10 important facts that affect mine drilling

Drilling Types

by Ina King (Potgieter) May 16, 2019

Drilling types and methods used in mining are many, and vary according to predetermined factors. High up on this list is whether it is open pit mining or underground mining. Generally speaking, the primary types of drilling are exploration drilling, production drilling and technical drilling. Drilling and blasting, the important ground preparation for any mining job, is necessary for explosives’ placement and detonation, and is required for any mining operation. The only instances where these may be omitted, is when the rock mass is highly weathered and not consolidated at all. There are modern machines however, like a continuous surface miner, that eliminate the need for drilling and blasting in certain surface mining operations.

Types of drilling

Exploration drilling

Exploration drilling is conducted for sample collections, which are used to estimate the quality and quantity of a mineral reserve. Samples are collected as core, and for this reason exploration drilling is sometimes referred to as core drilling. It is also sometimes referred to as diamond drilling because diamond bits are used for this type of drilling.

Production drilling

Production drilling is, essentially, making holes that are suitable for placing explosives used for blasting. The holes drilled for this purpose are commonly referred to as blast holes and are drilled in order to prepare fragmented loose rock that is amenable for excavation.

Technical drilling

Technical drilling is carried out during mine development and is conducted to facilitate drainage, slope stability and foundation testing purposes.

Factors that affect drilling

The performance of drills in the mining environment can be affected by several factors related to various aspects of drilling.

The performance of drills

Acknowledgements to Prof. K. Pathak, Introduction to drilling technology for surface mining.

Drilling method selection

Selection of machines for production drilling calls for value judgement. (Pathak quoting Capp [1962])

The following steps (i) are suggested as guide to select the right drill:

Determine the job factors

Labour and skill availability Site conditions: specify if site preparation for drilling requires a flat or inclined surface Weather Safety requirements Power availability

Define the job objectives of rock breakage in relation to other production cycle operations

  • Excavation constraints and restrictions
  • Haulage constraints and restrictions
  • Pit slope stability and pit geometry
  • Crushing capacity for size and rate
  • Production targets
  • Fragmentation demanded
  • Muck pile through expected
  • Ground vibration restrictions

Design the drill hole pattern based on the blasting requirements

  • Hole size
  • Hole depth
  • Hole inclination
  • Burden
  • Spacing
  • Toe burden

Determine the drillability factor and select a suitable drill

Specify the operating conditions.

Determine the coordination requirements

Estimate performance parameters, including

Estimations of machine availability, of costs and of operation Determine the specifications of power source used Identify the cost of items like bits, oil and lubricants Determine depreciation – costs of labour, maintenance, power, bits and spares Select a system based on optimal cost and maximum safe operation

Monitoring operations

Include monitoring of rotation and pull-down pressures Monitor gauges, alarms and other warning devices Inspect the conditions of rods, bits and associated equipment for optimal performance.

Hole cleaning and bailing velocities

To ensure efficient drilling and a higher penetration rate, the chips generated from drilling must be removed as soon as they are produced.

In blast hole drilling, air flushing is usually used. The return air velocity in the hole varies according to the drilling conditions such as the weight of the cuttings, the depth of the hole and amount of moisture encountered in the hole, among other factors. As a rule of thumb, 5 000 feet per minute represents a minimum value.

For the drill to perform better when heavy cuttings result from a drilling operation – or excessive hole depth, or a hole under high moisture content – higher return velocities are required.

Drilling trivia

This may not qualify as a mining project per se, but it is an incredible example of using skill to get something from deep within the earth. It wasn’t coal or oil or even gold that the drillers were searching for… it was knowledge and the project was called the Kola Superdeep Borehole.

The Kola Superdeep Borehole ii was a Soviet project that aimed to see how deep a hole could be drilled before things became impossible. While there are boreholes today which are longer in total length, none have gone deeper.

The Kola Borehole drilled through rocks greater than 2.5 billion years old and encountered temperatures of 180° C. The extreme conditions were eventually the end of the project, and it never reached its goal depth of 15 000 metres. However, at a total depth of 12.2 km (12 200 m), the achievement is quite remarkable.

The project was not only a matter of digging the deepest hole in the world – it has contributed some important scientific knowledge. Thanks to the Kola project, we now understand much more about the transitionary stages of the earth’s crust, as well as the rocks that make up this unseen part of our planet.

Today, the site is abandoned and decrepit, and the borehole itself has been welded shut.

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