When the A2 collapse occurred, the general assumption was that the road had fallen either into a natural chalk solution feature or ancient chalk mine workings.
In fact the site investigations and associated desk study revealed an altogether different situation. The A2 turned out to lie precisely over the alignment of a Roman road. From the Middle Ages the chalk underlying the ground surface either side of the road was quarried and burnt in kilns to make lime. The chalk beneath the road was left intact, forming a thin, 20m high chalk 'spine', overlain by a few metres of dense Thanet Sand.
Archive maps dating from 1695 to 1870 show quarrying activity. The full 300m by 250m extent of the quarrying to the south of the road can be seen, which probably explains why there are no Victorian houses in the area.
Since then the quarries have been backfilled and the area developed with housing. The road, has been widened so its shoulders extend over the edge of the chalk spine. This means the edge of the road and surrounding areas sit on a largely granular quarry backfill - typically mixed sand and chalk fragments but with the occasional brick giving away its man-made origin.
Within the area of the quarrying, the road is arguably the safest area - even before the remedial grouting work and not withstanding the collapse.
Greenwich and Lewisham borough councils are participating and extending the ground investigation to explore the implications further.
There is a history of recent movement in the area, evidence of which includes two abandoned blocks of flats and a derelict pub. The issue now is that these instances of structural movement can no longer be assumed to be associated with isolated chalk solution features. They could instead be the legacy of quarrying, an activity which, despite persisting for hundreds of years, appears to have been forgotten less than a century after it ceased.