A dramatic ground collapse occurred immediately in front of a house leading to evacuation of the residents. The subsidence enlarged to threaten the stability of the house and the adjacent highway which was partially closed to traffic. Utility services also crossed the collapse but were undamaged. The situation was monitored and risk assessed by PBA.
PBA designed a suitable investigation to determine the cause of ground movement. The subsidence occurred in an area where sands overlay chalk and many local chalk mines (deneholes) were recorded in the PBA Mining Cavities database.
We worked with the council to close the road to provide access for rotary drilling angled boreholes below the house and front garden, finishing with vertical drilled holes in the highway. Interpretation of the drilling results showed that the collapse was caused by an unstable denehole mine. Because of the emergency nature of the works to identify the cause and to stabilise both the road and property at the same time, the same boreholes were utilised to carry out compaction grouting of the loose, disturbed ground extending to 25m depth below the surface.
The ground treatment was successfully completed preventing the loss of the property. The garden, road, pavement and utility services were reinstated and the road re-opened to traffic again.