Unexpected piezometer trends in a tailings dam triggered an investigation that uncovered an overlooked variable in dissipation testing – the type of saturation fluid.
At the Life of Mine | Mine Waste and Tailings Conference 2025 in Brisbane, Ricardo Dellamea, senior consultant at SRK Consulting, shared a case study titled “Evaluating Unexpected Piezometer Trends in a Tailings Dam: Insights from Dissipation Testing”, co-authored with J. Eldridge, J. Rola, and M. Sottile.
The dam in question was built of liquefiable materials and undergoing construction and stabilisation works. Heavily instrumented with vibrating wire piezometers (VWPs) and other devices, the facility had two alert levels – one tied to construction activities and one linked to dam safety.
“Some VWPs began showing unexpectedly high pore water pressures that didn’t align with nearby instruments or with historical data,” Ricardo explained. “They weren’t just off – they were inconsistent with our understanding of the pore pressure profile in the dam.”
Initially, the anomalous readings were monitored but set aside. That changed when one of the VWPs tripped a construction alert, prompting a temporary halt to works and a thorough investigation. The review considered construction influences, rainfall, water levels and stratigraphy, but no clear cause emerged.
The next step was a targeted site investigation, including cone penetration testing (CPT) with dissipation tests and installation of new VWPs adjacent to the problematic ones. Over 50 dissipation tests were carried out – some run overnight – using silicone oil to saturate the pore pressure filters.
That’s when the team noticed something unusual. “Many of these tests didn’t stabilise – instead, pore water pressure would drop and then slowly increase, never reaching a steady value,” Ricardo said. “When we switched to glycerine, the dissipation curves stabilised much faster and aligned with our expectations.”
To confirm the effect, the team repeated tests in the same locations. Again, silicone oil produced atypical, drifting readings, while glycerine yielded stable values consistent with the new VWPs and the dam’s known pore pressure profile.
Although literature and field experience suggest that fluid properties – such as viscosity, surface tension, and insolubility – can affect dissipation behaviour, Ricardo cautioned against a blanket conclusion. “We can’t say glycerine is always better than silicone oil, but in certain conditions, the choice of fluid can influence the results.”
The investigation ultimately confirmed that the anomalous VWPs were unreliable, and they were removed from operational decision-making. The case reinforced two lessons: the importance of cross-checking data with multiple instruments, and the need to consider test setup details – even down to the saturation fluid – when interpreting geotechnical monitoring results.
“Dam monitoring requires a holistic approach,” Ricardo concluded. “Relying on a single instrument can lead to unnecessary actions or missed warnings. This case shows why it pays to question unexpected results – and to keep an open mind about where the answers might lie.”
First presented in The Rock Wrangler article.
