Shear strength, consolidation and drainage of colliery tailings lagoons
Colliery tailings are laminated sediments which vary from coal-rich horizons of coarse sand size, to fine silt horizons composed mainly of quartz, illite and kaolinite. The proportion of finer laminae increases away from the inlet of the containing lagoon, although both fine and coarse bands are found everywhere in the lagoon. Coal itself has a low specific gravity and high friction coefficient. The density increases and the shear strength decreases away from the inlet. Both the average coal content (47%) and friction angle (35º) are higher for tailings than for coarse colliery discard (14% and 31º respectively). The permeability of the contrasting laminae differs greatly, and consolidation and drainage in lagoons is therefore dominated by the horizontally laminated structure. Much of the water in lagoons drains laterally to the embankments. This water contains dissolved solids which reflect the groundwater chemistry of the Coal Measures at depth, being both saline and rich in sulphates. Overtipping lagoons with coarse discard is being used increasingly for waste disposal purposes. It is possible to overtip with a 1.5m high layer of discard using a D6 vehicle at a sediment shear strength of 3 KN/m(_2) . However, to include a safety margin, 4.5 KN/m(_2) should be the lower bound. An effective stress stability study of overtipping indicates that a desiccated surface is necessary; the operation cannot progress where supernatant water remains on the lagoon. In terms of liquefaction hazards, vehicle vibration levels are not high enough to be of concern. Similarly, measured ground vibrations produced by explosives did not liquefy a lagoon being overtipped. It is suggested that a 200-year return period earthquake will not cause problems in this respect.