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Method Statement for Piling Works Constructions – Drilling

A crawler mounted piling rig will be used to install bored cast in situ piles of dia 1200 mm. The procedure for the construction of bored piles, with temporary casing under bentonite / water  support system, comprises the following key elements:

  • Setting up at the pile location
  • Installation of temporary guide casing
  • Drilling of the pile borehole
  • Recycling of Bentonite and checking
  • Reinforcement cages installation
  • Sonic logging Tube installation
  • Concreting
  • Removal of temporary guide casing

Setting Up At The Pile Location

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Prior to setting up of the drilling rig at the pile location all the existing utilities, obstacle and any under ground services (telephone lines, electric cables, drainage system, concrete parts, fuel line etc.) are to be removed by the Main Contractor.

Temporary Guide Casing

A temporary guide casing of internal diameter 1200 mm according to the approved pile layout drawing shall be installed to the pile center as previously marked on the working platform.  The casing shall be lowered to the ground with the help of turntable of the rotary unit or with vibrator. Engineer’s approval will be sought prior to the use of vibrator.

The temporary casing used will be free from any distortion and shall be uniform cross-section throughout the length. The length of the casing will be approx. 6.0 meters, depending on the cut-off level.  The temporary steel casing will be driven in the pile center with a minimum over lap of 50 cm below cut-off level.

Two numbers reference steel pins will be installed equidistant from the pile point.  The distance of the steel casing is measured from the reference pins and adjustment made so that the reference pins are equidistant from the steel casing.  The verticality of the steel casing is checked with a spirit level in two perpendicular directions. Further the verticality can be controlled by an in-built inclinometer fixed to the boom of machine. The verticality of drilled borehole will be recorded in the pile record sheet. The piles drilled from –13.33 m DMD and below will be drilled using temporary long casing and ground water subject to achieving satisfactory performance from the trial drilling. All other piles that drilled from –2.25 m to -12.35m working platform level shall be drilled with temporary casing and bentonite slurry support system. A trial bore hole to be constructed to check the feasibility of drilling with water.

Drilling With Water And Temporary Casing

A trial bore hole shall be carried out at the required working platform level (-13.33 m DMD) to check the feasibility of boring using water as drilling fluid. The water level inside the bore shall be kept minimum 1.50 m higher than the surrounding ground water level. The bottom of the borehole to be cleaned using cleaning bucket. The bore hole to be monitored for minimum 12 hours, if excessive sedimentation or collapse is noticed during the trial drilling the bore hole will be back filled immediately and the working pile installation will be carried out using temporary casing and bentonite slurry support system.

DRILLING OF PILE BOREHOLE USING BENTONITE ( For piles drilled from Higher working Platform level Above –13.33 m DMD)

After installation of the temporary guide casing, the borehole will be drilled using appropriate drilling tools and the verticality of the drilled borehole shall be ensured to its full depth. The Slurry Support System relies on the presence of a thixotropic mud-water mix to maintain the stability of the sidewalls in a drilled borehole. Further the positive hydrostatic pressure developed by the slurry also avoids ground water penetration in the drilled pile shaft.

Bentonite complying with Specification DFCP-4 drilling fluid materials will be used to form the Slurry. Control tests will be performed on the bentonite suspension as per specifications. The drilling fluid is prepared in mixers and kept in tanks for storage and hydration purpose for a minimum period of 6 hours. After fixing of guide casing, the borehole will be drilled down to its final level by means of bucket and auger.

The drilled borehole will be filled with bentonite slurry and the slurry level maintained at working platform level at all times.  Care shall be taken to maintain the slurry 1.50 m above the ground water table.  In the event of a rapid loss of drilling fluid, the borehole will be backfilled without delay using the drilled material from the heap near the boreholes.  The Engineer’s approval must be obtained before resuming boring in that location.

Drilling penetration rate and soil cuttings will be kept under observation to notice any change in the sub-soil conditions from previous soil investigation.  The record of strata will be made during excavation of the pile and recorded on the pile record sheet.

Any noticeable variation shall be brought to the attention of the Engineer. The depth of borehole shall be measured using an end weighted measuring tape and to be recorded in the pile record sheet. The verticality of the borehole will be guaranteed in two ways:

  1. A hand held spirit level of 1.0 m length shall be used to check the verticality of mast and thereby that of the casing at two directions of 90o to one another at intervals.
  2. Some machines are equipped with in built inclinometers fixed to the boom of the machine, which can monitor the drilling operations to keep the verticality within the specified limits. The maximum out of verticality allowed as per specification is 12.5% of the pile diameter or 1.5% of pile length which ever is smaller.

Excavate shafts for closely spaced drilled piles (< 3 x diameter), only after adjacent drilled piles are filled with concrete and allowed to set. (Normally after 12.0 hours)

RECYCLING IN CASE OF BENTONITE DRILLING

The process of cleaning the bentonite suspension from sands and silts to keep its quality within the allowable limits is called recycling.  The bentonite quality will be verified by checking the sample taken from the top and bottom of the borehole. If excessive sand contents are noticed in the bentonite sample then fresh bentonite sample shall be pumped through the sealed tremie pipe. The tremie pipe shall extend to the bottom of the borehole. The sample from the borehole shall be checked for its compliance with specification requirements.

Recycling will be performed after excavation has been completed and before the bentonite slurry has been replaced by concrete. The suspension from the borehole will be pumped to the desanding plant during concreting.  After being cleaned in the plant, the bentonite will be pumped to the bentonite tank for further use.  For cleaning operation of the bottom of the borehole a cleaning bucket will be used.

BENTONITE SUSPENSION

We propose a bentonite concentration of 5%. Bentonite slurry will have the following properties.

Supply to                        Sample from

                                                                 Pile Borehole                 Pile Borehole

 

Density                                                   :  < 1.10 g/cm3                 <1.15 g/cm3

Plastic Viscosity                                    :  < 10 CP                         < 20 CP

Viscosity                                                 :   30 to 40 seconds        <60 seconds

Ph value                                                :  9.5 to 10.80                  9.5 to 11.70

Sand content                                        :  < 1%                              < 3%

Shear Strength                                     :  4 – 25N/m2                           4 – 25 N/m2

(10 Min.) Gel strength

Fluid loss                                               :  < 30 mil.                        < 40 mil.

In order to achieve a concentration of 5%, 50 kg of bentonite will be added to approximately 1000 liters of water.

Mixing will take place in a mixing plant designed by BAUER for this purpose.  The bentonite slurry shall rest in a tank for at least 6 hours to allow proper hydration.

The schematic layout of the bentonite mixing arrangement is attached.

Capacity of storage tank                 =  4 no. x (12 x 2.5 x 2.5) m

 

=  300.0 m3

 

The maximum number of piles to be drilled at one time  =  5 nos.

 

Required volume of suspension  =  5 x  {  p (1.20)2 x 41.0m x 1.2  } / 4

=  278 m3

 

Hence required volume of suspension  < capacity of storage tank. Hence safe.

TESTING FREQUENCY OF BENTONITE

The bentonite slurry shall be checked for specification compliance at suitable intervals. One sample shall be taken from the top of the borehole and another sample from the bottom to check compliance with the specification requirements.  We propose the following sequence of bentonite slurry testing for each pile.

  1.  Sample from the tank to the borehole.
  2. The sample during drilling of the borehole.
  3. The sample prior to concreting (one sample from the top & one sample from the bottom).

The bentonite powder is supplied with the test certificate from independent agency. However occasional testing can be carried out on powder sample if required by Engineer.


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