Abstract
Engineering practices indicate that narrow braced excavation exhibits a clear size effect. However, the slip circle method in the design codes fails to consider the effect of excavation width on basal heave stability, causing waste for narrow excavation. In this paper, numerical simulation for basal heave failure of excavation with different widths was performed by FEM with SSRT (shear strength reduction technique). The results revealed that the failure mechanism of narrow excavation is different from the complete slip circle mode. In addition, the safety factor decreases increasingly slowly as the excavation widens and stabilizes when approaching the critical width. Subsequently, the corresponding computation model was presented, and an improved SCM (slip circle method) was further developed. Finally, the engineering case illustrated that it can effectively optimize the design, which exhibits clear superiority.
Highlights
In recent years, the construction of urban underground space in China has rapidly developed, and the proportion of narrow excavations, such as underground integrated pipe galleries, has been increasing sharply. is type of excavation shows an obvious size effect, with smaller deformation and better stability [1,2,3,4,5,6,7]
In essence, when the pressure difference between the inside and outside of the excavation exceeds the ultimate bearing capacity, the excavation will be unstable. erefore, the damage can be attributed to excessive load or insufficient shear strength. e finite element method (FEM) with shear strength reduction technique (SSRT) exactly simulates the failure process by reducing the strength parameters of the soil. e basic principle is that after c and tanφ are divided by the same reduction coefficient Fr [34], they are resubstituted into the numerical model as a new set of strength parameters for calculation to determine whether the excavation has reached the ultimate failure state, and the process is repeated until the excavation is in the limit equilibrium
factors of safety (Fs) determined by FEM with SSRT is the ratio of the original shear strength to the reduced shear strength when large nodal deformation occurs
Summary
The construction of urban underground space in China has rapidly developed, and the proportion of narrow excavations, such as underground integrated pipe galleries, has been increasing sharply. is type of excavation shows an obvious size effect, with smaller deformation and better stability [1,2,3,4,5,6,7]. Because of the inherent variability of soil parameters (such as undrained shear strength and unit weight), which may lead to a slight difference between the actual circumstances and the theoretical assumptions, failure may still occur [28,29], even though the factors of safety (Fs) is greater than 1 or the minimum value specified in the design codes, such as undrained shear strength and unit weight [29] To this end, probabilistic approaches have been developed based on the limit equilibrium method and limit analysis method.
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