Application and efficacy of excavation compensation method in shallow, large-span hard rock tunnel

Abstract

The construction of shallow, large-span tunnels has become a standard practice in urban development, presenting considerable engineering challenges and construction risks in conditions of hard rock. In order to address the limitations of existing support theories and construction designs, this paper introduces a support method based on the Excavation Compensation Method (ECM). The study develops a theoretical model of excavation compensation suitable for hard rock tunnels. This is achieved by analysing the effects of excavation and large span sizes, and by establishing trends in the variation of Mohr’s circle during the unloading compensation process. The study emphasises the pivotal role of 2G-NPR bolts, selected for their elevated tensile and shear strength and elongation, as a fundamental component of the compensation support system. Furthermore, the utilisation of a multi-source monitoring system throughout the construction phase permitted the implementation of high-frequency monitoring, thereby facilitating the prompt identification of any instability signals. The data obtained from the field monitoring demonstrate that the application of the 3D-ECM significantly reduces the deformation of the surrounding rock. In particular, there was a 27.6% reduction in crown settlement and a 69.7% reduction in surface subsidence, thereby confirming the effectiveness of this approach. The findings offer a theoretical basis for the construction of shallow, large-span hard rock tunnels and provide valuable insights into the optimisation of support systems.