Abstract:

At present, the mining method, with high risk and low advancing speed in loess, is the main construction method for railway tunnels. However, the shield method has been widely used in metro and underwater tunnels. In this paper, the sandy new loess tunnel on Menghua Railway is taken for example. 1) The shield method is chosen by comparing to mining method. 2) Comparison is made between horseshoeshaped shield tunnel and circular shield tunnel in terms of excavation inner contour, excavation characteristics of cuterhead, segment assembling mode and stress and steel bar arrangement of segment. It is shown that the horseshoeshaped shield tunnel has a higher utility rate of crosssection comparing to circular shield tunnel; the stresses of horseshoeshaped segment and circular segment are different; but the steel bar arrangement of horseshoeshaped segment is lower than that of circular segment. 3) The general situation of horseshoeshaped shield is presented; and the design of horseshoeshaped shield segment is studied. 4) The difficulties, i.e. freezing prevention, cracks on segment bottom and calcareous concretion in old loess tunnel, encountered during horseshoeshaped shield tunneling and corresponding countermeasures are put forward. The practice and application of horseshoeshaped shield indicate that the horseshoeshaped shield tunneling in loess tunnel can reduce the construction risk, improve the construction quality and guarantee the construction safety.

Key words: loess tunnel, horseshoeshaped shield, circular shield, inner contour of excavation, excavation blind area, segment assembly, segment stress, segment steel bar arrangement, low temperature frozen, segment crack, calcareous concretion