卸荷岩体突发性破坏的宏细观动态失稳断裂机制

The burst failure of unloaded rockmass is a worldwide difficult problem in the geomechanical engineering that has not been well solved yet. In this project it is pointed out that such failure behavior is induced by the unstable dynamic propagation of fractures on the meso and macro scales driven by the strain energy stored in the unloaded rockmass. From the standpoint of dynamic fracture the burst failure mechanism of unloaded rockmass is studied. It is hypothesized that the energy used to drive unstable dynamic fracturing in a local rockmass is provided by a finite volume of surrounding rockmass. Such finite volume of surrounding rockmass is defined as the characteristic volume of rockmass in this project. The strain energy-driven dynamic fracturing process is realized by a self-developed loading-unloading device and the spring loading device in the material testing machine. By using such testing devices, the strain energy-driven dynamic fracturing process under simple conditions is investigated. To further study the strain energy-driven dynamic fracturing process under complex conditions, the numerical simulation is employed. To multiscale model the rockmass, the virtual multidimensional internal bond and the discretized virtual internal bond are used to model the rock matrix. The constrained cohesive finite element method and the element partition method are used to model the meso and the macro fractures, respectively. By numerical simulation, the evolution of macro-meso fracture, the critical condition for unstable dynamic fracturing onset, the dissipation mechanism of unstable fracturing process and the extension degree of unstable fracture are investigated. By this research, the unstable dynamic fracturing mechanism driven by strain energy will be revealed. The determination method of characteristic rockmass will be developed. The criteria for the trigger and evolution of unstable dynamic fracturing process of the macro-meso fracture system will be developed in terms of strain energy density of the characteristic rockmass. It is promising to reveal the mechanism of the burst failure of unloaded rockmass from the standpoint of dynamic fracture and provide a new fundamental theory for prediction and control of such failure process.

卸荷岩体突发性动力破坏是岩土工程界尚未解决的世界性难题。本项目明确提出该破坏行为是应变能驱动下的宏细观裂纹动态失稳扩展所致，从动态断裂角度对其进行研究。假设局部岩体裂纹失稳扩展所需的能量由周围有限体积的岩体提供，提出了特征岩体的概念。采用自主研发的试件加卸载装置和弹簧装置来实现应变能驱动下的裂纹动态扩展实验，并对简单应变能驱动下的裂纹动态扩展规律进行研究。采用多维虚内键(VIB)和离散虚内键力学模型对岩石基质进行多尺度建模，采用单元劈裂法和约束型粘结单元法分别对岩体宏、细观裂纹进行建模。应用数值模拟手段对复杂应变能驱动下的裂纹演化、失稳扩展临界条件、能耗机理及扩展程度进行研究。揭示应变能驱动下裂纹失稳扩展机制，明确特征岩体的确定方法，以特征岩体应变能密度为基本参量建立裂纹演化和失稳扩展准则。有望从动态断裂角度揭开卸荷岩体突发性动力破坏的发生机制，为预测和控制该破坏行为提供新的理论依据。

卸荷岩体突发性动力破坏是一个世界性难题。本项目明确提出了该破坏行为是应变能驱动下的宏细观裂纹动态失稳扩展所致，从动态断裂角度对其进行研究。主要内容包括：应变能驱动下的裂纹动态应力强度因子(DSIF)、宏细观裂纹演化、失稳扩展及其能耗机理。.研究发现围岩刚度、裂纹体模量、地应力大小、卸荷速率和裂纹特征皆对卸荷DSIF有不同程度的影响。其中围岩刚度比(围岩与裂纹体模量比)尤为显著，其值越小，DSIF越大，越易诱发岩爆。同样，应变能驱动下的卸荷裂纹演化也主要与这些因素有关，声发射表现出很强的规律性。当围岩刚度比小于一临界值（约为8.0）时，其值越小，围岩所提供的能量就越多，裂纹扩展越剧烈，声发射与刚度比呈指数关系。研究结果证实了局部卸荷岩体裂纹扩展所需的能量主要由其周围有限岩体（特征岩体）提供，其尺度约为裂纹体的4倍。在此尺寸范围内，声发射与围岩尺寸呈很强的指数关系。.当初始轴压比超过一临界值时（约为0.7），突然卸荷围压会导致裂纹失稳扩展，并且声发射与初始地应力呈幂函数关系。当有顶板下沉时，卸荷裂纹面的增加与时间具有明显的三阶段特征，并满足指数型函数关系。卸荷岩体破裂区以纵波速度向岩体内部扩展。岩石粘性会对动态裂纹扩展有较强的迟滞效应，揭示了岩爆迟滞的粘性机理。.应变能驱动下的裂纹扩展总面积与应变能密度及扩展时间均呈很强的幂律关系，与残余应变能密度呈线性关系。裂纹面增长速度与应变能耗散率呈很强的线性关系。揭示了应变能驱动下的裂纹扩展、能量演化和能量耗散关系。无外界能量输入的卸荷条件下，存在临界侧压系数（约为0.509）使得裂纹扩展最为复杂；预裂缝可减弱裂纹动态扩展程度。当预裂纹倾角由45°分别向0°或90°过渡时，裂纹扩展越剧烈。.研究成果揭示了卸荷岩体突发性破坏的宏细观动态失稳断裂机制，为岩爆预测和控制提供了理论依据。