Shaw Contracting (Aust) Pty Ltd is a leader in Tasmania’s road construction, dam building, irrigation, civil infrastructure and mining industries.

Renison D Dam - Tailings Storage Facility

December 2016 - April 2018

Task

In December 2016, Shaw Contracting commenced construction of a new tailings dam to support existing operations at the Renison Tin Mine. The Renison mine is an underground tin mine located near the town of Rosebery on the West Coast of Tasmania. It is the largest operating tin mine in Australia.

With an extremely high annual rainfall, a remote location, challenging geotechnical conditions, program constraints and an environmentally sensitive location surrounding the worksite, the dam construction works made for a challenging but rewarding project. The project involved the construction of a 700,00m³ earthen dam wall with a pressurized grout curtain, clay core, horizontal and vertical filters, PAF waste rock cell, concrete decant tower structure, monitoring instrumentation and an emergency spillway. 

Total excavation volume was 1,200,000m³.

Challenges

Low strength liquefiable silt up to 10m deep was encountered in the dam foundation excavations. This silt proved very challenging to excavate as it tended to slump and could not support the weight of earthmoving plant during excavation. Shaw Contracting engaged a geotechnical engineer to complete a temporary slope design to reduce the risks associated with this work which allowed the excavation works to continue safely.

With up to 50 items of mobile plant on site at any one time, one of the major risks to be mitigated was the interaction between mobile plant. This was achieved by seeking input from the whole site team which meant that practical control measures could be developed and implemented by the team.

Availability, quality and conditioning of clay for the dam’s core was also an issue which was overcome by the site team. After additional geotechnical investigation, Shaw found a suitable source of clay 2km outside of the dam’s impoundment area. 

Annual rainfall averaged 2,200mm which is over 3 times more than Hobart and Launceston’s average. This presented a significant challenge to clay core placement which was overcome by using weather forecasting data in combination with detailed long term and short-term program planning.

Vertical sand filter boxes were used to place sand filter material and prevent contamination between materials, a total of 10,000m³ of filter was placed using the boxes. Excavators used to place the boxes were fitted with GPS allowing a final construction tolerance of +/-50mm. There was very little contamination or wastage of materials and the use of the filter boxes effectively segregated filter materials from other material zones in the dam wall.

Sediment control both at the construction site and external borrow areas was a high priority given the close proximity of Lake Pieman with all discharge from site flowing into the lake. The management of water and sediment was made more difficult due to high rainfall on the West Coast of Tasmania with water from the dam footprint flowing through a natural water course in the centre of the new dam wall. Sediment was controlled through the construction of a coffer dam to capture water and sediment upstream of the wall construction zone with the water pumped around the wall before flowing into Lake Pieman. There was little sediment transport into the Lake and no observable sediment plume.

The construction of the dam also incorporated a cell within the impoundment footprint to encapsulate potential acid forming (PAF) waste material generated through historical mining activities. The PAF had previously been stored in a general waste storage area at the mine. The new cell was clay lined and compacted to prevent water and oxygen from reacting with the PAF in order to avoid the generation of acid rock drainage. Potential leachate from the cell, while under construction, was captured within another smaller control structure to ensure there was limited impact outside of the dam footprint. The discovery and subsequent removal of lacustrine material from the dam wall area introduced another unexpected source of PAF material that also required storage and containment within the new dam footprint.
 

Outcome

The site management team sought input from all site personnel by encouraging open and informed conversations with regards to risk management. This promoted a culture on site where safety has priority over all other considerations.

Shaw did not have one major OHS or environmental incident over the 75,000 man hours worked during the project. Shaw did not receive any major quality non conformances on this project, this is a credit to the skill of the operators and management team on site.

Shaw employed local West Coast operators to supplement the team, some of which had been out of work for a considerable time. Shaw was able to help support the west coast economy by creating job opportunities for local operators and contractors.

Shaw monitored production of all materials on a daily basis and presented this in a graphical format to the client. This daily monitoring helped to identify construction program issues early and deploy additional resources as needed to maintain the program and ensured completion by the contracted date.