7.8 Specific design related considerations

7.8.1 General Where the following matters are of importance to the quality or performance of the displacement piles they should be specified in an early stage of the design, prior to or during the execution stage. Items include:

  • method of joining;
  • quality of the weld for splices;
  • method, minimum length and tolerances of cutting back pile elements;
  • shape and structure of shoe or other precautions if necessary to protect and secure the toe of the pile in the bedrock;
  • effects of time on pile capacity;
  • restrike testing of prefabricated piles;
  • adjustment of the driving criteria when a follower is used.

7.8.2 Reinforcement of cast in place piles The reinforcement cage for cast in place piles shall be designed not only to have adequate strength in the final pile, but also adequate strength and stiffness during handling of the cage and construction of the pile. It shall also allow the fresh concrete to flow easily around each of its components. Starter bars or dowel bars installed into fresh or hardened concrete for connection to a superstructure shall be in accordance to ENV 1992-3. Where steel tubes or sections are used for the reinforcement of piles their design shall be in accordance with ENV 1994-1-1. Unless otherwise agreed in the design, cast in place concrete piles shall be reinforced over their full length. A cast in place displacement pile may be designed as an unreinforced concrete element if:

  • actions caused by the construction; and/or
  • actions resulting from the ground;
  • actions resulting from the ground produce only compressive stresses in the pile; and
  • the pile is not located in a seismic area. In order to cater for accidental loads (e.g. resulting from construction works on the site, pile eccentricity, ...) the minimum reinforcement as given in shall however be used over the top upper 4 m for bearing piles. Piles should also be reinforced over any length through soft or loose soil unless otherwise specified. Tension piles shall in any case be reinforced over their whole length. Where reinforcement is required and unless otherwise proven by design the minimum amount of longitudinal reinforcement shall be:

  • 0,5 % of the pile nominal cross section;
  • minimum four bars of 12 mm nominal diameter. The clear distance between the longitudinal reinforcement cage bars shall be at minimum:

  • 100 mm;
  • 80 mm when using d ≤ 20 mm aggregate.

The spacing may be reduced along the lap length of the bars. The transversal reinforcement shall fulfil the following values:

  • minimum diameter of the bars: 5 mm;
  • minimum distance between the bars: as for the longitudinal bars. The cover to all reinforcement in cast in place piles shall be not less than:

  • 50 mm for piles with temporary casing;
  • 75 mm in case of exposure class 5 following ENV 206 or when reinforcement is installed subsequent to concrete placement;
  • 40 mm to the internal face of a permanent casing or lining.

7.8.3 Pile shoe Pile shoe of displacement piles shall be manufactured from durable material capable of withstanding the stresses caused by the installation method and ground conditions without damage. The pile shoe for cast in place piles shall be designed to prevent water from entering into the drive tube during construction. The pile toe of prefabricated concrete and steel displacement piles should be designed when driving into hard rock, onto a sloping rock surface, suspected hard rock or when driving in soil with hard boulders.

NOTE In weak rock or soils the pile toe can be protected with other methods e.g. bands, extra reinforcement, plates.

Typical examples of pile and rock shoes are shown in Figure A.4 of annex A. When driving timber piles:

  • In soft soils no toe protection is normally required.
  • In other soils the toe should have protection unless comparable experience shows otherwise.

7.8.4 Pile joint

The joints of prefabricated displacement piles and combined displacement piles shall be capable of maintaining the alignment and position of the pile elements during installation. They shall also safely resist the stresses from handling, driving and actions from the structure and surrounding soil.

Typical examples of pile joints are shown in Figures A.7, A.8 and A.9 of annex A.

7.8.5 Pile enlargement Where pile enlargements are considered, the method of forming the enlargement and the bearing area and shaft parameters to be used in the design shall be agreed.

Examples of enlargements are shown in Figure A.2 of annex A. When redrives are used to form enlarged bases or enlarged shafts on cast in place piles the method used to form the pile and the nominal value of base and shaft perimeter to be used in the design shall be agreed before commencement of the work.

7.8.6 Spacing of piles The spacing of piles shall be considered in relation to pile type, length of pile and the ground conditions and their behaviour in groups. The possible interference of one pile with another during installation should be considered when determining pile type, pile spacing, orientation and installation sequence.

7.8.7 Combined piles

Careful consideration shall be given to the joining of elements and to the method of construction of combined piles to ensure adequate bearing capacity, structural strength and durability.

Examples of joints can be seen in Figure A.8 of annex A.

EN 12699:2001 Execution of special geotechnical work – Displacement piles