-Joel Conte, Ph.D., Professor at UC San Diego
The researchers wanted to experiment these helical piles, since these worked really well with structures in New Zealand and Japan that withstand large earthquakes. However, only qualitative studies were observed for the seismic structural behaviour of the structures in New Zealand and Japan. Experimental data and quantities studies are needed to prove the effectiveness of the helical pile seismic resistance, such that engineers can design with them and build safer and sustainable structural systems.
Helical piles, also called screwpiles, are like large steel screws. They are made out of slender steel shafts that anchor the superstructure into its foundation. These deep foundation piles can be various sizes of tubular hollow sections.
The helices consist of flat and round steel plates welded at different pitches along the long shaft. These do most of the resistance of axial loads, providing both tensile and compressive bearing resistance. Normally, the helices should be spaced 3 diameters apart along the shaft to account for uniform stress distribution between helices. The standardized spacing can make the helices act as individual bearing elements, so there is no need for the helices to depend on helices that are above or below to support the loads.
Here’s how you install helical piles.
- Attach a torque motor, drilling equipment, hydraulic equipment or other rotary equipment to an excavator
- Install the helical piles to the rotary equipment
- Mechanically rotate and penetrate helical piles using hydraulic pressure into the ground
Torque capacities can range from 5kNm to 500kNm.
- Screw the piles into the ground until the bearing capacity is achieved.
- Provide resistance by soil bearing on plates and along shaft
Helical piles can be used to retrofit existing structures since the piles have high anchoring capacity, damping ratio, slenderness, ductility and good resistance to uplift forces. They provide good support for both tensile and compressive vertical structures.
The ease of installation and removal of helical piles also makes it a good choice for construction. They can be used in urban areas, since they only take up a small area and produce low noise and vibration. Screwing helical piles does not produce a lot of soil displacement; therefore its application saves costs and is environmentally sustainable. Moreover, installation of helical piles reduces time, since no concrete is required for this process, which means there is no need to wait for curing.
The tests for the seismic effect of helical piles are now completed. Now, analysis will have to be done. Researchers from the University of Oklahoma and University of California, San Diego are looking to analyze the following topics:
- Behaviour of different helical piles with varying characteristics and installation methods
- Quantifying the strength of helical piles by measuring their moments, shear, axial loads
- Behaviour of helical piles due to different end support conditions
- Differences between helical piles and other types of piles
- Relationship between slenderness and pile behaviour
The earthquake simulator can help us understand the seismic effect of different earthquakes on different structures. Understanding more of the uses and behaviour of helical piles will make their use more common. Both the earthquake simulator and the helical piles can ultimately have big impacts on building regulations. Both can lead designers to design safer buildings in initial stages of planning for the safety of the public.