What is a screw combination pile?
A screw combination pile (Dutch: schroef-combinatie-paal, also combi-schroefpaal or schroef-combi-paal) is a displacement foundation pile that combines the advantages of a precast concrete pile with those of a pile formed in the ground. The pile is screwed into the ground without vibration.
The method in outline:
- A steel casing with a loose, sacrificial screw tip at its base is rotated and pushed into the ground. The soil is displaced sideways, not removed.
- At the required depth, a precast reinforced concrete pile is placed inside the casing.
- The space around the precast pile is filled with grout or a concrete mix.
- The casing is withdrawn; the screw tip remains in the ground.
The result is a precast pile with a rough, thick shell of grout. The precast core provides the structural strength; the rough shaft and larger diameter give high friction with the soil.
Application
Screw combination piles are used where driven piles are undesirable and high loads still have to be carried:
- In urban areas and next to existing buildings: installation is vibration-free and relatively quiet, which reduces the risk of damage to neighbouring properties.
- As tension piles: thanks to the rough shell and large cross-section they are suitable, for example, for basements and underwater concrete floors that are subject to uplift. The pile head can be ribbed for a better connection with the floor.
- In soft ground and where there are hard intermediate layers: stiff layers can be screwed through without risk of pile fracture, because the precast pile only goes into the casing once the tip is at depth.
Points to note:
- Pile length and bearing capacity follow from cone penetration tests and foundation advice; always have these determined by a structural engineer or geotechnical consultant.
- Provide enough working space and a firm working platform for the (heavy) screwing rig.
- Record the condition of adjoining buildings beforehand with a condition survey, even with low-vibration pile systems.
Related Terms
- Driven pile
- DPA pile
- Grout
- Soil investigation
- The bearing capacity of a pile
- Screw displacement pile
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