Double Head Drilling
Double head drilling as the name implies utilizes two drill drill heads in tandem that drive an inner drill rod and outer casing simultaneously. The method is primarily employed to maintain accurate linearity for deep anchors in unconsolidated formations.
The inner rods rotate in the opposite sense to the outer casing thereby compensating for angular drift of the drillstring. The use of a hydraulic drifter to drive the inner string is very useful to overcome problems encountered with wash boring in formations containing boulders etc. Suitable machines to utilize this method is the HD180 series. Another method of double head drilling can be employed for vertical piles using an outer casing and inner auger. The same method is suitable for soil mixing using the machines HD200, HD205P and HD325P.