Bored Cast in Situ Concrete Pile, Its Construction and Applications

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What is cast in situ concrete pile?

Bored cast in situ pile is constructed by digging a hole in the ground by suitable means such as percussive or rotary method with the use of temporary or permanent casing or drilling mud. After that, the construction is finalized by filling the hole with reinforced concrete. In this article, different aspect of bored cast in situ concrete pile construction along its application conditions will be discussed.

Procedure for Construction of bored cast in situ pile

In this section, procedure used for the bored cast in situ pile construction is presented:

Prior to the commence of pile construction operation

• Prepare construction program that involves main project milestones.
• Prepare and maintain all templates and quality check formats

Setting out pile points

• Set out pile points accurately based on the design drawings. According to IS 2911, the largest allowable tolerance for 60cm pile diameter or more is the greater of 75mm or D/2. For piles with diameter up to 600mm, the tolerance limit is 50mm.
• Mark set out points properly so as the points are clear and fix to avoid detrimental effect of other activities around the area.
•  Use total Station or theodolite to set out pile position. Make sure to recheck these points before starting the piling activity. Establish temporary reference points (TR) for this purpose.

Plan preparation for rig movement

• Prepare a layout that illustrate the expected path for rig movement based on the work program and the planned deployment of resources.
• The layout shall guarantee easy movement of all rigs without causing disturbances and problems to other rig operations.
• If the layout is prepared according to the above guidelines, then the team will have a clear picture on planning the work of next day.

Boring process

1. Position the rig over the established pile point. Check the accuracy of the point by comparing it with other reference points.

2. Diameter of the cutting tool shall not less than the required pile diameter by more than 75mm.

3. After rig positioning, drive the casing into the ground. Drive steel casing to a minimum a depth of 1 m below the ground level to take lateral loads and movements at site. The steel casing also combat difficulties due to groundwater during piling.

4. Additionally, provde permanent steel casing or liners in the case of loose soils as per consultant’s advice.

5. stabilize the borehole during boring process using bentonite or other suitable means.

6. Estimate specific gravity of bentonite at regular intervals. Consistency of the drilling mud shall be controlled throughout the boring as well as concreting operations. This measure used to stabilize the borehole as well as avoid concrete getting mixed up with the thicker suspension of the mud.

7. Practiced great care to guarantee straight piles driven.

8. It is recommended to carry out penetration tests to estimate ‘N’ value of the founding strata. In addition to take and maintain soil or rock samples from the founding strata, for future reference.

9. It is also advisable to take soil samples from each intermediate soil strata at the depths specified in the soil test report.

10. Finally, end the boring process upon reaching the designated boring depth. Check the borehole depth through measuring bailer pipe length after its removal from the borehole, and use sounding technique to recheck the borehole depth.

Reinforcement placement

• lower reinforcement cage into to the borehole vertically, without disturbing the sides of the hole.

• Provide sufficient cover blocks all around the cage to ensure sufficient cover.

• Equip the reinforcement cage with adequate stiffener bars to avoid sideways sway.
• weld stirrups, stiffeners and laps to prevent breakage.

Bored cast in situ pile Concreting

• According to IS 2911, slump of concrete employed for pile concreting ranges from 150mm to`80mm.
• Prevent interruption of concreting from the start of the process till the end of the work.
• Concreting begins by lowering tremie pipes in to the borehole. Common tremie pipe diameter is around 200mm.

• clean borehole bottom adequately before concrete pouring.
• Connect a hopper to the top of tremie pipe. After that, close the joint between hopper and tremie pipe with a steel plug before concrete first charge.
• Then, fill the hopper with concrete to its full capacity. Once filled, remove the steel plug to allow the concrete to flow down, and replace the bentonite slurry present in the pipe.

• Keep the bottom end of the tremie pipe embedded, at least 2m, within the laid concrete. To replace bentonite from bottom upwards and avoid mixing concrete with water or bentonite.
• The tremie pipe remains hollow after the first charge and each subsequent charge gets deposited within the already laid concrete.
• extend pile Concreting at least 60cm to 90cm above the cutoff level to ensure good concrete for proper embedment into pile cap.
• If the cutoff level is at ground level, then permit spill over of concrete till good concrete is visible.

Applications of bored cast in situ concrete pile

bored cast in situ pile is a strong choice in the following conditions:

• If noise prevention to the buildings located around the construction is mandatory or undesired, then the construction of bored cast in situ pile is the most suitable option.
• Bored cast in situ pile is ideal as end bearing pile, exclusively when it need to be socketed into rock.
• When high capacity pile is required, then this type of pile is an ideal choice. According to IS 2911 part 01- Section 02- 2010, such type of pile preferred for weights ranges from 150tonnes to 300tonnes.