Abstract
To investigate the instability of two neighborhood tunnels with large crossing section during the construction, the Tushan subway station was taken as study background, which was built in steeply jointed rock mass. Based on the excavation method called traditional double side drift, numerical simulations of four different face excavation sequences in the two neighborhood tunnels were conducted to optimize construction sequence to improve the stability during tunneling. The results show that first excavation of the right tunnel produced less deformation of the tunnels due to joints dip. The effects of rock mass discontinuities on the stability of the tunnels were studied through comparison between the real condition with joints and the assumed condition without joints. Furthermore, six initial supporting systems with different parameters were compared, and the field observations of deformations along tunnel profile show good agreement with the numerical results. Based on the numerical simulation, the length of rock anchors could be designed asymmetrically, which is more economical than the symmetrical design. The optimized thickness of shot concrete and spacing of steel sets was 35 cm and 60 cm, respectively.
Highlights
Rock mass always contains some discontinuities such as joints which are considered as a softer and weaker region than surrounding intact rock [1]
Due to sliding or detaching along the discontinuity plane, many unstable accidents of underground openings were reported to be closely related to joints [2,3,4]. e joint size is negatively correlated to tunnel stability when the other conditions are identical [5]
E stability is often significantly influenced by the size of tunnel crossing section. e subway station is a typical kind of tunnel with large crossing section, which is always more than 300 m2 in Chongqing of China and has a very high risk during construction by subexcavation especially in the soft/ weak ground [6]
Summary
Rock mass always contains some discontinuities such as joints which are considered as a softer and weaker region than surrounding intact rock [1]. Tsesarsky and Hatzor [11] studied the stability of underground openings excavated in a blocky rock mass considering the joint spacing and friction. Most of studies using continuum-based method are focused on some cases of intact rock mass or highway tunnel [14, 15], whereas studies such as the type in this paper are rarely reported. Is paper concerns steeply jointed rock mass and supercrossing section of subway station constructed by the subexcavation method. Two neighborhood tunnels of subway station with superlarge section at steeply jointed rock mass are taken as study example. Numerical simulations are carried out to compare different construction methods and find out the best face excavation sequence and support system parameters, which will significantly help to decrease the risk, make money, and reduce the duration of this project construction
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