Meso-Mechanical Testing Study On Characteristics And Mechanism Of Water Damage Of Gypsum Breccia

The softening mechanism when soft rock encounters water is an important subject in the field of water-rock interaction research. The internal damage in mesostructure of soft rock, which is caused by a watery environment, is the fundamental reason for a decrease of rock mechanical properties. Therefore, it's significant for engineering designs and constructions to find a reliable research method to quantitatively describe the change of rock mesostructure and make the relationship between mesosturcture characteristics and its macro mechanical properties.Gypsum breccia which is newly discovered during the construction of Taihangshan Tunnel, is taken as the experimental object. The mesostructure and mechanical properties of gypsum breccia with different initial water contents are discuessed. Furthermore, systematically quantitative analysis has been carried out on the basis of rock meso-mechanical test, using Digital Image Processing Technique (DIPT) and 3D Modeling, and the fractal theory has been also introduced into the research.In sum, the main work and conclusions including:Using a new type rock meso-mechanical testing system, an observation and experiment on whole process and real-time of gypsum breccia is carried out under the condition of uniaxial compress load. Digital images and videos of germination, extension, juncture, perforation till macroscopical fracture of micro-crack which is the rock initial damage, are obtained under the condition of different water contents. The stress-strain curve, the deformation and strength parameters of gypsum breccia with different initial water contents are determined, and the change laws are generalized,and a new damage factor is defined. On the basis of stress-strain curve and the mechanism of meso-damage caused by water, the process of damage and fracture is divided into four phases. Considered the condition of water damage, rule of rock damage evolvement is researched. A damage threshold value of stress is determined. The mechanism of three different damage patterns of specimens is dicussed.A great deal of mesostructural images of gypsum breccia of different initial water contents have been obtained by means of scanning electron microscopy (SEM) test. Modalities of rock mesostructure under different initial water contents are described. Based on Digital Image Processing Technique (DIPT), the algorithm program is compiled by Matlab to process the original SEM images to binary images. Then the parameters of rock pore mesostructure such as area, perimeter, diameter, quantity, shaper factor, etc. are obtained from binary images. The distribution of mesostructural parameters and their variation with increase of water content are analyzed according to quantitative information of digital images. Furthermore, the fractal characteristics of pore distribution under different initial water contents are studied based on fractal theory, and then the relationship between fractal dimension and macro mechanical properties is discussed.The morphology of rock fracture surface under the breakages of uniaxial compression and triaxial compression is observed and measured by means of a new-type 3D laser scanning system. Based on Geographic Information System (GIS) technique, the fracture surfaces are 3D visualized and reestablished. According to GIS 3D statistics, the geometrical characteristics of fracture surfaces under different breakage conditions are analyzed, and then based on fractal theory, a fractal model of fracture surface is established, the change laws of fractal dimension of fracture surfaces are discussed under the conditions of different cell pressures and initial water contents of rock. Furthermore, the relationships between characteristics of fracture surface and mechanical properties of rock are discussed.