Geoscience Reference
In-Depth Information
Table 21.6
Variation of bearing capacity coefficient (N
c
)
with the
depth (Skempton, 1951).
Embedment
Bearing capacity coefficient (N
c
)
ratio (z/B)
Strip footing
Circular or square
0
5.14
6.28
1
6.4
7.7
2
7.0
8.4
3
7.3
8.7
4
7.4
8.9
5
7.5
9.0
•
z
=
Depth from surface to underside of footing.
•
B
=
Width of footing.
Figure 21.2
General shear failure.
21.7 Bearing capacity of granular soils
•
In granular soils, the friction angle is often determined from the SPT N - value.
Methods that directly use the N - value to obtain the bearing capacity, therefore
can provide a more direct means of obtaining that parameter.
•
The table below assumes the foundation is unaffected by water. Where the water
is within B or less below the foundation then the quoted values should be halved.
This practice is considered conservative as some researchers believe that effect may
already be accounted for in the N - value.
•
The allowable capacity (FS
3) is based on settlements no greater than 25mm.
For acceptable settlements of 50mm say, the capacity can be doubled while for
settlements of 12mm the allowable capacity in the Table should be halved.
=
•
The footing is assumed to be at the surface. There is an increase bearing with
embedment depth. This can be up to 1/3 increase, for an embedment
=
Footing
width (B).
•
The corrected N - value should be used.
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