earthmoving operations. It is possible to install the piling for piers of bridges with
spill-through abutments at the toe of the cutting, and in the median strip from plant
operating from ground level before bulk excavation is commenced for the bridge. This
operation is similar to that adopted for ‘top-down’basement construction in building work
(see Section 5.9). It may also be possible to excavate the cutting to a temporary steep slope
to enable piles to be driven at the toe of a cutting using trestle guides (Figure 3.6), the piles
being pitched by a crane standing at the crest of the cutting. However, such operations
involve a risk of instability of the slope due to surcharge load, and, in the case of clay
slopes, to excess pore pressures caused by soil displacement.
Bridge construction, or reconstruction, in urban areas involves piling in severely restricted
sites with the likely imposition of noise abatement regulations. Driven types of pile have the
advantages of speed and simplicity. Compliance with noise regulations may be possible by
adopting a bottom-driven type (see Sections 2.3.2 and 3.2) in conjunction with sound absorbant
screens surrounding the piling equipment. If possible pile caps should be located above ground-
water level in order to avoid pumping from excavations which could cause loss of ground or
settlement of adjacent buildings due to general drawdown of the groundwater table.
Piling over or beneath railways involves special difficulties. The presence of overhead
electrification cables will probably rule out any form of bored or driven pile requiring
the use of equipment with a tall mast or leaders. The railway authority will insist on piling
operations being limited to restricted periods of track possession by the contractor if there
is any risk of equipment or materials falling on to the track. Soil disturbance by large-
displacement-driven piles may cause heave or misalignment of the rails. If it is at all possible
the design of the bridge should avoid the need for piling the foundations.
As noted above, many of the constraints described in the preceding paragraphs do not
apply to bridges in undeveloped territories. However, conditions of access to remote bridge
sites should be investigated. Equipment should be capable of being transported over poor
roads and across weak bridges of limited width.
9.5.2 Imposed loads on bridge piling
The various types of loading imposed on bridge foundations have been reviewed by
Hambly(9.22)in a wide-ranging report published by the Building Research Establishment:
Dead and live loads on superstructure
Dead load of superstructure
Earth pressure (including surcharge pressure) on abutments
Creep and shrinkage of superstructure
Temperature variations in superstructure
Traffic impact and braking forces on bridge deck (longitudinal and transverse)
Wind and earthquake forces on superstructure
Impact from vehicle collisions, locomotives and rail wagons
Construction loads including falsework.
In UK practice the loading requirements are specified in British Standard 5400, and the rel-
evant parts of Actions on Structures as given in Eurocode 1 (BSEN 1991: 2003). The Highways
Agency’s Design Manual for Roads and Bridges gives guidance on the use of BS5400.
Dead and live load combinations should be considered in relation to permissible differential
settlements between piers or between piers and abutments in longitudinal and transverse
directions. Permissible settlements are often poorly defined or not defined at all by
454 Miscellaneous piling problems