技术报告-尾矿库设计及评估 (英文)

Design and Evaluation of Tailings Dams

Figure 7. Phreatic Surface Through a Tailings Impoundment

(Source: CANMET 1977)

Creating a relative increase in permeability downstream can be accomplished in one of two ways, or a

combination of the two: by incorporating lower permeability zones in the upstream areas of the embankment

(typically by constructing embankments with low permeability cores) and by using higher permeability zones

downstream (typically using internal drainage zones). The selection of which technique to use is often based

on the availability of materials, such as clays for cores and/or clean sands for drains. The use of cores and

drainage zones to maintain embankment stability are further discussed in a later section. It should be kept in

mind, however, that major changes in phreatic surface require permeability differences in adjacent zones to be

two or more orders of magnitude (Vick 1990).

The low permeability layer generally controls the overall flow rate through the impoundment. This allows

higher permeability layers located downstream of the low permeability layer to drain and avoid increased pore

pressure. The rule on increasing permeability in the direction of flow only applies in areas near the

embankment face; if a low permeability core in the center of the embankment is used and permeability

increases downstream toward the face, permeability of the material on the upstream side of the embankment

may have little effect on the phreatic surface downstream of the low permeability core (Vick 1990).

In most embankments, materials in the various zones are also arranged to meet filter requirements, which are

designed to prevent migration of tailings and finer materials into coarser zones. Otherwise voids will be

produced that can form a pathway through the dam along which water can escape. As seepage rates

accelerate along the pathway, erosion of the dam material occurs leading to failure of the dam. Such failures

are referred to as piping failures, because of the natural "pipe" that is formed through the embankment.

Piping failures can be avoided by the proper application of various filter rules that have been established in

the design of water-retention dams. (Vick 1990)

Factors that affect the phreatic surface in the embankment affect its stability. These factors include the

depositional characteristics of the tailings (permeability, compressibility, grading, pulp density, etc.) and

site-specific features such as foundation characteristics and the hydrology and hydrogeology of the

impoundment area and its upstream catchment area. Changes in the phreatic surface in a waste embankment

will change the pore water pressures and consequently the resistance of the dam materials to sliding. Changes