Classification of the loosening zones and estimation of the loosening pressures of tunnels in layered jointed rock strata.

Abstract

Due to the complexity of tunnels, accurate prediction of their loosening pressures in layered jointed rock strata is a very difficult engineering task. To recognize loosening patterns and estimate loosening pressures, numerical tests were employed in areas where tunnels were excavated in layered jointed rock strata. A total of 12 influential factors, including joints, tunnel depth, and strata, were considered in each of the numerical models. Three loosening patterns were found in the numerical testing: a ringent trumpet-shaped boundary, an arch-shaped boundary, and a closing-trend trumpet-shaped loosening zone. Empirical expressions for the loosening zone boundaries were further established and, in the form of the 12 influential factors, considered in the simulation. Given the boundary function, the loosening pressures were further deduced, which were categorized according to loosening pattern, i.e., ringent trumpet shape or arch shape, and the excavation condition of whether or not the embedded depth was deeper than the soft layer. Two case studies were used to test this method. The newly-proposed method was found to perform better than existing methods, with loosening pressure values that were slightly larger than, but very close to, actual measured field data.