Noteworthy is that most 20th-Century liquefaction mitigations are executed in an open land where there is no structure. Therefore, big construction machines can easily improve vulnerable subsoils at a reasonable cost. In the 21st Century, the increased desire for safety has raised the intensity of design earthquake actions and many structures that were once safe under less intense design actions started to be considered unsafe. Because those structures already exist on the ground surface and owners do not want to demolish them, such a difficult soil improvement as grouting or installation of drainage pipes under existing structures is now required. Note that densification under existing structures is not recommendable because it causes ground subsidence that seriously affects the existing super-structures on the ground surface.
The situation of people’s house is very complicated. Because of the financial restrictions, foundation of most houses are not prepared for subsoil liquefaction. Because of the shortage of structural strength, soil improvement under existing houses requires special care. Conditions of houses and foundations are different from house to house and therefore soil improvement under existing houses cost significantly in spite of the house size and limited financial resource. With these in mind, the present paper is going to review the current situations of unprotected structures in more details.