Pile Design and Construction Practice, Fifth edition

Miscellaneous piling problems 455
bridge designers. Hambly (9.22) states that foundations for simply supported deck bridges are
frequently designed for differential settlements of up to 1 in 800 relative rotation (25 mm in
a 20 m span). In reasonably homogeneous soils, differential settlements between adjacent
foundations are often assumed to be half of the total settlement, thus a total settlement of
50 mm would be permissible under this criterion. Differential settlements of the order of 1 in
800 in a continuous deck bridge are required to be treated as a load producing bending
moments in the superstructure. This can add to the cost of the bridge, but it should also be
noted that limitation of total settlement to 5 to 10 mm is difficult to achieve with spread
foundations on soils of moderate to low compressibility. Some designers expect the rotation
to be limited to 1 in 4000, which is equivalent to a differential settlement of only 5 mm in
a 20 m span bridge. This would be difficult to ensure for bridges with longer spans even
when supported by piles taken down to a competent bearing stratum. Larger rotations have
to be anticipated in special conditions such as bridges in mining subsidence areas.
The distribution of live load when assessing total and differential settlement is usually a
matter of judgement. Full live load on the whole or part of the spans should be allowed for
calculating immediate settlements but the contribution of live load to consolidation settlement
may be small in relation to that from the dead loading. Figure 9.14 shows the loading
on a typical pier foundation for the 4 km-long elevated section of the Jeddah–Mecca
Expressway designed by Dar al-Handasah, consulting engineers. The piers support the 36 m
continuous spans of the three-lane carriageway. It will be noted that the predominant horizontal
force on the piers was in a longitudinal direction, the resulting bending moments
increasing the loads on the outer piles of the eight-pile group by about 25% above the
combined vertical dead and live loads. It was possible to carry the horizontal forces and
bending moments by 770 mm diameter bored and cast-in-place base-grouted piles of the
type described in 3.3.9 using the ‘flat-jack’ process.
6.50 m
Moment in direction of span � 6750 kN-m
" transverse to " � 4500
Dead load � 14 500 kN
Live � 2200 kN
Total�
16 700 kN
7.50 m
H transverse to span � 100 kN
H in direction of span � 450 kN
770 mm bored and cast-in-place piles
with grouted base
Bending moment in pile (max)
� 220 kN-m
P v max 2510 kN
Figure 9.14 Vertical and horizontal loads on viaduct piers of Jeddah–Mecca Expressway.