METHODOLOGY FOR BORED CAST IN SITU PILE WITH BENTONITE SLURRY

FLOW CHART FOR PILING WORKS DURING CONSTRUCTION

EQUIPMENT AND MATERIALS WE USE FOR PILING WORK

EQUIPMENT AND MATERIALS WE USE FOR PILING WORK

1. MAIT HR 130 Piling Rig ( fig.1)

Piling Dia.: 500 mm To 1200 mm , Piling Depth : 40 m

2. Hydra Crane (fig. 2)

Capacity : 08 to 14 tone

3. Tremie Pipe Set 

Diameter: 150 mm

Length : 1.0 m To 2.4 m Each pipe

4. Back Hoe Loader 

For Excavate Soil And Shifting Of soil

5. Temporary Casing 

For Support Earth soil And remove after casting

6. Bentonite Set up 

For mixing Bentonite And returning of bentonite

METHODOLOGY

1. SETTING OUT:

Necessary setting out and survey is to be done for pile points marking. The pile location is set out with respect to the control points by suitable peg marked at location, which shall be checked by client. The permission to start boring will be obtained in standard format as per the checklist. The guide points (min. 3 nos.) are fixed at suitable location outside the casing for future reference and the reference points are guarded/barricaded properly.

2. DRIVING OF TEMPORARY CASING:

Rotary drilling rig fitted with suitable tool (soil boring tool) to be positioned at the pile point. After necessary checks for verticality of the mast, boring shall be started. On completion of boring up to about 1m on top, temporary steel casing pipe of required dia. & thickness will be installed in position using attachment at the rotary head. The casing pipe shall be gradually pushed to the required depth in advance of boring; ensuring verticality. Casing shall be driven preferably in one piece only. The casing pipe shall be lifted by piling rig and lowered in the borehole. The diameter of cutting tool shall in no case be less than the diameter of the pile minus 75 mm. The working level should be 0.3m to 0.5 meter above ground level. The center line of the casing pipe is then checked with respect to the

reference points. Casing pipe is driven in by hammering / tempering / rotating by hydraulic rig. will be provided depending upon the diameter of the pile and the subsoil conditions. After concreting the same is extracted.

3. BORING:

Excavation of the soil inside the casing/bored holes will be carried out using the rotary Kelly bar and the auger or bucket method. The drilling process will be continued to the designed founding depth or require bore depth by using soil augers and drilling buckets.

4. REINFORCEMENT CAGE LOWERING:

The bar bending schedule based on approved drawing is prepared. Reinforcement is cut and bent to required size and shape. Reinforcement cage shall be fabricated at fabrication yard and transported to location or fabricated at site as may be convenient. Specified approved cover blocks shall be provided to reinforcement cage for the pile. Reinforcement cage shall be lowered in 2 pieces or more depending on the available length of steel bars & length of pile. Bottom cage is lowered inside the borehole and temporarily supported on the casing pipe top, keeping the dowel length projecting above and second cage is lifted and lapped as per approved/ construction drawings. Helical / rings are tied as per approved drawing and main reinforcement laps are welded with another cage using special electrodes provide more rigidity to the cage. The cage is further lowered in the borehole.

5. REINFORCEMENT COVER BLOCK:

Circular Cover Block of required dia. shall be made with the same grade of concrete as that of pile concrete and fixed to helical rebar cage. The cover block shall be provided at approximately 2.00 to 2.50 meters intervals and suitably staggered.

6. FLUSHING OPERATION:

Before the concreting starts, the bore shall be flushed with bentonite slurry. Immediately after lowering of the cage, tremie pipes of minimum 150 mm dia. of suitable length are joined together and lowered into the bore and the bentonite slurry shall be pumped through the tremie pipe.

For Bentonite:

The bentonite powder is delivered to the site in 50 kg bags which are stored under covered GI shed and bentonite suspension to be used for piling work shall satisfy the following requirements: a) The liquid limit of bentonite when tested in accordance with IS 2720 (Part 5) shall be 400 percent or more. b) The bentonite suspension shall be made in a Tank with proper safety measures, by mixing it with fresh water using a pump for circulation. The density of the freshly prepared bentonite suspension shall be between 1.03 and 1.10 g/ml depending upon the pile dimensions and the type of soil in which the pile is to be bored. The density of bentonite after contamination with deleterious material in the bore hole may rise up to 1.25 g/ml. This should be brought down to at least 1.12 g/ml by flushing before concreting. The bentonite slurry collected during the flushing operation shall be collected in the compartments of the pit of storage capacity of about 400 cu.m. The contaminated bentonite slurry will be discarded by mixing with the earth and transported by tractor to the dumping area as specified by client during entire piling work. The pH value of the bentonite suspension shall be between 9 and 11.5.

In case, the pile remains without concreting for more than 6-hrs., the slurry in the pile bore is to be maintained in circulation by carrying out flushing operation or as may be required due to site condition.

7. CONCRETING:

Once the flushing operation is complete, funnel or hopper shall be kept on top of the tremie pipe for pouring of concrete. The concrete shall be placed in the pile bore through the tremie. The bottom of the tremie shall be 300 mm above the bottom of the bore. During concreting care has to be taken to see that the tremie pipe is always embedded minimum 1.5-2.0mtr in concrete. Check workability of concrete & prepare concrete cubes as per specification requirements. Test cubes for 7 days and 28 days strength done on a regular basis. On satisfactory completion of concreting to the required elevation, temporary casing will be removed either by crane or the drilling rig. Piling rig will be shifted to next pile location.

8. PILE TESTING

Pile head preparation is important in order to impart a clear impulse in the pile and allow the detection of the returning reflected signals. The sensor (usually accelerometer) is fixed to the pile head while the pile is struck with the hand held hammer preferably at or near the pile center. The induced pile head velocity records and the subsequent reflections from either pile toe and/or discontinuities are graphically displayed on the screen of the PIT collector. Several hammer blows are usually recorded on each pile for averaging the signals and to confirm that a consistent repeatable response is being obtained.

The signal records obtained are may also be exponentially magnified with time. This enhances the identification of relevant reflections in records, which have low energy due to signals being dampened out by skin friction.