Several Quality Problems About Tunnel Support of Self-drilling Anchor Bolt System

In the current tunnel support system, rock bolts, as a very effective support method, have been widely used in recent years. The anchor rod not only maintains the stability of the surrounding rock during the construction process, but also plays an important role as a part of the permanent support structure after the construction is completed. Therefore, it is very important to ensure the construction quality of the anchor bolt. However, from the current domestic construction situation, the construction of hollow bolts has the following quality problems:

1. The water-cement ratio is incorrect, and the amount of cement is insufficient. Field testers and supervisors should always measure the consistency of the cement slurry to ensure that the consistency is controlled within the design range before grouting, otherwise the reason must be found out. The general reason is that there is less cement, or the cement is agglomerated, and the mixing is uneven.

2. The angle of the anchor rod is horizontal. At present, most of the bolts are built using excavation drilling trolleys, and pneumatic rock drills (air guns) are used to form holes. However, the upper and lower heights of the trolley cannot be adjusted. The general rock drill is too long and not upright. It often hits the top platform of the trolley. In this way, the bolt above the spandrel is easy to be placed at a horizontal angle, and it cannot be set into and open. The digging contour line is vertical, which makes the phenomenon of leaking the top of the vault very common.

3. The positioning deviation of the anchor rod is large, and the depth of the hole is insufficient. Some construction workers are eager to save trouble and fail to manually lay out, mark, and fix the line according to the regulations. Instead, the workers drill the holes intuitively, resulting in a large deviation of the hole position. The lack of depth of holes is caused by unclear technical disclosure or laziness of workers. The site should be randomly inspected, and the frequency should not be less than 10% of the number of bolts.

4. The number of anchor rods is insufficient, and most of them are deliberately cutting corners. It should be counted once before the bolts are grouted, and after all the grouting is completed. Finally, it should be counted again before the initial spray. On-site managers must be familiar with the design drawings and the number of bolts to be set in each cycle. The question of whether the number of bolts is sufficient is entirely a matter of responsibility of the on-site management personnel, because there are many opportunities and sufficient time to count the number of bolts.

5. No grouting, false grouting or insufficient grouting. No grouting or false grouting is a serious act of deliberately cutting corners. It is mainly manifested in intentionally plugging the bolt grouting port with anchoring agent, resulting in the phenomenon of false grouting. When checking, check whether there is overflowing slurry around the slurry stop plug at the orifice, or you can judge whether it has been grouted by tapping the anchor rod and listening to the sound. The main reason that the grouting is not full is that the stopper is not tightly plugged, resulting in too large pores and insufficient pressure, or after the grouting is terminated, the bolt mouth is not blocked in time, causing the cement slurry to flow back. On-site managers must monitor the whole process during grouting.

The above quality problems seriously affect the safety of the project and must be eliminated. The constructors must carry out the construction in strict accordance with the standardized operation procedures, and the site management personnel must also do a good job in supervision and inspection. Only in this way can the construction of anchor rods truly become an effective form of support.

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Created on Apr 8th 2022 21:28. Viewed 173 times.

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