BC-II lec 6 pierand deep fdn.pptx

FOUDATION WEOIFHOLKSEF

1. BUILDING CONSTRUCTION-II BEG 116 AR LECTURE-6 DATE:

2. 2. Pier foundation: A Pier foundation consists of a cylindrical column of large diameter to support and transfer large super-imposed loads to the firm strata below. Piers are holes filled with concrete which act as leg for foundation Piers are used when a heavy structure is to be built and the firm/hard strata is at reasonable depth Construction process comprises sinking vertical shafts and filling with concrete

3. 2. Pier foundation: Fig: Pier foundation with arch

4. 2. Pier foundation:……………… Difference between Pile and Pier foundation: Difference in methods of construction pile foundations transfer the load through friction and/or bearing but pier foundations transfer the load only through bearing pier foundation is shallower in depth than the pile foundation Pier foundation require less excavation

5. Types of Pier foundation: ( i )  Masonry or concrete pier  (ii)  Drilled caissons

6. ( i )  Masonry or concrete pier : When a good bearing stratum exists upto 5 m below ground level, brick, masonry or concrete foundation piers in excavated pits may be used The size and spacing of the piers depends upon the depth of hard bed, nature of overlying soil and super-imposed loads

7. ( i )   Drilled caissons  : The terms drilled caissons, foundation pier or sub-pier are also termed as cylindrical foundation. Drilled caissons are generally drilled with the mechanical means     A drilled caisson is largely a compressed member subjected to an axial load at the top and reaction at the bottom

8. Drilled caissons may be of three types :  concrete caisson with enlarged bottom   caisson of steel pipe with concrete filled in the pipe  caisson with concrete and steel core in steel pipe  

9. Drilled caissons   

10. 3. Well foundation or caissons: “Caisson”- french word “ caisse ”- means box Well foundations or caissons are box like structure-circular or rectangular-which are sunk from the surface of either land or water to the desired depth Adopted for the construction of foundation of bridge , pier They are much large in diameter than the pier foundations or drilled caissons

11. 3. Well foundation or caissons:……….. Caisson foundations are used for following purpose(or in following case): To transmit load of the structure to hard bearing strata lying below deep waters box

12. 3. Well foundation or caissons:……….. Caisson foundations are used for major foundation works, such as for: bridge piers and abutments in rivers, lakes etc Wharves, quay walls, docks Break waters and other structures for shore protections Large water front structures such as pump houses, subjected to heavy vertical and horizontal loads

13. Well foundations or caissons are hollow from inside, which may be filled with sand and are plugged at the bottom. The load is transferred though the perimeter wall, called steining Well foundations are not used for buildings 3. Well foundation or caissons:……

14. 3. Well foundation or caissons:……….. Types of caissons: Box caissons Open caissons or wells or well foundation Monolithic caissons Pneumatic caissons

15. 3. Well foundation or caissons:………..

16. a. Box caissons: Box is open at top and closed at the bottom May be of RCC, steel or timber It is first built on land and then sunk in position The cavity is generally filled with sand Then the top of caisson is filled with concrete

17. a. Box caissons:……….

18. a. Box caissons:………. Source: Gurucharan pg 112

19. a. Box caissons:……………….. Box caisson is used in following case: When the depth of water is not more than 6 to 8m When the bed material is soft or loose

20. a. Box caissons:……………….. Procedure: First of all the base where caisson is to rest is leveled and consolidated If caisson is to rest on group of piles, the head of piles are leveled such a way that heads of pile lie on same plain The caisson is floated on site of use and slowly sunk by filling the cavity of box caisson with sand or gravel

21. a. Box caissons:……………….. Procedure:………… Sand filling helps in uniform distribution of load on soil The sand bed is sealed with concrete in order to confine soil in its position

22. b. Open caissons or well or well foundation Unlike box caisson open caisson is open at both top and bottom end Open caisson is a box of timber, metal or RCC Open caissons are known as well and if no. of well is two it is known as twin well and if more wells than these are called monolith Uneconomical – as heavy timbering is required Slow in construction- since greater earthwork required

23. b. Open caissons or well or well foundation Types of open caisson: Single well Multiple well or monoliths Cylinder Gurucharan pg 113 to 116

24. c. Pneumatic caisson Closed at the top and open (during construction) at the bottom The basic principle is to isolate the working chamber from the surrounding atmosphere Working process similar to the open caisson but in this caisson the working chamber is kept air tight

25. c. Pneumatic caisson The pressure of air in the shaft is kept higher than that of the water at the working depth so that the subsoil water may not enter the working chamber

26. FOUNDATION IN BLACK COTTON SOIL What is Black cotton soil? Having properties of shrinkage and swelling due to moisture movement through them Swells excessively when wet(wet during rain) and shrinks when dry This results cracks in wall and damage lintels, beams Have poor bearing capacity

27. Things to be considered while designing footing in black cotton soil: The safe bearing capacity of soil should be properly determined The foundation should be taken at least 50cm lower than the depth of moisture movement If the black cotton soil layer is at the top(1 to 1.5m) deep, the layer can be completely removed and foundation can be laid over non-shrinkable and non-swelling soil

28. Things to be considered while designing footing in black cotton soil:………………………… But if the black cotton soil is at greater depth than the foundation laid should not be allowed to be in contact with soil In this case the excavation of large width and depth is dug and insert the layer of sand around and beneath the footing Deeper piles and piers are used In case the bearing capacity of soil is low the foundation trench should be made hard

29. Things to be considered while designing footing in black cotton soil:…………….. Better to construct this type of foundation during dry season Types of foundation in black cotton soil: Strip or pad foundation Pier foundation Under reamed pile foundation

30. 1. Strip or pad foundation: Some special design features as follows are adopted: When soil has little swelling pressure When soil has relatively high swelling pressure When soil has high swelling pressure When soil is soft and loose and bearing capacity is low

31. 1. Strip or pad foundation:………….. When soil has little swelling pressure: A layer of of 60cm thick cohesion less sand is filled below the foundation bed and compacted Sand is also filled around the footing

32. 1. Strip or pad foundation:………….. When soil has relatively high swelling pressure: By keeping alternate layers of sand and ballast/ mooram

33. 1. Strip or pad foundation:………….. When soil has high swelling pressure : 25 to 30 cm thick and wide strips of concrete is laid over the compacted base of trench The space or gap between the two strips of concrete is filled with sand and is equal to the width of bottom of masonry Above this strip the foundation bed is laid Side of foundation laid with sand

34. 1. Strip or pad foundation:………….. When soil has high swelling pressure:……….. 80mm dia pipe @ spacing 1.5m are placed through masonry and concrete reaching sand fill and pipe is filled with sand Plug may placed at the top of the pipe which can be removed and placed as per the work required

35. 1. Strip or pad foundation:………….. When soil is soft and loose and bearing capacity is low: First of all 30 cm thick layer of ballast is rammed Over it 30cm thick layer of sand is filled Over it lies the concrete bed of foundation Sides of foundation filled with sand as usual

36. 2. Pier foundation with arches: Piers are dug at regular interval and filled with cement concrete Piers rest on hard bearing strata Piers at the top are connected through concrete or masonry arch Above masonry arch the walls rest Sometimes beams are provided over the arch in case the arch is constructed of masonry

37. 2. Pier foundation with arches:……… The arches are constructed high above ground level The gap allows the free vertical movement of soil during swelling and shrinkage operation

38. 3. Under-reamed pile foundation: Under reamed piles are bored cast in situ  concrete piles having bulb shaped enlargement near base A Pile having one bulb is called single under reamed pile and having two is double underreamed pile The bearing capacity of the pile is increased by increasing the number of bulbs at the base The basic principle of under reamed pile is to anchor the structure at a depth where ground movement are negligible due to moisture variation or other reasons

39. Simple tools are required for construction of under-reamed  piles like spiral auger, under reaming tool, and boring guide For speeding up the construction bore and under ream for large diameter and deeper pile a  mechanical rig can be used 3. Under-reamed pile foundation:…………….

40. Diameter of the bulb is 2.5 times dia of piles stem or may vary from 2.5times the dia of piles The c/c distance vertical spacing between two bulbs(in case of multi reamed piles) vary from 1.25 to 1 .5 times the diameter of bulb Length of under reamed piles varies from 3 to 8m 3. Under-reamed pile foundation:…………….

41. FOUNDATION IN SLOPING SITE Generally, it's most economical to place wall footings at a constant elevation If the site or finished grade slopes along the length of the wall, however, the footing may end up a considerable distance below finished grade This is clearly not economical, as it requires extra excavation and material

42. Slope the footing with the site so its depth below the finished grade is nearly constant along its length FOUNDATION IN SLOPING SITE Step the footing so its depth below finished grade is not excessive at any point along its length Following options are preferred

43. FOUNDATION IN SLOPING SITE Sloped Footing Issues simple geometry and apparent ease in formwork construction. But create the following construction issues

44. FOUNDATION IN SLOPING SITE Sloped Footing Issues Vertical wall bars above the footing will have different lengths - increase the quantity of vertical reinforcement Horizontal reinforcing bars in the lower portion of the wall will also have different lengths because they are interrupted by the sloped footing will require trapezoidal formwork- so need modifications to standard rectangular formwork A sloped footing could be unstable, particularly on a very steep slope and Concrete placement and finishing could be difficult, and a stiff concrete mixture might be required to prevent the concrete from flowing downhill

45. FOUNDATION IN SLOPING SITE Considerations For Stepped Footings For a very long wall- stepped footing more economical

46. FOUNDATION IN SLOPING SITE Considerations For Stepped Footings It's generally more cost effective to minimize the number of steps. For example, it may be more economical to design for a 6 ft (1.8 m) change in elevation using three 2 ft (0.6 m) steps or two 3 ft (0.9 m) steps rather than six 1 ft (0.3 m) steps- This minimizes the number of wall sections to be detailed and formed

47. CAUSES OF FAILURE OF FOUNDATION AND THEIR REMEDIAL MEASURES ……………… Bc punmia pg 63

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