At present, the commonly used technical methods to improve the vertical bearing capacity of single pile of grouted pile are bottom expansion method and post-pressure method of bored grouted pile.
1. Bottom-expanding drill bit bottom-expanding method
In the same stratum, the pile end resistance is directly proportional to the pile bottom area. Under the premise that the pile diameter remains unchanged, increasing the bottom area of the pile end can substantially increase the end-bearing force of the pile. Therefore, using bottom-expanding drill bit to enlarge the bottom size of pile hole to form bottom-expanding pile with small top and big bottom is an effective technical method to improve the vertical bearing capacity and pullout resistance of single pile.
In Japan, most of the rotary drilling rigs are equipped with tubing tray under the power head related to the bottom-expanding drill bit, and the use of bottom-expanding piles can save 50% of concrete, so 95% of the pile holes in Japan are bottom-expanding piles.
2. Post-pressure method
This method enables the hidden problem of slag at the bottom of the pile to be eradicated, the pile body to be reinforced, the soil around the pile to be compacted, and the bond between the pile and the soil around the pile to be improved, thus increasing the single pile bearing capacity, which can generally be increased by 30-90%.
The effect of post-pressing process is that the pressed cement slurry is combined with the pile bottom slag to form concrete, which eliminates the influence of pile bottom slag.
The pressed cement slurry fills the "false tip" and "dry slag stone" formed due to segregation and improves the concrete strength of the pile tip.
After the pile bottom pressure slurry makes the soil at the bottom of the pile get compacted, and the cement slurry penetrates into the soil at the bottom of the pile to form a solid body, which improves the strength of the soil at the bottom of the pile, thus increasing the pile end bearing capacity of the bored pile.
High-pressure slurry injection can form upward prestressing force at the bottom of the pile, which is beneficial to the end-bearing pile.
The high-pressure pressurized slurry on the pile side has an extrusion compacting effect on the soil around the pile, and penetrates into the soil around the pile, forming bamboo-like slurry bubbles, which has a bamboo pile effect and improves the pile side frictional resistance.
After the pile hole is drilled and the reinforcement cage is placed, the slurry pipeline is pre-set, the pile concrete is filled and the slurry is pressed after solidification. Before grouting, carry out clear water injection test to confirm that the pipeline is smooth before grouting. Pile side first, then pile bottom, group filling first, then group grouting.
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