Drilled Shafts (Caissons)

When general contractors are looking for an experienced and consistent drilled shaft contractor with the capability of both land-and water-based installation, they frequently look to Moretrench.

Drilled shafts (also called caissons, drilled piers or bored piles) are often the deep foundations of choice for buildings with high or concentrated column loads. These large diameter reinforced concrete foundation elements are capable of supporting large axial and lateral loads and resisting overturning forces. They can be installed to depths of more than 300 ft through a wide range of subsurface conditions, from loose sands to hard rock, and through water. Construction can be by the dry, wet or cased method, depending on the subsurface conditions and groundwater table elevation.

Installation

Construction may be by any of four methods, depending on the subsurface conditions.

  • Dry Shaft Construction, as the name implies, relies on the soil/rock to be self-supporting during excavation, reinforcement placement and concreting.
  • Wet Construction involves the use of a slurry to maintain hole stability during excavation and completion of the shaft element.
  • Temporary Casing is often used where the soils are dry, but their characteristics are such that caving or sloughing is anticipated during excavation, or where clean granular soils are present below the water table and the shaft must be seated into an impermeable layer to prevent water flow into the borehole.
  • Permanent Casing may be used for shafts constructed through water, avoiding the need for a cofferdam to be constructed. This can be particularly advantageous in highway bridge construction where new foundations are installed ahead of demolition of an existing structure and access is limited.

The shaft is augered to design depth, either a suitable bearing formation or seated on/ keyed into underlying rock. If temporary or permanent casing is to be in installed, it is advanced with a vibratory hammer. Following augering and cleaning out of the shaft, a reinforcing cage is installed and high-strength concrete is tremie-placed to complete the pile. If resistance to uplift forces is the objective, an anchor bolt assembly is set at the top of the shaft before concrete placement.

Applications

  • Building foundations
  • Bridge pier and abutment foundations
  • Retaining walls and soundwalls
  • Transmission lines
  • Signs

Advantages & Limitations

Individual drilled shafts have a far greater load-carrying capacity than any other type of deep foundation element, eliminating the need for a pile cap. Construction is rapid, is accomplished using low vibration techniques, and drilled shafts can be installed in most soil conditions. However, obstructions such as boulders must be removed for pile continuity, and this can be costly. The method also requires unrestricted headroom and a large lay-down area for materials and reinforcing cage fabrication.

Moretrench has completed drilled shaft projects for a range of applications, including New England’s National Grid, the PA Turnpike soundwall installation, and Jacksonville’s challenging Overland Bridge project.