Edinburgh Napier University

  Modern approaches for the design of drains for liquefaction mitigation are based on the work of Seed and Booker (1977) for infinitely permeable drains and uniform soil with purely horizontal drainage. More recently, Bouckovalas et al. (2009) revisited this seminal work, and showed that it is systematically conservative by overlooking the “shake-down” effects in plastic strain or excess pore pressure accumulation exhibited during cyclic loading of cohesionless soils. After reviewing the basic assumptions and results of the revised methodology of Bouckovalas et al. (2009), this paper presents results from 3-D numerical analyses of the excess pore pressure response of a thin liquefiable sand layer improved with gravel drains. The numerical predictions verify the revisions introduced to the original Seed & Booker (1977) formulation, but also reveal a second source of considerable conservatism hidden into the conventional methods of drain design: the coefficient of volume compressibility,, as proposed in the original methodology and adopted in relevant seismic codes world wide, is grossly overestimated leading to a rather delayed reaction of the drains to the earthquake-induced excess pore pressure build up.