C.
M. JEWELL
Coffey Partners International Pty Ltd, Sydney, Australia
Abstract
Mardi Dam is a large earthfill embankment located near Wyong on the
Central Coast of New South Wales, Australia. The dam was constructed
by the NSW Public Works Department in the early 1960s; neither a foundation
cut-off nor a chimney drain were incorporated in the design. Shortly
after the dam was filled, artesian groundwater pressure above ground
level was observed at the downstream toe of the main embankment. Although
various remedial measures, including a weighting berm and a system of
pressure relief wells were installed, these proved ineffective in reducing
the artesian pressures. In particular the relief wells rapidly became
clogged by bioaccumulations of iron. In 1987 a study was carried out
to assess the situation and design new remedial works. A number of investigation
bores were drilled: these showed artesian pressures more than 5m above
ground level at the downstream toe. Gallionella were identified in the
blocked relief wells. A new system of pressure relief wells, constructed
from corrosion resistant materials and equipped with an integral cleaning
system utilizing hydroxyacetic acid as the main biocidal and cleaning
agent, operated by gas pressure, was designed.
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