Question

Question 5: Students are to undertake a tunnel blast design for the following conditions: Tunnel is 5.5 m wide by 5.5 m high Rock is granite with a UCS of 300 MPa Drill hole diameter is 51 mm, hole length is 4 m The cut is to contain 2 relief holes of 75 mm diameter . Dry conditions are anticipated, ANFO to be used with a density of 800 kg/m3 and a powder factor of 0.6 kg/m3 Nonel detonators are to be used Students have to determine: 1. The design of the cut, including burden and spacing for each element of the cut. 2. Burden and spacing of the easer, perimeter and lifer holes 3. Stemming length required 4. Method of hole priming 5. Firing sequence timings for the holes 6. Explosive utilisation Results should be presented in a report format, including diagrams as required and details of calculations undertaken.

Answer 1

Given, blasthole diameter (D)= 51mm

UCS= 300 MPa

HOle length = 4 m=4000mm

Blast density = 800 kg/m3

Powder factor (pf)= 0.6 kg/m3

1. Parallel hole cut design is assumed for the current design.

Burden = 25 to 40 D

Assume burden= 30D= 30*51= 1550 mm

Spacing = 12D=12*51=612 mm

For smooth blasting,

Spacing= 15*D=15*51= 765 mm

Burden= 1.25 Spacing= 956.25 mm

2.Easer holes:

Burden = 1100mm

Spacing = 612 mm

Perimeter holes:

In these holes ,decoupling is provided which prevents additional breakage of surrounding rock

Burden is same as easer holes

Hence, burden=1100 mm

Spacing = 612 mm

Lifter holes:

Burden is 65% more than other holes, hence burden= 1550*1.65= 2557.5 mm

Spacing=612mm

3. Stemming length >= 20D

Let stemming length = 22D= 22*51= 1122mm

4. Axial priming technique is used

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