ABSTRACT:

In Northern Bohemia, a new motorway crosses non-engineered landfills of clayey spoil from open cast coal mines. Recent fills are soft, unconsolidated, double porosity medium, originally consisting of lumps of typical dimensions from a few millimetres to a few hundred millimetres. The age of the fills is about 20 to 30 years and over this period they generally remained unsaturated down to the depths of 15 to 20 metres. Laboratory experiments with a model double porosity clayey material revealed high susceptibility to collapse. The site investigation involved both laboratory and in-situ testing. However, neither standard in-situ nor laboratory testing was believed to be representative enough for the field behaviour of the fill. Therefore an instrumented trial embankment was built on a site with about 30 metres of clayey fill. The settlement was found to be confined to the upper 20 metres of the fill.

INTRODUCTION

In Northern Bohemia, a new motorway between Prague (Czech Republic) and Dresden (Germany) has to cross a coal basin. The coal seam, which is up to 30 metres thick, is overlain by Tertiary overconsolidated clays and claystones. In the 20th century, the original roof-fall underground exploitation of the coal was replaced by large scale open cast mining, which reaches down to depths of about 150 metres. The spoil of the Tertiary clay and claystone has been dumped either in the open pits or in their neighbourhood. Some of these non-engineered fills reach a height over 100 metres. In some places, fly ash and municipal waste had been also deposited in the pits. On the periphery of the landfills, landslides are frequent. The largest one, involving about 140 million cubic metres, occurred in the coal basin area in 1985, as a consequence of the pore pressure build-up in a recent landfill.

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