Design and construction of tailings dams in Madagascar tropical wet conditions for the Ambatovy Project

Abstract

The Tailings Facility of the Ambatovy Project, located on the east coast of Madagascar, required the construction of three embankments in the first project phase. These embankments, completed in December 2009, included a starter dam with a maximum height of 33 m and two smaller separator dams. The major construction constraint was high rainfall intensity in the area with the mean annual precipitation of 3330 mm. The lateritic clayey soils, used both in the embankment fills and on the access roads, added to the construction challenges due to the wet conditions. Special measures had therefore to be taken in the borrow areas, in the embankment zoning and during the construction of the dams. The design of dams was dictated by the non-bedrock foundation conditions, the abundance of residual soils and the lack of granular materials in the project area. The dams were mainly built with locally available residual soils though chimney drains and a drainage blanket were also incorporated in the design. The relatively fine dune sand present in the area was employed in the chimney drains and drainage blankets; though coarser and more competent manufactured crushed rock zones were used in critical drainage areas. Special problems encountered during the construction of the starter dam included the presence of up to about 3 m thick alluvium layer comprising soft clay and loose sands and gravels. The soft soil areas were excavated to a competent stratum, which was confirmed by extensive field vane testing. The commonly used nuclear density meter was found unreliable for the compaction control of the residual soils. Field shear vane, sand cone and water balloon testing was employed for quality control of fill material placed in the dam.