Bored piles as mentioned in this post “What is Bored Piles?“ are constructed following the sequence below.
- The first step is to drill a hole of the require diameter into the ground.
- When the design depth or foundation bearing layer is reached, drilling stopped.
- The hole is then cleaned.
- Steel reinforcement cage is lowered into the hole.
- Concrete is then poured into the hole until the designed cut-off level of the pile.
- After Step5, the construction of bored pile is deem completed.
Thus it can be seen that bored piles are constructed by first drilling into ground. Drilling is one of the most important step in bored pile construction. Only successful drilling can the next construction step be carried out. Thus drilling requires experience and the use of the right tools and equipment. In this post we will take a look at the types of drilling machines used.
Bored piles can be drilled by using an auger ( see attached picture of an auger) that is powered mechanically. There will be an auger attached to a the tip of a telescopic Kellybar.
Telescopic means the Kellybar is extensible, there by enabling drilling to greater depths.
Drilling rigs can be crane mounted or as a single unit known as rotary drilling rig. Both of which is discussed in greater details below.
Crane Mounted Drilling Rig
The drilling rig is a power unit that rotates the Kellybar which in turn rotates the auger. The power unit is normally diesel powered. The Kellybar is slotted through the power unit at the front end known as the “Table”. Horizontal rotation of the diesel engine is converted to vertical rotation which rotates the Kellybar.
There is also a pair hydraulic arms at the “table”. These arms can clamped to the Kellybar and are use to raise or lower the the it slowly over a short lenght. Rotation can be either anticlockwise or clockwise. The whole power unit, Kellybar and auger assembly is then mounted onto a base crane.
Drilling starts by pressing and rotating the auger at the same time. Once the initial hole is formed and depending on the soil material, the weight of the Kellybar pushes the drilling deeper.
The Kellybar is raised and lowered into the hole by a separate lifting cable of the base crane. This enables the Kellybar and auger to be lifted up quickly to the surface to dispose of the drilled material and later lowered back again to the hole to continue drilling. This processes is repeated until the desired depth is reached.
Below is a picture of a crane mounted drilling rig.
Name of Parts
- Diesel Powered Drilling Rig
- Base Crane
- Hydraulic arms
The drilling rig can also be mounted onto a heavy truck like what is shown in the picture below.
Rotary Drilling Rigs
A compact design in which the drilling rig is permanently mounted onto a crawler unit. The drilling rig is hydraulic powered giving it a very powerful drilling capability. The drilling process is the same as what is mentioned above.
However rotary drilling unit is easy to deploy. There is no setting up required compared to crane mounted drilling rig.
A crane mounted rig requires assembly on site assembly and requires the help of another crane to mount and set up. As for for rotary drilling rigs, there is no such requirement.
Once mobilized to site it is just a matter of raising up the leader , attached the auger and drilling can start. On the left is an example of a rotary drilling rig.
Drilling By Using Continuous Flight Auger
Drilling of bored piles can also be carried out by using continuous flight auger (CFA). CFA is used when bored piles depths are fixed and the diameter not larger than 1500mm. The construction of bored piles using CFA is different from the auger and Kellybar drilling.
The major difference is this, when the bored pile desired depth is reached, the hole can be filled by injecting concrete down the hollow stem of the CFA. In this way the bored pile is concreted as the auger is slowly raised.
After completing the concreting and the auger had been removed, a reinforcing cage is pushed down the shaft while the concrete has not set. It is not uncommon to push down cage length of 12m. Thus it can be seen that the final steps of bored piles construction by CFA is different from the Kellybar and auger process.
However if the drilled hole depth is shallow and the ground stable ( the drilled hole does not collapse), the CFA can be completely removed. In this case, the concreting can be carried out like those of the Kellybar and auger method.
On the left is a picture of a Continuous Flight Auger Rig.
Modern drilling rigs can drill bored piles of diameters ranging from 600mm to 4000mm. Due to the convenience of the rotary drilling rigs, they have become more common nowadays as compared to the crane mounted drilling rigs. Rotary drilling rigs are self-mounting and also easily transportable. On the whole this capability which reduces cost of mobilization.
In the next post we will look at the types of drilling tools used.
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