Design And Construction Of An Impermeable Silt Curtain In A Tidal Zone

A new design and construction solution was needed for a sediment remediation project in Homebush Bay, NSW. When faced with the requirement to remove the top 0.5 m thickness of deep very soft sediments containing persistent organic pollutants (including dioxins) and replace this with inert material laid on a geofabric, the ‘traditional’ earth bund coffer dam and/or sheet pile approach had an elevated risk of contaminant migration, and was a significantly more expensive methodology than the Impermeable Silt Curtain (ISC) designed by GHD and constructed/managed by Thiess Services. This is understood to be the first occasion on which an ISC was used for such purposes within a tidal zone (tidal range approximately 2 m). The final constructed length of the ISC was approximately 1100 m. The environmental advantages of this system over the ‘hard’ forms of construction originally proposed included: much less disturbance to the contaminated soft sediments, control of odours by conducting all works beneath water cover and an effective (impermeable) barrier against sediment migration into the bay (not able to be obtained by conventional silt curtains). The design utilised a tough impermeable geotextile, mounted on single piles at regular spacings and bottom weighted by chain link into the underlying ‘muds’. It also included ‘windows’ with flap covers in the top 300 mm, opened at a calculated distance from the dredging/placement work, which allowed tidal water transfer while ensuring that the sediment had settled to a level below the window. The work was a resounding success, albeit it required a high degree of management as part of the design.