As an economical and efficient reinforced method, rock bolt has been applied largely in civil construction, tunnel, hydroelectric station and coal mine, etc., which can save plenty of materials and cost in actual projects.
Rock bolt combines drilling, grouting and anchoring together. Drilling machine pushes rock bolt into stratum, injection pump helps slurry full of drill holes and have good anchoring effect. While there are several questions need to be noticed when rock bolt grouting.
1. The ultimate load of rock bolt when grouting.
The depth of drill hole should exceed rupture surface and reach enough length into stable stratum, that’s efficient anchorage section. The maximum tension of efficient anchorage section is called ultimate load.
When efficient anchorage section was forced, tension was delivered into slurry by steel rod’s bond stress, and then arrived into stratum by friction force. So rock bolt’s ultimate load should meet following conditions: ① The slurry’s ultimate load should bear steel rod’s bond stress. ② The ultimate load of anchorage section should bear slurry’s friction. ③ The soil of anchorage section can still keep stable in disadvantageous conditions.
2. The bond stress of anchorage section.
Generally speaking, the fiction force of hole’s wall is bigger than that of slurry ( the cement mortar force≧30MPa ), so rock bolt’s ultimate load usually depends on slurry’s bond stress. And bond stress between steel bar and common concrete is one tenth of mortar’s standard.
3. The shear strength of anchor bolt’s hole wall in anchorage section.
In soft rocks or soil, the fiction force of rock bolt’s hole wall is less than the bond stress of slurry, so the ultimate load of rock bolt depends on the biggest fiction force of anchorage section’s stratum.
Besides the properties of stratum, it also concerns with construction method and grouting quality. If technology is good, the friction force of rock bolt’s hole wall depends on the shear strength of surrounding soil.
When calculating the anchorage length, the shear strength of rock bolt’s hole wall plays a quite important role, but the calculated value is only assessment, which will be a reliable basis only after testing on the spot. Safety factor needs to be considered when the ultimate load is depending on rock bolt’s bearing force.
In conclusion, choosing anchor bolt with appropriate ultimate load should base on different geological conditions, which finally aims to support project and reach stable effect.