Behaviour of sparsely fractured rock in low confining stresses

岩石破坏的产生和扩展是岩石工程中的一个重要问题。为了了解岩石和岩体作为工程材料的特性，有必要从完整岩石的行为开始。从工程的角度来看，这涉及研究实验室规模的样品，例如直径在50 mm范围内的金刚石钻芯。对于许多岩石类型，粒度足够小，因此可以认为这种规模的样品代表了较大的完整岩石体积。在确定岩石的完整强度时，采用了几种实验室试验。比较常见的试验是确定抗拉强度的巴西试验，确定约束为零时单轴抗压强度的单轴试验，以及确定约束条件下剪切强度的三轴试验。几十年来，剪切强度的结果被认为是建立岩石破坏包络线的最重要的试验。现在公认的是，完整岩石的抗拉强度是控制完整岩石的最终行为的基本材料性质。广泛使用的Hoek-Brown经验破坏准则是基于岩石三轴压缩破坏的剪切准则。该准则的参数来自于室内压缩试验。初步结果表明，Hoek-Brown不足以预测在拉伸区域的强度时，只有剪切强度作为输入。在本提案中，我们计划开发一种约束拉伸试验，用于跟踪完整岩石从拉伸到压缩的行为。从这些测试的结果将被用来作为修改的Hoek-Brown准则的条件下，拉伸应力可以主导的破坏过程的基础。大型岩质边坡的稳定性主要取决于岩体的粘聚力。这种凝聚力可被张应力破坏，张应力是由完整岩石和岩体的自然非均匀性产生的。用抗拉强度作为代用指标，可以有效地量化岩石粘聚力的大小。内聚力的另一个替代指标是抗压强度与抗拉强度之比，该替代指标用于Hoek-Brown准则。最近的研究表明，这个比率实际上是一种材料属性。将使用侧限拉伸试验来编制各种均质和各向异性岩石的该比率。一个新开发的边坡稳定性数值程序，利用这个比例作为输入参数将被用来评估几个案例的历史，边坡的稳定性一直难以评估。这些案例历史有大量的文件，传统分析的结果并没有对它们的行为提供充分的解释。室内试验结果，结合新开发的边坡模型和实例，将为应用该方法评估岩石边坡的稳定性和变形行为提供指导。研究结果将使采矿业和民用工业发展合理的设计方法，边坡开挖节理岩体。

The initiation and propagation of failure in rock is a matter of fundamental importance in rock engineering. In order to understand the characteristics of rock and rock masses as engineering materials, it is necessary to start with the behaviour of intact rock. From an engineering point of view, this involves studying laboratory scale samples, such as diamond drill core, with dimensions in the range of 50 mm in diameter. For many rock types, the grain size is small enough that samples of this scale can be considered representative of larger intact rock volumes.When establishing the intact strength of rock, several laboratory tests are employed. The more common tests are the Brazilian test to establish the tensile strength, the uniaxial test to establish the uniaxial compressive strength when confinement is zero, and the triaxial test to establish the shear strength under confined conditions. For decades, the results from shear strength were considered the most important tests in establishing the failure envelope for rock. It is now accepted that the tensile strength of intact rock is a fundamental material property that controls the ultimate behaviour of intact rock. The widely used Hoek-Brown empirical failure criterion is a shear criterion based on the triaxial compressive failure of rock. The parameters for the criterion are derived from laboratory compression tests. Preliminary results show that the Hoek-Brown is not adequate for predicting the strength in the tensile region when only the shear strength is used as input. In this proposal, we plan to develop a confined-tension test that will be used to track the behaviour of the intact rock from tension to compression. The results from those tests will be used as the bases for modifying the Hoek-Brown criterion for those conditions where tensile stresses can dominate the failure process. The stability of large rock slopes is dominated by the cohesion of the rock mass. This cohesion can be destroyed by tensile stresses, which are created by the natural heterogeneity of the intact rock and rock mass. Quantifying the magnitude of the rock cohesion can be effectively carried out using the tensile strength as a proxy. Another proxy for cohesion is the ratio of the compressive strength to the tensile strength, and this proxy is used in the Hoek-Brown criteria. Recent research suggests that this ratio is in fact a material property. The confined tension tests will be used to compile this ratio for various homogeneous and anisotropic rocks. A newly developed slope stability numerical program that utilises this ratio as an input parameter will be used to evaluate several case histories where the stability of the slope has been difficult to assess. These case histories have extensive documentation, and the results from traditional analyses have not provided adequate explanations of their behaviour. The laboratory test results, in combination with the newly developed slope model and case histories will provide guidance on the application of this methodology for assessing the stability and deformation behaviour of rock slopes. The results from this research will enable both mining and civil industries to develop rational design approaches for slope excavations in jointed rock masses.