Engineering Geology

Chapter 9
to tensile and compressive stresses. Although a bridge can have a rigid design to resist such
ground movements, it usually is more economic to articulate it, thereby reducing the effects
of subsidence.In the case of multi-span bridges, the piers should be hinged at the top and
bottom to allow for tilting or change in length. Jacking sockets can be used to maintain the
level of the deck. As far as shallow, abandoned room and pillar workings are concerned,
it usually is necessary to fill voids beneath a bridge with grout.
Seismic forces in earthquake-prone regions can cause damage to bridges and must therefore
be considered in bridge design (Fig. 9.24). Most seismic damage to low bridges has been
caused by failures of substructures resulting from large ground deformation or liquefaction
(Kubo and Katayama, 1978). Indeed, it appears that the worst damage is sustained by
bridges located on soft ground, especially that capable of liquefaction. Failure or subsidence
of backfill in a bridge approach, leading to an abrupt change in profile, can prevent traffic from
using the approach even if the bridge is undamaged. Such failure frequently exerts large
enough forces on abutments to cause damage to substructures. On the other hand, seismic
damage to superstructures due purely to the effects of vibrations is rare. Nonetheless, as
a result of substructure failure, damage can occur within bearing supports and hinges, which
combined with excessive movement of substructures, can bring about the collapse of
a superstructure. By contrast, the effects of vibrations can be responsible for catastrophic
failures of high bridges that possess relatively little overall stiffness. Arch-type bridges are the
strongest, whereas simple or cantilever beam type bridges are the most vulnerable to seismic
effects. Furthermore, the greater the height of substructures and the greater the number of spans,
the more likely is a bridge to collapse.
Foundations for Buildings
Types of Foundation Structure
The design of foundations embodies three essential operations, namely, calculating the loads to
be transmitted by the foundation structure to the soils or rocks supporting it, determining the engineering
performance of these soils and rocks, and then designing a suitable foundation structure.
Footings distribute the load to the ground over an area sufficient to suit the pressures to the
properties of the soil or rock. Their size therefore is governed by the strength of the foundation
materials. If the footing supports a single column, it is known as a spread or pad footing,
whereas a footing, beneath a wall is referred to as a strip or continuous footing.
The amount and rate of settlement of a footing due to a given load per unit area of its base
is a function of the dimensions of the base, and of the compressibility and permeability of the
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