Showing papers by "Jian Zhao published in 2008"

Dynamic model of fracture normal behaviour and application to prediction of stress wave attenuation across fractures

Abstract: The purpose of this paper is to establish a dynamic constitutive model of fracture normal behaviour, based on laboratory tests of artificial fractures cast by cement mortar. A series of tests are systematically carried out under quasi-static (10−1 MPa/s) up to highly dynamic (103 MPa/s) monotonic loading conditions. The normal stress-fracture closure response is measured at different loading rates. Based on the measured curves, a nonlinear (hyperbolic) dynamic model of fracture normal behaviour, termed as dynamic BB model, is proposed. The dynamic model is modified from the existing BB model of static normal behaviour of fractures by taking into account the loading-rate effect. Two important dynamic parameters of fractures, FSC d (dynamic fracture stiffness constant, which describes the incremental ratio of dynamic initial stiffness) and FCC d (dynamic fracture closure constant, which describes the decremental ratio of dynamic maximum allowable closure), are identified. They indicate the quantitative degree of loading-rate effect on fracture normal behaviour subjected to dynamic loads. For practical application, the new model is incorporated into the Universal Distinct Element Code (UDEC) and subsequently, UDEC modelling of normally incident P-wave transmission across single fractures with the dynamic BB model is conducted. Wave transmission coefficient is obtained for various combinations of fracture dynamic parameters, as well as different wave amplitudes and frequencies. The numerical results show that wave transmission coefficient for a fracture with the dynamic BB model is greater than that for a fracture with the static BB model. In addition, a fracture with higher values of FSC d and FCC d leads to higher transmission (lower attenuation).

Simulation of failure process of jointed rock

Abstract: A modified discontinuous deformation analysis (DDA) algorithm was proposed to simulate the failure behavior of jointed rock. In the proposed algorithm, by using the Monte-Carlo technique, random joint network was generated in the domain of interest. Based on the joint network, the triangular DDA block system was automatically generated by adopting the advanced front method. In the process of generating blocks, numerous artificial joints came into being, and once the stress states at some artificial joints satisfy the failure criterion given beforehand, artificial joints will turn into real joints. In this way, the whole fragmentation process of rock mass can be replicated. The algorithm logic was described in detail, and several numerical examples were carried out to obtain some insight into the failure behavior of rock mass containing random joints. From the numerical results, it can be found that the crack initiates from the crack tip, the growth direction of the crack depends upon the loading and constraint conditions, and the proposed method can reproduce some complicated phenomena in the whole process of rock failure.

Model experiments on the broken zone in intermittently jointed surrounding rock

Abstract: A triaxial roadway model and digital photogrammetry were used to analyze the deformation of the surrounding rock that had intermittent joints.The effect of the intermittent joints on the production,growth and size of the broken zone in the rock was investigated.Crack initiation,propagation and coalescence mechanisms as well as failure behavior of the rock are discussed.The results show that crack propagation and coalescence within the joints produce interlaced cracks in the surrounding rocks.Intermittent joints influence the development of a broken rock zone in a usually unsymmetrical way.The broken rock is produced primarily nearby the roof joints of the roadway and then it expands into the conjoint part.The maximum thickness of the broken rock zone parallels the joint while the minimum thickness is normal to the direction of the joint.