What is a tension pile?
A tension pile is a foundation pile that is loaded in tension: it stops a structure from being lifted or pulled out of place. Ordinary bearing piles work in compression, carrying the weight of a building down to a firm layer. A tension pile works the other way round: it anchors a structure into the ground. In Dutch it is called a trekpaal.
Why tension piles are needed
- Uplift (buoyancy): a basement, tunnel, underground car park or swimming pool below the water table is pushed upwards by groundwater, like a boat. If its own weight is not enough, tension piles hold the base slab down.
- Excavations: an underwater concrete slab in a cofferdam is often anchored with tension piles, so the excavation can be pumped dry without the slab blowing up.
- Quay walls and sheet piling: together with raking compression piles, tension piles resist the horizontal earth pressure behind the wall.
- Tall, slender structures: masts, wind turbines and light canopies can be put into tension on one side in strong winds.
Types
- Steel pile with grout: a hollow steel tube is driven or screwed into the ground and grout (cement and water) is injected through small holes. Steel and grout body act together as the tension pile.
- Concrete pile with tension reinforcement: a precast or cast-in-place pile with extra reinforcement or a cast-in or later drilled-in tension bar. Shaft friction alone is often enough to carry the tension force.
- Grouted anchor: a raking drilled anchor with a grout body, often used to tie back sheet pile walls.
Points to watch
- Tension capacity depends heavily on soil type and shaft friction. Site investigation and design by a structural or geotechnical engineer are essential.
- The connection between pile and slab or wall must also transfer the tension; the reinforcement has to be properly anchored into the structure.
- When dewatering stops, groundwater rises again. The uplift risk therefore mainly applies to the final situation and wet periods.
Related terms
- Driven pile
- Grouted anchor
- Grouted pile
- Buoyancy uplift
- Excavation pit
- Sheet pile wall
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