Investigation On Fracture Characteristics Of Layered Slate

Among rocks,sedimentary and metamorphic rocks with layered structure are the most commonly encountered rock materials in rock engineering.Layered rock that exhibits bedding and other structural planes usually hold the transverse isotropic behavior.Such bedding planes often lead to complex deformation and even rock failure.As a consequence,fracture of layered rock definitely affects the stability of rock and rock engineering in practice.In geological environment,layered rocks are also strongly acted by water and air-weathering,which induce more complex mechanical responses.To date the tensile and shear fracture characteristics of layered rocks have not been clearly understood so that there are rather more knowledges concerned failure perfformances of layered rocks to be developed in terms of experimental tests,theoretical analysis,and numerical modeling.In the present Ph D thesis,the layered carbonaceous slate cored from the Muzhailing tunnel of Lan-hai expressway in Min county is comprehensively investigated by means of field geological survey,laboratory tests,theoretical analysis,and finite-discrete element simulations.The related research perspectives cover the conventional mechanical properties of layered slate,the tensile mechanical properties of immediate drying and air-weathering layered slate,the experimental research on opening tensile fracture of layered slate using semi-circular bend method,and systematical finite-discrete element simulations for the layered slate,respectively.The main conclusions are summaried as follows.（1）The basic mechanical properties of layered carbonaceous slate under different bedding angles that include uniaxial compressive strength and elastic modulus are significantly affected by the weak planes of bedding.With the increase of bedding angle from 0°to 90°,the uniaxial compressive strength exhibits a typical U-shape trend.At the same time,the elastoplastic damage constitutive model for the layered slate is proposed,and its prediction for the uniaxial compression test is basically agreed with that of the experimental test.（2）Based on the Brazilian test for the immediate drying and air-weathering layered carbonaceous slate,the tensile properties of layered slate under two conditions are systematically analyzed.The results show that the two kinds of layered slate all exhibit brittle failures,while the mechanical response and peak load are very different,and the tensile strength is greatly anisotropic due to the influence of bedding.Moreover,the deterioration effect is significant due to water-rock interaction and air-weathering.And its mechanism is mainly governed by the physical and chemical reaction formed after water intrusion that changes the mineral micro-composition and structure,and the cumulative deterioration damage of mineral skeleton.（3）Based on the SCB test of layered slate,the opening tensile fracture characteristics under different bedding angles are systematically studied by finite-discrete element calculations.The mode I fracture toughnesses of typical specimens with related to arrester（0°）,short converse（90°）,and divider,are 0.168,0.097,and 0.195 MPa√m,respectively.Meanwhile,the fracture modes of layered slates with different bedding angles are also of rather difference.For example,vertical cracks appear along the bedding plane at the straight crack tip and propagate to the upper loading end in the short transverse specimen;stepped toughening cracks form in the arrester specimen;zigzag crack evolution trend presnts in the divider specimen.（4）Combined with the SCB experimental tests,the displacement fields of crack surfaces for different layered carbonaceous slates are analyzed by digital image correlation（DIC）.Moreover,the empirical relation for calculating the mode I fracture toughness of layered rock is proposed,which can be approximately used to estimate the fracture toughness of layered rock,K_IC=0.094σ_t+0.036.（5）Both the uniaxial compression test and the Brazilian test of layered slate are simulated by the finite-discrete element method where the cohesive zone model is used to characterize the transverse isotropy.The predictions by the numerical simulations are compared with the experimental results and it is found that their failure patterns are fairly the same.By conducting the SCB simulations of layered slate,the characteristics of fracture evolution is analyzed in detail.（6）With the help of the simulations for the SCB tests of layered slate with different bedding angles,it is found that a range of S/2R as 0.6-0.8 is suitable and feasible to gain the fracture toughness of layered slate in laboratory SCB tests.（7）In view of the fact that tensile and shear failure is a common phenomenon in rocks,the asymmetric SCB numerical simulations of layered slate for the pure shear fracture are carried out by changing the span form.The mechanical response and fracture patterns with different bedding angles are summarized and analyzed,in which the mode II fracture toughnesses of arrester,short transverse and divider type are 0.215,0.125 and0.289 MPa（?）,respectively.