5.7 Bearing capacity from Standard Penetration Test (SPT) results
When coarse-grained soils are encountered during the geotechnical investigation, it is often extremely difficult to obtain undisturbed samples for determining their effective shear strength parameters in the laboratory. Hence, it is common practice to use directly results from in situ tests, like the Standard Penetration Test (SPT) together with empirical correlations, to determine the bearing capacity of footings resting on coarse-grained soils.
When using methodologies such as the one presented here (AASHTO 2012) we should consider the average SPT value for a depth 1.5B below the foundation base, corrected for overburden stress according to Chapter 1.5 as:
(5.51) 
where the overburden stress correction factor CN is (Eq. 1.2):
(5.52) 
For footings on coarse-grained soils subjected to vertical loads of zero eccentricity, the AASHTO methodology provides the bearing capacity in terms of stress as:
(5.53) 
where Cw1, Cw2 are factors accounting for the depth of the groundwater table below the foundation level, and are provided in Table 5.7.
| Depth of water table below foundation level (m) | Cw1 | Cw2 |
|---|---|---|
| 0 | 0.5 | 0.5 |
| Df | 0.5 | 1.0 |
| 1.5B + Df | 1.0 | 1.0 |
In the above expressions, z is the depth to the groundwater table measured from the ground surface, B is the footing width, Df is the embedment depth and σ′z0 is the effective geostatic vertical stress. Eq. 5.53 provides the pressure on the footing corresponding to a settlement less than 25 mm. Even when used for cases of embedded footings, the weight of the excavated soil γDf should not be added to Eq. 5.53, as the latter provides directly the bearing capacity qf.
An application of this method is provided in the following Example 5.6.
