Abstract:

To address the challenge of determining the optimal timing for initial support during layered excavation in large underground powerhouses, we present a concept of plastic volume ratio to quantify the volume of rock mass undergoing plastic deformation to the total volume of excavated rock mass. Based on the lateral deformation characteristics of the entire cross-section during tunnel excavation, we propose a method for determining the timing of initial support in layered excavation processes based on the plastic volume ratio. This method takes into account the displacement release rate, along with the characteristics of the transverse displacement release rate and the longitudinal convergence deformation curve(LDP) under different stress release rates. The research demonstrates that the evolution of rock mass deformation, as described by the plastic volume ratio, offers a more intuitive understanding. This method overcomes the challenges associated with identifying the critical state of displacement evolution and the difficulty in determining the timing of initial support for different layers during layered excavation, as encountered when using the displacement release rate method. By conducting two-dimensional and three-dimensional numerical calculations for excavations in the underground main powerhouse of a pumped-storage power station, we demonstrate consistent performance of the proposed method. It can serve as a valuable reference for the design and construction of layered excavation in large underground projects.

Key words: underground plant, timing of initial support, plastic volume rate, stratified excavation, displacement release rate