If the pile can still be connected, it is best to carry out the connection treatment. When the earthwork excavation is completed, extend the steel bars into the interior of the pipe pile and pour concrete to connect the pile. The pile should be extended and anchored into the foundation 50~100mm. The length of the steel bars extending into the pipe pile, the strength grade of the concrete and the pouring length should refer to the local drawing collection.

But what should be done if there are no more conditions for pile driving? Some suggest trying to negotiate with the design team to add composite foundations; others propose lowering the cap height and making the columns longer; still others suggest driving another pile nearby. Although answers vary, one thing is certain: when driving PHC pipe piles, it is essential to pay attention to the following issues.

Water is discharged from the pile

[analysis of causes]

The opening part of the pipe pile continuously squeezes into the soil during driving, forming a soil plug at the pipe mouth. The length of soil entering the pipe is approximately 1/3 to 1/2 of the pile length. Under normal circumstances, there is no water inside the pipe pile. However, due to construction quality issues (such as: ① poor welding quality at the pipe pile joints, with porosity and slag inclusion; ② excessive hammering causing cracks at the joints; ③ cracks in the pile body), groundwater enters the pipe pile before the overconsolidated pore water has dissipated, driven by the pressure difference.

[treatment measure]

When the water is discharged from the pile, the groundwater in the pipe should be pumped out after the pore water pressure dissipates, and the hollow part of the pipe pile should be filled with C30 concrete to prevent the groundwater from flowing out of the pipe, so as to improve the durability of the pipe pile and improve the designed service life.

The top of the pile floats

[analysis of causes]

There are three main reasons for the floating of pile top.

(1) Soil Compaction Effect. During the process of pile driving, the soil is compacted, reducing its porosity. After being compacted horizontally, the soil is then passively pushed upward or creeps upward. Due to the relative reduction in vertical stress, the soil moves upward, causing the ground to rise. The pile body experiences upward friction, lifting the pile. Additionally, in saturated cohesive soil, the overconsolidation pore water pressure generated by pile driving can also cause the pile body to rise.

(2) Interaction of Stress Waves. When using the hammering method for pile driving, when the falling hammer acts on the top of the pile, the top of the pile moves downward, generating vertical stress waves in the pile body. At this point, these stress waves propagate through the soil around the pile to all sides. Simultaneously, the soil around the pile also generates upward stress waves, causing other adjacent piles to be subjected to upward forces and move.

(3) Soil Rebound. According to design requirements, the top elevation of some piles is below natural ground level. For example, when using pipe piles for reinforcement at the base of a box girder bridge, the top elevation of the pile is below natural ground level. A pile driver must be used to drive the pipe pile down to 5-6 m below ground level. After excavation of the foundation pit, due to unloading at the base, the soil rebounds, causing the pile top to rise.

[treatment measure]

When the phenomenon of pile top floating occurs, the re-driving should be carried out immediately. If the effect of re-driving is not obvious, the re-driving can be carried out after a certain period of time. The hammer weight or re-press value should be increased appropriately when re-driving.

The technical specifications for prestressed concrete pipe piles require that the allowable deviation of pile top elevation is ±50.000mm. When negative deviations are used to control the pile top elevation, although it can alleviate the phenomenon of pile floating, the impact on the load-bearing capacity of the pile after floating is significant. Therefore, using negative deviations to control pile floating is not advisable. After driving the pile, the pile top elevation should be monitored in a timely manner, and measures should be taken to address any detected pile top floating.