With the development of mine supporting technology, anchor bolt support has became a more popular and easier method. But there are also some problems during practical process, which causes the ineffective factors of anchor bolt, such as: the length of anchor bolt is not enough, the anchoring length is not enough, etc.
Firstly, we should understand the supporting principle of anchor bolt: turning the uni-axial or bi-axial compression of rock mass’s stress state into triaxial compression through axial force of anchor bolt, which can enhance its strength of compression and bear certain loads from interior or exterior, then reach the goal of controlling surrounding rocks’ deformation efficiently.
There are several factors influencing the ineffective of rock anchor:
1. The parameter design of anchor bar is unreasonable.
The parameter design of anchor rod includes bolt’s type, geometrical parameter, mechanical parameter, density of rock bolt, angle of installation, diameter and depth of drilling hole, etc.
If the design of geometrical parameter and mechanical parameter is unreasonable, in most cases, it means that mechanical property is small. In certain supporting conditions, if parameter and anchoring force can’t reach project’s demand, it will finally cause the ineffective of anchor bar.
The stratum’s stability and integrity are different, so supporting density has distinction. During the construction, anchoring force and supporting radius are different, it demands appropriate supporting density. If hollow bar’s density is too big, it will not enhance supporting effect and will cause damage for roof.
In conclusion, the unreasonable anchor bar design can’t ensure supporting effect. So the anchor bolt’s designed parameters should be corrected according to actual conditions in time.
2. The quality problems of hollow rock anchor.
During the actual construction, if hollow bar anchor’s quality has problems, it could not bear the force of surrounding rocks and cause ineffectiveness, which result in collapse. So projects must choose the high-quality anchor system.
Above are main factors that affect the quality if self drilling anchor bolt. To reach the purpose of efficient support for surrounding rocks, the geological structure and outer factors should be considered when designing. The interaction theory and stress-transferring rule have many uncertainties, so the method of empiric and semi-empiric are always adopted to make anchoring support perfect and realize the ultimate goal of security.