Physical and Numerical Modelling of Drainage in Liquefied Soils

Over the last 20 years, dynamic centrifuge modelling has contributed greatly to investigating the nature of liquefaction phenomena but of ultimate importance is the development of measures to protect people and structures against the negative effects of such behaviour. One such strategy that has been adopted is the installation of vertical drains to relieve excess pore pressures generated by earthquakes. However, question marks still exist surrounding their performance in practice.This paper presents results from centrifuge tests on small groups of drains in liquefiable deposits, and compares these to results obtained from a finite-difference solution to the diffusion equation in 3 dimensions. Both were seen to develop similar excess pore pressure regimes, in which the excess pore pressures within the group are consistently lower than outside, but fluid from places that were far from the group was also attracted, and by occupying the drain so delayed drainage of soil nearer the surface. Dynamic centrifuge modelling has the advantage that there is no selection of material parameters involved. There is also no reliance on empirical relations. It is shown that using the dynamic centrifuge test data the excess pore pressure dissipation regimes in liquefied soils are such that numerical procedures cannot model this accurately simply with the consolidation equation