Gold producer Newcrest has received the final report from the Independent Technical Review Board (ITRB) appointed to investigate the technical root cause of the tailings dam embankment slump in March 2018 at its Cadia operation in New South Wales, Australia.

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Location Map showing the NTSF and STSF. Image courtesy ©Newcrest

The ITRB comprised international experts and was chaired by Dr Norbert Morgenstern.

Newcrest Managing Director and Chief Executive Officer, Sandeep Biswas, said the ITRB report provides a great deal of technical insight which the company intends to share with the industry.

“We accept all the findings and recommendations of the ITRB and will work with our stakeholders, including the NSW regulators, and continue the extensive drilling and geotechnical analysis of the foundations of the northern and southern tailings facilities which commenced following the slump,” said Mr Biswas.

“We will continue to place the safety of our workforce and local community, together with care for the environment, at the forefront of our mind in operation of the southern tailings storage facility and as we progress work on the repair plans for the northern tailings storage facility. We are targeting completion of a study on the repair plan in Q2 FY20.”

On 9 March 2018, a slump occurred in the southern wall of Cadia’s NTSF, causing it to lose containment of tailings from part of the NTSF. The slump did not result in any injuries or environmental damage as the tailings released were captured in the abutting Southern Tailings Storage Facility (STSF). According to the Newcrest reports, no abnormal movement in the NTSF wall, or release of material from the NTSF, has occurred over the past year.

The ITRB report concluded that the dominant factor determining the location of the slump was the existence of a low-density foundation layer in the vicinity of the slump. Other factors that contributed were the local height of the dam, the prevailing phreatic conditions, and excavation at the toe of the structure in the area of the slump.

The ITRB report noted that low-density foundation layer material, which had not previously been identified, is relatively weak and highly compressible and brittle when subjected to significant load. It was determined that the failure of this weak foundation material, when placed under load accumulated through the construction history, resulted in deformation of the wall. This then triggered liquefaction of part of the tailings behind the embankment, causing it to slump forward. The ITRB report noted that this material has to date only been found in close proximity to the area of the slump.

Detailed seismic response analyses were conducted, with the ITRB concluding that the small seismic events which occurred the day prior to the slump did not contribute to the slump.

The ITRB made the following key recommendations in respect of Newcrest’s intentions to restore operation of the NTSF and maintaining operation of the STSF, all of which have been accepted and will be actioned by Newcrest:
•    Continue to work on ensuring that the design and maintenance of the foundations consider any weak material comparable to that in the area of the NTSF slump, as well as the limited drainage within the body of both the NTSF and STSF and the potential for liquefaction of the tailings
•    Enhance the level and type of monitoring equipment, including monitoring within the foundations of the TSFs, to ensure that the foundation is behaving as intended
•    The design, construction and operation of upstream tailings dams should be approached with a more precautionary view