approximately as √𝑇. For liquids, on the other hand, the shear stress is caused more by the intermolecular cohesive forces than by the thermal motion of the molecules. These cohesive forces, and consequently µ for a liquid, decrease with temperature.

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Capillarity

If molecules of certain liquid possess, relatively, greater affinity for solid molecules, or in other words the liquid has greater adhesion than cohesion, then it will wet a solid surface with which it is in contact and will tend to rise at the point of contact, with the result that the liquid surface is concave upward and the angle of contact θ is less than 90° as shown in Fig. 1. Capillary action is the result of adhesion and surface tension. Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward. The surface tension acts to hold the surface intact, so instead of just the edges moving upward, the whole liquid surface is dragged upward.

For example, if a glass tube of small diameter is partially immersed in water, the water will wet the surface of the tube and it will rise in the tube to some height, above the normal water surface, with the angle of contact θ, being zero. The wetting of solid boundary by liquid results in creating decrease of pressure within the liquid and hence the rise in the liquid surface takes place, so that the pressure within the column at the elevation of the surrounding liquid surface is the same as the pressure at this elevation outside the column.

Fig. 1 Capillarity in Circular Glass Tubes

On the other hand, if for any liquid there is less attraction for solid molecule or in other words the cohesion predominates, then the liquid will not wet the solid surface and the liquid surface will be depressed at the point of contact, with the result that the liquid surface is concave downward and the angle of contact θ is greater than 90° as shown in Fig. 1. For instance, if the same glass tube is now inserted in mercury, since mercury does not wet the solid boundary in contact with it, the level of mercury inside the tube will be lower than the adjacent mercury level, with the angle of contact θ equal to about 130°. The tendency of the liquids which do not adhere to the solid surface, results in creating an increase of pressure across the liquid surface, (as in the case of a drop of liquid). It is because of the increased internal pressure, the elevation of the meniscus (curved liquid surface) in the tube is lowered to the level where the pressure is the same as that in the surrounding liquid.

Such a phenomenon of rise or fall of liquid surface relative to the adjacent general level of liquid is known as capillarity. Accordingly, the rise of liquid surface is designated as capillary rise and the lowering of liquid surface as capillary depression and it is expressed in terms of m or mm of liquid in SI units, in terms of cm or mm of liquid in the metric system of units and in terms of inch or ft of liquid in the English system of units.

The capillary rise (or depression) can be determined by considering the conditions of equilibrium in a circular tube of small diameter inserted in a liquid. It is supposed that the level of liquid has risen (or fallen) by h above (or below) the general liquid surface when a tube of radius r is inserted in the liquid. For the equilibrium of vertical forces acting on the mass of liquid lying above (or below) the general liquid level, the weight of liquid column h (or the total internal pressure in the case of capillary depression) must be balanced by the force, at surface of the liquid, due to surface tension ‘σ’. Thus equating these two forces we have

where ‘w’ is the specific weight of water, ‘s’ is specific gravity of liquid, and ‘θ’ is the contact angle between the liquid and the tube. The expression for ‘h’ the capillary rise (or depression) then becomes

As stated earlier, the contact angle ‘θ’ for water and glass is equal to zero. Thus the value of cos θ is equal to unity and hence h is given by the expression

This equation for capillary rise (or depression) indicates that the smaller the radius r the greater is the capillary rise (or depression). The above obtained expression for the capillary rise (or depression) is based on the assumption that the meniscus or the curved liquid surface is a section of a sphere. This is, true only in case of the tubes of small diameters (r < 2.5 mm) and as the size of the tube becomes larger, the meniscus becomes less spherical and also gravitational forces become more appreciable. Hence such simplified solution for computing the capillary rise (or depression) is possible only for the tubes of small diameters.

However, with increasing diameter of tube, the capillary rise (or depression) becomes much less. It has been observed that for tubes of diameters 6 mm or more the capillary rise (or depression) is negligible. Hence in order to avoid a correction for the effects of capillarity in manometers, used for measuring pressures, a tube of diameter 6 mm or more should be used. Another assumption made in deriving this equation is that the liquids and tube surfaces are extremely clean. In practice, such cleanliness is virtually never encountered and h will be found to be considerably smaller than that given by the above equation.

If a tube of radius r is inserted in mercury (specific gravity s1) above which a liquid of specific gravity s2 lies, then by considering the conditions of equilibrium it can be shown that the capillary depression h is given by

in which ‘σ’ is the surface tension of mercury in contact with the liquid.

Further if two vertical parallel plates ‘t’ distance apart and each of width ‘l’ are held partially immersed in a liquid of surface tension σ and specific gravity ‘s’, then the capillary rise (or depression) ‘h’ may be determined by equating the weight of the liquid column h (or the total internal pressure in the case of capillary depression) (swhlt) to the force due to surface tension (2σl cos θ). Thus we have

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Engineering Surveys for Highway Alignment

Before a highway alignment is finalized in a new highway project, engineering surveys are to be carried out. These engineering surveys may be completed in the following four stages.

i) Map Study

ii) Reconnaissance Survey

iii) Preliminary Surveys

iv) Final Location and Detailed Surveys

1) Map Study

If the topographic map of the area is available, it is possible to suggest the likely routes of the road. In India, topographic maps are available from the Survey of India with 15 or 30 meter contour intervals. The main features like rivers, hills valleys, etc. are also shown on these maps. By careful study of such maps, it is possible to have an idea of several possible alternate routes so that further details of these may be studied later at the site. The probable alignment can be located on the map from the following details available on the map.

1. Alignment avoiding valleys, ponds or lakes
2. When the road has to cross a row of hills or mountains, possibility of crossing through a mountain pass
3. Approximate location of bridge site for crossing rivers, avoiding bend of the river, if any
4. When a road is to be connected between two stations, one of the top and the other on the foot of the hill, then alternate routes can be suggested by keeping in view that the design or ruling gradient and the maximum permissible gradient.

2) Reconnaissance

The second stage of engineering surveys for highway alignment is the reconnaissance survey. During the reconnaissance, the engineer visits the site and examines the general characteristics of the area before deciding the most feasible routes for detailed studies. A field survey party may inspect a fairly broad stretch of land along the proposed alternative routes of the map in the field. Only very simple survey instruments are used by the reconnaissance party to collect additional details rapidly, but not accurately. All relevant details which are not available in the map are collected and noted down. Some of the details to be collected during reconnaissance are given below.

1. Valleys, ponds, lakes, marshy land, ridge, hills, permanent structures and other obstructions along the route which are not available in the map.
2. Approximate values of gradient, length of gradients and radius of curves of alternate alignments.
3. Number and type of cross drainage structures, maximum flood level and natural ground water level along the probable routes.
4. Soil type along the routes from field identification tests and observation of geological features
5. Sources of construction materials, water and location of stone quarries
6. When the road passes through hilly or mountainous terrain, additional data regarding the geological formation, type of rocks, dip of strata, seepage flow etc. may be observed so as to decide the stable and unstable sides of the hill for highway alignment.

3) Preliminary Survey

The main objectives of the preliminary survey are:

1. To survey the various alternate alignments proposed after the reconnaissance and to collect all the necessary physical information and details of topography, drainage and soil.
2. To compare the different proposals in view of the requirements of a good alignment
3. To estimate quantity of earth work materials and other construction aspects and to work out the cost of alternate proposals
4. To finalize the best alignment from all considerations the preliminary survey is carried out to collect all the physical information which are necessary in connection with the proposed highway alignment.

The preliminary survey may be carried out by any one of the following methods.

i) Soil Survey

Soil survey is an essential part of the preliminary survey as the suitability of the proposed location is to be finally decided based on the soil survey data. The soil survey conducted at this stage also helps in working out details of earth work, slopes, suitability of materials, subsoil and surface drainage requirements and pavement type and the approximate thickness requirements. All these details are required to make a comparative study of alternate proposals.

ii) Material Survey

The survey for naturally occurring materials likes stone aggregates, soft aggregates, etc. and identification of suitable quarries should be made. Also, availability of manufactured materials like cement, lime, brick, etc. and their locations may be ascertained.

iii) Traffic Survey

Traffic surveys conducted in the region form the basis for deciding the number of traffic lanes and roadway width, pavement design and economic analysis of the highway project. Traffic volume counts of the classified vehicles are to be carried out on all the existing roads in the region, preferably for 24 hours per day for seven days. Origin and destination surveys are very useful for deciding the alignment of the roads. This study may be carried out on a suitable sample of vehicle users or drivers. In addition, the required traffic data may also be collected so that the traffic forecast could be made for 10 to 20 year periods.

iv) Determination of Final Centre Line

After completing the preliminary surveys and conducting the comparative studies of alternative alignments, the final centre line of the road is to be decided in the office before the final location survey. For this, the preliminary survey maps consisting of contour plans, longitudinal profile and cross sections of the alternate alignments should be prepared and carefully studied to decide the best alignment satisfying engineering, aesthetic and economical requirements. After selecting the final alignment, the grade lines are drawn and the geometric elements of the horizontal and vertical alignments of the road are designed.

v) Rapid Method Using Aerial Survey and Modern Technique Using GPS

Aerial photographic surveys and photogrammetric methods are very much suited for preliminary surveys, especially when the distance and area to be covered are vast. The survey may be divided into the following steps.

Taking aerial photographs of the strips of land to be surveyed with the required longitudinal and lateral overlaps. Vertical photographs are necessary for the preparation of mosaics.

1. The photographs are examined under stereoscopes and control points are selected for establishing the traverses of the alternate proposals. The control points are located on the maps.
2. Using stereo-pair observations, the spot levels and subsequently contour details may be noted down on the maps
3. Photo-interpretation methods are used to assess the geological features, soil conditions, drainage requirements etc.

4) Final Location and Detailed Survey

The alignment finalized at the design office after the preliminary survey is to be first located on the field by establishing the centre line. Next detailed survey should be carried out for collecting the information necessary for the preparation of plans and construction details for the highway project.

i) Location

The centre line of the road finalized in drawings is to be transferred on the ground during the location survey. This is done using a transit theodolite and by staking of the centre line. The location of the centre line should follow, as closely as practicable, the alignment finalized after the preliminary surveys. Major and minor control points are established on the ground and centre pegs are driven, checking the geometric design requirements. However, modifications in the final location may be made in the field, if found essential. The centre line stakes are driven at suitable intervals, say at 50 metre intervals in plain and rolling terrains and at 20 metre in hilly terrain.

ii) Detailed Survey

Temporary bench marks are fixed at intervals of about 250 m and at all drainage and under pass structures. Levels along the final centre line should be taken at all staked points. Levelling work is of great importance as the vertical alignment, earth work calculations and drainage details are to be worked out from the level notes. The cross-section levels are taken up to the desired width, at intervals of 50 to 100 m in plain terrain, 50 to 75 m in rolling terrain, 50 m in built up areas and 20 m in hilly terrain. The cross sections may be taken at closer intervals at horizontal curves and where there is abrupt change in cross slopes. All river crossing, valleys etc. should be surveyed in detail up to considerable distances on either side. All topographical details are noted down and also plotted using conventional signs. Adequate hydrological detail is also collected and recorded.

Drawings and Report

Drawings

The following drawings are usually prepared in a highway project.

i) Key Map

Key map should show the proposed and existing roads, and important places to be connected. The size of the plan generally should not exceed 22 x 20 cm. The scale of the map is chosen suitably depending upon the length of road.

ii) Index Map

Index map should show the general topography of the area. The details are symbolically represented. The index map should also be of suitable scale, the size being 32 x 20 cm.

iii) Preliminary Survey Plans

Preliminary survey plans showing details of the various alternate alignments and all information collected should be normally drawn to scale of 10 cm = 1 km to 25 cm =1 km.

iv) Detailed Plan and Longitudinal Section

Detailed plans show the ground plan with alignment and the boundaries, contours at intervals of 1 to 2 m in plain terrain and 3 to 6 m in hills, showing all details including existing structures. A scale of 1/2400 in close country and a scale of 1/1200 may be adopted for detailed plans. The size of the drawing may be A2 size or 60 x 42 cm approximately.

Longitudinal sections should be drawn to the same horizontal scale of the ground as in detailed plan. Vertical scale may be enlarged 10 times of the longitudinal scale. The longitudinal section should show the details such as datum line, existing ground surface, vertical profile of the proposed road and position of drainage crossings.

v) Detailed Cross Section

Detailed cross sections are generally drawn to natural scale of 1 cm = 2.0 to 2.5 m. Cross section should be drawn every 100 m or where there are abrupt changes in level. In hill roads the cross sections should be drawn at closer intervals. The cross-section drawings should extend at least up to the proposed right of way. The cross-section number, the reduced distances and the area of filling and/or cutting should be shown on cross section drawings.

vi) Land Acquisition Plans

Land acquisition plans and schedules are usually prepared from the survey drawings for land acquisition details. These plans show all general details such as buildings, wells, nature of gradients and other details required for assessing the values. The scale adopted may be 1 cm = 40m or less.

vii) Drawings of Cross Drainage and Other Retaining Structures

Detailed design for cross drainage and masonry structures are usually drawn to scale of 1 cm = 1 m. For details of any complicated portion of the structure enlarged scales up to 8 cm = 1 m or up to half full size may be employed. However, the size of drawing should not exceed the standard size. Cross sections of streams should be to a scale of not less than 1 cm = 10 m.

viii) Drawings of Road Intersections

Drawings of road intersections should be prepared showing all details of pavement, shoulders, islands etc. to scale.

ix) Land Plans Showing Quarries, etc.

Where quarries for construction materials are to be acquired for new projects, separate land plans should be prepared. The size of these maps and scales may be similar to those suggested under land acquisition.

Estimates

The project estimates should consist of general abstract of cost and detailed estimates for each major head. If the project work is proposed to be executed in stages, the estimate should be prepared for each stage separately.

Project Report

The first phase of project report soon after completing the preliminary surveys, feasibility and EIA studies is to prepare a 'Feasibility Report'. The Detailed Project Report (DPR) should be prepared after completing all the detailed studies including final location survey, preparation of longitudinal and cross sections, soil and material surveys, drainage studies, etc. The design details of the pavements and all Cross Drainage structures including major bridges should be carried out and the relevant drawings prepared as specified in the terms of reference for the project preparation.

Highway Projects

In a new highway project, the engineer has to plan, design and construct either a network of new roads or a road link. There are also projects requiring redesign and realignment of existing roads of upgrading the geometric design standards. Once a highway is constructed, development takes place along the adjoining land and subsequent changes in alignment or improvements in geometric standards become very difficult. A badly aligned highway is not only a source of potential traffic hazard, but also causes a considerable increase in transportation cost and strain on the drivers and the passengers. Therefore, proper investigation and planning are most important in a road project, keeping in view the present day needs as well as the future developments of the region.

New Highway Project

The new highway project work may be divided into the following stages.

1. Selection of route, finalization of highway alignment and geometric design details.
2. Collection of materials and testing of sub grade soil and other construction materials, mix design of pavement materials and design details of pavement layers.
3. Construction stages including quality control.

i) Route Selection

The selection of route is made keeping in view the requirements of alignment and geological, topographical and other features of the locality. However special care should be taken as regards the geometric design standards of the road for possible upgrading of speed standards in future, without being necessary to realign the road. After the alignment if finalized, the plans and working drawings are prepared.

ii) Materials and Design

The soil samples collected from the selected route during the soil surveys are tested in the laboratory in order to design the required pavement thickness and the design of embankment and cut slopes. The basic construction materials such as selected soil, aggregates etc. are collected from the nearest borrow pits and quarries and stacked along the road alignment after subjecting these materials to the specified laboratory tests. In order to design the mixes for the pavement component layers and to specify quality control test values during road construction, mix design tests are carried out in the laboratory.

iii) Construction

The construction of the road may be divided into two stages,

i) Earth work

ii) Pavement construction

The earth work consists of excavation and construction of the embankments. During the excavation for highway cuts, the earth slopes, their protection and construction of drainage network are taken care of. Highway embankments may be best constructed by rolled fill method by compacting the soil in layers under controlled moisture and density using suitable rollers. In the case of high embankments, the stability of the embankment foundation and slopes and the possible settlement of the embankment with time are to be investigated. The pavement construction is subsequently taken up starting with the preparation of sub grade and the construction of sub base, base and surface courses of the pavement.

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Highway Alignment

Once the necessity of the highway is assessed, the next process is deciding the alignment. The highway alignment can be either horizontal or vertical.

Alignment

The position or the layout of the central line of the highway on the ground is called the alignment. It is an arrangement in a straight line or in correct relative positions. Horizontal alignment includes straight and curved paths. Vertical alignment includes level and gradients. Alignment decision is important because a bad alignment will enhance the construction, maintenance and vehicle operating cost. Once an alignment is fixed and constructed, it is not easy to change it due to increase in cost of adjoining land and construction of costly structures by the roadside. A new road should be aligned very carefully as improper alignment would result in increase in one or more of the following

a) Construction cost

b) Maintenance cost

c) Vehicle operation cost

d) Accident rate

Requirements

The basic requirements of an ideal alignment between two terminal stations are that it should be:

a) Short

b) Easy

c) Safe and

d) Economical

a) Short

It is desirable to have a short (or shortest) alignment between two terminal stations. A straight alignment would be the shortest, though there may be several practical considerations which would cause deviations from the shortest path.

b) Easy

The alignment should be such that it is easy to construct and maintain the road with minimum problems. Also, the alignment should be easy for the operation of vehicles with easy gradients and curves.

c) Safe

The alignment should be safe enough for construction and maintenance from the view point of stability of natural hill slopes, embankment and cut slopes and foundation of embankments. Also, it should be safe for the traffic operation with safe geometric features.

d) Economical

The road alignment could be considered economical only if the total life cycle cost considering the initial cost, maintenance cost and vehicle operation cost is lowest.

Factors Controlling Alignment

It is not always possible to satisfy all these requirements mentioned above. Hence we have to make a judicial choice considering all the factors. For an alignment to be shortest, it should be straight between the two terminal stations. This is not always possible due to various practical difficulties such as intermediate obstructions and topography. A shortest route may have very steep gradients and hence not easy for vehicle operation. Similarly, there may be construction and maintenance problems along a route which may otherwise be short and easy. Roads are often deviated from the shortest route in order to cater for intermediate places of importance or obligatory points.

A road which is economical with lowest initial construction cost, need not necessarily be the most economical in maintenance or in vehicle operation cost. It may also happen that the shortest and easiest route for vehicle operation may work out to be the costliest of the different alternatives from construction view point. Thus, it may be seen that an alignment can seldom fulfil all the requirements simultaneously; hence a judicial choice is made considering all the factors. The various factors that control the alignment are as follows.

1) Obligatory Points

These are the control points governing the highway alignment. These points are classified into two categories. Points through which it should pass and points through which it should not pass. Obligatory points through which the road alignment has to pass are generally due to the topographic and other site conditions including natural obstructions. Some of the examples of this category include location of a mountain pass, suitable location of bridge to cross a river, presence of quarry or an intermediate town to be connected. These obligatory points necessitate deviation of the road alignment from the straight alignment with shortest or easiest path. Some of the examples are:

i) Bridge Site

The bridge can be located only where the river has straight and permanent path and also where the abutment and pier can be strongly founded. The road approach to the bridge should not be curved and skew crossing should be avoided as possible. Thus to locate a bridge the highway alignment may be changed.

ii) Mountain

While the alignment passes through a mountain, the various alternatives are to either construct a tunnel or to go round the hills. The suitability of the alternative depends on factors like topography, site conditions and construction and operation cost.

iii) Intermediate Town

The alignment may be slightly deviated to connect an intermediate town or village nearby.

These were some of the obligatory points through which the alignment should pass.

There are obligatory points through which the road should not pass and these locations may make it necessary to deviate from the proposed shortest alignment. The obligatory points which should be avoided while aligning a road include religious places, very costly structures, unsuitable land etc. Religious places like temple, mosque, church, grave or tomb have been protected by the law from being acquired for any purpose. Acquiring costly structures would mean heavy compensation resulting in increased cost. Marshy, peaty and water logged areas are generally unsuitable for road construction and should be avoided as far as possible. If a marshy land with peaty soil falls on the path of a straight alignment, it may be necessary to deviate the road alignment from the straight path and go around the unsuitable land or pond. The other alternative method is to resort to very expensive construction techniques. The points through which the alignment should not pass are given below.

i) Religious Places

These have been protected by the law from being acquired for any purpose. Therefore, these points should be avoided while aligning.

ii) Very Costly Structures

Acquiring such structures means heavy compensation which would result in an increase in initial cost. So the alignment may be deviated not to pass through that point.

iii) Lakes or Ponds etc.

The presence of a lake or pond on the alignment path would also necessitate deviation of the alignment.

2) Traffic

The road alignment should be decided based on the requirements of road traffic. Origin and Destination study should be carried out in the area and the desire lines be drawn showing the trend of traffic flow. The new road to be aligned should keep in view the desire lines, anticipated traffic flow, classified traffic volume, their growth and future trends.

3) Geometric Design

Geometric design factors such as gradient, radius of curve and sight distances also would govern the final alignment of the highway. If straight alignment is aimed at, often it may be necessary to provide very steep gradients. As far as possible while aligning a new road, the gradient should be flat and less than the ruling or design gradient. Thus, it may be necessary to change the alignment considering the design speed, maximum allowable super elevation and coefficient of lateral friction. It may be necessary to make adjustment in the horizontal alignment of roads keeping in view the minimum radius of curve and the transition curves. The absolute minimum sight distance, which should invariably be made available in every section of the road, is the safe stopping distance for the fast moving vehicles. Also, there should be enough distance visible ahead for safe overtaking manoeuvres of vehicles moving at design speed on the road. Hence the alignment should be finalized in such a way that the obstructions to visibility do not cause restrictions to the sight distance requirements.

4) Economics

The alignment finalized based on the above factors should also be economical. While working out the economics, the factors to be considered are,

i) initial construction cost of the road,

ii) regular and periodic maintenance cost of the road and

iii) vehicle operation cost in future years.

While trying to decrease the initial construction cost, either the future road maintenance cost or vehicle operation cost or both of these may increase considerably. Therefore, while carrying out economic analysis, it is essential to work out overall economics based on life cycle cost of the road project and not consider the initial cost of the road project only.

5) Other Considerations

Various other factors which may govern either the horizontal or vertical alignment of the road are drainage considerations, hydrological factors, political considerations and monotony. The vertical alignment is often guided by drainage considerations. The sub-surface water level, seepage flow and high flood level are the factors to be kept in view, while deciding the highway alignment.

Types of Alignment

1) Horizontal Alignment

Horizontal alignment in road design consists of straight sections of road, known as tangents, connected by circular horizontal curves. It is the design of the road in the horizontal plane. It consists of a series of tangents (straight lines), circular curves and transition curves. It should provide safe travel at a uniform design speed.

2) Vertical Alignment

Vertical alignment is the longitudinal section and it consists of straight grades joined by vertical curves. Vertical alignment specifies the elevations of points along the roadway. Once the road is aligned and constructed, it is not easy to change the alignment due to increase in cost of adjoining land and construction of costly structures by the road side, as the land value increases manifolds once the road is opened to traffic.

Principles of Highway Alignment

The alignment of a highway is a three-dimensional problem measured in x, y, and z coordinates. This is illustrated, from a driver’s perspective, in Fig.1. However, in highway design practice, three-dimensional design computations are cumbersome and the more important thing is the actual implementation and construction. As a consequence, the three-dimensional highway alignment problem is reduced to two two-dimensional alignment problems, as illustrated in Fig. 2. One of the alignment problems in this figure corresponds roughly to x and z coordinates and is referred to as horizontal alignment. The other corresponds to highway length (measured along some constant elevation) and y coordinates (elevation) and is referred to as vertical alignment. The horizontal alignment of a highway is referred to as the plan view, which is roughly equivalent to the perspective of an aerial photo of the highway. The vertical alignment is represented in a profile view, which gives the elevation of all points measured along the length of the highway.

Fig.1 Highway Alignment in Three Dimensions

Fig. 2 Highway Alignment in Two-Dimensional Views

Special Considerations while Aligning Roads on Hilly Areas

During alignment of hill roads, special care should be taken on the following points which pertain to the hill roads.

i) Stability of Hill Side Slopes

While aligning hill roads, special care should be taken to align the road along the side of the hill which is stable. A common problem in hill roads is that of landslides. The cutting and filling of earth to construct roads on hill-side causes steepening of existing slopes and this affect its stability of the hill slopes.

ii) Drainage of Surface and Subsurface Water Flowing from the Hill Side

Numerous hill-side drains should be provided for adequate drainage facility across the road. But the cross-drainage structure being costly, attempts should be made to align the road in such a way that the number of very expensive cross drainage structures is kept minimum.

iii) Special Geometric Standards for Hill Roads

Different sets of geometric design standards are followed on hill roads with reference to gradient, curves and speed, and they consequently influence the sight distance, radius of curve and other related features. The route should enable the ruling gradient to be attained in most of the length, minimizing steep gradients, hair pin bands and needless rise and fall.

iv) Resisting Length

The resisting length of a road may be calculated from the total work to be done to move the loads along the route taking the horizontal length, the actual difference in levels between the two stations and the sum of ineffective rise and fall in excess of floating gradient. In brief, the resisting length of the alignment should have kept as low as possible. Thus, the ineffective rise and excessive fall should be kept minimum.

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Classification of Roads

The roads are generally classified into two categories, depending on whether they can be used during different seasons of the year.

i) All-weather Roads

All-weather roads are those which are negotiable during all seasons of the year, except at major river crossings where some interruption to traffic is permissible up to a certain extent, but the road pavement should be negotiable during all weathers.

ii) Fair-weather Roads

Roads which are negotiable only during fair weather are called fair weather roads. On fair weather roads, the traffic may be interrupted during monsoon season at causeways where streams may overflow across the road.

Based on the type of the carriageway or the road pavement, the roads are classified as `paved roads' and `un-paved roads'.

i) Paved Roads

The roads with a hard pavement surface on the carriageway are called 'paved roads'.

E.g. Stones, Water bound macadam (WBM), Bituminous macadam (BM), Concrete roads

ii) Unpaved Roads

The roads without a hard pavement surface on the carriageway are called 'unpaved roads'. Earth roads and gravel roads may be called unpaved roads.

Based on the type of pavement surfacing provided, the roads may be classified as `surfaced roads' and `un-surfaced roads’.

i) Surfaced Roads

Road pavements with any type of bituminous surface or cement concrete are called surfaced roads.

E.g. BM road, Concrete road

ii) Un-Surfaced Roads

The roads which are not provided with a bituminous or cement concrete surfacing are called un-surfaced roads.

E.g. Soil or Gravel road

Methods of Classification of Roads

The roads are generally classified based on the following.

a) Traffic volume

b) Load transported or tonnage

c) Location and function

The classification based on traffic volume or tonnage has been arbitrarily fixed by different agencies and there may not be a common agreement regarding the limits for each of classification group. Based on the traffic volume or flow, the roads are classified as heavy, medium and low volume roads. These terms are relative and so the limits under each class should be clearly defined and expressed as vehicles per day or 'annual average daily traffic'.

The classification based on load or tonnage is also relative and the roads may be classified as class I, II etc. or class A, B etc. and the limits may be expressed in terms of tones per day. The classification based on location and function should therefore be a more acceptable classification method for a country as they may be defined clearly.

Road Classification Based on Location and Function

1) Road Classification as per Nagpur Road Plan

The Nagpur Road Plan classified the roads in India based on location and function into following five categories and are described below.

i) National Highways (NH)

National Highways (NH) are main highways running through the length and breadth of India, connecting major ports, foreign highways, capitals of large states and large industrial and tourist centres including roads required for strategic movements for the defence of India. All of the national highways are assigned the respective numbers. For example, NHI is the national highway connecting Delhi, Ambala, Jalandhar and Amritsar (up to Pakistan border); NH-4 connects Thane, Pune, Belgaum, Hubli, Bangalore, Chittoor and Chennai.

ii) State Highways (SH)

State Highways (SH) are arterial roads of a state, connecting the national highways of adjacent state, district headquarters and important cities within the state and serve as the main arteries for traffic to and from district roads. These highways are considered as main arteries of commerce by road within a state or a similar geographical unit.

iii) Major District Roads (MDR)

Major District Roads (MDR) are important roads within a district serving areas of production and markets and connecting with other major roads or main highways of a district. The MDR has lower speed and geometric design specifications than NH or SH.

iv) Other District Roads (ODR)

Other District Roads (ODR) are roads serving rural areas of production and providing them with outlet to market centres, taluk headquarters, block development headquarters or other main roads. These are of lower design specifications than MDR.

(v) Village Roads (VR)

Village Roads (VR) are roads connecting villages or groups of villages with each other to the nearest road of a higher category. It was specified that these village roads should be in essence farm tracks, but it was desired that the prevalent practice of leaving such tracks to develop and maintain by themselves should be replaced by a plan for a designed and regulated system.

2) Road classification as per third 20-year road development plan, 1981 -2001

The road classification system was modified in the third 20-year road development plan. The roads in the country are now classified into three classes, for the purpose of transport planning, functional identification, earmarking administrative jurisdictions and assigning priorities on a road network.

i) Primary System

ii) Secondary System

iii) Tertiary System or Rural Roads

Primary system consists of two categories of highways.

a) Expressways

b) National Highways (NH)

Expressways are a separate class of highways with superior facilities and design standards and are meant as through routes having very high volume of traffic. The expressways are to be provided with divided carriageways, controlled access, grade separations at cross roads and fencing. These highways should permit only fast-moving vehicles. Expressways may be owned by the Central Government or a State Government, depending on whether the route is a National Highway or State Highway. The National Highways form the other main category of primary system in the country.

The Secondary system consists of two categories of roads.

a) State Highways (SH)

b) Major District Roads (MDR)

(Explained above)

The Tertiary systems are rural roads and these consist of two categories of roads.

a) Other District Road (ODR)

b) Village Roads (VR)

(Explained above)

The roads have classified as follows in the order of increased accessibility and reduced speeds.

1) Freeways

Freeways are access controlled divided highways. Most freeways are four lanes, two lanes each direction, but many freeways widen to incorporate more lanes as they enter urban areas. Access is controlled through the use of interchanges and the type.

2) Expressways

They are superior type of highways and are designed for high speeds (120 km/hr is common), high traffic volume and safety. They are generally provided with grade separations at intersections. Parking, loading and unloading of goods and pedestrian traffic is not allowed on expressways.

3) Highways

They represent the superior type of roads in the country. Highways are of two types - rural highways and urban highways. Rural highways are those passing through rural areas (villages) and urban highways are those passing through large cities and towns, i.e. urban areas.

4) Arterials

It is a general term denoting a street primarily meant for through traffic usually on a continuous route. They are generally divided highways with fully or partially controlled access. Parking, loading and unloading activities are usually restricted and regulated. Pedestrians are allowed to cross only at intersections/designated pedestrian crossings.

5) Local Streets

A local street is the one which is primarily intended for access to residence, business or abutting property. It does not normally carry large volume of traffic and also it allows unrestricted parking and pedestrian movements.

6) Collectors Streets

These are streets intended for collecting and distributing traffic to and from local streets and also for providing access to arterial streets. Normally full access is provided on these streets. There are few parking restrictions except during peak hours.

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