Research On Excavation Methods And Supporting Structure System Of Tunneling Schemes In Crushed Phyllite

With the accelerating process of Chinese Western Development Project, tunnels constructed in high-intensity crushed areas and under complicated geological condition emerge in large number. Affected by strong earthquake and frequent aftershocks, mountains in these regions are even difficult for phyllite tunnel constructions. Therefore, how to establish security controlling criteria in these constructing regions and how to choose the feasible excavation methods and supporting structure system are critic questions in front of many scholars in this field. However, the excavation methods and supporting structure system have not been formed into unified design standards. Therefore, forming a security controlling system for the construction of tunnels surrounded by shattering rock is critical to ensure the construction safety of numerous phyllite tunnels in crushed region of western China.In the engineering background of Dujia mountain phyllite tunnel which is under construction and affected by Longmen Mountains Fault, a research based on Sichuan province transportation science and technology project of-’Key Technique of Tunnel Construction Guanyuan-Gansu Expressway’-was carried out in the thesis. On the basis of data collection and field survey, theoretical analysis, similar model tests, numerical stimulation and field tests were used to study the typical traits of rock affected by earthquake, deformation features of tunnel surrounded by shattering rock mass and its formation mechanism. After that, research on management indictors of security controlling criteria during the construction period, ultimate displacement of linings, excavation method and key parameter of supporting structure system was carried out. Major achievement of the thesis could be concluded as following:1. Results reveal that the phyllite mass in intense crushed areas bears several properties such as:low strength, low density, high attenuation of elastic wave velocity, high permeability and lots of hollow parts. The failure features of surrounding rock mass and supporting structures as well as the development process of lining deformation during the construction period were analyzed. For the structural deformation and mechanical behaviors of crumbling sections, formation mechanism of structural failure were analyzed by taking consideration of prosperities and deformation features of shattering rock. Moreover, ultimate deformation of tunnel lining was proposed in this thesis,2. Statistical analysis on structural failure cases of numerous phyllite tunnels in crushed region were carried out, which reveals the main associate indictors and controlling values of security controlling criteria for phyllite tunnels in crushed areas. According to evolvement rules of tunnel structural deformation, safety deformation, crack deformation and deformation at serviceability limit state as well as at ultimate limit states were clearly categorized. Meanwhile, the safety deformation and crack deformation were incorporated in management levels for security controlling of tunnel construction in crushed areas.3. Model tests assisted by "tunnel-stratum simulation test system" was combined with numerical stimulation to analyzed the correlation effect between safety criteria and management indictors, such as buried depth, excavation method, section form, supporting strength. Results show how the failure laws and ultimate deformation of supporting structure vary with different impact factors. Based on the result and combined by statistic analysis of deformation monitoring data, the safety controlling values for Grade V soft rock mass represented by phyllite were finally determined. Meanwhile, safety controlling criteria of phyllite tunnel under different buried depth were established according to controlling values for different factors. Also the allowable deformation values were proposed correspondingly.4. Field tests and FEM were adopted to analyze the deformation and mechanical characteristics of both surrounding rock mass and supporting structure. The interaction between rock mass and supporting structure were studied and results revels the technical applicability and feasibility different excavation methods for shattering rock stratum. Then favour excavation methods for phyllite tunnel construction was determined. Meanwhile, with adaptation of the favour excavation method, structural deformation and mechanical characteristics by using different excavation parameters, such as:footage, step height, width-to-height ratio of core soil, were compared and analyzed, proposing the key design parameters of favour excavation methods.5. Model tests on parameters optimization of supporting structure for phyllite tunnel were carried out and results reveal the impact of different parameters, such as thickness of shotcrete layer, span of steel arc, length and arrangement of rock bolts, secondary lining thickness, on structural deformation and mechanical characteristics of phyllite tunnel in crushed region, proposing the key parameters of supporting structure system for this project.6. Field tests, applying the proposed values of supporting structure parameters, were carried out in the classic sections of this tunnel. After analyzing the test data systematically, results verify the supporting effect of proposed scheme to phyllite tunnel in crushed areas. Meanwhile, results also reveal the long-term variation laws of structural deformation and mechanical behavior under the favor supporting structure system. Also, comparison of structural mechanical characteristics between absence of and adoption of system rock bolts show the supporting effect of rock bolt and its mechanism. Moreover, results above are also useful for adjudging structural safety of phyllite tunnel in crushed areas.Shattering soft rock mass affected by intense earthquake is different from usual soft rock mass in physical and mechanical property, which causes huge distinction of the security controlling system between crushed areas and acrushed regions. The results of this thesis are expected to theoretically support the structural design of phyllite tunnel in crushed areas. Meanwhile it could also be served as references for the construction of numerous phyllite tunnels in intense crushed areas of western China.