Study On Time-lagged Fracture Characteristic Of Brittle Rock Under High Stress Condition

In recent years, with the development of our national economy and science and technology, we will face more and more deeply rock engineering. A series of challenging rock mechanics and engineering issues such as high stress, high external water pressure due to structural safety, rock burst and long tunnel rapid construction will be faced. Study the mechanical properties of deep rock through experimental has become an urgent task. Key issues such as micro-fracture mechanism and strength deterioration with time of brittle deep rock under high stress is the deep rock mechanics theoretical study of international topics at the forefront currently. Most of the western hydropower project has a large underground cavern depth and high stress. The mechanical response of caverns is significantly different from the shallow rock mass. It has significant difference in aspects of disaster mechanism, engineering stability control and design theory. Study mechanical characteristics of surrounding rock under high stress conditions and clarify its deformation and fracture mechanisms is an urgent scientific issue need to solve.In this paper, typical brittle rock such as marble and limestone are chosen for the study. The effects of time-logged fracture, the law of destruction driving stress level evolution with damage-time and the law of strength parameters deterioration with damage-time of high stress brittle rock are studied by experiment, theoretical analysis and numerical simulation. Consider the impact of high stress and damage-time, selecting appropriate constitutive model, numerical simulation of excavation and maintenance analysis are studied on a hydropower station on the underground caverns. Distribution characteristic and evolution law of the displacement field, stress field, plastic zone, etc. of surrounding rock during excavation are researched. The regular pattern and the amount of deformation, potential failure mode and key location of the mechanical behavior of surrounding rock during the excavation of underground powerhouse is grasped. It can provide evidence for the underground plant support design improvements, monitoring arrangements, etc..The research wok mainly concentrated in the following aspects in this paper.(1) Based on conventional uniaxial testing and time-lagged fracture uniaxial compression testing of high stress brittle rock are carried out. This paper analyzes the strength characteristics, stress-strain characteristics and failure mode to obtain the relationship curve between stress intensity and destruction driving stress level with the damage-time. This paper gets a conclusion that the relationship curve between stress intensity and destruction driving stress level with the damage-time are exponential relationship by curve fitting. This paper alse compare and analyze the similarities and differences of high stress rock failure mode between conventional uniaxial testing and time-lagged fracture uniaxial compression testing.(2) Typical brittle surrounding rocks like marble and limestone which under high stress conditions of underground caverns are chosen as testing sampling. Based on MTS, different stress state (low pressure, medium pressure, high pressure) under triaxial compression plus, uninstall destruction tests are carried out. This paper analyzes the strength characteristics of different pressure and different stress paths. The differences of rock strength and deformation characteristics under normal stress levels and high levels are discussed. The results show that marble and limestone at low pressure loading path exhibite obvious brittle. But with the pressure increasing it shows some ductility characteristics. The peak brittleness under unloading stress paths is more significant than that under loading stress paths. The triaxial unloading strength parameters of two types of rocks show that the internal friction angle is increase and the cohesion is drop.(3) Based on Mohr-Coulomb strength criterion, the rock strength characteristics under low, medium and high pressure are analyzed. The strength parameters under different pressure are so different between each other. It indicates that strength parameters under high stress conditions are not constant with the pressure change. So that a linear relationship can’t be a good reflection of the rock sample pressure and variation of strength. And nonlinear strength criterion like Hoek-Brown criterion and power function criterion has advantages to describe nonlinear characteristics for the rock under high stress conditions.It can reflect the strength variation characteristics change from low pressure to high pressure of surrounding rock. Deformation and strength characteristics of rockmass are nonlinear under high stress conditions. Constitutive relation and strength criterion of rock under high stress is analyzed in this paper. Failure mode of the marble and limestone in different stress states (low pressure, medium pressure, high pressure) under triaxial compression loading and unloading test is analyzed.(4) In this paper, marble is chosen as the research object. The time-lagged triaxial compression test under high stress is analyzed. The test results show as follow:It is brittle fracture of small deformation in the confining pressure time-lagged fracture trials of brittle rock. It shows significant time-lagged fracture characteristic. The lag time of rock sample failure is up to few hours or even days. The lag time of rock destruction is greatly increased because the confining pressure. With extending damage-time the strength parameters like friction angle and cohesive of rock variation occurs in the following:The cohesion with the development of damage declines quickly from peak and soon reach the residue limits. The friction angle with the development of damage increases first and then reduces. Where in the increased phase is gradually increased to a peak when the most cohesive force losses. The crack of time-lagged triaxial compression test of rock samples are mostly along the axial extension which is not showing chunks of damage, but fragmented into a number of relatively thin sheet and a lot of flaky debris.In this paper, the relationship of damage-time with destruction driving stress level (partial stress/strength ratio) of high stress brittle rock are acquired. It can predict rock failure time under a relatively low level of destruction driving stress level.(5) In this paper failure mechanism through mesoscopic experiment of brittle rock under high stress loading and unloading stress paths is studied. The law of rock fracture morphology, micro-crack growth and the number of cross-sectional CT scan under different stress paths is analyzed. From Microscopic view, macro fracture mode of the rocks under high stress is discussed. The results show as follow. Fracture of marble under loading condition its macroscopic crack mainly due to mineral grains sliding movement and mineral crystal cleavage displacement. When fracture of marble under unloading condition, there are more burst fracture propagating crack. It is mainly the friction sliding between the crack surfaces. Under the condition of unloading marble sample is easy to led tensile shear failure. When fracture of Limestone under loading condition, microporous crack fracture aggregate in crack surface under compression-shear fracture state. It is obvious that scratches and clay minerals are alignment and worn-off crystal aggregates along the shear plane. When fracture of Limestone under unloading condition, mainly shows the state of tension and shear fracture cleavage plane between the hidden bedding and intergranular fracture as well as their mutual coupling form. Micro-cracks are along quartz or feldspar cleavage fracture randomness distributed and develop along the initial defects. Mesoscopic testing through a variety of means, including the fine scanning electron microscope, CT scans and other tests and acoustic emission AE. Characteristic and mechanism of micro-crack of brittle rock under high stress is studied.(6) Deep buried brittle rock will be influenced by the time and surrounding high stress. In the long process of loading, rock strength can be reduced to approximately 60% of the instantaneous strength. According to time-logged uniaxial compression test and time-logged triaxial compression test as well as the existing basic theory, the relationship between stress intensity and damage-time and the relationship between driving stress level and damage-time is established. The function of strength parameters c,φ over time is fitted. On this basis, a rock strength parameters over time weaken model is established. This model can reasonably describes the rock strength weakening process with continuous loading. It can obtain the law of rock strength with time weakening. It can explain the effect of time-lagged fracture of rock from the theoretical mechanism.(7) Surrounding rock of deep tunnel will be damaged and stress will be redistributed after the excavation. This stress change produces a different in-site phenomenon in the different kind of surrounding rock. The rock mechanics parameters (elastic modulus E, cohesion c, internal friction angle cp) will continue to decrease with the extension of damage time. The characteristic of surrounding rock within a certain region will be deteriorated after excavation. A reasonable constitutive model is selected which consider not only the high stress but also damage-time to reflect the deterioration characteristic of brittle rock damage zone. With the numerical simulation analysis, rock mechanics parameters is dynamically updated following the unit to reflect the continued deterioration of the surrounding rock during the cavern excavation. It can evaluate and predict the scope and depth of surrounding rock damaged area (loose circle) by analyzing the stress and displacement field after fractured.(8) Marble and limestone under high stress have the characteristic that brittleness can change to plastic. It can exhibit brittleness when close to the low confining pressure condition. It is a very widespread phenomenon when damage occurs in brittle rock under high stress. Fracture began to appear in the stress level under relatively high (such as 60% of peak stress) conditions. Although this damage is not yet cause a sharp change in rock strength and mechanical properties, but with the progress of the project develop, the damage expand in a direction of fracture that may give a hidden danger during tunnel project operation. Through this study, time-lagged fracture of the surrounding rock can be predicted and forecasted. It can provide a theoretical basis for its prevention.