4.8 Dump drainage condition
Piezometric conditions of a dump and its associated foundation are of primary importance for the assessment of dump stability. In most cases, large coarse dump materials have sufficient permeability so as to render the dump an effectively drained structure. The material placed using the end-dumping technique segregates and coarse material arrive at the toe/foundation contact thus enhancing the permeability. End-dumping from large heights and simultaneous construction of a flow-through drain to enhance permeability are apparently co-benefits of the most cost efficient dumping methods.
Percolation of rainwater through the dump carries existing fine particles towards the dump foundation level. The generation of fine particles over time within the dump by natural slaking, weathering, and high point to point loads adds to the fine particle load. Ultimately these fines settles and cause blinding of the foundation interface contact within the dump, severely reducing the capacity of the drained structure. The presence of perched water levels may also be created as fine grained, poor quality material is placed in one area or elevation of the dump. This may lead to localized areas of deformation and instability. The most common solution has been to construct diversion ditches around the dump. Occasionally, culverts or decant systems have been built to carry water under a dump. If surface water flows on dumps are unavoidable, contouring and ditching should be provided to channel the water. Ditches should be lined with coarse rock armouring to control erosion, designed according to hydrological requirements. Low permeability linings to control seepage may also be required (figure 6).
Figure 6: Drainage layout and Interception