Much less pushdown is required for DTH hammer drilling since the air compressor is providing much of the force. A general rule of thumb is 500 pounds of pushdown per inch of bit diameter. However, the air compressor specifications will greatly affect penetration rates. Pullback is vital in determining depth capacity as the rig must have sufficient power to be able to retract the drill string. The required pullback depends primarily on the weight of the drill string, which is determined by the drill rod weight and bore depth; however, extra power is often necessary when trying to recover a jammed or backfilled drill string due to the borehole collapsing and to allow for a reasonable margin of error. A rig that can drive casing and drill simultaneously is advantageous such as GL-6000 from our company, and there are several ways to accomplish this from dual rotary heads, which require lots of torque, to bottom drive casing systems.