6.8 Serviceability limit state design

6.8.1 General

6.8.1.1 Serviceability limit states should be verified according to BS EN 1997-1:2004+A1:2013, 2.4.8(1)P, by ensuring:

where:

E_d is the design effects of actions specified in the serviceability criterion; and

C_d is the limiting value of the relevant serviceability criterion.

6.8.1.2 Where appropriate, allowance should be made for elastic shortening of the pile shaft under axial loading.

6.8.2 Individual piles

COMMENTARY ON 6.8.2

In order to keep settlement to a minimum, it is common practice in the UK to limit the representative load on a friction pile to its characteristic shaft resistance (thereby discounting its base resistance). This is not a requirement of BS EN 1997-1:2004+A1:2013.

In this context, the relevant serviceability criterion C_d in equation (82) is the shaft resistance of the pile calculated for ultimate limit state conditions.

6.8.2.1 Settlement of a pile foundation may be verified by satisfying the following serviceability criterion:

(83)

where:

F_c,rep is the representative value of the compressive force applied to the pile in its serviceability limit state;

R_s,k is the characteristic value of the pile's ultimate shaft resistance; and

γ_s,SLS is a partial factor for shaft resistance in the serviceability limit state.

NOTE The symbol γ_s,SLS is introduced here to prevent potential confusion with the value of γ_s used under ultimate limit state conditions.

6.8.2.2 The value of γ_s,SLS should be taken as a minimum of 1.2.

NOTE 1 With γ_s,SLS ≥ 1.0, the settlement of an individual pile can be limited to less than 3% of its diameter, D; with γ_s,SLS ≥ 1.2, the settlement can be limited to less than 1.5% D.

NOTE 2 Use of equation (83) can lead to uneconomic designs when:

• the serviceability limit state has been otherwise verified by calculation or load testing;
• settlement of the pile is not a concern;
• the pile is installed by driving into competent ground; or
• the stiffness of the ground below the pile toe has been improved significantly (for example, by base grouting).

6.8.2.3 The design value of the compressive force applied to an individual pile at its serviceability limit state should be calculated from equation (69) with the values of the partial factors γ_F,i and γ_G normally taken as 1.0.

NOTE The symbol γ_s,SLS is introduced here to prevent potential confusion with the value of γ_s used under ultimate limit state conditions.

6.9 Structural design

6.9.1 General

6.9.1.1 The structural design of pile foundations should conform to 4.8.

6.9.1.2 The structural design of individual piles should consider:

• compressive and tensile resistance of the pile shaft;
• shear resistance of the pile shaft;
• bending resistance of the pile shaft;
• torsional resistance of the pile shaft;
• buckling resistance of the pile shaft, particularly in the absence of lateral restraint from the ground, for example if a gap opens up around the pile during installation (so-called «post-holing»);
• weakening of the pile material as a result of corrosion or other forms of deterioration;
• connection of the pile to a pile cap or slab;
• flared pile heads;
• combinations of axial load and moments; and
• concrete crack widths.

6.9.1.3 For the purposes of its structural design, the design value of the compressive force applied to an individual pile at the ultimate limit state should be calculated from equation (64).

6.9.1.4 The likelihood of some degree of eccentric loading on foundations consisting of only one or two piles should be considered. The piles should be designed to resist the bending which results or the pile cap should be effectively restrained from lateral or rotational movements. The restraint and the pile section or both should be sufficient to resist the moments due to eccentric loading or other causes.

6.9.2 Bored cast-in-place concrete piles

COMMENTARY ON 6.9.2

BS EN 1992-1-1:2004, 2.3.4.2 provides supplementary requirements for cast-in-place piles without permanent casing. These supplementary requirements involve reducing the diameter of these piles by 5% for the purposes of their structural design. This reduction is unnecessary when the execution of these piles conforms to BS EN 1536 or BS EN 14199.

BS EN 1992-1-1:2004, 2.4.2.5 provides partial factors for materials for foundations, including cast-in-place piles without permanent casing. A factor k_f is introduced as a multiplier to the partial factor for concrete γ_C, with a recommended value of 1.1. UK National Annex to BS EN 1992-1-1:2004, Table NA.1 specifies use of this recommended value in the UK. Thus, in permanent and transient design situations, the «effective» partial factor on cast-in-place concrete in piles without permanent casing is:

BS EN 1992-1-1:2004+A1:2014, 9.8.5 provides application rules for detailing cast-in-place concrete piles, in particular with regard to the minimum number and minimum diameter of longitudinal bars. These rules differ from the requirements of BS EN 1536:2010+A1:2015, which states:

• 7.5.2.3 For reinforced piles the minimum longitudinal reinforcement shall be four bars of 12 mm diameter; and
• 7.5.2.5 Spacing of longitudinal bars should always be maximized in order to allow proper flow of concrete but should not exceed 400 mm.

6.9.2.1 The design compressive resistance (R_c,d) of the reinforced length of a cast-in-place pile may be calculated from:

(85)

where:

f_ck and f_cd are the characteristic and design compressive strengths of the reinforced concrete, respectively;

f_yk and f_yd are the characteristic and design yield strengths of the steel reinforcement, respectively;

A_c,d and A_s,d are the cross-sectional area of the reinforced concrete and compressive steel reinforcement, respectively;

α_CC is a factor taking into account the long-term reduction in strength, etc., of reinforced concrete; and

γ_C and γ_S are partial factors on the strength of the reinforced concrete and the steel reinforcements, respectively.

6.9.2.2 The design compressive resistance (R_c,pl,d) of the unreinforced length of a cast-in-place pile should be calculated from:

(86)

where:

α_CC,pl is a factor taking into account the long-term reduction in strength, etc., of plain concrete; and

the other symbols are defined for equation (85).

6.9.2.3 Values of γ_C, γ_S, α_CC,pl, and k_f should conform to the UK National Annex to BS EN 1992-1-1.

6.9.2.4 A cast-in-place pile (without permanent casing) that conforms to BS EN 1536 or BS EN 14199 should be considered to meet the «other provisions» specified in BS EN 1992-1-1:2004, 2.3.4.2(2). Therefore, the diameter used in design calculations of cast-in-place piles without permanent casing should be as given in Table 14.

6.9.2.5 Longitudinal bars in cast-in-place concrete piles should conform to BS EN 1536, as follows:

• the minimum diameter should be no less than 12 mm (instead of 16 mm, as stated in BS EN 1992-1-1);
• the minimum number of bars should be no fewer than 4 (instead of 6, as stated in BS EN 1992-1-1).
• the clear distance between bars should be no greater than 400 mm (instead of 200 mm, as stated in BS EN 1992-1-1).

6.9.2.6 Depending on the type and magnitude of loading, a cast-in-place concrete pile may be reinforced over its whole length, over part of its length, or merely provided with short splice bars at the top for bonding into a pile cap. If a cast-in-place concrete pile is required to resist tensile forces, its reinforcement should extend over the full length of pile that is subjected to those tensile forces (including into any enlarged base, if necessary).

6.9.2.7 Reinforcement should be provided to resist tensile forces that might arise due to swelling of unloaded ground. In the temporary condition, it might be acceptable not to fully reinforce the pile.

6.9.3 Driven cast-in-place concrete piles

The structural design of driven cast-in-place concrete piles should conform to 6.9.2.

6.9.4 Prefabricated piles

6.9.4.1 Precast concrete piles

COMMENTARY ON 6.9.4.1

The structural design of precast concrete piles is often governed by the stresses that occur during handling (lifting, stacking, and transporting) and installation into the ground.

6.9.4.1.1 The manufacture of precast concrete piles should conform to BS EN 12794.

6.9.4.1.2 Precast concrete piles should be designed to withstand the stresses that occur during handling (lifting, stacking, and transporting) and installation into the ground.

6.9.4.1.3 Lifting points should be clearly marked on all precast concrete piles.

NOTE Guidance on the manufacture of precast concrete piles can be found in the ICE Specification for piling and embedded retaining walls, Sections B2 and C2 Driven pre-cast concrete piles (SPERW [N3]).

6.9.4.2 Steel bearing piles

COMMENTARY ON 6.9.4.2

The structural design of steel bearing piles is often governed by the stresses that occur during installation into the ground.

Steel bearing piles should be designed to withstand the stresses that occur during handling (lifting, stacking, and transporting) and installation into the ground.

NOTE Guidance on a suitable sheet pile section to withstand driving in different ground conditions can be found in the Piling Handbook [90].

6.9.4.3 Helical steel piles

The structural design of helical steel piles should conform to A.6.

6.9.4.4 Timber piles

NOTE Guidance on the design of timber piling can be found in BRE Digest 479 [43].