Damage Weakening And Fracture Failure Mechanism For Rock Mass Concerning Influence Of Water And Initial Defects

The rock mass contains a large number of initial defects, including holes, pores, microcracks, and joints, etc., the existence of initial defects increase the level of water-rock contact and change the mechanics and deformation characteristics of rock mass. The project practice indicates, water-rock interaction is one of the important factors to influence the stability of rock mass engineering, the effect of water-rock interaction does not only includes seepage pressure, but also exists the effect of complex water-rock chemical corrosion, research shows that most rock mass instability and faluire is closely related to water-rock interaction. Nowadays, with the development of large-scale construction project, the studies on mechanical characters of rock mass under the action of water have important theoretical significance and practical value to ensure the long-term stability of rock engineering, The results can provide scientific bases for engineering stability research, such as slope, dam, tunnel, underground mine, oil exploitation.This work has been supported by the National Natural Science Foundation of China (NO.10972238), Hunan Provincial Innovation Foundation for Postgraduate (No.CX2009B046), Hunan Safety Production Technology Development Guidance Project (2009) and the Graduate Degree Thesis Innovation Foundation of Central South University (No.2009ybfz06&No.1343-0024). The paper has carried out the research work involving several aspects as follows:(1) In order to research the corrosion behavior of hydration effect in rock damage, the dynamical reaction process of rock chemical damage effect has been analyzed. The calculation of dissolution rate of mineral components after the contact of water and rock has been studied. And in theory, it has been discussed the quantification analytical method of equivalent crack propagation under the chemical corrosion. By adopting the Law of conservation of mass, it has been worked out the variables in the directions of crack length and width and the hydration corrosion has been equivalent to the effective crack length. Finally it has been established the calculated formula of effective crack length under the chemical water-rock interaction and the stress intensity factor has been worked out.(2)In order to study the time-dependent of damage and degradation effect for soft rock under water corrosion, subcritical crack growth of soft rock was studied by double torsion test using Instron1342type electro hydraulic servo test machine and the mechanics parameters test using RYL-600rock test machine after marinated soft rock in water for different time. Based on testing results, the rate of water content (ω), the mechanics parameters, the subcritical crack growth velocity (V) and the stress intensity factor (KI) and the fracture toughness (KIc) were obtained respectively, the results show that the rate of water content(ω) VS the time of water-rock interaction relations accord with good linear rule and could established the function relationship of damage and degradation velocity(D) for soft rock by means of the rate of water content(ω) and elasticity modulus(E); And the results analysis for the relationship of subcritical crack growth velocity(V)and stress intensity factor(KI) of soft rock in different time were studied by double logarithm coordinate, the results show that lgKrlgV relations accord with good linear rule and the fracture mechanics effect of water corrosion action of swelling soft rock is evident and time-dependent, and the water corrosion damage can accelerate subcritical crack growth velocity(V) and can weaken fracture mechanical characteristic of swelling soft rock.(3) In order to study the gradual damage fracture mechanism of initial damage rock mass, the macro-mechanical experiments on regular cracks body in initial damage rock-like materials have been made. Using white cement as the similar materials, the rock-like specimens in which the cracks were arranged in different echelon design through pre-installed steel slice were produced, the research on the models of fracture propagation and coalescence, as well as the law of strength variation with the cracks collocation and crack numbers under uniaxial loading were summarized systematically. (4) For studying the fracture evolution of rock mass that contained initial damage under the osmotic pressure, the influence of seepage pressure to rock fracture and damage has been investigated. For those frictional cracks with filler, it has been deduced the stress intensity factor of crack under the osmotic pressure with leading the damage concept into the crack model and new definition of damage variable. For the rocks that have single crack under both the compression-shear and tensile-sheared stresses, their initiation damage has been discussed under water condition. Finally, the fraction criterion of single crack rock has been given with consideration about water damage.(5) In order to study the evolutionary process of brittle rock damage and fracture under impact loading, the dynamic damage was studied by variable angle impact test using compressed gas of SHPB to launch the ball to impact the lateral margin of rock plate, when a source of impact loading is applied to a rock plate specimen, damage cracks are developed from the source to the boundaries, and the surface cracks are able to effectively represent sample internal cracking condition and can effectively reflect the impact energy dissipation in these tests. Test results show that the cracks propagation length and break area are not arbitrary, but are influenced by the angle of dynamic impact loading. The crack length and the break area increase with the increase of the impact loading, when the break area decreases abruptly, the crack length increases suddenly, it shows that there was an obvious incubation period of crack forming, at first some pits appeared on the verge of the rock plate and the propagation rate of crack length and break area decreases with increasing the impact velocity.(6)In order to reveal blast-induced damage cumulative effect of rock mass, based on crack expansion and sonic speed variation, the calculation method about the rock mass damage and its cumulation has been discussed to discover the explosion damage and the damage accumulation on effect. The numerical damage cloud chart has been analyzed. The result shows that the same explosion energy can cause different damage areas with different cushion, and the damage of water-saturated rock mass is bigger than that of air cushion rock mass. Field monitoring result has been indicated that the source distance determines the rock damage degree:with the distance getting closer, the damage increment and accumulation are getting more. The error between numerical model and field monitoring is in the engineering permissible range. Thus, the calculation method of rock explosion damage in the paper can offer the compared basis for field monitoring. This research puts forward a new calculation method for measuring the damage of rock mass. So, this new calculation method can be used as a comparison basis for site monitoring of blast-induced damage of rock mass.