Abstract: In order to investigate protection against the frost heave of seasonal frozen soil behind precast L-type retaining walls in cold regions, test walls are installed in the campus of Kitami Institute of Technology (KIT, Hokkaido, Japan) in two sections. One is incorporated countermeasures against frost heave and the other has no such countermeasures. The freezing front distribution and ground temperature within the backfill are observed, and deflections of the walls are measured over three freeze-thaw seasons. Some understanding of the mechanisms of the build-up of frost heave pressure is gained, and the effectiveness of replacement method is observed. A simulation is performed to predict the shape of the freezing front in the backfill behind L-type walls with various cross sections. These findings are employed to propose a method for determining the appropriate zone for replacement with frost unsusceptible backfill materials in cold regions.