Time-Dependent Behaviour Of Rock Mass And Its Reliability On Anchoring In Underground Engineering

The failure of caverns due to creep behavior of rock mass is common in underground engineering. Bolt and pre-stressed bolt to prevent long-term creep deformation is a good measurement in practice. However, there is few achievements on time-dependent behaviour of rock mass and its reliability on anchoring in underground engineering. With the background of creep properties of undeground powerhouse in DAGANGSHAN hydropower station. The main achievements are conducted as follows:1. Anchorage mechanism in rheological rockA differential equation describing load transfer mechanism between bolt and rock mass is established. Variations of axial stress distribution with time in bolt are deduced by both General Kelvin model and an empirical power index creep model, it is concluded that axial force in bolt increases with developing of creep deformation when it is in tensile stress state while decreases when it is in compressive stress state. Through analyzing the relation between axial stress of bolt and creep strain of rock mass, we make a conclusion that the location of peak stress of anchorage are consistent with that of length of plastic zone.2. Nonlinear damage creep constitutive modelA nonlinear damage model for creep rock mass is established. The parameter identification of the creep model has been done based on experimental and field creep test results. In addition, the creep damage characteristics have been analyzed, which indicate that the damage is stablely developing with time and viscous strain increasing. Furthermore, the damage model can show the failure process from stable creep stage to accelerating stage.3. Experimental study on anchorage interfaces and constitutive modelThe shear and shear creep experiments about interface mechanics of anchorage system are carried out. The shear creep strain grows with time and shear stress increasing. On the basis of experiment, a constitutive model which have a softened pressure-overclosure relationship in normal direction and a continuous contact formulation in tangential direction is proposed. The model is more reasonable because of its consideration of nomal stress. Using this model, it can be concluded: there is a peak stress along bolt when bolt passes through plastic zone of surrounding rock mass. By considering creep effect of rock mass, the anchorage peak stress moves towards the boundary as prestress increases, and toward inside as creep of rock mass occurs.4. Time-dependent reliability analysis of anchorage in underground engineeringIn this paper, a method analyzing time-dependent reliability of anchorage is proposed. On the basis of improved monte-carlo method, a new named Stratified—Latin hypercubic sampling is introduced which is more precise and less time-consuming. By using MATLAB and ABAQUS code, a program calculating the anchorage reliability is developed. With the program, Load-bearing capacity of anchorage is studied. The results indicate that the failure probability of anchorage system varies with rheological properties and amplitude of anchorage prestress.5. Long-term stability and time-dependent reliability analysis of anchorage for DAGANGSHAN underground powerhouseBased on design of DAGANGSHAN underground powerhouse, a numerical model considering nonlinear creep characteristics of granite, constitutive model of anchorage interfaces and the reliability of anchorage load-bearing capacity is established. The achievements from the numerical work can be concluded as follows:The stress of opthread-prestressed strand cables is bigger at two ends and smaller at intermediate parts. The axial force of opthread-prestressed strand cables increases due to creep of rock mass. The performance of strand anchor system is reliable for the failure probability is 0.85% and reliability is 2.39 correspondingly. The powerhouse is safety in long-term even considering creep and variation of mechancial parameters.