Bore pile, also known as drilled pile or auger-cast pile, is a method of deep foundation construction that entails drilling a hole into the ground and filling it with reinforcement steel and concrete. It is a type of construction piling that is commonly used to support heavy structures such as high-rise buildings, bridges, and towers. It entails drilling a cylindrical hole into the ground with a drilling rig or an auger. The needs of the structure determine the diameter of the pile.
Bore piling has several benefits over other foundation techniques, including high load carrying capacity, the ability to penetrate various types of soil and rock, and the ability to be relatively quiet and vibration-free, reducing the risk of harm to nearby structures. It is, however, a comparatively expensive method that necessitates specialized equipment and skilled operators.
Today we are going to discuss on bore piling method. It may varies from projects to projects. But the main work procedure is constant for bore piling.
MATERIALS FOR BORE PILE CONSTRUCTION
Bentonite Slurry& Polymer:
For the preparation of Bentonite Slurry and Polymer,storage and handing shall be as per the manufacture recommendations and as well as the project specification requirements .
Bentonite Slurry
We’ll use the Pilo-gel Bentonite Powder or another approved brand. Main Contractor will submit the details of the Bentonite including characteristic, certificate and samples for approval prior to commencement of the works.
We’ll prepare the slurry by mixing fresh water with the appropriate bentonite powder, or approved equivalent product.
The slurry will have the following composition (1 cubic meter):
| Material | Unit | Quality |
| Water | L | 970-985 |
| Bentonite | Kg | 40-60 |
The main characteristics of the bentonite at various stages during the construction process will be
as follows：
| Property | Reconstituted | Prior to Concreting |
| Density | 0.5~0.8 | KG |
| Viscosity | 8~11.5(by ph dispenser) | – |
| Sand Content | 30~50 | Sec |
| pH | ≤ 1 % | – |
The contractor will calibrate the drilling fluid testing equipment the utilization at site.
Polymer Slurry
In our project, We proposed to use Ionic synthetic polymer based drilling slurry. We prepared the slurry by mixing fresh water with appropriate polymer. The polymer material for the slurry shall be POLYACRYLAMIDE, or approved equivalent product.
The fresh slurry shall have the following properties:
| Material | Quality | Unit |
| Water | 0.5-0.8 | Kg |
| PH | 8-11.5 | – |
| Viscosity | 30-50 | Sec |
| Sand Content | <1% | – |
The main characteristics of the Polymer Slurry at various stages during the construction process will
be as follows:
| Property | Prior to concreting |
| Density | <1.05 g/ml |
| Viscocity | 40-90 sec |
| Sand Content | <1% |
| PH | 8-11.5 |
The contractor will calibrate the drilling fluid testing equipment the utilization at site.
Concrete for bore pile construction
The characteristics of concrete will be as per the Contract Documents and Technical Specifications. The concrete shall be rich coherent mix of high work-ability. In our project we used the Concrete of Class 30 when test according to AASHTO T-22, ASTM C39 or requires 28-days compressive strength at 30-MPa base on cylinder specimen. Concrete slump was as follows:· -Dry placement methods 6.0-8.0-in. (150-200-mm.)
· -Casing removal methods 8.0-10.0-in. (200-250-mm.)
· -Tremie placement methods 8.0-10.0-in. (200-250-mm.)
Though we can extend the Slump life of concrete using approved retarders and mid-range water reducers.
Reinforcement
The characteristics of reinforcement must be as per the Contract Documents and Technical Specifications of the project. Reinforcing steel used in the construction of bored-pile shall conform to AASHTO M-31 or ASTM A615 as according of the specification.Reinforcing steel shall be bundled in order to meet the clear spacing requirements between the vertical reinforcement bars. Rolled hoops or bundled spirals shall be used in order to maximize clear space between horizontal reinforcement.
Spacers
The contractor will make the spacers by concrete as shown on the drawing. The spacer shape will be circle.The contractor will use concrete spacers or other approved noncorrosive spacing devices at sufficient intervals (near the bottom. the top, and at intervals not exceeding 10.0-ft.(3-m.) vertically) to ensure concentric spacing for the entire cage length.
The number of spacers required at each level shall be one spacer for each 1.0-ft.(300-mm.) of excavation diameter with a minimum of four spacers at each level. The spacers shall be of adequate dimension to ensure an annular space between the outside of the reinforcing cage and the side of the excavation along the entire length of the pile. The contractor shall provide acceptable feet made plastic or concrete (bottom supports) to ensure that the bottom of the cage is maintaining at the proper distance above the base of the excavation, unless the cage may suspend from a fixed base during the concrete pour.
RESOURCE, EQUIPMENT AND FACILITIES FOR BORE PILE CONSTRUCTION
Crane
We’ll use a service crane of appropriate load capacity (25T),to install the reinforcing steel cage.
Drilling Equipment
For Pile boring we will use Hydraulic Rotary Drilling Rig. The equipment consists of a rotary table which imparts a rotary movement to a kelly bar to the bottom which attached a drilling tool (bucket or auger).
Tremie Pipe
All temporary casing shall be smooth wall structure steel. All temporary casing shall be of ample strength to resist damage and deformation from transportation and handling, installation and extraction stresses, and all pressures and forces acting on the casing. During the removal of the casing, we should be careful for not deforming and causing damage to the completed pile, and not disturbing the surrounding soil.
When placing concrete underwater. the Contractor shall use a concrete pump or gravity tremie. A tremie shall have a hopper at the top that empties into a watertight tube at least 8.0-in. (200-mm.) in diameter. If we use a pump, it should be a watertight tube with a minimum diameter of 4.0-in. (100-mm.).
Slurry Mixing and Storage Plant
Generally, the contractor will place the plant within the limits of the working areas. They’ll use the following equipment for preparation and storage of the slurry:
a. Mixer, 8-10 m3 / hour capacity and/or similar;
b. Tanks for the hydration;
c. Tanks for the regeneration;
d. Circulation pumps (to maintain the slurry in agitation).
The tanks are interconnected with pipes and connected to the excavation with pipes and pumps, in order to maintain the constant level of the slurry into the excavation. The amount of reserve slurry will be sufficient to allow the excavation of bored pile with a safe stock.
Survey Tools
Relevant survey tools are Theodolite, Level, Measuring Steel Tape and Staff.
PPE
Relevant Personal Protective Equipment is Helmets, Safety Shoes, Goggles, Reflective vest and Masks.
PROCEDURE OF BORE PILE CONSTRUCTION
We shall set and take approval of survey marks, to control the position of each pile before starting boring operations. We’ll use a reasonable firm and level area for the piling plant. We’ll submit a plan of the site layout, including stabilizer slurry plant, office areas and areas for the stockpiling of materials etc. for approval prior to the commencement of the works.
Preparation
Site Installation
Prior to the commencement of the Works, we’ll need a platform suitable to bear all the equipment involved in the construction of the bored pile. This platform will consist of a reinforced-concrete slab or steel decking platform.
We’ll install all site facilities for the construction of the bored piles including areas required for polymer plant,cage fabrication and storage.
Setting Out
The consultant must check the availability of updated and accepted construction drawings Prior to each setting out phase.
Reinforcing Cage Fabrication
the contractor will fabricate the steel reinforcement cages according to the relevant Technical Specification and approved drawing of the project. They’ll assemble and properly store the steel cages, close to the fabrication area. Site team will use a crawler crane of suitable lifting capacity for handling and placing of cages. They’ll use U-clips or by other approved means to join cage elements together with such capacity that the full strength of the Bar is effective across the joint so there is no relative displacement of the reinforcement during construction of the pile. They shall use Stiffener Ring (φ16mm) at every 2m in the entire cage length. The lap length of Stiffener Ring shall be 0.3m.
They’ll maintain a minimum 75mm Concrete cover to the reinforcement. The cover blocks shall be made with at minimum class 30 Concrete and cured properly before utilizing.
The contractor should remove the loose rust on the reinforcement with suitable means before lowering the cage into the borehole.
During fabrication, they should maintain careful controls to ensure the cage within the allowable specification tolerances.
Construction of Bore Pile
Temporary Casing Installation
After setting out of the pile location by a surveyor, the contractor engineer will install a temporary casing prior to the excavation to support the upper layer of weak soil.
They’ll use a hydraulic vibrohammer zhengzhong DZ90A or similar, fixed on the top of the casing and lifted by a 50 tons capacity service crane to install the casing.
The vertical accuracy of the casing will be checked during the installation process with a level device.
After the temporary casing installation, they’ll check the center location of the casing by the surveyor. Ensure the deviation between center location and pile site is within the allowable range of specification.
Drilling Process
We’ll stabilize the bored holes with a temporary casing in the upper soft layer and with bentonite slurry from bottom of casing until final depth. After the installation of temporary casing, boring will commence with auger and will continue with same until the bottom of the temporary casing. Then we’ll fill up the bored hole with bentonite slurry before advancing the drilling process. During the boring operations, the drilling tool is rotated and lowered into the ground. As it digs into the soil, the bucket or auger gradually fills with the soil and when it is full, the kelly bar and drilling tool are lifted and the drilling tool emptied.
We must maintain the level of the fluid in the excavation so that the fluid pressure always exceeds the pressure exerted by the soils and external ground water. We use slurry to maintain a stable excavation, so shall maintain the slurry level in the excavation to obtain hydrostatic equilibrium throughout the construction operation at a height required to provide and maintain a stable hole, but not less than 5.0-ft. (1,500-mm.) above the water table.
Special care must be taken in the horizontal position of the crane/rigs and vertical positioning of the kelly bar to ensure that vertical drilling will be achieved.
The contractor shall not perform borehole excavation operations within three diameters of a newly poured bored-pile within 24.0 hour of the placement of concrete and only when the concrete has reached a minimum compressive strength of 1,800psi. (12.5-MPa.)
Final Pile Toe Level Measurement
The bottom of the excavated borehole shall be sounded with an airlift pipe, a tape with a heavy weight attached to the end of the tape, or other means acceptable to the Employer/Employer’s Representative to determine that the borehole bottom meets the requirements of depth as shown in the approved drawing or as directed by the Employer/Employer’s Representative.
Pile Base Cleaning
Before placing concrete, measures shall be taken to ensure that there is no accumulation of sediment at the base of the boring which could impair the free flow of concrete from the tremie pipe.
For wet process excavations in soils, the base of the excavation shall be covered with not more than 3.0-in.(75- mm.) of sediment or loose or disturbed material just prior to placing concrete.
Clean bucket/wash bucket should be used to remove the extra sediment that found at the pile bottom which can be measured using the sounding.
(If used Bentonite Slurry)
Bentonite Slurry
If the density of the suspension exceeds 1.2 g/ml the placing of concrete shall not proceed.
Before placing concrete, measures shall be taken to ensure that there is no accumulation of sediment at the base of the boring which could impair the free flow of concrete from the tremie pipe.
For wet process excavations in soils, the base of the excavation shall be covered with not more than 3.0-in.(75- mm.) of sediment or loose or disturbed material just prior to placing concrete.
Bentonite desanding units, Stores or similar, are specially designed for separation of sand, silty and organic particles in the mud recovery system.
The sediment from the slurry can be efficiently eliminated by the combined operation of the hydrocyclone and vibroscreening system of the desanding equip.
Installation of Reinforcement Steel Cage
The steel cage preassembled in as many sections as needed and complete with the appropriate spacers and stiffeners will be lowered into the hole by a 25-ton service crane. The amount, type and size of steel will be according to the approved drawing and specifications. Lap splices as shown in the drawings shall be connected by U-Clips.
Fig: Sketches for lowering of the cage
Concreting of bore pile construction
The pouring operation is a very important aspect of the pile execution and must be performed with special care.
Concrete placement shall commence as soon as possible after completion of borehole drilled excavation by the Contractor and inspection by the consultant engineer.If water exists in amounts greater than 3.0-in. (75-mm.) in depth or enters at a rate of more than 12.0-in./h. (300-mm./h.) then the borehole excavation shall be filled with slurry to at least 2.0 m from the top casing level and concrete shall be placed by tremie methods.
The discharge end of the tube shall remain submerged in the concrete at least 5.0-ft. (1.5-m.) and the tube shall always contain enough concrete to prevent slurry from entering.
The bottom of the lower hopper is blocked with a cover plate (a little larger than the inner diameter of the pipe), and the top is provided with a hanging ring which is connected with the crane hook through a steel wire rope.
The concrete mixer into the appropriate location discharge concrete into the funnel, when injected into the hopper in the concrete to satisfy the amount of the first concrete, immediately pull the cover, the concrete is quickly fall into the bottom of the hole, at the same time, the speed of concrete pouring is accelerated, until the tremie embedment depth more than 1m.
The depth of the tremie pipe should be controlled at 2 ~ 6m below the concrete level, and before each lifting of the tremie, the elevation of the concrete surface in the detecting hole and the lifting height are determined.
Concreting will proceed continuously until the designed head of pile level is achieved.
The over casting pile head concrete height above the design cut off level shall be 1.5m. If the concrete placement operation is interrupted. The Contractor shall prove by core drilling or other tests that the bored-pile contains no voids or horizontal joints. If testing reveals voids or joints, the Contractor shall repair them or replace the bored pile at no expense to the Employer.
Before pouring concrete, the performance of the mechanical equipment shall be checked to prevent failure during the filling process. Preparing some spare equipment is an effective way.
Before taking out the tremie should measure and review the hole depth, tremie length.
The slurry out of the pile need pumped to the mud pit in time with a mud pump, to prevent slurry pollution pavement.
TEMPORARY CASING EXTRATING
Temporary casings will be extracted immediately after the completion of concreting using the service crane or similar.
RECORDS OF BORE PILE CONSTRUCTION
The following records will be kept for each wall panel and will be available for inspection by the Engineer,
a) Bored pile no.
b) date and time of start of pile excavation
c) date and time of finish of pile excavation
d) details of any obstruction encountered
e) date, place and time of slurry control tests and recorded results
f) Diameter and depth of pile from top of casing level.
g) date and time of completion of cage installation
h) details of reinforcement and cage type
i) date and time of start and completion of concrete placement
j) Volume of concrete used and time of any interruption in concrete supply, volume of normal
and lean mix concrete.
k) log of soil encountered
QUALITY CONTROL FOR CONSTRUCTION OF BORE PILE
All work procedures are required to follow the attachment ITP sheet. Inspection and test plan of each activity shall be strictly followed and the contractor shall ensure that all the witness /hold points shall be attended by supervision consultant before going to the next activity.
CONCLUSION
As you can see, bore piling is a complicated and costly process that necessitates specialized equipment and experienced personnel. It is also necessary to plan and design carefully to ensure that the foundation is strong enough to support the intended structure. Despite these difficulties, bore piling is still a popular foundation method in many construction projects.
To summarize, bore piling is a valuable construction technique for creating deep and stable foundations. It, like any other method, has advantages and disadvantages, and its effectiveness is determined by a variety of factors such as soil conditions, site location, and the design and execution of the piling process.
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