Published: 4th October 2016.
Screw piles are often used on sites where there is a need to install deep foundations with a minimum of noise or vibration. This could be because of the proximity of a site to a residential neighbourhood, where the contractor wishes to keep disruption to local homeowners to a minimum, or it could be because there are many other buildings with deep foundations in close proximity, which might be disturbed by the installation process. The design, fabrication and installation of screw piles is discussed below.
Individual screw piles consist of a hollow central shaft with a splicing collar at the top and one or more helices near the toe. The helices allow the piles to cut into the soil as they are wound and are normally designed to suit the ground conditions on each particular site where screw piling is used. The shafts are also commonly designed specifically for the prevailing soil conditions.
Screw piling is normally fabricated from hollow steel sections. The helices are then attached to the outside of the sections, toward the end. A splicing collar is attached to the top of each pile, featuring drive lugs that enable the pile to be attached to a hydraulic torque drive for installation purposes. The type of steel to be used, along with the shape of the shaft and helices, will be specified at the design stage, as mentioned above.
A hydraulic torque drive is used to screw the piles into the ground. The exact type of drive that is used for the installation process will depend on the length and design of the piles and the nature of the soil into which they are being screwed. In certain cases, site access limitations may also need to be taken into account when specifying installation plant and machinery. Having screwed each pile to the desired depth, the resulting borehole is then filled with concrete and the pile will remain in place for the lifetime of the building that it supports. The tops of the piles are covered by a concrete mat, known as a pile cap, providing a stable base for the new construction. To form a solid connection, steel bars are cast into the top of the piles before the concrete cap is poured over them.
Given that the majority of applications for screw piling involve permanent structures such as tall residential or commercial buildings, it is necessary to consider the future degradation of the materials used in the fabrication of the piles. Because all steel eventually corrodes, the most common way to address this issue is to make allowances for sacrificial corrosion, i.e. to over specify the thickness of the pile shafts in order to ensure they are still able to provide the necessary support once they start to corrode.
At Belowground, we provide screw piling services to the construction industry and would be happy to answer any questions you may have about this type of deep foundation.