Geology Reference
In-Depth Information
standard approach to design in many parts of the world, the
following de
nitions are used:
Bearing pressure
The net loading pressure: load from structure, divided by the
area of the foundation, minus the weight of material removed
from the excavation.
Ultimate bearing
capacity
The loading pressure at which the ground fails. This is the same as
the ultimate limit state in the limit state approach (Eurocode 7).
Allowable bearing
pressure
The maximum loading pressure that meets two criteria:
1. An adequate Factor of Safety against failure.
2.
Settlement within tolerance of the structure (speci
c to the
particular structure).
Presumed bearing
pressure
A net loading pressure considered appropriate for a given ground
condition, based usually on local experience and incorporated in
building regulations or codes of practice such as BS 8004 (UK)
(BSI, 1986) and CP4 (Singapore Standard, 2003).
Typicalvaluesarepresentedin
Table6.1a
ndcanbeusedfor
preliminary design purposes. They allow the practicability of
foundation options to be assessed and to select appropriate ground
investigation, testing and design methods. Presumed values are
only appropriate if the site is approximately level (not, for example,
at the top of a steep slope) and where the geology is relatively
uniform and isotropic with no lenses or layers of signi
cantly
weaker or compressible material within the zone of ground that
will be stressed. Such tables are generally very conservative and
economies can be made by conducting more detailed characteri-
sation with testing and analysis, although sometimes regulating
bodies (building authorities) may be loathe to allow higher values
to be used without considerable justi
cation.
In Europe, since 2010, Eurocodes have replaced national standards and
should be used for design (BSI, 2004). The ultimate limit state (ULS) is
essentially the same as ultimate bearing capacity but with possible fail-
ure modes spelt out, including sliding resistance and structural capacity,
heave, piping, and so on, which were implicit in the BS 8004 approach
as factors that a responsible geotechnical engineer should consider. The
serviceability limit state (SLS) of Eurocode 7 is de
ned as:
'
states that
correspond to conditions beyond which speci
ed service requirements
for a structure or structural member are no longer met
, and this equates
effectively to the idea of allowable bearing pressure, as far as settlement
is concerned, but includes other considerations such as vibration annoy-
ance to neighbours, and so on
'
again, factors that would usually be
considered automatically by experienced and responsible geotechnical
engineers when adopting a traditional approach to design.
From
Table 6.1
it can be seen that, for rock, the two governing
parameters are generally taken to be uniaxial compressive strength
(UCS =
-
c
) and degree of fracturing. This is expressed in charts presented
