Monday, 21 April 2014

Installation vs Capacity: The balancing act of screw pile design

Ryan Fleming - Site Engineer

Piles are used where the ground in the near surface is poor and will not support the weight of a structure. On many sites an intermediate dense stratum must be penetrated by the piles in order to reach a more consistent founding layer. Screw piles are typically more sensitive to this than other types of piles; the screw pile designer must walk a tight rope between installation and bearing capacity, whilst providing a cost effective, low risk solution.

When designing a screw pile, we need to consider the amount of torque required to achieve the design load with a specific helix size as well as the torque needed to install the helix to the correct founding depth. As screw piles are self-tapping, when the density of the ground increases, so does the amount of torque required to install the pile. Even though the torque required to achieve the design load in the founding layer may be low, the section size of the shaft may need to be increased to provide enough torque capacity to penetrate through a dense intermediate layer.

As a screw pile designer it is critical to understand the relationship between the permanent performance requirements and the installation process. However, these are not the only inputs into defining the shaft and helix ratio. Others include:

Composite moment capacity
Lateral loading / spread

These various aspects should all be considered to develop a robust piling solution; the probability of success is improved with good geotechnical information and specific experience – get it wrong and the piles will fail to penetrate during installation, causing significant delays to the project.

Walking the tight rope of screw pile design is a real balancing act – remember, it’s a long way down……

Monday, 3 February 2014

Good Connections

James Wood – Piletech Manager

One of the benefits of screw piling is the speed with which it is delivered on site.

The final process on site within the piling sub-contract usually involves the connection detail.  This is typically achieved by placing reinforcing bars into the concrete infill within the steel tube of the screw pile.

A typical Screw Pile Connection

However, there are many ways to form a connection, dependent on load requirements.

A few of the more common examples are:
  • Reid bars with flange plate nuts
  • Annuli - welded to the pile
  • Cage style arrangement where pile allows
  • Shear studs or dowels
Where possible, we will propose the use of straight bars - this makes it easier for following trades to place cages, saving further valuable time on site.

Typically the screw pile designer will propose a connection detail.  However as the connection detail can affect what happens above it, the structural engineer's involvement is required to ensure design requirements are met.  A collaborative approach to this element ensures Good Connections - a technically suitable and economical result.

Factors to consider when developing connection:
  • Design loads
  • Bar development length
  • Bond between concrete and pile wall (typically this is greater than bar development)
  • Minimum steel to concrete ratio
  • Constructability for following trades
For more information about design and connection of screw piles, contact one of our team or visit: