Back-Calculation Method of Rock Mass Pressure in a Shallow-Buried Super Large-Span Tunnel Using Upper-Bench CD Method

Rock mass pressure has always been a research hotspot in the field of tunnel engineering, especially in the super large-span tunnel, which is characterized by flat section, large excavation span, and complex stress field. Based on the Letuan Tunnel (a bi-directional tunnel with eight traffic lanes) of Binlai expressway expansion project in Shandong Province, China, this paper focused on the calculation method of rock mass pressure and the evolution law of load release in the construction process of the super large-span tunnel excavated by upper-bench central diaphragm (CD) method. Based on field measured data of Letuan Tunnel, the deformation behavior of primary lining and the distribution state of rock mass pressure during the tunnel construction were analyzed. According to the bearing mode of supporting structure, the mechanical models of different construction stages were established. Then, the rock mass pressures in different construction stages were back-calculated using mechanical models and compared with the measured values, and the evolution law of load release during the tunnel construction was discussed. The study results show that the tunnel deformation and rock mass pressure were significantly affected by the construction process and support form, and the excavation span was the key factor affecting the stability of rock mass. For the shallow-buried super large-span tunnel constructed by upper-bench CD method, the primary support of upper bench was under eccentric pressure. The comparison between the back-calculated value and the field measured value indicated that they were similar, and the average relative error was 17.23%. According to the concept of load release coefficient proposed in this paper, the load release coefficient after the pilot tunnel ahead (Part I) excavation reached 63%, and the load release coefficient after the pilot tunnel behind excavation (Part II) was 37%, which means that the rock mass pressure of Part I is increased about 59% due to the excavation of Part II.