The Cause of Shallow Tunnel Collapse and Simulation Analysis of Consolidation for a Collapsed Tunnel

In order to ensure the safety of the shallow tunnels construction, it is very important to find the reasons of the collapse. During Fujiachong tunnel construction, collapse of the section Yk25 + 547~Yk25 + 552. Distributions of cracked surrounding rock, rainfall, geological forecast were discussed for reason of tunnel collapse. According to in-situ construction condition, reinforced support program was applied to the surrounding rock consolidation of the tunnel. In order to assess consolidation effect, a cross-sections of collapse zone measurement was increased and simulation analysis was applied to the section. The results of in-situ measurement and simulation results show that tunneling collapse zone was consolidated by reinforced support program successfully.


INTRODUCTION
With the rapid development of underground engineering construction in China, the engineering geological conditions encountered complexity problems facing the increasingly challenging (Li et al., 2007(Li et al., , 2008;;Zhang et al., 2009;Qian and Rong, 2008).Tunnel collapse is one of the most common accidents in the tunnel construction.Serious tunnel collapse accidents at home and abroad bring much personal injuries and economic losses (Chen et al., 2009;An et al., 2011).
The possible collapse of a tunnel is a rather complex problem because it is strongly affected by the random variability of the mechanical properties of the rock in situ and from the presence of cracks and fractures in the rock banks (Hyu-Soung et al., 2009).It can be inferred that uncertainties, which are embedded intricately in tunnel construction as well as given ground conditions, would be major causes for tunnel collapse (Fraldi and Guarracino, 2011).
Fujiachong Tunnel in Yi (chang) -Ba (dong) Expressway is 650 M long, the maximum burial depth is 82 M, so it is shallow tunnel.The geological conditions around it were surveyed before its excavation.It was found that the overall geological conditions predicted from the geological survey were closely matched the actual conditions observed during the tunnel's excavation.The surrounding rocks of the tunnel are Conglomerate and Shale.The rock mass can be classified as a soft rock.In the tunnel area, one joint set N35/50 of with a joint spacing of 20-50 mm were observed.More weathered zone could be found around the joints through which a groundwater flow at the access tunnel could occur.During Fujiachong tunnel construction, collapse of the section Yk25 + 547～Yk25 + 552, as shown in Fig. 1.In this study, distributions of cracked surrounding rock, rainfall, geological forecast were discussed for reason of tunnel collapse.According to in-situ construction condition, reinforced support program was applied to the surrounding rock consolidation of the tunnel.In order to assess consolidation effect, a crosssections of collapse zone measurement was increased and simulation analysis was applied to the section.This measurement method is successfully applied in the Fujiachong tunnel of Yi-ba expressway and has provided a reference for the similar engineering constructions.

CAUSE ANALYSIS OF SHALLOW TUNNEL COLLAPSE
Ground penetrating radar prediction: GPR working principle is based on the electromagnetic waves and the structure and distribution of unfavorable geology body are analyzed and judged by the waveform.The reflected wave energy is determined by reflection coefficient R. For the tunnel project, the unfavorable geology body and surrounding rock all have high impedance, so the reflection coefficient R and refraction coefficient T can be represented below (Cardarelli et al., 2003;Kasper and Meschke, 2006): ε 1 and ε 2 are relative dielectric constant of media before and after the interface.
The GPR prediction uses the SIR3000 radar and the frequency is 100 MHz The prediction results show in Fig. 2.
The GPR prediction method was used at the tunnel face of YK25 + 530 sections, the detection range is YK25 + 530～YK25 + 560.Based on the tunnel geological condition and prediction results, the surrounding rock in this section was cracked and the water content increased.
Monitoring measurement analysis: According to relevant design requirements, the monitoring was set up in section YK25 + 540.The sedimentation value of monitoring section and the sedimentation rate are shown in Fig. 3 and 4.
In the monitoring time, the final sedimentation value is 20.90 mm and the maximum sedimentation rate is 2.67 mm/day.Therefore, the sedimentation value and sedimentation rate are change frequently.It is easy to collapse under the influence of construction disturbance.

Cause analysis:
According to GPR prediction, surrounding rock, rainfall, the cause of collapse can be represented below.
The surrounding rock is weathering shale with layer structure and fracture development.In no effective bound, the layer will damage which led to the collapse eventually.
Continuous rainfall in the tunnel area, the earth's surface did not take timely measures of drainage.Large groundwater was rich in geotechnical layer causing the surrounding rock instability.
The result of GPR prediction shown that surrounding rock in this section was cracked and the analysis of monitoring measurement shown that it was easy to collapse under the influence of construction disturbance.Therefore, collapse is the result of many factors working together.
Collapse treatment scheme: The collapse treatment scheme of tunnel is shown in Fig. 5.The collapse of the ground is maintained security and rainproof cover and all around to dig the drain.
The thickness of 10 cm C25 shotcrete is used to close the collapse section and tunnel face.After the completion of the closed, the support of No. 16 joist steel is used to prevent deformation.
No. 16 joist steel acing of 50 cm is used in the bottom of collapse cavity and the thickness of 80 cm

SIMULATION ANALYSIS
Overview of FLAC 3D: FLAC 3D is a threedimensional explicit finite-difference program for engineering mechanics computation.FLAC 3D extends the analysis capability of FLAC into three dimensions, simulating the behavior of three-dimensional structures built of soil, rock or other materials that undergo plastic flow when their yield limits are reached.Each element behaves according to a prescribed linear or nonlinear stress/strain law in response to applied forces or boundary restraints.The material can yield and flow and the grid can deform (in large-strain mode) and move with the material that is represented.The explicit, Lagrangian, calculation scheme and the mixeddiscretization zoning technique used in FLAC 3D ensure that plastic collapse and flow are modeled very accurately.FLAC 3D offers an ideal analysis tool for solution of three-dimensional problems in geotechnical engineering.
Calculation principle: With tetrahedral as an example, Nodes number from one to four and surface L represents the surface relative to node L. The velocity component of any point in this tetrahedral is v i , in the case of constant strain unit, v i is shown as Linear distribution, in this case, we can known from Gaussian formula: where, v = The volume of the tetrahedral s = The external surface area of the tetrahedral where, l : The variable of node m (l) : The variable of surface l FLAC 3d is a way which regards nodes as objects of calculation.Where the forces and weights are concentrated in the nodes and then, The Calculation Results in time domain can be find by the motion equation.The motion equation of the nodes can be expressed as follows: The strain increments of units in some step can be expressed as follows: , ,    And the rate of final sedimentation value was 0.05 mm/day.The rate of final sedimentation value less than and the rate of final sedimentation value was 0.05 mm/day.The rate of final sedimentation value less than 0.20 mm/day was satisfied the standard requirement.The horizontal convergence value was 61.91 mm and the rate of final horizontal convergence was 0.08 mm/day.The rate of final horizontal convergence value less than 0.10 mm/day was satisfied the standard requirement.

CONCLUSION
Based on the study above, some important conclusions can be summarized as follows: Distributions of cracked surrounding rock, rainfall, geological forecast were discussed for reason of tunnel collapse.In the shallow tunnel construction, the reinforced support should be make to broken layer surrounding rock and increases the drainage measures to prevent collapse.The reinforced support including joist steel, shotcrete and steel mesh reinforcement was the effective method to the shallow tunnel.
According to in-situ construction condition and simulation analysis show that tunneling collapse zone was consolidated by reinforced support program successfully, so it can be further used for other tunnel engineering.

Fig. 5 :
Fig. 5: Collapse treatment scheme C20 shotcrete and Φ 22 steel mesh reinforcement of 20*20 cm are used in protect arch.The reinforced lining is made after the arch finished.

Fig. 6 :
Fig. 6: Computation model of tunnel n j = The component of the unit normal vector of the external surface v i can be expressed in such a way by Formula (4): 4