Controlled Blasting Techniques.pptx

controlled blasting techniquest

1. MALLESWARARAO DODDAKA 1 CONTROLLED BLASTING TECHNIQUES

2. MALLESWARARAO DODDAKA 2 There are several blasting techniques used to control the blasting results. The technique selected is dependent on the nature of the project or end use of the rock face, geology, drilling equipment available and associated cost. Some common controlled blasting techniques are_ Line Drilling Pre-splitting Cushion Blasting Smooth Blasting Air-Decking, and muffle Blasting. The techniques most commonly used to control damage in the final wall of excavations are Smooth blasting, Pre-splitting and Air-decking.

3. MALLESWARARAO DODDAKA 3 Line Drilling: Line drilling is a drilling of a row of closely spaced holes along the final excavation line. The line drilled holes are not loaded with explosive . These holes provide a plane of weakness to which the final row of blast holes can break. This technique is used mostly in small blasting jobs and involves small holes in the range of 50-75 mm dia.

4. MALLESWARARAO DODDAKA 4 Line drilling holes are spaced 2-4 times of the dia . of drill bit used. The distance between these unloaded row to last row of the production blast is approximately 0.4 – 0.6 times the Burden of the production hole. This distance is termed as BUFFER DISTANCE. The depth should be maximum include sub-drilling. The Back row of production holes should be charged 50% only. The holes in the line drilling system help to reflect the shock wave and help to reduce the shattering effect of the rock out side the periphery. It is very difficult to maintain the accuracy of drilling and cost involved in this technique is also very high . In this system a grater amount of drilling is required. it is applied only in HOMOGENEOUS Rock . this type practice shall not be done in in-homogeneous rock since plane of weakness in this case may extend in the finished wall .

5. MALLESWARARAO DODDAKA 5 Pre-splitting: In Pre-splitting, a row of holes are drilled along the final excavation line . These holes are loaded with light charge of explosives and fired prior to the primary blast. The line of pre-splitting holes can be fired separate from the primary blast or simultaneously with primary blast with early delay period.

6. MALLESWARARAO DODDAKA 6

7. MALLESWARARAO DODDAKA 7 This creates a fracture zone between the holes may be a single narrow crack or a thick zone of several cracks. This crack creates a artificial discontinuity along the periphery of designed and expected limit of the main blast fragmentation to isolate the blasted zone from the remaining rock mass. It helps to prevent over break from the subsequent main primary blast and creates smooth & stable finished wall. Pre-splitting is best carried out when the burden is composed of homogeneous consolidated rock. In badly fractured rock, un-loaded guide holes may be drilled between the holes. The light explosives charge can be obtained by using specially designed pipe cartridges. A heavier bottom charge is used in the bottom part of the hole.

8. MALLESWARARAO DODDAKA 8 This picture shows three charge loading methods for pre-splitting_

9. MALLESWARARAO DODDAKA 9 In homogeneous rocks, The Drill holes are charged up to 75-80% of the hole depth. If the rock is highly fractured, the charge can be reduced up to 55%. The stemming used consists of the locking devices to prevent the pipe charges being blown-out. When special charges (pipe charges) are not available, high strength cartridges taped on detonating cord can be used at a distance of 0.7-1.0 m for up to 87mm drill, 0.8-1.2 m for up to100 drill and 1.5 to 2.3 for up to 200 mm drill. The presplit charges are initiated simultaneously with d-cord and trunk lines or instantaneous electric detonators . Where a number of holes are to be fired, portions should be delayed with MS delays. For pre-splitting, the drill holes spacing is normally 08-12 times the drill hole dia. The distance between the row of pre-split holes to last row of the production blast is approximately 0.4 – 0.6 times the Burden of the production holes.

10. MALLESWARARAO DODDAKA 10 Hole dia. (mm) Loading density (kg/m) Spacing (m) 38-44 0.12 – 0.38 0.30 – 0.45 51-64 0.12 – 0.38 0.45 – 0.60 76-89 0.20 – 0.75 0.45 – 0.90 102 0.38 – 1.12 0.60 – 1.20 This table shows the drilling and explosives charge parameters: Dr. HOLMBERG (1982) indicates, that the minimum required linear charge concentration for pre-splitting is a_ W=90d 2 Where, w is the linear charge concentration of ANFO/equivalent in kg/m d is the hole dia. In meter. If the drill holes are close enough to each other and over charges in pre-splitting operation, it can be seen that the crack formation deviates from the direction between the holes and instead follows the natural cleavage direction of the rock.

11. MALLESWARARAO DODDAKA 11 In pre-splitting technique it is extremely important to find the right hole spacing and charges for the type of rock. The characteristics of the rock influence the result to a grater extent than in most other blasting methods. It is advisable to carry out a sample blasting in a short span (about 5m length) before drilling a long distance. Optimum distance and charge values can be found by blasting with variable inter hole distances and charges.