There are three major rock drilling methods which are as follows
- DTH (down the hole) Drilling methods
- Rotary drilling method
- Top Hammer Drilling method
Choice of Different drilling method depends on the rock and soil structure and the drilling application.
Down the hole (DTH) drilling:
Down-the-hole (DTH) drilling is to drive the hammer which behind the drill bit by compressed air via the drill pipe. The piston strikes the bit directly, while the hammer external cylinder gives straight and stable guidance of the drill bit. This makes the impact of energy is not lost in joints and allowing for much deeper percussion drilling. The impact force acts on the rock at the bottom of the hole, which is more efficient, and straighter than other methods of the drilling operation.
DTH is more suitable for the big hole of hard rock drilling, special for rock hardness over 200Mpa. However, for the rock below 200 MPa, it will not only energy waste, but also in low drilling efficiency, and serious wear to the drill bit. It’s because while the piston of the hammer strike, the soft rock cannot absorb the impact completely, which seriously lower the efficiency of drilling and slagging.
The most suitable hole diameter is 100–254 mm, in special applications can be up to 2M And the minimum can be around 50 mm as well.
Theoretically drilling depth is limited by back pressure only (can be up to several kilometers).
Adapt to hard rock and rock formation conditions (compared to top hammer).
Mainly use pressurized air for slag flushing, and sometimes air foam for special application.
Highly efficient in hard rock drilling (compared to the rotary drilling).
Drilling straightness is best in these three methods.
Rotary drilling Method:
In rotary drilling, the rig provides enough shaft pressure and rotary torque. The bit drills and rotates on the rock at the same time, which exerts both static and dynamic impact pressure on the rock. The bits rotate and grind continuously in the bottom of the hole to make the rock to fracture. Compressed air under a certain pressure and flow rate are sprayed from the nozzle through the inner of the drill pipe, to make the slag continuously blown from the bottom of the hole along the annular space between the drill pipe and the whole wall to the outside.
Mainly suitable for large holes (coal, limestone) usually diameter larger than 200 mm.
Drilling depths up to several thousand meters or even tens kilometers.
Adapted to soft and medium hard rock.
The slag flushing medium can be compressed air, air foam, mud, etc.
Inefficient for hard rock drilling.
Drilling straightness is lower than the down the hole (DTH) drilling type.
Top Hammer Drilling Method:
The percussive force of the top hammer drilling produced by the piston of the pump in the hydraulic drilling rig, it is transmitted to the drill bit via shank adaptor and drill pipe, This is the difference between DTH drilling. Meanwhile, the percussion system drives the drilling system rotation. When the stress wave reaches the drill bit, the energy is transmitted to the rock in the form of bit penetration. The combination of these functions enables drilling holes into the hard rock, and the air compressor only performs dust removal and slagging in top hammer drilling.
The combination of these functions enables drilling holes into the hard rock, and the air compressor only performs dust removal and slagging in top hammer drilling.
Impact energy multiplied by impact frequency together creates the percussive output of the drifter. However, usually, top hammer drilling used for hole diameter maximum 127mm, and hole depth less than 20M, which in high efficiency.
The most suitable for hole diameter between 25–127 mm.
Generally, the hole depth is less than 25 meters.
Adapt to a stable and uniform hard rock.
The slag flushing medium is compressed air or water.
High efficiency in shallow holes drilling, the deeper the lower efficiency.
Most easily to skew in these three drilling methods.
In india, DTH method is widely used for borewell driling and rotary drilling for mining application. Top Hammer drilling for piling and construction of pillars for bridges.