7 Design related considerations

7.1 General

7.1.1 The basic European Standards for the design of bored piles shall be:

  • ENV 1991-1 Eurocode 1: Basis of design and actions on structures - Part 1: Basis of design;
  • ENV 1992-1-1 Eurocode 2: Design of concrete structures - Part 1-1: General rules and rules for buildings;
  • ENV 1994-1-1 Eurocode 4: Design of composite steel and concrete structures - Part 1-1: General rules for buildings;
  • ENV 1997-1 Eurocode 7: Geotechnical design - Part 1: General rules;

NOTE: Clause 7 relates to matters, resulting from the execution of bored piles which can affect the design.

7.1.2 The design can be affected by

  • the specific needs of the project as stated in clause 4,
  • the site investigation as described in clause 5 and/or
  • the material requirements as set out in clause 6.

7.1.3 The design shall take into account

  • the geometrical construction tolerances and
  • execution conditions as set out in 7.2 and clause 8.

7.1.4 A bored pile may be designed as an unreinforced concrete element, if

  • the design actions and/or
  • actions caused by the construction and/or
  • actions resulting from the ground produce only compressive stresses in the pile and
  • the pile foundation is not located in a seismic area.

7.1.5 Reinforcement of pile heads is recommended for unreinforced piles to cater for accidental loads (e.g. resulting from construction works on the site).

7.1.6 Piles should be reinforced over any length through soft or loose soil unless otherwise specified.

7.1.7 Piles designed for bending shall be provided with reinforcement.

7.1.8 Tension piles shall be provided with reinforcement over their whole length.

7.1.9 If there is no design requirement for reinforcement, starter bars should be provided in the pile head for the connection to the superstructure.

NOTE: Base enlargements of piles are usually constructed without reinforcement beyond that required (if any) in the shaft.

7.1.10 Where reinforcement cages are installed after concrete placement, a special and rigid design can be necessary.

7.2 Geometrical construction tolerances

7.2.1 Piles shall be constructed within the following geometrical tolerances:

  • a) plan location of vertical and raking piles referenced to the working level:
    • e ≤ emax = 0,10 m for piles with: D ≤ 1,0 m;
    • e ≤ emax = 0,1 × D for piles with: 1,0 m < D ≤ 1,5 m;
    • e ≤ emax = 0,15 m for piles with: D > 1,5 m;
  • b) deviation of inclination of vertical piles or piles with an inclination of n ≥ 15 (Θ ≥ 86°):
    • i ≤ imax = 0,02 (0,02 m/m);
  • c) deviation of inclination of piles raking 4 ≤ n < 15 (76° ≤ Θ < 86°):
    • i ≤ imax = 0,04 (0,04 m/m);
  • d) deviation in plan of centres of enlargements in relation to the pile axis:
    • e ≤ emax = 0,1 × D.

(see figure 8)

NOTE: For the recording of construction deviations the centre of the pile is considered the centroid of the longitudinal reinforcement or, for unreinforced piles the centre of the largest circle which can be drawn within the section at the section at the pile head.

Definition of geometrical construction deviations
Vertical pile Raked pile Deviation of location

E1: Working level

E2: Cut-off level

X1: Design centre line

X2: As build centre line

i: Tangent of deviation angle (between the designed and the as built centre line of the pile)

n: Rake of the design centre line against the horizontal

Θ: Angle of the design centre line against the horizontal

L1: Design location

L2: As built location

e: Plan deviation at working level

Figure 8: Definition of geometrical construction deviations

7.2.2 Where tolerances other than those stated are required or allowed in regard to

  • constructional demands,
  • ground conditions,
  • available piling equipment or
  • a very deep cut-off level

they shall be agreed before the commencement of the work.

7.3 Piles forming a wall

7.3.1 Piles forming a wall can be designed for axial and/or lateral loads (e.g. in the case of ground or water retaining walls or of underpinning).

7.3.2 The geometrical construction tolerances for piles forming a wall can be more demanding than the values indicated at 7.2.1, in particular when soil or water tightness is required.

7.3.3 The rake, spacing, geometrical construction tolerances/overlap and requirements for water tightness of joints in walls shall be specified.

7.3.4 The rake of a wall shall be n ≥ 8 (Θ ≥ 83°) (see figure 4).

7.4 Excavation

7.4.1 When piles are to be socketed into a bearing stratum or into rock, the design shall specify the shape, the minimum depth of penetration and the quality of the material in which the socket is to be formed.

7.4.2 Where ground conditions differ from those stipulated in the design, the designer shall be notified and appropriate action shall be taken to provide the required bearing resistance of the pile or of the foundation.

7.4.3 Compression piles shall not be founded on obstructions unless

  • sufficient bearing resistance is proved,
  • full face seating, and
  • similar deformation behaviour with respect to adjacent piles can be achieved.

7.4.4 If piles encounter an imenetrable obstruction prior to reaching their designed founding depth, the design shall be reviewed in the light of any available knowledge about the obstruction.

NOTE: Additional or supplementary piles of equivalent performance can be necessary in this case.

7.4.5 Enlargements of a pile base or shaft shall be designed only when the intended shape can be constructed in a controllable way and checked by suitable methods.

7.4.6 Base enlargements shall not be specified in unstable soils such as:

  • loose sands;
  • uniform sands below the ground-water table;
  • soft or sensitive clays.

7.4.7 Shaft enlargements shall be specified only for vertical piles in stable ground.

7.5 Precast concrete elements

7.5.1 The design, execution and supervision of precast concrete elements shall be in accordance with ENV 1992-1-1 and ENV 1994-1-1.

7.5.2 The design shall consider the cases of handling, transportation and installation; any restrictions shall be marked on the element.

7.5.3 The concrete cover shall be in accordance with the requirements for the respective environmental conditions.

7.6 Reinforcement

7.6.1 General

7.6.1.1 Starter bars or dowel bars for connection to a superstructure shall be in accordance with ENV 1992-1-1.

7.6.1.2 Where steel tubes or sections are used for the reinforcement of piles their design shall be in accordance with ENV 1994-1-1.

7.6.1.3 An allowance for corrosion shall be made in the design where a steel reinforcement pipe or a permanent casing is used as a structural member, unless protection is already naturally present or the entire surface is protected by a sufficient concrete or grout cover or other protective measures.

7.6.1.4 All necessary measures to provide cage rigidity should be shown on the working drawings.

7.6.2 Longitudinal reinforcement

7.6.2.1 Where a

  • bentonite,
  • clay or
  • polymer

suspension is used as a stabilizing fluid, only ribbed bars shall be used for main reinforcement.

7.6.2.2 Unless otherwise specified by design the minimum amount of longitudinal reinforcement shall be as indicated in table 4 where reinforcement is required.

Table 4: Minimum longitudinal reinforcement
Nominal pile cross section: AC Area of longitudinal reinforcement: AS
AC ≤ 0,5 m2 AS ≥ 0,5 % AC
0,5 m2 < AC ≤ 1,0 m2 AS ≥ 0,0025 m2
AC > 1,0 m2 AS ≥ 0,25 % AC

7.6.2.3 The minimum longitudinal reinforcement shall be four bars of 12 mm diameter.

7.6.2.4 Spacing of longitudinal bars should always be maximized in order to allow proper flow of concrete but should not exceed 400 mm.

7.6.2.5 The minimum clear distance between longitudinal bars or bundles of bars of one layer shall be not less than 100 mm.

7.6.2.6 The minimum clear distance between longitudinal bars or bundles of bars of one layer may be reduced to 80 mm when using d< 20 mm aggregate.

7.6.2.7 Concentric layers of longitudinal bars should be avoided where possible.

7.6.2.8 Where concentric bar layers of longitudinal bars are used

  • the maximum number of layers shall be two for circular bored piles,
  • bars of the layers shall be placed radially behind each other, and
  • the minimum clear distance between bar layers shall be equal to 2 times the bar diameter or 1,5 times the size of the coarse aggregate, whichever is the greater.

7.6.2.9 The spacing of the reinforcement may be reduced along the lap length of bars.

7.6.2.10 Where longitudinal bars are not evenly spaced, special methods are required to hold the correct positioning of the reinforcement cage during installation and concrete placement.

7.6.3 Transverse reinforcement

7.6.3.1 Transverse reinforcement is to be designed in accordance with ENV 1992-1-1.

7.6.3.2 The diameters of the transverse reinforcement should be in accordance with table 5.

Table 5: Recommended diameters of transverse reinforcement
Links, hoops or helical reinforcement > 6 mm and
> one quarter of the maximum diameter of the longitudinal bars
Wires of welded mesh transverse reinforcement >5 mm

7.6.3.3 The minimum clear distance of transverse bars shall not be less than the clear distance as set out for the main reinforcement in 7.6.2.

7.6.3.4 Where the shear stress in a pile exceeds 0,5 times the shear strength of the concrete alone, then the transverse reinforcement should be designed in accordance with the appropriate code practice.

7.6.3.5 Longitudinal bars or longitudinal bar bundles placed in a corner of a reinforcement cage should be restrained by the transverse reinforcement.

7.6.3.6 Stiffening rings or other means of support for the assembly of reinforcement cages may be recognized as a part of the transversal reinforcement only where properly connected to the longitudinal bars.

7.6.4 Concrete cover

7.6.4.1 The cover to all reinforcement in cast-in-place piles shall comply with ENV 1992-1 -1 and shall be not less than

  • 60 mm for piles with D > 0,6 m or
  • 50 mm for piles with D < 0,6 m

unless otherwise agreed.

7.6.4.2 The minimum cover should be increased to 75 mm where:

  • piles penetrate soft soil and are constructed without a casing;
  • exposure class 5 in accordance with ENV 206;
  • submerged placement of concrete with 32 mm max. aggregate is used;
  • reinforcement is installed subsequent to concrete placement; or
  • the borehole walls have uneven surfaces.

7.6.4.3 The concrete cover may be reduced to 40 mm to the external face of a permanent casing or lining, where used.

7.6.5 Reinforcement tubes and sections

7.6.5.1 The design of tubes or sections as special reinforcement shall be in accordance with ENV 1994-1-1.

7.6.5.2 The design value for bond stress between the concrete and the embedded section shall have been agreed before the commencement of the works and the concrete cover or thickness shall be specified in the design documents.

7.6.5.3 Site or laboratory tests can be required for this purpose.

7.6.5.4 An installation procedure shall be determined to maintain the alignment of the special reinforcement with the pile axis and the correct concrete cover over its entire length.

EN 1536:1999 Execution of special geotechnical work – Bored piles