Method of Folding of Drawing Sheets

When drawings sheets are in more numbers, they have to be folded and kept in order to save the trace required for preserving them. Folding of drawings applies to only the drawings which are released for shop floor for manufacturing of components/reference. Original drawings will never be taken out of drawing office and they should be kept under safe custody. Drawings which are prepared on tracing sheets/transparencies like cloth, polymer, acrylic polymer transparencies should never be folded. They should be kept in polythene folders and kept in filing cabinets. Sometimes the blue prints/photo copies of drawings which are released to shop floor are also laminated for extending their life. While folding the drawings following care should be taken.

• It is required to the fold the drawings such that, they should not get defaced damaged.
• Drawing sheet to be folded such that the title block is easily visible to retrieve it and keeping it back.

Folding Principle of Drawings

The following is the method of folding printed drawing sheets for drawing sheet of size A1, A2 and A3 as recommended by as per IS 11664 - 1986. There are two methods of folding of drawing prints. The first method is intended for drawing prints to be filed or bound, while the second method is intended for prints to be kept individually in filing cabinet. Depending on the method of folding adopted, suitable folding marks are to be introduced in the tracing sheets as guide.

The basic principles in each of the above methods are to ensure that

• All large prints of sizes higher than A4 are folded to A4 sizes.
• The title blocks of all the folded prints appear in topmost position
• The bottom right corner shall be outermost visible section and shall have a width not less than 190 mm.

Fig. 1 Method of Folding of Drawing Prints

Fig.2 Folding of Prints for Sorting in Filing Cabinet as per IS 11664 - 1986

Fig.3 Folding of Prints for Filing or Binding as per IS 11664 - 1986

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Drawing sheet

Different qualities of drawing sheets are available in the market. Depending upon the nature of the drawing, the qualities of drawing papers are selected. The drawing paper should be of uniform thickness and of such quality that erasing should not have leave any impression on it. For ordinary pencil drawings, the paper selected should be tough and strong. It should be uniform in thickness and as white as possible. One of the sides of the drawing paper is usually rough and the other smooth. The smooth surface is the side for the drawing work. Good quality of paper with smooth surface should be selected for drawings which are to be inked and preserved for a long time. It should be such that the ink does not spread. These are of two types.

1) Hand-Made Paper

Hand-made papers have rough surfaces, pale in colour and not used for regular work, but meant for charts.

2) Mill-Made Paper

Mill-made papers are most commonly used for regular work, and are available in different sizes and rolls. They are specified by their weight in kg per ream or density in grams per square meter.

Designation of sheets

The drawing sheets are designated by symbols such as A0, A1, A2, A3, A4 and A5. A0 being the largest. Table 1 gives the length and breadth of the above sizes of sheets. For class work use of A2 size drawing sheet is preferred.While working or handling, the papers are liable to tear on the edges. So slightly large size (untrimmed) sheets are preferred. They are trimmed afterwards. IS: 10811:1983 give the designation of preferred trimmed and untrimmed sizes.

Table 1: Standard Sizes of Trimmed and Untrimmed Drawing Sheets

Sl. No.	Designation size in mm	Trimmed size in mm (Width x Length)	Untrimmed size in mm (Width x Length)
1	A0	841 x 1189	800 x 1230
2	A1	594 x 841	625 x 880
3	A2	420 x 594	450 x 625
4	A3	297 x 420	330 x 450
5	A4	210 x 297	240 x 330
6	A5	148 x 210	165 x 240

Fig. 1 Standard Size of Drawing Sheets 

Fig. 2 General Features of a Drawing Sheet

Basic Principles

Surface area of A0 size is one square meter. Successive format sizes (from A0 to A5) are obtained by halving along the length or doubling along the width. The areas of the two subsequent sizes are in the ratio 1:2. The basic principles involved in arriving at the sizes are:

(a) x:y = 1: √2

(b) xy =1

Where y and x are the sides and having a surface area of l m² so that x=0.841 m and. y =l.l89 m.

Fig. 3 Relationship between Two Sides

Quality Drawing Paper

The drawing papers should have sufficient teeth or grain to take the pencil lines and withstand repeated erasing. A backing paper is to be placed on the drawing board before fixing drawing/tracing paper, to get uniform lines. Before starting the drawing, the layout should be drawn. White drawing papers which do not become yellow on exposure to atmosphere are used for finished drawings, maps, charts and drawings for photographic reproductions. For pencil layouts and working drawings, cream colour papers are best suited.

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Construction Engineering

Construction Engineering is a professional discipline that deals with the designing, planning, construction and management of infrastructures such as roads, tunnels, bridges, airports, railroads, facilities, buildings, dams, utilities and other projects. Construction engineering is similar to civil engineering, which also focuses on infrastructure design and development, but with more emphasis on managing the construction process on project sites. It is an important field because it ensures structures are safe, well-made and dependable. It also makes sure construction projects get finished by a set date and according to strict plans and building codes.

Construction engineers are involved in nearly every step of a construction project, from its design to its implementation. They manage building projects and maintenance, often being present to oversee workers and activities on-site. Projects and infrastructure that construction engineers might work on include:

• Roads and highways
• Bridges
• Tunnels
• Railroads
• Housing projects
• Airports
• Energy sources like dams
• Facilities such as wastewater treatment plants
• Utilities
• Drainage and sewage systems
• Public buildings such as hospitals and sports stadiums

The typical duties of a construction engineer include:

• Calculating the cost of inspections, testing, materials, equipment and labor to create a budget for each project
• Managing funds appropriately to stay within budget
• Using computer software and simulations to create project designs and 3D models
• Performing risk analysis
• Surveying potential construction sites and planning their layouts
• Preparing bids from contractors and managing the contracting firms they hire
• Choosing and acquiring materials and equipment
• Hiring and overseeing workers and setting their schedules
• Making sure projects follow environmental laws, government regulations and building codes
• Designing and overseeing the construction of temporary structures needed on-site
• Using engineering and business skills to solve any problems that might occur during construction
• Staying up-to-date on the latest technology, building laws and construction processes 

Successful construction projects require a highly coordinated team effort. Builders and skilled trade’s people are required to lay brick, construct frames, install plumbing and electrical systems and ensure completion of a long list of other elements. With a large-scale construction project, construction engineers play an essential role in designing and implementing complicated building plans. They may also oversee the development or maintenance of critical infrastructure, ranging from roads and bridges to dams and water supplies.

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Water

Water is mainly used for construction purposes, preparation and curing of concrete and mortar, preparation of cement paste etc. Water is an important ingredient of concrete as it actively participates in the chemical reaction with cement. Since it helps to form the strength giving cement gel, the quantity and quality of water required must be checked.

Quality of Water

It should be noted that if water is fit for drinking it is fit for making concrete. This does not appear to be a true statement for all conditions. Some waters containing a small amount of sugar would be suitable for drinking but not for mixing concrete and conversely water suitable for making concrete may not necessarily be fit for drinking. Some specifications require that if the water is not obtained from source that has proved satisfactory, the strength of concrete or mortar made with questionable water should be compared with similar concrete or mortar made with pure water.

Some specification also accept water for making concrete if the pH value of water lies between 6 and 8 and the water is free from organic matter. Instead of depending upon pH value and other chemical composition, the best course to find out whether a particular source of water is suitable for concrete making or not, is to make concrete with this water and compare its 7 days and 28 days strength with companion cubes made with distilled water. If the compressive strength is upto 90%, the source of water may be accepted. This criterion may be safely adopted in places like coastal area of marshy area or in other places where the available water is brackish in nature and of doubtful quality.

Carbonates and bi-carbonates of sodium and potassium effect the setting time of cement. While sodium carbonate may cause quick setting, the bi-carbonates may either accelerate or retard the setting. The other higher concentrations of these salts will materially reduce the concrete strength. If some of these salts exceed 1000 ppm, tests for setting time and 28 days strength should be carried out. In lower concentrations they may be accepted. Brackish water contains chlorides and sulphates. When chloride does not exceed 10,000 ppm and sulphate does not exceed 3,000 ppm the water is harmless, but water with even higher salt content has been used satisfactorily.

Salts of Manganese, Tin, Zinc, Copper and Lead cause a marked reduction in strength of concrete. Sodium iodate, sodium phosphate and sodium borate reduce the initial strength of concrete to an extra ordinarily high degree. Silts and suspended particles are undesirable as they interfere with setting, hardening and bond characteristics. A turbidity limit of 2000 ppm has been suggested. Algae in mixing water may cause a marked reduction in strength of concrete either by combining with cement to reduce the bond or by causing large amount of air entrainment in concrete. Algae which are present on the surface of the aggregate have the same effect as in that of mixing water.

The initial setting time of the test block made with a cement and the water proposed to be used shall not differ by ±30 minutes from the initial setting time of the test block made with same cement and distilled water.

Table 1 Permissible limit for solids as per IS 456 of 2000

Material	Tested as per	Permissible limit Max.
Organic	IS 3025 (pt 18)	200 mg/l
Inorganic	IS 3025 (pt 18)	3000 mg/l
Sulphates (as SO3)	IS 3025 (pt 24)	400 mg/l
Chlorides (as Cl)	IS 3025 (pt 32)	2000 mg/l for concrete work not containing embedded steel and 500 mg/l for reinforced concrete work
Suspended	IS 3025 (pt 17)	2000 mg/l

Use of Sea Water for Mixing Concrete

Sea water has a salinity of about 3.5%. In that about 78% is sodium chloride and 15% is chloride and sulphate of magnesium. Sea water also contains small quantities of sodium and potassium salts. This can react with reactive aggregates in the same manner as alkalies in cement. Therefore sea water should not be used even for PCC if aggregates are known to be potentially alkali reactive. It is reported that the use of sea water for mixing concrete does not appreciably reduce the strength of concrete although it may lead to corrosion of reinforcement in certain cases.

Sea water slightly accelerates the early strength of concrete. But it reduces the 28 days strength of concrete by about 10 to 15%. However, this loss of strength could be made up by redesigning the mix. Water containing large quantities of chlorides in sea water may cause efflorescence and persistent dampness. When the appearance of concrete is important, sea water may be avoided. The use of sea water is also not advisable for plastering purpose which is subsequently going to be painted.

Divergent opinion exists on the question of corrosion of reinforcement due to the use of sea water. Some research workers cautioned about the risk of corrosion of reinforcement particularly in tropical climatic regions, whereas some research workers did not find the risk of corrosion due to the use of sea water. Experiments have shown that corrosion of reinforcement occurred when concrete was made with pure water and immersed in pure water when the concrete was comparatively porous, whereas, no corrosion of reinforcement was found when sea water was used for mixing and the specimen was immersed in salt water when the concrete was dense and enough cover to the reinforcement was given. From this it could be inferred that the factor for corrosion is not the use of sea water or the quality of water where the concrete is placed. The factors effecting corrosion is permeability of concrete and lack of cover. However, since these factors cannot be adequately taken care of always at the site of work, it may be wise that sea water be avoided for making reinforced concrete.

For economical or other passing reasons, if sea water cannot be avoided for making reinforced concrete, particular precautions should be taken to make the concrete dense by using low water/cement ratio coupled with vibration and to give an adequate cover of at least 7.5 cm. The use of sea water must be avoided in prestressed concrete work because of stress corrosion and undue loss of cross section of small diameter wires. The latest Indian standard IS 456 of 2000 prohibits the use of sea water for mixing and curing of reinforced concrete and prestressed concrete work. This specification permits the use of sea water for mixing and curing of plain cement concrete (PCC) under unavoidable situation.

(Ref : Concrete Technology Theory and Practice by M.S. Shetty)

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Highway Planning in India

Excavations in the sites of Indus valley, Mohenjo-dero and Harappan civilizations revealed the existence of planned roads in India as old as 2500 - 3500 BC. The Mauryan kings also built very good roads. Ancient books like Arthashastra written by Kautilya, a great administrator of the Mauryan times, contained rules for regulating traffic, depths of roads for various purposes and punishments for obstructing traffic.

During the time of Mughal period, roads in India were greatly improved. Roads linking North-West and the Eastern areas through Gangetic plains were built during this time. After the fall of the Mughals and at the beginning of British rule, many existing roads were improved. The construction of Grand Trunk road connecting North and South is a major contribution of the British. However, the focus was later shifted to railways, except for feeder roads to important stations.

Modern Developments

The First World War period and that immediately following it found a rapid growth in motor transport. So, the need for better roads became a necessity. For that, the Government of India appointed a committee called Road Development Committee with Mr. M.R. Jayakar as the chairman. This committee came to be known as Jayakar committee.

1) Jayakar Committee (1927)

In 1927 Jayakar committee for Indian road development was appointed. The major recommendations and the resulting implementations were given below.

• Committee found that the road development of the country has become beyond the capacity of local governments and suggested that Central government should take the proper charge considering it as a matter of national interest.
• They gave more stress on long term planning programme, for a period of 20 years (hence called twenty year plan) that is to formulate plans and implement those plans within the next 20 years.
• One of the recommendations was the holding of periodic road conferences to discuss about road construction and development. This paved the way for the establishment of a semi official technical body called Indian Road Congress (IRC) in 1934.
• The committee suggested imposition of additional taxation on motor transport which includes duty on motor spirit, vehicle taxation and license fees for vehicles plying for hire. This led to the introduction of a development fund called Central Road Fund in 1929. This fund was intended for road development.
• A dedicated research organization should be constituted to carry out research and development work. This resulted in the formation of Central Road Research Institute (CRRI) in 1950.

2) Nagpur Road Congress (1943)

The Second World War saw a rapid growth in road traffic and this led to the deterioration in the condition of roads. To discuss about improving the condition of roads, the government convened a conference of chief engineers of provinces at Nagpur in 1943. The result of the conference is the Nagpur plan.

• A twenty year development programme for the period (1943-1963) was finalized.
• It was the first attempt to prepare a coordinated road development programme in a planned manner.
• The roads were divided into four classes:
• National Highways which would pass through states, and places having national importance for strategic, administrative and other purposes.
• State Highways which would be the other main roads of a state.
• District Roads which would take traffic from the main roads to the interior of the district.
• According to the importance, some are considered as Major District Roads and the remaining as Other District Roads.
• Village Roads which would link the villages to the road system.
• The committee planned to construct 2 lakh kms of road across the country within 20 years.
• They recommended the construction of star and grid pattern of roads throughout the country.
• One of the objectives was that the road length should be increased so as to give a road density of 16kms per 100 sq.km

3) Bombay Road Congress (1961)

The length of roads envisaged under the Nagpur plan was achieved by the end of it, but the road system was deficient in many respects. The changed economic, industrial and agricultural conditions in the country wanted a review of the Nagpur plan. Accordingly, a 20 year plan was drafted by the Roads wing of Government of India, which is popularly known as the Bombay plan. The highlights of the plan were

• It was the second 20 year road plan (1961-1981)
• The total road length targeted to construct was about 10 lakhs.
• Rural roads were given specific attention. Scientific methods of construction were proposed for the rural roads.
• The necessary technical advice to the Panchayath should be given by State PWD's.
• They suggested that the length of the road should be increased so as to give a road density of 32kms/100 sq.km
• The construction of 1600 km of expressways was also then included in the plan.

4) Lucknow Road Congress (1984)

This plan has been prepared by keeping in view of the growth pattern envisaged in various fields by the turn of the century. Some of the salient features of this plan are as given below.

• This was the third 20 year road plan (1981-2001). It is also called Lucknow road plan.
• It aimed at constructing a road length of 12 lakh kilometers by the year 1981 resulting in a road density of 82kms/100 sq.km
• The plan has set the target length of NH to be completed by the end of seventh, eighth and ninth five year plan periods.
• It aims at improving the transportation facilities in villages, towns etc. such that no part of country is farther than 50 km from NH.
• One of the goals contained in the plan was that expressways should be constructed on major traffic corridors to provide speedy travel.
• Energy conservation, environmental quality of roads and road safety measures were also given due importance in this plan.

Current Scenario

About 60% of freight and 87% passenger traffic is carried by road. Although National Highways constitute only about 2% of the road network, it carries 40% of the total road traffic. Easy availability, adaptability to individual needs and cost savings are some of the factors which go in favour of road transport. Road transport also acts as a feeder service to railway, shipping and air traffic. The number of vehicles has been growing at an average pace of around 10% per annum. The share of road traffic in total traffic has grown from 13.8% of freight traffic and 15.4% of passenger traffic in 1950-51 to an estimated 62.9% of freight traffic and 90.2% of passenger traffic by the end of 2009-10. The rapid expansion and strengthening of the road network, therefore, is imperative, to provide for both present and future traffic and for improved accessibility to the hinterland.

Road Development Plan Vision: 2021

The Government of India takes up the development works of National Highways through five year plans. However, the Ministry in 2001 had prepared, through Indian Roads Congress (IRC), `Road Development Plan VISION: 2021’ for a period of 20 years (2001-2021). This document provides the vision for the next 20 years for development and maintenance of all categories of roads i.e. National Highways, State Highways, Major District Roads and Rural Roads. The urban roads as well as the roads for specific need e.g. tourism, forestry, mining and industrial areas etc. have also been considered. The research and development, mobilization of resources, capacity building and human resources development, quality system, environment and energy considerations for the highway sector and highway safety are also included in this document which serves as only a valuable guide to the Centre and the State Governments for planning purpose. Salient features of Vision: 2021 are given below.

• To construct National Highway such that, it forms 100 sq.km network.
• To construct Express Highway for fast moving vehicle and Four-lane road having maximum traffic density.
• To connect District Head quarter by four lane, Taluk head quarters, Industrial centre, Tourist centre by two lane State Highways.
• To connect Village having population more than 1500 by MDR (Major District Road).
• To connect Village having population 1000 to 1500 by ODR (Other District Road).
• To connect remote Village by all weather road.

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Fire Demand

Fire demand is the amount and rate of supply of water that are required to extinguish the largest probable fire that could occur in a community. Usually, a fire occurs in factories and stores. The quantity of water required for firefighting purposes should be easily available and always kept stored in the storage reservoir. When population increases number of building also increases and the risk also increases. Fire demand is considered in designing pumps, reservoirs and distribution system.

Following requirement must be met for fire demand.

• Three jet streams are simultaneously thrown from each hydrant; one on the burning property and one each on adjacent property on either sides of the burning property. The discharge of each stream should be about 1100 liters/minute.
• The minimum water pressure available at fire hydrants should be of the order of 100 – 150 kN/m2 and should be maintained even after 4 to 5 hours of constant use of fire hydrant.

In case of public water supply, fire demand is treated as a function of population and some of the empirical formulae, commonly used for calculating the fire demand are as follows.

1) Kuichling’s Formula

Q = 3182 √P

Where, Q = Quantity of water required in liters per minute

             P = Population in thousands

2) Freeman Formula 

 Where, ‘P’ and ‘Q’ have the same meaning as above.

3) Buston’s Formula

Q = 5663 √P

Where, ‘P’ and ‘Q’ have the same meaning as above.

4) National Board of Fire under Writers Formula

A) For a central congested high valued city

(i) When population ≤ 200000

Q = 4637√P [1−0.01√P ]

Where, ‘P’ and ‘Q’ have the same meaning as above.

(ii) When population > 200000

A provision for 54600 liters per minute may be made with an additional provision of 9100 to 36400 liters/minute for a second fire.

(B) For a residential city

(i) For small or low buildings: Q = 2200 liters/minute.

(ii) For large or higher buildings: Q = 4500 liters/minute.

Example Question

Compute the ‘fire demand’ for a city of 200000 populations by all the above formulae.

Solution

1) By using Kuichling’s Formula

                                                               Q = 3182 √P

                                                                   = 3182 √200

                                                                   = 45000.275 l/min

2) By using Freeman Formula 

                                                                               Q = 56800 l/min

3) By using Buston’s Formula

                                                              Q = 5663 √P

                                                              Q = 5663 √200

                                                                  = 80086.914 l/min

4) By using National Board of Fire under Writers Formula

                                                             Q = 4637√P [1−0.01√P ]                                                 

                                                                    = 4637√200 [1−0.01√200 ]

                                                                    = 56303.080 l/min

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Weathering of Rock

Weathering is the process of breaking down rocks by mechanical and chemical processes into smaller pieces. Weathering occurs in situ with no visible movement and thus should differ from erosion that, it involves the movement of rocks and minerals by agents such as water, ice, wind and gravity. The weathering of the rocks might be by physical disintegration, and/or chemical decomposition.

1) Physical/Mechanical Weathering

Physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions such as heat, water, ice and pressure, without any change in chemical condition. The soil formed due to physical weathering will be cohesionless (sand and gravel). It reduces the size of the parent rock material, without any change in the original composition of the parent rock. Mechanical weathering may be caused by the expansion and contraction of rocks from the continuous gain and loss of heat, which results in ultimate disintegration. Frequently, water seeps into the pores and existing cracks in rocks. As the temperature drops, the water freezes and expands. The pressure exerted by ice because of volume expansion is strong enough to break down even large rocks. Other physical agents that help disintegrate rocks are glacier ice, wind, the running water of streams and rivers, and ocean waves.

The main processes involved are exfoliation, unloading, erosion, freezing, and thawing. The principal cause is climatic change. In exfoliation, the outer shell separates from the main rock. Heavy rain and wind cause erosion of the rock surface. Adverse temperature changes produce fragments due to different thermal coefficients of rock minerals.

Temperature changes of sufficient amplitude and frequency bring about changes in the volume of the rocks in the superficial layers of the earth's crust in terms of expansion and contraction. Such a volume change sets up tensile and shear stresses in the rock ultimately leading to the fracture of even large rocks. This type of rock weathering takes place in a very significant manner in arid climates where free, extreme atmospheric radiation brings about considerable variation in temperature at sunrise and sunset. Erosion by wind and rain is a very important factor and a continuing event. Cracking forces by growing plants and roots in voids and crevasses of rock can force fragments apart.

In summary, the physical agencies causing mechanical weathering of rocks are;

• Daily and seasonal temperature changes.
• Flowing water, glaciers and wind, which produce impact and abrasive action on rock.
• Splitting action of ice.
• Growth of roots of plants in rock fissures and to a minor degree burrowing activities of small animals like earthworms.

2) Chemical Weathering

Chemical weathering (decomposition) can transform hard rock minerals into soft, easily erodible matter. The principal types of decomposition are hydration, oxidation, carbonation, desalination and leaching. In chemical weathering, the original rock minerals are transformed into new minerals by chemical reaction. Oxygen and carbon dioxide which are always present in the air readily combine with the elements of rock in the presence of water. Water and carbon dioxide from the atmosphere form carbonic acid, which reacts with the existing rock minerals to form new minerals and soluble salts. Soluble salts present in the groundwater and organic acids formed from decayed organic matter also cause chemical weathering.

Chemical weathering changes the composition of rocks by decomposing the parent minerals, transforming them into new compounds such as clay silica particles, carbonates and iron oxides. An example of the chemical weathering of orthoclase to form clay minerals, silica, and soluble potassium carbonate follows:

                                             H₂O + CO₂  → H₂CO₃  →  H⁺  +  (HCO₃ ) ^–

                              2K(AlSi₃O₈)  +  2H⁺  +  H₂O  →  2K⁺  +  4SiO₂ + Al₂Si₂O₅(OH)₄

 

                                                                                                 H₂CO₃  -  Carbonic acid

                                                                                                 2K(AlSi₃O₈)  -  Orthoclase

                                                                                                 SiO₂  - Silica

                                                                                                 Al₂Si₂O₅(OH)₄  - Kaolinite (Clay mineral)

Most of the potassium ions released are carried away in solution as potassium carbonate is taken up by plants. The chemical weathering of plagioclase feldspars is similar to that of orthoclase in that it produces clay minerals, silica and different soluble salts. Ferromagnesian minerals also form the decomposition products of clay minerals, silica and soluble salts. The iron and magnesium in ferromagnesian minerals result in other products such as hematite and limonite. Quartz is highly resistant to weathering and only slightly soluble in water.

The effects of weathering and transportation mainly determine the basic nature of the soil (size, shape, composition and distribution of the particles). The environment into which deposition takes place, and the subsequent geological events that take place there, determine the state of the soil (density, moisture content) and the structure or fabric of the soil (bedding, stratification, occurrence of joints or fissures). The decomposition of rock is the result of the following reactions.

1) Oxidation

Within the weathering environment, oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe2⁺ (iron) and combination with oxygen and water to form Fe3⁺ hydroxides and oxides such as goethite, limonite and hematite. This gives the affected rocks a reddish-brown colour on the surface which crumbles easily and weakens the rock. This process is better known as ‘rusting’.

2) Carbonation

Carbonation of rock material is caused by carbon dioxide in the presence of water. Limestone is very much affected by carbonation.

3) Hydration

Mineral hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral. When rock minerals take up water, the increased volume creates physical stresses within the rock. For example iron oxides are converted to iron hydroxides and the hydration of anhydrite forms gypsum. Another example of hydration is the chemical decomposition of mineral feldspar in granite to form kaolite.

4) Leaching

Leaching is the process in which percolating water washes out water-soluble salts from the soil. Soil produced by chemical weathering of rocks will be cohesive (silt and clay).

Transportation of Weathering Products

The products of weathering may stay in the same place or may be moved to other places by ice, water, wind and gravity. In water or air, the grains become sub-rounded or rounded and the grain sizes get sorted so as to form poorly graded deposits. In moving ice, grinding and crushing occurs, size distribution becomes wider forming well graded deposits. In running water, soil can be transported in the form of suspended particles, or by rolling and sliding along the bottom. Coarser particles settle when a decrease in velocity occurs, whereas finer particles are deposited further downstream. In still water, horizontal layers of successive sediments are formed, which may change with time, even seasonally or daily. Wind can erode, transport and deposit fine grained soils. Wind-blown soil is generally uniformly graded. A glacier moves slowly but scours the bedrock surface over which it passes. Gravity transports materials along slopes without causing much alteration.

The soils formed by the weathered products at their place of origin are called residual soils. An important characteristic of residual soil is the gradation of particle size. Fine-grained soil is found at the surface, and the grain size increases with depth. At greater depths, angular rock fragments may also be found. The transported soils may be classified into several groups, depending on their mode of transportation and deposition.

1. Glacial soils - formed by transportation and deposition of glaciers

2. Alluvial soils - transported by running water and deposited along streams

3. Lacustrine soils - formed by deposition in quiet lakes

4. Marine soils - formed by deposition in the seas

5. Aeolian soils - transported and deposited by wind

6. Colluvial soils - formed by movement of soil from its original place by gravity, such as during landslides

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Origin of Soil/Formation of Soil

Soil is formed by the process of ‘weathering’ of rocks, that is, disintegration and decomposition of rocks and minerals at or near the earth’s surface through the actions of natural or mechanical and chemical agents into smaller and smaller grains. The factors of weathering may be atmospheric, such as changes in temperature and pressure, erosion and transportation by wind, water and glaciers, chemical action such as crystal growth, oxidation, hydration, carbonation and leaching by water, especially rainwater, with time.

Soil is formed from rock due to erosion and weathering action. Igneous rock (e.g. granite) is the basic rock formed from the crystallization of molten magma (underground) or lava (above the ground). This rock is formed either inside the earth or on the surface. These rocks undergo metamorphism under high temperature and pressure to form Metamorphic rocks (e.g. marble). Both Igneous and metamorphic rocks are converted in to sedimentary rocks (e.g. limestone, shale) due to transportation to different locations by the agencies such as wind, water etc. Millions of years of erosion and weathering convert rocks in to soil.

Soils formed by mechanical weathering bear a similarity in certain properties to the minerals in the parent rock, since chemical changes which could destroy their identity do not take place. It is to be noted that 95% of the earth’s crust consists of igneous rocks and only the remaining 5% consists of sedimentary and metamorphic rocks. However, sedimentary rocks are present on 80% of the earth’s surface area.

Feldspars are the minerals abundantly present (60%) in igneous rocks. Amphiboles and pyroxenes, quartz and mica come next in that order. Rocks are altered more by the process of chemical weathering than by mechanical weathering. In chemical weathering some minerals disappear partially or fully, and new compounds are formed. The intensity of weathering depends upon the presence of water and temperature and the dissolved materials in water. Carbonic acid and oxygen are the most effective dissolved materials found in water which cause the weathering of rocks.

Chemical weathering has the maximum intensity in humid and tropical climates. ‘Leaching’ is the process whereby water-soluble parts in the soil such as Calcium Carbonate, are dissolved and washed out from the soil by rainfall or percolating subsurface water. ‘Laterite’ soil, in which certain areas of Kerala abound, is formed by leaching. Harder minerals will be more resistant to weathering action, for example, Quartz present in igneous rocks. But, prolonged chemical action may affect even such relatively stable minerals, resulting in the formation of secondary products of weathering, such as clay minerals— illite, kaolinite and montmorillonite.

Soils are primarily of two types: residual or transported. Residual soils remain at the location of their geologic origin when they are formed by weathering of the parent rock. When the weathered soils are transported by glacier, wind, water, or gravity and are deposited away from their geologic origin, they are called transported soils. Depending on the geologic agent involved in the transportation process, the soil derives its special name: glacier— glacial; wind— aeolian; sea— marine; lake— lacustrine; river— alluvial; gravity— colluvial. Human beings also can act as the transporting agents in the soil formation process, and the soil thus formed is called a fill.

The nature and structure of a given soil depends on the processes and conditions that formed it: 

• Breakdown of parent rock: weathering, decomposition, erosion. 
• Transportation to site of final deposition: gravity, flowing water, ice, wind. 
• Environment of final deposition: flood plain, river terrace, glacial moraine, lacustrine or marine. 
• Subsequent conditions of loading and drainage: little or no surcharge, heavy surcharge due to ice or overlying deposits, change from saline to freshwater, leaching, contamination.

Soil Profile

A deposit of soil material, resulting from one or more of the geological processes is subjected to further physical and chemical changes which are brought about by the climate and other factors. Vegetation starts to develop and rainfall begins the processes of leaching and eluviations of the surface of the soil material. Gradually, with the passage of geological time profound changes take place in the character of the soil. These changes bring about the development of ‘soil profile’.

Thus, the soil profile is a natural succession of zones or strata below the ground surface and represents the alterations in the original soil material which have been brought about by weathering processes. It may extend to different depths at different places and each stratum may have varying thickness. Generally, three distinct strata or horizons occur in a natural soil-profile; this number may increase to five or more in soils which are very old or in which the weathering processes have been unusually intense.

From top to bottom these horizons are designated as A-horizon, B-horizon and C-horizon. The A-horizon is rich in humus and organic plant residue. This is usually eluviated and leached; that is, the ultra fine colloidal material and the soluble mineral salts are washed out of this horizon by percolating water. It is dark in colour and its thickness may range from a few centimeters to half a meter. This horizon often exhibits many undesirable engineering characteristics and is of value only to agricultural soil scientists.

The B-horizon is sometimes referred to as the zone of accumulation. The material which has migrated from the A-horizon by leaching and eluviation gets deposited in this zone. There is a distinct difference of colour between this zone and the dark top soil of the A-horizon. This soil is very much chemically active at the surface and contains unstable fine-grained material. Thus, this is important in highway and airfield construction work and light structures such as single storey residential buildings, in which the foundations are located near the ground surface. The thickness of B-horizon may range from 0.50 to 0.75 m.

The material in the C-horizon is in the same physical and chemical state as it was first deposited by water, wind or ice in the geological cycle. The thickness of this horizon may range from a few centimeters to more than 30 m. The upper region of this horizon is often oxidized to a considerable extent. It is from this horizon that the bulk of the material is often borrowed for the construction of large soil structures such as earth dams. Each of these horizons may consist of sub-horizons with distinctive physical and chemical characteristics and may be designated as A1, A2, B1, B2, etc. The transition between horizons and sub-horizons may not be sharp but gradual. At a certain place, one or more horizons may be missing in the soil profile for special reasons. A typical soil profile is shown in figure.

Fig. 1 Typical Soil Profile

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Precipitation

Precipitation refers to all the moisture that comes to the earth from atmosphere. This may be in the form of rain, snow, sleet, fog, dew or hail. Precipitation can occur only when the air containing moisture is cooled sufficiently to condense a part of the moisture which is present in the atmosphere. The atmospheric air always contains moisture. Evaporation from the oceans is the major source (about 90%) of the atmospheric moisture for precipitation. Continental evaporation contributes only about 10% of the atmospheric moisture for precipitation. The atmosphere contains the moisture even on days of bright sun shine.

For the occurrence of precipitation, some mechanism is required to cool the atmospheric air sufficiently to bring it to (or near) saturation. This mechanism is provided by either convective systems (due to unequal radiative heating or cooling of the earth’s surface and atmosphere) or by orographic barriers (such as mountains due to which air gets lifted up and consequently undergoes cooling, condensation and precipitation) and results into, respectively, convective and orographic precipitations. Alternatively, the air lifted into the atmosphere may converge into a low pressure area (or cyclone) causing cyclonic precipitation. Artificially induced precipitation requires delivery of dry ice or silver iodide or some other cloud seeding agent into the clouds by aircraft or balloons.

Forms of Precipitation

1) Drizzle

A light steady rain in fine drops (0.5 mm) and intensity <1 mm/hr.

2) Rain

The condensed water vapour of the atmosphere falling in drops (>0.5 mm, maximum size - 6 mm) from the clouds.

3) Glaze

Freezing of drizzle or rain when they come in contact with cold objects.

4) Sleet

Frozen rain drops while falling through air at subfreezing temperature.

5) Snow

Ice crystals resulting from sublimation (i.e., water vapour condenses to ice).

6) Snow flakes

Ice crystals fused together.

7) Hail

Small lumps of ice (>5 mm in diameter) formed by alternate freezing and melting, when they are carried up and down in highly turbulent air currents.

8) Dew

Moisture condensed from the atmosphere in small drops upon cool surfaces.

9) Frost

A feathery deposit of ice formed on the ground or on the surface of exposed objects by dew or water vapour that has frozen.

10) Fog

A thin cloud of varying size formed at the surface of the earth by condensation of atmospheric vapour (interfering with visibility).

11) Mist

A very thin fog.

Classification of Rainfall Based on Rate

Table 1: Classification of Rainfall Based on Rate

Sl. No.	Type of Rainfall	Rate (mm/hr)
1	Light	upto 2.5
2	Moderate	2.5  - 7.5
3	Heavy	>7.5

Types of Precipitation

Adiabatic cooling resulting from the vertical transport of air mass is the primary cause of condensation and hence of precipitation. Depending on the conditions responsible for the vertical motion of the air mass, precipitation can be classified into the following four types.

1) Convective Precipitation

Convective precipitation results from the heating of the earth's surface. The warm ground heats the air over it. As the air warms, the air molecules begin to move further apart. With increased distance between molecules, the molecules are less densely packed. Thus, the air becomes “lighter” and rises rapidly into the atmosphere. As the air rises, it cools. Water vapour in the air condenses into clouds and precipitation.

Fig. 1 Convective Precipitation

2) Orographic Precipitation

Orographic precipitation results when warm moist air moving across the ocean is forced to rise by large mountains. As the air rises, it cools because a higher elevation results in cooler temperatures. Cold air cannot hold as much moisture as warm air. As air cools, the water vapour in the air condenses and water droplets form. Clouds forms and precipitation (rain or snow) occurs on the windward side of the mountain.

This air is now dry and rises over top the mountain. As the air moves back down the mountain, it collects moisture from the ground via evaporation. This side of the mountain is called the leeward side. It receives very little precipitation.

Fig. 2 Orographic Precipitation

3) Cyclonic Precipitation or Frontal Precipitation

Cyclonic or frontal precipitation results when the leading edge of a warm, moist air mass (warm front) meets a cool and dry air mass (cold front). The molecules in the cold air are more tightly packed together (i.e., more dense) and thus, the cold air is heavier than the warm air. The warmer air mass is forced up over the cool air.

As it rises, the warm air cools, the water vapour in the air condenses, clouds are formed and it result in precipitation. This type of system is called Frontal Precipitation because the moisture tends to occur along the front of the air mass. In another words, unequal heating of the earth’s surface creates low and high pressure regions. Movement of air masses from the high pressure regions to low pressure regions displaces the low pressure air upward to cool and cause precipitation. Cyclonic precipitation can be of two types.

Fig. 3 Cyclonic Precipitation

a) Frontal Precipitation

A frontal is called as the hot moist air mass boundary. This precipitation is caused by the expansion of air near the frontal surface.

b) Non-Frontal Precipitation

This is a cold moist air mass boundary that moves and results in precipitation.

There are two main types of cyclones namely Tropical Cyclone (also called hurricane or typhoon) of comparatively small diameter of 300-1500 km causing high wind velocity and heavy precipitation and the Extra-Tropical Cyclone of large diameter up to 3000 km causing wide spread frontal type precipitation.

When two air masses due to contrasting temperatures and densities clash with each other, condensation and precipitation occur at the surface of contact. This surface of contact is called a ‘front’ or ‘frontal surface’. If a cold air mass drives out a warm air mass’ it is called a ‘cold front’ and if a warm air mass replaces the retreating cold air mass, it is called a ‘warm front’. On the other hand, if the two air masses are drawn simultaneously towards a low pressure area, the front developed is stationary and is called a ‘stationary front’. Cold front causes intense precipitation on comparatively small areas, while the precipitation due to warm front is less intense but is spread over a comparatively larger area. Cold fronts move faster than warm fronts and usually overtake them, the frontal surfaces of cold and warm air sliding against each other. This phenomenon is called ‘occlusion’ and the resulting frontal surface is called an ‘occluded front’.

Characteristics of Precipitation in India

India receives more than 75% of its annual precipitation during the monsoon season (June to September). The monsoon (i.e., south-west monsoon) originates in the Indian Ocean and appears in the southern part of Kerala by the end of May or the beginning of June. Monsoon winds, then, advance and cover the entire country by mid-July. The monsoon season is not a period of continuous rainfall. The temporal and spatial variability of the magnitude of rainfall results into regions of droughts and floods. Assam and the north-eastern region are the heavy rainfall regions (with average annual rainfall ranging from 2000-4000 mm) and Uttar Pradesh, Haryana, Punjab, Rajasthan and Gujarat constitute low rainfall regions (with average annual rainfall less than about 1000 mm). Western Ghats receive about 2000-3000 mm of annual rainfall.

Around mid-December, the western disturbances cause moderate to heavy rain and snowfall (about 250 mm) in the Himalayas and Jammu and Kashmir and other northern regions of the country. Low pressure areas formed in the Bay of Bengal during this period cause some rainfall in the south-eastern parts of the country.

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