Study On Mechanical Characters Of Watery Weakly Consolidated Sandstone During Tunnel Construction And Its Support Countermeasure

During the crossing poorly cemented and water-rich sandstone in Humaling tunnel of Lanzhou-Chongqing railroad, there are big deformation of tunnel surrounding rock,distortion and instability of steel frame, being easy to collapse of tunnel face, burst mud and sand gushing, and so on.Because of tunneling difficult and tremendous construction risk, it is rare in China and be a worldwide problem. Base on this project, this paper research around the surrounding rock deformation features and its control technology by means of theoretical analysis, numerical simulation, indoor model test and field monitoring, then further and systematiac studies the mechanical properties of poorly cemented and water-rich sandstone and its control technology. The several main achievements in the research are as follow:(1) It obtains parameters of physical properties for poorly cemented and water-rich sandstone from indoor experiments, and interactional relationship among them, which is a basis for numerical simulation and the support structure design. The results showed that the poorly cemented and water-rich sandstone have a weak cohesion force and permeability; there are much difference between water-cut poorly cemented sandstone and dry poorly cemented sandstone, which uniaxial compressive strength of water decrease sharply, and the elastic modulus and coefficient of viscosity decreases sharply with the increasing of water content.(2) Through the rheological test of weakly consolidated sandstone which is under load, we found the rheological strain is accounted for40%of the total strain, and when the moisture contents are not the same, the instantaneous strain, the initial rheological strain and the total rheological proportion of the creep curves are also different. In dry surrounding, the initial rheological of unconsolidated sandstone accounts for about53%of the total rheological deformation, and it would be over80%in moisture surrounding. Based on the above test results, the relationship between each coefficients of the general kelvin creep equation and moisture content is fitted, and the creep equation which contains the w coefficient is also derived, the equation aimed at describing the change of the unconsolidated sandstone creep property along with the moisture content; at the same time, the expression of the interaction between weakly consolidated sandstone surrounding rock and supporting is deduced, and through the related expression, the stress, strain and deformation of supporting structure which is under released load are also got.(3) The watery weakly consolidated sandstone prones to heavily deformation failure duced by the influence of tunnel construction, and the failure always has the characteristics of large deformation, quickly deformation development, long lasting time and the distinctly asymmetry and inhomogeneity in time-space effect. For the same cross section, the periodicity of the surrounding rock deformation with the excavation can be obviously divided to three stage by deformation rate: the sharp deformation stage, the stable deformation stage and the rheological stage. After the excavation of tunnel, the surrounding rock deformation has the distinctly time-space effect, the time effect mainly occurs in the rheological stage, the space effect mainly occurs in the sharp deformation stage, while the stable deformation stage is the transition period between the two above.(4) The loose range of watery weakly consolidated sandstone under the influence of tunnel construction can be confirm by the seismic refraction layer analysis method. This method firstly estimate an initial model by the time field method and the delay time method, based on this model, modeling analysis adopts the minimum travel time tree method, inversion analysis adopts the damping least square method, this paper has calculated the ray path through several times of iteration based on the initial model, the fitter the calculation time-distance curve and the measured curve, the better it is, the best velocity model can be the explanation result. The relaxation zone is the low velocity zone which the surface wave velocity value is less than1000m/s; the loose zone is the gradient zone which the central wave velocity value is between1000m/s and2500m/s; the undisturbed zone is the relative stable zone which the bottom wave velocity value is less than2500m/s; the basis of the boundaries among each zone is the speed gradient and the slide interface position of the wave radiation.(5) Based on the special mechanical properties of Humaling with watery weakly consolidated sandstone, advanced support and reinforcements measures are adopted to avoid the deformation of surrounding rock. Support structures are closed as soon as possible. Tunnel is excavated one block with another by step. And the concept of monitoring measurement is also used. Numerical simulation and engineering analogy method are used to ensure the excavation method of the Humaling tunnel and the construction method of the inclined hole to the tunnel. And also some optimization analysis is conducted.(6) The finite element method is used to analysis the application time of primary liner of the watery weakly consolidated sandstone tunnel. We can conclude that the deformation control of surrounding rock should be the center of construction. The timeliness of primary liner should be emphasized under the premise of the safety of the primary liner. With the field monitoring of the typical section of watery weakly consolidated sandstone tunnel, we can conclude that the pressure between primary liner and rock mass has great time effect. So the design of support should consider the long-term rheological effect. The distribution of pressure presents the heterogeneity in spatial regularities. The contact pressure between primary liner and rock mass increases at first and decreases later, and trend to be stabilize at last. The factors that caused the change include the strength and stiffness variation after the form removal, the compactness of the concrete, stress release after the form removal and also the influence of the shrinkage and creep of the concrete. The contact pressure between the primary and secondary liner is asymmetry. It gradually increased from crown, to spandrel, to haunch, to the top of arch-wall and to the bottom of the arch-wall as a whole. But the measure value is relatively small for the springing and invert of the tunnel.(7) The Humaling tunnel with watery weakly consolidated sandstone has a lower strength, it will be soften with water and has a bad stability, and it also can easily collapse. The tunnel often appears the trouble of sand gushing, water burst, large settlement, the deformation of primary liner, etc. The rock mass has the engineering properties of little particle, water sensitivity and also has the liquidation effect with watery unconsolidated conditions. This article intensively studies the physical and mechanical properties of this kind of ground, and forms the technologies of mild type well point dewatering based and deep negative pressure precipitation on step method. Dual fluid back fracturing grouting technology is adapted based on aquaporin and local plastic clay rock mass. The auxiliary construction technologies help to solve the stability problem of the rock mass and control the deformation of the ground effectively, and also guarantee the project schedule.