深埋洞室劈裂破坏形成机理的试验和理论研究

【作者】 李晓静；

【导师】 朱维申；

【作者基本信息】 山东大学 ， 工程力学， 2007， 博士

【摘要】 地下洞室群在开挖时，洞室之间的岩柱往往会出现纵向的劈裂裂缝现象，这种裂缝对于洞室的稳定性造成威胁。尤其对于处于大埋深、高地应力的脆性围岩中的地下洞室，开挖时更容易发生这种裂纹，且通常会有更为剧烈的脆性变形破坏发生，如岩爆等。少有学者针对这种现象展开深入研究。本文以该现象为研究对象，通过实验室模拟试验，能量耗散理论分析以及数值计算分析等手段，深入地研究和探讨了劈裂裂缝的形成条件与机理，取得了一系列有意义的研究成果。试验方面，首先对取自工程现场的真实岩石进行了室内单轴压缩、常规三轴试验以及用体积应变作为损伤变量的加卸载试验，测得了脆性岩石力学参数，并分析其强度特性。基于这些参数，选定了一定配比的水泥砂浆材料作为模拟脆性岩石的类岩石材料来模拟地下工程开挖时岩柱的加载—卸载过程。试验采用自行研制的真三轴加卸载装置，该装置可有效的实现对试件的三向加载和卸载，较好的模拟了地下洞室因开挖出现劈裂裂缝的过程。得出卸载使得岩柱内应力场发生重分布，部分区域出现应力集中。开挖面附近近似单轴受压的受力特点使得试件出现了劈裂裂纹。基于试验结果，从能量耗散理论出发，认为卸荷引起弹性能释放用于克服裂纹的表面能，引起裂纹开裂。分别推导得出了拉伸和压缩应力作用下，裂纹扩展所需的能量的表达式。结合试验室加卸载试验分析，得出脆性岩石中裂纹扩展受到颗粒粒径的影响的结论，并据此建立了沿颗粒粒径扩展形成劈裂破坏的细观能量模型。通过压缩试验获得的脆性岩石的全应力应变曲线，利用体积变量和裂纹体积变量等损伤变量分析得出裂纹发展的五个阶段即：压密、弹性、微裂纹稳定扩展、裂纹非稳定扩展和峰后失稳等。并得出随着加载的进行，脆性岩石内部会同时引起粘聚力和摩擦力的改变。基于断裂力学理论，采用简化的直线型滑移裂纹组进行分析，得出了微小裂纹扩展贯通形成纵向劈裂裂缝的临界长度、间距和应力。劈裂裂缝将岩柱分割成为多个厚度很小的岩板，在轴向压缩荷载的作用下，其受力状态类似于弹性力学理论中的薄板压曲问题。借鉴薄板压曲的相关理论，结合能量耗散分析方法，研究了出现宏观劈裂裂缝之后，岩柱失稳破坏的机理，得到了岩板发生压曲破坏的临界荷载，并且推导得出了劈裂裂缝条数的计算公式。根据前述工作，提出了劈裂破坏的稳定性判据、经验裂纹密度公式和围岩位移的预测分析方法。基于本文所提出的能量方法，结合数值软件FLAC，通过记录单元发生脆性破坏前后的弹性能密度差，得到单位体积岩体突然释放的弹性能量，最终获得整个洞室围岩的总能量耗散值。据此，可以有效判断围岩的破坏区域，为洞室稳定性提供判别依据。并利用本文提出的劈裂破坏判据和位移预测方法，针对二滩工程进行了对比分析取得较好的一致性效果。利用可以描述脆性岩石非均匀性的三维有限元程序RFPA^3D，模拟了二滩地下工程洞间岩柱的破坏过程，证实了在高地应力作用下，劈裂裂缝这种脆性破坏形式是造成洞室失稳的重要因素，同时也对物理力学试验的结果进行了验证。更多还原

【Abstract】 While the underground openings are being excavated, the rock pillars between the caverns are apt to appear longitudinal splitting cracks, which constitute a threat to the stability of the caverns. Especially the deeply embedded caverns under high in situ stresses, they are more readily to appear this kind of cracks and even intense brittle deformation failure, like rock blast etc. Only a few scholars are interested in further studying these phenomena. This paper mainly studies these phenomena by means of the tools like experimental model tests, energy dissipation theory analysis, and numerical simulation and so on. This paper further studies and probes into the shape conditions and mechanism of the splitting cracks, consequently attains a series of meaningful research achievements.From the experiments aspect, we first use the in-situ rock specimens to do uniaxial compressive test, triaxial compressive test and load-unload test with the volumetric strains as damage parameter, so we got the physical mechanical parameters of the brittle rock and analyzed the characteristics of the strength. According to the parameters, we chose a type of mortar as a rock-like material to model the load-unload process of rock pillars when excavating the underground openings. We have developed a triaxial load-unload apparatus by ourselves, which can effectively realize the triaxial load-unload test perfectly model the process of splitting cracks owing to excavating underground openings. The experimental results are as follows: unload can cause the stress redistribution in the rock pillars and stress concentration in part of them. The splitting cracks emerge in the excavation plane which is similar to the characteristics of uniaxial compression. Based on the experimentation result and the energy dissipation theory, the elastic energy change is absorbed by cracks for growth. The energy expressions are obtained under tension and compression conditions respectively. According to the load-unload experimental data analysis, the growth of crack is considered to be influenced by grain scale. As a result, the microcosmic energy model is founded.According to the stress-strain complete process curve, the total volumetric strains and crack induced 1 volumetric strains is adopted as damage parameters for analyzing the five phases during the crack growth （crack closure , linear elastic, stable crack growth, unstable crack growth and post failure）. The cohesion and friction is mobilized in the brittle rock.Based on the fracture mechanics, the linear-sliding cracks groups are adopted for analyzing the critical length, interval and stress. Splitting cracks divide the rock pillars into a few thin slabs, whose stress state is similar to the buckling of slabs. According to the corresponding theories on slab buckling and energy dissipation analysis, failure mechanism of rock pillars is studied after appearing the splitting cracks. The critical buckling failure load of slabs is achieved and the predictive formulas about the numbers of splitting cracks are deduced.According to the above result, the criterion of splitting failure, crack density formula and displacement forecasting method are put forward.Based on the energy method in this article and numerical procedure FLAC3D, the total dissipation energy is obtained through tracing the variety of elastic energy density of each element and recording the maximal fluctuation when brittle failure happens during numerical calculation. According to this method, the intension of rockburst and the position and extent of failure zone can be predicted during rockmass excavation under high in situ stress. The criterion of splitting failure and displacement forecasting method is used for contrastive analysis of Er’tan project. The result is accord with in-suit data.Making use of the RFPA^3D which can simulate the heterogeneous of brittle rocks, the failure process of rock pillars between the caverns in Er’tan project is simulated. It is proved that the brittle failure forms of splitting cracks under high in situ stress is the key factor which causes instability of caverns. In the meantime, the physical model test result is testified.更多还原