Diamond drilling

This method is commonly adopted where cores of rocks passed through are desired for accurate records of the strata or for testing the rocks for their strength, composition, porosity, etc. Fig. 3.5(a) shows the common type of drill bit which consists of a cylindrical cast steel shell having in its lower face a number of small sockets in which pieces of black diamonds are set. These diamonds are not useful as jewellery but are used in the drill bits for their hardness and the bit is suitable for the hardest rocks. The hole sizes in diamond drilling are designated as NX, BX, AX, and EX. The drill rods and the driii bits are specified under two main groups, X series and W series, as per the standards laid down by DCDMA (Diamond Core Drill Manufacturers' Association), an international Association. The drill hole diameters and core sizes (in mm) available are given below.

 

diamonds

                       (a) Diamond drill bit Fig. 3.5 (b) Tri-cone rock roller bit

Standard	Drill rod outside dia		Hole dia. mm	Core dia. mm.
	X series in inches	W series in mm.
NX .....	2.3/8	NW - 67	75	54
BX .....	1.29/32	BW - 54	60	40
AX .....	1.5/8	AW - 44	47	28
EX....	1.5/16	EW - 35	38	21

NW series rods are of, W series and conform to international standards for conventional drilling. NQ series drill rods are manufactured by Long year for wire line drilling technique. There are Q series standards for wire line drilling rods but some manufacturers have their own sizes.

The core sizes where wire line drilling technique is adopted are:

NX holes - 44 mm; BX holes - 25 mm.

The surface arrangements for diamond drilling include, as shown in Fig 3.6,

1.  a derrick,

2.  an engine for supplying power,

3.  a winch,

4.  a pump for supplying water under pressure for flushing,

5.  a settling tank,

6. .a platform for keeping the drilling rods lifted for removal of core or changing the bit,

7.  core boxes for keeping the cores, and

8.  a driving and feed mechanism for the drill rods.

Fig. 3.6. Surface Arrangement for Rotary Drilling

The diamond drill bit is rotated at a speed of nearly 300_rijiJT)-and the pressure on the diamonds is between 1.5 and 2 kgf/cm2. The pressure acting upon the diamonds of the drill bit and the rate of advance of the drill bit into the rock are controlled by an arrangement known as "feed mechanism". The feed mechanism is hydraulic for deep holes, but may be replaced by screw feed for shallow holes. Beyond a depth of nearly 60 m, the weight of the rods keeps the bit pressed against the rocks and the feed mechanism may not be necessary. At greater depth the feed mechanism is operated in such a way that the weight on the drill bit is not excessive.