Deep excavation clearly impacts existing underlying subway tunnels and threatens their operational safety. The prediction of existing tunnel deformation induced by nearby excavation is a major concern for urban construction. This research presents an analytical calculation method for predicting tunnel deformation induced by upside excavation and also discusses the role of dewatering in the deformation mechanism. First, the existing tunnel is assumed to be nonexistent in the soil, and the vertical unloading stress at the location of the existing tunnel caused by the upper excavation and associated dewatering is calculated. Second, the existing underlying tunnel is simplified as an elastic beam on a Pasternak foundation to calculate its vertical deformation. The proposed method was verified by the good agreement found between the predictions and the field measurements of the construction in the Shenzhen Chegongmiao hub project. Such an analytical method can provide fast and accurate evaluation resul...

The excavation of a deep foundation pit adjacent to an existing tunnel may lead to the large deformation and induce damages in the tunnel structure. However, the influence on existing tunnel structure from nearby excavations has not been understood clearly, since it is affected by complex influencing factors of not only the geological and topographical conditions but also the construction method and positional relationship of the adjacent structures. This paper presents a numerical investigation into an existing underground rail transit line during the excavation of an adjacent deep foundation pit, in which the behavior of the existing tunnel structure from excavating the aforementioned foundation pit is clarified, and the effectiveness of the adopted three-dimensional model is confirmed by comparison between the numerically calculated and field-measured ground settlement of the monitoring point. The results demonstrate that the deformation of the existing tunnel structure is mostly induced by the excavation of the deep foundation pit. This study can provide a reference of deep excavations adjacent to existing infrastructures.

In this study, the stability of an elliptical tunnel in cohesionless soils was determined by an upper-bound theorem in combination with triangular rigid translatory moving elements. The elliptical tunnel had a height D under a depth of cover C and a span B. The lining, used to support the tunnel, was equivalent to applying uniform internal compressive normal pressure on its periphery. In the proposed method, the nodal coordinates and velocities of rigid elements are treated as unknowns, without considering the rotating freedom. Upper bounds on the internal tunnel pressure were proposed using nonlinear programming, and the optimal geometry of the collapse mechanism was determined by removing nonactive velocity discontinuities where the two adjacent elements had no relative movement or zero velocities. The variation of the stability numbers (N_r) with dimensionless spans (B/D) is presented for various combinations of dimensionless depths (C/D) and internal friction angles (ϕ). The stability number increased obviously with C/D in cases where ϕ ≤ 25°, and it decreased with ϕ. Compared to the case where B/D = 0.5, N_r was found to increase approximately in a range of (1) 6.8–27.6% for ϕ = 10°, and (2) 94.1–134.9% for ϕ = 25° with B/D = 2. The collapse mechanisms of elliptical tunnels comprising two groups of slip lines are also presented. The results show that ϕ has a significant effect on the collapse mode, and the collapse zone was sensitive to ϕ and C/D. To verify the solutions, the computed stability numbers are compared with those reported in literature.

The dynamic compression properties of transversely isotropic rocks and their dependence on the confining pressure and bedding directivity are important in deep underground engineering activities. In this study, a slate is characterized using a split Hopkinson pressure bar (SHPB) test. Five groups of samples with preferred bedding directions (dip angles of 0°, 30°, 45°, 60°, and 90°) are subjected to coupled axial impact loading (low, medium, and high) under confining pressure (0, 5, and 10 MPa). The failure mode, dynamic strength, and Young’s modulus are investigated. The test results show that the tensile splitting effect is significant when there is no confining pressure. However, under a confining pressure (5 and 10 MPa) condition, the cracks that develop along the loading direction can be significantly constrained and the samples are forced to fail along the bedding plane. With increasing confining pressure, the critical dynamic strength significantly increases, and Young’s modulus increases when θ ≥ 45^° while it decreases when θ ≤ 30^°.

The blast vibration attenuation will change as the tunnel cut hole is blasted to create a new free surface. To guarantee safe tunnel construction and optimize blast design parameters, it is crucial to understand the impact of the free surface on blast vibration attenuation. Previous studies have often only qualitatively evaluated the effect of the free surface on blast vibration, failing to quantify these impacts on vibration attenuation. In this paper, by analyzing the decay law of the blast vibration velocity under different free surfaces, we quantitatively assessed the effect of the number of free faces and burden distance on the peak vibration velocity. Then, we introduced free-face parameters to enhance the current formula for predicting blast vibration velocity. The results show that the peak vibration velocity decreases with the increasing free surface number and increases with increasing burden distance; the free surface factor cannot be ignored in the decay of vibration velocity and contributes about 21% more to the total decay factor than the scale distance factor. The average tolerance rises from 24.79% in the Sadovsky formula to 13.32% in the correction formula, and the correction formula more precisely predicts the peak vibration velocity. The accuracy of the correction formula provides a good scientific basis for the design of tunnel blasting parameters. The research results were successfully applied to the Jiujiawan proximity tunnel on the Duyun-Anshun highway in Guizhou, effectively guaranteeing blasting safety with a minimum clear distance of 17.2 m.