Bearing Capacity for Shallow Foundations by Prof E.Saibaba Reddy & Eadala Rakesh Reddy

1. 4
Bearing Capacity of Foundation
Prof. E.Saibaba Reddy
B.Tech, M.E.(Hons) Roorkee, Ph.D (Nottingham, UK)
Post Doc,(Halifax Canada), Post Doc (Birmingham UK)
&
Eadala Rakesh Reddy
B.Tech (JNTUH), M.Tech (VSSUT-Gold Medal),
(Ph.D)- Andhra University-DST-Inspire Fellow
Chief Consultant –EE Engineering Construction Services

2. References
ACI 318M-14 Building Code Requirements for Structural
Concrete ( ACI 318M -14) and Commentary, American
Concrete Institute, ISBN 978-0-87031-283-0.
Bowles , J.,E.,(1996) “Foundation Analysis and Design” -5th
ed. McGraw-Hill, ISBN 0-07-912247-7.
Das, B., M. (2012), “ Principles of Foundation Engineering ”
Eighth Edition, CENGAGE Learning,
ISBN-13: 978-1-305-08155-0.
2

3. Bearing Capacity ofFoundation
The soil must be capable of carrying the loads from
any engineered structure placed upon it without a
shear failure and with the resulting settlements
being tolerable for that structure.
This lecture will be concerned with evaluation
of the limiting shear resistance, or ultimate
bearing capacity quSt of the soil under a
foundation load.
3

4. Bearing Capacity ofFoundation
It is necessary to investigate both base shear
resistance and settlements for any structure.
In many cases settlement criteria will control
the allowable bearing capacity; however, there
are also a number of cases where base shear (in
which a base punches into the ground - usually
with a simultaneous rotation) dictates the
recommended bearing capacity.
4

5. Bearing Capacity ofFoundation
Structures such as liquid storage tanks and mats are
often founded on soft soils, which are usually more
susceptible to base shear failure than to settlement.
Base shear control, to avoid a combination base
punching with rotation into the soil, is often of more
concern than settlement for these foundation types.
5

6. Allowable Bearing Capacity
The recommendation for the allowable bearing
capacity qaSSto be used for design is based on the
minimum of either:
1. Limiting the settlement to a tolerable amount
2.The ultimate bearing capacity, which considers soil
strength, as computed in the following sections
6

7. Allowable Bearing Capacity
The allowable bearing capacity based on shear
control qaSSis obtained by reducing (or dividing)
the ultimate bearing capacity quSt (based on soil
strength) by a safety factor SF that is deemed
adequate to avoid a base shear failure to obtain
qaSS = qult
S.F
(2-1)
The safety factor is based on the type of soil (cohesive or cohesionless),
reliability of the soil parameters, structural information (importance,
use, etc.), and consultant caution.
7

8. Allowable Bearing Capacity
Most building codes provide an allowable
settlement limit for a foundation, which
may be well below the settlement derived
corresponding to qaSS given by equations( 2-1).
Thus, the bearing capacity corresponding to the
allowable settlement must also be taken into
consideration.
8

9. BEARING-CAPACITY EQUATIONS
Terzaghi’s Bearing Capacity Theory
Terzaghi (1943) was the first to present a comprehensive
theory for the evaluation of the ultimate bearing capacity
of rough shallow foundations. According to this theory, a
foundation is shallow if its depth, Df (Figure slid 11), is
less than or equal to its width. Later investigators,
however, have suggested that foundations with Df equal
to 3 to 4 times their width may be defined as shallow
foundations.
9

10. BEARING-CAPACITY EQUATIONS
Terzaghi’s Bearing Capacity Theory
Terzaghi suggested that for a continuous, or strip, foundation
(i.e., one whose width-to-length ratio approaches zero), the
failure surface in soil at ultimate load may be assumed to be
similar to that shown in Figure on Slide 11.
The effect of soil above the bottom of the foundation may also
be assumed to be replaced by an equivalent surcharge, q = y ∗ Dƒ
(where y is the unit weight of soil).
10

11. BEARING-CAPACITY EQUATIONS
Terzaghi’s Bearing Capacity Theory
The failure zone under the foundation can be
separated into three parts (see Figure 4.6):
1.The triangular zone ACD immediately under
the foundation
2.The radial shear zones ADF and CDE, with the
curves DE and DF being arcs of a logarithmic
spiral
3.Two triangular Rankine passive zones AFH
and CEG
11

12. BEARING-CAPACITY EQUATIONS
Terzaghi’s Bearing Capacity Theory
The angles CAD and ACD are assumed to be equal to the
soil friction angle ∅u.
Note that, with the replacement of the soil above the bottom
of the foundation by an equivalent surcharge q, the shear
resistance of the soil along the failure surfaces GI and HJ
was neglected.
12

13. BEARING-CAPACITY EQUATIONS
Terzaghi’s Bearing Capacity Theory
Dr. Abdulmannan Orabi IUST 13

14. BEARING-CAPACITY EQUATIONS
Terzaghi’s Bearing Capacity Theory
14

15. BEARING-CAPACITY EQUATIONS
Terzaghi’s Bearing Capacity Theory
The ultimate bearing capacity,quSt, of the foundation
now can be obtained by considering the equilibrium of the
triangular wedge ACD shown in Figure below
15

16. BEARING-CAPACITY EQUATIONS
16

17. BEARING CAPACITY EQUATIONS

20. As per IS 6403:1981

21. BEARING CAPACITY EQUATIONS

22. BEARING CAPACITY EQUATIONS

24. Types of Failure in Soil

25. General Shear Failure

26. Local Shear Failure

27. Punching Shear Failure

28. Punching Shear Failure

29. Modes of Failure in Sand

31. Effect of Water Table on Bearing Capacity
31

32. Effect of Water Table on Bearing Capacity

33. Effect of Water Table on Bearing Capacity

34. Effect of Water Table on Bearing Capacity

35. Effect of Water Table on Bearing Capacity

36. Effect of Water Table on Bearing Capacity

37. Foundation Settlements

38. Foundation Settlements

39. Foundation Settlements

40. Foundation Settlements

41. Foundation Settlements

42. Foundation Settlements

43. Foundation Settlements

44. Foundation Settlements

45. Foundation Settlements

47. Plate Load Test

48. Plate Load Test

49. Plate Load Test

50. Plate Load Test

51. Plate Load Test

52. Plate Load Test

53. Plate Load Test

54. Plate Load Test

55. Bearing Capacity From SPT

56. Bearing Capacity From SPT

57. Bearing Capacity From SPT

59. Effect of Footing Size on Settlements

60. Bearing Capacity From SPT

61. BEARING-CAPACITY ofMat Foundation
The net allowable soil bearing capacity
qaSS (net)
quSt(net)
=
S. F
For mats on clay, the factor of safety should not be less
than 3 under dead load or maximum live load.
Under most working conditions, the factor of safety
against bearing capacity failure of mats on sand is
very large.
61

62. BEARING-CAPACITY ofMat Foundation
Bearing Capacity from SPT
The net allowable bearing capacity for mats constructed over
granular soil deposits can be adequately determined from the
standard penetration resistance numbers
qaSS (net) =
55
0.08
Fd
N Se (nn)
qaSS(net) = 16.63 N55
25
Se(n n )
25
62