The end bearing element of a screw in foundation can consist of single or multiple helical steel plates formed to a specified pitch or flat bladed plates of various profiles welded to either side of the piles shaft.
Only end bearing plates formed into a spiral can be classified as a helix. A flat bearing plate is simply that when used on a pile, therefore only piles with helix's can be truely classed as a screwpile. Piles that soley use flat plates are simply bladed piles that are wound into the ground. They are not screwpiles!
A true helix is formed to a specified pitch and is actually part of a spiral. It is important the pitch of a helix is uniform in shape so a single track entry is produced through the soil. A single track allows a helix to pare through the soil, reducing any disturbed natural soils shear values. This combined with the fact that a true screwpile is a dual acting displacement end bearing pile enhances overall pile capacities.
The same is not the case for bladed end bearing piles as blade bearing plates are not helix's, they are simply flat plates welded at opposing angles along a piles shaft. Opposing angle flat bearing plates generate two soil entry paths and as such core rather than pare through soil. Like an auger, bladed bearing plates have the ability to cut into soft rocks such as coffee rocks and limestones due to the dual soil entry configuration. The harder the rock the higher the steel grade necessary for the bearing plates leading edges.
The jury is still out as to whether there is any significant performance advantages of one configuration over the other, but it stands to reason that a single track entry causes less soil disturbance and thereby achieves greater pile capacity performance particularly when the pile is acting in tension. Comprehensive testing will determine performance comparitives.
Since 1999 we have use a variety of end bearing elements from helical shapes to flat plates for clay and sand sites. At sites where hard ground such as limestone rock was to be encountered a combination of helical and flat bladed end bearing elements are used such as our RET Technology.
Some examples of the various helical and bladed end bearing elements are show.