SOIL MECHANICS AND THE OBSERVATIONAL METHOD:
CHALLENGES AT THE ZELAZNY MOST COPPER TAILINGS DISPOSAL FACILITY
By M. Jamiolkowski
Abstract:
Geotechnical and geophysical methods used for site characterization, as well as instrumentation for
geotechnical monitoring, have advanced enormously in recent decades. As a consequence, a deeper
understanding of the mechanical and physical properties of natural deposits, including their spatial
variability in situ, is now possible. Modern monitoring systems allow the observational method to
be applied to a greater range of situations with increasing confidence.
The combined use of modern soil mechanics and the observational method often represent the only
viable option for tackling the challenges posed by large or unusual structures in difficult geological
and geotechnical environments. This is particularly the case when the duration of the project allows
one to maximise the benefits of applying the observational method.
The lecture will illustrate this with reference to the long-term project at Zelazny Most, Poland, the
largest tailings storage facility in Europe and one of the largest its kind in the world. Zelazny Most
tailings dam, located in the SW of Poland near the city of Wroclaw, extends over an area of 14 km2.
Its construction started in 1977 with the aim of storing the tailings from three copper mines of the
government-owned ‘Copper and Mill Company’ (KGHM in Polish). Up to 2012, 506 million cubic
metres of tailings have been stored within the 14.3 km long ring-dam, which has reached heights
varying between 31 and 60 metres. To continue operations until the ore-body is exhausted, which is
forecast to occur in 2042, KGHM plans to raise the existing dams by a further 15 metres. Since
1992 KGHM has been assisted by a four-member board of international experts (the ‘IBE’),
supported by a Polish geotechnical expert as local liaison.
The most significant geotechnical hazard for the Zelazny Most tailings disposal is a result of its
location in an area that, during the Pleistocene, experienced at least three major glaciations.
The ice sheets, over 1000m thick that overrode the Zelazny Most area, induced
substantial glacio-tectonic phenomena, the most geotechnically important being the formation of
extensive sub-planar deep shear planes along which the shear strength is close to the residual
strength.
Since the mid-Nineties monitoring of the highest dam section, at present 63m high, and more
recently another dam section, now 48m high, has showed the commencement of horizontal
displacements along the shear planes. These displacements are still continuing. This is presently
the phenomenon of greatest concern for all involved in the project. If these displacements are not
arrested by the stabilization measures that have been introduced, it may be necessary to decide to
cease, at least temporarily, any further dam rising so as to allow the dissipation of the excess pore
pressure in the underlying Pliocene clay.
The disposal facility itself, constructed by the upstream method, is also of special concern as the
tailings discharge by spigotting is about 45,000m3 / day.
The quantity of the stored tailings, the height of the ring dam and the presence within a radius of
3km of a number of villages and small towns, have drawn the designers’ attention to the risk of
flow failure of the tailings dams which could cause both huge environmental problems and
catastrophic inundation of the surrounding area. Detailed investigation of this issue, however, using
in-situ geophysics and dedicated laboratory tests, shows that, thanks to the very conservative
construction practice and the efficient drainage system within the dam, the potential susceptibility
of the tailings to static liquefaction does not control the stability of the dams.