Dam Safety: The Question of Tailings Dams

Abstract

Tailings’ Dams are mining waste impounding structures. They differ from conventional dams in purpose, design and operation. Percentage wise their failures are higher and posing considerably more safety concerns, causing long lasting devastation on communities, environment, and animal and plant ecosystem. Two basic types of embankment tailings structures are used for tailings impoundments; the Retention Dams and the Raised Dams. Retention dams are built in one operation to a full height, while construction of Raised Dams is a continuous process lasting for the whole useful life of the mine. Raised Dams are favored over Retention dams as they can be enlarged and expanded as the extraction works continue with time. Raised embankment dams themselves can be of three alternative designs according to the method used in construction; the Downstream, Upstream and Centerline structures. This designates the direction in which the embankment crest moves in relation to the initial embankment at the base as successive lifts are added. Resulting from the used method of tailings weight disposition, the Upstream Raised Dams are the least safe in earthquake prone areas as compared to the other two types due to its higher possibility of liquefaction, so they are not favored in highly seismic areas. The disadvantage of Downstream Raised Dams is their use of larger land areas. Centerline Raised Dams are a compromise between the other two. Tailing Dams failure may occur due to: dam instability, overtopping, internal erosion, or combination of these. Instability can result from faulty design and/ or faulty tailings deposition method. Internal erosion can follow saturation of the fill due to fast rate of work and close proximity of the water pond to the dyke combined with downstream gullying, and overtopping happens in case of faulty water management and/ or inoperable decan system. Careful analysis of historic failures and drawing out new lessons from them can help reducing failure probability and enhance tailings’ dams’ safety.