Field monitoring and analysis of the stress variation relationship between fiberglass bolts and face deformation in tunnels based on the ADECO-RS construction approach

One of the main causes for excessive deformation within a tunnel is due to the instability of the soil or soft rock ahead of the excavation face. Fiberglass bolts have been shown to be a useful advance reinforcement method for the excavation face. In this paper, an improved ADECO-RS (Analysis of controlled deformation in rock and soils) method have been proposed for soft rock mountain tunnels, in terms of the partial (mainly the upper bench) excavation face reinforcement and also for the bench excavation method. Strain gauges were used to test the micro-strain in the fiberglass bolt to investigate how the axial force of the fiberglass bolt varied during the tunnel excavation. In addition, combined with the field tunnel deformation monitoring data, the relationship between the reinforcement parameters of the fiberglass bolts and the tunnel construction phase has been discussed. The conclusions of this study are: (1) The redistribution of the stress after the excavation led to an increase in the stresses of the surrounding rock near the excavation face, and there was a significant stress increase in the fiberglass bolt; (2) The excavation during the lap area of the fiberglass bolt led to an increase in the axial force in the bolt, however, the excavation outside the lap area of the fiberglass bolt didn’t show any influence on the bolt; (3) The deformation of the tunnel met the construction specification by ensuring the stability of the excavation face and closing the loop of each initial support construction as soon as possible; (4) In a future project with similar conditions, the recommended lap length of the fiberglass bolt was 3 m utilizing the fiberglass bolt grouting face reinforcement method.