Drilled Shafts (also known as bored piles or caissons) have been used as deep foundations elements for over 100 years. They have been developed for the transfer of high vertical and lateral loads and as a vibration less alternative to driven piles, which require larger pile caps and have certain installation limitations in difficult ground conditions.
Drilled Shafts are well-suited to cope with a variety of ground conditions ranging from very loose cohesive or soft non-cohesive soils to hard rock. Expandable clays as well as caving sands, gravels, cobbles, and large boulders can be drilled utilizing state-of-the-art high torque and crowd top-drive rotary drilling rigs. Malcolm’s expansive fleet of modern equipment, utilizing the most advanced drilling and excavation techniques, ensures that projects can be constructed in highly variable ground.
Shaft diameters typically range from 24 inches to 120 inches, with extreme sizes of 144 inches or more. Standard shaft lengths vary from 20 feet if bearing layers are relatively shallow to a common depth of 100-150 feet. Extreme depths of more than 250 feet are only possible with specialized drilling equipment.
Uncased Drilled Shafts are the option of choice when the soil is stable enough to support itself or caving of the borehole can be prevented with the use of a support fluid (Water, Bentonite, or Polymer).
Cased Drilled Shafts are required when ground conditions are so unstable that drilled holes cannot safely be stabilized with support fluid or where loss of
ground must be controlled. 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 steel casings provide a more cost-effective option than using permanent steel casings. Casings can be installed by high-capacity impact or vibratory hammers when noise and vibration are of no concern. In all other cases, the use of oscillator or rotator machines is the only remaining option.
The Oscillator/Rotator method 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 using specialty grab tools without major interruption to the excavation process. Since only water can be used for drilling, environmental concerns are minimized or eliminated using this technique. The Oscillator/Rotator method provides a superior method for drilled shaft construction with high shaft integrity, ensuring an uninterrupted construction schedule through the elimination of anomalies. Carbon teeth on the casing tip allow for greater installation depth and enable the advancement through obstructions as well as penetration into rock.
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. Onshore as well as offshore applications of this technique have been successfully executed by Malcolm in North and Central America.