Settlement sinkhole

Site Geotechnical were asked to determine reasoning for sinkholes, building / pool distress and subsidence related to an existing property in the south east suburbs of Melbourne. Our forensic geotechnical investigation discovered a broken concrete sormwater pipe, deep uncontrolled & unsuitable fill (associated with backfilling of a former watercourse) and significant voids within the fill.

The problem and associated defects has been ongoing for years with the problem not fully understood or realized until after our investigation. The ultimate solution would be to remove and replace the existing uncontrolled fill with suitable material, underpin structures so they are supported by the underlying naturally occurring soils or undertake continual maintenance/repairs.

In order to avoid history repeating its self for new projects serious consideration needs to be given to a detailed investigation and subsequent backfill specification when undertaking fill earthworks. This should also be followed up with full time inspection and testing by a trusted Geotechnical Testing Authority (GTA) together with a statement of compliance. High levels of testing such as settlement monitoring and Insitu testing can further support GTA inspection and testing.

Poor quality earthworks like the one illustrated can take years to realize (well after earthworks have been completed) and are very difficult to repair with 100% satisfaction.

The picture illustrates a void within the poor quality fill and fully disintegrated concrete pipe with remnants of steel reinforcing.

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