If there is a defect in the bottom of the pile foundation, the stress can be simply calculated according to the stress nature of the pile foundation, and then whether it needs to be treated. If the pile foundation of the pile foundation construction is a friction bearing pile, the length of the defect position will not affect the overall bearing capacity, that is to say, the defect is not included in this part of the pile length, and the friction force can meet the load requirements, so no further processing is required, otherwise it must be processed. If the pile foundation is rock-socketed and the bottom is defective, it must be treated. Pile bottom grouting for pile foundation construction is a feasible treatment method.

(1) Preparation for pile bottom grouting in pile foundation construction

Reasonable grouting design is the premise of grouting. In order to make the pile bottom grouting construction reasonable and effective, it is necessary to conduct a detailed investigation and analysis of the rock geological conditions, groundwater conditions, defect degrees and the surrounding environment of the pile foundation, calculate the amount of cement slurry, scientifically prepare the grouting slurry of high-grade pile concrete, and determine parameters such as grouting pressure.

(2) Drilling

Three grouting holes shall be drilled in the pile shaft of the defective pile. During the drilling process, verticality must be ensured to ensure that the drilling is parallel to the axis of the pile foundation to prevent the drilling from deviating, contacting the reinforcement of the pile foundation or penetrating the pile foundation. The drilling depth is 1m longer than the pile.

(3) Hydrostatic test

The water pressure test can not only dredge the grouting pipeline, but also adjust the operation experience parameters of the grouting according to the results of the water pressure test. Hydrostatic testing can also remove sediment and mud from the bottom of the pile.

(4) Control grouting parameters and improve grouting effect

In the grouting process, the grouting pressure, slurry consistency, injection rate and grouting volume will change during the grouting process, so it is very important to control their changes reasonably to improve the grouting effect. As the amount of grouting increases, the grouting pressure changes from low to high. If the grouting pressure drops suddenly and sharply during the grouting process, it indicates that there is local slurry leakage or air leakage. Check the grouting pipeline in time, it may be damaged, or there are cracks around the pile foundation. In order to ensure the quality, admixture shall be added in time to grout for intermittent grouting and multiple grouting.

(5) Principle of pile foundation grouting to improve bearing capacity

Improve the original geological conditions, improve the bearing capacity of the cast-in-place pile hole in the process, the machine has a certain disturbance effect on the formation around the pile, reducing the bearing capacity of the whole pile foundation. In the process of concrete pouring, the friction force of mud on the whole pile foundation is weakened, which also affects the bearing capacity of the whole pile foundation. Pile bottom grouting can form a combination of slurry and soil in the pile end bearing layer through penetration, splitting, compaction and other effects within a certain range, thereby improving the physical properties of the bearing structure, repairing and increasing the strength of the bearing soil.

Because the hole cleaning of the cast-in-place pile before pouring concrete is not clean, the existence of sediment at the bottom of the pile directly affects and seriously reduces the bearing capacity of the pile bottom. Through grouting, the sediment at the bottom of the pile can be replaced, compacted and consolidated, and the adverse effects of sediment at the bottom of the pile on the bearing capacity can be improved and eliminated.

Dramously increase the frictional resistance of the pile foundation. Under the action of high-pressure grouting, the slurry can penetrate, split, fill, compact and grout from the pile end along the pile side. The mud skin and retaining wall around the pile are replaced by mud skin, and the gap is filled to form gangue around the pile, which greatly improves the side friction resistance of the pile foundation.

The stress state and load transfer performance are improved. The enlarged head of the pile end is formed by the pile foundation grouting through infiltration, splitting, compaction and cementation, which increases the force area of the pile end, and strengthens the bearing layer of the pile foundation through grouting, and improves the stress state. After the pile bottom is grouted, the increase of the side friction resistance of the pile foundation precedes the increase of the pile end bearing capacity. In the friction pile, the effect is very good.