8.3 Concreting

8.3.1 General The interval between completion of excavation and commencement of concrete placement is required to be kept as short as possible. Prior to concrete placement the cleanliness of the bore shall be checked. If the bore is supported by a stabilizing fluid, the properties of the fluid shall be checked before concrete placement (see 6.5).

NOTE 1: Water inside the bore can contain in suspension a significant quantity of fine sand or silt that can settle on the pile base during the interval preceding concrete placement. Cleaning or replacement of the water may be required.

NOTE 2: The possibility of trapped fluid or segregated concrete is increased when piles are constructed with base enlargements. Special precautions should be taken in the cleaning of base enlargements. The concreting of an enlarged base shall be in one continuous operation without interruption. The bore shall be partly or wholly filled with concrete in such manner as to form a continuous sound monolithic shaft of the full required cross section and height. No soil, liquid or other foreign material which can adversely affect the performance of the pile shall be permitted to contaminate the concrete. The workability of the concrete shall be such as to allow the whole concreting procedure to be carried out satisfactorily. An adequate supply of concrete shall be available throughout the whole placement process to enable a smooth operation. Fresh concrete shall always be poured into concrete which retains its full workability. When determining the workability time of the concrete, allowance should be made to cater for potential interruptions in the supply and the time required for the placement process. Internal vibration is not permissible for the compaction of the concrete. Appropriate precautions shall be taken to prevent the fines of the concrete being washed out from the shaft surface by flowing groundwater. It can be necessary to contain the fresh concrete in soft ground (cu ≤ 15 kN/m2) along a part or the whole of the pile length by the installation of sacrificial linings or permanent casings. During the concreting the volume placed and the level of concrete inside the bore shall be checked and recorded. The method and the sequence of the checking and recording shall suit the dimensions and type of pile and shall be agreed prior the beginning of the work. The levels shall be checked at least once

  • after every pour, or
  • before or after a temporary casing is lifted. For piles with diameter less than 0,6 m it may be sufficient to record the concreting of the first 10 piles of a site and a percentage of the remaining piles. The placement shall continue until any contaminated concrete in the upper part of the concrete column has risen above cut-off level. The height of the casting level above the cut-off level should be increased when

  • the cut-off level lies deep below the working platform,
  • concreting is carried out in submerged conditions, or
  • temporary casings are recovered. At an ambient air temperature less than 3 °C and falling, the heads of newly cast piles are to be protected against frost. When the final casting level is below the working platform, the fresh concrete should be protected against contamination from above:

  • by concreting above the cut-off level;
  • by backfilling the empty bore with suitable material; or
  • by maintaining a stabilizing fluid inside the empty bore until the concrete has set. In circumstances where the casting level is below the groundwater level, a pressure on the unset concrete equal to, or greater than, the external groundwater pressure shall be maintained. The pile trimming operation

  • shall be carried out only when the concrete has obtained sufficient strength,
  • shall remove all concrete which is contaminated or of lower quality than required from the top of the pile, and
  • shall continue until sound concrete over the whole cross section is revealed. Trimming and cutting off of pile heads with mechanical equipment shall be done with the greatest possible care. The risk of extensive cracks, caused by mechanical equipment used for cutting off, can require restriction of the type and size of concrete breaker employed.

8.3.2 Concreting in dry conditions The procedure for placing concrete in dry conditions shall not be followed if there is standing water at the base of the pile bore. A check shall be carried out immediately before the placement. If water is recognized concrete should be placed as for submerged conditions. Concreting shall be carried out in such way as to avoid segregation. The concrete shall be directed vertically into the centre of the bore by means of a funnel and an attached length of pipe so that the concrete does not hit

  • the reinforcement, or
  • the walls of the pile bore. The internal diameter of the concreting pipe shall not be less than 8 times the maximum size of the aggregate.

8.3.3 Concreting in submerged conditions Where concreting is carried out under water or a stabilizing fluid, the consistency shall be in accordance with table 2 and a tremie pipe be used for the placement.

NOTE: The main purpose of the tremie pipe is the prevention of segregation of the concrete during placement or its contamination by the fluid inside the bore. Submerged concrete shall not be compacted by internal vibration.

NOTE: Compaction is dependent on the flowability of the concrete in relation to its self weight and the surcharge of the fluid above the concrete column. The tremie pipe, including all its joints, shall be water tight. It shall be equipped at its upper end with a hopper to receive the fresh concrete and prevent spillage of concrete which otherwise could fall freely into the hole, segregate or become contaminated. The tremie pipe shall be smooth to allow free flow of concrete and have a uniform internal diameter of at least

  • 6 times the maximum size of the aggregate, or
  • 150 mm

whichever is the greater. The external shape and dimension of the tremie pipe, including its joints, shall allow its free movement inside the reinforcement cage. The maximum outside diameter of the tremie pipe including its joints should be not more than:

  • 0,35 times the pile diameter or the inner diameter of a casing;
  • 0,6 times the inner width of the reinforcement cage for circular piles; and
  • 0,8 times the inner width of the reinforcement cage for barrettes. The tremie pipe shall be cleaned of all encrusted concrete or mortar prior to its use. The tremie pipe shall extend to the bottom of the pile at the commencement of the concreting. A bung or plug of suitable material, to prevent mixing of concrete with any fluid in the tremie pipe, shall be inserted into the pipe before the commencement of concrete placement. As the first batch, a cement enriched mix or a charge of cement mortar may be used to lubricate the tremie pipe. To allow the first concrete to leave the tremie pipe, the pipe shall be lifted slightly, not exceeding a value equal to the inner diameter of the tremie pipe. Placement shall then proceed quickly to fill the entire base of the pile so that no concrete which may have segregated at the beginning of the discharge is trapped. During subsequent placement the tremie pipe shall be withdrawn progressively as the concrete rises in the bore. The pipe shall at all times remain immersed in unset and workable concrete which has previously been placed and shall not be withdrawn from the concrete until the completion of the concreting process. The immersion of the tremie pipe into the concrete should be not less than 1,5 m, particularly when disconnecting sections of the pipe and when recovering and disconnecting sections of temporary casing. For piles with a diameter D ≥ 1,2 m the immersion should be at least 2,5 m and for barrettes at least 3,0 m, particularly when two or more tremie pipes are used. After completion of the placement the tremie pipe should not be extracted too quickly as the resulting suction can lead to pile imperfections. When concrete is placed under stabilizing fluid

  • a sample of the fluid shall be taken from the base of the bore, and
  • any major filtercake or debris shall be removed from the bottom of the bore

immediately before the start of the placement. Concreting shall proceed only when the properties of the suspension are satisfactory. Otherwise additional recycling and cleaning or substitution of the suspension is necessary.

8.3.4 Extraction of casings The extraction of temporary casings shall not be started unless the concrete column has reached a sufficient height inside the casing to generate an adequate excess pressure

  • to protect against inflow of water or soil at the tip of the casing, and
  • to prevent the reinforcement cage from being lifted. The extraction shall be carried out while concrete is still of the required workability. During the continued extraction a sufficient quantity and head of concrete shall be maintained inside the casing to balance the external pressure so that the annular space vacated by the removal of the casing is filled with concrete.

  • The supply of concrete, and
  • the speed of extraction of the casing

shall be such that no inflow of soil or water occurs into the freshly placed concrete, even if a sudden drop of concrete level should occur when a cavity outside the casing is uncovered.

NOTE: This is particularly important in loose or soft ground or close to the pile top. In addition to the general requirements, the depths of casing and of the tremie pipe shall also be recorded.

8.3.5 Permanent casings or linings The installation of permanent casings or linings can be necessary to confine fresh concrete in the pile bore.

NOTE: In cases where sacrificial linings are installed into temporarily cased or uncased excavations or piles are constructed with permanent casings, voids can remain in the ground outside the pile shaft. If the presence of voids, which can cause possible ground settlements affecting adjacent structures, is known or suspected, measures shall be taken to fill them.

8.3.6 Concreting of continuous flight auger piles Concreting of piles excavated with continuous flight augers may be carried out by placing concrete through the hollow central stem of the auger, the stem being closed at its base, to avoid entry of water or soil until concrete placing commences. Once boring has reached the final depth, concrete shall be placed through the stem to fill the pile while the auger is withdrawn. If concrete flow cannot be initiated, it is necessary to completely remove the auger by backscrewing it from the ground, backfilling the hole so that no voids or collapses occur. The pile may then be rebored at the same location to at least the original depth. During withdrawal and concrete placement, the auger

  • shall not be rotated, or
  • shall be rotated only at low speed in the same direction as for excavation. During continuing placement, the concrete at the tip of the auger shall be kept under a pressure exceeding the external pressure, so that the volume vacated by the auger's extraction is concurrently and completely filled. So as to control continuity, monitoring of pile construction shall comprise

  • the control of concrete supply,
  • the concreting pressure,
  • the rate of extraction, and
  • the record of rotation of the auger. An adequate concrete supply shall be maintained to fill the pile section until the tip of the auger has reached the working level.

NOTE: It is generally necessary to bring concrete to working level in order to insert the reinforcement cage.

8.3.7 Prepacked piles Before constructing prepacked piles, trials shall be carried out for the determination of

  • the composition, flowability and setting time of the grout,
  • the spread of the grout in the packed aggregate, and
  • the necessary number and the distribution of grouting pipes. The completed and clean pile bore shall be filled with clean coarse aggregate of 25 mm size or greater with an open structure and a sufficient void ratio to allow full penetration with grout. Grouting shall be effected through grouting pipes which shall initially extend to the bottom into the pile. The grouting pressure and rate shall be such as to penetrate the open pores of the aggregate completely with grout. Where grouting pipes are to be recovered simultaneously with the grouting progress, sufficient immersion shall be maintained to provide uniform distribution of grout over the entire cross section of the pile.

8.3.8 Loss of immersion of tremie or casing When the immersion of a tremie pipe is accidentally lost during concreting, further placement shall not proceed unless:

  • concrete into which fresh concrete is to be placed has retained its workability;
  • the tremie pipe is re-immersed sufficiently deep into the previously placed concrete;
  • no water and no contamination is introduced into concrete which will remain below the final cut-off level. Otherwise the placement shall be suspended, the tremie pipe removed and alternative measures taken to form a sound pile as required. In any case where the immersion of the casing is lost and/or an inflow of foreign material into the freshly concreted section of the pile is likely to have occurred, the placement shall be suspended. The pile may be completely replaced or reformed in the original position if reinforcement can be extracted and concrete bored out, and provided these actions can be taken in time. Piles may be recovered by the formation of a construction Joint after all concrete of insufficient quality has been removed and sound concrete over the full section of the pile has been exposed forming a faultless interface. Where the preparation of a construction joint is not possible, the pile shall be abandoned and the empty bore above the concrete column be backfilled with suitable material. Integrity tests should be carried out to document the quality of any pile where the tremie pipe was re-immersed or a construction joint was made (see also 9.3).

8.3.9 Piles containing precast concrete elements, reinforcement tubes or permanent casings Precast concrete elements or reinforcement tubes shall be centred inside the bores to give symmetry of section and sufficient cover. The annulus around the element or the tube shall be grouted upwards from the bottom unless a self hardening supporting fluid is used during excavation. Where reinforcement tubes are to be filled with grout or concrete, this may be done after installation of the tube. Where precast elements or reinforcement tubes are installed in fully cased excavations and the annulus is completely filled with grout, the pile may be designed as a composite structure in accordance to ENV 1994-1-1. The bond stress between the external grout and the element or tube shall be demonstrated.

8.3.10 External grouting of cast in place piles Shaft and/or base grouting shall be carried out to cast-in-place bored piles only after the concrete has set. Only permanent grouting pipes are allowed and their arrangement shall be appropriate to the zones and materials to be grouted. Base grouting can be effected:

  • by means of a flexible box structure installed with the reinforcement, allowing the spread of grout over the whole base area of the pile; or
  • with sleeved perforated cross pipes arranged at the pile bottom, (see figure A.10). Shaft grouting shall be carried out through grouting pipes fixed to the reinforcement cage or tube or a precast concrete element as applicable (see figure A.11). Grouting shall proceed at appropriate pressures and grouting rates

  • to allow the spread of grout at the interface of the pile with the ground, and
  • to avoid hydrofracture of the surrounding ground. After the initial grout has set, second stage grouting may be carried out. Where grouting of a pile base and shaft is to be carried out, the shaft grouting is to be carried out before the base grouting, unless otherwise agreed prior to commencement of the process.

8.4 Bored pile walls

8.4.1 A template of steel or concrete should be installed at the working platform for the maintenance of the pile positions where specified accuracy requires.

8.4.2 Excavations should be supported by temporary casings in the construction of secant pile walls.

8.4.3 Normally in the construction of secant pile walls, alternate piles only should be reinforced. These reinforced piles should be constructed after the initially installed unreinforced bored piles are in place.

8.4.4 Where all piles are to be reinforced, the primary piles shall be constructed so as not to impair the later alternate pile installation.

8.4.5 The construction sequence of secant and contiguous pile walls, and the concrete composition employed, shall be chosen as such that the concrete of the primary piles has achieved sufficient strength for stability but has not developed a strength that would be too high for an intersection to be achieved.

NOTE: Otherwise imperfections of the wall (e.g. deviations or leakages) might result.

8.4.6 In the construction of secant pile walls, self-hardening slurry may be used for primary piles instead of concrete.

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