↑ Click for a larger version (427k 1000 x 674)
Wednesday, 16 March 2011
Royal Hospital - 1700 groundwater level investigation
An unusual early geotechnical investigation of about 1700 into groundwater levels at the river wall of the Royal Hospital (subsequently the Royal Naval College and now Greenwich University) shows test pits that prove water level.
Royal Hospital - King Charles Quarter - sewer mouth & cellarage
Two beautiful 1700-ish drawings of the King Charles Quarter of the Royal Hospital (subsequently the Royal Naval College and now Greenwich University) have turned up. One shows a sewer mouth in the river wall (now obscured), the other a salami-style cut through the block including cellarage - of which there were thousands of square feet under all four blocks.
↑ Click for a larger version (630k 1000 x 881)
↑ Click for a larger version (627k 1000 x 825)
Thursday, 10 March 2011
Maidenstone Hill - intriguing account of subsidence and passageway
Robert Williams (80) wrote to the Kentish Mercury in 1933 after its coverage of the Trinity Grove subsidence. He recalled a major collapse that happened in the garden of a house at the Point Hill end of Maidenstone Hill when he was a child (the 1860s), well away from the known Cavern. His account is intriguing, though should be viewed with at least a little scepticism. The house number has been censored because we are still investigating. Click for a large version (529 x 1500 545k):
Wednesday, 9 March 2011
Blackheath Cavern - the Trinity Grove and Undercliff problems
Our new overlay of the wartime plan of the Cavern's location onto a very modern (June 2010) OS map is the most accurate ever done.
The Kentish Mercury of January 27 1933 reported a subsidence in Trinity Grove, which is located between the Blackheath Cavern and an old quarry now partially occupied by a tennis court. Five families had to be evacuated. It's highly likely that there were walk-in mine entrances in this quarry which headed towards the Cavern, though any possible underground connection is pure speculation. But it suggests a risk to property in a broad zone from the tennis court right through to the cavern; the corridor of risk should perhaps be expanded somewhat north to include the fork of Trinity Street and Maidenstone Hill.
Undercliff, the block of flats at the junction of Blackheath Hill and Maidenstone Hill, is partially built on shallow foundations, but the northern half - closer to the Cavern - was piled because a hole was discovered during foundation works. The piled section of Undercliff is some 30m away from the wartime shaft dug by Legrand Sutcliff and Gell, yet not above any known section of the Cavern; the implication is that the Cavern originally extended further towards the roadway of Blackheath Hill than the galleries surveyed by LSG. This is consistent with early descriptions of rooms in the Cavern (with at least two collapse domes in the ceilings) that were not found by LSG, probably because they had already fallen in. In 1914 John Stone mentions: "There have of late years been several landslips in the neighbourhood of Blackheath Hill... a small one occurred a few months ago just behind Trinity Church." The church was located where Undercliff now stands, at the corner of the main road. It seems logical to conclude that the Undercliff hole and the 1914 subsidence were closely related due to their extreme proximity.
Here is the overlay, showing additionally the piled section of Undercliff and Trinity Grove (click for a larger version 1336 x 1308 760k):
At the same time there were more problems occurring just over the main road behind the Horse and Groom pub:
The Kentish Mercury of January 27 1933 reported a subsidence in Trinity Grove, which is located between the Blackheath Cavern and an old quarry now partially occupied by a tennis court. Five families had to be evacuated. It's highly likely that there were walk-in mine entrances in this quarry which headed towards the Cavern, though any possible underground connection is pure speculation. But it suggests a risk to property in a broad zone from the tennis court right through to the cavern; the corridor of risk should perhaps be expanded somewhat north to include the fork of Trinity Street and Maidenstone Hill.
Undercliff, the block of flats at the junction of Blackheath Hill and Maidenstone Hill, is partially built on shallow foundations, but the northern half - closer to the Cavern - was piled because a hole was discovered during foundation works. The piled section of Undercliff is some 30m away from the wartime shaft dug by Legrand Sutcliff and Gell, yet not above any known section of the Cavern; the implication is that the Cavern originally extended further towards the roadway of Blackheath Hill than the galleries surveyed by LSG. This is consistent with early descriptions of rooms in the Cavern (with at least two collapse domes in the ceilings) that were not found by LSG, probably because they had already fallen in. In 1914 John Stone mentions: "There have of late years been several landslips in the neighbourhood of Blackheath Hill... a small one occurred a few months ago just behind Trinity Church." The church was located where Undercliff now stands, at the corner of the main road. It seems logical to conclude that the Undercliff hole and the 1914 subsidence were closely related due to their extreme proximity.
Here is the overlay, showing additionally the piled section of Undercliff and Trinity Grove (click for a larger version 1336 x 1308 760k):
In Trinity Grove house numbers 2-5 (numbers are consecutive in this street) were affected, with considerable subsidence of foundations and buckling of walls, doors not shutting, and windows shattering under pressure. The subsidence occurred over a number of hours during 20-21 January 1933. Water and gas mains were severed. No geotechnical investigation of any kind was carried out, and the houses were repaired - although the leaning out-of-vertical front wall of no 3 is very obvious when compared with the newly-rebuilt no 2 (blue door), whose brickwork is true to the plumb line.
Click for larger version (1000 x 750 692k)
At the same time there were more problems occurring just over the main road behind the Horse and Groom pub:
"It is a curious thing, however, that within two or three hundred yards of Trinity Grove is Harwich-place, where, a few days earlier, similar cracks were found in a number of houses.
Harwich-place is on the other side of Blackheath Hill, behind the Horse and Groom public house, and one of the householders here told the reporter that on Monday week cracks were noticed in the fabric of one of the houses. These seem to have spread quickly to the adjoining properties, and at first it was thought it would be necessary to evict the tenants on the ground that the condition of their homes was dangerous.
It is gathered that the necessity for this has been averted. Eight of the houses have been shored up, and it is understood that the tenants will now only be required to move to the backs of the houses while the repairs to the fronts are carried out."
Blackheath Hill 2002 - the boreholes
A number of boreholes were carried out during 2002 on Blackheath Hill after the major subsidence that caused its closure. This plan of 10 May 2002 shows the 9 boreholes (click for a larger version 1059 x 1500 1.1 MB):
Results were generally inconclusive, with evidence of much disturbed ground. Borehole 3 was very interesting, however - located in the westbound roadway slightly downhill of the junction with Maidenstone Hill. A curious metal item was hit at 16.5m (54 feet) which could not be passed and indeed broke various drill bits. The driller's interpretation of this is that it was the roof of passageway. We believe this to be incorrect, and consider it to be metallic junk left on the floor of a mine passageway that subsequently became choked or collapsed. The depth to the floor of the supposed passageway would put the gallery on the same level as that of the Blackheath Cavern, although we are not suggesting there was ever a through trip possible underground between the two.
The driller describes it (quoted verbatim):
Results were generally inconclusive, with evidence of much disturbed ground. Borehole 3 was very interesting, however - located in the westbound roadway slightly downhill of the junction with Maidenstone Hill. A curious metal item was hit at 16.5m (54 feet) which could not be passed and indeed broke various drill bits. The driller's interpretation of this is that it was the roof of passageway. We believe this to be incorrect, and consider it to be metallic junk left on the floor of a mine passageway that subsequently became choked or collapsed. The depth to the floor of the supposed passageway would put the gallery on the same level as that of the Blackheath Cavern, although we are not suggesting there was ever a through trip possible underground between the two.
The driller describes it (quoted verbatim):
At 16.50m an obstruction was encountered. Various drill bits tryed + damaged. beleived to be metal. Possible roof of mine /Tunnel!!!Here is the log sheet (click for a larger version 1066 x 1500 1.2MB):
Other interesting subsidences in the vicinity have come to light:
In 1974 Glennie House (on the south side of Blackheath Hill directly opposite the main 2002 roadway subsidence) had a sudden collapse of surrounding grounds.
In 1998 the Horse and Groom pub, downhill from Glennie House, suffered severe subsidence after a "sink hole" developed. It was subsequently closed after becoming structurally unsound.
Downhill from the Horse and Groom was Braid House, where a "sink hole" appeared during 1999/2000 - the hole was partly filled.
Downhill from Braid House, on the southern side of Blackheath Hill, is Robertson House. During 1999/2000 a "sink hole" appeared which caused 100mm of settlemnet of the flats - they have now been demolished.
"Sink holes" may well have been air pockets from collapsed mine galleries working their way up to the surface. What is certain is that Blackheath Hill is an area heavily undermined by old chalk mine galleries which are collapsing in a chaotic fashion; more collapses will surely happen in the future. A more systematic approach from local and central government is required to address the problem, which should include driving extensive underground headings to intercept unsurveyed/orphaned mine galleries.
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