What is a ribbed pile?
A ribbed pile is a precast concrete foundation pile with horizontal ribs on the shaft or in the pile head, which improve the bond so that permanent tension loads can be transferred. Where an ordinary pile works mainly in compression, a ribbed pile is intended for situations where the pile has to hold the structure down.
Why ribs?
A smooth concrete pile derives its tensile resistance only from shaft friction and from the reinforcement in the head. That is limited and prone to creep. The ribs increase the contact area and create a mechanical key, so the force transfers reliably between pile and soil, or between the pile and the slab cast onto it.
On precast piles the ribs are often formed internally, on two, three or four faces of the pile head, so that the slab cast against it locks into them.
Where they are used
Ribbed piles are used wherever uplift from groundwater pressure or wind would lift the structure:
- Approach ramps to aqueducts and tunnels, which sit below the water table and would float without tension anchorage
- Tanked basements and underground car parks below the water table
- Light structures with large wind loads, such as sheds, halls and masts
Points to watch
- Tension piles need their own geotechnical design. The design value in tension is well below that in compression; never use the compression capacity
- Crack control governs durability under tension: the reinforcement has to be sized to limit crack width
- The pile-to-slab connection must be detailed as a full structural joint, with adequate cover to reinforcement
- Check whether the temporary construction condition or the permanent condition governs for groundwater level
Designing a tension pile foundation is work for a structural engineer, not a DIY exercise.
Related terms
- Driven pile
- Reinforcement
- Formwork
- Compressive strength
- Crawl space
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