Study On The Tensile Properties Of Chert Considering Three-Dimensional Laminae Effect

Tensile strength is one of the weakest strength properties of rock,and the judgment of the distribution range of tensile zone and stability control generated by excavation and unloading of rocky slopes or underground caverns is often the key to engineering stability.The deformation and damage of laminated structural rock masses under load are closely related to the three-dimensional spatial output state of laminated surfaces,and the strength and deformation properties show obvious anisotropy.With the change of the angle between the main stress direction and the laminae,the emergence,expansion and penetration of tension cracks in the rock mass can occur both in the block and between the layers.Therefore,to carry out the study of tensile anisotropy of laminated structural rock masses and to accurately understand the stress state and deformation and damage mechanism of the internal structure of rock masses under the action of loads is essential to ensure the stability of rock projects.Based on the high ground stress section in the back-sloping core of a section of Tongzi Tunnel,the thick-layered Limestone with laminae is the object of study,and the Brazilian splitting test is used as a means to study the tensile properties and damage mechanism of the Limestone under different combinations of laminae inclination angles and dip angles.Determining the ultimate tensile strain values of laminated Limestones in combination with finite difference numerical simulation methods.Based on the acoustic emission signal values during the test,define the tensile damage parameters and establish the tensile cracking criterion.Finally,the criterion is applied to the tunneling project,and the distribution of stress,displacement and damage zone of the surrounding rock on both flanks of the back slope of the high stress section is analyzed.The main contents and results are as follows.（1）Petrographic polarized light microscopy mineralogical identification of laminated Limestones.Brazilian disc standard specimens of laminated Limestones with different inclination anglesφ（φ=0°,30°,45°,60°,90°）and dip anglesθ（θ=0°,15°,30°,45°,60°,75°,90°）were prepared,and indoor Brazilian splitting tests were conducted,and the tensile properties（tensile strength,tensile modulus,maximum principal strain）and damage modes（tensile damage through the laminae,tensile-shear composite damage and tensile damage along the laminae）of laminated Limestones with different tendencies and dip angles showed obvious anisotropic characteristics.Combined with the rock acoustic emission test,the acoustic emission signal values were collected throughout the test and analyzed to obtain the anisotropic characteristics of the tensile strength parameters with different inclination dip laminar angles.（2）Using FLAC^3D finite-difference numerical software,combined with the Ubiquitous-Joint model,numerical tests of Brazilian splitting were conducted to determine the maximum principal stress,maximum principal strain and displacement vectors of laminated tuffs with different inclination dips in a three-dimensional spatial distribution.According to the limit analysis method,the abrupt change point between slowly increasing displacement（or strain）and non-convergence is taken as the critical state for judging the pulling fracture,and the limit tensile strain values of laminated Limestones with different inclination angles and dip angles laminar Limestone are determined.（3）Based on the Weibull distribution of rock micro-element strength and the Lemaitre strain equivalence principle,and the evolution equation of the damage variable with the growth of the maximum principal strain is derived based on the Weibull distribution of rock micro-element strength and the Lemaitre strain equivalence principle,and the damage variable increases with the growth of the accumulated strain,and the tensile cracking criterion based on the ultimate tensile strain and the tensile damage stress calculation method are established.The secondary development of the FISH language embedded in FLAC^3D software was used to construct a numerical simulation program for dynamic damage under tension.（4）Apply the ultimate tensile strain damage criterion considering the three-dimensional effect of laminae and the all-over joint model to the high ground stress section in the back-slope core of a section of Tongzi tunnel,and use Midas GTS NX and FLAC^3Dnumerical software to establish a three-dimensional numerical model reflecting the topography,geological structure and structural characteristics of the rock body.Using the three-step method,five different excavation options with different step heights were designed,and option4 was found to be the best.Support measures for Option 4,analysis of the tunnel surrounding rock stress,displacement and damage area distribution,after comparison to optimize the construction method and recommendations,that is,in the construction process need to cross the monoclinic rock layer,should try to make the tunnel boring direction and rock inclination consistent,more conducive to the stability of the surrounding rock after excavation.In summary,this paper uses indoor tests and numerical simulations to study the three-dimensional tensile effects of laminated Limestone,and establishes the ultimate tensile strain damage criterion,the results of which can provide theoretical guidance for the identification of the tensile fracture range of excavation and unloading of tunnel projects,and provide an important reference for the analysis of tunnel envelope stability.