Experimental Study On The Instantaneous And Rheological Mechanical Properties Of Soft Rock

With rapid economic development, a great number of geotechnical engineering such as slope, underground cavern and dam foundation had appeared in engineering fields such as transportation, mine, water conservancy and hydropower engineering. It is noteworthy that the rock stability in these geotechnical engineering is usually the important restrictive factors of the construction process and safety. As we all know there are many factors which can affect the rock stability, such as stress state, anisotropy, size effect, joint inclination, the rheological effects appeared with time, and so on.In the construction of the underground works the weak rock are frequently encountered, the large-scale underground excavation and backfill make the research on the stability of surrounding rock foundation and mechanical properties of soft rock foundation become quite important and urgent. In this paper, the typical soft rock-muddy siltstone and soft interlayer were took as the main study objects to study the instantaneous mechanical properties of soft rock and the rheological mechanical properties under the long-term effect of constant external load, considering the influence of stress path, joint inclination, unloading rate and some other factors. The main research work is as follows:(1) The instantaneous triaxial compression test was carried out on the intact soft rock mass to study the strength, deformation and damage characteristics of soft rock mass under different confining pressures. Test results show that in the process of instantaneous triaxial compression the typical ductile failure features is obvious, however with increase of the confining pressures the obvious brittle-ductile transition points didn't appeared.(2) The instantaneous triaxial compression test has also been carried out on the soft rock masses with three different inclinational intermittent joints; the test results show that the instantaneous triaxial compression strength of the jointed soft rock is significantly lower than that of the intact soft rock. What's more the strength of rock mass with40°joint inclination is the highest, the strength of rock mass with0°joint inclination is the second, and the strength with60°joint inclination is the lowest. Moreover the influence of joints on rock stress-strain feature reduces with the increase of confining pressures. In the destruction process for rock mass with60° joint inclination, the penetrating failure surface was extended from the precast joint surface, for rock mass with0°and40°joint inclinations one or two set penetrating shear failure surfaces was finally formed from the end of the left and right precast joint surfaces along the direction of the axial stress, however the precast joint surface didn't run-through, and didn't form the failure surface.(3) The confining pressure unloading test with constant axial compression pressure was carried out on the intact soft rock which considered the different unloading rates, the test results show that the unloading confining pressure amount required in destruction increases in logarithmic form with the increase of the unloading rate, what's more both the stress-strain behavior and damage characteristics are influenced by the unloading rate significantly. According to the test results, the formula which can reflect the variation law formula E(σU)=a*e(b/σuc+d) of deformation modulus with the confining pressure unloading amount during unloading was proposed. In the formula, the parameter C represents the unloading rate.(4) The confining pressure unloading test with constant axial compression pressure was also carried out on the soft rock masses with three different inclinational intermittent joints; the test results show that the sample strength were significantly affected by the precast joint. For rock samples with joints, the confining pressure unloading amount required in destruction is about58%-84%the amount of the intact rock mass, what's more for rock samples with60°joint inclination the strength decrease is the most significant. For joint rock samples, the deformation modulus dropped with large amplitude in destruction, and the decrease extent is2-3times of the intact rock samples in same condition. CoMPared with the intact rock samples, the peak deformation and residual deformation of rock samples with0°or40°joint inclination is close to that of the intact rock mass, while the value of rock samples with60°joint inclination is significantly less than that of the intact rock mass. All in all, the influence of the joint inclination on failure mode is remarkable; however the relationship between the joint inclination and the shear failure angle is quite complex without obvious pattern.(5) According to the actual engineering stress state of the dam foundation in Tingzikou project, a group of shear rheological tests on the weak intercalation under four normal stress levels was carried out, the test results show that the shear rheological deformation properties of the weak intercalation is significant, and the rheological deformation amount at all shear stress levels is large. According to the test results analysis, the exponential function u=m×enτ was proposed to express the change law of the average and state rheological rate, and on base of the isochronous curve method a new method which use the curves u-Δ τ/Δ u to certify the yield inflection point was proposed. Finally the calculation results show that the method is quite convenient and intuitive, and can calculate the long-term shear strength of the weak intercalation accurately, what's more the method has a certain reference value for the actual engineering.(6) According to the actual engineering stress state of the dam foundation in Tingzikou project, a group of shear rheological tests on the soft rock mass under four normal stress levels was also carried out, the test results show that the rheological deformation properties of the soft rock is similar to that of the weak intercalation, but the rheological deformation of the soft rock is far less than that of the weak intercalation. According to the rheological curve of the soft rock, the shear rheological empirical equation u=u0+A (1-exp (-Bt))+Ctπ was proposed, and the fitting results show that the empirical equation can reflect the rheological deformation characteristic of soft rock in various stages better.(7) According to the compressive strength of the intact soft rock at different confining pressures which were got in the instantaneous triaxial compression test, the axial stress level classification scheme of triaxial compression rheological samples under different confining pressures was set up. The triaxial compression rheological test of the intact soft rock under different confining pressures was conducted with RLW-2000, the change law of the axial and lateral deformation with time under different confining pressures was analyzed, the develop trend and characteristics of the rheological deformation and rate in rheological deformation accelerated phase was researched, and the triaxial rheological rupture form and mechanism under different confining pressures was analyzed.(8) Based on the triaxial compression rheological experimental results of soft rock, the triaxial unloading rheological test of the intact and joint soft rock under different confining pressures was conducted with RLW-2000, the change law of the axial and lateral deformation with time under different confining pressures was analyzed, the develop trend and characteristics of the rheological deformation and rate in rheological deformation accelerated phase was researched.(9) On base of the triaxial rheological complete curve of the soft rock samples, and refer to the selection and the parameter identification process of the rheological constitutive model, the linear rheological constitutive model Burgers was used to identify the soft rock parameters by the nonlinear optimize least squares Levenberg-Marquardt, what's more the change law of the identified axial and lateral rheological parameters with the variation of stress level and confining pressure was also been analyzed.(10) On base of the linear rheological model Burgers, A new viscous element was cascaded with Burgers model, and a damage factor D was introduced to the element viscosity coefficient ηD, and then a new non-linear rheological damage model was established, which can better describe the nonlinear accelerated rheological deformation stage in the rheological complete curves. The verification results show that the model can close fit to the rheological test curves, reflecting the accuracy and reasonableness of the model.(11) The nonlinear damage rheological model was used to simulate the dam foundation of Tingzikou project, and the rheological mechanical characteristics and long-term stability of the rock mass in dam foundation were analyzed, the research results have an a certain reference value and guiding significance to the long-term stability and security of the rock mass in Tingzikou project.