Diamond Drilling

Drilling a hole is the best way to discover what lies hundreds of meters below the surface. Ideally, a geologist or engineer needs a large, solid sample of the subsurface in order to assess it.. To do this, special core drilling equipment has been developed to make a circular cut in the rock and extract a continuous, cylindrical sample. This non-technical document is intended for anyone desiring fundamental information regarding how diamond drilling is done in Canada.


Hole Sizes & Equipment

The size of a drill hole and core sample depends upon the size of the machine used to obtain the sample, and the intended final hole depth.

    The most common hole sizes are
    (diamond drilling uses letters to designate sizes)

  • “A” 27 mm core, 48 mm hole
  • “B” 36.5 mm core, 60.0 mm hole
  • “N” 47.6 mm core, 75.5 mm hole
  • “H” 63.5 mm core, 96.0 mm hole
    The drilling rig consists of

  • a power unit which can simultaneously rotate and push steel tubes
  • flush jointed threaded steel tubes which connect to core catching equipment
  • a diamond bit/ reaming shell
  • a water pumping unit which pumps water (and additives) through the rotating steel tubes.

The water keeps the bit cool and removes rock cuttings. The water returns to the surface between the rock bore and the O.D. of the steel tubes (called rods in the industry)

Power unit

The power unit is either a mechanically powered unit, a hydraulically powered unit, or a combination of both. . Power (diesel engine or electric motor) is required to rotate the rods either through a transmission or hydraulic motor, hold the rods while rotating (a “hydraulic chuck” is commonly used) , push the rods while rotating (now accomplished with the use of hydraulic push/pull cylinders), and remove the rods from the hole. The rods are removed from the hole either by a hoist/cable system, or with a hydraulically powered cylinder.

Drill rods and corebarrel

The flush jointed drill rods are generally in 1.5 or 3 m lengths, and are threaded together as the hole is advanced. The rod threads must be machined concentrically and the rods themselves must be very straight in order to minimize vibration when rotated at speeds up to 2000 rpm. The core is retrieved into a corebarrel assembly which must be emptied of core from time to time. A drilling technique known as wireline is commonly used to remove the core. The core catching assembly (the “inner tube”) can be retrieved from the surface using an “overshot” which is lowered into the drill rods attached to the end of a wire cable. The innertube is winched to the surface, and an empty innertube is put into place. Drilling can proceed, the rods never leave the hole.

Diamond bit and reaming shell

The cutting of the rock is done with a diamond bit. Diamond bits have changed dramatically from the early hand set days. Now, the diamonds are synthetically manufactured and are distributed evenly throughout a metal binder. {Click here for more information} As the bit cuts the rock, the OD gets smaller and smaller, and eventually a new bit cannot be put into a hole to replace a worn out bit. The solution to this problem is to use a reaming shell, a cutter placed behind the bit, to ensure the hole is always larger than a new bit.

Water pump

The water pump pumps water under pressure to the drill bit. The water is under pressure, up to several hundred psi or 40 bar, and with flow rates up to 12 gallons/min (45.4 l/m). Sometimes additives are added to the water to increase core.


Steps in Drilling a Hole

Finding bedrock

Before drilling can begin, it is necessary to drill a “protection system” through the overlying soil (overburden). Threaded steel tubes, called casing, is drilled into the overburden until it reaches bedrock. The type of overburden varies from place to place, from glacial tills to sand and clay. The depths of overburden range from a few meters to over 200 m. At the end of the casing, a diamond casing shoe is fitted so that when bedrock is encountered, it can drill a few inches into the bedrock and form a seal to prevent soil from entering the borehole.

Drilling in solid rock

Once the casing is placed (set), the corebarrel and bit are lowered into the hole, and drilling solid rock can proceed. Either the wireline method or conventional method is used. Wireline corebarrels enable the driller to keep the rods and bit in the hole, only the inner core tube is removed after each core run. Conventional corebarrels require the entire length of rods to be removed after each core run. Conventional drilling is commonly used for very short holes. The time it takes to remove the rods after each core run quickly renders conventional drilling expensive after a certain depth, plus there is danger the hole will collapse (“cave”) when the drill rods are removed. The one advantage conventional tools have over wireline tools is penetration rate. Typically, conventional tools have a thinner kerf (kerf is the thickness of the cut) than wireline, hence they will drill faster.

Core runs will vary depending upon the rock. Solid, competant rock can be drilled without interruption and fill the core tube each time. On the other hand, fractured, broken rock can wedge (or “block”) the core tube, and prevent further advance. When a core block is encountered, drilling has to stop, and the core tube has to be removed and emptied. Drilling in fractured “blocky” ground is substantially slower than in solid, unfractured rock, because of the need to frequently remove and empty the core tube.

Information can be gathered at this early stage of drilling regarding the degree of rock fracturing, and is designated the RQD or Rock Quality Designation. The RQD can be used to estimate rock strength when designing underground structures such as tunnels or mine drifts.