Since the type of hollow grouting bolt is a set of drilling, grouting, anchoring function in one of the new bolt, it can ensure high efficiency, safety, and reliability in the complex formation. The application of this drilling method in tunnel surrounding rock support is described in detail in this article.
Bolt anchoring support can give full play to the stability of the surrounding rock. It is widely used in road tunnel surrounding rock support.Permanent anchor bolts use ordinary mortar anchor mostly. But the grouting of ordinary mortar anchor is not sufficient, which leads to the actual effective anchorage zone cannot meet the design requirements.
In addition, it is hard to drill holes with the ordinary mortar injection bolt in the broken, loose rock condition. It results in failure of the bolt anchorage support.
Self-drilling hollow grouting anchor is a new set of drilling, grouting and anchoring functions into one. It can ensure the construction efficiently under the complex formation and reliable anchoring effect.
In the same construction condition, respectively install Ф25 ordinary mortar anchor, hollow grouting anchor and self-drilling anchor (hereinafter referred as IBO hollow bar anchor) in the length of 2m and 5m. Mortar strength grade M30, anchorage by 150mm*150mm*60mm plates. The test results were analyzed as follows.
As Table 1 shows, in the general crushing zone, 3 kinds of 2m anchor bolt tensile limit values are between 65-70 kN. The different from each other is not very big.
The pullout strength limit of 5m ordinary mortar anchor bolt is between 60-90 kN. The strength limit of two hollow IBO anchor types is between 140-150 kN. The increased pull resistance of Self-drilling anchor rods is not obvious.
In the severe crushing zone, the pullout strength limit of 2m ordinary mortar anchor bolt is between 45-70 kN. For hollow grouting anchor and self-drilling anchor, the limit is between 60-75 kN. A decrease is not obvious.
For mortar grouting anchor bolt in the length of 5m, the strength limit is between 55-70 kN. And IBO bolt can achieve 130 to 145 kN. The pull strength advantage is very obvious.
In general, as the integrity of the rock mass decreases, the pull-out limit of the bolt decreases, and the ordinary mortar anchor decreases with the increase of the rock mass, and the hollow grouting anchor is more serious The loss of drawing limit in the broken rock mass is small.
Table 1: Comparison of anchor bolt pull strength
|Anchor Bolt Type||2m||5m|
|General broken area||Severe broken area||General broken area||Severe broken area|
|Ordinary mortar anchor||65-75||45-70||60-90||55-70|
|Ordinary hollow bar anchor||65-75||65-75||140-150||130-145|
|IBO self-drilling anchor||65-75||65-75||140-150||130-145|
The grouting quality of the anchor is very important, which directly affects the effect of supporting the surrounding rock. Grouting pressure and grouting volume are two main indicators that reflect the effect of grouting.
The degree of crushing of the rock mass seriously affects the grouting pressure. In the general crushing zone, the grouting pressure of the hollow grouting anchor is slightly larger than that of ordinary mortar, and in the severe crushing zone, the grouting pressure of the hollow grouting anchor is obviously larger than that of the ordinary mortar.
The amount of grouting in the crushing zone is 4-6 times higher than that in the general crushing zone. The grouting amount of hollow grouting anchor is also significantly 1.5-2 times larger than that of ordinary mortar.
The construction efficiency for ordinary mortar anchor and hollow grouting anchor in the general crushing area is almost. Self-drilling can increase the efficiency of boring work by as mush as 20%.
Analysis from the cost per KN pull force, IBO self-drilling hollow grouting anchor is more competitive than the ordinary anchor bolt in the severe crushing of surrounding rock support.
Self-drilling anchor hollow bar is especially suitable for weak surrounding rock support. It combines drill pipe and anchor bolt together. The anchor bolts can be directly into the rock and soil, the hollow part can be used as a grouting channel, from the inside to the outside for grouting.
The grouting method is back bottom hole injection. Due to stop-grouting plugs are assembled to the self-drilling system, it can use pressure grouting.
With ISO standard thread, the anchor bolt body processing, jointing in length and assembling drill bits becomes easy and convenient. After the bolt is in place, the hole can be cleared, grouting, without the need to pull out the drill pipe and install the bolt
Continuous thread makes the IBO anchor bolt can be any cutting and connection. It achieves the purpose of strengthening the rock anchor bolt in the narrow space. At the same time, the continuous thread brings a stronger bond resistance than the smooth steel pipe.
The typical construction process for self-drilling anchor hollow bar (see Fig. 1).
Figure 1: Process flow for the installation of IBO self-drilling anchor system
According to the terrain and geological condition, adjust the bolt drilling angle of the drill rig. The anchor should be exposed at least 10cm at the end of drilling when it reaches the designed depth. Thus makes it easy to install arched and spherical nuts.
Start the mortar mixer, input the mixer into the cement and other admixture materials for water mixing. The grouting should be full and the grouting pressure should reach the design value to ensure the quality of the injection.
After the construction of the anchor bolting, due to blasting vibration or other factors, it may make the plate loose and anchorage failure. Therefore, it should be checked regularly. If the tray is found loose, should tighten the nut promptly.