The technology using mortar-bolts to pre-reinforce the surface of shallow tunnel now has been applied to a certain extent. This engineering problem is analyzed in this paper by using spatial elastic-plastic numerical method and coupling with finite element, infinite element and 3-D bolt element. Numerical computation module is developed, and corresponding computational program is complied. The displacement and the stress in the surrounding rock of shallow tunnel pre-reinforced with surface mortar-bolts are analyzed.
The technique of pre-reinforcing with surface mortar-bolts utilized before digging shallow tunnels has many advantages, such as convenient construction, prominent reinforcing effect and economic benefit. It can strengthen the stability of surrounding rock, and it is an assistant construction measure which is valuable to be popularized overall. In railway and highway tunnel construction after the late of the 1980, this technigve has been widely applied. All kinds of numerical methods are widely used in tunnel and underground engineering and they have good conducting effect on the design and construction of many practical engineering. To realize the pre-reinforcing practical effect of surface mortar-bolts acting on allow tunnel stratum, spatial elastic-plastic coupled numerical method together with finite element, infinite element and 3-D bolt element is used in this paper. Numerical computation module is developed and the corresponding computational program is compiled. Then the influence on surface subsidence, stratum moving rule and deformation status of shallow tunnel caused by surface mortar-bolts is discussed and the change of stress states in tunnel surrounding rock is analyzed.
A portal of shallow tunnel pre-reinforced with surface bolts is shown in figure 1. In the figure, the whole calculated region is divided into three sections (surface bolts, tunnel lining and surrounding rock). So it is better to simulate the spatial effect of portal and the action of surface bolts when coupled numerical method is applied. In the computation module, tunnel lining is imitated with 20 nodes isoparametric finite element and surface mortar-bolt is imitated with three-dimension bolt element in which shear action between bolt and grout is considered. As to surrounding rock of tunnel, the region near tunnel is analyzed with elastic-plastic theory and simulated with 8–20 nodes isoparametric finite element, because the change of displacement and stress is large and nonlinear deformation is caused as tunnel being dug. The region far from tunnel is simulated with 8–12 nodes isoparametric infinite element because surrounding rock is in elastic status. The detailed discussion is listed below. 2. 1 3-Dbolt Element modle in which Shear Effect is Considered The results of measuring in situ and model test in laboratory indicate that when tunnel has been dug, besides axial normal stress and transverse shear stress caused by longitudinal and transverse displacement in mortar-bolts, there are also tangential shear stress caused by opposite displacement of bolt/grout and grout/rock in mortar-bolts. The shear action between bolt and grout is very important to prevent stratum from sinking and decrease rock pressure. To reflect bolt's practical effect.
