8 Execution

8.1 Excavation

8.1.1 General

8.1.1.1 When constructing bored piles measures shall be taken to prevent uncontrolled inflow of water and/or soil into the bore.

NOTE 1: An inflow of water and/or soil could cause for instance:

  • a disturbance to or instability of the bearing stratum or the surrounding ground;
  • loss of support by the removal of soil from beneath adjacent foundations;
  • unstable cavities outside the pile;
  • damage to the unset concrete in the pile or piles recently installed nearby;
  • voids in the shaft during concreting;
  • washing out of cement.

NOTE 2: There are increased risks in:

  • loose granular ground;
  • soft cohesive ground; or
  • ground which is variable.

8.1.1.2 In soils liable to flow into the pile bore or where there is a risk of collapse, means of support shall be used to maintain stability and thereby prevent the uncontrolled entry of soil and water.

EXAMPLE:

Common means of support of a pile bore are

  • casings,
  • stabilizing fluid,
  • soil-filled auger flights.

8.1.1.3 Pile bores shall be excavated until they reach

  • the specified bearing stratum, or
  • the anticipated founding level,

and shall be socketed into the founding material where and as required by the design.

8.1.1.4 In cases of

  • unfavourable stratification of the bearing layers,
  • founding on bedrock, or
  • sloping surface of the bearing layers

the excavation shall be carried down to provide full face contact.

8.1.1.5 In case of an inclined rock surface, the bottom of the excavation should be levelled for fixing of the pile base and for preventing the pile from sliding.

NOTE 1: In the case of a steep rock surface or other unfavourable stratification it can be necessary to excavate deeper or to provide the pile-bases with fixing dowels.

NOTE 2: It can be necessary to install a casing down to full contact and to seal it into the rock.

8.1.1.6 Where the ground conditions differ from those stipulated in the design, appropriate action shall be taken in agreement with the designer.

8.1.1.7 Completed excavations shall be left open only for the time necessary to allow

  • cleaning and/or desanding,
  • the various checks, and
  • installation of reinforcement, if any.

8.1.1.8 Where piles are constructed in ground which is likely to deteriorate with time and it is not possible to finish the pile by the end of the working day, a depth equivalent to

  • at least twice the shaft diameter but
  • not less than 1,5 m

shall be bored the following working day immediately before concrete placement.

8.1.1.9 If a pile bore encounters an impenetrable obstruction prior to reaching its designed founding depth, the designer shall be informed of further actions required to continue the work.

8.1.1.10 The use of explosives

  • for removing obstructions, or
  • for socketing piles into bedrock

shall not be allowed unless damage will not result to neighbouring piles or structures.

8.1.1.11 The construction sequence of piles shall be chosen so as to avoid damage to neighbouring piles.

8.1.1.12 Disturbed soil, debris or any other material that could affect the pile performance shall be removed from the base prior to concrete placement (cleaning of bases).

8.1.2 Methods and tools

NOTE: Piles can be excavated in an intermittent or continuous process:

a) tools for intermittent excavation are for example: grabs, shells, augers, boring buckets and chisels (see figures A.3 to A.6);

b) tools for continuous excavation are for example: augers, drilling or percussion tools for excavation combined with augering or flushing methods for soil removal (see figures A.7 to A.9).

8.1.2.1 The employment of

  • temporary or permanent casings
  • supporting fluids, or
  • soil-filled flights of a continuous flight auger

can be necessary to support the excavation walls.

8.1.2.2 The type of boring tool shall

  • be appropriate to the given soil, rock, groundwater or other environmental conditions,
  • be selected with a view to preventing loosening of material outside the pile bore and below its base, and
  • allow the bores to be excavated quickly.

8.1.2.3 It can be necessary to change the method or tool employed to meet the requirements.

8.1.2.4 Special tools and/or techniques other than those used for excavation may be used for the cleaning of bases.

8.1.2.5 In situations where water or supporting fluid is present inside the bore, the choice and operation of tools shall not impair bore stability.

NOTE: A piston effect with negative influence on the stability of the pile bore walls can occur and the operating speed of the tool should be adapted accordingly.

8.1.3 Excavations supported by casings

8.1.3.1 Raking piles shall be cased over their entire length if their inclination is: n ≤ 15 (Θ ≤ 86°) unless it can be shown that uncased bores will be stable (see figure 4).

8.1.3.2 Casings may be installed during the excavation process using:

  • oscillating or
  • rotating equipment

or they may be driven prior to the excavation using:

  • piling hammers or
  • vibrators or other.

8.1.3.3 The casings shall allow their safe installation and subsequent recovery during or after the concreting process, unless casings are required to be permanent.

8.1.3.4 For that purpose

  • casings shall be cylindrical and without any significant longitudinal or diametrical distortion,
  • casings shall be designed to withstand the external pressure and the forces of installation and recovery,
  • temporary casings shall be free of significant internal projections or encrusted concrete,
  • casing joints shall permit the transfer of longitudinal forces and torsion moments without significant play.

8.1.3.5 Where a cutting ring projects at the bottom edge of the casing it should be kept as small as possible, but sufficient for the safe installation and recovery of the casing (see figure A.2).

8.1.3.6 Where a pile is excavated

  • below the groundwater table in permeable ground, or
  • in artesian conditions

an internal excess pressure shall be provided within the casing by a head of water or other suitable fluid of not less than 1,0 m which shall be maintained until the pile has been concreted.

8.1.3.7 The excess pressure may be reduced if

  • a sufficient casing advancement is provided, or
  • a sufficient head of concrete is achieved during placement.

8.1.3.8 In unstable bores the casing shall be maintained in advance of boring.

8.1.3.9 The advancement in relation to the excavation shall be adjusted to suit the ground and groundwater conditions.

NOTE: The insertion of the casings ahead of boring is necessary to prevent an inflow of soil and disturbance below the pile base which can affect the pile performance ("caving in", "bottom heave"). The creation of a cavity outside the casing can endanger the integrity of a concreted pile if and when the casing is withdrawn ("necking"). Zones of loosening can also move upwards to the surface and can there cause subsidence.

8.1.3.10 The amount of casing advance or the internal excess pressure shall be increased if instability of the bottom of the excavation is likely.

8.1.3.11 Temporary casings shall not be installed into pre-excavations stabilized by supporting fluids unless special precautions are taken to prevent contamination of the concrete by that fluid.

NOTE: Otherwise "locked pockets" of that fluid might form outside the casing and could contaminate the concrete during the placement process.

8.1.4 Excavation supported by fluids

8.1.4.1 The properties of a stabilizing fluid shall be in accordance with subclause 6.5.

8.1.4.2 The fluid shall be completely or partially replaced if one of the properties of the fluid is outside the specified ranges of table 3.

8.1.4.3 Fluid recovered during excavation work or during concrete placement may be reused, after suitable processing.

8.1.4.4 The upper part of an excavation shall be protected by a lead-in tube or guide wall

  • to guide the boring tools,
  • rotect the pile bore against collapse of upper loose soils, and
  • for the safety of site personnel.

8.1.4.5 The level of the stabilizing fluid shall be such that at all times sufficient internal pressure is provided to maintain the stability of the walls and prevent migration of soil particles into the bore hole.

8.1.4.6 At all times during boring and concrete placement the level of stabilizing fluid shall be maintained

  • within the lead-in tube or the guide wall, and
  • at least 1,5 m above the external ground-water level.

8.1.4.7 The head of the stabilizing fluid may be reduced based on experience or calculations.

8.1.4.8 An adequate supply of stabilizing fluid shall always be kept available to cater for regular consumption and any potential loss of suspension into the ground.

NOTE: In cases of sudden outflow of fluid from the excavation, it can be necessary to backfill the bore.

8.1.4.9 The operating velocity of the tool shall be controlled and adjusted as necessary in order to avoid a "piston" effect that can affect pile bore stability.

8.1.4.10 Stabilizing fluids should not be used for support of excavations for raking piles with an inclination of n ≤ 15 (Θ ≤ 86°) unless special precautions are taken in installation of reinforcement and concrete placement.

8.1.5 Boring with continuous flight augers

8.1.5.1 Piles may be formed without other means of support of the bore, by using a continuous flight auger in such a way that the stability of the bore is preserved by the material on the flights.

8.1.5.2 Continuous flight auger piles shall not be constructed with inclinations of n ≤ 10 (Θ ≤ 84°), unless measures are taken to control the direction of the excavation and the installation of the reinforcement.

8.1.5.3 Boring with continuous flight augers shall be carried out as fast as possible and with the least practical number of auger rotations in order to minimize the effects on the surrounding ground.

8.1.5.4 Where layers of unstable soil are encountered with a thickness of more than the pile diameter, the feasibility of the construction shall be demonstrated by means of trial piles or local experience before the commencement of the works.

8.1.5.5 Unstable soils are considered to be:

  • uniform non-cohesive soils (d60/d10 < 1,5) below the groundwater table;
  • loose non-cohesive soils with relative density Dr < 0,3 or having a corresponding pressuremeter results;
  • clays with high sensitivity;
  • soft soils with undrained shear strength cu < 15 kPa.

8.1.5.6 During excavation the advance and speed of rotation of the auger shall be adjusted in accordance with the soil conditions so that soil removal is limited to such an extent that

  • the lateral stability of the bore wall will be preserved, and
  • over-excavation will be minmized.

8.1.5.7 For this the boring tool shall be provided with sufficient torque and traction power.

8.1.5.8 The pitch of the flights shall be constant over the whole length of the auger.

8.1.5.9 A system of closure shall be provided in the hollow auger stem to prevent the entry of soil and inflow of water during drilling.

8.1.5.10 The auger shall be lifted from the pile bore only if

  • the surrounding ground remains stable, or
  • the required depth has been reached and the surrounding ground is stabilized by the rising concrete.

8.1.5.11 If a pile can not be completed and the auger has to be removed, the auger shall be withdrawn by back-screwing and the hole shall be back-filled with soil or stabilizing fluid.

8.1.6 Unsupported excavation

8.1.6.1 Excavation without the provision of support to pile bore walls is permissible in ground conditions which remain stable during excavation and where a collapse of ground material into the bore is not likely.

8.1.6.2 The upper part of the excavation shall be protected by a lead-in tube unless

  • the excavation is carried out in firm soil, and
  • the diameter D is smaller than 0,6 m.

8.1.6.3 Piles raking n ≤ 15 (Θ ≤ 86°) or less shall not be constructed with an unsupported excavation and a full length casing shall be provided unless it can be shown that the pile bore remains stable, as for example in firm or stiff cohesive soils or rock.

8.1.6.4 If unsupported excavations pass through unstable ground strata, this part of the pile bore shall be stabilized.

8.1.7 Enlargements

8.1.7.1 The proper formation of an enlargement requires

  • a stable excavation, and
  • complete filling with sound concrete.

8.1.7.2 Enlargements should be constructed using mechanical tools allowing control of their operation from the surface.

8.2 Reinforcement

8.2.1 General

8.2.1.1 Steel reinforcement shall be stored in clean conditions and shall be

  • clean,
  • free from loose rust, and
  • loose mill scale

at the time of installation and concreting.

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

8.2.1.3 Where raking piles are constructed without a casing, suitable means of support shall be employed for the installation and the position control of the reinforcement.

8.2.2 Joints

8.2.2.1 Joints in reinforcement bars

  • shall be such that the full strength of each bar is effective across the joint, and
  • shall be made so that there is no detrimental displacement of the reinforcement during construction of the pile.

8.2.2.2 Joints between sections of reinforcement cages can require additional fixing, (e.g. by clamps or tack welding).

8.2.2.3 Reinforcing bars shall not be welded at or near bends.

8.2.2.4 Spot welding is permissible within the requirements laid down in the particular specification for the steel used.

8.2.3 Bending of reinforcement

8.2.3.1 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 stipulated by ENV 1992-1-1.

8.2.3.2 No reinforcement shall be bent at a temperature lower than 5 °C without prior approval.

8.2.3.3 Before bending, reinforcement may be warmed to a temperature not exceeding 100 °C.

8.2.4 Assembly of cages

8.2.4.1 The assembly of cages and the fixing together of bars shall be such that

  • cages can be lifted and installed without permanent distortion, and that
  • all bars remain in the correct position.

8.2.4.2 Transverse reinforcement shall

  • fit closely around the main longitudinal bars, and
  • be bound, or
  • otherwise fixed to them.

8.2.4.3 Ties or fixing shall be carried out as necessary using

  • wire,
  • clips, or
  • welding.

8.2.4.4 Additional support such as

  • stiffening rings, and/or
  • lacings, and/or
  • oblique bars

can be necessary.

8.2.5 Spacers

8.2.5.1

  • The concentric position of the reinforcement cage in the bore, and
  • the necessary concrete cover

shall be provided by spacers unless the position and the cover are otherwise provided.

8.2.5.2 Spacers shall be designed and manufactured using durable materials which will not lead neither

— to corrosion of the reinforcement nor

— to spelling of the concrete cover.

8.2.5.3 Metal pads may be used as spacers.

8.2.5.4 Where bores are uncased, large size spacers with suitable shape shall be used, so that no collapse from the walls is caused during the installation of the reinforcement.

8.2.5.5 Spacers shall be arranged symmetrically around the cage with

  • at least 3 number at each level,
  • at level intervals of not more than 3,0 m, and
  • sufficient tolerance to the inner wall of a casing or the wall of the pile bore to allow safe installation and avoid damage to the bore walls.

8.2.5.6 The number of spacers should be increased:

  • for piles of diameter D ≥ 1,2 m; and
  • for raking piles.

8.2.6 Installation

8.2.6.1 The reinforcement shall be installed as soon as possible after the cleaning of the pile bore.

8.2.6.2 The installation of the reinforcement has to provide for its alignment with the pile axis and maintain the correct concrete cover over its full length.

8.2.6.3 During concrete placement, the reinforcement level shall be maintained to provide the specified projection above the final cut-off level.

8.2.6.4 The elevation of the top of the cage after concrete placement shall be equal to the nominal value with a maximum deviation of ± 0,15 m.

8.2.6.5 For piles constructed by continuous flight augers, reinforcement installation subsequent to concrete placement is permitted if the method has been proved in the same ground conditions.

8.2.6.6 This subsequent installation shall take place as soon as possible after the completion of the concreting operation.

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

8.2.6.8 The subsequent installation may be assisted by light vibration or the reinforcement may be pulled-in.

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