A novel in-situ tunnel expansion method based on removable prestressed active support technology for rapid construction

Abstract

With the continuous increase in vehicle ownership and travel demand, the need for in-situ expansion of existing tunnels has grown annually. However, current expansion methods are predominantly confined to the traditional approach of ‘initial backfilling followed by sequential excavation with temporary supports.’ While these methods effectively control surrounding rock deformation, they suffer from complicated construction procedures, limited workspace, operational complexity, and high costs. To address this, this paper proposes a novel in-situ tunnel expansion method based on removable prestressed active support technology (hereinafter referred to as the novel method). The novel method employs removable prestressed anchor as the primary supporting components, with active pre-support as its core design principle. By replacing traditional ‘inner lining’ structures with an ‘outward pulling’ mechanism, it fundamentally overturns the traditional construction approach, eliminating the need for backfilling and temporary supports. Compared to current expansion methods, the novel method offers advantages including streamlined construction procedures, high efficiency, superior economic viability, and ample construction space. A case study on Tangling Tunnel validates this method through a removable scheme. Numerical simulations were conducted to compare this scheme with the original design. The key findings are as follows: (1) both schemes exhibit comparable performance in controlling surrounding rock deformation; (2) the removable scheme reduces the plastic zone volume above the invert arch and support structure stresses by over 20%; (3) it also reduces cost by 17,867 CNY per meter and shortens construction time by 10 hours per meter. Aligning with contemporary principles of green, low-carbon, and sustainable development, the novel method exhibits substantial potential for in-situ tunnel expansion projects.