Extracting underground core


Lesedi Drilling and Mining Contracting Company extracts underground core samples from deposits, enabling mines to determine the quality and economic viability of their reserves. Lesedi is remunerated for its services per metre of rock sample delivered to the geologist’s surface core yard.

The mine geologist instructs the Lesedi team about the site of a hole and the approximate distance and angle the hole must be drilled in order to achieve a desired reef intersection from the collar position of the hole. Work starts when the mine supervisor signs off the site as safe to work in , that compressed air and water are available and that the air quality is on standard.

Under ideal drilling conditions, and once the drill bit is lowered to the bottom of the hole and the drill started, the bit will cut a core, consisting of a cylindrical piece of rock. Rotating at a high rpm, the bit is forced downward by the action of hydraulic or pneumatic cylinders on the drill. As it moves through the rock, it pushes the core up into the core barrel or, in the case of continuous coring, straight into the rod string. 

The rods are withdrawn at intervals of 1,5 or 3,0 metres and the core is removed from the core barrel for examination and storage.  

Continuous coring 

Lesedi also uses a drilling method known as continuous coring. In this method the core produced by the cutting action of the bit goes straight into the rod string. Water is pumped down to the bit on the outside of the rod string and returns to the drill via the inside of the rods. The water pressure from the return water is enough to push the core to the top of the rod string where it is retrieved by the driller. Continuous coring results in much higher levels of productivity as there is no down-time when core is retrieved from the core barrel.

The core presents a tangible and accurate record of the various rock formations through which the bit has passed.

Cylindrical core samples

Generally, in South Africa, a rock sample is a cylindrical core, 30,5mm in diameter, with a length equal to the thickness of the deposit requiring evaluation. This is obtained by drilling into the deposit with an AXT diamond drill bit, reaming shell and AQ rods. The drilling process leaves a hole with a 48mm diameter.

A Lesedi drill crew will complete a 100-metre hole in, on average, two weeks. During drilling operations, relentless precautions are taken to ensure that there is no uncontrolled ingress of water or methane. As core boxes are filled up, the driller and his assistant deliver them to the mine’s core yard, where the mine geologist inspects and logs the core.

If required, the completed hole is surveyed by Lesedi, using the latest electronic survey equipment.

Operating structure

 Lesedi’s organisational structure on a typical mine, where ten to fifteen drills are deployed, is: 

1 contract foreman

1 supervisor for every seven drills 

1 storeman

1 fitter

An operator and assistant for each machine

 A transport team of two or three people



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