Banks, J.R, Bloodworth, A.G., Knight, T. and Young, J.
Integral bridges – development of a constitutive soil model for soil structure interaction.
In, 2008 Structures Congress - Crossing Borders, Vancouver, Canada,
24 - 26 Apr 2008.
Traditionally, engineers have used bearings and expansion joints to accommodate bridge
expansion and contraction caused by daily and seasonal temperature fluctuations. Studies carried
out in the late 1980s showed durability problems can be associated with bearings and expansion
joints [Wallbank, 1989]. Since the mid twentieth century Integral Bridges with no expansion
joints or bearings have been used. Deck expansion and contraction is accommodated by
movement of the abutments into the retained fill. This eliminates the problem of durability but
the movement of the abutments has been thought to cause a build up of horizontal pressures,
particularly in the case of full height abutments. In the United Kingdom BA42/96 [Highways
Agency, 2000] was issued and gave guidance on the soil pressures that should be adopted in
design. The validity of the work on which the code of practice was based is a subject of
continued debate by both researchers and practicing engineers. For this reason Integral Bridges
have been used much less widely than conventional bridges.
As part of a strategy by the University of Southampton to further investigate the occuring soil
pressures, Xu  carried out radial controlled triaxial tests of granular material under cyclic
loading. The applied strain and stress path used represented that typically experienced by an
element of retained material behind an integral bridge abutment. This was the first time that the
fundamental behaviour had been investigated in this way.
The further research discussed in this paper builds upon this by use of numerical modelling.
The fundamental behaviour of granular material under this particular loading could not be
represented by any available constitutive model and therefore a new model was be developed
based on this behaviour. The basis of the model and initial validation process are discussed. The
first stage of the validation process was implementation in a commercially available spreadsheet
package. This was then used to develop a model in the Finite Difference Method package FLAC.
Once this was implemented, the triaxial tests were modelled and the results compared to
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