A simple method of drilling through the overburden is with the so-called drive drilling system. The system makes use of the principle of displacement, whereby a single casing is driven through the overburden with the use of a hydraulic drifter. The drill string thus experiences both rotation and percussion.
The displacement work is carried out by a cone shaped tip that fits perfectly to the end of the drill string drill, and is also constrained in the direction of rotation. Once the desired depth of the borehole has been reached, the tip is pushed off. By the pushing off of the sacrificial bit, the casing interior is freed up to allow the introduction of temporary or permanent anchors, injections, Gewi piles etc.
The drill bit remains in the borehole and is considered to be lost. Should the ground formations contain rock deposits, the cone is replaced by a lost or sacrificial percussion drill bit made further wear-resistant via hard armouring, carbide tipping etc. Flushing can then always be used to discharge the drilled cuttings as long as inclusions, the presence of caverns or sub-surface erosion may be excluded.
Also, drill bits can be adapted during project execution to perform more effectively against water pressure or different ground conditions, thereby increasing penetration rates.
