Gladstone Harbour bund wall a lesson in what not to do
FAILURE to contain dredged spoil material within a designated bund wall in Gladstone Harbour has provided the perfect case study of what not to do.
Senior engineers Bill Service and Warren Hornsey believe the failure of the bund wall at Fishermans Landing in 2011, during the dredging of 25 million cubic metres of sediment to accommodate LNG projects on Curtis Island, could be attributed to a lighter weight geotextile than that designed for the project, and the layering of the protective geotextile cloth on the inside of the bund wall, rather than the outside as originally specified.
Run-off effects were exacerbated through tidal changes causing stress to the protective layer.
A presentation about the bund wall failure was shown to the Great Barrier Reef Marine Park Authority on Friday.
The authority is now asking questions about the scrutiny of future dredging operations in Queensland, and has decided not to give a permit to allow dredging spoil to be dumped in reef waters until further tests are finished in late January.
Brisbane Times reports Mr Service and Mr Hornsey, say the leaking was caused by problems with the lining - which was thinner than originally proposed - and not being placed within the bund wall.
"This resulted in a porous wall, which relied on a geotextile fabric lining material to prevent fine dredged spoil from leaking through the wall," the authors said.
"Unfortunately, problems arose with the lining, resulting in significant leakage of dredged spoil."
"The rising tide caused the geotextile cloth to split horizontally; in some cases the geotextile was not secured at the bottom and simply lifted up; and there were crumpled areas, tears, rips and holes due to the water movements," the report said.
State and federal laws designed to protect marine fauna and flora state the wall must be structurally sound and have minimal impact on nearby seagrass beds.
Permit requirements to proceed stated "contaminants resulting from dredge spoil disposal ... must be released only to surface waters at the north-east corner of the reclamation area".
A concluding comment attributed to Dr JP Giroud provided a sombre reflection, which might indicate Gladstone has earned its place as a dredging dilemma.
"Geotechnical engineers who do not learn from the mistakes made by others, will learn from the mistakes made by themselves."