Annex A (normative)
Helical steel pile foundations
COMMENTARY ON Annex A
This annex applies to the design and construction of helical steel piles.
Helical (screw) piles generally comprise a lead section, which comprises helix plates attached to a central steel shaft, and extension sections.
Helical steel piles are typically suitable for use in loose to medium dense coarse soils and low to medium strength fine soils.
The design of helical steel pile foundations should conform to
A.2 Design considerations
A.2.1.1 The design of helical steel pile foundations should conform to 6.3.1.
A.2.1.2 The helix pitch should not be less than 75 mm.
A.2.1.3 The gradient of the helix plates should be constant throughout their pitch.
A.2.1.4 The outer diameter of the helix should be circular in plan and undamaged by the forming process.
A.2.1.5 The shaft of a helical pile should be manufactured from circular hollow sections (CHS) or square sections, with the former being preferred.
A.2.1.6 To prevent formation of an annulus around the helical pile shaft, the diameter of any shaft extensions should be constant or decrease with depth. The diameter of any oversized connections should also be constant or decrese with depth.
A.2.1.7 Connections between shaft extensions should allow full transfer of bending moments (i.e. they should not be a pinned connection). Connections should not oversize the hole formed during installation to create an annulus around the helical pile shaft.
A.2.1.8 Where threaded connections are used, the direction of the thread should be opposite to the direction of rotation during installation. A locking device for the connection should be used.
A.2.1.9 The design of helical piles should be based on a conventional soil mechanics approach supported by pile testing data where this is combined with an empirical approach.
A.2.1.10 Helical piles should not be designed solely on empirical rules relating soil resistance to the applied torque measured during pile installation.
A.2.1.11 Particular consideration should be given to the effects of the following:
- actions that are applied repeatedly;
- actions with varying intensity;
- actions that produce a dynamic response in the structure or the ground; and
- water pressures.
A.2.2 Ground investigation
Ground investigation for helical pile foundations should conform to BS EN 12699.
A.2.3 Pile spacing
A.2.3.1 The plan location and inclination of helical piles should conform to BS EN 12699.
A.2.3.2 The bearing resistance of helical piles spaced closer than 4 helix diameters (centre-to-centre on plan) should be reduced to account for interaction between the helices.
A.2.4 Calculation models
NOTE 1 Guidance on the design of helical piles can be found in Helical piles – a practical guide to design and installation .
NOTE 2 The tensile resistance of a helical pile is typically calculated as the sum of the resistances of its helix plates acting in pull-out. This is known as the «reverse bearing capacity model». This calculation model ignores any beneficial effect of shaft friction.
A.3.1 Materials used to fabricate helical steel piles should conform to 6.5.2.
A.3.2 Hot finished helical steel piles should also conform to BS EN 10210.
A.3.3 Cold formed helical steel piles should also conform to BS EN 10219.
A.3.4 Helical steel piles should be manufactured in accordance with BS EN 1090.
The durability of helical steel piles should conform to 6.6.
A.5 Ultimate limit state design
A.5.1.1 For the purpose of selecting partial factors from BS EN 1997-1, helical piles should be classified as shown in Table 8, Table 9 and Table 10.
A.5.1.2 For helical piles, the following potential ultimate limit states should be checked:
- failure of a helix in bearing;
- a combination of helix bearing failure and plugged shaft failure;
- base punching failure into underlying weak layers; and
- rotational failure mechanisms caused by the application of torque to the pile head (typically only relevant to single piles or pairs of piles).
A.5.1.3 The potential contribution to bearing resistance from friction on the pile shaft should be ignored, unless there is definitive evidence from pile load tests that the soil between the helices does not form a plugged shaft.
NOTE Plugged shaft failure might not need to be checked if there is sufficient vertical spacing between the helices to ensure that a plugged shaft does not form.
The potential contribution to pull-out resistance from friction on the pile shaft should be ignored, unless there is definitive evidence from pile load tests that the soil between the helices does not form a plugged shaft.
A.6 Structural design
The structural design of helical piles should consider:
- compressive, tensile, and bending resistance of the shaft;
- torsional resistance of the shaft;
- buckling resistance of the shaft, particularly in the absence of lateral restraint from the ground, for example if installation postholing occurs;
- failure of the connections between shaft segments;
- structural failure of the helix plate;
- failure of the welds between the shaft and the helix; and
- weakening of the pile as a result of corrosion.
A.7.1 The penetration rate of a helical pile per revolution of the pile (v) should satisfy:
p is the pitch between adjacent helices.
A.7.2 A variable axial force should be applied to the pile head to ensure that (A.1) is satisfied.
NOTE The axial force applied to the pile head to advance the pile into the ground is commonly known as the «crowd force» (or just «crowd»).
A.7.3 If the penetration rate falls outside the limits given above, the design resistance of the pile should be re-assessed and downgraded appropriately unless testing validates the pile's resistance.
A.7.4 Installation of a helical pile should cease immediately if its penetration rate falls below 0.5p. The pile's design resistance should be reassessed and downgraded appropriately.
A.7.5 If the rotation of a helical pile is reversed during installation (i.e. the pile is «back-spun»), then its design resistance should be re-assessed and downgraded, if appropriate, unless testing validates the pile's resistance.
A.7.6 Installation of a helical pile should cease immediately if the installation torque exceeds its maximum design value before the pile has reached its design depth of penetration.
A.7.7 The rotation speed during installation of a helical pile should:
- not exceed 20 revolutions per minute;
- be appropriate to the anticipated ground conditions;
- be appropriate for the responsiveness of the installation equipment; and
- be reduced appropriately where unacceptable penetration rates are observed.
A.7.8 If pre-drilling is used to assist pile installation, the diameter of the pre-drilled hole should not exceed the pile's shaft diameter. To prevent gaps occurring between the pile shaft and the surrounding ground, pre-drilling should not result in the removal of material.
A.7.9 Helical piles should be removed from the ground by unscrewing, in a similar manner to their installation.
A.7.10 A helical pile should not be re-used after removal without it first being confirmed by inspection (by either the manufacturer or installer) that the pile's condition is such that it continues to provide the required design strength and durability. The results of this inspection should be recorded on the piling record.
A.7.11 If a new pile is to be installed in the same location as a previous removed helical pile, proper allowance should be made in the design for the disturbance of the ground caused by the previous pile.
A.7.12 The mean installation torque measured over the final 1 m of installation should be used to check compliance with the design torque requirements.
A.7.13 If the torsional resistance of the pile shaft is exceeded prior to achieving the required depth of installation, either the installation should be terminated at the depth reached or the pile should be replaced by a new one with smaller diameter helices, fewer plates, or a larger shaft diameter.
A.7.14 If the minimum design installation torque is not achieved at the required depth of installation:
- the depth of penetration should be increased by adding extension sections to the pile; or
- the pile should be replaced by a new one with additional and/or larger diameter plates; or
- the pile should be relocated.
A.8.1 The following information should be collected during the execution of helical piles:
- installation torque should be monitored throughout installation and recorded during the last 1 m of penetration (at intervals not less than 0.5 m);
- rotation speed and penetration rate should be monitored throughout installation to ensure the pile is not flighting; and
- depth and diameter of any pre-drilling, details of any pile flighting, and details of any pre-augering should be recorded.
A.8.2 This information should be collected at intervals not exceeding 500 mm of penetration.
A.8.3 Appropriate adjustments should be made to the installation procedure and, when necessary, the pile resistance, based on the information recorded.
A.9.1 Ground Investigation Report
The Ground Investigation Report for a helical pile foundation should conform to 4.12.1.
A.9.2 Geotechnical Design Report
The Geotechnical Design Report for a helical pile foundation should conform to 6.13.2 and should also include, where appropriate, assumed design values for torque and any correlations on which these values are based.
A.9.3 Geotechnical Feedback Report
The Geotechnical Feedback Report for a helical pile foundation should conform to 6.13.3 and should also include, where appropriate:
- installation torque records and associated calibration data;
- number of pile rotations per minute;
- rate of penetration (distance travelled per revolution and per unit of time);
- depth and diameter of any pre-drilling;
- details of any pile flighting or augering;
- final depth of bottom plate.