Solid blasting

In a development gallery, coal can be blasted without giving an undercut, by the use of explosives of P₅ type. The blasting is known as blasting-off-the-solid or, in short solid blasting. P₅ explosive is used along with Carrick short delay detonators (of I.E.L.) or coal delay detonators (of I.D.L.). Introduction of such blasting in mines needs approval from the D.G.M.S. for exemption under the following regulations:—

(a) Reg. 173: To blast off the solid.

(b) Reg. 175: To use delay detonators.

(c) Reg. 168: (5): To fire rounds of more than 6 shots in coal.

The pattern of shot-holes drilled for solid blasting in coal in galleries of bord and pillar working is:

(1) Wedge cut, or

(2) Fan cut

Drivage of a narrow gallery in coal without an undercut can be compared with drivage of a drift in stone. Blasting out coal in the centre of the face in the wedge cut pattern gives free face for the remaining coal. The maximum permissible charge to be used per hole therefore varies when blasting out the central wedge with P₅-explosives and when blasting out the remaining coal (after the central wedge is blasted out thereby providing free face) with P₁/P₃ explosives if these are used. On a long wall face, the shot holes are drilled at an angle of 45° to 60° to the face, a coal face 2.4 m high requires7 rows of shot holes and distance of holes in the same row should be nearly 1.2 m.

Powder factor with solid blasting is 1.5 to 2.5 te of coal per kg of explosive and 0.8 to 1.25 te of coal per detonator.

DGMS stipulations on maximum permissible charge in a shot-hole, delay intervals, etc. in u/g coal mines.

1. Explosives

Type of Explosives

P_I 

P₃ 

P₅ 

P₅

Degree of gassiness/Type of Application

Degree 1 mines, cut face

Degree 1, II & III, cut face

Degree I- 'BOS'

Degree II & III-'BOS'

Max. Permissible charge per shot-hole, (gms)

800

1000

1000

565

2.  Delay Detonators

(a) While using non-incendive delay detonators in 'BOS' applications the maximum delay period between the 1st & last shot in a degree I and II gassy coal seam will not exceed 150 milli-seconds.

(b) While using non-incendive delay detonators in 'BOS' applications, the maximum delay period between the 1st & last shot in degree III gassy coal seems will not exceed 100 milli-seconds.

(c) The delay period between 2 consecutive shots with different delay numbers will not exceed 60 milli-seconds.

3.  Distance between 2 adjacent shots with different delay numbers will not come closer than 0.6m at the explosive-charged ends.

The advantages of solid blasting are:

(1) It eliminates the use of coal cutting machines. This is particularly important in steep seams where flitting and control of coal cutting machine is difficult and risky.

(2) Skid mounted coal cutters are flitted with the help of anchor pipes which can be used in seams of upto 2.2 m height. Moreover, the roof and floor of the seam to use such pipes should be hard. Where these working conditions do not exist the skid mounted coal cutter cannot be used and solid blasting is an advantage.

(3) Saving in capital expenditure on equipment at the face, like coal cutting machine, gate-end boxes, cables, etc.
Compared to the undercut faces, the amount of drilling on a solid blasted coal face is high but the provision of one or two extra drills at the face is not very costly, considering the cost of coal cutting machines, gate-end-boxes and medium tension cables of special construction for remote control operation.

(4) If tubs are used for transport of coal, track can be laid right up to the face in the absence of coal cutting machines.

(5) When working a seam over a caved area, the floor is irregular and coal cutting machines cannot be used. Solid blasting offers a definite advantage in such cases and in other cases where floor may be irregular for some other reasons.

For Deg. 1 gassy mines at least 284 m³ of air per mm. shall be conducted in the ventilation connection out bye of every face where solid blasting is to be practised.

For Deg. 2 gassy mines at least 284 m³ of air/min. shall be conducted upto every face where solid blasting is to be carried out.

The results obtained by solid blasting in one mine in Hazaribagh area (C.C.L.) are given below for guidance:

Gradient of seam

Category of gassiness

Working method

Gallery size

Shot holes at a face

Explosive charge

Pull per round.

Coal yield/kg of explosive

Coal yield/detonator

— 1 in 3.7

— Degree 1.

— Bord & pillar development.

— 4.2 m wide x 2.4 m high.

— 1.4 m deep, wedge cut pattern total No. 12

— 500 g/hole; total charge 6 kg.

— 1.06 m.

— 2.0 te (soligex)

— 0.9 te

One driller and two helpers drill 60—70 holes in a shift of 8 hours; one shotfirer, 2 explosive carriers and 1 stemming material / carrier constitute the blasting crew and fire nearly 60 shots in a shift of 8 hours.

Blasting patterns for shot holes in coal for solid blasting are shown in fig. 8.11 and 8.12 in a narrow heading and in fig. 8.13 for a long wall face.