Drilling mud or slurry such as bentonite is mostly used for drilling support in situations where the bore hole is unstable. A positive head of bentonite suspension above the water table is maintained during the drilling process, thus providing continual support to the borehole walls. Fluid concrete displaces the clean bentonite which is collected, cleaned and reused several times before it is discarded.
The auger is simultaneously withdrawn at a rate synchronized with the concrete pumping process such that the pumped concrete promptly occupies the displaced volume exerting a positive pressure, which assists in the extraction process whilst maintaining lateral support to the surrounding soils. The steel reinforcing cage (if required), is then immersed into the wet concrete.
