Future Port Expansion Seawall
Client name: Port of Brisbane Corporation
Duration: 2002 - 2005
Location: Brisbane, Queensland, Australia
Located adjacent to an active shipping channel and on the boundary of the environmentally sensitive Moreton Bay Marine Park, the Alliance team delivered exceptional results in terms of savings and benefits.
Building a seawall on soft marine clays posing a high geotechnical risk
The seawall site has very soft marine clays, extending in places to over 30m below the seabed, where the seabed itself is 1 to 3.5m below the lowest tide level. In this uncertain environment, designing a safe, efficient, cost effective and constructable design was extremely challenging.
Close to land the seabed was covered with sand but this thinned out towards the Bay. The very soft mud found where the clay was exposed was almost a liquid and posed a high geotechnical risk during construction.
One part of the alignment seabed was 1m below low tide and therefore draft was insufficient to use a barge for construction activities. In the remaining area, where the seabed was up to 3m below low tide and 6m below high tide, land based construction was difficult.
The project was further complicated by the fact that the Seawall footprint is close to the Moreton Bay Marine Park and Ramsar site, and surrounded by extensive seagrass beds. Project approvals severely restricted the turbidity the construction works could generate.
Sand pancake on high strength geotextile for structural support
After considering the 2D geometry of the bund and fill submergence during settlement, our design avoided expensive ground improvement by allowing embankments to settle. Long-term settlement was factored into the design with simple maintenance such as topping up over the design life.
In order to better manage the risks involved in this program we used an observational method. This enabled the successful completion of the project and in one instance, this technique allowed timely intervention to eliminate the risk of a catastrophic failure affecting the Marine park.
Where the clay was very soft, we used a sand pancake above the geotextiles, placed under water, to spread the loads. Our design placed 1.2 million tonnes of rock, 425,000m3 of sand and used 220,000m2 of high strength geotextile for structural support. Without this technique the construction would have been delayed by several years. Five metre wide rolls of geotextiles were stitched to make 40m x 100m panels. They were then folded and placed on a barge to be fed into the water during high tide to access shallow waters.
A simple innovative system to counteract these conditions was developed.
In the shallow seabed area, the bund construction progressed from land by pushing rockfill and maintaining an embankment height to be just above the mean seawater level (msl). When the water rose above the msl during the day, the construction crew proceeded with the next lift in the dry area. This allowed the construction to proceed throughout the entire day.
In the deeper seabed area, a barge was used to construct a wide sand pancake until it rose above low tide mark so that land based equipment could move in and complete the works in the dry.
We carried out trials in the water to assess turbidity generation with rock brought from different quarries and selected a source which generated very low turbidity. During construction, real time 24 hour monitoring is carried out provide alerts if adverse behaviour is detected.
Extremely cost-effective and approved turbidity level solution
Exceptional savings – this innovative embankment design saved the Port of Brisbane millions of dollars from the ground improvement budget and allowed the project to be completed ahead of schedule.