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 bonding stress, then arrived into anchoring stratum through friction force. So, rock bolt’s ultimate load should meet following conditions:
① The slurry’s ultimate load should bear steel rod’s bonding 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.
Generally speaking, the fiction force of hole’s wall is bigger than that of slurry (the cement mortar force≧30MPa), so self drilling rock bolt’s ultimate load and minimum anchorage length usually depend on slurry’s bonding stress.