Dear All,
i have a doubt, regarding the field application of use of quantity of explosives, as we progress with the drilling and blasting method. Assuming that the tunnel sections are of equal length, which need to be excavated, so with an initial blast, the some energy would be transferred to the rock behind. This energy should create some vibrations which would affect the intact rock.Hence the amount of explosive required should be less?
is this the case n field?
Awaiting Replies,
Regards,
Partha
Partha! A simple answer could be that the blast gasses, not just the high vibration level, must obviously add some blast-induced 'radial' fractures to whatever the natural RQD, RMR, Q value is immediately ahead of the face. So the second blast (and all following) will actually require less kg of explosives per m3 of rock mass broken, because - in Q-terminology - there will be an artificial 'effective' increase in Jn, the number of apparent joint sets (Of course 'set' is better described in this case as extra random fractures). Since overbreak is very likely when Jn/Jr >/= 6, the energy needed to produce x m3 of broken rock / or m of tunnel advance is likely to be less - when there is some natural jointing. The radial fracturing is perhaps of less help when excavating in a very high-Q massive rock mass.
Dear Sir,
Thank you for the answer, it clears the doubt regarding what happens in field in terms of rock mass parameters.
Dear Partha,
The requirement of quantity of explosives depends on several factors, such as type of geomaterial (Rock/Soil/Soft rock/Hard rock etc..) as well as on the type of geological structures (Folds/faults/any discontinuities/bedding/gneissosity/shear zones etc.) present within the rock or geomaterial. These all geological structures requires different safety measure and quantity of explosives as well. Further, Dr Barton answer also add on on the quantity of explosives in progressive drilling.
Regards!
Dear Sir,
Thank You for the reply.
As per Dr Barton's reply my understanding is considering an Intact rock , with a high Q value- as blasting occurs, fractures follow, this increases the Jn , which is weakening of the rock and hence for successive excavations lesser energy needs to be provided for the breakage meaning lesser quantity of explosives.
Yes definitely there are lot of parameters taken into considerations as you rightly mentioned needs attention while calculating the explosive quantity.
Regards,
Partha
Holding all other conditions and blast variables constant, the explosive requirement should ordinarily reduce. More artificial discontinuities and micro fractures have been created in the rock mass weakening its compressive strength. Additionally, free face has been created by the first blast and the energy required for movement of rock fragments is thus lessened. This assumes that the rock mass properties are the same which is hardly true because rocks are heterogeneous in nature. It is therefore imperative to test the rock strength and rock mass properties as drilling continues for better blast planning.
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