Mechanical characteristics and mechanical calculation model of tunnel feet-lock bolt in weak surrounding rock

The feet-lock bolt is widely used in weak surrounding rock tunnel. In order to study the mechanical characteristics, based on the Baojiashan extra-long highway tunnel in Baomao line Xikang highway, the field test method was used to test the bolt axial stress at the typical section. The results show that the stress of arch department bolts is small, the maximum stress is only 11.5% of the bolt’s ultimate stress and the effect is not big; the stress of feet-lock bolts is larger, the maximum stress is 55.4% of the bolt’s ultimate stress and the effect is very big; and the anchorage effect cannot be used very much at the feet-lock bolts as a result of the length and the angle limitations. Based on the analysis of its mechanical characteristic, the mechanical calculation model of the steel frame and feet-lock bolt is established using structural mechanical and elastic foundation beam methods. And the model is used to analyze the stress of feet-lock bolts at the tunnel test section, and the bending moment distribution of feet-lock bolt is given. The strain value of bolt is calculated from the bending moment and compared with measured values. The results show that the calculated values are consistent with the measured ones, that the strain of bolt is the largest at the closest point to the orifice and that the strain decreases gradually with the increasing distance between orifice and anchor body.