SPY EYE IN THE SKY

This waste is usually dumped by constructing impoundments to retain the slurry and allow the solids to settle out. The embankments are often built from loose, end dumped coal refuse with little or no compaction, and the process might continue until disaster threatened; in two fairly recent cases, it actually set in.

The first disaster happened on Oct. 21, 1966, in the small mining community of Aberfan in Wales, Britain. A 120 ft high coal tip failed and the resulting mudflow traveled about 2000 ft downhill into the town, where it buried a school and killed 144 people, most of them schoolchildren.

The second disaster occurred at Saunders, W.Va, on Feb 26, 1972. A poorly constructed and overstressed coal waste embankment failed on Buffalo Creek and the flood of the impounded slurry killed 125 people; it also caused $65 million in damage to homes and roads.

In both disasters, subsequent investigation revealed that there had been ample signs of instabilities long before the disasters occurred. The tips at Aberfan had had a long history of small slides, rotational slips and settlements. When the dam at Buffalo Creek was constructed, 100 ft of it sank and disappeared in the fine coal waste foundation, and one year before final failure, boils of black water were reported downstream from the dam whose main purpose was to impound coal wastes.

Both disasters could surely have been avoided with careful and frequent monitoring and evaluation of even the slightest shifts in earth structures.

Until recently, such monitoring was not easy, or at least, not cheap. It is easy enough to record the movement of something mobile against a rigid background, but if a pile of mud moves in loosely compacted surroundings that impound slurry whose solids are settling out, what, if anything, is moving with respect to what?

The answer is triple aerial photogrammetry with computerized evaluation of the photographs.

The system uses 11 inch diameter aluminum disks attached to pipes and anchored in the ground as markers. Four such markers are placed in stable ground outside the area to be monitored for slight movements. The other markers are the targets placed at critical points in the area to be surveyed.

An airplane then takes photographs of the target area, plus surrounding terrain with the standard markers, from three angles. The area that can be monitored from a height of 2,700 ft above the terrain is about 4,000 by 4,000 ft.

The three photographs are then superimposed to determine the coordinates of the targets with respect to the stable markers. Target movement is detected and monitored by recording these coordinates from successive photographs taken over a period of time.

The cost of monitoring coal refuse embankments this way is only about 25% of the costs in regular ground survey methods, and the accuracy is about 1/35,000 times the flying height, or in practice, a fraction of an inch.

[More: "Sky eye monitors tailing dams," Coal Age, August 1977.]