Publication: Mining Weekly
Issue: February 22, 2019

Multidisciplinary engineering and environmental consultancy Knight Piésold aims to improve project sustainability this year and lower the possible risks that can arise from tailings facilities. This includes regular inspections, geotechnical investigations  and necessary slope analysis, says Knight Piésold consulting lead engineer Veronique Daigle.

“It is important for a mine to take the necessary steps such as the assessment of piezocone penetration test (CPTu) readings and laboratory tests to ensure the correct global practices and software are used for correct geotechnical interpretation. We are very proud to have clients in Namibia that are taking the necessary steps to mitigate risk and aim to improve their practice towards responsible and sustainable mining.”

CPTu tests measure friction and pore pressure through the tailings layers during sounding, which enables Knight Piésold to interpret how consolidated the material is and if it shows drained or undrained properties or any excess pore pressures. The material strength parameters and behaviours, as well as the position of the phreatic surface, are key inputs to the slope analysis model.

She tells Mining Weekly that Knight Piésold recently completed inspection and geotechnical site investigation of several tailings and water storage facilities in Namibia. The inspection of the tailings and water facilities typically entailed dam safety and environmental review, compliance to design intent, local and regional guidelines as well as international best practice. Through the environmental review, the company identified opportunities for cost savings and operational improvements, and looks closely at risk identification and management.

The company recently completed CPTu soundings, drilling for sampling and the installation of vibrating wire piezometers to better understand the shearing behaviour of tailings and monitor pore pressure in large tailings dams. 

“We are very proud to work with our clients to better understand and mitigate risks at their facilities. This is a fully win-win situation, where we get more tools to advise them appropriately on future design and they actively mitigate risk, which ultimately enables them to operate safely and maintain a good reputation in the industry,” elaborates Daigle.

She notes that it is very important to take on-site measurements of the soil, especially if a company suspects different material layering and possible pore pressure build-up. If there are no records of the previous history of the properties of the soil or if the tailings facility is being operated at a high rate of rising, the underlying properties cannot be detected.

“Upstream wall raises are very popular in Southern Africa, owing to the predominantly semi-arid to arid climate, and most areas are not very active seismically. CPTu tests are an economical way to assess the tailings properties,” explains Daigle.

However, consolidation of the tailings becomes very important to ensure that the wall raises remain stable. CPTu between major raises or stages is a good way of confirming if the material has sufficiently been consolidated. Piezometers installed in the perimeter embankment are also essential in understanding the position of the phreatic surface and possible build-up in pore pressure during loading. The company provides advice on all these aspects on how to contain tailings to ensure wall raises remain stable for copper, gold, zinc, uranium and graphite mines.

She explains that complications can occur if a mine does not do the necessary tests, but this varies from mine to mine, and depends on the site, the deposition and construction method used (downstream, centre-line or upstream tailings raise).

Further, not knowing the deposition history and tailings properties of a historical site  would be a significant risk, while a site that has a low tailings height and load, functioning underdrainage system, low rate of rising and free-draining tailings, would be a much lower risk.

“We generally initiate the work with a site visit and inspection, followed by a review of available site monitoring documentation and a risk assessment prior to recommending a site investigation programme,” elaborates Daigle.

Meanwhile, the company has worked on the slope stability of a range of tailings storage facilities in Namibia, as well as a variety of geotechnical assessments. “When looking at potentially liquefiable tailings, in addition to the sands and silts we will analyse  fine sensitive contractive tailings to understand the shear strength behaviour and particularly their undrained shear strength ratios,” says Daigle.

She further explains that it is essential to understand the liquefaction potential of the tailings  –  dilative versus contractive properties  – and events that might trigger liquefaction. This involves in situ CPTu tests, sampling and laboratory tests.

The laboratory tests involve standard drained and undrained triaxial tests on undisturbed samples, but also a range of triaxial tests at various void ratios to understand the material’s critical state line. This type of assessment and slope analysis is essential to verify actual conditions, manage risks during operation and avoid failures.

In addition to mine residue, Knight Piésold offers heap leaching and water management as well as a wide range of services covering transportation, onshore and offshore geotechnical studies and foundation design, pit geomechanical design, infrastructures and utility services design, renewable power supply, dam design, and environmental permitting and impact assessments. We can assist our clients with a wide range of applications and assist them in mitigating risks for a sustainable operation.

“We are also looking at heap leach stack design and slope stability analysis. Ultimately, the geotechnical properties of the material will dictate how high a stack can go and how big the heap leach pad must be to optimally leach the ore,”  concludes Daigle.

Read the article on Mining Weekly.