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DESIGN OF PRESSURE RELIEF WELLS WITH AN INTEGRAL CLEANING SYSTEM FOR A LARGE EARTH-FILL DAM

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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