Unusual rainfall-induced cut-off slope failures and their countermeasures

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During 2004.2009 in Mito, Ibaraki, Japan, several locations of cut-off slopes of the Northern Kanto Expressway collapsed. These collapses were caused mainly by scour or erosion at shallow depth and piping phenomena at deep locations of sandy deposits underneath cohesive materials constituting cut-off slopes and by topographical characteristics,.. which caused collection of surface water from rainfall. Whenever a collapse occurred at these locations, traffic was blocked. Therefore, the highway function was compromised. The piping causing slope collapses at the deep location was triggered by water seepage through an intrusion from the surface, which had originated from continuous rainfall and heavy intervening rainfall after a side ditch failed because of seepage of continuous rainfall water intruding from behind. The remediation of collapsed slopes and countermeasures against the additional collapse expected in the future were conducted depending on the severity of slope failures. Particularly in more severely damaged locations, two kinds of gabion-type reinforcement were adopted at both shallow and deep locations of slopes; plastic pipes of two kinds for accelerating drainage were buried at both shallow and deep locations. Along with these countermeasures, high-density polyethylene pipes were installed along longitudinal ditches and grass was planted on the slope surface. An index for engineering judgment was proposed to determine whether these countermeasures will be successful or not. The countermeasures were finalized in 2009: monitoring for slope movement started and has performed continuously up to the present.

Unusual rainfall-induced cut-off slope failures and their countermeasures

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