Experimental study on failure mode and mechanical characteristics of inclined and advanced rockbolts support system

To ensure the stability of the surrounding rock during mechanized excavation, the Chongqing-Kunming High Speed Railway has pioneered the use of pre-stressed inclined and advanced rockbolts (hereinafter referred to as “inclined rockbolts”). By adopting the “anchoring before supporting” method, it has postponed the installation of primary support, creating sufficient space for rock drilling jumbos to operate, thus minimizing over-excavation in the tunnel. This paper puts forward the concept of “anchoring before supporting” and innovatively proposes a new inclined rockbolt support system. The effectiveness of this support system has been demonstrated through field tests. Subsequently, based on the support stress field, a quantitative evaluation system for the rockbolt support effect is established, which quantitatively distinguishes the support effects of inclined rockbolts and radial rockbolts. Moreover, by utilizing tunnel horizontal loading tests, an equivalent stiffness test for simulating inclined rockbolts is proposed. This method explores the failure modes, deformation characteristics, and mechanical behaviors of different rockbolt support systems under various geo-stress fields, and the effectiveness of different support schemes are evaluated and compared. The research results indicate that, compared with radial rockbolts, the inclined rockbolt support system has a larger magnitude and wider range of the support stress field. This leads to improvements in the stress state of the surrounding rock, reduction in bending moment of the steel arch and tunnel deformation, and enhancement of the maximum bearing capacity and stability of the tunnel structure. In addition, the critical safety locations of each support scheme, along with the corresponding design and construction preventive measures, have been summarized. These research findings provide a reference for elucidating the load-bearing mechanism of the inclined rockbolt support system and its further application in tunnels and underground engineering.