What is more important than the drilling diameter itself is the equipment we drill with

Previously I mentioned that drilling is even more important than blasting itself. Today I am putting forward another thesis, that equipment we drill is more important than drilling diameter itself. In other words, we cannot only talk about what drilling diameter to choose, but also about what to drill with. This is where a big part of our success lies.

World top manufacturers of drilling rigs offer us a choice between nearly 80 models of drill rigs and range of drilling 64 to 406 mm. Basically. Which to choose? In this article a TH and DTH methods will be discussed without rotary drilling.

Previously in post I asked which drill rig to choose depending from the deposit type properties, technical parameters of excavation and annual production.

Having those information we can start to play a game.

Deposit type and its properties

Sedimentary, igneous, methamorphic rock - each has different origin physical and mechanical properties. For a engineering evaluation it is not necessary to know all parameters. From experience I can say that origin information combined with average density, compressive strenght and silica content are sufficient to pre-condition the choice of drilling equipment (drill rig + tools).

• Compressive strenght Interesting parameter thanks to which it is possible to easily assess the rock mass.There is a relation between UCS and tensile strenght. Tensile strenght has a lower value and is about 10x less than UCS. Tensile stresses are one of the mechanisms that break the rock. When stresses caused by the detonation reach values below compressive strenght they are not able to break the rock by compression. Therefore, the further mechanism of rock failure results from the action of stresses exceeding the tensile strength. Researches provide the information about relation between tensile strenght and rock characteristic impendance where tensile strenght increase and also characteristic impendance increase. Acoustic impedance of rock (rock density x p-wave velocity) and acoustic impedance explosive (explosive density x VOD) ratio should take values as close as possible to 1. UCS is also combined with powder factor. As the UCS increase the required powder factor also increase. And at the end the penetration rate which shows a simple relationship with the UCS. Penetration rate decreases with growth of the UCS.
• Silica content Can be treated as an additional parameter to complete the picture of rock properties. The high content of silica significantly affects the wear of drilling tools. Studies also show that with the increase in silica content, an increase in the strength parameters of the rock can be expected.

What is the conclusion from this? Explaining it directly - as the rock is harder it require higher VOD explosive and its drilling penetration speed will drop. Faster wear of drilling tools is also to be expected.

Therefore, for hard rocks, it is recommended to use TH drilling rigs to maintain reasonable drilling progress and reduce tool wear costs. In addition, in igneous rocks with a block structure, the drilling pattern should be sufficiently dense at a fairly high PF to avoid oversizes. At the same time, a sufficiently small diameter should be used to reduce the unfavorable share of the rock crushed by explosive detonation. The size of this zone will result from the area of contact between the explosive and the walls of the blast hole.

For soft rocks as sedimentary TH and DTH can be used. For sure foch soft rocks the diameter can be larger and PF lower. Additionally it is necessary to mention that in very soft rocks rotary drilling is enough.

Technical parameters of the excavation

Bench height is crucial in case of drill rig choose. TH offer quite good drilling progress and it can be used for hard rock conditions but the higher the wall, the worse it is for him. There are three main reasons: percussion energy looses on the lenght of the drill string, air flush capability and blasthole deviation.

• Percussion energy loosesEach connection between drill rods, shank and drill rod, drill rod and drill bit is a place where percussion energy escape. It is estimated that each connection is about 6% energy loss.
• Air flush capability Drill cuttings have to be removed from the hole bottom upward, outside the hole. Amount of air and pressure is crucial. Depending from lenght and drilling diameter total weight of drill cuttings to be removed is between 100 kg for small diameters and 1 500 kg for large diameters. Rule is simple - as the diameter is larger and depth of the hole higher the air flush capability should increase. For TH drill rigs available air flush capability is much lower than for the DTH drill rigs. The conclusion is that TH ability to drill long holes is less than DTH. A good information is that usualy air flush capability is going hand in hand with drill rod and drill bit lenght diameter range available for drill rig model so it is hard to set up drill rig by putting too high diameter which will exceed its ability to remove drill cuttings from the hole bottom. Of course there are boundary conditions in which the problem of drill cuttings transport may occur: water presence; long blasthole; cracks through which air flush escapes; bad general setup of drill rig, which results in too large drill cuttings. Those combined with a large space between the wall of the hole and the drill rod (highest drilling diameter for lowest available drill rod diameter) will result in jamming of the drill string.
• DeviationOf course we can not forget about blasthole deviation. It is not a secret that TH drill rigs they have "better predispositions" to make the blastholes not straight. Deviation depends from technical, geological and executive factors. Here I just want to focus on following technical factors: percussion and feed from rock drill transmitted through drill string. Imagine how many times you have bent a long nail by hammering it in. It's similar here for TH, except that a hammer is a rock drill, drill string is a nail and a bend is a deviation.

Number of production sites, distance between them, amount of rock to blast. Industry know many examples and variations of this factor. From single blast site from which quarry takes whole material to couple blast sites. Distance between blast sites is crucial and it can kill whole service. Drill rig should drill, not travel around the quarry. A simple OEE indicator comes to the rescue in this situation. Overall Equipment Effectiveness (Availability * Effecriveness * Quality) define how efectivelly our drill rig is used taking into account (among others) availability time. OEE value should reach >60% to speak about any sense of utilisation of the equipment. For example if the drill rig has OEE 75%, additional 0,5 hour of travel around the quarry decrease this value to 68%. Another 0,5 hour effect that OEE is 62%. If 1 of 10 holes will need to be re-drilled OEE will be 56%. Operation of each machine, including the drilling rig, consists of factors controlled and not controlled by us. We do not fully control when a random failure will occur or when the machine's service will be extended. We can, however, control where and for how long the machine will travel. So if the distance has a significant impact on OEE there are following options:

• increase drilling diameter to reduce number of blastholes necessary - problem will occur when next available diameter exceed maximum supported diameter. For TH it is a specific problem due to rock drill power and air flush capabilities. For DTH it is necessary to choose right drill rods and hammer. Anyway jump 10 mm upwards diameter allow to save 10-16% of drilling time, but you need to pay attention to possible toes and floor unevenness.
• use faster drilling rig - use a TH in place of DTH. Depending from sources you will drill 10% faster, but pay attention to the possibility of hole deviation. Sometimes it is possible to use larger diameter and thicker wall drill rods which can reduce negative impact of faster drilling in above two examples.
• do not look on the quality and speed up drilling drilling at the expense of drilling quality and hole deviation using current drilling tools.
• place another drill rig.
• do not change nothing and suffer from OEE.

Annual production

It is a factor that complements the criteria for the technical parameters of the deposit. It can be treated as a scaling factor that largely helps to initially determine what technical parameters the drilling rig/rigs should have. Each drill rig model has its production capacities - number of maximum meters to be drilled. Of course there are additional factors as for example number of drill rig operators and number of shifts. For same wall height this number of meters varies over a wide range and nothing stands in the way of the difference in practice being 20 - 30%.

Summary

Drill rig choose is a process which should be carried out carefully and precisely. We have to be aware that mining equipment as drill rig should be tailor made and in some cases reasonably universal. It will allow to reach satisfying results with relative low cost. At the end we have got our drilling diameter! In oposite direction the selection of the diameter entails the selection of the drilling rig. Difference is when we choose drill rig we can easily choose drilling diameter regardless of whether it is TH or DTH method. If we focus only on a drilling diameter itself without understanding which drill rig to choose we expose ourselves to losses that we may not be aware of.