Helical Screw Pile Foundations
Helical piles also called helical piers or helical screw piles are manufactured steel foundations that are grounded into the support structures. They are circular and hollow steel screw tubes that hold one or more helices on their outer surface to penetrate into the soil at a constant pitch.
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| Helical Screw Pile Foundations |
Helical piles are an example of an end-bearing deep foundation. It displaces the soil during its installation, hence it is an example of a displacement pile.
Alexander Mitchell is the man behind the development of this revolutionary engineering technology, that was used from 1836 as the foundation for lighthouses, bridges, and piers.
This article takes through the components, features, and applications of helical screw pile foundations in the construction industry.
Components of Helical Screw Pile Foundation
The main components of a helical screw pile are: |
Components of a Helical Screw Pile |
- Lead: It is the first section of the pile that penetrates or enters the ground. Lead possesses a tapered pilot point with one or more helical bearing plates.
- Extensions: The extension section of a helical screw pile comprises of central shaft and couplings. In addition to these, additional bearing plates can also be placed based on the type of pile.
- Couplings: The coupling consists of bolted male and female sleeves.
- Central shaft: It is a solid square bar or a hollow tubular round section.
Helical piles can be constructed with casing or without casing as shown in figure below.
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| Cased and Uncased Helical Steel Piles |
How to Install Helical Piles into the Ground?
The designed helical screw pile is drilled into the ground by applying torsion. This torsion is applied using any of the following methods:
- Truck-mounted auger
- Forklift
- Front-end loader
- Derrick truck
- Hydraulic torque motor attached to a backhoe
- Other hydraulic machines
The principle component of the above-mentioned machines is the hydraulic torque motor that applies a rotational force or torque to the top of the helical screw steel pile. The helical piles must be penetrated into the ground with high torque and low speed to reduce soil disturbances.
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| Helical Pile Installation Equipment Image Courtesy: Hubbell, Inc |
Torque for Helical Pile InstallationThe torque motors used for helical pile installation usually produce a torque of 4500 to 85,000 ft-lbs or higher. These motors must have clockwise and counterclockwise rotation capability as well as must be adjustable with respect to revolutions per minute during the installation.
The torque-driven helical piles rotate and penetrates the strata until the required depth is reached. If the total pile depth is large, multiple steel sections are connected using bolts to cover the total depth.
Loads on Helical Piles
The four main loads coming on helical piles are:
- Compression loads: This is the load on a helical pile from the above superstructure unit that is placed over the pile.
- Tension loads: This is the tension load experienced by the helical pile due to uplift when the pile is subjected to pulling from the ground.
- Torque: Helical piles are subjected to torque during their installation process to tightly drill into the ground.
Capacity of Helical Piles
Helical piles compared to other pile foundations provide a known capacity value once after installation. The torque during the installation of the helical pile confirms the pile capacity. The resistance required to torque the helical pile into the soil directly correlates to its capacity - the greater the resistance (torque) the denser the soil and the higher the load carrying capabilities.
For example, if the soil that is encountered for pile drilling varies, then the torque reading reveals this variation. Based on this, the helical pile can be modified by adding or removing extensions to make the pile deeper or shallower.
Benefits of Helical Piles
The main benefits of helical piles are:
- Helical pile foundations resist scour and undermining issues when used for bridge foundation applications
- It can be removed easily for temporary applications or when the installation location goes wrong.
- Helical pile units can be easily transported to the remote sites
- Easy installation
- Installation time is less
- Do not produce any drill spoil
- Undergo less noise and minimal vibrations
- There is no concrete curing or formwork requirement
- Minimize disturbances caused to the environment
- It can be grouted in place after installation
- Easy installation of groundwater water without the need for casing
- Helical steel piles can be galvanized for corrosion resistance
- Torque is a strong verification of capacity
- It is installed at a batter angle to add lateral resistance
- It can be installed with smaller and more accessible equipment
- Reduce the truck transportation
- It is cost-effective.
Applications of Helical Piles
Helical piles are used as:
- Foundation for residential and commercial buildings, light poles, sound walls, bridges, etc.
- Used as underpinning elements for the repair of failed foundations.
- It is installed horizontally at any angle to take tensile loads in addition to compressive loads
- It is used as a tensile member in the construction of retaining wall systems, membrane roof systems, utility guy anchors, transmission towers, and many other structures.
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