AJAY GHOSH Associate Prof. works 7420041131

1. Indian Standard IS:1080-1985CODE OF PRACTICE FORDESIGN AND CONSTRUCTION OFSHALLOW FOUNDATIONS IN SOILS (OTHERTHAN RAFT, RING AND SHELL )( Second Revision )First Reprint DECEMBER 1988UDC 624.151.5.04:006.76 AJAY GHOSH Associate Prof. works 7420041131

2. 2. TERMINOLOGY 2.2.5 of IS 6403: Shallow Foundation: - A foundation whose width is greater than It’s depth. The shearing resistance of the soil in the sides of the foundation is generally neglected.

3. Types of foundation • Foundations are broadly classified under two heads: shallow foundation and deep foundation. • According to Terzaghi for a shallow foundation D ≤ B. However • in practice, it is widely accepted that the above criterion may be modified as D≤2B for shallow foundations.

4. TYPES OF SHALLOW FOUNDATIONS • 3.1 Shallow Foundations - These cover such types of foundations in which load transference is primarily through shear resistance of the bearing strata ( the frictional resistance of soil above bearing strata is not taken into consideration ) and are laid normally to depth of 3 m. • 3.1.1 The various types of shallow foundations are as under: • a) Spread or pad - IS : 1080-1986:. • b) Strip - See IS : 1080-1986 • c) Raft foundation - See IS : 2950 ( Part 1 )-1981. • d) Ring and shell foundation --see IS : 11089-1984 and IS : 9456- 1980.

6. 3. GENERAL • a) Pad or Spread - In such type of foundation, which is constructed of masonry and/or concrete ( plain or reinforced ) and is isolated, the loads of a structure is transferred to the ground in such a manner that the safe bearing pressure is not exceeded. • b) Strip-- Such type of foundation provides continuous and longitudinal bearing for loads carried by vertical elements, such as continuous wall foundation beams or the like.

7. Shallow foundation

10. Mat or raft foundation • Large slab supporting number of columns and walls under the entire structure

11. RING FOUNDATION • Ring foundation is provided for tall structures like water tank, chimney, silows etc. which have vertical non uniform loads

12. 4. DESlGN CONSIDERATIONIS-1080-1985 • 4.1 In such type of foundations wherever the resultant of the load deviates from the centre line by more than l/6 of its least dimension at the base of footing, it should be suitably reinforced. • 4.2 For continuous wall foundations ( plain or reinforced ) adequate reinforcement should be provided particularly at places where there is abrupt change in magnitude of load or variation in ground support.

13. 4. DESlGN CONSIDERATION • 4.3 On sloping sites the foundation should have a horizontal bearing and stepped and lapped at changes of levels for a distance at least equal to the thickness of foundation or twice the height of step whichever is greater. The steps should not be of greater height than thickness of the foundations.

14. 4. DESlGN CONSIDERATION • 4.4 Ground Beams The foundation can also have the ground beam for transmitting the load. The ground beam carrying a load bearing wall should be designed to act with the wall forming a composite beam, when both are of reinforced concrete and structurally connected by reinforcement. The ground beam of reinforced concrete structurally connected to reinforced brick work can also be used.

15. 4. DESlGN CONSIDERATION • 4.5 Dimensions of Foundation • 4.5.1 The dimensions of the foundation in plan should be such as to support loads as given in IS : 1904-1985*. The width of the footings shall be such that maximum stress in the concrete or masonry is within the permissible limits. The width of wall foundation shall not be less than that given by: • B= W + 30 cm where • B = width at base in cm, and • W = width of supported wall in cm.

16. 4. DESlGN CONSIDERATION • 4.6 In the base of foundations for masonry foundation it is preferable to • have the steps in multiples of thickness of masonry unit. • 4.7 The plan dimensions of excavation for foundations should be wide enough to ensure safe and efficient working ( see IS : .7764-1966 ).

17. 4. DESlGN CONSIDERATION • 4.8 Unreinforced foundation may be of concrete or masonry ( stone or brick ) provided that angular spread of load from the pier or bed plate to the outer edge of the ground bearing is not more than 1 vertical to a horizontal to masonry or 1 vertical to I horizontal for cement concrete and 1 vertical to 2/3 horizontal for lime concrete. • The minimum thickness of the foundation of the edge should not be less than 150 mm. • In case the depth to transfer the load to the ground bearing is less than the permissible angle of spread, the foundations should be reinforced.

18. 4. DESlGN CONSIDERATION • 4.9 If the bottom of a pier is to be belled so as to increase its load carrying capacity such bell should be at least 30 cm thick at its edge. The sides should be sloped at an angle of not less than 45° with the horizontal. The least dimension should be 60 cm ( circular, square or rectangular ). The design should allow for the vertical tilt of the pier by 1 percent of its height.

19. 4. DESlGN CONSIDERATION • 4.10 If the allowable bearing capacity is available only at a greater depth, the foundation can be rested at a higher level for economic considerations and the difference in level between the base of foundation and the depth at which the allowable bearing capacity occurs can be filled up with either: • (a) concrete of allowable compressive strength not less than the allowable bearing pressure, or

20. 4. DESlGN CONSIDERATION • (b) incompressible fill material, for example, sand, gravel, etc, in which case the width of the fill should be more than the width of the foundation by an extent of dispersion of load from the base of the foundation on either side at the rate of 2 vertical to 1 horizontal. • 4.11 The cement concrete foundation ( plain or reinforced ) should be designed in accordance with IS : 456-1978’ and masonry foundation in accordance with IS : 1905-1980.

21. Factors affecting choice of foundation • Function of the Structure – Residential, Commercial, Bridges etc. • Loads coming from the structure • Subsoil conditions • Relative cost of foundation in relation to superstructure

22. Factors affecting Depth of Shallow Foundations • Depth of top soil or filled-up soil • Depth of poor surface deposit such as peat, muck, sanitary land fill • Location of ground water table and its seasonal variation • Depth of poor or better underlying strata • Depth of adjacent footing

23. 5. CONSTRUCTION • 5.1 The cement concreting ( plain and reinforced ) in the foundation should be done in accordance with the provision given in IS : 456-1978*. • 5.2 The stone masonry construction should conform to IS : 1597 ( Parts 1and 2 )-1967: and brick masonry construction should conform to IS : 2212-19629.

24. 5. CONSTRUCTION • 5.3 The lime concrete should be done in accordance with the provisionsgiven in IS : 2541-197711 or IS : 5817-19701. • 5.4 Masonry should be constructed over the base concrete after curing the base of concrete for at least 3 days. Before laying concrete, the bed of the foundation pit/trench should be thoroughly compacted by manual Ramming.

26. Share failure or bearing capacity criteria Foundation should be designed such that soil bellow does not fail in shear

27. Maximum gross intensity of load that soil can support before it fails in shear The maximum net intensity of loading at the base of foundation that the soil can support before it fails in share The maximum net intensity of loading that the soil can safely support without the risk of failure

28. The maximum gross intensity of load that the soil can carry without failing in shear

29. Settlement criterion • Safe bearing pressure: the maximum safe intensity of loading that can be allowed on the soil without the settlement exceeding the permissible limit

30. Modes of failure • General shear failure • Local shear failure • Punching shear failure

32. General shear failure

33. Consistency chart for clay If undrained cohesion value of clay is 50-100 kpa it is stiff clay and shear failure is expected to be general shear failure

34. Consistency chart for sand If relative density of sand is 65-85 it is dense sand and failure is expected to be general shear failure

35. Local shear failure • Medium or relatively loose sand/ medium or relatively soft consistency clay

36. Nature of load settlement curve for local shear failure Failure load can be estimated approximately by single tangent method or double tangent method

37. Punching shear failure • Very loose san/ very soft clay

38. Load settlement curve for punching shear failure Ultimate load can be approximated by double tangent method

39. Terzaghi’s bearing capacity theory (1920) • The footing is a long strip or continuous footing resting on a deep homogeneous soil having shear parameter C and • Analysis is a 2 D condition • Soil fails in general shear failure mode • The load is vertical and concentric • The ground surface is horizontal • The base of footing laid at a shallow depth • The shearing resistance of soil between the surface and depth Df is neglected and the footing is considered resting on surface with a surcharge loading of at the level of footing

40. Terzaghi’s shear failure zones

41. Terzaghi’s shear failure zones

42. Terzaghi’s shear failure zones • There will be friction between soil and base of foundation for Ø soil ( granular soil) • Ø- soil-soil friction • δ – soil and other material friction • There will be cohesion between clayee soil particles • C - cohesion for clay-clay particle • Ca - Adhesion for clay and other materials • For C , phy soil there will be friction soil and foundation and adhesion between soil and foundation because of this phenomena zone-I will not deform laterally outside foundation

46. As per terzaghi for local shear failure value of cohesion is 67% of that in general shear failure

50. The effect of submergence is to reduce undrained shear strength. The shear strength parameters should be determined in laboratory under saturated condition.