A study on the influence of the location of fracture zones to the displacement of tunnel face

Abstract

When a fracture zone with directional characteristics and a certain range is anticipated in the front of the tunnel face, significant changes in vertical displacement occur through internal measurements. At this point, by utilizing the measured values, it is possible to predict the presence of a fracture zone ahead through influence lines, trend lines, and stereographic projection methods. This study proposes a method for predicting the presence of a fracture zone around the tunnel using these methods based on the measurements of displacements obtained from horizontal boreholes at the tunnel face during each excavation. Additionally, the research employs 3D numerical analysis using the Midas NX program to compare vertical and horizontal displacements at the tunnel face with measured data. This study aims to present a practical method for predicting the occurrence of a fracture zone in tunnel construction field. The distinctive feature of this study is the use of the total station for displacement measurements in the tunnel face by conducting horizontal drilling on-site. In previous studies, displacement was measured at the tunnel crown face using 3D displacement sensors or tape extensometers. However, in this study, horizontal holes were drilled through the center of the tunnel face in the direction of the tunnel excavation progress. moreover vertical and horizontal displacements were measured using sockets. Through this method, 3D displacement can be measured on-site using the total station, allowing the prediction of fracture zones in regions of anomaly ahead of the tunnel face from the electrical resistivity survey. This study was conducted at a tunnel construction site in the Chungcheong region, which was instrumental in providing valuable insights. Then, following an electrical resistivity survey, certain anomalies were detected, indicating the potential presence of fracture zones or cavities. Subsequently, a horizontal borehole of 60 meters (Ø100.0mm) was drilled; through the borehole, the coordinates (x, y, z) of the tunnel face were measured at each excavation stage to determine how much displacement occurred at the tunnel crown. The results showed that among homogeneous rocks, there was minimal displacement at the tunnel crown during excavation. However, when the fracture zone was located ahead of the tunnel face, the magnitude of displacement was significantly affected depending on the location and size of the fracture zone. Based on the measured data, a 3D numerical analysis was conducted, evidencing a similar trend to the measured data. From these results, it can be concluded that the displacement of the tunnel face varies in the direction from the fracture zone to the tunnel face, and this displacement is proportional to the size and location of the fracture zone. From this study, a method for predicting the size and location of a fracture zone ahead of the tunnel face during borehole tunnel excavation has been established, providing the ability to anticipate the location of the fracture zone in front of the tunnel face. Keywords: fracture zones, influence lines, trend lines, stereographic projection method, tunnel measurement, 3D numerical analysis