Geofabrics spent six months working with a consultant on a liner design for a series of lifts to the upstream face of an existing Tailings Storage Facility (TSF) in Western Australia. The primary design objective was to prevent seepage through the upstream face, thereby reducing the risk of structural instability and environmental issues associated with cyanide and acid formation.
The subgrade material was composed of mine waste rock, including a roller-compacted 100-200mm layer of suitable fines material on top. Given the project constraints, the client faced a tight budget and the low-permeability clay material required was unavailable on-site. After evaluating several options, we finally settled on using a single layer of High-Density Polyethylene (HDPE) for the liner.
To further cut costs, the client decided against using a geotextile cushion layer underneath the liner and opted for a 1.5mm thick geomembrane instead of the recommended 2.0mm thickness. Considering the harsh environmental conditions of Western Australia, characterised by extreme UV exposure and heat, along with the average quality of the subgrade, the lack of a cushion geotextile, and the reduced liner thickness, we emphasised the importance of specifying a suitable grade of HDPE.
Figure 1: Typical waste rock from a mine site
Our extensive experience indicated that a standard GRI-GM13 geomembrane would not provide adequate longevity in this application. Therefore, it was determined that a geomembrane resin with a high Stress Crack Resistance (SCR) and an elevated antioxidant package (OIT/HPOIT) exceeding GRI-GM13 standards was required. These measures were necessary to lower the risk of strain-related stress cracking and premature oxidative degradation, which could lead to the formation of holes.
The project eventually went to tender. As is often the case during the tender stage, the initial design intent was overshadowed by the commercial aspects of the project. The focus shifted from technical considerations to a competitive commercial struggle. Competing contractors began to question product specifications and cast doubt on our recommendations, while the client continued to seek additional cost savings.
Just before the contract was awarded, we received a phone call from the client questioning the specified grade of HDPE. According to the client, who claimed to have done his "research" and was influenced by a contractor, high SCR was not considered overly important. He suggested relaxing the liner specification, stating, "All plastic liners behave the same and GRI-GM13 will suffice." This client was effectively inviting problems, such as the development of holes in the liner and subsequent regulatory issues with the Department of Water and Environmental Regulation (DWER).
Fortunately, the design consultant, who understood the underlying engineering principles and the importance of risk management, did not approve the use of a lower-specification geomembrane. This incident highlights the constant tug-of-war between design intent and cost-saving exercises in engineering projects.
Our extensive experience with geomembrane liners has repeatedly shown that cutting corners to save costs can lead to significant issues down the line. The decision to use a geomembrane with high SCR and an enhanced antioxidant package was not made lightly. It was based on empirical evidence and a thorough understanding of the environmental conditions and material performance requirements.
High Stress Crack Resistance in geomembranes is crucial in applications where liners are subjected to mechanical stress and harsh environmental conditions. Standard GRI-GM13 geomembranes, while suitable for many applications, may not provide the necessary durability in more demanding environments. The risk of stress cracking and oxidative degradation increases with lower-quality materials, leading to premature failure and potential environmental contamination.
In this case, the client’s attempt to reduce costs by compromising on the material specification could have resulted in severe consequences. The long-term performance of the liner is critical to maintaining the integrity of the TSF and protecting the surrounding environment from harmful seepage. By insisting on the use of a higher-quality geomembrane, we aimed to ensure the longevity and reliability of the liner system.
This scenario is a common challenge in the engineering and construction industry. Clients, often driven by budget constraints, may not fully appreciate the technical nuances and long-term implications of material specifications. It is the responsibility of the engineering team to communicate these risks effectively and advocate for solutions that prioritise safety and performance over short-term cost savings.
Ultimately, the success of a project depends on a balanced approach that considers both technical and financial aspects. While it is essential to manage costs, it should not come at the expense of compromising the structural integrity and environmental safety of the project. In this instance, the collaboration between our team and the design consultant ensured that the appropriate material specification was upheld, safeguarding the project’s objectives and mitigating potential risks.
In conclusion, stress crack resistance is a critical factor in the performance of geomembrane liners. When clients think they know best and push for cost-saving measures that undermine technical specifications, engineering professionals must stand firm and advocate for the right solutions. The case of the TSF liner in Western Australia serves as a reminder of the importance of maintaining rigorous material standards to ensure the long-term success and safety of engineering projects.
Comments