What kinds of construction projects can use screw pile foundations?
Owing to the broad range of sizes available, both small and large construction projects can use screw pile foundations. The most popular small-scale applications are terraces, fences, canopies, garden buildings, piers and landing stages, etc. For small buildings, the foundations can use hand-installable screw piles, which can be easily installed with a crowbar or other bar. Applications for larger screw piles include summer cottages, detached houses, halls, riding rings, pipelines, etc. Larger constructions also require larger screw piles, which are always machine installable models. In larger applications, it is advisable to conduct a ground survey (vane auger) and make use of a structural engineer.
What types of soil are most favourable for the use of screw piles?
The most favourable soils for screw piles are clay and sandy soils. Owing to the large surface area of the helix, a screw pile has a better carrying capacity than standard steel piles. The good carrying capacity means that adequate capacity is attainable even in layers of clay or sand. Screw piles do not often need to be installed as deep as in pile driving, and thus both the length and costs of the piles are often lower than in pile-driven foundations. Also, installation of screw piles succeeds best in clay and sandy soils. With machine installation, screw piles can be installed also in moraines.
What is the difference between hand and machine installable screw piles?
Hand-installable screw piles can be installed in the ground using the torque from a crowbar or similar lever. Hand-installable screw piles are well suited for applications where only a few screw piles are needed, or when the installation location is less accessible (e.g., on an island). Machine-installable screw piles are installed using a hydraulic auger attached to an appropriate site vehicle. Machine-installable screw piles are suitable for applications in which larger numbers of piles are used. Also, all piles with 250 mm and 400 mm helices are machine-installable models.
Why do some screw piles have more than one helix?
The more helices a screw pile has, the better is the compressive and tensile strength achieved in cohesive soils (clay soils). By increasing the number of helices, it is possible to achieve higher load capacities with shorter piles. Screw piles with more than one helix are always machine-installable models.
How deep should screw piles be installed?
The helix of the screw pile should be installed at least below the frost line. This ensures that the foundations will not be susceptible to frost movements. The installation depth is also determined by the soil at the place of installation, together with the desired load capacity and lateral support.
How close to each other should screw piles be installed?
The installation distance is normally decided by a structural engineer. For small buildings such as terraces and courtyard buildings, the screw piles are normally installed with a span of about 1.5-3 m, depending e.g. upon the weight of the building, the soil type, and the strength of the lower beam.
How is a screw pile installed to the right depth?
Screw piles are installed to the right depth by turning in a clockwise direction, like tightening a screw. It is not recommended to adjust the height by screwing counter-clockwise, because the screw pile's helix has already broken the soil below the pile, and the pile might therefore settle. A screw pile can also be cut to the required height with an angle cutter, if rotating it to the correct height for some reason does not succeed.
Does the pipe of a screw pile need to be filled with concrete after installation?
Normally, the pile pipe does not need to be filled with concrete after installation, but if the water table is closer than 1 m from the ground surface, it is recommended that the pile pipe be filled with grouting, urethane foam, or sand. Filling the pile pipe with concrete might also improve the pile's resistance to buckling and avoid one-sided pile pipe corrosion.
How can screw piles be extended?
Hot-dip galvanized screw piles are extended with extension sleeves attached by bolted joints, and untreated screw piles by welding. Pile extensions are available in multiples of 1 m.
How does one build up from a pile?
A variety of fasteners is available for screw piles, such as U-shoes, cap plates, bolt fixtures and pile caps. U-shoes, bolt mounts, and flat steel are well suited for fixing timber to screw piles. When the screw pile is left in the concreting, pile caps should be used over the piles. It is also possible to weld steel beams onto the screw piles, on top of which e.g. a composite slab or hollow-core slab can be installed.
Can the plinth of the building be covered so that the screw piles cannot be seen below the building?
The plinth can be covered, for example, with wooden lathing or an acrylic plinth that looks almost like a real plinth. The parts of screw piles that remain on the surface can also be painted, plastic heat-shrink tubing can be installed over them, or the pile pipes can be clad by adding hollow concrete blocks around them.
What surface treatment is applied to screw piles?
All screw piles with 60.3 x 2.9 mm tubes are delivered hot-dip galvanized (zinc layer thickness about 110-140 µm). The larger screw piles have a 6.3 mm wall thickness and are provided without surface treatment, since allowance for corrosion has been included in the material strength.
What kind of installation equipment is used to install screw piles?
Hand-installable screw piles can be installed using a crowbar or similar lever. The installation can also be done using a portable auger. Machine installable screw piles are usually installed using an excavator, wheel loader or similar vehicle, to which a hydraulic auger is attached. Installation equipment comes in a wide variety of sizes, from 0.8-ton mini diggers to over 15 ton excavators. Most installation is done using smaller machines that do not damage lawn areas and plantings.
Is a sizing guide available for screw piles?
Paalupiste Oy can provide structural engineers with Finnish-language geotechnical sizing guides for screw piles. Designers can also get the PaPi-CAP pile-calculating computer program, which automatically calculates the correct sizes of screw piles from the supplied target loads and site investigation results.