Start Date: April 2007
Finish Date: September 2008
Value: $21 million
Customer: Land Development Agency
Type of Contract: Design and Construct
The project: Construction of a boat harbour and outdoor space that in the future will house boardwalks, fine dining, cafes, accommodation and the like.
The project involves 150,000m3 of landscaping, treatment of contaminated materials, dredging of navigation channels and construction of retaining lake walls.
Macmahon won the contract with the innovative design and construction methodology offered in the tender submission.
The construction activities present a complex sequence of operations that has required detailed planning and coordination to ensure the project objectives are achieved.
The scope of works includes dredging and dewatering of harbour silts using a suction cutter dredge that selectively extracted silt from the lake bed.
The sediment and water were then processed in two centrifuges to separate the water from the silt. Water is then collected in a sed pond until tests ensured it was safe to re-enter the lake system.
The silts were selectively removed to extract the contaminated portion, tested, dried and land farmed.
Macmahon installed and operated a ‘wellpoint’ dewatering system of over 30 wells to maintain a dry construction environment.
This dry environment is achieved by drawing down the natural water table by up to 10m within the construction area, allowing conventional excavation techniques to be employed.
The dewatering system operates 24 hours per day / 7 days per week, and to limit noise generation to the surrounding residential zone, the well pumps are compressed air driven via a common electric compressor.
Earthworks activities using ‘wet’ and ‘dry’ construction techniques are being employed to create new landforms, including the formation of a new island and canal.
Given the previous industry that occupied the land, identifi cation, extraction and treatment of contaminated material was required to be carried out.
Decontamination and isolation of contaminants via land farming and selective placement of fill materials was required to meet strict remediation guidelines.
A key element of the Macmahon solution in this D&C project was the development of an installation process for precast concrete retaining walls in a ‘submerged’ environment.
This removed the need for temporary works, also removing the need to install sheetpiles walls that has a signifi cant environmental benefit.
The project is being managed to the highest environmental compliance scrutiny with all works requiring approvals from the ACT Planning and Land Authority, Land Development Agency, Environmental Protection Authority, Department of Environment and Heritage and The National Capital Authority who have been actively engaged in the design process.
The contract between Macmahon and the Land Development Agency is unique in as much as the project risks are shared between Macmahon and the LDA.
Utilising a collaborative contract framework (GC21), all parties are encouraged to work together in overcoming project issues.