Numerical Analysis On Deformation And Internal Force Of Underground New Tubular Roof Structure During Excavation

In recent years, the underground structures with deep height and large cross-section were used by designers and became a trend with the deep development of urban underground space for the effective use of underground space resources. For the construction of large underground space, the use of a single construction method may have technical or economic limitations. Therefore, based on the existing technology and technology expansion, further researches on new construction methods, by which large underground structures can be built, needed to be conducted. In response, this paper presents a new construction method for large section underground structures– New Tubular Roof (NTR) Method. The structures constructed by NTR method were usually named NTR structures. The NTR method originated in Korea and was firstly applied for construction of Metro Station in Shenyang City. However, the theoretical research on the NTR structure lagged behind the practice, and the theoretical study was relatively weak although the application of the NTR method was increasing. Therefore, a systematic study on the NTR structure and NTR method was need.The FLAC~3D commercial software was used to numerically analyze the strength and deformation of a selected NTR structure during the total construction process by the nonlinear finite element method (FEM). The main contents and method were included as following.(1) Deformation analysis on the NTR structure and soil during the excavation processDeformation of NTR structure may generate during construction process. In general, the NTR structure had relatively larger deformation during the first excavation than that during the later. Different displacement appeared in the different sites of the structure. There existed vertical deformations towards the inner direction of the arch at both the top and the bottom of the structure, and there were also horizontal displacements in the middle part of the arch. Therefore, the first excavation had a great effect on deformation of structure which was the key points to monitor. The vertical displacement of the middle plate is large because of relatively large span of the middle plate, and the vertical support of the middle plate should be made, reducing the vertical displacement of the middle plate in the excavation of the soil below the middle plate.The development of the plastic area of the nearby soil had certain rule for NTR structure during the excavation. After the excavation, starting points of the two plastic bands of soil surrounding the structure appeared in the soil on the shoulders of the structure because of the sinkage of the soil above the arch head, extending to ground along 45 degree angle with the horizontal plane. A large volume of plastic zone occurred inside the arch and there was also a small number part of plastic zones below the arch. The plastic bands had no further development, and the plastic zone inside the arch was gradually gouged out. The plastic zones below the arch finally became larger than that at the beginning.(2) Inner force distribution analysis on the NTR structure during the evacuation processThe inner force distribution and its transfer path for the NTR structure were kept changing during the excavation process. The location of the maximum principal stress in the ring beams moved downward from the top to the bottom, and appeared at the following three points: the section of the structure near the bottom plane of the first step excavation, the location of the middle plate and the position slightly below the intersection. The force in the ring beam above the middle plate had no obvious change during the excavation the low part of soil. The spatial stiffness was great and the distribution of the forces trended to uniformity because the middle plate and the ring beams became a whole structure after the middle plate was made.The numerical simulation showed that the distribution of the inner forces and deformation of the NTR structure had certain rule during the excavation process. The developed analysis method and the numerical results can be used as a reference for the design and construction of an NTR structure.