Experts on flooding at Williamson mine

Dar es Salaam. The Williamson Diamond mine’s Tailings Storage Facility (TSF) breach, which caused flooding, would have been discovered if the facility’s standard best practice of “observational method” (OM) had been followed.

The OM is a technique used for assessing the stability of the structural system, the rock mass and support, as well as for controlling the design requirements related to durability and serviceability.

This is according to findings recorded from Interferometric Synthetic Aperture Radar (InSAR) analysis undertaken by Value.Space, a UK based tech company which uses satellites to conduct risk assessments on Critical National Infrastructure and detect deformation of assets that can be the early warning signs for infrastructure failures.

Ms Lindsay Bowker, an executive director for World Mine Tailings Failures (WMTF) exclusively explained to The Citizen that the InSAR finding covered a period from June last year to November this year, when the failure occurred.

“The accelerated deformation which led to the facility’s failure, began immediately on resumption of depositions at an expanded rate, in fact; five months prior to failure. Value.Space’s InSAR has indicated 15 areas on the TSF that have notable deformation,” she observed.

“These 15 deformed conditions were detectable through normal TSF best practice of observational method life of the facility from design to closure,” she explained.

According to the expert, InSAR technology uses satellite images for monitoring assets such as the Williamson mine and has reliably shown subsidence on tailings containment and dormant waste piles, thus providing an early indication of slope failure, which would allow the monitoring of TSF’ stability.

Media information indicated that the high level of production through bulk mining, generates higher rate of deposition of tailings and perhaps a different character that was not assumed in the original design.

Therefore, an assessment related to the facility’s capacity to manage expanded volume of tailings, was vital, as it could have shown whether the already constructed facility could absorb a faster rate of deposition.

“Petra Diamonds had acquired the facility in 2009 from DeBeers who had reacquired it from the Government who had owned and operated it for the 19 years after DeBeers 1973 exit,” Ms Bowker observed.

Adding: “We have not been able to find any details on when the failed facility was placed in service or on who designed it and on what assumptions. But Google Earth has helped us to go back - far enough to see that the breached TSF did not exist in 2005.”

Although there is no available information that directly indicate persons behind the designing and construction of the failed facility, but Ms Bowker was of the view that since it did not exist in 2005 then the facility might have been designed and built during or prior to Petra’s acquisition.

“The deformation acceleration seemed to briefly level out for the month from mid-December last year to early January this year, but then it slid into a ‘tertiary creep,’ a rapidly accelerating rate of deformation after that until the failure occurred,” she observed.

Therefore, the WMTF’s call is for an Independent Tailings Review Board (ITRB) to assess stability of remaining structures, establish a cause of the failure and develop specific tailings plan for the planned production.

“The ITRB should indicate whether there is a safe interim tailings strategy and operations should not resume unless there is both an immediate safe place for tailings and a viable preliminary plan for the remainder of its phase of life,” she noted.

Adding: “Given Value.Space’s credentials and expertise experience in InSAR, these data alone justify a call for an immediate ITRB with a primary mandate to assess the capacity of the remaining structure to retain emanating tailings.”

Information made available to this paper from Value.Space suggested that with InSAR technology, they are able to look back up to seven years and see prior risks that have been present in the past and if that affects the current risk status of the site.

Moreover, information available at Petra Diamonds maintained the firm’s commitment to implementing global standard on tailings management (GISTM) though the Mwadui mine does not require a mandatory code of practices (COP).