Rock bolt is a main supporting method in geotechnical project, which combines functions of drilling, grouting and anchoring together. It shows that there are three stages during the loading process of rock bolt:
1. At the beginning of rock bolt’s installation, grouting is finished, while the bonding force between grouting materials and soil layer is not formed. At this time, soil nail bears a little force.
2. Grouting materials bond rock bolt to stratum. Rock bolt produces tension gradually with the increase of excavation depth and then it concentrates tension on the position of bonding stratum. Closing to the stratum, the tension is maximum and it decreases gradually.
3. When rock bolt digging enough depth, most parts of rock bolt are in slip crack range. Internal force has decreased from middle part to both sides.
When rock bolt has got ultimate load, there are grip force of rock bolt body, friction force between soil and rock bolt and shearing strength, etc.
The grip force and shearing strength are smaller in soil than rock conditions. When rock bolt is in soil condition, it will be damaged by shearing strength, while in rock condition, it will be damaged by grip force.
Also, the friction force needs to be considered when confirming bearing capacity in soil conditions. It demands that the standard compressive strength of mortar stone body should not less than 20MPa in soil condition and 30MPa in rock condition.