In situ densification as a liquefaction resistance measure for bridge foundations

Paulo Alexandre Lopes de Figueiredo Coelho, Cambridge University
Geotechnical Engineering Group


Liquefaction is a major threat to small-span deck-bridges built on saturated deposits of sand in seismically active regions using shallow foundations. Failure of these structures causes great economic losses, serious impediment to post-earthquake emergency operations and long-lasting disruption of social and economic life. Densification is the most popular liquefaction resistance measure but its performance is poorly understood. Therefore, evaluation of if and of how densification should be carried out in a particular field situation is currently based on semi-empirical principles derived from post-failure analysis of liquefaction effects.

Considering the limitations of current understanding on the problems described, a wide-ranging research task was developed based on three different categories of research tools. Centrifuge modelling was used to identify the major features and effects of liquefaction in the field. Particular attention was given of the mechanisms governing the bridge performance, in some cases including densification under the footing, during and after seismic loading. Element tests were carried out to evaluate the performance of sand under controlled monotonic and cyclic loading in well-defined drainage and boundary conditions. Their results assisted in clarifying some doubtful aspects of liquefaction phenomena in the field, defining the critical capabilities of the numerical tools and identifying model parameters for numerical simulations of centrifuge experiments. The dynamic FE code Swandyne was used together with the Pastor-Zienkiewicz-Mark III soil model to assess the ability of advanced numerical tools to simulate the behaviour of shallow foundations built on liquefiable ground and to examine the use of densification as a liquefaction resistance measure.

The experimental and numerical results obtained highlight the multifaceted character of the liquefaction phenomenon in the field, especially when its effects on shallow foundations are mitigated through densified zones. The importance of considering soil-structure-interaction effects and understanding soil behaviour under different laboratory and field conditions is emphasized. The scientific approach followed in this research enables a logical understanding of the phenomenon and of the use of densification in mitigating its effects on shallow foundations. Apparently, liquefaction mitigation can be more efficiently achieved if hybrid resistance measures are used. Further development of the numerical tools is required before performance-based design can be used in sound assessment of liquefaction problems in the field.