How to Prevent the Steel Cage from Floating When Pouring Bored Piles?

During the pouring process of bored cast-in-place piles, the steel cage often floats up. In mild cases, the steel cage may float up by a few centimeters to tens of centimeters. In severe cases, the steel cage may float up by several meters or even float up completely, which is very harmful to the quality of the project.

1. Analysis of the Causes of Steel Cage Floating

1. The upper end of the steel cage is not fixed firmly at the hole. When the guide tube is lifted, it is easy to be caught by the guide tube and lifted together, causing the steel cage to float.

2. During the concrete pouring process, when the concrete surface returns to the bottom of the steel cage, due to the shallow depth of the guide tube, the amount of concrete poured is relatively large, resulting in excessive concrete return speed, generating a large upward force, thereby lifting the guide tube and the steel cage.

3. After the concrete surface and the bottom of the guide tube enter the steel cage, if the guide tube is buried too deep, it will easily cause the steel cage to float.

4. Since the initial concrete has been on the upper part of the concrete poured in the drill hole, once the concrete has quality problems such as easy segregation, short initial setting time, and large slump loss, the concrete fluidity will deteriorate over time, causing the initial concrete on the upper part to solidify. When the concrete surface with poor fluidity or even solidification contacts the bottom of the steel cage, it is very easy to lift the steel cage to float.

5. When there is a fine sand layer in the ground, if the mud density is too low, the collapsed fine sand will be spread on the concrete surface, thus forming a cushion layer with a certain thickness to support the steel cage. As the concrete surface rises, the steel cage will also be lifted up.

2. Measures to Prevent Steel Cage from Floating

1. When pouring pile foundation concrete, pay special attention to the changes in the hanging bars of the suspended steel cage. If you see that the hanging bars are slightly “swaying” upward, it means that the steel cage has floated up. At this time, take measures immediately to slow down the pouring speed of concrete, and repeatedly use the winch on the drilling rig to “slowly lift and quickly drop” the guide tube, that is, slowly bring the floating steel cage back to the poured concrete.

2. Increase the diameter and number of hanging bars, add counterweights at the wellhead, and weld them to the casing.

3. When concrete enters the steel cage, it is required to strictly control the common burial depth of the guide tube and the steel cage, and the maximum depth shall not exceed 6m. When the bottom end of the guide tube is raised above the safe distance, the burial depth of the guide tube should be properly controlled. At this time, as long as the concrete fluidity is good, the steel cage will generally not float up.

4. When pouring concrete, measure the height of the concrete surface in the hole at any time, and strictly control the concrete pouring speed, so as to control the speed of concrete return and reduce its carrying capacity for the steel cage.

5. Adjust the slump of concrete. Generally, the slump of underwater concrete should be controlled at 18-22cm. The concrete for pouring pile foundations is required to have good workability and fluidity to ensure that the concrete can have good “floating” during the pouring process. Otherwise, if the concrete has poor workability and fluidity, it will be very difficult to pour the pile foundation. The concrete poured first is about to solidify into a whole, and the steel cage is lifted as a whole, causing it to float.

6. When pouring concrete, control the mud density to about 1-1.1. If the specific gravity is too large, the buoyancy generated on the steel cage will increase, and a thicker slurry will be formed on the concrete surface. When the concrete surface rises, the slurry will float up with the steel cage. According to the actual situation on site, high-quality bentonite is used to make slurry.

7. Since the bending or improper overlap of the steel cage also has a certain impact on the floating of the steel cage, the construction personnel should strictly control it to eliminate such effects.

8. A steel bar pole can be welded on the steel cage to protrude from the mud surface. When the pole is found floating up, it also means that the steel cage has floated up. At this time, measures must be taken immediately to restore the floating steel cage to its original position.

====Thanks===