Pile Foundation Construction Process

During the construction of pile foundations, due to geographical environment, mechanical equipment, human factors and other reasons, it is inevitable that some defective pile phenomena will occur during construction. Once defective piles appear, different degrees of defects have different treatment methods, but low-cost and high-efficiency methods are what people have always been pursuing.

Types of Pile Foundation Defects

** First, Defects at the Top of the Pile Foundation

Due to mud sedimentation during underwater concrete pouring, the thickness of the floating slurry is difficult to estimate accurately. When the concrete at the pile top is insufficiently over-poured, mud inclusion in the concrete is inevitably produced, affecting concrete quality. Secondly, after the concrete pouring is completed, when the embedded steel casing is removed and pulled out, excessive force or unbalanced force from left to right will disturb the concrete at the pile top and affect concrete quality. Finally, during the process of chiseling away the pile head concrete, because the power of the pneumatic pick is too high, the concrete around the sonic logging pipes is disturbed to varying degrees, which also has a certain impact on concrete quality.

Second, Defects in the Middle of the Pile Foundation

First, due to poor geological conditions, local hole collapse occurs during the concrete pouring process, locally preventing the concrete from flowing back up, resulting in local defects. Secondly, during the process of removing and pulling out the tremie pipe, excessive force disturbs the continuity of the concrete and affects concrete quality. Finally, the tremie pipe has poor air tightness. Because the tremie pipe enters the mud during underwater concrete pouring, the pressure difference inside and outside the pipe is destroyed. In mild cases, the continuity of concrete quality is affected; in severe cases, the concrete feeding is hindered, normal backflow cannot occur, resulting in broken piles.

Third, Defects at the Bottom of the Pile Foundation

Before the start of concrete pouring for the pile foundation, incomplete cleaning of debris will cause defects of sediment at the pile bottom, or poor sealing at the bottom during the first concrete pouring will cause defects such as mud mixing into the pile bottom concrete or non-compact separation of the pile bottom concrete. The core-drilling and grouting treatment method is often used.‍

Pile Foundation Defect Treatment Methods

I. Treatment of Defects at the Top of the Pile Foundation

According to the test results, for defects existing at the top of the pile foundation, we can continue to chisel away the pile head concrete downward until reaching the position where the concrete quality is intact. However, during the process of breaking and chiseling concrete downward, attention should be paid to observation, timely discovery of abnormal defect positions in the concrete, correspondence with experimental test data, and finally connecting the pile with concrete of the same grade as the pile foundation. Note that during the process of chiseling concrete downward, the pile foundation reinforcement should be protected to prevent damage.

II. Treatment of Defects in the Middle of the Pile Foundation

This type of pile foundation defect treatment is difficult to measure uniformly. Small defects can be treated by grouting. For large defects, generally those of more than two meters, grouting treatment is difficult to guarantee quality, and the pile is usually discarded, with re-percussion drilling at the original position, installation of the reinforcement cage, and concrete pouring.

III. Treatment of Defects at the Bottom of the Pile Foundation

For defects at the bottom of the pile foundation, according to the load-bearing properties of the pile foundation, a simplified force calculation can be carried out first, and then it can be determined whether treatment is actually needed. If the pile foundation is a friction pile, the length of the defect position does not affect the overall load bearing; that is, within the pile length excluding the defective part, the friction force can already meet the load requirements, so no further treatment is needed; otherwise, treatment is required. If the pile foundation is a rock-socketed pile and there are defects at the bottom, treatment is required, and pile bottom grouting is a feasible treatment method.

(1) Preparation for Implementing Pile Bottom Grouting. Reasonable grouting design is the prerequisite for grouting. In order to make pile bottom grouting construction reasonable and effective, it is necessary to conduct detailed investigation and analysis of the rock stratum geological conditions, groundwater conditions, defect degree and surrounding environment of the pile foundation, calculate the amount of cement slurry, scientifically prepare the grouting slurry using cement one grade higher than the pile body concrete, and determine parameters such as grouting pressure.

(2) Drilling. Drill 3 grouting holes in the defective pile body. During drilling, verticality must be ensured to ensure that the drilled holes are parallel to the pile foundation axis, preventing deviation of the drilled holes, contact with the pile foundation reinforcement, or penetration out of the pile foundation. The drilling depth is 1 meter deeper than the pile length.

(3) Water Pressure Test. The water pressure test not only serves to clear the grouting pipeline, but also allows relevant operational experience parameters for grouting to be adjusted accordingly based on the water pressure test results. The water pressure test can also remove sediment and mud at the pile bottom. As shown in the figure, inject clean high-pressure water into Hole No. 1, while closing Hole No. 2 and opening Pipe No. 3, until the water flowing out becomes clear; then open Hole No. 2 and close Hole No. 3, and repeat the above operation. Then inject the high-pressure water head into Hole No. 2 and Hole No. 3 respectively, and repeat the above cycle of operations.

(4) Grouting. After hole cleaning is completed, prepare the slurry according to the slurry mix proportion. During the slurry preparation process, carefully weigh the amount of water and admixture, stir evenly, and prepare test specimens. When all indicators of the slurry meet the requirements, inject cement slurry into Hole No. 1 while opening Hole No. 2 and Hole No. 3, and start grouting under pressure. Through grouting, the clean water in the other pipes is displaced until slurry flows out from the pipes, and drain off 100 L of slurry. Then inject cement slurry into Hole No. 2 while first opening Hole No. 1 and Hole No. 3, waiting until uniform slurry is discharged with a composition basically consistent with the prepared slurry. Finally, inject cement slurry into Hole No. 3 and repeat the above operation. Connect the grouting interface to Hole No. 1 while closing Hole No. 2 and Hole No. 3, and when the grouting pressure reaches 50 Pa, maintain it for 5 minutes. Then connect the grouting interface to Hole No. 2 while closing Hole No. 3 and Hole No. 1, and when the grouting pressure reaches 50 Pa, maintain it for 5 minutes. Finally, connect the grouting interface to Hole No. 3 while closing Hole No. 2 and Hole No. 1, and when the grouting pressure reaches 50 Pa, maintain it for another 5 minutes. Close all valves, remove the grouting equipment and clean it.

(5) Control Grouting Parameters and Improve Grouting Effect. During the grouting process, grouting pressure, slurry consistency, injection rate and grouting volume all change during the grouting process, so reasonably controlling their changes is very important for improving the grouting effect. Grouting pressure changes from low to high with the increase of grouting volume. If the grouting pressure suddenly drops sharply during grouting, it indicates that local slurry leakage or air leakage has occurred. Check the grouting pipeline in time; it may be that the grouting pipeline is damaged, or cracks may have appeared around the pile foundation. Admixtures should be added to the slurry in time, with intermittent grouting and multiple groutings to ensure quality.

(6) Principle of Pile Foundation Grouting to Improve Bearing Capacity:

Improving the original geological conditions and increasing bearing capacity. During the hole-forming process of cast-in-place piles, machinery has a certain disturbing effect on the surrounding strata of the pile, weakening the bearing force of the entire pile foundation. During the concrete pouring process, because the mud cake on the hole wall weakens the friction of the entire pile foundation, it also affects the bearing force of the entire pile foundation. Pile bottom grouting, through permeation, fracturing and compaction, causes the bearing stratum at the pile end to form a combination of slurry and soil within a certain range, thereby improving the physical properties of the load-bearing structure and repairing and enhancing the strength of the load-bearing soil body. Before concrete pouring for cast-in-place piles, because hole cleaning is not clean, the presence of sediment at the pile bottom directly affects and seriously reduces the exertion of bearing capacity at the pile bottom. Grouting can replace, compact and consolidate the sediment at the pile bottom, improving and eliminating the adverse effect of pile bottom sediment on bearing capacity. Sharply increasing the friction resistance of the pile foundation. Under the action of high-pressure grouting, the slurry can move upward from the pile end along the pile side through permeation, fracturing, filling, compaction and cementation. It replaces the mud cake around the pile and the protective wall and fills voids, forming vein-like consolidated bodies around the pile, greatly increasing the side friction resistance of the pile foundation.

Improving the stress state and load transfer performance. Pile foundation grouting forms an enlarged pile end through permeation, fracturing, compaction and cementation, increasing the load-bearing area at the pile end, and grouting reinforces the bearing stratum of the pile foundation and improves the stress state. After pile bottom grouting, the increase in side friction resistance of the pile foundation occurs earlier than the increase in pile end bearing capacity. In friction piles, the effect is the best.

Reinforcement Benefit Analysis. By comparing the ultrasonic testing data before and after grouting treatment of defective piles, it is found that the concrete at the pile bottom is significantly improved, and the benefit is considerable.‍