being raised in stages. A retention dam constructed in this fashion is
satisfactory provided the incremental raising is properly engineered
and controlled. Construction by hydraulic filling is also practiced, i.e.
a natural silt or the waste fines if suitable are fluidized and pumped
to hydro-cyclones located at intervals along the planned embank-
ment. These distribute the fluidized fines in controlled fashion, with
the coarsest fraction settling out first and leaving the finest material
to form a zone or ‘core’ of lower permeability, but it may be noted
that hydraulic fills of this nature can be prone to liquefaction under
dynamic loading.The large modern lagoon embankments associated with the operation of
many process industries generally conform with good embankment dam
standards and practice. Older and/or smaller lagoons, however, may be
much less capably engineered.
Common deficiencies in the design of older tailings dams and lagoon
embankments are poor control of the internal seepage regime and mar-
ginal freeboard, the last often combined with limited overflow capacity.
Excessive deformations and steep downstream slopes with marginal
stability are also common.
DoE (1991) and ICOLD Bulletin 106 (ICOLD, 1996) provide guide-
lines for good practice in the design and management of tailings dams and
storage lagoons; DoM and E, (1999) presents a comprehensive guide to
Australian construction and operating standards for tailings storage.2.12 Geosynthetics in embankment dams
Geosynthetics (geotextiles and geomembranes) have considerable potential
in dam engineering given that issues of durability in specific applications can
be resolved. A range of geosynthetics have been employed in a number of
different applications both in new construction and in rehabilitation projects
(Section 8.7). Their use in dams, commencing about 1970, has developed
relatively slowly by comparison with other geotechnical applications. This
is attributable to caution based on limited information as to the long-term
durability of man-made synthetics, particularly when used internally
where they cannot be readily inspected and, if necessary, replaced.
Geosynthetics can be employed to fulfil several different functions in
embankment dams.- Impermeable membranes (upstream or internal).Polyvinyl chloride
(PVC) and high-density polyethylene (HDPE) upstream membranes
have been successfully employed in dams up to 40 m in height. The
membranes, typically 3–4 mm thick, are laid in 4–6 m wide strips on a