Why do multiturn type helix work more effectively with better cost efficiency than traditional plate helix piles?
Traditionally Driven or Screwed piles have relied on the End Bearing principle for the capacity of the pile. A simple mathematical calculation for Engineers relates the Total End Surface Area of the pile end against the measured Load Bearing Capacity of the Footing ground material. Some piles have a rough external surface which creates a friction allowance that will increase the Compression capacity of the pile eg Driven Concrete or Timber Piles but this added capacity is very imprecise and uncertain. Steel piles, either driven steel beam piles or screwed piles have no extra load capacity for friction due to their smooth external surface and rely solely on End Bearing Capacity which is an accurate way of assessing the pile load capacity.
A Multiturn Type screw pile reacts similarly to a steel plate pile in lower strength soils but once the Founding Material becomes Medium Dense or greater, the Multiturn helix behaves quite differently to a Plate helix in that it rapidly increases in Compression Load capacity due to what is now known as the “ BULB EFFECT “, This phenomena, first explained by Sydney University School of Civil Engineering under the Leadership of Professor David Airey, showed from extensive testing that Dense Ground Material ( Usually designated by N values greater than 20 or DCP,S greater than 15 ) compacts between the Multiple steel flights of the helix to create a solid core of layered steel and founding material which then transferred some of the Compression load laterally out of the helix into the surrounding ground material, rather than all of the load being only transferred vertically down.
After exhaustive site testing over more than 20 years, the capacity increase of a Multiturn Steel Helix over a Single Turn Plate Helix of the same diameter, has been proven by Beam Load Testing to increase the load capacity by 1.8 times in dense clays and 2.2 times in dense sand. Using Strain gauges and multiple size helix, The University was able to show that the lateral load dispersion was not uniform in its spread but increased as the Gauges measured load transfer more closer to the vertical down, giving a dispersion of an egg shaped Bulb hence the nominated phenomena of the “ BULB THEORY “. This testing by Sydney University Civil Engineering is available as a complete Report, “ THE FINITE ELEMENT ANALYSIS OF SCREW PILES. REPORT S1622 “and is available from the Association and its Members.
In effect this testing and performance outcome clearly indicates that a Multiturn Type Helix transforms from a simple End Bearing load pile to a Conical Shaped Bearing pile with the Conical Surface Area being much greater ( approx double the area ) than the area of the original helix diameter.
Work continues to be done by Sydney University Civil Engineering to develop a simple Engineering Formula to be used in conjunction with the Range of Multiturn Type helix available, for all Engineers, to allow complete confidence in calculating Compression Load Capacities for Multiturn Helix Screw Piles.
Addendums to be read in conjunction with this Technical Comment are enclosed or available from the Multiturn Screw Pile Association of Australia Inc.to expand the understanding of the “ BULB EFFECT “ of Multiturn helix and their benefits to the Foundation and Building Industries.
Addenda.Why use a Multiturn Helix Screw Pile in lieu of other piling options.Current calculations used when designing with Multiturn helix.Calculations on Plate diameter areas, Cone areas and Multiturn flight areas.Benefits of Variable thickness with a Cast Steel helix over Single plate helix.
A Multiturn Type screw pile reacts similarly to a steel plate pile in lower strength soils but once the Founding Material becomes Medium Dense or greater, the Multiturn helix behaves quite differently to a Plate helix in that it rapidly increases in Compression Load capacity due to what is now known as the “ BULB EFFECT “, This phenomena, first explained by Sydney University School of Civil Engineering under the Leadership of Professor David Airey, showed from extensive testing that Dense Ground Material ( Usually designated by N values greater than 20 or DCP,S greater than 15 ) compacts between the Multiple steel flights of the helix to create a solid core of layered steel and founding material which then transferred some of the Compression load laterally out of the helix into the surrounding ground material, rather than all of the load being only transferred vertically down.
After exhaustive site testing over more than 20 years, the capacity increase of a Multiturn Steel Helix over a Single Turn Plate Helix of the same diameter, has been proven by Beam Load Testing to increase the load capacity by 1.8 times in dense clays and 2.2 times in dense sand. Using Strain gauges and multiple size helix, The University was able to show that the lateral load dispersion was not uniform in its spread but increased as the Gauges measured load transfer more closer to the vertical down, giving a dispersion of an egg shaped Bulb hence the nominated phenomena of the “ BULB THEORY “. This testing by Sydney University Civil Engineering is available as a complete Report, “ THE FINITE ELEMENT ANALYSIS OF SCREW PILES. REPORT S1622 “and is available from the Association and its Members.
In effect this testing and performance outcome clearly indicates that a Multiturn Type Helix transforms from a simple End Bearing load pile to a Conical Shaped Bearing pile with the Conical Surface Area being much greater ( approx double the area ) than the area of the original helix diameter.
Work continues to be done by Sydney University Civil Engineering to develop a simple Engineering Formula to be used in conjunction with the Range of Multiturn Type helix available, for all Engineers, to allow complete confidence in calculating Compression Load Capacities for Multiturn Helix Screw Piles.
Addendums to be read in conjunction with this Technical Comment are enclosed or available from the Multiturn Screw Pile Association of Australia Inc.to expand the understanding of the “ BULB EFFECT “ of Multiturn helix and their benefits to the Foundation and Building Industries.
Addenda.Why use a Multiturn Helix Screw Pile in lieu of other piling options.Current calculations used when designing with Multiturn helix.Calculations on Plate diameter areas, Cone areas and Multiturn flight areas.Benefits of Variable thickness with a Cast Steel helix over Single plate helix.
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