Substructure Development Through Bored Pile Construction

Introduction
1. Foundation is a significant part of construction. Among the several types of foundation, bored pile is a type of deep foundation that supports heavy structures by transferring their loads to deep, stable soil or rock layers. Bored piles are also known as drilled shafts or replacement piles, and they are crucial for constructing tall buildings, bridges, and large industrial complexes.

 2. Types of Piles. As per the installation methods there are different types of piles as follows:

 a. Driven Piles. These are pre-formed piles such as- timber, steel, or concrete piles and forced into the ground using hammers.
 b. Bored/Drilled Piles. Boring holes are created in the ground and filling them with concrete. Common types include Rotary Bored and Continuous Flight Auger (CFA) piles.

 c. Micropiles. These are small-diameter piles (100-400mm) and mostly used in restricted access areas for underpinning or slope stabilization.
 d. Screw Piles. Screw piles are driven with a helix (screw) mechanism for self-penetration into the ground.

 Cause of Choosing Bored Piles Over Other Types of Piles at Site 
 3. Bored piles have some advantages over other types of piles. Key advantages for choosing bored piles are:
 a. Low Vibration/Noise. It is ideal to use near existing structures and in urban areas. Also, in sensitive environments where driven piles cause disruption.
 b. Minimal Soil Displacement. As they are cast in place so they cause less soil disturbance of surrounding soils.
 c. High Load Capacity. Bored piles can achieve very high vertical and lateral load bearing capacities by extending deep into bedrock.
 d. Flexibility in Size and Depth. Flexibility of size and depth is a special characteristic of bored pile. 

Construction Methodology
 4. Flow Diagram of Bored Pile Construction.
Survey and Setting out
5.    Prior to construction, precise control survey network should be established. Control measuring points needs to be set up in every 500 - 1000m to ensure the precision of in-situ survey. During construction process, Total Station and RTK are being used to set out the bored pile positions, and forming two mutually perpendicular crosshair.

Prefabrication of Temporary Steel Casing
6. To prevent the loose top soil collapse of the borehole temporary steel casing is used. The temporary casing of the bored pile is made of steel plate (not less than 10mm) in the steel structure workshop, which is 6m long and with the outer diameter of 0.7 - 0.8m or 10 - 20 cm larger than pile diameter (For Pile Diameter 600mm).

 Driving of Temporary Steel Casing
7. First, the center point of pile should be set out. Secondly, centering on the center point a hole with depth of 0.5-1.0m needs to be excavated manually, of which the outer diameter is 10~20cm larger than that of the temporary steel casing. Then, after the temporary steel casing is positioned into the pit, and lowering with the assistance of vibratory hammers. At last, the displacement of the casing center and verticality is measured, the center displacement of the casing should not exceed 5.0cm, and the verticality should not exceed 1%. If the inspection result does not meet the requirements, the temporary steel casing shall be pulled out and re-drive.

 Mixing Drilling Fluid and Setting up Drilling Fluid Circulation System
8. Bentonite Slurry needs to be used in the drilling fluid. The bentonite slurry is mixed in a container by a machine that stirs it. After that the bentonite slurry is pumped into the hole that is drilling. While bentonite slurry is drilling it helps to keep the sides of the hole from falling in. Before putting the bentonite slurry into the drilling operation, it needs to take a sample of the bentonite slurry and do some tests on it. This helps to make sure that the bentonite slurry is good to use, from the beginning.

 9. The properties of bentonite slurry should meet the values shown in Table:

 Installation of Drilling Rig
10. Stability of the multi-functional rotary drill rig is of paramount importance prior to the operation. For attaining the stability, the original ground must be leveled and compacted. It prevents the rig from sinking during the drilling process. To ensure the stability of the drilling rig, verticality of the drilling pipe should also be tested in two directions. Later the drill pipe is checked with total station and when found okay the adjustment system is then locked. The top elevation of the steel casing should be measured before drilling. This measurement serves as a reference for depth measurement during the drilling process.

 Boring
 11. The low-speed drilling method is adopted for the beginning of drilling & make sure the drill bucket does not touch the bottom of the steel casing. The temporary bentonite slurry pond should be prepared near the pile, and the temporary bentonite slurry pond also should be connected with the steel casing, When the drill bit is not in the hole, the slurry stays near the top of the steel casing; when the drill bit and pipe are inserted, the slurry overflows into the temporary bentonite pond, and when the pipe is lifted out, the slurry refills the hole, ensuring cyclic utilization during drilling. The mud surface in the hole is always higher than the underground water level to prevent hole collapse. When the depth counter shows 2m above the design depth, drilling stops for a quality inspection i.e. Pile diameter, verticality etc. of the bored pile. After inspection, drilling continues to the design depth, then the hose bit is replaced with a slag removal bit to clear slags. Once cleaning is done, a lining rope and line hammer is used to check the pile bottom level.

  Borehole Quality Inspection
 12. After borehole completed drilling bit should be pulled out and drilling rig have to be removed, along with, a rebar cage detector used to check borehole quality such as the hole diameter and the verticality.

 Tremie Pipe Installation and First Time Borehole Cleaning
 13. The watertight integrity pressure test should be conducted before use of Tremie pipe. The base pipe section is 3m - 4m long and the middle pipe section is 2.7m - 3m long. The inner diameter is 300mm.The pipes are connected to each other, and water is supplied at one end. Each joint section is carefully inspected to ensure there are no leaks. While the tremie pipe is being lowered, the tremie pipe should be in the center of the hole and the axis is vertical. The bottom of the tremie pipe is 30 -50 cm above the bottom of the hole.

 14. Air-lift reverse circulation system adopted in the borehole cleaning.
After 3-4 hours cleaning, samples of slurry be taken in the vicinity of the borehole bottom, and the relevant tests (as described in table) should be carried out. After getting satisfactory result rebar case should be lowered in the borehole.

Fabrication of Rebar Cage
15.    The rebar cage needs to be made in parts in the yard where reinforcement work is done. It needs to test the rebar cage that is made in parts. We have to make sure the joints in the bars in the piles are strong. This can be done by welding or by using sleeves that are threaded. The number of joints in the bars in one section should not be more than half of the total number of main long bars. It also needs to check the rebar cage length, the number of rebars, the space between the spiral rebars, the blocks that keep things spaced and the wire that ties everything together at the site.

Installation of Rebar Cage
16.    Precision and care are important during installation of rebar cage. While reaching near the bottom lowering should be slower not to crash and collapse of the hole. If the rebar cage gets stuck and cannot be lowered, it should lift up a little. Then try again to lower it until the rebar cages at the height that it wants to be. This should be kept repeating until the rebar cages in the right position.

17. To stop the steel skeleton from floating during pouring concrete, placement of a hanging ring or fixing rod at the top of the steel case is important. These rings or rods need to be arrayed in a line so ensure even spacing. The steel skeleton and the support system must be aligned at the center to ensure stability. Bar lapping length and the welding property are another important criterion to ensure desired quality.

 Tremie Pipe Installation and Second Time Borehole Cleaning
 18. The purpose of the second time borehole cleaning is to remove the soil which is scraped off when the steel case is installed, as well as the drilled cuttings after drilling stops. During borehole cleaning, shake the tremie frequently to make it to change locations at the bottom of borehole, thus clear out the slag more thoroughly. Drilling fluid should be added in time and the height of water head in the hole be maintained. After completion of borehole cleaning, samples of slurry should be checked in the vicinity of the borehole bottom, and the relevant tests should be carried out. 

Concreting
19. During pouring of concrete in place the temperature of the concrete must be below 30ºC and the slump value has to be within 180-220mm. The tremie pipe should be buried more than 1.5m while pouring the 1st batch of concrete. Measurement of height of the concrete surface and depth of tremie pipe needs to be carried out simultaneously.

20. Care must be taken not to spill of the mixture over the top of the hopper. Concrete flow and the water level have to be monitored with precision during the process. The tremie pipe should be lifted and removed under correct instructions to ensure the tremie embedment of 2 - 6m. The removed sections should be cleaned immediately and stacked neatly. Tremie pipe should be removed quickly, the time should not exceed 15 minutes, and each pile should be poured within 6 hours.

Conclusion
21. So, on the basis of writing so far, it can be concluded that, proper geotechnical investigations are required- ground water level should be verified. Backup equipment for pilings is required during construction to avoid downtime during concreting. Proper alignment and centralization of reinforcement cage is essential. Concreting should be continuous and uninterrupted to avoid cold joints. Slurry disposal carefully handled to avoid any environmental issues.
 

Lieutenant Colonel Md Mahbubur Rahman 
Khan, psc, belongs to the Engineering 
Corps of Bangladesh Army.