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Bored Pile (Cast-in-Situ) Construction Method Statement

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The purpose of this document is to outline the proposed methodology for the construction of the cast-in-situ bored piles using temporary casings.

Aim of the procedure is to detail the sequence and method of pile construction, identify the hazard, assess the associated risks involved, develop and implement adequate control measures and to provide a safe place for safework.

The contractor will ensure that documentation is generated to record the installation activities and to monitor the applicable quality assurance requirements.

All personnel will receive a site specific induction. Before any work commences a method of statement briefing will be conducted by the site management. Operatives will confirm their understanding by signing the briefing register.

Under no circumstances shall any employee or subcontractor undertake works/activity they perceive to be unsafe or commence an operation they do not fully understand.

Relevant Documents, Standards & Codes

The Institute of Civil Engineers (ICE) – Specification for Piling and Embedded Retaining Walls

BS 8004: 1986– Code of Practice for Foundations

BS81 10: 1997 – Structural Use of Concrete

Particular Project Specification

Project Quality Plan

Project HSE Plan

Mandatory Plant & Equipment

Materials to be Used:

Concrete  fcu = 35 N/mm2 or as per bored pile specification

Steel Reinforcement Fy= 375 N/mm2 or as per bored pile specification

Binding Wire

Concrete Spacers (75mm) or equivalent

Flooring Steel Plates

Roles & Responsibilities

The project manager / civil engineer shall be responsible for overall operation activities for bored piles works, also ensuring that all safety measures are implemented.

Site engineer shall be responsible also for the coordination and supervision of construction activities and coordinate with other agencies and ensure that the work is done as per specification and approved drawings and ensuring all safety measures are implanted as per project HSE plan.

Owner’s / client’s designated site Engineer/ Supervisor shall be supervising closely the activities designated to them and ensure that all instructions are followed and all necessary documents are in place prior to commencing the works and safety procedures of the project is strictly adhered to.

Site QA/QC engineer shall be responsible for monitoring the quality of workman ship and implementation of project quality requirements.

Site safety personnel are responsible for the enforcement of safety and ensure that the works are carried out in accordance with company safety procedures and in compliance with the HSE authorities.

Health and Safety Requirements

A risk assessment for the bored piling works shall be prepared for the project.

All personnel involved in the above activities shall be introduced with the risk assessment in order to provide them with the required knowledge of the works and to prepare them for the activities. Workers / personnel shall respect the requirements of HSE plan.

All Operative shall be made aware of the HSE requirement for drilling, steel fixing and concrete casting works.

Appropriate Personal Protective Equipment’s (PPE) shall be available and worn by all operatives on the project site.

All non-motorized equipment’s and tools will be mechanically lifted into the work area.

Only trained and competent slingers will sling and direct any loads lifting activities.

Bored Pile Construction Work Method

The method of installation of the Bored Cast-In-Situ Piles is detailed below.

Working Platform:

The drilling works will be carried out from a levelled working platform.

Working platform shall be prepared and maintained with suitable compacted material or steel plates matting for the movement of the equipment and the ready mix concrete trucks.

The site shall be levelled and spacious enough to allow the machineries to move freely to each point and every location without any obstruction.

During the preparation of the working platform and prior to the start of the drilling, an investigation should be carried out to assure the absence of any obstructions at the pile locations.

All utility services, old foundations, obstacles or any objects interfering with the drilling works shall be removed or diverted.

Survey Works:

A qualified surveyor will be employed to carry out all settings-out and surveying works.

The centre point of each pile shall be fixed by the surveyor using coordinate system. These points could also be checked and confirmed by the consultant/client.

The points will be marked by fixing steel pegs on the working platform.

After checking of points, the pegs will be driven into the ground to protect it from getting disturbed.

Bored Pile Construction Sequence

The procedure for the construction of the Cast-In-Situ Bored Piles comprised of the following key elements:

Setting up at the pile location:

Piles will be drilled in a set sequence that minimizes long movements and time wastage.

A working sequence will be prepared by the site engineer on a daily basis and submitted to the client/owner 24hrs ahead.

Piles drilled will be concreted on the same day.

Placement of the temporary casing:

The temporary steel casings are used for the pile alignment and for the control of the soft upper layer.

Wall Thickness of the pipes shall be around 14mm to allow for rigidity.

Length of these pipes will depend on the depth of the soft upper-layer and the water level.

The condition of these casings shall be free from significant distortion and shall have a uniform cross section throughout the length.

Casings shall be installed by vibration using a vibro-hammer of sufficient power or by rotation using the drilling rig.

The Casing will be installed within a maximum deviation from the center point of the pile of 75mm in any direction.

Drilling of the Piles:

After fixing the casing concentrically to the pile center, the borehole shall be drilled down to the set toe level in accordance with the approved pile design and shop drawings by the use of buckets and/or augers mantled on the Kelly bar of the drilling rig.

Verticality of the holes shall be continuously monitored by the on-board electronic inclinometer and/or the use of a spirit level placed on the Kelly Bar.

The alignment of the bored pile shall be made vertical to a tolerance of within 1 in 75.

Subsequent to the completion of the drilling, the borehole depth and coordinates shall be verified with respect to the shop drawings and the project’s coordinates by the client/owner.

If the consultant is also requiring verification, time shall be allowed for him to perform his inspection and provide approval.

Inspection time shall not exceed two hours to allow time for concreting at the same day.

Cleaning of the Socket:

There are two methods of cleaning the borehole socket i.e. by the use of Cleaning Bucket/ Sealed Bucket and by the use of Airlifting (Air Compressor).

Bored Piling Reinforcement:

Once all inspections are conducted and prior to concreting time, the bottom of the borehole (socket) will be cleaned by the use of a cleaning bucket.

Only after the cleaning of the socket is assured, can the reinforcement cage be lowered into the hole.

A reference site datum level shall be given by the surveyor to ensure the correct positioning of the cage with respect to the final cut off level.

The reinforcement for the piles shall be cut and bent on site and/or the workshop, according to the requirement of the specification and the approved design of the piles.

Reinforcement cages shall be fabricated as per the shop drawings.

The cages shall be securely fixed manually using tied galvanized wires.

Plastic spacer or equivalent (75mm) shall be tied to the cage to assure the adequate concrete cover of the pile.

The reinforcement cage shall be lowered to the required level by the use of a crane.

Cages will be securely suspended in the bore using tie rod and hook system to the temporary casing.

Hooks will only be removed after the concrete has been casted and prior to the extraction of the casing.

Casting of Concrete:

The concrete shall be delivered to site by means of truck mixers.

Concrete delivery tickets shall be checked for correctness in accordance with the approved mix design.

The concrete shall be checked for the slump and temperature.

Cube samples shall then be taken as per specification.

The Concrete shall only be allowed on-site after the quality has been found in compliance with the specifications.

Concrete shall be supplied to the pile by transit mixers and placed by the full length tremie pipe method.

A mobile concrete pump shall be used to transfer the concrete from the truck mixer to the pile.

The flexible hose of the concrete pump shall be connected to the tremie pipe that shall be lowered into the center of the pile by the use of a crane or direct from the truck mixer to the concrete hopper attached to the tremie pipe.

Concrete shall be poured without interruption to prevent the creation of joints.

The tremie tube size shall not be less than 150 mm.

Tremie pipe shall have sufficient length to reach the toe of the pile.

The tremie pipe shall be inserted at the centre of the pile to reach up to the toe.

Bottom of the tremie pipe shall remain throughout the casting process of at least 2m embedded inside the concrete.

The concrete shall be brought up to the working level or minimum 1.0m above cut-off level to ensure good quality and contamination free concrete at pile cut-off level.

After concreting, the temporary casing shall be withdrawn by rotation using the piling rig or a vibro-hammer.

When the casing is being extracted, a sufficient quantity of concrete shall be maintained within it to ensure that pressure from external water or soil is exceeded and that the pile is neither reduced in section nor contaminated.

Disposal of drilling spoils:

Spoils/slurry resulting from drilling of boreholes shall be stored on site and carted away at regular intervals.

Pile Head Breaking:

Upon achieving the required strength of the concrete for the piles, and in conjunction with the load testing program for the project, the pile heads can be trimmed to final level.

Piles were casted higher than the final level to control laitance (impurities which have migrated to the top of the piles) during the construction of the piles.

Cropping of the pile head is required to remove all the additional concrete to the specified level, while leaving reinforcement bars protruding to the designed length to allow adequate lap and/or anchorage in the Pile Cap.

Excavation around the piles and as per pile cap dimension shall be prepared.

A maximum of 200mm of soil under the cut-off level shall be excavated.

The cut-off level shall be marked on the pile and then saw cut 1 to 2 inches inside the pile to limit the breaking surface.

Demolition of the excess concrete above the cut-off level shall be carried out by operatives using jackhammers.

The use of a pile cropper can be allowed for the top part of the pile but not for final breaking to level.

Removal of the concrete to the correct level must not introduce damage to the pile concrete below the cut-off level, as this will affect the workability of the piles.

Piling Records:

Piling Records, provided by the foreman will be checked by the site in charge before final submittal to Engineer on the Bored Pile Record Sheet.

A sample pile report record includes the following data:

Any unexpected drilling condition encountered will be noted briefly in the records, and brought to the attention of the consulting engineer immediately.

Standard report forms are attached with the method statement submittals.

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