Subsequent to the completion of the pile drilling, the borehole depth will be checked and verified versus the depth given on daily piling sequence and pile data. Only after borehole depth check can the cage be lowered into the borehole. A reference site datum level will be given to ensure the correct positioning of the cage with respect to cut off level.
The reinforcement for the piles shall be according to the requirement of the specification and the design of the pile. The reinforcement shall confirm to the mill certificate approved by the Consultant.
The reinforcement cages will be fabricated up to 18 m length pre coupling of 6 m + 12 m. The 18 m length reinforcement fixed with helical shall be lowered to the borehole. The cage shall be arrested in position, when it reaches 2.0 m above casing top level. Suitable crossbars shall be provided to keep the cage hanging in position. The reinforcement segments for 1200 mm diameter piles will be connected together using the couplers fixed to the end staggered at a minimum clear distance of 70 cm c/c to receive the second segment pile cage. The helical reinforcement / rings shall be fixed in place after connecting the two segments of reinforcement for 1200 mm diameter piles. The reinforcement shall be lowered to correct location and cage top level and secured from movement using hangers and arresters. The sonic coring pipes shall be connected by hand to push in the socket. The 4 nos. pipes shall be fixed to the cage at diametrically opposite sides by tying with binding wires at close intervals (50 cm C/C) before lowering to the borehole. The sonic pipes shall be end capped before fixing to the cage.
The maximum weight of the reinforcement cage will be approx. 9.0 tons. A lifting platform shall be used for the lifting of the steel cage without distortion. The steel cages will be lifted with the help of main crane (50 ton capacity) and an auxiliary crane (service crane of 35 ton capacity) shall be used to lift the lifting platform along with the steel cage. The lifting platform shall be released once the cage reaches 900.
Durable spacers shall be used at proper interval to keep the reinforcement in position. The spacers shall be of PVC material or cement concrete and as per approved sample for a clear concrete cover of 75 mm to the reinforcement. The first layer of spacers will be placed at 20 cm below cut off level. The spacing between spacers should be within 3 to 4 m and should terminate only 30 cm above pile toe.
Sonic Logging Pipes Installation
The sonic logging will be carried out for preliminary piles and 100% of working piles. The sonic logging pipes of diameter 50 mm shall be fixed to the inner side of the cage at 4 diametrically opposite location for the full length of all the piles. Care shall be taken to avoid bend / damage to the pipes during installation and concreting. The top and bottom of the pipe to be capped in order to avoid ingress of grout to the pipe. The detailed method statement for sonic logging is available separately. The sonic coring pipes to be grouted after the testing using approved grout mix.
Method of Casting Concrete in Piles
The pile concrete is delivered to site by means of truck mixers. The concrete will be received on site and the delivery ticket will be checked for correctness in accordance with the approved mix design. The quality checking and testing of the concrete to be carried out by an independent testing agency (I.T.A)
The concrete will be checked for the slump and temperature. Cubes will then be taken as per specification. The Concrete will be allowed to the site only after concrete quality has been found in compliance with specifications. The concreting of the drilled piles should be completed on the same date of completion of borehole. If there is an elapsed time of more then 6 hours between the completion of borehole & cage installation, cleaning bucket shall be carried out to clean the borehole.
The concrete shall be poured from the concrete truck to the piles by using a concrete pump fixed to the tremie pipe and placed without interruption to prevent hardening of the previously placed batch.
The concrete shall be placed by tremie tube method, the tremie size will not be less than 150 mm. The tremie pipe is made up of sections with sufficient length to reach the toe of the pile and the joints provided with seals. The tremie pipe will be inserted at the center of the pile to reach upto the toe. The top of the tremie pipe will be connected to the concrete pump. The concrete shall be delivered directly from the transit concrete mixer to the concrete pump. If there is no pump available prior start of pouring, the concrete from the concrete mixer shall be delivered directly to the pile through the funnel connected to the tremie pipe of 250 mm diameter. The tremie pipe will be lifted 100 mm above pile toe level prior to concreting. A Styrofoam ball having a diameter slightly larger then the diameter of the tremie pipe shall be placed at the mouth of the tremie pipe to avoid contact of concrete with bentonite slurry / water in the tremie pipe. While concreting the length of the tremie pipe will be shortened if necessary but the tremie pipe will be maintained full time into the concrete of at least 2.0 m length. During the concreting the level of the concrete within the pile will be monitored by use of an end weight tape measure.
A sufficient quantity of concrete will be maintained within the tremie to ensure that the pressure from it exceeds that from the external water pressure. The concrete will be brought up to minimum 1.30 m above cut-off level and overflowing of concrete abovecasing top level will be carried out to ensure good quality and contamination free concrete at pile cut-off level. The concrete above the cut-off level will be chipped by the Main Contractor. The bentonite slurry / water displaced by the concrete will be pumped back to the desander or tank.
After concreting, the guide casing will be withdrawn by piling rig. When the casing is being extracted, a sufficient quantity of concrete shall be maintained within it to ensure that pressure from external water, bentonite slurry or soil is exceeded and that the pile is neither reduced in section nor contaminated. This applies for piles where the cut off level is below working platform.
The quantity of poured concrete volume will be measured & it will be compared with the theoretical quantity. The theoretical volume, actual volume & % over break will be recorded in the pile record sheet and plotted on a graph sheet for the submission to the main contractor.
Piles will be cast within the tolerances given under the specifications.
Disposal of drilling spoil
Spoil from drilling of boreholes shall be removed from site to designated disposal areas provided by the local Authorities and will be carried out by the Main Contractor.
Piling records, provided by the foreman to be checked by the site in charge before final submittal to Engineer on the Bored Pile Record Sheet. The record include the following data:
- Pile number
- Pile date of completion
- Pile diameter
- Working platform level
- Pile design cut off level
- Pile toe level
- Concrete top level
- Reinforcement cage details (length and bars diameters)
- Pile concrete length
- Pile bored length
- Ground water table
- Bentonite tests, if used
- Drilling time and piling rig in use
- Concreting time, concrete details and ticket no.
- Concrete cube Nos.
- Theoretical volume of concrete
- Actual volume of concrete
- Over break in percentage
- Drilling fluid.
Any unexpected drilling condition encountered will be noted briefly in the records, and brought to the attention of the Engineer immediately.
CUTTING OF PILE HEADS
The Main Contractor shall cut off all undesirable concrete min. up to C.O.L of the finished piles to carry out integrity testing on piles.