Updated November 4th, 2019
Based on the construction method, piles can be either cast in situ or precast driven. In this post, I’m going to share a step by step process of constructing bored cast in situ concrete piles.
What Are Bored Cast In Situ Concrete Piles?
Piles are basically columns built underground. When the top layer of soil is weak to carry the load of a structure then piles are made.
The purpose of pile foundations is to carry the load of a structure to the hard layer of underground soil.
When a cylindrical hole is bored on the ground and then concrete is poured into the hole to make a pile that is called bored cast in situ pile.
Now let’s see…
The Specification Of A Bored Cast In Situ Concrete Pile
You’ll get a drawing something like the image below:
From the image above:
- Dia of the pile is 24 inches.
- Length of CC below the pile is 2 feet.
- As per section 1-1, 9 numbers of 20 mm full-length bar is used for the top portion of the reinforcing cage.
- 6 numbers of 20 mm bars are used in the middle portion of the reinforcing cage as per section 2-2.
- And as per section 3-3, 6 numbers of 16 mm bars are used for the bottom portion of reinforcing cage.
- The reinforcing cage should be placed 6 feet below the existing ground level. And the cut-off level is 3 feet.
- The spiral spacing is 8″ c/c.
- Lap length is 2 feet.
These are the specifications we get from our example drawing.
You’ll get some other information from structural notes of the pile drawing. Such as strength of concrete, required concrete slump, and concrete clear cover, etc. You’ll also need these.
Now that you have the specification of the pile, you can go for constructing it.
Step By Step Construction Process Of Bored Cast In Situ Concrete Piles
I assume that you’ve already hired a contractor for piling work and he has gathered all the required tools and machinery on your project.
Now let’s do the work.
Step #1: Locate Pile Point On The Ground
You’ll have a drawing showing all the pile points in reference to gridlines like the image below:
Once you’ve set out the building, it’s easy to mark pile points on the ground. I’ve talked about this before in the post how to locate pile points on the ground and protect them from missing.
Pro Tip: Try to mark all the pile points at a time. If you left some points to mark in some other day you’ll have many hassles as there will be tools, machinery, and materials on the project.
Step #2: Start Boring
After marking all the pile points you can proceed to construct the first pile.
The purpose of boring is just to make a hole up to the hard layer of soil below the ground and wash out the mud from the hole.
Water is pumped through the boring bit to wash out the soil while driving chisel.
You have to have a mud tank ready to collect this mud-mixed-water.
Pro Tip: Mix bentonite powder with the water used to wash out the borehole. It’ll protect the hole’s wall from breaking down.
Now that you know the mechanism of boring a pile, let’s…
Set The Piling Rig To Bore A Pile
Now that you’ve marked all the pile points, you can now be ready for boring a pile.
Set the piling rig.
Make sure the chisel is centered properly with the pile point.
Drive Steel Casing
Now you need to drive the steel casing. The casing is driven to protect the lateral loads and prevent breaking top layer soil.
It also protects the boring bit from displacing.
After pouring concrete, the casing is removed and used for another pile.
Pro tip: There is a dispute regarding the length of the casing. Some say, 10 feet long casing is enough for quality piling works. But some say it should be at least 15 feet long. So it’s better to consult with your structural engineer before start piling work.
Complete The Boring
Now that you’ve driven the casing, you can now continue bore till the chisel nose reach the hard layer of soil.
But how long should a pile’s hole be?
You can get this from the structural drawings of piles.
That is 124 feet.
You need to bore a 124 feet long hole into the existing ground.
But the existing ground may not be the same level all through the plot.
So take a reference level from the entrance of the building and measure all the boreholes length with that reference level.
Now you can ask me how to measure a borehole below ground?
You don’t need to measure the hole. Just measure the boring bit.
Pro tip: It isn’t that you have to bore 124 feet long holes for all the piles as specified in the drawing. If you get hard soil layer at 110 feet you can stop boring. But if you don’t get hard soil layer even after boring 124 feet, you need to bore more till you get the hard soil layer. For this, you need to have a close look while boring.
Okay. Let’s say you get a hard soil layer in 124 feet and your boring is finished.
Step #3: Keep Washing The Bore Hole
During boring, the wash was also running. Even after completing the boring there must be mud in the borehole.
You shouldn’t do anything before completely clean the borehole.
So you have to continue to wash the borehole until the hole is completely cleaned.
But how do you make sure that the borehole is free from mud or loose soil?
We use a density meter to check the density of the mud slurry, which is coming out from the borehole while washing.
Collect mud slurry in a bucket and put the density meter in the liquid.
If the meter indicates the density of the liquid below 1.15 g/ml mark then you can stop washing.
And get ready for…
Step #4: Pouring Concrete
As washing borehole is finished, the next step is to remove the boring bits from the hole one after another.
Once the boring bits are removed, now it’s time to…
Insert Reinforcing Cages Into The Hole
We need to insert 3 numbers of reinforcing cages as per our structural drawing.
Insert the first one, which will be at the bottom of the hole, and hold it on the top of the casing to join the second reinforcing cage with it.
Again, insert the second reinforcing cage, which will be in the middle of the borehole, and hold it to join with the last reinforcing cage.
Finally, insert the last reinforcing cage.
Here you need to understand one thing that the reinforcing cages won’t go to the bottom of the borehole as there is a 2 feet long CC at the bottom of the pile.
So you need to hold it two feet above the bottom of the borehole.
You can hang the cage by welding a hook with the cage and hang it with casing.
Another thing, as per our example drawing, you need to put the cage 6 feet below the existing ground level.
You can do it while making the hook for hanging the reinforcing cage.
Pro tip: While lapping one reinforcing cage with another, don’t take much time for welding. Another thing, if the reinforcing cages don’t go into the borehole freely, lift them and wash the borehole again.
Once you inserted reinforcing cages successfully, the next step is to…
Insert Tremie Pipes
Tremie pipes are normally 8 inches dia hollow pipes. Concrete goes through these pipes into the borehole.
The mechanism of pile concreting is, the concrete goes directly to the bottom of the borehole through the tremie pipes and pushes the mud slurry to the top of the borehole.
That means the concrete you first poured into the borehole will be on the top of the borehole when you finish concrete pouring.
So, you need to join tremie pipes one after another till it goes to the bottom of the borehole.
Okay. Once you finished inserting required tremie pipes into the borehole, now it’s time to…
Pour Concrete Into The Borehole
You can use ready mix concrete or on-site machine mix concrete, it’s upon you.
If you use on-site machine mix concrete make sure you have available concrete ingredients on your project.
You can’t stop pouring concrete until the pile is completed.
As per our example drawing, you need to stop pouring concrete once the concrete reaches at 6 feet level below the existing ground.
You can use a bamboo-like thing to check the concrete level. Just mark a 6 feet length on the bamboo and put it into the borehole to check the concrete level.
Pro Tip: Depending on the length you may need two-three hours to finish pouring concrete into a bored cast in situ pile. As we know, the initial setting time of cement is 45 minutes. After 45 minutes the concrete will start to become harden. But we need to keep the first poured concrete in the plastic state till the concrete pouring is finished. For this, use admixture in the concrete mix to increase the initial setting time of concrete.
Remove Tremie Pipes
As you pour concrete you need to remove tremie pipes in a certain interval. Before removing a tremie pipe, make sure the bottom end of the tremie pipe in the borehole remain at least 10 feet inside the concrete.
The thing is that our purpose is to push the mud slurry up and remove through the top of the borehole. If the bottom end of tremie pipes come up above the concrete level, the mud slurry will fill the borehole. And thus there will be a concrete gap in the pile (which will be filled with mud). As a result, the pile will fail to carry the load of the structure. So, be careful with this thing.
As soon as concrete pouring ends all the tremie pipes removing will end too.
At last, remove the steel casing from the borehole.
With that, you’ve successfully finished the construction of a bored cast in situ concrete pile.
The construction process of a bored cast in situ concrete pile is actually easy. You first need to locate all the pile points on the ground. Then bore a hole for one pile into the ground till you get the hard layer of soil.
Once the boring is finished clean the borehole by washing with water.
Once the borehole is cleaned, just insert the reinforcing cage into the hole and pour concrete.
If you have experience of constructing bored cast in situ concrete pile, please let us know which chemicals do you use in the concrete mix to delay the concrete hardening process and what’s the quantity of that per cement bag? Please share in the comments below…