8.5 Cast in place piles

8.5.1 General

8.5.1.1 All plant, materials and operations employed in the formation of a pile shall be such as to ensure that the completed pile satisfies the minimum required cross section.

8.5.1.2 The driving of the drive tube shall be in accordance with relevant subclauses in 8.3 and 8.4.

8.5.1.3 The sequence for driving of temporary cased piles shall be such as to prevent damage to any recently completed piles before the concrete in these piles has reached sufficient resistance.

8.5.1.4 Unless otherwise specified or determined from site experience, no piles without permanent casing should be installed within 6 diameters centre to centre of adjacent piles until the concrete in these piles has reached sufficient resistance.

8.5.1.5 If the soil has a characteristic undrained shear strength of less than 50 kPa the distance centre to centre between temporary cased fresh cast in place piles should be increased in accordance with Figure 3 if the concrete has not reached sufficient strength.

Minimum distance between fresh piles without permanent casing in soft soil

Key

  • 1 Minimum distance centre to centre / Diameter
Figure 1 — Minimum distance between fresh piles without permanent casing in soft soil

8.5.1.6 When semi-dry compacted concrete is used for pile shafts, the recommended distances may be reduced to half the distances given in Figure 3.

8.5.1.7 Piles that have been lifted should not be redriven except piles in which concrete is placed in a permanent steel or precast concrete tube and in accordance with the specification. Piles without permanent casing should not be redriven unless it can be shown that the pile section can safely withstand the driving force.

8.5.2 Temporary cased piles

8.5.2.1 For a top driven drive tube, the base shall be covered with a shoe or other closure devise to prevent water and soil entering the tube.

8.5.2.2 If the pile shoe be displaced or damaged so that soil or water enters the drive tube the pile shall not be concreted before one or more of the following operations has been completed:

  • the tube shall be filled with free flowing material if necessary, withdrawn and redriven; or
  • the pile shall be repositioned; or
  • if an obstruction is present and can be practically and safely removed the pile shall subsequently be formed in its original location.

8.5.2.3 When the drive tube is impact driven from the bottom concrete, gravel or sand may be used as a plug at the bottom of the drive tube provided it is not damaged during driving.

8.5.2.4 Handling and installation of reinforcement

8.5.2.4.1 The reinforcement cage shall be constructed to enable it to be handled and lowered into the drive tube without damage or distortion.

8.5.2.4.2 Where reinforcement cages are formed or extended on site by welding, the area and quality of the welds shall be adequate for the forces applied during handling and under working conditions after the pile has been concreted, and shall be carried out in accordance with ENV 1992-3.

8.5.2.4.3 Reinforcement cages shall be suspended or supported so as to maintain their correct position during concreting.

8.5.2.4.4 The reinforcement should be placed in the tube prior to concreting.

8.5.2.4.5 Reinforcement installation subsequent to concrete placement may be done for vertical piles, if prior experience in comparable conditions or on trial piles has demonstrated the feasibility of the method of installation.

8.5.2.4.6 Such installation shall take place as soon as possible after the completion of the concreting operation.

8.5.2.4.7 Where reinforcement cages are inserted after concreting, it can be necessary to maintain their position by suitable support.

8.5.2.4.8 The installation process may be assisted by light vibration or the reinforcement may be pulled-in e.g. with a mandrel.

8.5.2.4.9 If reinforcement protruding from the concrete at the pile head is to be bent the internal radius of the bend shall be not less than that stipulated by ENV 1992-3.

8.5.2.5 Concreting

8.5.2.5.1 The concreting of cast in place displacement piles shall be done in dry conditions by one of the following three methods:

  • 1) using high workability concrete that is discharged in sufficient quantity into the drive tube before and during extraction of the tube;
  • 2) by pumping concrete of high workability into the drive tube;
  • 3) by using semi-dry concrete that is added in small charges in the drive tube, each charge being expulsed and compacted by internal tamping during the stepwards withdrawal of the tube.

The first and third methods may be combined e.g. fabrication of the pile base (eventual enlarged) with semi-dry concrete, and of the pile shaft with high workability concrete.

8.5.2.5.2 The procedure for placing concrete in dry conditions shall not be followed if there is standing water or soil at the base of the tube. A check shall be carried out immediately before concreting.

NOTE Where there is no soil in the tube, concreting under submerged conditions may be authorized, using a tremie pipe.

8.5.2.5.3 Fresh concrete shall always be poured into concrete which retains its full workability.

8.5.2.5.4 When determining the workability time of the concrete allowance should be made for potential interruptions in the supply and the time required for the placement process.

8.5.2.5.5 The concrete shall be placed in sufficient quantity and with sufficient workability and coherence in order:

  • to ensure that no significant quantities of air are entrapped;
  • to avoid lifting of the concrete during withdrawal of the tube;
  • to prevent separation of the concrete;
  • to prevent inflow of soil or water.

8.5.2.5.6 External vibrating or light tamping of the drive tube may be used during tube extraction to improve the concrete outflow and the concrete compaction.

8.5.2.5.7 An adequate head of concrete shall be maintained above the toe of the drive tube during extraction.

8.5.2.5.8 The level of concrete within the drive tube should be maintained at or above working level during the tube extraction operation.

8.5.2.5.9 The pile should be cast to working level unless relevant experience has proven that this is not necessary to ensure integrity and geometry.

8.5.2.5.10 If a semi-dry concrete mix is used, the method of casing extraction shall ensure that the semi-dry concrete does not lift and is adequately compacted and tamped.

8.5.2.5.11 During concreting the volume placed and the level of concrete inside the tube should be checked and recorded.

8.5.2.5.12 The method and the sequence of checking the concrete level should suit dimensions, type of pile and soil conditions and should be agreed prior to the beginning of the work.

8.5.2.5.13 In cold weather with ambient air temperature less than 3 °C and falling, the heads of newly cast piles shall be protected against frost.

8.5.3 Permanently cased piles

NOTE Permanent casing or linings can be used for cast in place piles e.g. to avoid pile necking, excessive pile enlargements or as protection in aggressive soils or groundwater.

8.5.3.1 Handling and installation of reinforcement and concreting shall comply with 8.5.2 and EN 1536.

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