Field tests on stress characteristics of support structures of unequal-span ten-lane four-arch tunnel during whole construction process

The excavation of multi-arch tunnels with large chambers and spans leads to frequent load transfers and complex stress characteristics on the support structures, and the field tests allow for a more accurate study on the stress distribution characteristics of this kind of support structures. Based on the Guanyinyan unequal span four-arch tunnel project in Changsha, the strain gauges are used to monitor the stress characteristics in typical sections during the whole construction process of the steel arches and secondary linings. The test results reveal that the main advance tunnel experiences greater pressure of the surrounding rock compared with the subsequent tunnels. The excavation of the right main tunnel and the left and right auxiliary tunnels occurs later, resulting in fewer disturbances and smaller internal forces of the support structures. The overall distribution of internal forces of the steel arches is asymmetrical, with higher forces at the right side. The maximum axial pressure of 402.71 kN and the maximum bending moment of 7.97 kN·m are observed at the right foot of the left main tunnel. The secondary linings exhibit low asymmetry, only with the left main tunnel showing significant asymmetrical distribution. The internal force disturbances in the support structures of the left main tunnel are primarily influenced by the excavation of the subsequent adjacent tunnels. During the excavation and support stages of the subsequent right main tunnel and left auxiliary tunnel, most positions in the left main tunnel experiences the internal force increment exceeding half of the total increment. In summary, the construction plan of "first main and then auxiliary, left main first" is found to be reasonable, and the support structures meet the specification requirements. The research findings can provide references for the design and construction of multi-arch tunnels under complex conditions.