Particular Specification for Tension Piles

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Structural Engineering Branch, Arch SD

Page
2

of 5

File Code: SEI0405.doc

Technical Instruction No. 4/2005


WWL/MKSW/WKF

Issue Date: 14 June 2005 Revision Date:

Particular Specification for Tension Piles



1.0 General

The following types of piles shall be permitted to be designed as tension
piles:

(a) Rock-socketed steel H-piles
(b) Minipiles
(c) Large diameter bored piles

The requirements stipulated in the Particular Specification for Rock-
Socketed Steel H-piles and the Particular Specification for Minipiles shall
be applicable except those modified by this Particular Specification.

Static loading tests are required to justify the tension capacity of piles.
Should tension piles be proposed, the contractor shall bear the cost and
time of the required tests.


2.0 Piles subjected to Uplift Forces


In general, the anchorage resistance (R
a
) of a pile may be taken as:

R
a
= allowable uplift resistance of pile shaft + effective self weight
of pile


3.0 Uplift Resistance of Individual Pile

The safe uplift resistance of individual pile shall be the least of the
following:
• the design bond stress between bedrock and grout/concrete
times the area of contact between the rock and
grout/concrete below the effective rock socket
top level; or
• the design bond stress between the pile and grout times the
total area of contact between the pile and grout below the
effective rock socket top level; or
• the allowable axial force of the steel H-section or steel
reinforcements forming the pile; or
• ½ of the dead weight of the mass of rock cone anchoring
the pile.

In assessing th
e socket bond strength between bedrock and
grout/concrete, the possible negative Poisson’s ratio effect should be
taken into consideration. This may result in a reduction of bond capacity

Structural Engineering Branch, Arch SD

Page
3

of 5

File Code: SEI0405.doc

Technical Instruction No. 4/2005


WWL/MKSW/WKF

Issue Date: 14 June 2005 Revision Date:

when compared with the bond capacity of piles in compression. This
reduction shall be taken as not less than 50%.



4.0 Uplift Resistance of a Pile Group

The safe uplift resistance of a pile group shall be the lesser of the
following:

(a) the sum of the allowable anchorage resistance of the piles in the
group;
(b) the allowable shear resistance mobilised on the surface perimeter of
the group plus the effective weight of soil and piles enclosed in this
perimeter; and
(c) 50% of the total weight of the rock/soil cones anchoring the piles
with an assumed half angle which should not be greater than 30°.

Due consideration should be given to the effect of overlapping of cones
due to proximity of adjacent piles. Submerged weight of rock/soil
should be used where appropriate. Shear at interface between cone
surface and surrounding rock should be neglected.



5.0 Pile Head Details

The pile head and pile cap connection shall be modified and be designed
to transmit the maximum pile loads in both tension and compression.



6.0 Static Loading Tests

The test load shall not be less than 2

times the design anchorage
resistance of the pile.

Loading test procedure given in the General Specification should be
followed.

If reaction piles are used for tension test, to minimize interaction effects,
the reaction piles shall be located as far from the test pile as practicable.
In any case, the reaction pile shall be at least 3 test pile diameters, or 2 m,
whichever is larger, from the test pile, measured centre to centre.

The test shall be deemed to be unsatisfactory if any one of the following
conditions applies :



Structural Engineering Branch, Arch SD

Page
4

of 5

File Code: SEI0405.doc

Technical Instruction No. 4/2005


WWL/MKSW/WKF

Issue Date: 14 June 2005 Revision Date:

(a)

the maximum extension at the head of the pile during the test exceeds
the elastic extension (PL/AE) of the net length of the pile plus 4 mm;

(b) when the rate of recovery after the removal of the maximum test load
is less than 0.1mm / hour observed in a period of not less than 15
minutes, the residual extension at the head of the pile exceeds the
greater of 4mm and 25% of the maximum pile head extension during
the test; or

(c) there is structural failure in the test pile.

The maximum test load shall not result in the test pile or anchor being
stressed beyond the yield stress.

In calculating the elastic extension of the pile, the net pile length (L) shall
be measured to the centre of the rock socket.



































Structural Engineering Branch, Arch SD

Page
5

of 5

File Code: SEI0405.doc

Technical Instruction No. 4/2005


WWL/MKSW/WKF

Issue Date: 14 June 2005 Revision Date:

Appendix 1

Internal Guidelines for allowable bond stresses in Tension Piles




Concrete/Grout
Strength (MPa)
Allowable bon
d
stress between
bedrock and
concrete/grout
(MPa)

Allowable bond
stress between
grout and steel
section (MPa)

Allowable bond stress
between
concrete/grout and
grade 460
reinforcement (MPa)
30

0.35

0.48

0.80

25

0.32

0.43

0.72




Note: 1. Bedrock should be grade III or better rock as defined in Geoguide 3,
with core recovery greater than 90%.
2. The allowable bond stresses between bedrock and concrete/grout are
based on Table 2.2 of the Code of Practice for Foundations. A
reduction of 50% has been taken when compared with the allowable
bond stresses for compression piles.
3. The allowable bond stresses between grout and steel sections are
based on clause 25.5(3) of the Code of Practice for Foundations.
4. The allowable bond stresses between concrete/grout and grade 460
reinforcement are based on ASD’s P.S. for Minipiles.