| Popis: |
The overburden materials of Canadian Plains coal strip mines comprise a sequence of overcompacted arenaceous and argillaceous strata of Cretaceous age. Mining is effected by casting the mined overburden across the pit into windrows. Some time after levelling and soiling, differential subsidence of up to 1.2 m over a 12 m distance can be observed. This paper explains the mechanics of subsidence in terms of clay mineralogy, degree of cementation and overcompaction, osmotic and matrix suction, cation geochemistry of the resaturation fluid, and the availability of water. Basically, in engineering terms, spoiling and levelling disaggregates the material, particularly in the valleys between windrows. In the absence of water, the high suction potential of the smectitic overburden generates a relatively high lump strength, so that a loose assemblage of disaggregated lumps is initially stable. Upon exposure to water from meteoric sources or from groundwater table recovery, the suction is destroyed, lumps lose strength, and subsidence occurs. The subsidence is delayed due to the volumes of water required and the low permeability of smectite, after the macroporosity has been destroyed. Processes were simulated in the laboratory, and tests with fluids of different salinity demonstrated that ionic make-up is a secondary variable at best; delayed access to water controls the process. |
