Screwpiles are wound into the ground using rotary hydraulics fitted to various earthmoving equipment. Helical formed or flat plates welded to the screwpiles steel CHS shaft utilize the soils bearing and shear capacities to support the required design load.
In compression screwpiles are classified as displacement end bearing piles, with the emphasis on end bearing. Due to the fact that a screwpiles shaft size is proportionately small, in comparison to other piles, the displacement effect and benefit is limited tends to be ignored in engineering design.
In tension screwpile ground anchors rely on soil overburden pressures, soil sheer angles of friction, soil density and saturation. Screwpiles work effectively in both tension and compression and once installed load capacity can be utilised immediately unlike conventional piling methods such as concrete or grout piles.
The connection between a structure and a screwpile is called the interface. The interface can be of direct weld connection via connecting brackets or by using the existing concrete footing with added reinforcement to transfer the loads and eliminate concrete crushing.
A screwpile may consist of one or more spliced CHS sections which combined are installed to the engineered specified minimum target depth. Helix's are positioned at the base and at other stratigic locations along the length of the piles shaft. The deepest screwpiling installed to date is 33m.
Where limited headroom and access exists multiple sectional piles are used. Sectional joint splicing is likely to consist of a bolted coupling for pile loads <200kN and welded joints for piles with loads >200kN and up to 2000kN.
The type of interface available is as varied as the size and capacity of screwpile available.