VT Report On Foundation
VT Report On Foundation
VT Report On Foundation
Before we get to more important things, we would like to thank numerous people, without whom our summer training would have been very difficult.
We would like to thank National Building Construction Corporation Limited for providing us with the necessary, whole-hearted guidance and valuable suggestions throughout our training period, without which this endeavor would not have been possible.
We are very grateful to Mr. M. Manna, Regional Manager & Mr. P.Saha, Project Incharge, for their continuous support. We thank Mr Sagar Sahoo & Mr. Rajiv Rao, Planning, for guiding us right from the start.
We profoundly express our gratitude to Mr. Kamal Bhattacharya, for being kind & helpful in getting us the summer training.
Finally, We acknowledge each and every member of staff at the construction site, for their wholehearted co-operation, which made our summer training comfortable and successful.
AREAS OF OPERATION
NBCC is one of the few public sector companies engaged in the business of (i) (ii) project management consultancy services for civil construction projects ("PMC") civil infrastructure for power sector and real estate development.
NBCC is headquartered in New Delhi and in addition has 10 regional / zonal offices across India. The projects undertaken by our Company are spread across 23 states and 1 union territory in India. In addition, NBCC has also have also undertaken projects overseas. NBCC's PMC business segment includes providing management and consultancy services for a range of civil construction projects including residential and commercial complexes, redevelopment of buildings and colonies, hospitals, educational institutions; infrastructure works for security personnel, border fencing as well as infrastructure projects such as roads, water supply systems, storm water systems and water storage solutions. NBCC's civil Infrastructure for power sector segment includes providing engineering and construction services for power projects, including design and execution of (i) civil and structural works for power projects (ii) Cooling towers (iii) Chimneys. NBCC's real estate development segment focuses on principally two types of projects, namely, (i) residential projects, such as apartments and townships and (ii) commercial projects, such as corporate office buildings and shopping malls.
COMPUTERIZATION AND TRANSPARENCY NBCC makes extensive use of information and communication technologies for the execution and management of its projects. NBCC has implemented Enterprise Resource Planning ("ERP") system in some business processes related to accounting, salaries, HRM system and e-Bidding system. In addition, NBCC's team has access to domain controller and additional domain controller, cluster servers, IT security management, network management etc. IT security management is also utilized for the continuous upkeep of security products , solutions, products, tools. The online computer maintenance services ensures accurate management of the organizations hardware and software complaints electronically, including hardware management, complaint tracking, minimum problem rectification time, better reports management and optimal performance. NBCC also hascentralised its database and has begun digitising its records and has business continuity servers to protect the electronic data and ensure maximum uptime.
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As of September 30, 2011 Number of Projects Contract Value (in million) Outstanding Contract Value / Order Book (in million)
PMC Projects Ongoing Projects Forthcoming Projects 130 59 162,764.24 23,786.40* 79,240.52 23,786.40*
Completed Projects
Ongoing Projects
Forthcoming Projects
No. of Developa No. of No. of Developa Saleabl Leasab Projec ble Projec Projec ble Area e Area le Area ts Area ts ts Residentia 3,410,4 Nil 4 3,732,053 0 6 l 03 Commerci 8 1,076,65 3 407,633 239,560 66,624 6 al 3,649,9 Total 8 1,076,65 7 4,139,686 66,624 12 63
Developa Saleabl Leasab ble Area e Area le Area 4,878,8 147,58 82 5 1,724,1 290,78 2,323,908 01 6 6,602,9 438,35 7,879,729 83 3 5,555,821
As of September 30, 2011 Number of projects Contract Value(` in million) Outstanding Contract Value / Order Book(` in million)
3. No window was there in staircases which lead to complete darkness, so it was decided to change the drawing by consulting the concerned authorities.
4. The depth if beam above the door was 35 earlier but to keep the size of the door as per the standard it was changed to 3.
5.Frequent power cuts lead to increase in the cost of construction as generators were used to meet the power requirements 6.Laying of foundations was postponed by 1 month due to the rainy season.
LAYING OF FOUNDATION
At our site, Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure. Normally raft foundation is used when large load is to be distributed and it is not possible to provide individual footings due to space constraints that is they would overlap on each other. Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. They are often needed on soft or loose soils with low bearing capacity as they can spread the loads over a larger area. In laying of raft foundation, special care is taken in the reinforcement and construction of plinth beams and columns. It is the main portion on which ultimately whole of the structure load is to come. So a slightest error can cause huge problems and therefore all this is checked and passed
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Apart from raft foundation, individual footings were used in the mess area which was extended beyond the C and D blocks.
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(iii) Do not be content with badly fitting windows and doors, make sure they fit properly and ensure that they are kept shut. (iv) Do not stack bags against the wall. Similarly, dont pile them on the floor unless it is a dry concrete floor. If not, bags should be stacked on wooden planks or sleepers. (v) Do not forget to pile the bags close together (vi) Do not pile more than 15 bags high and arrange the bags in a header-and-stretcher fashion. (vii) Do not disturb the stored cement until it is to be taken out for use. (viii) Do not take out bags from one tier only. Step back two or three tiers. (ix) Do not keep dead storage. The principle of first-in first-out should be followed in removing bags. (x) Do not stack bags on the ground for temporary storage at work site. Pile them on a raised, dry platform and cover with tarpaulin or polythene sheet.
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COARSE AGGREGATE
Coarse aggregate for the works should be river gravel or crushed stone .It should be hard, strong, dense, durable, clean, and free from clay or loamy admixtures or quarry refuse or vegetable matter. The pieces of aggregates should be cubical, or rounded shaped and should have granular or crystalline or smooth (but not glossy) non-powdery surfaces.Aggregates should be properly screened and if necessary washed clean before use. Coarse aggregates containing flat, elongated or flaky pieces or mica should be rejected. The grading of coarse aggregates should be as per specifications of IS-383.
After 24-hrs immersion in water, a previously dried sample of the coarse aggregate should not gain in
weight more than 5%. Aggregates should be stored in such a way as to prevent segregation of sizes and avoid contamination with fines.
Depending upon the coarse aggregate color, there quality can be determined as:
Black => very good quality Blue => good Whitish =>bad quality
FINE AGGREGATE
Aggregate which is passed through 4.75 IS Sieve is termed as fine aggregate. Fine aggregate is added to concrete to assist workability and to bring uniformity in mixture. Usually, the natural river sand is used as fine aggregate. Important thing to be considered is that fine aggregates should be free from coagulated lumps. Grading of natural sand or crushed stone i.e. fine aggregates shall be such that not more than 5 percent shall exceed 5 mm in size, not more than 10% shall IS sieve No. 150 not less than 45% or more than 85% shall pass IS sieve No. 1.18 mm and not less than 25% or more than 60% shall pass IS sieve No. 600 micron.
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Following table shows unit weight of materials used at construction site. Please note this is for reference purpose only and may vary from place and type of material. We are thankful to Engineer Prince Saha for submitting this very useful information to us.
S.No
Material
Theoretical Weight in(KG/M) 1440 7850 1600 1840 2850 to 2960 1000 2240 2420 1600 to 1920 1920 1760 1800 2080 1760 640 2530 670 to 830 990
Approx Weight at Site Remarks in Kg 50 d/162 50 to 55 57 to 63 48 to 52 1 8.24 to 8.5 Per Bag d -dia in mm farma farma farma liter Cube mould no no cft cft no cft cft bag sft cft cft no no 1 farma=1.25cft 1 farma=1.25cft metal 12mm to 20mm cube mould size=15x15x15cm 9x4x2 3/4 9x6x3 3/4 Black cotton 30x15x20 cm 30x10x20 cm
1 2 3
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Cement Steel SandDry River Stone(basalt) Water PCC RCC 2% Steel Bricks Brick Masonry Soil(damp) Cement concrete block(solid) Cement Mortar Lime Mortar Lime Glass Teak Wood Sal Wood Marble mosaic tile
4mm tk plain
25x25x22mm 30x30x25mm
15
19 20 21 22 23 24 25 26 27
Chequered tile Glazed tile15x15cm Marble Stone Granite Stone Coddappa A.C.sheet corrugated Bitumen Window frame (simple design) Door Frame a)30070 b)26x70
2.5 to 2.8 0.20 to 0.25 5.1 5.35 6.4 1.2 220 1.9 to2.1
25 to 27 24 to 26
no no
BRICKWORK
Brickwork is masonry done with bricks and mortar and is generally used to build partition walls. In our site, all the external walls were of concrete and most of the internal walls were made of bricks. English bond was used and a ration of 1:4 (1 cement: 4 coarse sand) and 1:6 were used depending upon whether the wall is 4.5 inches or 9 inches. The reinforcement shall be 2 nos. M.S. round bars or as indicated. The diameter of bars was 8mm. The first layer of reinforcement was used at second course and then at every fourth course of brick work. The bars were properly anchored at their ends where the portions and or where these walls join with other walls. The in laid steel reinforcement was completely embedded in mortar.
efflorescence. However at small construction sites the quality of bricks can be assessed based on following, which is prevalent in many sites.
Visual check Bricks should be well burnt and of uniform size and color. Striking of two bricks together should produce a metallic ringing sound. It should have surface so hard that cant be scratched by the fingernails. A good brick should not break if dropped in standing position from one metre above ground level. A good brick shouldnt absorb moisture of more than 15-20% by weight, when soaked in water For example; a good brick of 2 kg shouldnt weigh more than 2.3 to 2.4 kg if immersed in water for 24 hours. PRECAUTIONS TO BE TAKEN IN BRICK MASONRY WORK Bricks should be soaked in water for adequate period so that the water penetrates to its full thickness. Normally 6 to 8 hours of wetting is sufficient. A systematic bond must be maintained throughout the brickwork. Vertical joints shouldnt be continuous but staggered. The joint thickness shouldnt exceed 1 cm. It should be thoroughly filled with the
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cement mortar 1:4 to 1:6 (Cement: Sand by volume) All bricks should be placed on their bed with frogs on top (depression on top of the brick for providing bond with mortar). Thread, plumb bob and spirit level should be used for alignment, verticality and horizontality of construction. Joints should be raked and properly finished with trowel or float, to provide good bond. A maximum of one metre wall height should be constructed in a day. Brickwork should be properly cured for at least 10 days
REINFORCEMENT
Steel reinforcements are used, generally, in the form of bars of circular cross section in concrete structure. They are like a skeleton in human body. Plain concrete without steel or any other reinforcement is strong in compression but weak in tension. Steel is one of the best forms of reinforcements, to take care of those stresses and to strengthen concrete to bear all kinds of loads Mild steel bars conforming to IS: 432 (Part I) and Cold-worked steel high strength deformed bars conforming to IS: 1786 (grade Fe 415 and grade Fe 500, where 415 and 500 indicate yield stresses 415 N/mm2 and 500 N/mm2 respectively) are commonly used. Grade Fe 415 is being used most commonly nowadays. This has limited the use of plain mild steel bars because of higher yield stress and bond strength resulting in saving of steel quantity. Some companies have brought thermo mechanically treated (TMT) and corrosion resistant steel (CRS) bars with added features. Bars range in diameter from 6 to 50 mm. Cold-worked steel high strength deformed bars start from 8 mm diameter. For general house constructions, bars of diameter 6 to 20 mm are used Transverse reinforcements are very important. They not only take care of structural requirements but also help main reinforcements to remain in desired position. They play a very significant role while abrupt changes or reversal of stresses like earthquake etc. They should be closely spaced as per the drawing and properly tied to the main/longitudinal reinforcement
and bending of rebars. This schedule contains all details of size, shape and dimension of rebars to be cut.
LAP LENGTH Lap length is the length overlap of bars tied to extend the reinforcement length.. Lap length about 50 times the diameter of the bar is considered safe. Laps of neighboring bar lengths should be staggered and should not be provided at one level/line. At one cross section, a maximum of 50% bars should be lapped. In case, required lap length is not available at junction because of space and other constraints, bars can be joined with couplers or welded (with correct choice of method of welding). ANCHORAGE LENGTH This is the additional length of steel of one structure required to be inserted in other at the junction. For example, main bars of beam in column at beam column junction, column bars in footing etc. The length requirement is similar to the lap length mentioned in previous question or as per the design instructions
COVER BLOCK
Cover blocks are placed to prevent the steel rods from touching the shuttering plates and there by providing a minimum cover and fix the reinforcements as per the design drawings. Sometimes it is commonly seen that the cover gets misplaced during the concreting activity. To prevent this, tying of cover with steel bars using thin steel wires called binding wires (projected from cover surface and placed during making or casting of cover blocks) is recommended. Covers should be made of cement sand mortar (1:3). Ideally, cover should have strength similar to the surrounding concrete, with the least perimeter so that chances of water to penetrate through periphery will be minimized. Provision of minimum covers as per the Indian standards for durability of the whole structure should be ensured. Shape of the cover blocks could be cubical or cylindrical. However, cover indicates thickness of the cover block. Normally, cubical cover blocks are used. As a thumb rule, minimum cover of 2 in footings, 1.5 in columns and 1 for other structures may be ensured. Structural element Footings Columns Slabs Beams Retaining wall Cover to reinforcement (mm) 40 40 15 25 25 for earth face 20 for other face
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THINGS TO NOTE Reinforcement should be free from loose rust, oil paints, mud etc. it should be cut, bent and fixed properly. The reinforcement shall be placed and maintained in position by providing proper cover blocks, spacers, supporting bars, laps etc. Reinforcements shall be placed and tied such that concrete placement is possible without segregation, and compaction possible by an immersion vibrator. Three types of bars were used in reinforcement of a slab. These include straight bars, crank bar and an extra bar. The main steel is placed in which the straight steel is binded first, then the crank steel is placed and extra steel is placed in the end. The extra steel comes over the support while crank is encountered at distance of (1-distance between the supports) from the surroundings supports. For providing nominal cover to the steel in beam, cover blocks were used which were made of concrete and were casted with a thin steel wire in the center which projects outward. These keep the reinforcement at a distance from bottom of shuttering. For maintaining the gap between the main steel and the distribution steel, steel chairs are placed between them
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FORM WORK
Forms or moulds or shutters are the receptacles in which concrete is placed, so that it will have the desired shape or outline when hardened. Once the concrete develops adequate strength, the forms are removed. Forms are generally made of the materials like timber, plywood, steel, etc. Generally camber is provided in the formwork for horizontal members to counteract the effect of deflection caused due to the weight of reinforcement and concrete placed over that. A proper lubrication of shuttering plates is also done before the placement of reinforcement. The oil film sandwiched between concrete and formwork surface not only helps in easy removal of shuttering but also prevents loss of moisture from the concrete through absorption and evaporation. The steel form work was designed and constructed to the shapes, lines and dimensions shown on the drawings. All forms were sufficiently water tight to prevent leakage of mortar. Forms were so constructed as to be removable in sections. One side of the column forms were left open and the open side filled in board by board successively as the concrete is placed and compacted except when vibrators are used. A key was made at the end of each casting in concrete columns of appropriate size to give proper bondings to columns and walls as per relevant IS.
CLEANING AND TREATMENT OF FORMS All rubbish, particularly chippings, shavings and saw dust, was removed from the interior of the forms (steel) before the concrete is placed. The form work in contact with the concrete was cleaned and thoroughly wetted or treated with an approved composition to prevent adhesion between form work and concrete. Care was taken that such approved composition is kept out of contact with the reinforcement.
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DESIGN The form-work should be designed and constructed such that the concrete can be properly placed and thoroughly compacted to obtain the required shape, position, and levels subject ERECTION OF FORMWORK The following applies to all formwork: a) Care should be taken that all formwork is set to plumb and true to line and level. b) When reinforcement passes through the formwork care should be taken to ensure close fitting joints against the steel bars so as to avoid loss of fines during the compaction of concrete. c) If formwork is held together by bolts or wires, these should be so fixed that no iron is exposed on surface against which concrete is to be laid. d) Provision is made in the shuttering for beams, columns and walls for a port hole of convenient size so that all extraneous materials that may be collected could be removed just prior to concreting. e) Formwork is so arranged as to permit removal of forms without jarring the concrete. Wedges, clamps, and bolts should be used where practicable instead of nails. f) Surfaces of forms in contact with concrete are oiled with a mould oil of approved quality. The use of oil, which darkens the surface of the concrete, is not allowed. Oiling is done before reinforcement is placed and care taken that no oil comes in contact with the reinforcement while it is placed in position. The formwork is kept thoroughly wet during concreting and the whole time that it is left in place. Immediately before concreting is commenced, the formwork is carefully examined to ensure the following: a) Removal of all dirt, shavings, sawdust and other refuse by brushing and washing.
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b) The tightness of joint between panels of sheathing and between these and any hardened core. c) The correct location of tie bars bracing and spacers, and especially connections of bracing. d) That all wedges are secured and firm in position. e) That provision is made for traffic on formwork not to bear directly on reinforcement steel. VERTICALITY OF THE STUCTURE All the outer columns of the frame were checked for plumb by plumb-bob as the work proceeds to upper floors. Internal columns were checked by taking measurements from outer row of columns for their exact position. Jack were used to lift the supporting rods called props STRIPPING TIME OR REMOVAL OF FORMWORK Forms were not struck until the concrete has attained a strength at least twice the stress to which the concrete may be subjected at the time of removal of form work. The strength referred is that of concrete using the same cement and aggregates with the same proportions and cured under conditions of temperature and moisture similar to those existing on the work. Where so required, form work was left longer in normal circumstances Form work was removed in such a manner as would not cause any shock or vibration that would damage the concrete. Before removal of props, concrete surface was exposed to ascertain that the concrete has sufficiently hardened. Where the shape of element is such that form work has reentrant angles, the form work was removed as soon as possible after the concrete has set, to avoid shrinkage cracking occurring due to the restraint imposed. As a guideline, with temperature above 20 degree following time limits should be followed: Structural Component Footings Sides of beams, columns, lintels, wall Underside of beams spanning less than 6m Underside of beams spanning over 6m Underside of slabs spanning less than 4m Underside of slabs spanning more than 4m Flat slab bottom Age 1 day 2 days 14 days 21 days 7 days 14 days 21 days
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CONCRETE MAKING
Just mix cement, aggregates and water, cast this mix in a mould, open the mould next day. A uniform hard mass will be found, which is known as concrete, any body can make it. The simplecity in making concrete make this material to be look like very simple in its production, yet it as not so simple. Due to ignorance about concrete no other building materials ever mis-used as concrete in the construction. In India concrete is being used in the construction since the last 70 years. Yet 80% of the builders have no proper understanding of this materials. Go to any construction site (except big construction sites) you will find that sand and aggregates are being taken in iron tasla or cane baskets to charge the mixer without the consideration of site aggregates actual grindings, moisture content and bulking of sand. The water is poured in the mixer without any measured quantity. It could be well imagine what sort of concrete structure will be made with the concrete being produced in this crude method. Most of the contractors, builders, masons etc. still follow 1:2:4 or 1:1.5:3 mixes they are not aware of Design Mixes and Concrete Admixtures. This paper described how Design Mixes can be converted into volume with 1 Bag Cement, 2 Boxes of sand and 4 Boxes of Aggregate. The site practical problem is the dispersion of water and liquid admixtures into the mixer. For this the site should fabricate a plastic circular graduated measuring container of 30 lit capacity with a tap fitted at its bottom. This container is to be fitted on top of the mixer. From this container water and liquid admixtures can conveniently poured direct into the mixer in a measured quantity.
4. Cement will be used PPC, having 7 days average compressive strength of 37.5 N/mm2 5. Mean design target strength: 25 + 1.65 x 5 = 33.3 N/mm2 at 28 days age Table-1 Test Data of Dehradun Aggregates: I.S. Sieve Size Percentage Passing River Sand 40 mm 20 mm 10 mm 4.75 mm 2.36 mm 1.18 mm 600 micron 300 micron 150 micron Specific Gravity Water absorption % Bulk density kg/lit 100 100 95 79 72 56 47 27 6 2.65 0.80 1.78 2.65 0.50 1.40 20 mm Crushed Aggregate 100 86 3 0
Note : The sand is not falling to any grading Zone of IS : 383-1970. The aggregate grading is 20 mm single sized as per IS: 383-1970. If 95% this sand passes on 4.75 mm sieve, then the sand will become of Zone-II as per IS : 383-1970. The following mix is worked out as per Zone-II sand. For detail calculations refer reference of No. 1. a) For the target strength and given cement and Aggregate W/C ration found to be =
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0.49 b) Water for OPC 190 kg/m3. For PCC 5/100 x 190 = 9.5 , Say 10 190 10 = 180 kg/m3 to give 50 mm of Slump with the given aggregates. Normal Superplasticizer at a dosages of 7 ml/kg cement will give 15% water reduction without loss of workability. Water = 180 27 = 153 kg/m3 c) Cement = 153/0.49 = 312 kg/m3 d) Density of OPC concrete = 2405 kg/m3 Density for PPC Concrete = 2405 24 = Say 2380 kg/m3 e) Aggregates = 2380 -153 312 = 1915 kg/m3 f) Sand (Zone-II) = 1915 x 0.36 = 689 kg/m3 g) 20 mm aggregate = 1915 689 = 1226 kg/m3 Mix. No. 1 On the basis of saturated and surface dry aggregatesWater =153 kg/m3 PCC=312 kg/m3 Sand=689 kg/m3 20 mm Aggregate=1226 ml/m3 Mix No. 2 95-79 = 16% oversized particles in the sand is to be adjusted in the above mix. The modified mix on the basis of saturated and surface dry aggregates is given below: Water=153 kg/m3 PCC=312 kg/m3 Sand=820 kg/m3 20 mm Aggregate =1095 kg/m3 Normal Superplasticizer=2184 kg/m3 Accordingly mix ratio by weight on the basis of saturated and surface dry aggregates is given below: Cement : Sand : 20 mm Agg. 1 : 2.63 : 3.51 W/C Ratio = 0.49 Mix ratio by volume on the basis of room dry aggregates is given below: Cement : Sand : 20 mm Agg.
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1 : 2.14 : 3.63 Free W/C Ratio = 0.49 MIX RATIO BY VOLUME FOR ONE BAG OF CEMENT 1. Cement = One bag = 50 kg = 35 lit = 35000 cc 2. Sand (room dry) = 2.14 x 35 = 74.9 lit = 74900 cc 3. 20 mm Aggregate (room dry) = 3.63 x 35 = 127.05 lit = 127050 cc 4. Free Water = 24.5 lit 5. Normal Superplasticizer = 350 ml MEASURING BOXES TO BE MADE AT SITE 1. Cement = One bag = 50 kg 2. Sand (room dry) = 33 x 33 x 34.4 cm two boxes 3. 20 mm Aggregate (room dry) = 33 x 33 x 29.2 cm .. four boxes 4. Free Water = 24.5 lit 5. Normal Superplasticizer = 350 ml In the above example M-25 Design mix is converted to the familiar 1 bag cement : 2 boxes of sand and 4 boxes of aggregate. While making concrete at site the moisture content of site sand and aggregate must be taken into account in the mixing water and bulking of sand. In the field trial mixes are to be carried out to finalize the mix.
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FOUNDATION
The building foundation or sub structure is that part of a structure which is placed below the surface of the ground and which transmits the superstructure load to the underlying soil ultimately. It is the part of a structural system that supports and anchors the superstructure of a building. Foundation is the most important part of a building. Building activity starts with the formation of foundation. Main activities of building foundation are: To distribute building load to soil beneath To distribute the load uniformly To grapnel the structure to the ground to resist movement due to lateral force To prevent sinking of the structure.
Any part of a structure that serves to transmit the load to the earth or rock can be called foundation. The higher and heavier the building is to be, the wider and deeper the supports of footings for the foundation have to be.
PURPOSE OF FOUNDATION
To pass out building weight to the soil beneath evenly; To prevent differential settlement of building; To provide a plane surface for the convenience of construction; To make building substantial and durable by continuing the structure in the soil.
TYPES OF FOUNDATION:
Depending on the depth of the load-transfer member below the super-structure and the type of transfer load mechanism foundation can be classified into two types: a) Shallow Foundation b) Deep Foundation.
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Foundation
Shallow
Deep
Spread Footings
Grillage Foundation
Combined Footings
Mat Foundation
Pile Foundation
Cofferdams
Caisson
Wall Footings
Pre-cast Piles
Classification of Foundation
a)
SHALLOW FOUNDATION:
A shallow foundation is a type of foundation which transfers building loads to the earth very near the surface. The objective of shallow foundation is to distribute the structural concentrated load over a wide horizontal area at a little depth rather than a range of depths. Shallow foundation is often selected when the soil has a good bearing capacity and the structural load will not cause excessive settlement of the underlying soil layers. In general, shallow foundations are more simple and cost effective to construct than deep foundations because little soil is removed or disturbed. Shallow foundation construction is typically utilized for most residential and light commercial raised floor buildings.
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SPREAD FOOTINGS:
Spread footing is a common term used to refer shallow foundations which transfers concentrated loads to a wide range area. It includes reinforced concrete footings, wall footings, isolated column footings. Spread footing provides a stable base or platform at a low depth and it prevents the structure from settling into the ground.
Spread Footing
ISOLATED COLUMN FOOTINGS: Isolated column footings are used to support single columns. Each individual isolated footing provides support for each individual column, pier, post or other single concentrated load. So, they act as a base for a column. They transfer the superimposed structural load to a wide range of soil. They are the most economical types of footings and are used when columns are spaced at relatively long distances. They can be square, rectangular or even circular in plan view. Isolated footings can be of brick masonry, stone masonry or of Reinforced Cement Concrete (R.C.C.) depending on amount of load, project expanses and materials available. The base of the footing depends on the load bearing capacity of soil and the superimposed load. They are also known as pad footing or individual column footing. Generally isolated footings are used in case of reinforced concrete structure buildings. WALL FOOTINGS: This type of shallow footing supports a wall by providing footing beneath the entire wall structure. This type of footing is used when columns are made of bricks.
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GRILLAGE FOOTINGS:
In case of low bearing capacity of soil and avoiding deep excavation grillage foundation can be used. Grillage foundation can reduce the superimposed load of column within the safe bearing capacity of the soil. The grillage can be made of steel or timber.
COMBINED FOOTINGS:
A combined footing is nothing but a combination of pad footings having a common base. A combined footing is constructed when (i) a column lies very close to the property line and (ii) to prevent overlapping of footings when columns are very adjacent. They can be rectangular or trapezoidal in plan. Combined footing proves to be more cost effective than a single column footing.
MAT FOUNDATION:
A mat is a slab that supports multiple columns. It is typically used when the bearing capacity of soil is very low. When required footings will cover more than half the area beneath a structure, it is often desirable to enlarge and combine the footings to cover the entire area. A mat foundation may be cheaper than individual footings because of reduced forming costs and simpler excavation procedures. Although mat foundations are more difficult and more costly to design but they prove to be more effective.
b)
DEEP FOUNDATION:
A deep foundation is used when the bearing capacity of soil is low near ground. When a building structure transmits excessive loads to a soil with low bearing capacity near ground, settlement of the foundation takes place which endangers the stability of the structure. In that case deep foundation is used to ensure good bearing capacity of soil at a considerable amount of depth. A deep foundation requires considerable amount of materials and earthwork resulting increased cost and effort. Piles, cofferdams and caissons are some the familiar forms of deep foundations.
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BRICK PILLAR FOOTING: When the structural columns are made of bricks this type of
isolated footing is often used. This is the most economic type of isolated column footing. The width and the depth of the footing depend on load to be carried and bearing capacity of the soil. The width is found by offsets running symmetrical round the column. Stone Pillar Footing: Stone pillar footings are stronger than brick pillars. Construction of this type of footing is similar to brick pillar footing. The width and the depth of this footing is a bit bigger because of the stone sizes. This type of footing is weak in resisting bending. So they are not used against long heavy structural loads. R.C.C. Column Footing: In case of column subjected to heavy loading and bending this type of column footing provides a superior solution in shallow foundation. Made with reinforcement and concrete this type of footing is high on strength and bending. The footing is reinforced cross ways by re-bar placed at right angles to one another. This type footing is capable of transmitting massive loads with a reduced footing depth. This type is used as a most effective form of isolated footing. Due to its strong structure this footing is used against high building structures provided that the soil has sufficient bearing capacity.
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STEPS
OF ISOLATED FOOTING
SOIL EXCAVATION LEVELLING & DRESSING OF SOIL SURFACE PLACEMENT OF BRICK FLAT SOLING LAYER GIVING A CEMENT CONCRETE LAYER ON IT PLACEMENT OF CEMENT CONCRETE BLOCK PLACING OF REINFORCEMENT SHUTTERING CONCRETE CASTING REMOVAL OF FORMWORK CURING BACKFILLING OF EXCAVATED AREA
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SOIL EXCAVATION
To set a footing, the first step is to excavate the soil of respective area. At first the area should be located. The depth of excavation depends on the desired strength as strength increases with depth. We see the tools that are used in excavation below, PICS In most cases, baskets & spades are used for excavation. Deep excavation damages adjacent constructions. In this phase, shore piles or other preventive means are used. Soil sloping can also reduce this damage. The picture below shows how workers dig soil during excavation,
Fig: Excavation
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The area of footings are chosen according to design. First the C.G of footings is located. The excavated soil is kept near the footing as later it can be used for backfilling.
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Now a hard & plane surface is needed. For this purpose we place a brick layer on previous layer. It is called BRICK FLAT SOLLING. In this layer bricks are arranged in a regular combination. The upper surface of the layer should be uniformly planed. Below we see a brick flat soling.
Usually some CC blocks are kept on the CC layer to maintain the clear cover between CC layer & reinforcement. It is also very important to prevent the reinforcement from corrosion. There is no standard size of blocks. It varies with size of footing & clear cover. Sometimes brick or half brick can be used instead of CC block. But bricks are not uniformly smooth plane. Its strength is also less than CC block. So, it is better to use CC blocs as clear cover. Generally its size is 3x3x3.
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PLACING OF REINFORCEMENT
Now it is a very important step to place reinforcements. First we should maintain available clear cover by using CC blocks. We must tie rebars with G.I wires or welding. It is done so that rebars remains intact in all the time even during the casting. First a single frame of horizontal reinforcement is kept upon the CC block. The number & diameter of reinforcements depends upon the strength of construction. And it is determined by design engineer. The rebars are tied by hand at the outer place if the case is respectively small. But for large case, it is done on the brick flat soling. Because it is difficult to move & place a large case.
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Fig: Preparing column The case must remain horizontal. It is done by plumb bob. The figure below we see how a case is being horizontal by plumb bob.
Fig: Making horizontal After placing the case, the centre of case is marked as the column can be placed.
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Fig: Fixing the column position Now the column is placed upon the marking space. It must be vertical. Then it is also tied with case. After making vertical the column is supported to remain fixed.
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SHUTTERING
Shuttering means the solid boundary around the concrete. It resists water flow. It also bears the load of concrete. So a shuttering should be enough water tight & strong. The amount of water should remain constant in order to get a proper hydration. There are two kinds of shuttering, 1. Wooden shuttering & 2. Steel shuttering Wooden shuttering is more chip than steel shuttering. But it is less water proof & less strong. It also lasts a short time. On the other hand steel shuttering is strong enough. It also can be used
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more times. But it is costly. Side shuttering can be removed after 3 days as after this time concrete get required strength.
Fig: Wood shuttering The minimum requirements of a shuttering: *It should be enough strong to bear the load *It should be water tight *It should be economical *Its inner surface should be plane *Its joints should be made carefully *It should retain the concrete shape from all types of distortions.
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Figure: segregation. Segregation occurs due to the difference in particles size (sometimes in the specific gravity of the mix ingredients) .There are two types of segregation. In the first, the coarser particles tend to separate out since travel further long distance. In the second, segregation is occurred by separation of grout (water &cement). Here in our project aggregates were thrown from a remarkable height. As a result the ingredients were separated as they hit the lower rebar layer. This segregation will ultimately decrease the strength of the concrete. So proper arrange should be taken to prevent segregation.
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2. Compaction: In our observed site, for compacting concrete they used both rodding & vibration procedure. Roding with bamboo
In case of using vibrator, it should kept in mind that vibrator must be vertical. But in our project vibrator was used aligned.
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Aligned vibrating
Figure: Aligned vibrating. 3.Improper clear cover maintainance: Clear cover is a very important fact in structure. For the structure under the ground clear should be maintained at least 3. If clear cover is not maintained properly then the moisture content around the structure will penetrate into the structure & cause corrosion to the reinforcement . Finally collapse the structure. Improper clear cover
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4. Water logging during casting: Mix proportion is very much important for the design strength. In our site during casting, water logged continuously. The source of water may be rain water or leakage from nearby pipes or drains. Logged water
Figure: Water logging during casting 5. Brick as C.C block & no C.C layer: In isolated footing we normally use BFS & C.C layer to provide a flat surface. But in our project BFS was used with no mortar. Here they also did not use C.C layer as well as C.C block. For maintenance of the clear cover between the BFS bricks were directly used as C.C block that is shown in the following figure.
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