1.1.1 BEARING CAPACITY EQUATION METHOD
Allowable Vertical Bearing Pressure of Shallow Foundation founded on Soil
The allowable vertical bearing pressure of foundations founded on soils derived by bearing capacity equation may be taken as:
q = qu – qo + q
a F o
where qa = allowable vertical bearing pressure
qu = ultimate bearing capacity of the granular soil, which should be limited to 3,000kPa
qo = effective overburden pressure at the base of the foundation, i.e. qo = γs‘ Df , whereγs‘ and Df are respectively the effective unit weight and depth of the soil that originally exists above the base of the foundation
F = factor of safety not less than 3
The ultimate bearing capacity of the soil for shallow foundation may be estimated by the following equation:
| u |
q = Qu
= c’ N ζ ζ ζ ζ
+ 0.5 B ‘ γ ‘ N ζ
ζ ζ ζ
- q N ζ ζ ζ ζ
Bf ‘ Lf ‘
c cs ci ct cg
f s γ γs
γi gt γg
q qs qi qt qg
where Nc, Nγ, Nq = general bearing capacity factors which determine the capacity of a long strip footing
Qu = ultimate resistance against bearing capacity failure c’ = effective cohesion of soil
γs‘ = effective unit weight of soil
q = overburden pressure in the ground adjacent to the foundation and at same level as the base of the foundation (see Figure 2.2(a) for sloping ground)
Bf = least dimension of footing
Lf = longer dimension of footing
Bf‘ = Bf – 2eB
Lf‘ = Lf – 2eL
eL = eccentricity of load along L direction
eB = eccentricity of load along B direction ζcs, ζγs, ζqs = influence factors for shape of foundation ζci, ζγi, ζqi = influence factors for inclination of load ζcg, ζγg, ζqg = influence factors for ground surface
ζct, ζγt, ζqt = influence factors for tilting of foundation base Notes:
- A shallow foundation is taken as one in which the depth to the bottom of foundation is less than or equal to 3m.
- q should not include any overburden pressure that may be temporarily or permanently removed during the design life of the foundation. In its derivation, the maximum effective overburden depth of subsoil should not be greater than Bf and suitable adjustments should be made to discount any voids that may be allowed for underground utilities.
- Figure 2.2 shows the generalised loading and geometric parameters for the design of a shallow foundation and the bearing capacity factors are given in Table 2.3.
- Any weak geological features present in the ground may affect the validity of the bearing capacity equation. Therefore the geological characteristics of the ground should be considered in the evaluation of the bearing capacity.
- For shallow foundations on or near the crest of a slope, the ultimate bearing capacity may be obtained by linear interpolation between the value for the foundation resting at the edge of the slope and that at a distance of four times the foundation width from the crest. The latter may be assumed to be equal to that of a foundation placed on flat ground. Figure 2.3 summarizes the procedures for the linear interpolation. The effect of the foundation works on the overall stability of the slope should also be checked.
- The bearing capacity equation is applicable to rectangular shaped shallow foundations. For shallow foundation of an irregular shape, the calculation may be based on the largest inscribed rectangle as shown in Figure 2.4.
- The effective unit weight of the soilγs’ may be taken as follows:
- Dry condition (see clause 1.2 for definition):
γs‘ = γ
whereγis the bulk unit weight of the soil
- Submerged condition (see clause 1.2 for definition):
- For static groundwater:
γs‘ = γ’
whereγ’is the submerged unit weight of the soil
- For groundwater flows under an upward hydraulic gradient:
γs‘ = γ – γw (1 + ί)
where ί is the upward hydraulic gradient; and γw is the unit weight of water.
- For intermediate groundwater levels, γ s‘ may be interpolated between the above limits.
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