Types of foundation Prepared by Prof Alka M

Types of foundation Prepared by : Prof. Alka M. Shah Civil Engineering Department Nirma University

WHAT IS FOUNDATION ? Lowest part of a structure directly in contact with the ground and transmits all the loads to the sub-soil below it. Majority of structure fails due to failure of their foundation PURPOSE q q q To support structure Distributes the loads over a larger area Minimizes the differential settlements Increases stability & prevents overturning Distribute non-uniform load uniformly to the soil 2

Foundation and Footing are same? • Foundation: Part of the structure which is in direct contact with the ground to which the loads are transmitted. • Footing: Lower most portion of foundation which is in direct contact with the sub soil is called footing. 3

TYPES OF FOUNDATIONS SHALLOW FOUNDATION D≤B DEEP FOUNDATIONS D>B 4

SUITABILITY OF FOUNDATION SHALLOW FOUNDATION q Bearing capacity of soil is more. q ground water table(W. T) is low. q dewatering of foundation is not required. q top layers of soil are uniform and stable. q Load on the structure is less. DEEP FOUNDATION q Bearing capacity of soil is low. q ground water table(W. T) is high. q dewatering of foundation is costly and difficult. q top layers of soil are non uniform and unstable. q Load on the structure is more. 5

TYPES OF SHALLOW FOUNDATION 1) SPREAD FOOTING i) Continuous footing (strip or wall) ii) Isolated column footing a) Square b) Circular c) Rectangular iii) Inverted arch footing iv) Reinforced concrete footings 2) STRAP FOOTING 3) COMBINED FOOTING i) Rectangular ii) Trapezoidal 4) RAFT FOUNDATION 5) GRILLAGE FOUNDATION 6

1) SPREAD FOOTING ØCarries a single column ØUsed to spread out loads from walls of columns over a wider area ØConsists of concrete slabs located under each structural column and a continuous slab under load-bearing walls. ØCommonly used due to their low cost & ease of construction. ØUsed in small to medium size structures with moderate to good soil condition. ØFor high rise buildings it is not efficient. 7

Wall Footing Simple wall footing without masonry offsets (SBC is high) Wall footing with masonry offsets (Stepped wall footing) (SBC is low) 8

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STRIP/WALL FOOTING A spread footing for a continuous wall is called strip footing. (Used to support load bearing walls) 10

INVERTED ARCH FOOTING Used on soft soils to reduce depth of foundation. End columns must be strong to resist outward pressure caused by arch action. 11

REINFORCED CONCRETE FOOTING o Used where the walls are subjected to heavy loads & soil bearing capacity is low 12

ISOLATED COLUMN FOOTING Used to support individual columns. 13

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2) COMBINED FOOTING Supports two columns q the two columns are so close to each other that their individual footings would overlap q one column is placed right at the property line q C. G. of column load and centroid of the footing should coincide q 15

COMBINED FOOTING Rectangular footing Trapezoidal footing 16

A COMBINED FOOTING WILL DEFORM AS SHOWN IN FIG BELOW The eccentric loading condition upon the left end, due to the restrictions of a property line, will generate tensile stresses on the top of the footing. 17

3) CANTILEVER/STRAP FOOTING Two isolated footings connected with a strap or beam q Connects in such a way that they behave as one unit q The strap acts as a connecting beam and does not take any soil reaction. The strap is designed as rigid beam. q It does not provide bearing q 18

to connect an eccentrically loaded column footing to an interior column. Used to transmit the moment caused from an eccentricity to the interior Can be used instead of combined footing if the distance between two columns is large More economical than combined footings q 19

4) RAFT FOUNDATION Large reinforced concrete slab supporting no. of columns & walls area of spread footings or combined footings exceeds about 50 percent of the gross area of the building Greater weight & continuity of a mat provides sufficient resistance 20

RAFT / MAT FOUNDATION 21

RAFT / MAT FOUNDATION 22

Raft/Mat Foundation is suitable under following conditions q q q q Structural loads are heavy or the soil condition is poor Soft or loose soils having less bearing capacity Foundation soil is non-homogeneous and prone to excessive differential settlements Lateral loads are non-uniform Columns are so close that their individual footings would overlap To resist water pressures when groundwater extends above the floor level of the lowest basement of a building There is a large variation in the loads on individual columns 23

5) GRILLAGE FOUNDATION Used to transmit heavy loads from steel columns to a soil having low bearing capacity Avoids deep excavation Reduces pressure intensity Lighter & economical Used in masonry bridge seat, bus stands, railways etc Made up of a rolled steel joints (RSJ) known as grillage beams Types of Grillage foundation § § Steel grillage foundation Timber grillage foundation 24

Ø A type of foundation often used at the base of a column. It consists of one, two or more tiers of steel beams superimposed on a layer of concrete, adjacent tiers being placed at right angles to each other, while all tiers are encased in concrete. Ø This is dependable foundation and is used in those place where the load of the structure is pretty and bearing capacity of soil comparatively poor Ø The grillage foundation helps in distributing the load over a wider area of subsoil. Ø The grillage foundation helps in avoiding deep excavations as the necessary base area is provided for the load of transmission Ø This type of foundation generally used for heavy structure columns piers and steel stanchions etc. 25

GRILLAGE FOUNDATION 26

STEEL GRILLAGE FOUNDATION Consists of rolled steel joists in single or double tiers 27

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TIMBER GRILLAGE FOUNDATION Uses timber planks & timber beams No concrete is embedded between the timber joists Used in waterlogged areas 29

GRILLAGE FOUNDATION FOR WOODEN/TIMBER POST 30

SHALLOW FOUNDATION Advantages: a) Cost (affordable) b) Construction Procedure (simple) c) Material (mostly concrete) d) Labour (doesn’t need expertise) Disadvantages: a) Settlement b) Foundation gets subjected to pullout, torsion etc c) Irregular ground surface(slope, retaining wall) 31

TYPES OF DEEP FOUNDATION 1) PILE FOUNDATION 2) CAISSONS OR WELL FOUNDATION 3) COFFER DAMS 32

1) Pile foundation • Pile: Column-like foundations going deep into the subsoil. • Piles are structural members that are made of steel, concrete or timber. • Preferred: i)When soil bearing capacity is poor. ii) Non availability of proper bearing strata at shallow depth. iii) pumping of sub soil water would be too costly. iv)Heavy loads from super structure for which shallow foundation will be uneconomical. 33

1) Pile foundation Classification of pile foundation • Based on function: -End bearing: Transfer load to through soil or water… -Friction: Transfer load by means of skin friction… -Compaction: Compacting loose granular material and thus increasing SBC… -Tension(uplift): Anchor down the structures subjected to uplift due to hydraulic pressure or due to overturning… 34

1) Pile foundation Classification of pile foundation -Anchor: Provide anchorage against horizontal pull… -Fender and dolphins: Protect against impact from ships. -Sheet: To reduce seepage and uplift under hydraulic structures like dams… 35

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Under reamed piles Ø Under reamed piles are bored cast-in-situ concrete piles having one or more number of bulbs formed by enlarging the pile stem. Ø These piles are best suited in soils where considerable ground movements occur due to seasonal variations, filled up grounds or in soft soil strata. Ø Provision of under reamed bulbs has the advantage of increasing the bearing and uplift capacities. It also provides better anchorage at greater depths. Ø These piles are efficiently used in machine foundations, over bridges, electrical transmission tower foundation sand water 41 tanks

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1) Caissons o o o The word caissons derived from the French word caisse meaning a box Box like structure, round or rectangular which sunk from the surface of either land or water to some desired depth Used for placing foundation under water 43

Types of Caissons Box caisson (open at top and closed at bottom) Open caisson or wells (open at top and bottom) Pneumatic caisson (closed at top and open at bottom) Single, Multiple and Cylinder 44

Box caissons o Strong water tight vessel o Built on land floated up to site and then sink in to the water o. Used when loads are not heavy o Bearing stratum is available at shallow depth o. Water depth is 6 to 8 m o Foundation bed not required o. Material is loose 45

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Open or Well caissons ( Well Foundation) o Used for buildings and bridge where soil liable to scour o No firm bed is available 47

Pneumatic caissons o It is useful when well foundation is not possible o Depth of water 12 to 35 m 48

1) Cofferdams o o Cofferdam is a temporary structure which is built in a river, lake or dam to remove water from the area and make possible to construct Require in dams, docks and bridge pier 49

Types of Cofferdams 1. Earth fill cofferdam 50

2. Rock fill cofferdam 51

3. Rock fill crib cofferdam 52

4. Single wall cofferdam 53

5. Double wall cofferdam 54

6. Cellular cofferdam o o Mostly used for de-watering areas in places where depth of water is from 18 to 21 m Two types 1. Diaphragm cellular cofferdam 2. Circular cellular cofferdam 55

Diaphragm cellular cofferdam 56

Difference between caisson and cofferdam CAISSON o Permanent structure used for placing foundation in correct position o It is build on dry bed, floated to the desired site and sunk to desired depth o Preferred when work area is small and water depth is more o Used where driving of sheet pile is impossible COFFERDAM o Temporary structure used for construction work under reasonably dry condition and demolished after completion of work o It is build in river or lake o Preferred when work area is large and water depth is small o Used where driving of sheet pile is easily possible 57

FOUNDATION IN BLACK COTTON SOIL o Black cotton soil is very dangerous to place foundation and bearing capacity is about 50 to 100 kn/m 2 o Variation in the volume change of soil extent from 20 to 30 % of original volume o Alternate swelling and shrinkage results in formation of cracks which are of 15 to 20 cm wide and 2. 5 to 4 m deep 58

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Types of Foundation in black cotton soil 1. Strip (wall) or pad (column) foundation o Suitable when load is medium and swelling pressure is little Foundation in black cotton soil 60

o o Suitable when swelling pressure is high and having high shrinkage properties Used at place where high rainfall 61 Foundation in black cotton soil

2. Pier Foundation o o Suitable foe wall carrying higher load Gap between arch and ground would permit free vertical movement of soil during swelling and shrinkage 62

Precaution For The Safety of Foundation In Black Cotton Soil o For heavy structure raft or pile foundation is preferable o If possible remove black cotton soil o Construction work should be carried out in dry season o Black cotton soil not allowed to come in direct contact with the foundation o R. C. C. bands 10 to 15 cm deep should placed at plinth, lintel level o Gap on either side of foundation should filled with sand o Below foundation, 30 cm thick layer of moorum must provided 63

FACTORS AFFECTING FOUNDATION SELECTION PRIMARY FACTORS AFFECTING FOUNDATION CHOICE: Sub-surface soil Ground water table conditions Type of structure Magnitude of load SECONDARY FACTORS AFFECTING FOUNDATION CHOICE: Construction access, methods and site conditions Environmental factors Building Codes and Regulations Impact on surrounding structures Construction schedule Construction risks

GROUND WATER TABLE CONDITIONS Presence of groundwater near a footing is undesirable because: Footing construction below GWT is difficult and expensive, Groundwater around a footing can reduce the strength of soils, It may also cause hydrostatic uplift, Frost action may increase, Waterproofing problems. If Ground water table is high, raft or grillage foundation is desirable. If ground water table is low or at a high depth from foundation than column foundation is desirable. 65

SUBSURFACE CONDITION AND BUILDING STRUCTURAL REQUIREMENTS 66

T N A H U O Y K 67