Foskor is a producer and distributor of phosphate rock, phosphate-based fertilisers, sulphuric acid, phosphoric acid and magnetite locally and internationally. The company is South Africa based, with in excess of 1600 employees.
Foskor awarded Geomech Africa a contract to carry out a geotechnical investigation at its Mining Division is in the Limpopo Province of South Africa.
The image below shows a simplified geological map of the Phalaborwa Complex, with the mine located in the North.
Figure 1 Simplified geological map of the Phalaborwa Complex (from: Torsvik et al, 2011).
The current pit designs for Foskor’s north and south pits were based on very limited geotechnical data. This lack of data results in pit slope designs of low confidence. A geotechnical risk assessment and gap analysis of the South and North pyroxenite pits was carried out in 2010 and it was concluded that a number of oriented geotechnical boreholes should be drilled to ensure that there is sufficient discontinuity, rock mass and intact rock strength data to identify the continuity of the rock.
The main purpose of this drilling programme is to obtain sufficient geotechnical data for optimisation of the north and south pits. The project entailed drilling two holes in the North pit and five in the South pit. The holes which range from 300m to 500m deep are being drilled by LF90 Boart Longyear and CS1500 Atlas Copco rigs whose long reach booms are specially designed for exploration drilling.
Down the hole surveys are done at the end of each day to ensure that the direction of the holes does not deviate more than 2º from the required angle of -70º. The survey tool is connected to the overshot and dropped to the bottom of the hole to read the direction of the hole. Further, structural data comprising of structural identifications and their respective orientations is measured by means of Acoustic Televiewer, a down the hole geophysical method.
There are instances where the upper sections of the rock mass is highly fractured making the drilling and identification of strata more complex. The holes have had to be grouted in places to allow the drill to pass through larger cracks and specific drilling muds have been used in the production process to help prevent the collapse of unstable strata into the borehole and the escape of essential cooling and lubricating drilling muds into open cracks. The correct drilling muds were identified through a combination of experience and trial and error.
Drilling supervisor, Kiewiet Mahlangu has been in the exploration drilling division of Geomechanics for 14 years and using his knowledge, he tested various scenarios and their impact on the formations as the drill advanced, until the correct balance was found. The final solution involved Aus plug, Bentonite and CR650.
The contract lasted 3 months and was completed on time.
According to Nkosi Nene, Chief Rock Engineer at Foskor, “Our experience working with Geomech Africa has been a good and a professional one. Our working relationship and collaborative efforts and engagement has yielded good results and progress on the project and we are pleased thus far.”