What is a screw jack for foundation repair?
A screw jack for foundation repair, also called a screw prop (Dutch: schroefstempel or schroefvijzel), is a short, adjustable steel support used to transfer an existing wall onto a new foundation. The jack is placed in kassen: large pockets cut at regular intervals into the bottom of the wall being underpinned. Because the jack can be turned to exactly the right height on its thread, the wall can be supported without dropping. Most screw jacks for this purpose are about 200 to 400 mm high.
A screw jack in this sense should not be confused with the Archimedes’ screw used for lifting water (also called schroefvijzel in Dutch), or with the long adjustable steel props used in construction to support floors and formwork.
Application
In foundation repair, the old foundation (for example timber piles affected by pile rot, or a shallow spread foundation) is replaced by a new load-bearing structure, often piles with a new concrete beam or slab. The screw jacks transfer the load from the wall to this new structure:
- Pockets are cut into the wall, usually staggered so the wall always keeps enough support.
- A screw jack is placed in each pocket, often with a concrete tile or load-spreading plate above and below.
- The jacks are tightened until they carry the wall; this transfers the load.
- The pockets are then filled with concrete, often together with the new concrete beam or slab, so that the wall and new foundation form a single unit.
Points for the owner to watch:
- Work to a design and method statement from a structural engineer; the spacing of the pockets and the order in which they are cut are critical for safety.
- Permits: foundation repair counts as a structural alteration; check with the local authority beforehand which permit or notification is required.
- Monitoring: have level measurements taken before, during and after the work, so that settlement is spotted in time.
- Neighbours and masonry: in terraced or semi-detached houses, the junction with a neighbouring house that has not been repaired needs extra attention.
Related Terms
- Foundation repair
- Jacking up a building
- Propping
- Foundation problems with timber piles
- Foundation survey
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