Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 9 FOUNDATION ENGINEERING
The conventional approach of applying a global safety factor provides for variations in loads and
material strengths from their estimated values, inaccuracies in behavioural predictions, unforeseen
changes to the structure from that analysed, unrecognised loads and ground conditions, errors in
design and construction, and acceptable deformations in service.
b) Limit State Design Approach
A limit state is usually defined as 'any limiting condition beyond which the structure ceases to fulfil
its intended function'. Limit state design considers the performance of a structure, or structural
elements, at each limit state. Typical limit states are strength, serviceability, stability, fatigue,
durability and fire. Different factors are applied to loads and material strengths to account for their
different uncertainty.
c) Recommended Factors Of Safety
The following considerations should be taken into account in the selection of the appropriate
factors of safety:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
There should be an adequate safety factor against failure of structural members in
accordance with appropriate structural codes.
There must be an adequate global safety factor on ultimate bearing capacity of the ground.
Terzaghi et al (1991) proposed the minimum acceptable factor of safety to be between 2
and 3 for compression loading. The factor of safety should be selected with regard to
importance of structure, consequence of failure, the nature and variability of the ground,
reliability of the calculation method and design parameters, extent of previous experience and
number of loading tests on preliminary piles. The factors as summarised in Table 9.3 for
piles in soils should be applied to the sum of the shaft and end-bearing resistance (HONG
KONG GEO 2001).
The assessment of working load should additionally be checked for minimum 'mobilisation'
factors f s and f b on the shaft resistance and end-bearing resistance respectively as given in
Table 9.5.
Settlement considerations, particularly for sensitive structures, may govern the allowable
loads on piles and the global safety factor and/or 'mobilisation' factors may need to be
higher than those given in (ii) & (iii) above.
Where significant cyclic, vibratory or impact loads are envisaged or the properties of the
ground are expected to deteriorate significantly with time, the minimum global factor of
safety to be adopted may need to be higher than those in (ii), (iii) and (iv) above.
Where piles are designed to provide resistance to uplift force, a factor of safety should be
applied to the estimated ultimate pile uplift resistance and should not be less than the values
given in Table 9.4.
March 2009 9-7