WORK METHOD STATEMENT FOR BORED CAST-IN-SITU PILES
Purpose/Objective
To document a suitable methodology for carrying out pile foundation operations, which shall serve as a structural element to transfer the designed superstructure load to the required bearing strata.
Scope
This procedure is applicable for bored cast-in-situ piles for Fly-Over, ROB, and Minor & Major Bridge Construction of Road from Km. 9.282 to Km. 22.000, where the foundation is of RCC Bored Cast-In-Situ Piles.
Reference
- IS 2911 for Bored Cast-In-Situ Concrete Pile
- Applicable approved latest drawings
- MORT&H Specifications Sections 1100 and 1700
Materials:
The basic material shall confirm to the specifications for materials given in Section 1000. The specification for steel reinforcement, structural concrete and structural steel to be used in pile foundations shall be as given in the relevant sections.
Definitions/Abbreviation
- Bored Cast-in-situ piles : The bored cast-in-situ pile is formed by making a borehole into the ground with the use of temporary or permanent casing & subsequently placing the reinforcement and concrete. The M35 Piling grade of concrete to be used in cast-in-situ piles and the minimum slump of concrete for bored cast-in-situ piles 150 mm to 200 mm. The slump should not exceed 200mm in any case.
- Foundling Level : The level of the strata on which the pile rests.
- Cut off level : The level where the installed pile is cut off to support pile caps and other structural components.
Procedure of cast in situ pile
- Pile point: Pile points will be established on ground from the approved established survey station by using total station based on the approved drawing.
- Execution: Bored Cast-in-situ piles can be executed in any of the following two methods
Percussion method
Rotary Method of piling
- Shift and center the piling tripod over the pile point established on the ground.
- Foxing temporary casing pipe
- Normally temporary casing pipe will be pitched/installed by making a naked borehole of about 1 meter to 1.5 meter in the ground of the pille location and drive the temporary casing up to the required depth by hammering and boring either by conventional piling rig or by rotary drilling equipment.
- To position this temporary casing with exact pile location two reference points are to be fixed on the ground approximately perpendicular to each other from the center point of the pille. Normally these reference points will be distanced from the casing with the conventional measurement.
Pile boring methods:
Pile boring can be done either of the two following methods:
- Direct Mud Circulation Method (DMC Method)
- Reverse Mud Circulation Method (RMC Method)
Direct Mud Circulation Method
In DMC method boring will be done with the help of conventional piling rig i.e. heavy tripod coupled with power winch and an attachment of DMC rods and chisel. Chisel is normally 75mm lesser than the diameter of the pile. This reduction of diameter is for limiting the over breakage of the sides of the borehole which happens due to the oscillation of the chisel fitted to the long length of the DMC rods. With the help of the chisel boring will be done. In the process of boring bentonite slurry (drilling mud) will be continuously fed / injected in the bottom of the borehole through DMC pipes. Due to the penetration of the chisel and continuous flow of bentonite slurry soil will get agitated and the agitated soil will come out from the borehole along with the overflowing bentonite slurry. This forms the borehole.
Reverse Mud Circulation Method
In this method boring will be done either by Percussion or by Rotary method. In percussion method the bored material will be bailed out by suction either by bailor or by pumps
In rotary method bored material will be directly scooped out either by augur or by bucket.
In both the methods bentonite slurry is pumped at the top of the borehole.
Reinforcement
After reaching the final level boring is stopped and bentonite slurry is fed for some more time to allow the floating material to come out and the bentonite coming out reaches the standard specific gravity.
The reinforcement cages that are already made as per the design were lowered in suitable lengths and the pieces will be joined together by tack welding.
Concreting
Before concreting the borehole has to be flushed again with bentonite slurry to remove any sediment material left within the bore hole. After flushing tremie pipes will be lowered up to the bottom of the borehole for concreting. On top of the tremie pipe suitable capacity funnel is to be fitted. Before the first pour the funnel neck is covered by a metallic cover and it is removed after sufficient concrete has been poured into the funnel. This ensures that first pour concrete displaces the bentonite slurry and gives sufficient embedment to the tremie pipe. Likewise concrete is to be built up to the required level ie. above the cutoff level to clear off the contaminated top concrete mixed with bentonite and at each stage crosschecks will be done so that the minimum embedment of the tremie pipe is ensured. The laitance will be chipped off at a later stage so that sound concrete is ensured at the cutoff level.
EQUIPMENTS AND MACHINERY for pile work
- Heavy piling tripod with 5 ton capacity power winch
- Reciprocating/Centrifugal pumps
- Bentonite mixing plant
- Boring accessories and tools
- Concreting accessories
- Electrical Generator (Required capacity)
- Welding transformer/Gas cutting sets.
- DOCUMENTS
- Pile Register
- Concrete pour and inspection records
- Bar Bending Schedule
- Record for Concrete test
TESTS AND STANDARDS OF ACCEPTANCE : The materials shall be tested in accordance with these specifications and shall meet the prescribed criteria.The work shall confirm to these specifications and shall meet the prescribed standards of acceptance.
- SAFETY:-
1. During construction of piling work adequate safety & traffic sign shall be installed adjacent to the road shoulder to protect the ongoing work.
2. Piling location shall be provided with visible safety & Road signs.
3. During construction of Piling minimum 2 flagman with reflective vests and holding a red flag each shall be deployed to control the traffic, which may be effected by the construction operations.
4.The Safety Officer shall make frequent patrols of construction site to ensure that the safety equipments and signs are operational at all time.
5. Other safety arrangement shall be as per approved safety manual.