SUBDRILL: AN IMPORTANT PARAMETER IN BLASTING
BACKGROUND OF THE ARTICLE
To a non mining Engineer or Blasting Engineer, the question must have come to you most time that,
1. why subdrill in Blasting, what is the important?
2. Why most subdrill be included in of rock blast and which formation will subdrill not required?
This article provides brief answers to the above knowledge gap.
1. INTRODUCTION
Subdrill by definition is the additional hole length included to the Bench height to improve Blasting result and Bench safety. Most drillhole blasted in the mine especially in small scale miners and artisanal blasting Operation don't include subdrill because they tend not to know it important.
As I have define earlier, there is a specified bench height design to mine an orebody, but the extended hole length beyond the bench height is known as subdrill length. Subdrill is one of the most important Blasting parameters that is use to improve not just the fragmentation rate at the bench toe but also to enhance safety by producing a stable and smooth bench floor and toe.
What are the important of subdrill in Blasting?
The following are the major important of subdrill in blast design:
1. subdrill improve fragmentation at the bench floor
2.subdrill assist the blaster in planing for next blast operation.
3. It aid the mine slope from failing.
4. Subdrill produce a smooth and reliable new bench floor during blasting.
As I have explained in my previous video on Explosive and Rock Interaction part 1&2 and in my journal publication on effect of charge distribution on fragmentation (Taiwo, 2022), I explained that the efficiency of rock fragmentation depends on; powder weight, Burden distance which are few but the major factor that determines the rate of micro-fracturing in rock before explosion. Explosive release shock Energy for fracturing and gas Energy.
The efficiency and performance of the gas Energy depends on the fracturing rate during shock energy distribution according to Sushil Bhandari (1997). The fracturing rate is maximum at tensile slabbing because rock is weak in tensile than in compression (check Ash&Konya,1978 work)
At the toe of a mine bench, the tensile strength of the rock or ore deposit is high due to small or no free face available. This reduce the efficiency of Explosive shock an gas Energy at the toe resulting into poor fragmentation and rough floor. Most time the oversize and boulders are formed from the bench toe, in other cases like underground mine you have put out with sockets in Boulders.
You have boulder and sockets (hollow cavity) in rocks due to pull out of Explosive charge because the Explosive released shock wave Energy is lower than the rock intact Strength at the toe level. What about this! at the point of Detonation, the shock wave fracture by compression at the crushing zone round the hole collar because the Explosive compressive strength is high at initiation point, the wave further fracture by shearing due to rock Anisotropic and heterogenous nature then fracture by tensioning through tensile slabbing using change in medium impledence and Sonic Velocity. At the bench toe, the rock is intact at all dimension, there's less weak zone.
The Explosive energy is also weak and exhausted, because the primer charge or bottom charge make use of it release energy to initiate the column charge and also perform fracturing, this makes the toe fragmentation becomes difficult and in efficient. Also, note that, the base charge is initiate first and the energy only split without complete tensile slabbing because of unavailability of free face for shock wave reflection, this makes the bench toe yield poor fragmentation.
With this, subdrill provide additional charge length, Explosive charge weight at the bench toe to overcome the high tensile strength. It confirmed that the longer the drill hole the higher the powder factor, the higher the blast powder factor, the higher the Explosive energy (shock and gas) release at Detonation. Having subdrill help to provide sufficient energy which is ultilize to intoxicate initiation and also gear up tensioning time at the CJ plane to ensure completion of wave reflection within hole cracks before explosion and casting time of the insitu rock. Other important of subdrill in Blasting is that, subdrill help to ensure smooth and stable bench floor.
subdrill also ensure the production plan and schedule per blast is achieved.As a Mining Engineer we have plan and Target per blast, our mill and beneficiary plant must operate Continuous non stop, therefore all fleet operation had been planned with several target production per day. If for example 50holes are blasted and we expect 100Tonnes of material, and According to our plan, three haul trucks is needed with one loader to ensure 20 trips per day. Which is expected to keep the mill working for days. Assuming the blast failed at the toe, Producing oversize material, and only 60Tonnes are loadable. This will not only affect operation plan it will also affect operation cost. More diesel will be waste on selective loading, haul truck will be underused, plant will also operate in efficiently.
Subdrill is important, it makes the bench floor smooth and good for Tyre mounted trucks ploughing, in most case after Blasting a need for leveling and surveying to ensure pit is accessible for haul trucks, this becomes additional cost and stress. Subdrill will not just solve this challenge it will improve Blasting fragmentation.
Is subdrill needed for all type of rock formations?
No, not all formation requires subdrill.
Bedded sedimentary formation Like limestone among others don't require subdrill because of there stratigraphy.strata or layered formation response positively to bottom charge energy because if the weakeness status of the beds. The bed tend to shift under low Explosive Energy because the shock wave will propagate along the cleavage plane (weak plane) of the formation and improve fracturing at the bench toe. Also, the bedding nature of the rock will support creation of smooth bench floor after Blasting.
In conclusion, bedded formation will support high tensile slabbing with low Explosive Energy. This makes subdrill unimportant.
"How to design subdrill length for a typical Blasting operation" Subdrill is calculate according to rule of thumb as 0.3*Burden distance.
For Langford research work he proposed 0.3*Maximum Burden distance. Subdrill can also be calculated using drill bit diameter using 7.5*D. Subdrill increase charge length by 15.8%. subdrill is important and most be use to optimize Blasting result.
Blast it safely and profitably, all workers and blasters working on the mine site must return home safely to have a happy dinner with their families.. say no to all unsafe Blasting act and conditions...
® Giant Miner
