Toi San Public Housing Development

A SENSITIVE GEOTECHNICAL CONTEXT

The construction of public housing in Toi San has been problematic as the works had to be stopped in 2012 when the adjacent buildings started to crack. Some cracks in the neighbouring buildings were suspected to be caused by the soft soil conditions and the basement of the public housing building. While the original plan involved three underground floors, the revised design eliminated the basement area of the building in order to ensure the construction works will not affect adjacent residences.

A CROSS HOLE SURVEY TO INVESTIGATE BENEATH EXISTING BUILDING

An extensive evaluation of the ground condition beneath the surrounding buildings was needed. The solution proposed by Sixense was the cross hole seismic velocity imaging technique. Boreholes were drilled at the corners and at the edges of the buildings, and seismic surveys were carried out between this pattern of boreholes. The seismic velocity is linked to the contrast of “stiffness” between different materials, and finally provides an image of the soil constitution.

The accumulation of low velocity materials, especially when close to the surface below the buildings, and the heterogeneity of soil constitution, was correlated to the differential settlements of the buildings at surface. The absence of a deep foundation system, anchored onto materials of better geotechnical characteristics was also a factor that explained defects observed at surface.