Study On Large Deformation Control Technology Of Carbonaceous Slate Tunnel With High Crustal Stress In Suture Zone

As an important part of the Trans-Asian Railway middle line,the China-Laos Railway is a landmark project in the transformation of Laos from a"land-locked"country to a"land-union"country.It is also the first international railway directly connected with China’s railway and using Chinese equipment after the B&R was proposed.Due to the interwoven suture zone,the complex geological structure and the intense regional tectonic in Laos,the initial crustal stress has complex characteristics such as high stress and high strain,resulting in tunnel large deformation problems occurred frequently during construction,which seriously affects construction safety and progress.The dissertation focuses on the China-Laos railway large deformation tunnels with carbonaceous slate in Luang Prabang suture zone,a variety of research methods were adopted to explore the tectonic characteristics,the crustal stress characteristics and the mechanical properties of carbonaceous slate in Luang Prabang suture zone,analyzing the characteristics and mechanism of tunnel large deformation in suture zone.Final,the corresponding control technology were proposed.The main conclusions are as follows:（1）Through data investigation and analysis,it is clear that Luang Prabang suture zone has significant structural characteristics of deep and large active fault zone,high seismic intensity and high crustal stress.Through hydraulic fracturing method and stress relief method crustal stress test,it is concluded that the crustal stress characteristics of the Luang Prabang suture zone are mainly horizontal tectonic stress,the stress distribution difference and stress direction change due to the local influence of fault activity and topography.The crustal stress inversion of the Huifulai tunnel was carried out based on the measured crustal stress results,It shows that the crustal stress characteristic isσ_H>σ_h>σ_v,which is conductive to the development of reverse fault.The maximum horizontal principal stress is 24.2MPa,and the lateral pressure coefficient is between 1.5 and 1.8.The stress direction is deflecting about 10～20°because of the regional faults.（2）Based on the experimental analysis of point load strength test and uniaxial compression strength of the carbonaceous slate in Luang Prabang suture zone,the relationship expressionUCS=18.45I_{s（5） 0}is established.the structure of carbonaceous slate revealed that basically structure are soft argillaceous massive and thin layered.Argillaceous massive carbonaceous slate has obvious compressional and torsional properties,disordered bedding plane,and UCS is less than 5MPa.which belongs to extremely soft rock.The UCS of rock mass with thin layered structure is generally less than 25MPa,which belongs to soft rock.The anisotropy is obviously.Due to low strength rock and developed fissures,the tunnel is prone to large deformation under the action of high tectonic stress in suture zone.Based on the point load strength test and the inversion analysis of the crustal stress,it is preliminarily determined that the large deformation grade of the carbonaceous slate tunnel in Luang Prabang suture zone is mainly slight and medium.（3）Based on the statistics of tunnels deformation data,It is revealed that the main large deformation characteristics of the carbonaceous slate tunnel in Luang Prabang suture zone are serious horizontal convergence,non-uniform,significant space effect,large cumulative deformation,and structure failure gradually.The influence of the angleαbetween horizontal principal stress and tunnel axis on deformation under different tectonic stress combinations were analyzed by numerical calculation.It is found thatαis between 15°and 75°,the sensitivity to deformation is stronger.Whenα<15°orα>75°,the sensitivity to deformation is reduced.The largerαis,the greater influence of the horizontal principal stressσ_Hon tunnel deformation is.The smallerαis,the greater influence of the small horizontal principal stressσ_hon tunnel deformation is.Based on the normal fitting of tunnels deformation data,the tunnel allowance deformation of crown settlement and horizontal convergence for different large deformation grades in suture zone are proposed.By testing the tunnel surrounding rock loosen zone and internal surrounding rock deformation,it is found that loosen zone extends to deep part with the advance of tunnel face,and internal surrounding rock deformation increases gradually with the loosen zone extending.Therefore,the tunnel large deformation in suture zone is a gradual accumulation process of surrounding rock from shallow to deep.（4）Considering the stress release effect of tunnel excavation,the relationship between the tunnel stress release coefficient and the virtual supporting force is applied to the elastic-plastic solution of tunnel strain-softening surrounding rock,then the calculation method of the loosen zone in the tunnel construction process is obtained.Through single factor analysis of the relationship between surrounding rock loosen zone and related parameter factors,it is concluded that the larger crustal stress is,the formation of loosen zone is earlier,the expansion rate is faster,and the time to reach stability is longer,the range of stable surrounding rock loose zone is larger.The larger stress release coefficient of surrounding rock during excavation is,the range of surrounding rock loosen zone during excavation is larger.The lower the rock mass strength is,the range of surrounding rock loosen zone is larger and formation is earlier.Based on the theoretical analysis,the gradual expansion of the loosen zone of soft rock tunnel had been analyzed and demonstrated,and it was revealed that the large deformation of the soft rock tunnel in suture zone is the result of the continuous superposition of the dilatancy and crushing deformation of the surrounding rock during the progressive failure process.（5）The improvement on bearing capacity of tunnel supporting structure are finite by high stiffness steel and high strength shotcrete.The self-bearing capacity of surrounding rock in Luang Prabang suture zone are decreased with high tectonic stress,and the structure instability and failure are easy to occur when only passive supporting structure is used to resist large surrounding rock load.The bolt support could improve the self-carrying capacity of the surrounding rock effectively,the deformation of the surrounding rock and the expansion of the loosen zone could be limited,the force of the supporting structure could be improved and active control could be used.Based on large deformation control concept of surrounding rock bearing capacity mobilization and targeted local reinforcement optimization,Optimization of tunnel section height to span ratio,Full shot bolt supporting with local long bolt,and rapid supporting by short bench step are proposed.The large deformation control technology of carbonaceous slate tunnel in suture zone are formed.