Bored pile
1. Preparation works
The ground must be cleared, nearly leveled and compacted. The bentonite mixing tank has to be set up on free land, where it’s not disturbing for the pile construction.
The pile center is setting out by using Total Station survey equipment and from twos directions. This step must be checked by a surveyor.
2. Drilling
All of the drilling is made by a crawler.
Firstly, you have to pre-drill from 6 to 10 meters to install a temporary casing in the borehole. This temporary casing is used to maintain the stability and the verticality of the borehole during drilling. The borehole is continuously feed by bentonite to prevent against collapses during excavation. The bentonite solution shall be prepared and tested before the following drilling operations.
Bentonite solution requirements
· Density < 1.1
· Viscosity from 30 to 90 sec
· pH from 8 to 12
· Sand content < 2%
When the verticality and the position of the temporary casing are checked, the drilling excavation can continue. Then the crawler excavates the soil with its drilling bucket. The crawler puts its drilling bucket in the borehole, drills, and goes back to the top to remove the soil. The drilling process takes about 10 hours for 60 meters depth. The drilling excavation is surveyed by a record of the borehole. The surveyor has to check the depth of the borehole every 30 min and the verticality of the excavation during the drilling.
After that, the borehole is cleaned by the cleaning bucket and an airlift system. The bentonite is extract from the borehole and sand filtered. Then the bentonite solution is tested. If the test shows that the bentonite solution is not clean, you have to re-clean or replace with good slurry. If the test is ok, you have to measure the depth of the borehole in the 4 checking’s positions.
To survey the verticality of the borehole and the quality of the drilling, you have to do a Kooden test.
The Kooden test machine emits waves in 4 directions.
The waves hit the edge of the borehole and come back to the machine. The machine record the course time of the wave, and can calculate the distance between the center of the pile and the edge because the velocity of the waves is the same in the bentonite solution.
v = d / t => d = v x t
If the Kooden test shows that the slope of the edge of the borehole is more than 1%, you have to re-drill, to improve the verticality of the borehole.
3. Rebar cage fabrication and installation
The rebar cage is fabricated in many parts and assembled in the borehole with U-Bolts. All of the rebar cage fabrication is checked by the surveyor because it has to be like on drawings. The rebar cage inspection is recorded in a checking report. The sonic tubes are also inspected. When the rebar cage is installed, you have to re-clean the borehole with an airlift system and to test again the bentonite solution.
4. Casting concrete
Before casting concrete, you have to set up the tremie pipe. It serves to cast concrete from the bottom to the top of the borehole. The distance between the tremie pipe and the bottom of the borehole is about 20 or 30 cm. A sliding plug is placed in the pipe. This can guarantee a high pressure in the tremie pipe, to be sure that the sedimentation will get up. Moreover it’s preventing the concrete for a direct contact with the water of the bentonite.
The concrete is cast under the bentonite with the tremie pipe. The density of concrete is higher than the density of the bentonite, so the bentonite solution run out the borehole and the concrete take its place. The tremie pipes are removed after each concrete casting.
After each concrete truck (about 6m3), the depth of the concrete top is measured. Then these results are gathered in a concrete chart. It permits to compare the theory rapport volume/height with the actual. If the actual curve is under the theory, it means that you have more concrete than in the project, so it is good for safety. If you are upper the theory curve, it means that there is a problem in the borehole (maybe a collapse or a smaller borehole diameter).
Generally, the tip of the tremie pipe shall be 2 m under the concrete surface. These results also permit to know the length of tremie pipe which is in concrete. So you can know easily how many tremie pipes you can remove.
Every concrete truck is tested. The concrete temperature and slump are recorded. The temperature must be under 32°C, and the slump test 18 +/- 2.5cm. If one of these requirements is not respected the truck has to leave from the site. Moreover the ambient temperature must be under 32°C to cast concrete. If it’s not, concrete has to be made with glass or fresh water to be sure the casting concrete temperature will be fine.
Moreover it’s important to improve the elevation of the top of the pile, because this concrete is contaminated by bentonite. The height of extra or surplus concrete should be at least 1m from cut off level.
After the completion of pouring concrete, the temporary casing is removed by the crane. The concrete level decrease to nearly 1m, so this as to be considered as surplus concrete too.
Seven days after the concreting, there is the sonic test. This test permits to know the integrity of the pile. There are 6 tests plans per piles. The sonic test machine sends waves and measures the time between the emission and the reception. The sonic tubes distance is known, so you can calculate the velocity of waves in the concrete. If the wave speed is under 2000 m/s or evolves abnormally, there is a problem (maybe a borehole collapse, a concrete contamination, a concreting interruption…).
5. Check list for acceptance of construction work
All of these reports must be approved by the surveyor to accept the constructed pile:
· General report of bored pile work
· Checking record of center point co-ordinate before drilling
· Drilling record borehole
· Checking report of steel cage section
· Checking report of the completed borehole
· Checking report of join rebar cage
· Checking report of installation sonic duct for bored pile testing
· Checking report before concreting pile work
· Concreting record of bored pile
· Checking report of concreting for bored pile
· Report on filling of sonic testing duct by cement grout
· Report on concrete compressive strength test
Resume process
1-2. Commence boring
3. Install casing
4. Continue boring with drilling bucket (Bentonite stabilization)
5. Bore to required depth
6. Measurement of borehole depth
7. Cleaning the base with a cleaning bucket
8. The reinforcement cage is lowered into the borehole
9. Tremie pile & bentonite pump installation
10. Cleaning of base by air lift method
11. Measurement the level of concrete riser - The tremie will be withdrawn accordingly
12. The temporary casing is then withdrawn
13. Finish level