Particular Specification for Tension PilesRev1).pdf · resistance between the surface of the pile and the soil along the pile depth. The perimeter of the Steel ‘H’ pile for uplift

Structural Engineering Branch, Arch SD SE Instruction No. 04/2005 1st Issue Date: June 2005

Page 1 of 13 File code: SEI0405(Rev1).doc KLT/MKW/KWL Revision No : 1 Revision Date: June 2019

Particular Specification for Tension Piles 1.0 General

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

*(i) Rock-socketed Steel H-piles (RSSHP) *(ii) Minipiles *(iii) Large diameter bored piles *(iv) Steel ‘H’ piles

* delete as appropriate.

1.2 For large diameter bored piles, the requirements stipulated in the Particular Specification for Large Diameter Bored Piles Socketed into Bedrocks shall be applicable except those modified by this Particular Specification.

1.3 The allowable anchorage resistance Ra and the ultimate anchorage resistance Ru

of a pile shall be taken as:

Ra = allowable uplift resistance of pile + effective self weight of pile; and

Ru = ultimate uplift resistance of pile + effective self weight of pile The allowable uplift resistance and ultimate uplift resistance of pile may be derived from Clause 2 and 3 of this Particular Specification.

1.4 The following criteria under transient and permanent tension shall both be satisfied for the design: Under Transient Tension (a) Dmin + 0.9Ru - 2.0Ia - 1.5 Ua (or 1.1Up) - 1.5Wk ≧ 0

(b) Dmin + Ra - Ia - Ua - Wk ≥ 0



Under Permanent Tension (c) Dmin + 0.9Ru - 2.0Ia - 1.5 Ua (or 1.1Up) ≧ 0 (d) Dmin + Ra - Ia - Ua ≥ 0

where Dmin = Minimum dead load

Ra = Allowable anchorage resistance of the pile Ru = Ultimate anchorage resistance of the pile Ia = Adverse imposed load including soil loads Ua = Uplift due to the highest anticipated groundwater table Up = Uplift due to the highest possible groundwater table Wk = Adverse wind load

The highest anticipated groundwater table and the highest possible groundwater table shall be as specified on the drawings.

1.5 Except for large diameter bored piles, tension piles of the same type and size

shall be designed and constructed to the same Ra and Ru under the respective transient or permanent tension.

1.6 Tension piles under the same building structure shall be of the same type. Except for large diameter bored piles, tension piles under the same pile cap shall be of the same size and same Ra and Ru under the respective transient or permanent tension.

1.7 Definition of transient tension and permanent tension:

(a) Transient tension shall mean tension in a foundation element induced by wind load and load combination with wind.

(b) Permanent tension shall mean tension induced to the foundation that is not

categorised as transient tension.

2.0 Piles with rock socket

2.1 The allowable uplift resistance of individual pile shall not exceed the least of the



following: (a) the allowable bond stress between rock and concrete/grout times the area

of contact between the rock and concrete/grout below the effective rock socket top level; or

(b) the allowable bond stress between the steel H-section and grout times the total area of contact between the steel H-section and grout below the effective rock socket top level, for RSSHP; or

(c) the allowable bond stress between the reinforcement and concrete/grout times the effective area of contact between the reinforcement and concrete/grout, below the effective rock socket top level, for minipiles and large diameter bored pile. For minipiles, the effective area of contact shall be calculated from the effective diameter of a group of bars (with spacing between adjacent bars less than 25mm) which shall be equivalent to the diameter of a bar of equal total area; or

(d) the allowable axial force for the steel H-section or steel reinforcements forming the pile.

The ultimate uplift resistance shall be taken as 2 times the allowable uplift resistance in this sub-clause and shall not exceed the total effective weight of the rock cone and the soil column directly above the rock cone derived in accordance with Clause 2.3 and Figure 1. In case the ultimate uplift resistance is controlled by the total effective weight of the rock cone and the soil column directly above the rock cone, the allowable uplift resistance shall be limited by 50% of the total effective weight of the rock cone and the soil column directly above the rock cone.

2.2 Design Assumptions

2.2.1. Allowable bond stress between rock and concrete/grout for piles The presumed allowable values of bond or friction between rock and concrete/grout of the pile are given below:



Description of rock (granite and volcanic)

Pile Type

Presumed allowable bond or friction between rock and concrete/grout (kPa)

Under transient tension

Under permanent

tension Grade II or better rock with total core recovery greater than 95% for any one metre within the rock mass and minimum uniaxial compressive strength (UCS) not less than 50 MPa (equivalent point load index strength PLI50 not less than 2 MPa)

RSSHP, Minipiles and

Large diameter

bored piles

700 350

Grade III or better rock with total core recovery greater than 85% for any one metre within the rock mass and minimum uniaxial compressive strength (UCS) not less than 25 MPa (equivalent point load index strength PLI50 not less than 1 MPa)

RSSHP and Minipiles

700 350

Large diameter

bored piles 500 250

Notes: (i) Concrete/grout shall have a minimum cube strength of 30 MPa at 28 days.

(ii) The point load index strength of rock quoted is the equivalent value for 50 mm diameter cores.

(iii) The rock socket shall be formed in granite or volcanic rock. (iv) No deduction is required on the presumed allowable bond or friction

between rock and concrete/grout when concreting/grouting under water.

2.2.2. Allowable bond stress between grout and steel H-pile of RSSHP

The maximum allowable average bond stress between grout and steel H-pile shall be 0.48 MPa for grout having minimum cube strength of 30MPa at 28 days when the shear bars detail shown in Annex “E” of GS section 5 is applied. No deduction is required on this maximum allowable average bond stress when grouting under water.



2.2.3. Allowable bond stress between concrete/grout and reinforcement of

minipiles and large diameter bored pile The maximum allowable average bond stress between concrete/grout and reinforcement for ribbed steel reinforcing bars in Grade 500 shall not exceed 0.8MPa for concrete/grout having minimum cube strength of 30MPa at 28 days. No deduction is required on this maximum allowable average bond stress when concreting/grouting under water.

2.2.4. Allowable axial working stress of steel H-pile sections

The maximum allowable axial working stress of steel H-pile sections shall be 45% of the yield stress.

2.2.5. Allowable axial working stress of reinforcement.

The maximum allowable axial working stress of reinforcement shall not exceed 225 MPa for ribbed steel reinforcing bars in Grade 500.

2.2.6. The number of steel reinforcing bars for a minipile shall not be more than 5 with bar diameter not exceeding 50mm.

2.3 Assessment of the effective weight of the rock cone and soil column

For single or group of closely-spaced piles (i.e. with overlapping rock cone/soil column) that derive the ultimate uplift resistance from rock socket, the configuration of the rock cone/soil column as given in Figure 1 shall be used, and the assessment of the effective weight of the rock cone and soil column shall be based on the following assumptions:

(a) The half angle of the rock cone at the toe of the pile shall not exceed 30

degree measuring from the vertical.

(b) Only the column of overburden soil directly above the rock cone shall be considered, and the soil friction at the vertical face of such soil column above the rock cone shall be ignored.



(c) Effective weight of the rock cone and the soil column shall be adopted. Any

part of the rock cone or soil column falling outside the site boundary shall be ignored.

(d) For a group of closely-spaced piles subjected to tension, overlapping effect

shall be considered when assessing the volume of rock/soil cone to be used for resisting the combined uplift force.

(e) For a group of piles with same allowable uplift resistance of individual piles,

checking of rock/soil cone failure of individual pile is not necessary when the group effect has been considered as stated in (d) above.

(f) Where the allowable uplift resistance of individual piles within a pile group

are not the same, checking of rock/soil cone failure of individual pile is required. The effective weight of the overlapping part of rock cones between piles may be distributed to each pile on a pro-rata basis according to the allowable uplift resistance of the individual piles.



Figure 1 - Configuration of rock cone/soil column for rock socketed piles

3.0 Steel ‘H’ piles in granular soil The allowable/ultimate uplift resistance of individual pile may be derived from and shall not exceed the least of Clause 3.1, 3.2 and 3.3 of this Particular Specification. In addition, the ultimate uplift resistance of individual piles shall not exceed the value as derived from Clause 3.4 of this Particular Specification. In case the ultimate uplift



resistance is controlled by the total effective weight of the soil cone/soil column, the allowable uplift resistance shall be limited by 50% of the total effective weight of the soil cone/soil column. 3.1 The allowable uplift resistance of individual pile shall not exceed the allowable

axial force for the steel ‘H’ pile section. The maximum allowable working stress of steel ‘H’ pile section shall be 30% of the yield stress. The ultimate uplift resistance of pile shall be taken as 2 times the allowable uplift resistance of pile.

3.2 The allowable and ultimate uplift resistance of individual pile under transient tension loads Piles in granular soil may derive the uplift resistance from the frictional resistance between the surface of the pile and the soil along the pile depth. The perimeter of the Steel ‘H’ pile for uplift resistance assessment shall be taken as 2 x (breath + depth) of the H section. The allowable and ultimate uplift resistance of steel ‘H’ piles under transient tension loads may be assessed by one of the following methods:

(a) Uniform shaft friction method

A uniform allowable shaft friction of not greater than 10 kPa may be used for soil strata with SPT N values not less than ten. The ultimate uplift resistance of pile shall be taken as 2 times the allowable uplift resistance of pile.

(b) Effective stress method (or commonly known as the β-Method)

τs = β σv', but not exceeding 120kPa

where τs = ultimate uplift resistance under transient tension

β = shaft resistance coefficient σv' = mean vertical effective stress (kPa)

For steel ‘H’ piles in granular soil, β typically ranges from 0.1 to 0.4.



Verification by trial piles should be required unless the criteria for the soil parameters set out below are all complied with and τs is limited by not exceeding 60 kPa.

Soil parameters adopted in design Criteria SPT N-values not less than 20

Density not exceeding 20 kN/m3

Effective density not exceeding 10 kN/m3

β-value not exceeding 0.2

(c) Empirical method by correlation with SPT N-values. τs = 1.5N, but not exceeding 120kPa where τs = ultimate uplift resistance under transient tension

N = uncorrected mean SPT value in the soil strata where shaft resistance is being mobilized

Justification by trial piles should generally be required. However, verification by trial piles may be waived if τs is taken as 50% of the recommended value, i.e. τs = 0.75N, but not exceeding 60 kPa.

For method (c), before commencement of pile installation, additional drill holes shall be carried out to obtain sufficient information for determination of SPT N-values. The additional drill holes shall be in sufficient number such that any pile tip shall not be more than 5m away from the nearest drill hole. This predrilling shall be carried out by an independent Ground Investigation Contractor from the List of Approved Suppliers of Materials and Specialist Contractors for Public Works – Ground Investigation Field Work Category. The Contractor shall bear the cost and time of the required additional drill holes. Submit two copies of the drill-hole logs to the SO within 3 days of the predrilling.

For method (b) or (c) in Clause 3.2, the allowable uplift resistance of the pile under transient tension loads may be obtained by applying a factor of safety to the ultimate uplift resistance of the pile under transient tension loads. The applied factor of safety should be taken not less than 2.



3.3 The allowable and ultimate uplift resistance of individual pile under permanent

tension loads

The allowable and ultimate uplift resistance of individual pile under permanent tension loads shall be taken as:

(a) 100% of the allowable and ultimate uplift resistance calculated in

accordance with method (a) in Clause 3.2; or

(b) 50% of the allowable and ultimate uplift resistance calculated in accordance with method (b) or (c) in Clause 3.2.

3.4 Assessment of the effective weight of the soil cone/soil column

For a group of closely-spaced piles (i.e. with overlapping soil cone/soil column) that derive the ultimate uplift resistance from friction in granular soil, the configuration of the soil cone/soil column as given in Figure 2 may be used, and the assessment of the effective weight of the soil cone/soil column should be based on the following assumptions: (a) For single pile subjected to tension, checking on soil cone failure is not

required.

(b) For soil with an SPT N-value of not less than 30, the angle of dilation of the soil cone shall not exceed 1 in 4 (i.e. approximate 15 degree). For soil with an SPT N-value of less than 30, the angle of dilation of the soil cone shall be taken as zero.

(c) Skin friction on the face of the soil cone/soil column shall be ignored.

(d) Effective weight of the soil cone/soil column shall be adopted. Any part of

the soil cone/soil column falling outside the site boundary shall be ignored.

(e) For a group of closely-spaced piles with same allowable uplift resistance of individual piles, overlapping effect of the soil cones shall be considered when assessing the volume of soil cone/soil column to be used for resisting



the combined uplift force.

(f) Where the allowable uplift resistance of individual piles within a pile group are not the same, checking of soil cone failure of individual pile is required. The effective weight of the overlapping part of soil cones and columns between piles may be distributed to each pile on a pro rata basis according to the allowable uplift resistance of the individual piles.

Figure 2 - Configuration of soil cone/soil column for group of closely-

spaced friction piles in soil 4.0 Pile Head Details

The pile head and pile cap connection shall be modified and be designed to transmit the maximum pile loads on both tension and compression for the approval of the SO.

5.0 Tension Loading Test

Static loading tests are required to justify the tension capacity of piles, other than large



diameter bored piles. Should tension piles be proposed, the contractor shall bear the cost and time of the required tests.

The SO may order not less than 1% of nos. of piles installed of each Ra (minimum one number or more piles for each Ra), other than large diameter bored piles, to be load tested to twice the Ra. The maximum test load should not result in the test pile being stressed beyond the yield stress, and in no case shall the test load exceed Ru.

The pile should be load tested at the cut-off level with no allowance for group effect.

Loading tests of piles shall be carried out by a Testing Firm employed direct by the Employer. The Contractor shall co-ordinate the works with the Testing Firm and comply with the requirements as stated in Clause 5.29 of the GS. Notwithstanding GS Clause 5.29(i)(b)(ii), reaction piles may be used for tension loading test, the Contractor shall provide and install reaction piles and associated temporary works with adequate stability and safety as approved by SO. The resistance of the reaction pile system shall be at least 1.15 times the maximum test load and shall be placed such that the load centroid matches the centre of test pile. To minimize interaction effects, the reaction piles should be located as far from the test pile as practicable. The reaction pile should be at least 3 test pile diameters, or 2 m, whichever is larger, from the test pile, measured centre to centre. Working piles may be adopted as reaction piles for tension loading tests, however the testing loads as imposed on the reaction piles should not be greater than the design pile capacity of the working piles. Should reaction pad be proposed for tension loading tests, the reaction pads should be of sufficient plan dimensions to transfer the reaction loads to the ground without settling at a rate that would cause difficulty in maintaining the applied test loads and the ground should be adequately strong. The Contractor shall provide and erect the reaction pads with adequate stability and safety as approved by SO. The reaction pads should be located as far from the test pile as possible, and the Contractor shall submit proposal for SO’s approval to assess the interaction effects. Should the Contractor design the pile by adopting a Ra less than the maximum value allowed in this particular specification, the reduced Ra shall be used for the purpose of tension loading test.



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

(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 0.1 mm/hour observed in a period of not less than 15 minutes, the residual extension at the head of the pile exceeds the greater of 4 mm and 25% of the maximum pile head extension during the test; or

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

In calculating the elastic axial extension of minipile or RSSHP, the following criteria shall be adopted:

(d) the contribution from cement grout shall be ignored; and

(e) the contribution from steel casing if used shall be considered.

Documents

Particular Specification for Tension PilesRev1).pdf · resistance between the surface of the pile and the soil along the pile depth. The perimeter of the Steel ‘H’ pile for uplift