Performance of large cross-section tunnel constructed in loose ground by optimal multi-step excavation method

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Although an appropriate tunnel construction method can be conducive to safeguarding the deformation of surrounding rock mass, too much emphasis on controlling deformation leads to ignoring the importance of accelerating tunnelling speed and saving construction costs. Therefore, this paper presents a historical case of large cross-section tunnel in loose ground, which was excavated by optimal multi-step excavation method. First, to investigate the influence on rock mass deformation by the centre diaphragm (CD) method and optimal tunnelling method, comparative studies were performed by numerical simulation of the hypothetical construction schemes. Then, field monitoring was carried out to further reveal rock mass deformation and structural load characteristics with optimal excavation method. Numerical simulation results show that, compared with CD method, the optimal construction method is more helpful in controlling tunnel invert uplift but has minor disadvantages of increasing horizontal displacement of surrounding rock mass. However, tunnel crown settlement excavated by CD method is approximately similar to that of optimal construction method. Field test results confirm the effectiveness of optimal tunnelling method on controlling excavation-induced tunnel deformation; the maximum vault settlement and horizontal convergence of drift are approximately 57.5 mm and 8.5 mm, respectively. Moreover, the maximum inside and outside forces of steel set are 172.5 MPa and 151.5 MPa. The rock bolt is subjected mainly to a tensile force with a maximum value of up to 29.3 MPa. Therefore, as the crucial part of initial support structure, steel sets and rock bolts are under appropriate conditions and do not exceed their bearing capacity.