Issue 41

L. Zhang et alii, Frattura ed Integrità Strutturale, 41 (2017) 356-368; DOI: 10.3221/IGF-ESIS.41.47 363 -0.274 t -0.012t 23.5(1- 1.5e - 0.56e ) E(t) = (5) The Elasticity modulus of shotcrete with the new liquid accelerator associated with time is determined as -0.127t -0.013t 25.4(1 - 0.39e - 0.46e ) E(t) = (6) Model building and parameter selection (1) Mechanical Parameter Numerical simulation adopts FLAC3D numerical software to study the cross section of class IV surrounding rock of Changgang tunnel, Fushou Highway and consider the stress release rate of surrounding rock. The model dimension is twice of the diameter that is 12 m. Both surrounding rock and preliminary support adopt solid elements. Normal restraint is set on the lateral boundary and full restraint is set on the surface boundary of the model. Considering the hardening characteristics of shotcrete with time, set Poisson's ratio to 0.2 and volume density to 2300 kg/m 3 . Tab. 8 lists the physical and mechanical parameters of the selected surrounding rock. Surrounding rock class Unit weight (KN/m 3 ) Deformation modulus /GPa Poisson's ratio Cohesion /MPa Internal friction angle/° IV 20 1.3 0.3 0.2 27 Table 8 : Physical and mechanical parameters of surrounding rock. (2) Stress release rules of surrounding rock According to the NATM theory, after tunnel excavation, the stress of surrounding rock is gradually released with time under the action of the self-stabilization capacity of surrounding rock instead of being immediately released. Document [18] proposed the following expression of the load released by tunnel with time based on numerical simulation results:    0 ( ) (1 0.7 ) mt p t p e where:  3.15 2 V m a V : tunnel excavation progress, m/h a : tunnel excavation radius, m t : tunnel excavation time, h This formula can be used to resolve the following formula of stress release rate of surrounding rock:    1 0.7 mt n e This calculation adopts the preceding stress release rules of surrounding rock and simulates surrounding rock release of this expansive loess tunnel. Therefore,  1 / 6 V m h , a=6m, thus determining that the curve of surrounding rock release rules is as shown in Fig. 5. Calculation result analysis (1) Largest vault settlement displacement The shotcrete added with the traditional and new liquid accelerator respectively is studied. Fig. 6 shows the largest displacement of vault settlement when the shotcrete added with the conventional and new liquid accelerator respectively interacts with surrounding rock with the passage of time.