Method Statement

Construction of Large-diameter Bored Piles











1.      GENERAL

The method statement refers to the construction of mechanically excavated bored piles. In general the large-diameter bored piles will be excavated by driving the temporary casing using hydraulic oscillators. The principal methods in the following sections are subject to review during the progress of the construction and may be amended if so required.




2.1   All piling works will be performed in compliance with the requirements of the respective regulations, codes of practice and contract specifications.


2.2   In case any circumstances indicate different conditions from those expected or require special procedures for the execution of the works such circumstances will be reported to the Engineer.




3.1   Survey and record the existing ground level at each pile location.


3.2   Set out the pile location in accordance with the Engineer’s working drawings using a theodolite with the accuracy required in the specifications. All setting out should refer to the reference points and the related Contract Drawings.


3.3   In order to monitor the position of the steel casing, control pins will be established with two orthogonal positions offset from the centre of the pile.





4.1   Predrilling shall be done for every large diameter bored pile to identify the suitable grade of rock consistent with the design and to determine the founding level. 


4.2   The predrilling shall be sunk at least 5 meters below the founding level of each pile.


4.3   All pre-drilling and proof drilling as well as any core-drilling on the constructed piles shall be carried out by an independent Ground Investigation Contractor who should be registered under the Work Category "Ground Investigation Field Works" of Specialist Contractors for Public Works, and who is required to make a declaration that it is not a holding company, a subsidiary company, an associated company or a related party of the Contractor, and that it has no financial stake in the piling works to be tested.




5.1   After setting-up the casing oscillator over the pile location, the first section of casing will be inserted into the oscillator. Its position and verticality will be checked prior to commencing excavation and regularly during excavation.


5.2   The verticality will be checked by placing a spirit level against the casing; adjustments can be made by the oscillator to keep the vertical alignment and plan location within the allowable limits.


5.3   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.




6.1   Excavation of Pile Shaft and Installation of Permanent Liner


6.1.1        The location of the centre of each bored pile will be set out by reference to the working drawings and the site boundary markers. The shaft will be excavated within a heavy temporary steel casing with an outside diameter of 1.5m. The casing is composed of sections of 6 to 8m single lengths with 50mm wall thickness. There is a special cutting shoe at the toe of the casing for the bottom section. The casing section are connected together using special casing joints. The casing will be driven using hydraulic casing oscillator connected to a crawler crane.


6.1.2        After setting-up the casing oscillator over the piling location, the first section of the casing will be inserted into the oscillator and its position and vertically shall be checked prior to commencing excavation and regularity during construction.


6.1.3        The pile shaft excavation will be carried out using a single or hammer grab supported from a crawler crane. The casing will be advanced by means of the hydraulic casing oscillator and the toe of the temporary casing will be kept in front of the excavated level. Particular attention will be paid to this requirement when soft materials are expected or encountered in the pile shaft (e.g. marine clay, loose fill material). When the materials in the pile shaft cannot be excavated by hammer grab then either a star formed heavy drop chisel or reverse circulation drill will be used to overcome the obstructions.


6.1.4        The water level within the pile shaft will be maintained at or above the surrounding ground water level to ensure that there is no differential head encouraging piping of the soil at the base of the excavation.


6.1.5        The above procedures for the excavation of the pile shaft will be followed until the top of rock socket as pre-determined by site investigation is reached. The permanent casing will then be installed down to the top of rock socket level. The permanent casing will be cut off 750mm above the cut off level. Where required the permanent casing will be joined by continuous welding.


6.1.6        The required pile socket inside the permanent casing will be formed by either reverse circulation drilling or by heavy drop chisels. The rock fragments will be removed by hammer grab. All excavated material shall be stockpiled a minimum of 6.0m from the excavation.


6.1.7        On completion of the formation of the shaft of the pile, excavation will be thoroughly cleaned by means of an airlift.


6.1.8        Excavated material not suitable for incorporation into the works will be removed off site.


6.2   Cleaning of Pile Base

Following the completion of the excavation, the bored hole will be thoroughly cleaned by means of an airlift. Cleanliness of the bored hole will be monitored by dipping the base of the excavation with a weighted tare and by observing the colour of the water coming out from the airlift. Airlifting will continue until clean water or negligible debris in suspension is detected.


6.3   Installation of Reinforcing Cage


6.3.1        The reinforcement cage for the pile will be prefabricated on site in 12m lengths in accordance with the details shown on the drawings together with a make up top section of appropriate length for each pile.


6.3.2        In addition to the main bars and binders shown on the drawings 25mm diameter mild steel rings will be fixed inside the main bars at appropriate centres to act as stiffeners to prevent distortion of the prefabricated cages during handling. The stiffeners will be spot welded to the main reinforcement with approval from the Engineers.


6.3.3        Each 12m section of reinforcing 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 welding if the composition of the steel is such that the carbon content complies with the requirements of BS 4449 for welding of steels or alternatively rope clamps might be used, subject to approval from the Engineer.


6.3.4        Pre-cast concrete roller spacer blocks will be attached to the reinforcement to provide the correct cover from the inside of the permanent liner


6.3.5        When all the reinforcement has been installed in the pile the base on the pile will be dipped again and if necessary further airlifting will be carried out to ensure cleanliness of the bored hole prior to concreting.


6.3.6        Reinforcing bars protruding above the concreted level will be protected by cement paste or epoxy approved by the Engineer..



6.4   Installation of Access Tubes for Sonic Tests


Four sonic tubes will be fixed to the reinforcing cage by welding to the stiffener ring.


6.5   Placing of Concrete

6.5.1       Concreting to the pile will be carried out under water by tremie techniques maintaining the water head inside the casing at or above existing ground water level at all times.


6.5.2        The workability of the concrete will be tested by slump tests on samples taken from each batch. A set of two cubes will be made for each 25m3 of concrete for testing at 28 days as specified in the particular specifications.


6.5.3        A 250mm diameter tremie pipe will be used to place the concrete in the pile and this pipe will be placed initially on top of the base of the pile excavation. A conical hopper will be attached to the top of the tremie pipe and a plug of polystyrene chips will be placed on top of the water in the tremie pipe to form a seal between the water in the pile and the first batch of concrete.


6.5.4        All the joints of the tremie pipe are water-tight in order to avoid the inflow of water into the pipe and therefore segregation of the concrete. Concrete will then be introduced into the hopper of the tremie pipe in 6 cu m batches by use of a skip. As successive batches of concrete are installed into the tremie pipe the level of the concrete in the pile will be checked and a continuous record kept by means of weighted tapes.


6.5.5        As the level of the concrete in the pile reaches a suitable level, the tremie pipe will be withdrawn in section in such a way that a minimum immersion of 2m of the tremie pipe into the concrete is maintained at all times.


6.5.6        The concrete level will be brought to a level of minimum 750mm above cut off level, then backfilling the space between the temporary and permanent liners with sand will take place. If necessary, steel stabilizers will be used to fix the gap between the permanent casing and temporary casing at the top level prior to concreting to ensure the stability and verticality of the pile during concreting. The temporary liner will be removed using a hydraulic casing oscillator section by section, during which further backfilling between the casings will be carried out as necessary.




7.1   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 (2 days). The piling sequence will be submitted for Engineer’s approval.



8.      RECORDS


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


8.2   The format of the piling records are attached.





9.1   Testing of concrete

The quality of concrete will be tested on the site by measuring the slump and temperature of concreting at the time of discharge into the pile borehole recorded on a Concrete Delivery record. In addition to the site test, laboratory tests will be carried out in order to check the strength of the placed concrete in accordance with the requirements of the specifications, by an approved laboratory (HOKLAS).


9.2   Proof Drilling


9.2.1        Proof Drilling to be carried out on every constructed large diameter bored pile at the concrete/rock interface.


9.2.2        All proof drilling to be done by independent Specialist Contractors for Public Works registered under the category of "Ground Investigation Field Work";


9.2.3        One of the sonic test coring tubes shall be at least 150mm diameter and this tube shall be left at about 1000mm above the founding level of the pile.  The core-drilling shall be carried down to 1000mm below the concrete/rock interface.


9.2.4        Equipment for taking NX size drill coring through the full depth of piles, which are selected by the Engineer will be on site. When instructed by the Engineer certain piles will be cored.


9.2.5        Place cores in correct order and relative location in timber core boxes clearly marking the depths of cores and boxes and issue drilling logs. The cores will be photographed for record purposes and the bore logs together with the photographs will be submitted to the Engineer.


9.2.6        The testing of the cores should provide additional information about quality of concrete as well as the condition of the interface between concrete and rock. If instructed, certain portions of the core will be delivered to an approved laboratory for capping and testing the strength in accordance with the specifications.


9.3      Sonic Logging Test


9.3.1. In order to check the quality of the concrete as well as the integrity of the pile in its overall length sonic core testing will be carried out to the first pile of each pile cap and as instructed by the Engineer.


9.3.2  The testing shall be carried out not earlier than 7 days after concreting the pile. Tubes of minimum 50mm diameter as access for the sonic test will be installed with the reinforcement cage down to the bottom of the pile. The tubes will be closed at the bottom and they will be filled with water after concreting the pile. The top of the tube will be protected and sealed with a cap. Grouting of tubes after logging will be carried out as instructed by the Engineer.



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