Measurement and analysis of the internal displacement and spatial effect due to tunnel excavation in hard rock

Abstract

It is very difficult to observe the deformation of the surrounding rock within a very short time after excavation in hard rock such as granite. This paper reports an in situ monitoring test of the internal displacement of the surrounding rock conducted using multi-point extensometers that were pre-embedded before excavation in the Beishan Exploration Tunnel (BET) in China's Gansu Province. The internal deformation characteristics of the surrounding rock throughout the whole tunnel excavation process were analyzed, and an analysis of the spatial effect of the tunnel excavation face and an estimation of the loss displacement were carried out with 3 typical longitudinal deformation profile (LDP) curve equations: the Lee equation, the Hoek equation and the Vlachopoulos and Diederichs (V-D) equation. The results show that under the geological conditions of Beishan granite, the internal displacement of the surrounding rock throughout the whole process of blasting excavation passes through 3 stages, including initial growth, rapid growth and stable convergence. The temporal curve of the internal displacement of the surrounding rock near a measuring section shows stepwise characteristics, which are similar to the morphological characteristics of a creep test curve for rock subjected to graded loading. Measuring points farther from the side wall of the tunnel are less affected by the spatial effect of the tunnel excavation face and experience a larger displacement release ahead of the tunnel face relative to the total displacement. The loss displacement calculated with the Hoek equation is closest to the measured value; thus, the Hoek equation is the optimal estimation equation. When the measuring section is located 2 m from the tunnel face, the loss displacement accounts for more than 50% of the total displacement.