BORED PILE

GENERAL

The method statement refers to the construction of hydraulically excavated bored piles. In general the large-diameter bored piles shall be excavated by driving the temporary casing using high capacity drill rigs. The principal methods in the following sections shall be subject to review during the progress of the construction and may be amended, if so required.

STANDARDS AND SPECIFICATIONS

All piling works shall be performed in compliance with the requirements of ACI 336 code of practice and subcontract specifications.
In case any circumstances indicates different conditions from those expected or require special procedures for the execution of the works such circumstances shall be reported to the Engineer.

MATERIALS

Materials that shall be used during construction shall provide the specifications mentioned below;

*Materials*

Concrete		As per project specifications

Reinforcing Steel		As per project specifications

Cement		As per project specifications

*Slump Range*

Min. 175 mm		All conditions except placement under a drilling slurry

200 mm ± 25 mm		Placement under a drilling slurry (polymer or slurry)

To insure against segregation, the maximum aggregate size should be in the 19 mm range and the sand-cement content should be high compared to the coarse aggregate content.
The clear spacing between bars of the rebar cage should be at least four times the size of the maximum coarse aggregate. Bentonite and potable water shall be suitable for concrete and slurry. All materials shall be consistent with project specification for designing, furnishing, installing of piles also all testing and material specification.

SETTING OUT

Survey and record the existing ground level at each pile location.
Set out the pile location in accordance with the Engineer’s working drawings using a totalstation with the accuracy set by the specifications. Pile exact location shall be marked by fixing steel peg embedded properly in the ground and approved before the excavation (drilling). Steel peg shall be used to setting out for the drilling rig Kelly. Lines, levels, contours and datum shall be established from the existing onsite benchmarks. All exsiting monuments and benchmarks shall being preserved. Benchmarks disturbed by the Subcontractor shall be re-establihed by the Subcontractor as directed by the Contractor. All setting out should refer to the reference points and the related Subcontract Drawings.
In order to monitor the position of the steel casing, control pins shall be established with two orthogonal positions offset from the centre of the pile.

PREDRILLING (if necessary)

Totally three predrilling shall be done for bored piles to identify the soil profile and determine the method of drilling. This predrillings shall be positioned away from production piles in the location shown on the plans or as directed by the Engineer. The predrilling shall be sunk at least the pile bottom elevations in the aplication projects

TOLERANCES

The following construction tolerances apply to drilled shafts unless otherwise stated in the
Subcontract documents:

The center of the pile shall be within 76 mm (3 inches) of plan position in the

horizontal plane at the plan elevation for the top of the shaft.

The vertical alignment of a pile shall not vary from the plan alignment by more than 20 mm per meter.
After all the concrete is placed, the top of the reinforcing steel cage shall be no more than 152 rnm (6 inches) above and no more than 76 mm (3 inches) below plan position.
The top elevation of the pile shall have a tolerance of plus 25 mm (1 inch) or minus 76

mm (3 inches) from the plan top-of-shaft elevation.

The checks shall be repeated in cases where major underground obstructions are encountered. If the location of the excavation is so much outside the required tolerance it shall not be corrected, the hole shall be backfilled and the temporary casing shall be extracted. The excavation procedure shall be repeated again in order to satisfy the particular requirement.

BORED PILE CONSTRUCTION
General:

Drilling shall start after the verification of pile position and verticality of the drilling tools. After understanding of soil properties along the pile shaft and the depth of water table, specific piling construction method shall be chosen. The piles can also be executed by using temporary casing through the length of the pile, if necessary.

Protection of Existing Structures:

The Subcontractor shall control his operations to prevent damage to existing structures and utilities. Preventive measures shall include, but shall not be limited to, selecting construction methods and procedures that shall prevent caving of the shaft excavation, and monitoring and controlling the vibrations from construction activities such as the driving of casing or sheeting, drilling of the shaft, or from blasting, if permitted.

Excavation of Pile Shaft and Installation of Permanent Liner

The location of the centre of each bored pile shall be set out by reference to the working drawings and the site boundary markers. The shaft shall be excavated within a heavy temporary steel casing with an appropriate outside diameter. There is a special cutting shoe at the toe of the casing for the bottom section. The casing section shall be connected together using special casing joints. The casing shall be driven by drill rig or vibro-hammer, if needed. Additional casings shall be used if necessary for drilling.

Dry Construction Method

If the static ground water level (G.W.L) is very close to the approximate ground level and if highly permeable soil layers are available, dry construction methods shall not be allowed to use while drilling.

Casing Construction Method

Casing method could be used in order to prevent hole caving or excessive deformation of the hole. However in any case a temporary casing of minimum 3m long shall be used during the construction of each pile. The upstand of the casing above the surrounding ground level shall be 1.20 m minimum for safety and to prevent contamination of the concrete in the hole during concreting. The initial casing shall be driven into the ground as much as possible before starting to bore with auger.
Drilling of the piles will be accomplished by an adequate drilling equipment. Augers or buckets of right dimension will be used during drilling above and below groundwater level. The diameters of the drilling tools shall not be less than the pile diameter specified by the piling project drawing. The auger and bucket diameter shall be checked as necessary and recorded to ensure that specified diameter is achieved.
A tolerance of 5 % on the auger width (diameter) is permissible. The temporary casings shall be of uniform cross-section throughout each continuous length and be of quality and material, length and thickness adequate for the purpose of preventing water and unstable soil entering into the drilled shaft.

Bottom Cleaning

When the borehole is drilled down to desired depth, the bottom of the hole will be cleaned by a cleaning bucket (one of the drilling buckets modified for that purpose) at first. The material extracted out at cleaning will be examined and the decision to continue will be made by the engineer (depending on the extracted material is sludge or soil).

Construction Under Water/Bentonite Slurry(if necessary)

If necessary water or bentonite slurry shall be used to ensure bore stability during drilling and counteract any potential base heave due to artesian pressure. Borehole will be drilled dry by driving temporary casings down to the specified layer. Augers and buckets will be used respectively during drilling according to the soil conditions. Top of casing will be kept min. 1m above ground. Final height of the casing will be checked by Main Contractor’s surveyor
After, drilling reaches the bottom level of casing, borehole will be fulfilled with water/slurry and drilling will be then continued under a continous water/slurry pumping. Water/slurry head will be maintained at a level not more than 1m below existing ground level at all times during drilling. Drilling penetration will not be kept long and the kelly bar will be pulled out slowly to avoid turbulences and suction effects. Kelly bar will also be taken out slowly at the slurry head level (and may be stayed there for a while) in order to have enough time for pumping to replace excavated soil volume. Thus, sudden drawdown in slurry head will be prevented (speed of kelly bar should be 8-10m/min). Additional 3-4m long casing will be kept ready nearby the hole. In case of a touch to artesian level, drilling will be stopped, additional casing will be mounted and fulfilled with water/slurry immediately. Thus, a higher hydrostatic water/slurry pressure can be maintained. Drill depth will be checked by the deep meter in operating cabin and manually by tape measure as well. Drilling information will be recorded in pile reports. Existing fill material which is going to be extracted while drilling will be deposited on another side to re-use for backfilling after completion of concreting. • Then, slurry in the borehole will be sampled from the bottom by means of a sampler. The sampler is composed of one cylindrical steel can, two steel balls and a steel wire which goes through the center of both can and balls.
Then slurry will be tested in terms of sand content, density, viscosity etc. Testing of bentonite should comply EN 1538. In case test results from bottom sampling do not meet the requirements, slurry will be de-sanded by submersible pump. It will be lowered completely down to the bottom (50cm above the base). Desanding will last until the slurry tests meets the criterias. Slurry sample will be taken from the outlet of discharging pipe while pumping out. Then after, borehole bottom will be re-cleaned by cleaning bucket. Drill depth will be re-measured by the deep meter in operating cabin and manually by tape measure as well.

Excavation and Drilling Equipment

The excavation tools shall be of adequate design, size and strength to perform the work shown in the plans or described herein. When the material encountered cannot be drilled using conventional earth augers with soil teeth, drill buckets, grooving tools, the Subcontractor will provide special drilling equipment.

Obstructions

Surface and subsurface obstructions shall be removed by the employer. Such obstructions may include man-made materials such as old concrete foundations and natural materials such as boulders.
The drilling subcontractor will be responsible from drilling and excavating only the material that mentioned in the soil investigation report.

Casings

All subsurface casing shall be considered temporary unless specifically shown as permanent casing in the Subcontract documents. Casings shall be steel, smooth, clean, watertight, and of enough strength to withstand both handling and driving stresses and the pressure of both concrete and the surrounding earth materials.

Excavation Inspection

The Subcontractor shall provide equipment for checking the dimensions and alignment of each shaft excavation. The dimensions and alignment shall be determined by the Subcontractor under the direction of the Engineer. Final shaft depths shall be measured with a suitable weighted tape or other approved methods.
Shaft cleanliness will be determined by the Engineer, by visual inspection for dry shafts or other methods deemed appropriate by the Engineer for wet shafts. In addition, for dry excavations, the maximum depth of water shall not exceed 75 mm (3 inches) prior to concrete pour. For dry shafts, the sidewalls shall be visually free of cuttings that may have been smeared on the walls during the removal and insertion of drilling tools.

Construction Tolerances

The above mentioned ( in title6.) construction tolerances will be applied to piles unless otherwise stated in the Subcontract documents.
The checks shall be repeated in cases where major underground obstructions are encountered. If the location of the excavation is so much outside the required tolerance that it cannot be corrected, the hole will be backfilled and the temporary casing will be extracted. The excavation procedure will be repeated again in order to satisfy the particular requirement.

Reinforcing Steel Cage Construction And Placement

The reinforcement cage for the pile will be prefabricated on site in required lengths in accordance with the details shown on the drawings
Consisting of longitudinal bars, ties, cage stiffener bars, spacers, centralizers, and other necessary appurtenances, shall be completely assembled and placed as a unit immediately after the shaft excavation is inspected and accepted, and prior to concrete placement. Internal stiffeners could be removed as the cage is placed in the borehole so as not to interfere with the placement of concrete.
Each section of reinforcement cage will be lifted into the pile by crawler crane and the next section will be spliced together. Splicing of the main bars will be done by overlapping the longitudinal bars and attaching appropriately with tie wires at minimum 2 elevations. Also top and bottom stirrups of the each section will be overlapped together and firmly attached with tie wires. Tie wires shall be used at 4 points on spliced stirrups and 4 turns of tie wires shall be applied at each point.
In order to prevent elusion of the stirrups while hanging the complete reinforcement cage, the first three stirrups of the cage at the top shall be attached to the longitudinal rebars. Tie wires shall be used at 6 points on each stirrup and 15 turns of tie wires shall be applied at each point.
The reinforcement cage in the shaft shall be tied and supported so that the reinforcing steel will remain within allowable tolerances of this specification. Concrete spacers shall be used at sufficient intervals to ensure concentric spacing for the entire cage length. Spacers shall be constructed of approved material equal in quality and durability to the concrete specified for the shaft. The spacers shall be of adequate dimension to insure a minimum 75 mm annular space between the outside of the reinforcing cage and the side of the excavated hole. The elevation of the top of the steel cage shall be checked before and after the concrete is placed.orrections shall be made by the Subcontractor to the satisfaction of the Engineer.

Concrete Placement

Concrete shall be placed as soon as possible after reinforcing steel placement. Concrete placement shall be continuous from the bottom to the top elevation of the shaft.Concrete shall be placed by tremie pipes .
The elapsed time from the beginning of concrete placement in the shaft to the completion of the placement shall not exceed 2-hours. Admixtures such as water reducers, plasticizers, and retarders shall not be used in the concrete mix unless permitted in the Subcontract documents. All admixtures, when approved for use, shall be adjusted for the conditions encountered on the job so the concrete remains in a workable plastic state throughout the 2-hour placement limit.
Prior to concrete placement the Subcontractor shall provide test results of a slump loss test. The slump loss tests shall be conducted using concrete and ambient temperatures appropriate for site conditions. All concrete placing procedure should be in accordance with the ACI 309 code of practice.

Tremies

Tremies shall be used for concrete placement in all shafts. Tremies used to place concrete shall consist of a tube of sufficient length, weight, and diameter to discharge concrete at the shaft base elevation. The tremie shall not contain aluminum parts that will have contact with the concrete. Minimum and maximum tremie inside diameter (D) shall be 6in and 14in, respectively. The inside and outside surfaces of the tremie shall be clean and smooth to permit both flow of concrete and unimpeded withdrawal during concreting. The wall thickness of the tremie shall be adequate to prevent crimping or sharp bends, which restrict concrete placement.
Underwater or underslurry placement shall not begin until the tremie is placed to the shaft base elevation, and the concrete shall be kept completely separated from the water or slurry prior to the time it is discharged.
The discharge end of the tremie shall be constructed to permit the free radial flow of concrete during placement operations. The tremie discharge end shall be immersed at least 1.5 m in concrete at all times after starting the flow of concrete. The flow of the concrete shall be continuous. The level of the concrete in the tremie shall be maintained above the level of slurry or water in the borehole at all times to prevent water or slurry intrusion into the pile concrete.
If at any time during the concrete pour, the tremie line orifice is removed from the fluid concrete column and discharges concrete above the rising concrete level, the shaft shall be considered defective. In such case, the Subcontractor shall remove the reinforcing cage and concrete, complete any necessary sidewall removal directed by the Engineer and repour the shaft. All the defective shafts shall be replaced.

Sequence of Pile Construction

The sequence of construction of the piles will be chosen in such a manner that no damage can be caused to nearby piles still under construction or recently completed. It is not allowed to drive a casing or drilling a shaft closer than about six pile diameters clear spacing to the shaft with newly set concrete. The setting duration could be assumed as 24 h. The piling sequence will be submitted for Engineer’s approval.

MONITORING

For testing the constructed piles, pile integrity test shall be carried out. Detailed method of statement about type and process of integrity tests should be submitted before performing the tests. Pile load tests shall be performed according to the project specification. Quantity and type of load test, location and method of statement shall be submitted for approval before execution.

SAFETY PRECAUTIONS

All the staff working at the site including engineers, operators, foreman and the workers should obey the following instructions to provide safety at the site;

Not entering the working area of the rig
Not performing any activity around the rig unless the rig stop
Not making any operation at the back side of the rig
Not starting the activity unless making the eye contact with the operator
Not performing any activity with the self authority
Not allowing the entrance of unauthorised person to the working site
Protecting the empty holes and newly puored holes with the cap and by lining around
Not allowing to operate the ready mix concrete transmixers by unauthorised person
Always using self protective eguipments (gloves,air plugs,steel toe boots,dress,etc.)

In addition to these statements the staff shall obey the general Health and Safety
regulations of the contractor without any excuse.

RECORDS

Full records of the construction of the bored pile will be submitted to the Engineer after the completion of each pile.