What is a Cased Drilled Shaft?

Cased Drilled Shafts are required when ground conditions are so unstable that drilled holes cannot safely be stabilized with drilling slurry or where loss of ground must be controlled. Casing can be temporary or permanent steel pipe which provides a 100% stable excavation for the full length of the drilled shaft. Casings can be installed by high capacity impact or vibratory hammers when noise and vibration are of no concern environmentally or to surrounding structures. In all other cases the use of oscillator or rotator machines are the only remaining option.

This technology is the only proven method to drill large diameter shafts in caving conditions, such as loose sands and gravelly soil with cobbles and boulders. Boulders several feet in diameter can be removed safely by the use of specialty grab tools without major interruption to the excavation process. Since only water is used for drilling, environmental concerns are minimized or totally eliminated using this technique.

The Oscillator/Rotator method provides a superior method for drilled shaft construction with high quality ensuring an uninterrupted construction schedule through the elimination of anomalies.

This technology incorporates high torque drill rigs and/or casing Oscillators & Rotators to advance heavy wall steel casing into the ground concurrent with the excavation without any vibration or ground loss. Either permanent steel casings or sectional temporary (removable) casings can be installed over a specified depth or the full length of the drilled shaft.

Temporary drill casings are installed in segments of typically 10 to 25 feet and joined with a double wall bolted connector, providing a flush outside diameter of the casing. Carbon teeth on the casing tip allow for greater installation depth and enable the advancement through obstructions as well as penetration into rock.

The soil is removed from inside the casing with a spherical grab, auger or drilling bucket, without disturbing or loosening the surrounding ground.

The temporary casing is removed while always maintaining the concrete tremie pipe within the casing and thus creating a positive concrete head of approximately 8 to 10 feet inside the casing.
The specific casing teeth pattern and the oscillating motion during the extraction of the casing will eliminate the potential for a smooth wall surface by creating a grooved pattern during casing removal. The fully cased shaft hence results in a greater soil-­to-concrete shear resistance compared with all the other installation methods. Using temporary casing in combination with self-consolidated concrete will enhance overall shaft quality and reduces the risk of shaft non-conformities while providing a more cost effective option than using permanent steel casings.

In combination with specialized rock drilling tools like Down-Hole-Hammers (DHH) or Air Core Barrels, the powerful rotary drill rigs of Malcolm’s nationwide equipment fleet can be used to tackle very difficult overburden soils as well as the hardest rock. On shore as well as off shore applications of this technique have been successfully executed by Malcolm in North and Central America.