Abstract: The rock bolt foundation is a special type of wind turbine foundation. It shows a good pull-out performance, and can well demonstrate mechanical properties of rock mass in its original state. Recent studies indicate that the modified Mohr-Coulomb (M-C) failure criterion with a small but not zero tension cut-off is more applicable for describing failure characteristics of rock masses. According to the modified M-C failure criterion, an expression is derived with the energy dissipation rate per unit area. Furthermore, the planar translational failure mechanism of rock wedge is put forward, which is based on the upper bound theorem for the limit analysis of soil plasticity mechanics. A virtual power equation is deduced, when the work rate of external loads to self-weight of rock wedge equals to the rate of energy dissipation along the sliding surface of rock wedge. Then a formula is developed for calculating ultimate pull-out capacity of rock bolt. The prototype test results show that the calculated values by this formula are in good agreement with the measured values, which indicates the formula with certain reliability can meet the requirements of engineering application.

Key words: a small but not zero tension cut-off, Mohr-Coulomb failure criterion, limit analysis, rock bolt, ultimate pull-out capacity