Effects of Horizontal Drainage Layer for Seepage Control on Mitigation of Liquefaction of Levee Body

The 2011 Tohoku Pacific Earthquake of March 11 caused extensive damage due to soil liquefaction, triggering major deformations in and eventual collapse of levees. At the same time, the effectiveness of measures to counter the liquefaction of levee foundations was confirmed in many places. In addition, measures for seepage control, such as cut-offs with steel sheet piles for foundation under-seepage and horizontal drainage layers for seepage through embankments, were effective in mitigating large liquefaction-induced deformation of levees in some places. This paper focuses on the effects of the horizontal drainage layer for seepage control on the mitigation of the liquefaction of levees and reports the results of numerical analysis on a levee that survived the earthquake. Analysis results reveal that effects of the drainage layer on the mitigation of levee deformation were unexpectedly low, while a marked difference was seen at the site. Accepting the limitations of numerical analysis, it can be concluded that (1) the major function of the horizontal drainage layer on liquefaction-induced levee damage was to lower the water table in the levee and (2) drainage did not help much in the dissipation of excess pore water pressure in the levee during an earthquake.