text: A siphon drainage method is being implemented in several expressways as of 2022. Recent studies found that it can reduce groundwater level behind expressway retaining walls, and there was no indication of clogging. This new drainage system is being pioneered as a long-term method to limit leakage hazard in the retaining wall. Siphon drainage is also used in draining unstable slopes, and siphon roof-water drainage systems have been in use since the 1960s.