In geotechnical engineering, bearing capacity is the capacity of soil to support the loads applied to the ground. The bearing capacity of soil is the maximum average contact pressure between the foundation and the soil which should not produce shear failure in the soil. Ultimate bearing capacity (q_{f}) is the theoretical maximum pressure which can be supported without failure; allowable bearing capacity (q_{a}) is the ultimate bearing capacity divided by a factor of safety. Sometimes, on soft soil sites, large settlements may occur under loaded foundations without actual shear failure occurring; in such cases, the allowable bearing capacity is based on the maximum allowable settlement.
There are three modes of failure that limit bearing capacity: general shear failure, local shear failure, and punching shear failure.
Figure 1. Shear stresses based on Terzaghi’s soil bearing capacity theory.
Zone I: A relatively undeformed wedge of soil below the foundation forms an active Rankine zone with angles (45º + f'/2).
Zone II: The transition zones take the form of log spiral fans.
Zone III: The wedge pushes soil outwards, causing passive Rankine zones to form with angles (45º - f'/2).
Based on Terzaghi’s bearing capacity theory, column load P is resisted by shear stresses at edges of three zones under the footing and the overburden pressure, q (=gD) above the footing. The first term in the equation is related to cohesion of the soil. The second term is related to the depth of the footing and overburden pressure. The third term is related to the width of the footing and the length of shear stress area. The bearing capacity factors, Nc, Nq, Ng, are function of internal friction angle, f.
Strip footings:
Qu = c Nc + g D Nq + 0.5 g B Ng
Square footings:
Qu = 1.3 c Nc + g D Nq + 0.4 g B Ng
Circular footings:
Qu = 1.3 c Nc + g D Nq + 0.3 g B Ng
Where:
C: Cohesion of soil (apparent cohesion intercept); g: unit weight of soil; D: depth of footing (depth of embedment); B: width/breadth of footing; Nc, Nq, Nr: Terzaghi’s bearing capacity factors depend on soil friction angle, f:
K_{p}=passive pressure coefficient.
(Note: from Bowles' Foundation analysis and design book, "Terzaghi never explained..how he obtained K_{p} used to compute Ng")
Table 1. Terzaghi’s Bearing Capacity Factors.
f |
Nc |
Nq |
Nr |
0 |
5.7 |
1 |
0 |
5 |
7.3 |
1.6 |
0.5 |
10 |
9.6 |
2.7 |
1.2 |
15 |
12.9 |
4.4 |
2.5 |
20 |
17.7 |
7.4 |
5 |
25 |
25.1 |
12.7 |
9.7 |
30 |
37.2 |
22.5 |
19.7 |
35 |
57.8 |
41.4 |
42.4 |
40 |
95.7 |
81.3 |
100.4 |
45 | 172.3 | 173.3 | 297.5 |
48 | 258.3 | 287.9 | 780.1 |
Figure 2. Terzaghi’s bearing capacity factors.
Given:
Soil properties:
Soil type: cohesionless soil.
Cohesion: 0 (neglectable)
Friction Angle: 30 degree
Unit weight of soil: 100 lbs/ft^{3}
Expected footing dimensions:
3 ft wide strip footing, bottom of footing at 2 ft below ground level
Factor of safety: 3
Requirement: Determine allowable soil bearing capacity using Terzaghi’s equation.
Solution:
From Table 1 or Figure 1, Nc = 37.2, Nq = 22.5, Nr = 19.7 for f = 30 degree
Determine ultimate soil bearing capacity using Terzaghi’s bearing capacity equation for strip footing
Qu = c Nc + g D Nq + 0.5 g B Ng
= 0 +100x2x22.5+0.5x100x6x19.7
= 10410 lbs/ft^{2}
Allowable soil bearing capacity,
Qa = Qu / F.S. = 10410 / 3 = 3470 lbs/ft^{2} @ 3500 lbs/ft^{2}
Given:
Soil type: Clay
Soil properties:
Cohesion:2000 lbs/ft^{2}
Friction Angle: 0 (neglectable)
Unit weight of soil: 120 lbs/ft^{3}
Expected footing dimensions:
6 ft by 6 ft square footing, bottom of footing at 2 ft below ground level
Factor of safety: 3
Requirement: Determine allowable soil bearing capacity using Terzaghi’s equation.
Solution:
From Table 1 or Figure 1, Nc = 5.7, Nq = 1.0, Nr = 0 for f = 0 degree
Determine ultimate soil bearing capacity using Terzaghi’s bearing capacity equation for square footing
Qu = 1.3 c Nc + g D Nq + 0.4 g B Ng
= 1.3x1000x5.7 +120x2*1+ 0
= 7650 lbs/ft^{2}
Allowable soil bearing capacity,
Qa = Qu / F.S. = 7650 / 3 = 2550 lbs/ft^{2} @ 2500 lbs/ft^{2}
Given:
Soil properties:
Soil type: sandy clay
Cohesion: 500 lbs/ft^{2}
Friction Angle: 25 degree
Unit weight of soil: 100 lbs/ft^{3}
Expected footing dimensions:
10 ft diameter circular footing for a circular tank, bottom of footing at 2 ft below ground level
Factor of safety: 3
Requirement:
Determine allowable soil bearing capacity using Terzaghi’s equation.
Solution:
From Table 1 or Figure 1, Nc = 17.7, Nq = 7.4, Nr = 5.0 for f = 20 degree
Determine ultimate soil bearing capacity using Terzaghi’s bearing capacity equation for circular footing
Qu = 1.3 c Nc + g D Nq + 0.3 g B Ng
= 1.3x500x17.7 +100x2x7.4+0.3x100x10x5.0
= 17985 lbs/ft^{2}
Allowable soil bearing capacity,
Qa = Qu / F.S. = 17985/ 3 = 5995 lbs/ft^{2} @ 6000 lbs/ft^{2}
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