Characterising the size-dependent behaviour of rock has been a significant challenge in rock engineering particularly during the design of structures on or within a rock mass. Generally, the mechanical characterisation of rock starts at the laboratory scale where the intact rock is tested and then the resulting data is extrapolated to the field conditions using a suitable size effect model. Despite extensive research on the size effect of intact rock, very few have included the weak rocks with low uniaxial compressive strength (UCS). Thus, in this study, the size-dependent behaviour of two different weak rocks namely, Gambier limestone and artificial rock with uniaxial compressive strength of less than 10 MPa were investigated experimentally and analytically. The diameter of the cylindrical Gambier limestone samples varied from 26 to 285 mm while the diameter of artificial rock samples ranged between 26 and 139 mm. For Gambier limestone, the uniaxial compressive, Brazilian and point load experiments were carried out while for artificial rock, only the uniaxial compressive tests were conducted. In both rock types, the ascending and then descending size effect trend was a pronounced behaviour for UCS and Young’s modulus data while the size effect behaviour of Poisson’s ratio was inconclusive. Also, the tensile strength and point load index data obtained from Gambier limestone revealed only descending size effect trends. The unified size effect law and its improved version were fitted to the UCS and Young’s modulus data leading to a very good agreement between the data and the model predictions. It was confirmed that an improved version of unified size effect law can predict a more realistic strength value for a sample with an infinite size. Finally, the applicability of two descending size effect models to the tensile strength and point load data was assessed and concluded that the multifractal scaling law is a suitable model for the point load data while the size effect law can better predict the tensile strength data.

中文翻译：

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弱完整岩石的尺寸相关行为

表征岩石的尺寸相关行为一直是岩石工程中的一个重大挑战，特别是在岩体上或岩体内部的结构设计过程中。通常，岩石的力学特性从实验室规模开始，在此测试完整的岩石，然后使用合适的尺寸效应模型将所得数据外推到现场条件。尽管对完整岩石的尺寸效应进行了广泛的研究，但很少包括具有低单轴抗压强度 (UCS) 的软弱岩石。因此，在本研究中，通过实验和分析研究了两种不同的弱岩，即甘比尔石灰岩和单轴抗压强度小于 10 MPa 的人造岩石的尺寸相关行为。圆柱形甘比尔石灰岩样品的直径从 26 到 285 毫米不等，而人造岩石样品的直径则在 26 到 139 毫米之间。甘比尔灰岩进行了单轴压缩、巴西和点荷载试验，而人造岩石仅进行了单轴压缩试验。在这两种岩石类型中，UCS 和杨氏模量数据的尺寸效应趋势是先升后降，而泊松比的尺寸效应行为尚无定论。此外，从 Gambier 石灰岩获得的抗拉强度和点载荷指数数据仅显示尺寸效应下降趋势。统一尺寸效应定律及其改进版本适合 UCS 和杨氏模量数据，导致数据和模型预测之间非常吻合。经证实，统一尺寸效应定律的改进版本可以为具有无限尺寸的样本预测更真实的强度值。最后，评估了两种递减尺寸效应模型对抗拉强度和点载荷数据的适用性，得出的结论是，多重分形标度定律适用于点载荷数据，而尺寸效应定律可以更好地预测抗拉强度数据。