Analysis method and design charts for bolt reinforcement of the tunnel face in cohesive-frictional soils

• We analyze the stability of a tunnel face reinforced by bolts.
• We propose a computational method based upon the so-called “method of slices”.
• The method can consider heterogeneous layered ground and different bolt layouts.
• We show the effects of length, distribution and installation sequence of the bolts.
• We report design diagrams for homogeneous grounds with uniformly distributed bolts.

We propose a computational method for assessing bolt reinforcement of the tunnel face in cohesive-frictional soils. The method is based on limit equilibrium considerations and can be applied to heterogeneous, layered ground as well as to reinforcement layouts that are arbitrary in terms of the spacing, length, longitudinal overlapping and installation sequence of the bolts. A closed-form solution is presented for the special case of a homogeneous ground with uniform face reinforcement. An investigation is made into the influence of the unsupported span and the reinforcement scheme on face stability. The case of a stress-dependent bond strength is analysed and the results are compared with results of numerical analyses. Design nomograms are presented for the assessment of tunnel face stability in a homogeneous ground with uniformly distributed bolts, constant or variable bond strength and various installation sequences.