6.5 Materials

6.5.1 Concrete

6.5.1.1 The materials and products incorporated into concrete piles should conform to 4.3.6.

6.5.1.2 Materials and products incorporated into bored cast-in-place concrete piles should also conform to BS EN 1536 and BS EN 206:2013, Annex D.

6.5.1.3 Materials and products incorporated into driven cast-in-place concrete piles should also conform to BS EN 12699 and BS EN 206:2013, Annex D.

6.5.1.4 Materials and products incorporated into prefabricated concrete piles should also conform to BS EN 12699 and BS EN 12794.

6.5.1.5 Materials and products incorporated into micropiles should also conform to BS EN 14199 and BS EN 206:2013, Annex D.

NOTE 1 BS EN 206:2013, Annex D, includes the normative rules for concrete for special geotechnical work that were previously given in BS EN 1536:2010, BS EN 1538:2010, BS EN 12699:2001, and BS EN 14199:2005.

NOTE 2 At the time of publication, amendments to BS EN 1536:2010 and BS EN 1538:2010 are in preparation to remove the rules that are now contained in BS EN 206:2013, Annex D.

NOTE 3 Guidance on the use of low-strength concrete mixes for use in firm female secant piles can be found in BRE Information Paper IP 17/05 [85].

6.5.2 Steel

6.5.2.1 The materials used to fabricate steel piles should conform to 4.3.7.

6.5.2.2 Materials used to fabricate H-section and tubular steel piles should also conform to BS EN 12699.

6.5.2.3 Hot finished tubular steel piles should also conform to BS EN 10210.

6.5.2.4 Cold formed tubular steel piles should also conform to BS EN 10219.

6.5.2.5 Hot rolled steel sheet piles used as bearing piles should also conform to BS EN 10248.

6.5.2.6 Cold formed steel sheet piles used as bearing piles should also conform to BS EN 10249.

6.5.2.7 Steel reinforcement used in the fabrication of concrete piles should conform to BS EN 1992-1-1 and BS EN 10080.

6.5.3 Timber

COMMENTARY ON 6.5.3

Certain types of softwood and hardwood are suitable for use as permanent piles. The choice depends upon availability in suitable sizes, the expected useful life, and the relative cost, including preservative treatment. Availability in the required lengths is frequently a limiting factor.

Commonly used softwoods include:

  • Douglas fir, imported to the UK in sections up to 400 mm square and 15 m long (or longer to special order);
  • Pitch pine, imported to the UK in sections up to 500 mm square and 15 m long. Commonly used hardwoods include:
  • Greenheart (used for permanent works), imported to the UK rough-hewn in sections up to 475 mm square and up to 18 m long (or larger sections and lengths up to 24 m long to special order);
  • Tropical hardwoods, imported in sections up to 9 m long.

Straightness of grain is important in timber piles, particularly where hard driving is anticipated.

6.5.3.1 The materials used to fabricate timber piles should conform to 4.3.8 and BS EN 12699.

6.5.3.2 Timber piles should be free from any defects that could affect their strength or durability.

6.5.3.3 Piles should be ordered long enough to ensure that, after cut-off at the proper level, the top of the pile is clean, sound, and undamaged after driving.

6.5.3.4 When straight trunks of coniferous trees are used for timber piles, the bark should be removed and the timber pressure impregnated with preservative.

NOTE Strength classes for structural timber are defined in BS EN 338.

6.5.4 Cement grout

Cement grout incorporated into micropiles should conform to BS EN 14199.

6.6 Durability

6.6.1 Concrete

The durability of concrete piles should conform to 4.4.2.

6.6.2 Steel

6.6.2.1 The durability of steel bearing piles should conform to 4.4.3.

6.6.2.2 The durability of steel bearing piles should also conform to BS EN 1993-5.

NOTE 1 NA+A1:2012 to BS EN 1993-5:2007 gives UK values for loss of thickness per face due to corrosion of steel bearing piles in soils, with or without groundwater.

NOTE 2 A method of assessing the corrosivity of the ground surround buried steel pile can be found in Table 8.1 of the Design Manual for Roads and Bridges, Volume 2, Section 2, Part 6, BD 12/01 [86].

6.6.3 Timber

6.6.3.1 The durability of timber piles should conform to 4.4.4.

6.6.3.2 Measures taken to prolong the life of timber piles should conform to BS EN 1995-1-1.

NOTE Timber piles may be designed with no protection against decay if they are kept below a moisture content of 22% or they are buried in the ground below the lowest permanent water table.

6.6.4 Cement grout

The durability of cement grout incorporated into micropiles should conform to BS EN 14199:2015, 7.6.

6.7 Ultimate limit state design

6.7.1 General

The ultimate limit state design of a pile foundation should conform to 4.6 and this subclause (6.7).

6.7.2 Bearing

6.7.2.1 Individual piles

COMMENTARY ON 6.7.2.1

BS EN 1997-1 provides several alternative methods for verifying the ultimate compressive resistance of an individual pile, including methods based on:

  • static pile load tests;
  • ground test results;
  • dynamic impact tests;
  • pile driving formulae; and
  • wave equation analysis.

Traditional UK practice has been to verify the compressive resistance of an individual pile using ground test results in calculations based on soil mechanics theory. This approach is termed the «alternative procedure» in BS EN 1997-1:2004+A1:2013, 7.6.2.3.

6.7.2.1.1 The design value of the ultimate compressive resistance of an individual pile (Rc,d) should be verified according to the alternative procedure given in BS EN 1997-1:2004+A1:2013, 7.6.2.3, and conform to expression (7.5) of that standard, namely:

(68)

where:

Rb,k is the characteristic value of the pile's (calculated) ultimate base resistance;

Rs,k is the characteristic value of the pile's (calculated) ultimate shaft resistance; and

γb and γs are the partial factors given in the UK National Annex to BS EN 1997-1:2004+A1:2013, whose values depend on the level of pile testing that is performed to corroborate the calculations of resistance.

6.7.2.1.2 In the United Kingdom, the ultimate geotechnical compressive resistance of an individual pile may be verified using Design Approach 1 Combination 2 alone, since the values of the partial factors γb and γs given in the UK National Annex to BS EN 1997-1:2004+A1:2013 are such that Design Approach 1 Combination 1 cannot govern the design.

6.7.2.1.3 The design compressive force (Fc,d) applied to an individual pile at its ultimate limit state should be calculated from:

(69)

where:

Pc,k,i is the ith characteristic compressive force applied to the pile by the structure;

Wk is the characteristic self-weight of the pile;

Pdd,k is the additional characteristic compressive force owing to downdrag, given by equation (57);

ψi is the corresponding combination factor for the ith force;

γF,i is the corresponding partial factor on actions for the ith force; and

γG is the partial factor on permanent actions.

6.7.2.1.4 The additional characteristic compressive force owing to downdrag (Pdd,k) should only be included in equation (69) when the pile head displacement at the geotechnical ultimate limit state is less than the anticipated settlement of the ground surface. In that situation, the pile's bearing resistance should also be reduced accordingly, to:

(70)

where:

Rs,dd,k is the characteristic shaft resistance of the pile when subject to downdrag, given by equation (58); and

the other symbols are as defined for equation (68).

6.7.2.1.5 The method used to determine the ultimate limit state of a pile may be based on any of the following criteria:

  • the ultimate load measured at a settlement equal to 10% of the pile's diameter, as recommended in BS EN 1997-1:2004+A1:2013, 7.6.1.1(3) when it is difficult to define the ultimate limit state from the load settlement curve;
  • Chin's criterion [87], assuming a hyperbolic pile load v settlement curve; or
  • Fleming's method [81], assuming separate hyperbolic shaft and base load vs settlement curves and elastic pile shortening;
  • Davisson's offset limit [88];
  • Butler and Hoy's slope and tangent method [89].

6.7.2.2 Pile groups

COMMENTARY ON 6.7.2.2

Provided the pile cap has adequate structural strength to redistribute axial loads across the group, full mobilization of an individual pile's geotechnical resistance can occur within a large pile group (comprising 5 or more piles) without the pile group reaching an ultimate limit state.

6.7.2.2.1 The compressive resistance of a pile group should be verified assuming the individual piles and the ground between them act as a block.

6.7.2.2.2 Where a row of piles is used to form a retaining wall, particular attention should be paid to the possibility of failure of that row when the retaining wall carries significant vertical load.

6.7.2.2.3 The design value of the ultimate compressive resistance of a pile group (Rgroup,c,d) should be calculated from:

Rgroup,c,d = min(Rsum,c,d;Rblock,c,d;Rrow,c,d)
(71)

where:

Rsum,c,d is the sum of the ultimate compressive resistances of all the individual piles;

Rblock,c,d is the ultimate compressive resistance of a block that encloses the pile group;

Rrow,c,d is the ultimate compressive resistance of a row of piles within that block.

6.7.2.2.4 The design value of the ultimate compressive resistance of a large pile group subject to large horizontal loads and/or moments may alternatively be calculated from:

Rgroup,c,d = Rsum,c,d
(72)

NOTE For the purposes of this clause, a pile group is considered «large» if it comprises five or more piles and has a cap that can redistribute axial loads across the group.

6.7.2.2.5 The sum of the ultimate compressive resistances of the individual piles (Rsum,c,d) should be calculated from:

(73)

where:

(Rc,d)j is the design value of the ultimate compressive resistance of pile j; and

n is the total number of piles in the group.

6.7.2.2.6 In coarse soils, the design value of the ultimate compressive resistance of the block that encloses the pile group (Rblock,c,d) may be calculated from:

(74)

where:

K0 is the soil's at-rest earth pressure coefficient;

is the average vertical effective stress over the side of the block;

φ is the soil's angle of shearing resistance;

As is the total side area of the block (i.e. perimeter of block × pile length);

sblock is a shape factor chosen in accordance with 5.4.1.2;

Nq is a bearing coefficient chosen in accordance with 5.4.1.2;

σ'v,b is the vertical effective stress at the base of the block; and

Ab is the total base area of the block (i.e. width × breadth of block).

6.7.2.2.7 In fine soils, the design value of the ultimate compressive resistance of the block that encloses the pile group (Rblock,c,d) may be calculated from:

(75)

where:

is the soil's average design undrained shear strength over the side of the block;

As is the total side area of the block (i.e. perimeter of block × pile length);

sblock is a shape factor chosen in accordance with 5.4.1.3;

Nc is a bearing coefficient chosen in accordance with 5.4.1.3;

cu,b,d is the soil's design undrained shear strength at the base of the block; and

Ab is the total base area of the block (i.e. width × breadth of block).

NOTE 1 The design value of the ultimate compressive resistance of a row of piles within the block (Rrow,c,d) may be calculated in a similar fashion to the block as a whole, adjusting As and Ab accordingly.

NOTE 2 Except when the spacing between rows is variable or the action from horizontal forces or moments is large relative to the action from vertical forces, the compressive resistance of a row of piles does not normally govern the design and may therefore be ignored.

6.7.3 Tension/pull-out

6.7.3.1 Individual piles

COMMENTARY ON 6.7.3.1

BS EN 1997-1 provides two alternative methods for verifying the ultimate compressive resistance of an individual pile, including methods based on:

  • static pile load tests; and
  • ground test results.

Traditional UK practice has been to verify the tensile resistance of an individual pile using ground test results in calculations based on soil mechanics theory. This approach is termed the «alternative procedure» in BS EN 1997-1:2004+A1:2013, 7.6.3.3.

6.7.3.1.1 The design value of the ultimate tensile resistance of an individual pile (Rt,d) should be verified according to the alternative procedure given in BS EN 1997-1:2004+A1:2013, 7.6.3.3, and conform to expressions (7.15 and 7.16) of that standard, namely:

(76)

where:

Rt,k is the characteristic value of the pile's (calculated) ultimate tensile resistance;

Rs,k is the characteristic value of the pile's (calculated) ultimate shaft resistance; and

γs,t is the partial factor given in the UK National Annex to BS EN 1997-1:2004+A1:2013, whose value depends on the level of pile testing that is performed to corroborate the calculation of resistance.

6.7.3.1.2 In the United Kingdom, the ultimate geotechnical tensile resistance of an individual pile may be verified using Design Approach 1 Combination 2 alone, since the values of the partial factors γs,t given in the UK National Annex to BS EN 1997-1:2004:2013 are such that Design Approach 1 Combination 1 cannot govern the design.

6.7.3.2 Pile groups

6.7.3.2.1 The tensile resistance of a pile group should be verified assuming the individual piles and the ground between them act as a block.

6.7.3.2.2 The design value of the ultimate tensile resistance of a pile group (Rgroup,t,d) should be calculated from:

Rgroup,t,d = min(Rsum,t,d;Rblock,t,d;Rrow,t,d)
(77)

where:

Rsum,t,d is the sum of the ultimate tensile resistances of all the individual piles;

Rblock,t,d is the ultimate tensile resistance of a block that encloses the pile group;

Rrow,t,d is the ultimate tensile resistance of a row of piles within that block.

6.7.3.2.3 The sum of the ultimate tensile resistances of the individual piles (Rsum,t,d) should be calculated from:

(78)

where:

(Rt,d)j is the design value of the ultimate tensile resistance of pile j;

n is the total number of piles in the group.

6.7.3.2.4 In coarse soils, the design value of the ultimate tensile resistance of the block that encloses the pile group (Rblock,t,d) may be calculated from:

(79)

where:

K0 is the soil's at-rest earth pressure coefficient;

is the average vertical effective stress over the side of the block;

φ is the soil's angle of shearing resistance;

As is the total side area of the block (i.e. perimeter of block × pile length); and

sblock is a shape factor chosen in accordance with 5.4.1.2.

6.7.3.2.5 In fine soils, the design value of the ultimate tensile resistance of the block that encloses the pile group (Rblock,t,d) may be calculated from:

(80)

where:

is the soil's average design undrained shear strength over the side of the block;

As is the total side area of the block (i.e. perimeter of block × pile length); and

sblock is a shape factor chosen in accordance with 5.4.1.3.

6.7.3.2.6 The design value of the ultimate tensile resistance of a row of piles within the block (Rrow,t,d) may be calculated in a similar fashion to the block as a whole, adjusting As accordingly.

NOTE Except when the spacing between rows is variable or the action from horizontal forces or moments is large relative to the action from vertical forces, the tensile resistance of a row of piles may be ignored since it does not normally govern the design.

6.7.4 Transverse resistance

6.7.4.1 Individual piles

COMMENTARY ON 6.7.4.1

Although BS EN 1997-1:2004+A1:2013, 7.7.1(2)P, requires verification that an individual pile «will support the design transverse load with adequate safety against failure», it gives no details of which partial factors are to be applied or their recommended values.

6.7.4.1.1 The design value of the ultimate transverse resistance of an individual pile (Rtr,d) should be calculated from:

    
in coarse soils
in fine soils
(81)

where:

φd is the soil's design angle of shearing resistance;

γd is the soil's design weight density;

cu,d is the soil's design undrained shear strength;

MRd is the pile's design ultimate bending resistance; and the other symbols are as defined for equation (66).

6.7.4.1.2 The design values of the soil parameters to be used in equation (81) should conform to BS EN 1997-1:2004+A1:2013, 2.4.6.2, with the partial factors γcu, γφ, and γγ as specified in the UK National Annex to BS EN 1997-1:2004+A1:2013 for Design Approach 1, Combinations 1 and 2.

6.7.4.1.3 The effect of ground movement of the transverse resistance of an individual pile should be considered.

BS 8004:2015 Code of practice for foundations