Helical Pile Design

DESIGN ENGINEERING

INTERNATIONAL LEADERS IN HELICAL PILE TECHNOLOGY

DESIGN

Helical (screw) Pile designs are developed by GTL’s in-house Engineers taking into the account the performance specifications of the Client and the available geotechnical information for the project.

The design of the GTL Helical (screw) pile utilises the same conventional geotechnical theory as used for the design of CFA or rotary bored piles.

GTL do not use torque as a basis of design nor for pile set. Torque versus soil stiffness is used on site to monitor variable soil conditions that may affect the performance of the piles. Piles are designed and plate spacing and depths set on consideration of the geotechnical site investigation information.

Helical (screw) piles can be designed to take into account compression, tension loads and load reversals.

The nature of the Helical (screw) pile is such that it accommodates the forces from expansive soils and negative skin friction. Lateral loads can be designed for using pile groups, single piles with larger diameter steel sections or GTL’s centrifugal pile to enhance lateral capacity.

SOIL INVESTIGATIONS

Geotechnical Engineering plays an essential role in Helical (screw) Pile engineering works. 

A good ground investigation is key to providing a sound engineered design and to reduce risk at all stages of the project. 

Therefore GTL recommend that a ground investigation is carried out at the earliest possible stage and to a sufficient depth to thoroughly explore the soil conditions under the site.

DESIGN FACTS

  • Circular-shaped Helical pile shafts offer higher resistance in loading than square-shaped shafts
  • 3 main factors affecting the ultimate bearing capacity are helical spacing, pile embedment and soil properties
  • Cohesive soils under  compressive  loading  with  larger helical spacing have larger ultimate capacities, whereas cohesionless soils under compressive loading with smaller helical spacing have larger ultimate capacities
  • The individual bearing method affirms that the more helical plates there are, the greater the resistance, whereas the cylindrical shear method increases the resistance of Helical pile which have increased helical spacing
  • An increase in the embedment ratio is known to increase bearing capacity in both cohesive and cohesionless soils under both compressive and tensile loads; under tensile loads, this increase is larger
  • The larger the  embedment  ratio,  the higher the resistance under compressive than uplift loads due to effect of the end- bearing resistance
  • The cylindrical shear  method  reaches  its  peak  at  a  lower displacement than the individual bearing method which relies on individual bearing as opposed to a shear failure zone
  • Helical piles in compression offer higher resistance than tension due to an increased surface area through the soil plug
  • The relationship between the installation torque and the bearing capacity is evidentiary with the average installation torque representing the soil conditions of the screw pile under tension. In compression, the soil below the lowest helix is undisturbed; and therefore, the installation torque is not a true indicator of the soil and its bearing capacity.
  • Once the number of helices surpasses the sufficient minimum, there is no effect on the pile capacity when dealing with correlations with the installation torque and the ultimate bearing capacity.

GROUND COMPATIBILITY

  • Slender shaft reducing negative skin friction
  • Ability to spread load in weak strata
  • Ability to develop high tension loads
  • Ability to develop high end bearing loads
  • Ease of extension and reaching greater depths
  • Ductile geotechnical failure mechanism
  • Cyclical loading performance
  • to deal with the unexpected while minimising the costs

STRUCTURE COMPATIBILITY

  • Designing to resist fatigue
  • Designing to tight deflectioN criteria
  • Ductility performance
  • Elastic shortening
  • Durability
  • Seismic failure mechanism
  • Simplicity of connection
  • Being able to align loads with anchors

CORROSION

Corrosion of piles is assessed by studying the available soil investigation information, the contamination testing results and the soil resistivity readings.

Generally GTL calculate sacrificial steel loss. If little or no information is available regarding the

potential corrosion risk to the piles, a worst case scenario will be assumed based on the published guidance and estimated corrosion rates stated in BS8004 and Eurocode 3 Part 5 – Steel Piling.