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

Chapter 9 FOUNDATION ENGINEERING
Table 9.3 Minimum Global Factors of Safety for Piles in Soil and Rock
Notes:
Mobilization Factor for Shaft Mobilization Factor for Endbearing
Resistance, f b
Material
Resistance, f s
Granular Soils
1.5
3 – 5
Clays
1.2
3 – 5
1. Mobilization factors for end-bearing resistance depend very much on construction.
Recommended minimum factors assume good workmanship without presences of
debris giving rise to a ‘soft’ toe and are based on available local instrumented loading
tests on friction piles in granitic saprolites. Mobilization factors for end-bearing
resistance. The higher the ratio, the lower is the mobilization factor.
2. Noting that the movements required to mobilize the ultimate end-bearing resistance
are about 2% to 5% of the pile diameter for driven piles and about 10% to 20% of
the pile diameter for bored piles, lower mobilization factor may be used for driven
piles.
3. In stiff clays, it is common to limit the peak average shaft resistance to 100 kPa and
the mobilized base pressure at working load to a nominal value of 550 to 600 kPa for
settlement considerations, unless higher values can be justified by loading tests.
4. Where the designer judges that significant mobilization of end-bearing resistance
cannot be relied on at working load due to possible effects of construction, a design
approach which is sometimes advocated (e.g. Toh et al, 1989, Brooms & Chang, 1990)
is to ignore the end-bearing resistance altogether in determining the design working
load with a suitable mobilization factor on shaft resistance alone (e.g. 1.5). .Endbearing
resistance is treated as an added safety margin against ultimate failure and
considered in checking for the factor of safety against ultimate failure.
5. Lower mobilization factor for end-bearing resistance may be adopted for end-bearing
piles provided that it can be justified by settlement analyses that the design limiting
settlement can be satisfied.
9.2.3.4 Pile Capacity
a) Design of Geotechnical Capacity in soil
Pile capacity can be divided into 2 main components, namely;
• Shaft resistance; Qs
• End bearing resistance; Qb
The ultimate capacity of the pile is the sum of both the shaft resistance and the end bearing resistance;
Qult = Q s + Q b (9.6)
As for allowable pile capacity;
Where,
Q allow = Q s /F s + Q b /Fb (9.7)
F s = safety factor for shaft resistance. The common F s adopted in design is 2.0
F b = safety factor for end bearing. The common F b ranges from 2.0 to 3.0 subjected to
availability and sufficiency of soil parameters. Higher safety factor shall be used when
limited soil information is made available. As for bored pile, normally Q b is ignored.
9-8 March 2009