A New Approach for Quantitative Definition of Asymmetrical Loading Tunnels

Asymmetric loads often occur at tunnel entrances or exits during shallow buried tunnel construction. Due to asymmetrical loads, structures are subjected to complex forces, and the design process of these tunnels is different from that of regular tunnels. Therefore, it is necessary to discriminate asymmetric tunnels quantitatively. Currently, determining whether a tunnel is subject to asymmetrical loads is based on the empirical maximum burial depths and surface slope angles listed in the Chinese Code for the Design of Railway Tunnel. However, in real geological conditions, the maximum burial depths and surface slope angles of the tunnel are not necessarily completely consistent with the Chinese Code; in other words, the continuous discrimination of asymmetric tunnels is not realized. In this paper, the surrounding rock stress ratio of the arch shoulder was treated as index to define asymmetrical loading tunnels and a new approach to achieve the continuous discrimination of asymmetric tunnels was proposed. Compared to the empirical values in the Chinese Code, the results show that the asymmetric stress ratio tends to decrease with the decrease in the surrounding rock grade. Regardless of whether the grade is III, IV Rock , IV Soil or V, the asymmetric stress ratios of the surrounding rock tend to converge and stabilize with increasing burial depths and decreasing dip angles, and the sensitivity of the slope angle to the stress ratio is bounded by 20°. The asymmetric stress ratio of the surrounding rock arch shoulder can be used as an index to ascertain whether a tunnel is subjected to asymmetric loads. When the surrounding rock grades are III and IV Rock , the values calculated by the proposed approach are lower than the empirical values, and the approach is more conservative than the empirical method. When the surrounding rock grades are IV Soil and V, the values calculated by the proposed approach are in good agreement with the empirical values. The approach of continuous discrimination of asymmetric tunnels proposed in this paper is reliable. The findings of this study can help for better understanding of the identification of asymmetrical loading tunnels, the design of support structures and the safety risk assessment of tunnels.