What is a screw displacement pile?
A screw displacement pile, or screw pile (Dutch: schroefpaal), is a foundation pile formed in the ground using a screw auger (a drill head with a screw flight). Most screw piles are installed by rotating and pushing them into the ground, without driving. This makes installation quiet and vibration-free. With many types, the soil is pushed aside (displaced) rather than removed.
Note that this is not the same as a steel helical pile (screw anchor), which is a steel shaft with welded-on helical plates screwed into the ground as a whole. The Dutch schroefpaal mainly refers to piles formed in the ground, such as cast-in-place screw displacement piles.
The term is used broadly. True screw piles include:
- Mortar screw pile (concrete mortar screw pile): after screwing in, concrete is placed while the auger is withdrawn; the reinforcement cage is inserted afterwards. There is no casing between the concrete and the soil.
- Tube screw pile: the reinforcement is inserted through the hollow tube before concreting, after which a screw-shaped concrete pile is formed as the tube is unscrewed.
- Screw combination pile: a precast concrete pile in a grout casing, formed using a temporary auxiliary tube.
- DPA pile and other systems with a lost or recovered screw tip.
There are also rotary piles without a screw auger, such as the screw injection pile (grout injection pile), which are also commonly called screw piles.
Application
Screw piles are especially suitable for:
- inner-city construction and building next to existing houses, where vibrations from pile driving can cause damage or nuisance;
- locations with limited space or headroom, because some systems only need a relatively small rig;
- foundation repair and extensions, where piles need to be installed close to an existing building;
- structures that resist tension forces, such as basements in groundwater.
Points to watch:
- Cone penetration tests first: the pile type, length and bearing capacity follow from cone penetration tests (CPTs) and foundation advice from a structural or geotechnical engineer.
- Quality control: with piles formed in the ground, withdrawing too quickly can cause necking. Ask for the record of installation parameters (depth, torque, concrete pressure) for each pile.
- Condition survey of neighbouring buildings: even with vibration-free systems, record the condition of adjoining buildings beforehand.
- Cost: screw piles are usually more expensive per pile than driven precast piles, but are often the only option where driving is not possible. Get several quotes based on the same foundation advice.
Related Terms
- Driven pile
- Screw combination pile
- DPA pile
- Grout injection
- Foundation repair
- Foundation survey
Discover our DIY plans at fredsdiyplans.com