DTH hammer drilling depth capability

The drilling depth capability with down-the-hole hammers is governed by two main factors; sufficient air volume to keep the hole clean and the drill rig's lifting power i.e. its ability to withdraw the drill string from the finished bore hole.

Crawler mounted drill rigs working on quarries or open pit mining applications are generally designed with sufficient lifting power or pullback capability to lift the weight of the drill string from the completed bore hole which on these types of applications rarely exceeds 35 Metres(115ft.) On deep hole applications, such as water well drilling, it is essential that the selected drill rig has sufficient lifting power with a reserve of power (safety factor) for contingencies such as the drill rig's hydraulic system inefficiency, the weight of the rotary head, friction in the bore hole, potential hole collapse etc. The above factors combine to reduce the amount of lifting power available for drill tubes and consequently the achievable drilling depth with any particular drill rig.

- Hydraulic system inefficiency ..........................................................................20%
- Weight of rotary head / hammer / drill bit......................................................... 5%
- Contingencies for friction / potential hole collapse...........................................25%
Total reduction..................................................................... .......... ....... ............50%

In reality only around half the gross lifting power 1can be used. The average weight of 90 - 114mm (3.1/2" - 4.1/2") dia. drill tubes around 20kgs.per metre (13 lbs per ft) therefore to account for the 50% reduction for safety factor, this figure of 20kgs. per metre should be doubled to 40kgs. per metre (26 lbs per ft). For example therefore a drill rigs with a gross lifting power of 8000kgs. (17,600 lbs) would have a safe theoretical depth capability of 200Metres (660 ft). Equally to drill a 350 Metre (1150 ft) deep well with a DTH hammer would require a rig with a gross lifting power of 14000kgs.(30,800 lbs). A safety factor of 50% should be considered as a maximum. With good drilling conditions and/or an experienced drilling crew, theoretical depths will be regularly exceeded. On very deep hole applications a common practice in order to increase the drilling depth capability of the drill rig is to attach the casing winch cable to the rotary head and pull simultaneously with the feed system and the winch. Whilst the theory behind this practice appears sound, the reality is that no make of drill rig can "holdback" more weight during drilling, than it can actually lift out on completion of the bore hole. Damage to DTH hammers & bits due to excess thrust(pulldown) being applied during drilling is a common result of this practice.