Fd shallow is1904_cmt_9-aug-2013


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Fd shallow is1904_cmt_9-aug-2013

  1. 1. 8/12/2013 Design and Construction of Shallow Foundations: General requirements(IS1904-1986) 1
  2. 2. 8/12/2013 Shallow Foundations:.. • Usually the more economical option • As a general rule, consider deep foundations only when shallow foundations do not give satisfactory design • Types of Shallow foundations • • • • Spread footings (square, circular, rectangular) Combined Footings Continuous (strip) Footings Mat or Raft Foundations 4 2
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  6. 6. 8/12/2013 Combined/Strap Footing 6
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  8. 8. 8/12/2013 Mat (or Raft) Foundation Excavation, Layout and Foundation • Excavation is a process of making trenches by digging up of earth for the construction of foundations and basements. • Excavation is done by the Backhoe (JCB) on the hourly basis • After the excavation the surface is leveled called surface dressing • Layout is done on the PCC poured over leveled surface. • Column and foundation (raft ) steel is then laid as per drawings. 8
  9. 9. 8/12/2013 Points to take care:• layout should be checked properly. • Check any difference between architectural and structural drawings regarding location of column. • After excavation check the stability of temporary structures built near the excavated ground. • Before laying raft reinforcement, shuttering wall which is mainly brick wall should be built and should be filled with soil on other side. • Check the direction of chair bars in the raft Construction Methods • Excavation; Backhoe followed by handwork o Neat excavation-no formwork used o Formed footing 9
  10. 10. 8/12/2013 Depth and location of foundation Depth and location of foundation depends on 1. Zone of significant volume changes in clayey soils. 2. Adjacent structures and property lines. 3. Ground water 4. Underground defects (root holes, cavities by burrowing animals etc.) 5. Zone of frost action 6. Maximum depth of scour 7. Filled up ground, ditch, pond, excavation etc. 10
  11. 11. 8/12/2013 Depth and location of foundation The following considerations are necessary for deciding the location and depth of foundation • As per IS:1904-1986, minimum depth of foundation shall be 0.50 m. • Foundation shall be placed below the zone of the frost heave • Topsoil or organic material • Peat and Muck • Unconsolidated material such as waste dump Depth and location of foundation Zone of significant volume changes in soil : Clays having high plasticity shrink and swell considerably upon drying and wetting respectively (expansive soil). Volume change is greatest near ground. Decreases with increasing depth. Volume changes usually insignificant below a depth from 1.5-3.0 m and does not occur below volume changes. No tree should grow to a large size shall be planted within 8 m of foundations of buildings in such type of clayey soils (IS 1904) 11
  12. 12. 8/12/2013 Depth and location of foundation Adjacent structures and property lines. Structures may be damaged by the construction of new foundations, as a result of vibrations, undermining by excavation or lowering of the water table. . After new foundations have been constructed, the (new) loads they place on the soil may cause settlement of previously existing structures as a result of new stress pattern in the surrounding soil. 12
  13. 13. 8/12/2013 14. Spacing Between Existing and New Foundation The deeper the new foundation and the nearer to the existing it is located, the greater the damage is likely to be. The minimum horizontal spacing between existing and new footings shall be equal to the width of the wider one. While the adoption of such provision shall help minimizing damage to adjacent foundation, an analysis of bearing capacity and settlement shall be carried out to have an appreciation of the effect on the adjacent existing foundation. 13
  14. 14. 8/12/2013 a > width of wider footing Depth and location of foundation New Footing 26.60 Limit for bottom of deeper Footing Existing Footing i.e. 26.60 i.e. 450 14
  15. 15. 8/12/2013 the line from the base of the new footing to the bottom edge of the existing footing should be 45° or less with the horizontal plane If the soil is a sand (does not have cohesion) one cannot excavate to a depth greater than that of the existing foundation. 15
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  18. 18. 8/12/2013 26.60 2H:1V 2 18
  19. 19. 8/12/2013 2H:1V 3 Ans1. On sloping sites, stepped foundations at all changes of levels, they shall be lapped at the steps for a distance at least equal to the thickness of the foundation or twice the height of the step, whichever is greater. The steps shall not be of greater height than the thickness of the foundation, unless special precautions are taken. 19
  20. 20. 8/12/2013 5.1 Dewatering During Construction When the foundation is placed under water table, frequent pumping from a sump in the excavation is unacceptable because of the danger of the collapse of the excavation as a result of the lowered effective stress due to the rising water. The use of well point of good control is better option. Make an attention to nearby building not to be affected by lowering the water level beneath the building. 20
  21. 21. 8/12/2013 Loads on foundation Dead Load : Refers to the overall weight of the structure. Includes weight of the materials permanently attached to the structure (such as flooring) and fixed service equipment (such as air conditioning) Live load : Refers to the weight of the applied bodied that are not permanent parts of the structure. Applied to the structure during part of its useful life (e.g. people, warehouse goods). Specified by code. Wind loads : Acts on all exposed parts of the structure. Calculated using building codes. Earthquake Forces : Building code is consulted. 15. LOADS ON FOUNDATIONS • 15.1.3 Foundations shall be proportioned for the following combination of loads: • a) Dead load + live load, and • b) Dead load + live load + wind load or seismic load. 21
  22. 22. 8/12/2013 • 15.1.4 Dead load also includes the weight of column/wall, footings,foundations, the overlying fill but excludes the weight of the displaced soil • 15.1.6 Where wind or seismic load is less than 25 percent of that due to dead and live loads, it may be neglected in design and first combination of load shall be compared with the safe bearing load to satisfy allowable bearing pressure. • 15.1.7 Where wind or seismic load is more than 25 percent of that due to dead and live loads, foundations may be so proportioned that the pressure due to combination of load ( that is, dead + live+ wind load ) does not exceed the safe bearing capacity by more than 25%. • When seismic forces are considered, the safe bearing capacity shall be increased as specified in IS : 1893-(2002)-Part1. • In non-cohesive soils, analysis for liquefaction and settlement under earthquake shall also be made. 22
  23. 23. 8/12/2013 Increase in allowable pressure in soils When earthquake forces are included, the allowable bearing pressure in soils shall be increased as per Table 1, depending upon type of foundation of the structure and the type of soil. 23
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  25. 25. 8/12/2013 1. The allowable bearing pressure shall be determined in accordance with IS 6403 or IS 1888. 2. If any increase in bearing pressure has already been permitted for forces other than seismic forces, the total increase in allowable bearing pressure when seismic force is also included shall not exceed the limits specified above. PART II FOUNDATIONS FOR IMPACT TYPE MACHINES (HAMMER FOUNDATIONS) 25
  26. 26. 8/12/2013 Factor of Safety Depends on:  Type of soil  Level of Uncertainty in Soil Strength  Importance of structure and consequences of failure  Likelihood of design load occurrence Minimum Factor of Safety 26
  27. 27. 8/12/2013 17. STABILITY AGAINST OVERTURNING AND SLIDING FOR FOUNDATIONS) Stability condition Factor of safety IS1904 IRC78 Sliding (DL+LL+EP) Sliding (DL+LL+EP+EQ/WL) 1.75 1.5 1.5 1.25 1.5 1 1.5 1.125 Overturning(DL+LL+EP) Overturning(DL+LL+EP+EQ/WL) 2 1.5 2 1.5 2 1.5 2 1.5 Bearing Capacity (static) Bearing Capacity (with seismic) IS 14458 AAAHTO 3 1.5 27
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