Construction Monitoring and Numerical Simulation Analysis of the Yangwujian Soft Rock Tunnel

Tunnel construction monitoring plays a key role in excavation construction of soft surrounding rock tunnel, it can be accurately judged stability of surrounding rock, its provide more priority information to design and construction. This paper adopts to construction of New Austrian Tunnel-ling Method (NATM), by strengthening the supporting and actual measurement on the spot, obtained displacement curve above vault settlement and clearance convergence. Selection parameter of supporting structure property by using Midas-GTS finite element analysis software, as well it has obtained both upper guide and down guide clearance convergence displacement-excavation step curve and valut settlement-excavation step curve. It’s found that: the faster stage of changing velocity is the first 10 days; the optimal execution time of the secondary lining is that the tunnel was excavated 30 days. It’s almost anastomosis both numerical simulation analysis and actual monitoring, it is better confirmed correctnass to establish the tunnel model, which provide reference for design and construction.


Introduction
With the rapid development of economy and invest heavily in the construction of infrastructure, tunnels are also in the process of prosperous construction. Monitoring is an indispensable in the construction process, because most of the tunnels are in the mountains and it has complex geological conditions. The stability of surrounding rock and the working state of support and protection can be judged by monitoring [1]. the information of clearance converges and vault settlement can reasonably adjust the construction operation. so long as the relevant parameters are fully considered, the tunnel excavation support can be well matched with the actual results by three dimensional numerical simulations [2]. In the process of weak surrounding rock construction, large deformation, the accident of collapse, and the slow construction progress, which seriously affect the construction period and safety [3].
In this paper, taking Yangwujian double-hole tunnel as an example, the large deformation mechanism of soft rock is fully considered, improving the construction safety by increasing the number of bolts, adding strength and thickness of shotcrete, and raising the field monitoring.  Table 1. The tunnel's lining structures is three-center curved walls. The location of the tunnel is medium weathering, fine sand structure, muddy cementation and slightly developed joints and fissures.

Construction Excavation Monitoring
The monitoring points are arranged at intervals of 10m in the longitudinal direction of the four grade surrounding rock. the surrounding rock is constructed by short bench method. The step length is controlled to be 10 ~ 15m.when the deformation of the surrounding rock basically stable, the secondary lining is carrying out. the buried depth of the double -line tunnel is less than 100 m, the standard of controlling the deformation of surrounding rock is 120 mm of the vault settlement or 150mm of the clearance convergence [4]. At present, our country puts forward the basic stability standard of the surrounding rock of composite lining tunnel: when the tunnel's width is less than 10 m, the horizontal convergence rate is 0.1 mm / d, the horizontal convergence rate is 0.2mm/d when the tunnel's width is more than 10 m [5].
The cumulative value of the vault settlement(mm) c. the vault Figure 1. different displacement curves From these curves we can conclude that accumulated value is far less than the control value. The clearance converges rapid change in the first 10 days, it has little change between 10days and 15days and keep stable after 15 days. The vault settlement rapid change in the first 10 days, it has little change between 10days and 30days and keep stable after 30 days. This also accord with the field conditions. The down guide is excavated after 15 days for the primary support of the upper guide. The construction period is exactly right. When the surrounding rock is still an unstable condition, the monitoring data can directly reflect actual situation and then the excavation is delayed. It can be explained that the monitoring reflects the surrounding rock situation in time and conducts the construction through the analysis of the data.

Numerical Simulation
The section is selected by ZK7+945~ZK7+975, YK7+895~YK7+925, building the finite element analysis model as shown in figure 2. The Midas-GTS calculation model is established in an area with a size of 90m×30m×70m (length×width×height). from top to bottom the rock formation is silty clay 2m, strong weathered fine sandstone 4m, medium weathered fine sandstone 64m.The tunnel structure material parameter table is shown in Table 2. Mohr-Coulomb constitutive model is applied to the soil material, established a 3D soil unit for the soil material and 1D implantable truss unit which a solid circle diameter d = 0.022m for the bolting support. Built a 2D plate unit and a thickness of 0.3m for the sprayed concrete, it's an elastic constitutive model.

Numerical Simulation Results
The upper and down guide curve of the excavation steps and horizontal convergent displacement are illustrated in Fig.3 (a), (b).
The cumulative value of upper guide clearance convergence(mm) construction step(step) The cumulative value of down guide clearance convergence(mm)  According to the Fig.3(a), we can conclude that: The upper guide YK905 converges faster from step S5 excavation to step S20, the displacement reaches 97% of the total displacement, and then tends to be stable, the cumulative convergent displacement is -5.05 mm, and the upper guide YK915 converges faster from the S10 excavation to the S20 step. The displacement reaches 96% of the total displacement and then tends to be stable. The cumulative convergent displacement is -5.38mm. This also shows that the excavation of the right line has a great influence on the convergence displacement of the clearance of the right line section, and the excavation of the left line has a slight effect on the clearance displacement of the section of the right line. The upward guide ZK950 converges faster from step S25 to step S38, the displacement reaches 99% of the total displacement, and then it tends to be stable, the cumulative convergent displacement is -4.81 mm, and the upper guide ZK960 converges faster from step S29 to step S40. The displacement reaches 99% of the total displacement and then tends to be stable. The cumulative convergent displacement is -5.41 mm.
According to the Fig.3(b), we can conclude that: The down guide YK905 converges faster from step S14 to step S20, and the displacement reaches 92% of the total displacement, and then tends to be stable, the cumulative convergent displacement is -4.96mm, and the down guide YK915 converges faster from step S18 excavation to step S20. The displacement reaches 96% of the total displacement and then tends to be stable. The cumulative convergent displacement is -4.07mm. ZK950 converges rapidly from step S30 to step S35, the displacement reaches 99% of the total displacement, and then tends to be stable, the cumulative convergent displacement is -3.79mm, and the down guide ZK960 converges faster from step S38 to step S40. The displacement reached 99% of the total displacement and then tended to be stable. The cumulative convergent displacement was -3.82 mm.
The vault curve of the excavation steps and the vault settlement displacement is shown in Fig.3(c). According to the Fig.3(c), we can conclude that: The settlement velocity of YK905 is faster from S5 excavation to S20 step, the displacement reaches 88% of the total displacement and then tends to be stable. The cumulative settlement displacement is -4. 28mm.YK915 from S10 excavation to S20 step, the displacement reached 88% of the total displacement, and then tended to be stable. The cumulative settlement displacement was -4. 37mm.The settlement speed of ZK950 reached 98% of the total displacement from the S25 excavation to the S38 step, and then it tended to be stable. The cumulative settlement displacement is -4.16mm. The settlement velocity of ZK960 is faster from S29 excavation to S40 step, the displacement reaches 99% of the total displacement, then tends to be stable, and the accumulative settlement displacement is -4.16mm.

Conclusions
The change rate of displacement is the first 10 days and the best time of the second lining construction is the 30th day of tunnel excavation. The construction period can be shortened and the cost can be saved according to this construction.
The numerical simulation results are basically consistent with the measured data. The numerical simulation can better reflect the tunnel excavation process and provide reference and assistance for similar engineering cases.