Thermal impact on soil-structure interaction for integral bridges.

Integral bridges are generally considered an attractive alternative to conventional bridges presenting the economic advantage of lower construction and maintenance costs. However, the concept of the integral bridge presents other challenges primarily arising from the monolithic connection that exists between the superstructure and the substructure. Thermal loading leads to daily cycles of expansion and contraction superimposed on seasonal cycles. This results in significantly higher soil-structure interaction activity that may lead to excessive earth pressures behind the abutment and potential failure of the soil and structure. A parametric study was carried out to evaluate the impact of change in the backfill soil
parameters and change in the season of construction on the earth pressures developed behind the abutment. The frequency of the daily and seasonal cycles of expansion and
contraction is such that granular soils respond as fully drained materials. This is seldom the case for fine grained soils. Excess pore pressures are developed and some drainage may occur. However, data and resource limitations make it not feasible to accurately model this over the long term. Further the need to make assumptions about the temperature cycles and the permeability characteristics weakens the strength of the analysis. Therefore, an
envelope of earth pressure generation was created in these parametric studies by modelling fine grained soils as fully drained and fully undrained. Plaxis 2D was used to model the
bridge and surrounding soil. In developing a realistic model of an integral bridge, the first stage was to simulate a constructed instrumented integral bridge which presented measured values of temperature, deformation and earth pressures in time. This allowed the model to be validated and the sensitivity of the analysis to the parameters assessed. A second simulation was undertaken
to compare the output of an integral bridge analysis using Plaxis 2D finite element software with a published study output carried out using the finite difference method.
There were a number of challenges to overcome in modelling an integral bridge. These are described in some detail, highlighting the impact the assumptions made within this studies, had upon the output. It was found that the backfill stiffness parameter was the dominant factor that controlled the magnitude of earth pressure. The parametric study revealed that the season of construction affected the earth pressures generated behind the abutment with autumn and summer construction often leading to cumulatively lower earth pressures than spring and winter respectively. In integral bridge construction, it is common to use granular soils in backfill construction. However, the use of granular soils in foundation construction may not be sustainable as a result of material availability and construction cost. Fine grained soils are alternatively used
where granular soils are not. It was found that modelling fine grained foundation soils as fully drained and fully undrained produced significant variations in the behaviour of the backfill soil and the resulting earth pressure pattern. It is therefore necessary to take into account the impact of thermal loading on the envelope of earth pressure to ensure that the capacity of the structure and soils are not exceeded or underutilised.