Pile Design and Construction Practice, Fifth edition

reached the point of ultimate resistance at a failure load of 4.2 MN. A safety factor of 2 on
this condition gives a working load of 2.1 MN, under which the settlement of the pile will
be nearly 5 mm. This is well within the settlement which can be tolerated by ordinary build-
ing structures. The full shaft resistance will have been mobilized at the working load, but
only 22% of the ultimate base resistance will have been brought into play. For economy in
pile design the settlement at the working load should approach the limit which is acceptable
to the structural designer, and this usually involves mobilizing the full shaft resistance.
By using partial safety factors on the ultimate shaft and base resistances, Burland
et al.(4.38)have presented a simple stability criterion for bored piles in clay which states that
if an overall load factor of 2 is stipulated, together with a minimum factor of safety in end
bearing of 3, then the maximum safe load on the pile is the lesser of the two expressions
(Qp) and (Qs Qb), where Qpis the ultimate resistance of the whole pile, Qsis the ultimate
resistance of the shaft, and Qbis the ultimate resistance of the base.
Burland et al.state that the first expression is nearly always dominant for straight-sided
piles and for long piles with comparatively small under-reams, whereas the second expres-
sion often controls piles with large under-reamed bases. Satisfaction of the above criteria
does not necessarily mean that the settlement at working load will be tolerable. Experience
based on loading tests on piles in similar soil conditions may give a guide to the order of

1

3

1

2

Resistance of piles to compressive loads 193

Load (MN)

0

25

50

75

100

Settlement (mm)

125

150

12345

Working load

2.1 MN

Total pile

load

Base

resistance

Shaft

resistance

Figure 4.27Load/settlement relationships for large-diameter bored piles in stiff clay.