Drilling patterns in stone.

Drilling patterns, also called shot-hole patterns, are named after the type of cut holes used and the principal patterns are:

(1) Pyramid cut or cone cut

(2) Wedge cut.

(3) Drag cut.

(4) Fan cut.

(5) Burn cut.

(6) Coromant cut,

(7) Ring, drilling.

In general each hole in a round covers an area of 0.3 to 0.5 m²; cut holes are located about 0.45 m vertically apart) and easers 0.5 to 0.6 m apart; trimmers are drilled at intervals of about 0.6 m round the perimeter of the drift.

Pyramid cut or cone cut: — Pyramid cut consists in drilling holes (in the centre of the drift axis) at corners of a square, 0.7 to 1 m sides, almost to meet at a point at the back of the round. In a modified design known as cone cut, illustrated in fig. 8.6, holes are drilled forming corners of a polygon with a centre holes, all nearly meeting at a point at the, back of the cut. The depth of pyramid cut is generally restricted to 50% to 60% of the width of the drift.

Wedge cut: Wedge cut takes the form illustrated in fig. 8.6 in which 2 to 4 pairs of holes are drilled to form a wedge, each pair starting from two sides of the drift centre anq¹ inclined at an angle less than 45° towards the centre almost meeting at the back of the cut along a line.

Pyramid and wedge cuts, the most commonly used forms, are suitable for uniform, thickly bedded and hard rocks. They consume the least total quantity of explosives, but the depth of pull is restricted by the width of the drift.

Drag cut: Drag cut, used for small drifts, 1.8 to 2.4 m wide, consists in drilling holes at an angle to the cleavage so that strata break along the cleavage planes (fig. 8.6)7 This pattern, being dependent on the direction of cleavage planes, calls for frequent changes, which are detrimental to systematic work and the pattern is, therefore, not favoured for large excavations.

Fan cui:-  Fan cut, favoured for laminated strata, mostly soft, covers the face with a-fan-like pattern as shown in fig. 8.6 As each shot has to act for itself, charge in each hole is heavy. This cut is not recommended for hard ground.

Various drilling patterns.

With the above mentioned cuts it is normally difficult to drill deeper than half the width of the drift because of the angle of the drill, but with the burn cut, advance equal to the width of the drift can be obtained.

Burn cut: In Burn cut, parallel holes at right angles to the face are drilled in a cluster which may take several forms one of which is illustrated in fig. 8.7. Some of the holes (which are sometimes of larger dia. than the rest) are left uncharged to give relief to the heavy concentration of explosives in charged holes. This cut is effective in hard, brittle, homogeneous ground which breaks evenly; but cannot be used in springy plastic ground. The advantages claimed for this cut are:

(1) Drilling time is considerably reduced and supervision in drilling is easier as boles are straight.

(2) Depth of pull is independent of the size of the drift, and

(3) The quantity of blasted material is not projected far with a suitable form of the cluster.

 

Fig. 8.8. Drill guide for hotel on hum cut pattern

All the holes, charged as well as uncharged ones, may be drilled by the same drill and those to be left uncharged may he reamed by a reaming bit. At Mosaboni mine shot-holes for burn cut in the drives are 32 mm dia. and the central hole which is not charged is reamed to 58 mm. dia.

The coromant cut: The coromant cut is a new type of parallel hole cut which has been worked out with the object of achieving greater advance per round in tunnels or drifts of small area. In principle, the Coromant cut consists of a slot, which is left unloaded, together with 6 outer cut holes, the locations of which are carefully calculated. All the drilling is done with the same pusher-leg drill. The slot is produced by the contiguous drilling of two holes. Drilling is carried out using a 20 mm drill rod with taper and a special drill bit of 57 mm diameter. The upper hole is drilled first, being directed slightly upwards to assist in the disposal of cuttings. Contiguous drilling calls for guidance in drilling the second hole. For this purpose a guide tube is used, being inserted into the first hole and secured by means of an expander.

Burn cut and coromant cut are known as parallel-hole cuts. In drilling parallel-hole cuts, precision is always an important factor. A special drill jig or template which gives the desired precision and facilitates drilling, has, therefore been designed for the outer cut holes. This template also makes drilling considerably faster. The template normally used is the 6-hole template which fits in with the standard drilling pattern of the Coromant Cut. The two centre tubes of the template are slid into the finished slot, and the template is then secured in place by means of an expander in one of the tubes. The drilling of peripheral holes in the cut is then carried out using ordinary integral steels.

It may be possible to have only one centre hole depending on the nature of the rock. The charge varies with the dia. of the empty hole and in the case of the pattern shown, the recommended charge is 0.3—0.4 kg/m length of the hole. The holes are fired in a sequence shown by delay numbers in the pattern.

Fig. 8.9.   Coromant Cut pattern

The cut holes and the rest of the holes in the drift are normally blasted in the same round with the help of millisecond delay detonators. The standard coromant cut pattern covering six cut holes has been found to be good for the great majority of rocks. The arrangement of the holes and the firing order has been so chosen that the dislodged rock is given sufficient room to expand.

Ring drilling: — For some, a type of sloping operations in metalliferous mines ring drilling with long holes is practised. There are 2 types of ring drilling,

(a) Vertical ring drilling where rings are drilled in vertical planes, radially like a fan to break to a vertical face; applicable in sub-level sloping, illustrated in fig. 8.10

(b) Horizontal ring drilling where the holes are fanned out radially in horizontal -planes to break to a horizontal face; applicable in shrinkage sloping. The principle is the same as for vertical drilling and blasting Short delay interval of 25 milli-seconds is usually employed between holes in each row or ring, starting from the easiest breaking section n the middle and progressing towards the walls.

Long hole blasting is done in wide ore bodies with strong walls so that dilution is minimal. In narrow ore bodies, on the other hand, long hole blasting in not generally economical due to the high cost of drilling. The method is also not quite applicable in irregular ore bodies because long holes would either tend to leave ore un blasted in places or to break into waste rocks.

Fig. 8.10 Ring Drilling (figures in metres)