Bored and cast in-situ piles or bored & cast in place piles carries the process of installation in the name it self. A bored and cast in-situ pile is installed by boring the soil and or rock using heavy machinery and pouring concrete to the bored hole with or without steel reinforcement.
Figure: Rotary drilling machines in the process of pile boring
You can watch the detailed process in video form the following link
You can check more details from the following link
MOS for Bored Cast In situ Pile Installation
https://engcivilstuff.blogspot.com/2020/03/bored-piling-installation-by-rotary.html
Normally, Bored piles are constructed in the range of the diameters 500mm to 3000mm. The same machinery can be used with different sizes of tools.
Boring process can be done either as Dry boring or as Wet boring with use of a drilling fluid. Or it can be done with a full casing if the soil condition is not reliable (i.e. very thick silt or peat layers).
Pile Toe
Bottom of the pile which touches rock or hard soil layer and provide end bearing to the pile is called the pile toe. It is very important that the pile toe is clean from loose particles, mud, etc. while concreting to provide a solid strong toe and to prevent settlement of pile after loading.
Toe flushing technologies can be used to prevent loose material to retain at pile toe.
Pile Termination or Socketing
Pile termination is depend on the design and engineers decision at the field. Basically a pile will be stopped at rock or at a dense soil. The bearing layer will be based on the design. The pile design can be either End bearing type of Friction type of a combination of both in most cases. In an area where the good rock formation can be found in lower depths, it is better to design for End bearing type of piles. And in areas where no rock is presence or the depth of the rock is very high (ex: 50m below ground level) it is better to go for Friction piles. The end condition in designing is depending on the loads or reaction on piles and the diameter of piles.
Socketing is the length of pile in the termination layer (i.e. either in hard rock or hard soil as per design). For an example if the design says the pile must be socketed to a 1m length in a rock having 75 RQD or higher, then the pile must be drilled up to 1m depth in the hard rock layer. In practice, piles socketing length is not less than the diameter of the pile (i.e. 600mm dia. pile should be socketed minimum 600mm depth in hard layer)
End bearing Piles
Piles design with mainly considering the end bearing provide by rock layers are known to be the end bearing piles. Most of the load carrying capacity will be taken by the toe of the pile in this type of piles. End bearing type pile designs are more common in areas with hard rock in lower depths.
Friction Piles
In Friction piles, the load carrying capacity is mainly developed by the shaft of the pile. The frictional resistance developed by the friction force in concrete to soil or concrete to rock against the settlement of the pile under loading is the main consideration in designing frictional piles. In areas where hard rock is difficult to encounter in lower depths, frictional piles are a feasible solution.
Machineries & Tools
There were different type of machineries used in the construction of bored piles in past. Currently the Rotary Drilling Rigs are used mostly in constructing bored piles around the world. These Rotary drilling machines are with different capacities/ powers. And the possible depths of boring of a pile is depend on the Kelly Bar length of the machine and the capacity of the machine to provide required torque at higher depths. The capacity of the machine will also matter when drilling the rock or hard rock to higher depths to provide required socketing length.
Figure: Boring Rig
Tools
Soil/ Rock Auger
Soil or rock auger is basically use to drill the soil or rock. Soil auger is use for the installation of temporary casing after setting out of the pile. Basically soil auger is same as above picture and consists of less or no bullet teeth (pointed parts or pins at the bottom of the auger). Rock auger mainly use to drill and penetrate the soft or hard rock layers.
Soil/ Rock Boring bucket
The picture above shows a Rock boring bucket. For soil boring buckets, the same type tool is used without the bullet teeth (pointed parts attached to edge of the tool). The bullet teeth are basically to cut the rock or hard layers of soil. After cutting rock or soil the bucket will be fill with rock or soil material while rotating the shaft (Kelly bar) of the rotary drilling machine. Then they can be removed after taking out of the pile bore by simply releasing the bottom panel of the bucket releasing pin or lever in the bucket.
Rock Core barrel
Rock core barrel is used to cut the rock in the perimeter of the pile. Normally the rock is cut to 500mm or 1000mm depths depend on the rock types. This make it easy for the Rock boring buckets to remove the rock in pile area. if the core cut depth is high, then it will be difficult to remove the core barrel sometimes since it stuck in the rock core.
Cleaning bucket
Cleaning bucket is used to clean the pile toe after boring work is completed. Rock particles, mud, etc. which were not cleaned by the boring bucket is removed by the cleaning bucket. Minor particles may not be cleaned by the cleaning bucket and need pile toe flushing to fully clean the pile in some locations.
Different types of tools using in piling industry
Dry Boring
In the absence of water (i.e. water levels are much lower) and if the soil existing are not collapsible, the dry boring method can be used. In this method no drilling fluid is use. After boring of pile and cleaning of pile toe, the concreting process can be started. This method is easy in piling construction and less troubles to construction teams in removal of soil, maintaining site clean, etc. But its applicability is very limited since most of the locations will consists with high water levels and collapsible type soil.
Wet Boring
Wet boring is the technique using in boring of piles with the use of a drilling fluid which needs to keep the pile bore without collapsing. Mainly the drilling fluid is providing stability to the core by applying more pressure than the pressure applying from water in the soil side. Wet boring makes piling sites muddy and needs regular cleaning process.
Drilling Fluid
Use of Bentonite or Polymer is common in current piling industry as the drilling fluid. the selection of type of drilling fluid is depend on the soil type mainly. Both above types of drilling fluids are having higher densities than water. Bentonite mainly use its density and clayey properties to protect the pile bore without collapsing. Polymer is having less density than bentonite. It makes some polymer chain after mixing and creates a layer of polymer in pile bore to keep it stable without collapsing. These properties can be vary with different types of material available in the market.
Toe Cleaning or Flushing of Pile Toe
After completing the boring process of pile, toe of the pile need to clean thoroughly. initially the large particles and mud are removing using the cleaning bucket. But there are mud or other small rock particles which are not possible to clean using a bucket. Hence there are different technologies used in the piling industry to clean the pile toe. For example flushing with pressurized bentonite using heavy pumps, air flushing by using air compressors or using submersible pumps connected to the tremie pipe bottom and clean by suction.
Basically the bored piles are circular. Hence the reinforcement cages also need to be circular. The reinforcement cages consists of main bars, circular spirals and circular rings to keep the cage stability.
Cover block are also attached to the rebar cages with the shape of a wheel to allow rotation and avoid any block when lowering the cages in to pile. Installation of rebar cages is done by using cranes.
Pile Concreting
Pile concreting is done by using Tremie pipes (series of pipes connected together with the diameter of around 200mm). And a large funnel is fixed in the top of tremie pipes which is used for the pouring of concrete from concrete truck mixers. In concreting it should be done in a way that all the mud in the bottom of pile should come out with concrete without creating a soft toe (concrete mixed with mud) after concreting.
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