SITE INVESTIGATIONS have revealed that the huge hole in the A2 road in south east London was not caused by collapse of old chalk mine workings as first thought.
Microgravity surveys and 105 boreholes identified a spine of solid chalk running down the centre of the carriageway, following the line of a Roman road, with loose fill either side.
While some evidence of quarrying was uncovered, including a backfilled quarry tunnel, Road operator Transport for London (TfL) believes the subsidence was caused by the loose material slipping down the sides of the spine.
Erosion and loss of fines due to water inundation were the likely triggers rather than self-weight compaction and vibrations from traffic or construction, it said.
However, TfL did not discover whether erosion was caused by gradual rainwater infiltration, or a water main burst that coincided with the collapse.
TfL spent three months investigating the ground beneath the Blackheath Hill section of the A2 after a 6m diameter hole suddenly opened in April (GE June 02).
The road, a major artery in and out of London, was immediately shut in both directions and Greenwich council evacuated 37 households as a precaution. The road remains closed, and some residents have still not been allowed back into their homes.
Keller Ground Engineering began grouting a 180m length of the A2 in mid-July. Repairs are expected to take at least four months and cost £2.25M, TfL said.