When the great Blackheath hole first appeared in April this year I confidently expected KURG to headline it in every subsequent newsletter. It is with regret that I see you haven't even mentioned it ..... and now Mike has suggested that I write it up myself! I am only too aware of my deficiencies in this respect since my only qualification is being local. However I will do my best- but believe me, every word that follows comes from someone else. I must also apologise that this is a bit long.
The hole first appeared on the evening of Sunday 7th April when the road began to heave and some brave woman ran out to stop the traffic. A few days later Greenwich Council's long serving Transportation Officer, Phil Thompson retired- and the speeches at his leaving party noted that his departure was marked by the collapse of the Dover Road 'after 2000 years'.
Initially there was a great deal of speculation about the cause. It was assumed to be old chalk workings, which had subsided, and the local papers were full of articles drawn largely from the works of Harry Pearman and the Chelsea speleologists. Some asked if it was the legendary Blackheath plaque pits, which had collapsed. Eventually the guilty parties were identified as the Steer family.
It was noted that William Steer had been fined a total of £95 in 1666 for mining activities in the area. By 1677 his family had lime kilns in the area of Greenwich South Street where he was "filling up, supporting and making good, safe and secure the King's Highway there ... which he hath undermined by digging, taking and carrying from thence great quantities of chalk. whereby the said common highway is become very unsafe, and very dangerous for all the King's Liege and over the said highway". Mining ceased around 112.5 and the entrance in Maidenstone Hill was blocked up., In due course this cavern became a nightclub, was closed, opened again and is now the stuff of legends as the Blackheath Cavern.
Back in the real world the hole grew and grew. Thirty seven households from surrounding blocks of flats evacuated but the 'for the first few days we didn't date get an heavy equipment on site. All we could do was survey up and down the road to detect movement' said the Transport for London engineer in charge of the scheme 'there was a total ground loss below a section of the road and adjacent pavement. It was clear it was gradually enlarging as the collapsed faces assumed their angle of repose'. Eventually 100 tons of pea gravel was put into the void to stabilise it but within a week Transport for London felt able to investigate further.
The investigation covered a 600m section of the A2 on the hill between Blackheath and Deptford. Two main techniques were used to investigate the problem- micro gravity and borehole sampling. The micro gravity survey stretched along the whole area of concern and used the services of Keele University's Microsearch. A total of 105 boreholes were sunk and three more to monitor the ground water. Samples were taken down to 13m and the rotary cores reached down as far as 40m. There was some concern over the weight of the drilling equipment and it was necessary to find a company, which could undertake a lightweight percussive window sampling- and this was done by Bristol based Structural soils. The engineers were still concerned about the overall stability of the area and Cementation Skanska used a 3dT borehole seismic tomography technique to produce a 3D image of the ground. Four boreholes were drilled around the area of the collapse and by sending seismic signals between them a 3d velocity model was drawn up. This could be viewed from many angles on a PC. This work took many weeks.
I am assuming that KURG readers are not interested in the social repercussions of all this work. Suffice it to say that many people remained in temporary accommodation and that many local businesses suffered. At the same time as nightmare of traffic congestion has built up in Greenwich and Blackheath villages.
So- what did they find? Basically they found that the Roman Road descending down the hill on a stable spine of chalk is substantially intact but the areas on either side of it have been disturbed- and the road widened onto these areas. The disused railway tunnel under the hill is well known and stable- but another gallery has been identified further up the hill.
The cause of the collapse has been identified as water running down the hill. At the bottom of the hill is the Kent Well from which vast quantities of water have been extracted for over two hundred years. There were stories that a lot of water was running down the hill before the hole appeared which would imply that a water main had burst- but, since the main was ruptured anyway, that is difficult to prove. No doubt the lawyers are working on it since someone's insurance company has a pile of claims waiting.
As I write this in late September the road is still far from being opened. It is now being stabilised with 'grouting' on a 3m grid around the area penetrating the chalk to 2 m. This work started in July and hopefully will be finished by November. The traffic remains a nightmare and some people are still not back in their homes. A number of crowded public meetings have been held at which the engineers have done their best to explain their findings- and an exhibition was held over a week and very well attended.
I realise that this article is probably a bit low tech for most KURG readers- I am happy to get more detail if people want. It seems unlikely that historical investigations or archaeology were done on the site and researchers will only have the findings of the micro gravity survey to go on. It was thought the Blackheath Cavern might be opened but this now seems increasingly unlikely. I
t should perhaps also be noted that a letter in the Blackheath experience; Guide quoted a civil engineer of 'many years as saying that the hole could have been further excavated and backfilled over a weekend- and that this had not been done in order to 'deliberately cause congestion' Perhaps we should have listened to him! Or again, perhaps not
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