Damage to residential retaining walls at the Genkai-jima Island induced by the 2005 Fukuoka-ken Seiho-oki earthquake

The 2005 Fukuoka-ken Seiho-oki earthquake caused heavy damage to residential areas at the Genkai-jima Island. A field survey was carried out to investigate the damage of retaining walls in the residential areas. From the survey of 218 retaining walls, it appeared that 83% of the retaining walls were damaged and 61% of the whole lost their functions (collapsed or distorted/cracked). Especially in masonry retaining walls using natural rocks and boulders, the damage was serious. The damage features of dry masonry retaining walls (collapsed: 62%) and wet masonry retaining walls (collapsed: 25%) were different, and the difference in the earthquake-resistant capacity between the two retaining walls became obvious. Moreover, it appeared that most of the residential houses are built on transitions of cut slopes and fill embankments. It seems that this condition is one of a great factor which causes the heavy damage. Only 17% of the retaining walls escaped from any damage. Furthermore, to evaluate the collapse mechanism of masonry retaining walls during earthquake, a theoretical model based on the earth pressure theory was proposed. Through the theoretical considerations, it became possible to quantitatively evaluate the stability of the masonry retaining walls.