The Effect Of Primary Micro-cracks On The Mechanical Properties Of Dolomite Under Uniaxial Compression

Rock is a complex natural medium. Due to the difference of diagenesis and diagenetic environment, there are a large number of defects, such as joints, cracks, micro cracks exist inside the rock. The study of the mechanics properties of cracked rock are focused on by academic circles of engineering and rock mechanics in recent years. The experimental study on mechanical properties of rock containing natural micro cracks has important practical significance for ensuring the stability and safety of rock engineering.Combining with engineering practice, this paper bases on the National Natural Science Fund Project" the accumulative deformation conversion characteristics and instability mechanism of deep jointed rock roadway in the frequent strong vibration and high stress environment (project number:41362013)". In order to study the influence rule of native microcracks on the mechanical properties and failure mode, a series of uniaxial compression test on dolomite rock specimens containing native microcracks were carried out. Before the experiment, acoustic wave detector and natural water absorption method were used to test sonic wave velocity and open porosity of dolomite specimens, and the development degree of rock specimens containing natural microcracks can be by reflected. On this basis, the macroscopic mechanical properties, acoustic emission rules and deformation characteristics of dolomite under uniaxial compression are studied. The relationship between compression strength, elastic modulus, Poisson ratio, acoustic emission (AE) and failure characteristics of dolomite and fracture development were analyzed. And the influence of native micro cracks on the mechanical properties of dolomite under uniaxial compression was revealed. The main conclusions are as follows:(1) The existence of micro cracks actually reduces the velocity of acoustic wave propagation in the rock medium. And with the increase of open porosity, the effect on the compressional wave velocity is weakened. The acoustic wave propagation in fractured rock show obvious "crack effect". By fitting curve found that, between the longitudinal waves velocity of dolomite samples with open porosity has a good relationship of logarithmic decrement.(2) The initial configuration and parameters of the circle surface microcracks of dolomite samples are analyzed. The research methods of fractal geometry is utilized to calculate micro cracks fractal dimension of dolomite samples, and draw out the curves between fractal dimension with number density and length density of microcracks. It is found that between fractal dimension and number density or length density of micro cracks has a linearly increased variation relationship, which shows micro cracks on the rock surface have good fractal characteristics.(3) The test results of dolomite specimens under uniaxial compression are analyzed. Through the curves between stress and strain, the strength characteristics and deformation characteristics, acoustic emission characteristics under uniaxial compression are studied. The microcracks have a significant impact on strength characteristics and deformation characteristics of dolomite. Both the uniaxial compressive strength and elastic modulus of dolomite show decrease in general trend with open porosity increases, and the lower amplitude is changing gradually from big to small. But Poisson ratio do not show obvious trend with the increase of open porosity from this experiment data.(4) Through analyzing destruction forms of dolomite samples, it is found that there are three damage forms of tensile failure, tensile shear compound fracture failure and sliding along the crack surface damage. The findings after correlated analysis discover that the failure modes are closely related to the initial state of the fracture distribution of samples surface.(5) The damage evolution process of dolomites’microcracks under uniaxial compression is analyzed. It is found that due to the difference of initial geometric distribution of microcracks, the cracks’ macroscopic condition of cracking, extension and linking has bigger difference.