Page:VCH Kent 1.djvu/546

 A HISTORY OF KENT ft. in. Oldhaven or Blackheath Beds. Ferruginous quartzose sand [? with pebbles in the lower part]. . . . . . . -44 — „, ... I'Yellow mottled clay with white fragments of shells. . . 6 — Woolwch U.eensand lo - ^^'^^ i Black pebble-bed 10 — Thanet Sand, nearly white. . .... . . . 52 — Chalk While the Thanet Sand stands so well that the foot-holes in the sides of the shafts at Hang- man's Wood are still visible, the variable nature of the strata overlying this sand at Eltham would alone necessitate the steining or lining of the shaft there to ensure stability. In addi- tion, a difficulty arises from the fact that the 6 ft. of yellow mottled clay at the top of the Woolwich Beds would hold up the water percolating through the sand and gravel of the Old- haven Beds at the surface, and cause its concentration towards the base of those beds. This would make necessary a specially well-made and watertight lining for the shaft from the sur- face to a few feet below the top of the Woolwich Beds, and is, no doubt, the reason why, at Eltham, the shaft was lined with bricks only down to a depth of about 50 ft., the alternating courses of brick and chalk beginning below that depth. The Eltham denehole was discovered towards the end of January, 1878. On 12 April, 1878, after a night of extremely heavy rain, it was found that at a spot on Blackheath, south of the Shooters Hill Road and west of that between the south-eastern corner of Greenwich Park and Blackheath Village, the earth had sunk in to a depth of about 20 ft., leaving a nearly circular hole 7 ft. to 8 ft. in diameter. Its sides were vertical except near the bottom, where they gradually expanded, so that, at the lowest point visible, the diameter of the hole was about 14 ft. After some little delay this hole was filled up by the Metropolitan Board of Works. But early in November, 1880, a second subsidence took place on Blackheath more than 600 yds. south-west of the first ; and later in the same month a third hole about no yds. south-east of that which appeared in 1878. The hole of 1878 and that south-east of it were almost identical in size and shape. But that to the south-west, near Eliot Place, was much less deep than the other two, and expanded much more at the bottom. Its shape suggested that the subterranean hole, filled up as the result of the subsidence at the surface, might be much less deep than in the two other cases. The positions of all three are now marked by metal plates fixed on the ground. The Lewisham and Blackheath Scientific Association formed a committee, early in the year 1881, for the purpose of making an exploration of the more recent and more easterly of the two deeper holes, Prof. J. K. Lrughton, then at the Royal Naval College, Greenwich, being chairman of the Committee, which began work on 4 April, 1 88 1. The beds between the surface of Blackheath and the chalk are, apart from slight variations in their respective thicknesses, identical with those at Eltham Park ; the depth of the chalk at Blackheath being probably 15 or 20 ft. less. The Oldhaven (or Blackheath) pebble beds in which the workmen began to dig were throughout broken and disturbed, and yielded readily to the spade. At a depth of 34 ft. from the surface water began to appear, and as the depth increased became more and more troublesome. When speaking of the steining of the Eltham shaft, I men- tioned the concentration of water towards the base of the Oldhaven Beds owing to the pres- ence beneath them of the clayey beds of the Woolwich series. But in the case of a small shaft in unbroken ground the water difficulty would be trifling compared with that encoun- tered in a very large shaft in loose and shattered ground, like that of the Blackheath pit. For there the earth behind the timber framework had no cohesion, and it speedily became ap- parent that the timber ' might prove to be of insufficient strength. At a depth of 43 ft. the earth on the south side of the hole became markedly hard, that on the north side remaining soft and broken. This caused the framework to cant, and its shape to alter till, from being a rectangle, it became a rhomboid or diamond. This alteration in shape increasing, the Committee determined, on account of the great expense which would have attended a con- tinuation of the digging, to drive a tube vertically down, to test the hardness or softness of the earth, and to ascertain if any cavity existed underneath. Accordingly, an iron gaspipe of I J in. diameter, having a loose steel-pointed plug at the lower end, was driven down, with some difficulty, to the depth of 70 ft., the ground through which it passed being hard. Thence to the depth of 82 ft. the pipe was driven with very great ease. Owing, however, to a frac- ' This timber framework was introduced into the pit to ensure safety when 452