Bored Piling Methodology

The purpose of the method statement is to present the operations process and methods engaged in bored pile construction works. The bored pile works is defined by the specific project element design, are necessary for the retaining support requirements of the project. It also incorporates safe work practices for bored pile construction.

This method statement intends to outline the sequence of the activities, safety considerations, and resources required for the bored piling. Periodic reviews on the specific details at the site shall be carried out and necessary adjustments may be made based on these findings.

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CONSTRUCTION PROCEDURES

Preparation and Setting Out Works

Figure 1: Construction Sequence for Bored Piles

Generally, safety preparation works must be conducted prior to any bored piling activities on site, which includes carrying out cable detection works upon piling boundary set out by Licensed Surveyor, applying necessary permits, such as permit to dig, permit to excavate, piling PTW and etc. to relevant safety department, and organizing tools box meeting to brief all the workers involved before commencing the bored piling works.

Regarding the setting out works, the procedures have been listed below. Pile location must be set out accurately at the start to all pile installation processes.

Figure 2: Setting Up of Pile Location Through Offset Method

Figure 3: Verticality Checks Via Plumb-Line Method

The surveyor will peg the position (N&S coordinates) of every pile prior to the commencement of the drilling operation.
The competent Supervisor will offset the position of the pile to X and Y direction with adequate distance. Offset surveyed position in 2 perpendicular directions by placing 2 offset markers at appropriate distances from the pile position, to facilitate the installation of temporary steel casing.
Level platform with hardcore and top with steel plate to where the boring rig will park by using an excavator
Check the verticality of the Kelly Bar of the boring rig by banks-man prior to commencement of boring.
Position the boring rig at the designated location (over the steel plates, next to the pile point) to commence boring works.
The as-built survey will be provided before and after drilling in order to check the correct casing position. The as-built coordinates will capture in the pile setting out the record.
Single piles shall not deviate more than 75mm from the designed position at the cut-off level. During casing installation, vertically is checked with a precise spirit level. Required corrections can be easily made by moving at the top end of the casing with a casing drive adapter. It should be noted that is possible to keep the mast of the rig at a vertical position at any time.
The as-built survey of pile position and pile top level will be carried out after exposed of pile head.

Installation of Temporary Steel Casing

To carry out the excavation works for bored piles, a temporary steel casing with a wall thickness of 20mm shall be installed.
The insertion of the temporary casing is to ensure the loose and soft soil at ground level does not erode and collapse into the bore-hole, thus interfere with the boring process later.
Installation of temporary casing will be carried out with vibro-hammer or turn-table depending on the site condition.
During casing installation, checks are made to correct positioning and verticality. The necessary adjustment shall be made by extracting and re-driving the casing until the deviation is within tolerance.
The surveyor shall verify the position of the steel casing through a series of verticality checks. The supervisor shall employ the Plumb-line method and spirit level to measure the steel casing’s verticality, project downward, and verify the correct pile position.
Tolerance allowance in the eccentricity of piles/steel casing to within ±75 mm at cut-off-level in any direction on the plan, and verticality of 1/75.
Casing length uses approximately 3 to 6 metres (until stable soil strata).
The casing top shall be left a minimum of 600mm protruding above ground subject to site condition and discretion.

Construction Procedures

Preparing of Drilling Fluid/Slurry

Drilling fluid will be supplied from the site plant for the construction of bored piles. Water will be used as drilling fluid and it is depends on soil condition. If necessary, polymer is used in the construction of the bored piles.

Specific recommendations of the stabilizing fluid quality parameters are shown on the table below.

Formation of Borehole

Upon completing the casing installation, fill the empty borehole with drilling fluid to maintain the hydrostatic pressure within the soil.
Commence the boring process until it has reached the design pile toe level.
Collect soil sample at intermediate of 3m depth from the cut-off level to the pile toe level by the geologist on-site to confirm the toe level.
Maintain drilling fluid level at about ground level during drilling as well as drilling fluid properties during boring.
Stop the boring works when the designated toe level has been achieved.

Possible Obstructions

Following the design note of the construction of pile, we are requiring to do rock socketing. The rock shall be determined with the Point Load Test equipment. Point Load Index need to be determined before commencement for every 2m drilling in rock. Further to the design requirement, the rock socket is determining when the Point Load Index is exceeding 1.5MPa.

Cleaning Works of Base

Clean the base of the borehole to remove any loose, disturbed or remolded soil with a cleaning bucket.
Measure the depth from the base of the borehole to the top of the temporary steel casing, then confirm the depth of the borehole.
Conduct final cleaning works with cleaning buckets and re-confirm the depth. Record pile toe as measured.

Circulation Method

Figure 4: Circulation Method

The purpose of circulation is to separate out heavier and lighter particles in suspension using vibration and screen mat at the circulation plant. Silo plant and the circulation plant will be set up for the slurry supply, as well as to carry out circulation for the test piles and bore piles. For polymer, do not need a recirculation process.
After boring works are completed at the desired depth, test the slurry property in use according to the drilling fluid Test Form stipulated on density and viscosity.
If the slurry collected in the bore pile has failed the stipulated criteria, lower submersible pump with tremie pipe into the bore pile to carry out circulation works.
If the slurry passes within the range of stipulated criteria, no circulation works would be required to perform.
Retrieve all the tremie pipes and submersible pump from the bore-hole
Prepare to lower steel cage.

Table 4_ Characteristics for Drilling Fluid Suspensions (extracted from BS EN 1538_2000).p

Installation of Steel Reinforcement Cage

Figure 5: Cage Installation Process

Figure 6: Details of Cage Installation

The fabricated steel cage comprises of straight main bars spaced evenly and bounded by links spirally as per approved shop drawings.
The cages with stiffening rigs (if required) and other accessories to enable handling, lifting, and installation without permanent deformations. The cages will be securely fixed using manually tied galvanized wire and subsequently bolted with “U” bolt at predetermined cages, which are required to be pre-fabricated in lengths exceeding 12 meters.
Hook the wire sling with shackles onto the temporary support.
The service crane will hoist the steel cage via the wire sling to the vertical position, swing over to the top of the borehole, and lower the cage through the temporary casing into the borehole.
Spacers are secured to the links at a regular interval of no more than 3m. The spacer is to ensure positioning of reinforcement cage with appropriate concrete cover (approximately 75mm cover space).
A reinforcement cage is held at the temporary steel casing top by inserting T25 or T32 short rebars below the temporary support links. Dislodge the wire sling with the shackles from the support links.
When another cage is hoisted, lower and lap to the cage below using U-clips at every main bar. Place additional U-clips at the section of the reinforcement cages where the main lifting point will be located. If multiple cages are required in the bored pile, the reinforcement cage will be joined by U-clips.
When both cages are secured, the service crane will lift up the cages at a short distance while the T32 support bars will be then removed. Service crane will then lower down the cages into the borehole.
The process is repeated until the required length is achieved.
For the installation of the last reinforcement cages, support the cage on steel casing with the pre-determined rebar for connection. Dislodge the wire sling with shackles from the main links.
Hoist and lower the hangers together with the reinforcement cages. The hangers will be made up of a hook, hangers of different lengths will depend on the rebar top level.
Suspend the cages by hooking the hanger onto the casing at the top. This is to secure the reinforcement cage in position and maintain the verticality of the reinforcement cage during concreting works.

Pile Toe Cleaning Prior to Concreting

Upon completing the jointing and lowering of tremie pipes, sounding check on pile toe will be carried out to ensure no soil erosion occurs during cages installation.
Sediments will present due to the time taken for the installation of the cage/or high pore-water pressure presence beneath.
Sounding checks at pile toe shall not exceed 500mm difference with the earlier recorded depth. If exceeding 500mm, the air-blow method to pile toe will be carried out to stir up the sediments and to provide a reasonably clean pile toe.
Insert a steel tube or use a tremie pipe to the bottom of the pile toe, pump air with water hose inside using an air compressor to stir up the sediments.
Compress airflow within the tube is controlled to mix with water to reduce the “blowing-effect‟ created by compressor air, thus minimize the chance of further erosion caused by air-bubbling along the shaft.
Move the steel tube to stir up the sediments at the toe to create the up-flow movement of sediments with air and water continuously.
Overflow slurry at the top will be siphoned back to the plant.
Air-blown processes take about 5 minutes and commence discharge concrete at the tremie pipe, retrieve the steel tube while discharging concrete if steel tube is used.

Concreting Process

Figure 7: Concreting Process of Bored Pile

Use a tremie pipe of a diameter of 203mm OD for 800mm diameter pile, but may vary at site fabrication to suit discretionally by the construction manager on the sizes of the pile.
Join sections of the tremie pipe prior to lowering the pipe into the borehole until the tremie toe is approximately 500mm above the pile toe.
Introduce vermiculite into the tremie pipe to function as the separator between the concrete and the drilling fluid.
Place concrete into the borehole via a hopper attached to the top section of the tremie pipe. The concrete slump for the tremie method to be confirmed with material use for drilling fluid and concrete mix.
Withdraw the tremie pipe from the borehole as the concrete level rises. This is to facilitate the flow of concrete. However, it is important to note that the tremie pipe is to be embedded not less than 3m below the concrete level.
Measure and record the concrete level in the borehole when discharging concrete from each truck. Dismantle the tremie pipe from the top, sections by sections.
Pump the slurry back to the storage silos by a submersible pump when the slurry is displaced by the volume of concrete. However, it is important to note that the pumping rate of the slurry shall not exceed the discharging rate of the concrete.
The level of slurry shall be maintained at ground level to prevent erosion of the borehole during concreting works. Terminate the concreting works when the sound concrete level in the borehole is over-flown from the casing top.

Extraction of Pile Casing

When steel casing is required to be extracted, a sufficient amount of concrete (3~4m depth) is maintained within the casing in the last few metres to ensure pressure from external soil and groundwater is exceeded to deter pile from reducing in section nor contaminated. This is when the pile cut-off- level is designated to be at the casing top level for the test pile.
Before casing extraction by Vibro-hammer, the site supervisor shall check the concrete level inside the casing and topping up from the concrete truck upon necessary.
Cut loose the cage hanger bar from the casing top before the extraction of the temporary casing. After extraction, 1.5m to 2.0m length of permanent casing is installed at the surrounding of the test pile. The permanent casing top shall be protruded 300~500mm above the ground level. This is for setting up the load test equipment at a later stage. The empty space inside the permanent casing shall be filled with concrete from the truck until it is over-flown.
The surface of the test pile is leveled and the vicinity of the test pile is barricaded with a warning sign put up.
Record all field-check information into record form.
The above method and procedure for the test bored piles construction including the casing installation method, cage lowering process, casting of concrete process and its placing methods shall be at the discretion of the construction manager, based on his professional judgment to adapt the methodology based on actual site conditions.